Operator Generic Fundamentals

Operator Generic Fundamentals
K1.01 Purpose of racking out breakers (de-energize components and associated control and indication circuits) K1.02 Local indication that breaker is open, closed or tripped K1.03 Loss of power supply circuit breaker indicator lights and capability in remotely open and close K1.04 Operation of various push buttons, switches and handles and the resulting action on breakers K1.05 Function of thermal overload protection device K1.06 Interpretation of symbols for breakers, relays and disconnects in a one-line diagram K1.07 Safety procedures and precautions associated with breakers, including MCC bus breakers, high, medium and low voltage breakers, relays and disconnects K1.08 Effects of closing breakers with current out of phase, different frequencies, high voltage differential, low current, or too much load K1.09 Effect of racking out breakers on control and indicating circuits and removal of control power on breaker operation K1.10 Function, control, and precautions associated with disconnects K1.11 Control room indication of a breaker status K1.12 Trip indicators for circuit breakers and protective relays Operator Generic Fundamentals Components – Breakers, Relays, and Disconnects

Terminal Learning Objectives
At the completion of this training session, the trainee will demonstrate mastery of this topic by passing a written exam with a grade of ≥ 80% on the following areas: Explain the purpose, safety precautions, and operation of electrical circuit interrupting and circuit switching devices. Explain the construction, operation, and indications for electrical circuit breakers. Describe the conditions that must be met prior to paralleling two generators, including effects of not meeting these conditions. TLOs

Circuit Protection TLO 1 – Explain the purpose, safety precautions, and operation of electrical circuit interrupting and circuit switching devices. 1.1 Explain the principles of circuit protection and their application, including selective tripping. 1.2 Describe the protection provided by each of the following: fuses, protective relays, circuit breakers, and overload devices. 1.3 Describe the function of the following types of switches: disconnect switch, automatic transfer switch and manual transfer switch. 1.4 Describe the personnel safety and equipment protection precautions associated with circuit interrupting devices and relays. 1.5 Interpret symbols for breakers, relays, and disconnects in a simple one-line diagram, and explain operation of the control circuit. 1.6 Explain the purpose and function of normal and power seeking automatic transfer switches. 1.7 Describe the functions, operation, and protective features of motor controllers. TLO 1

Circuit Protection ELO 1.1 – Explain the principles of circuit protection and their application, including selective tripping. Designed to de-energize circuit to protect equipment and circuit from electrical faults Uses various types of fault sensors Overcurrent, undervoltage, underfrequency, etc. Uses various types of circuit interrupting devices Fuses, breakers, etc. Designed to maximize system reliability Avoid unnecessary trips Isolate only portion of circuit necessary, continuing service to remainder Not Related to any specific KAs ELO 1.1

Figure: Selective Tripping
Protective device closest to fault operates to remove fault from system Maintaining largest possible portion of system energized In example below, fuses open circuit at 50 amps to Load 1 Output breaker for generator set to trip at 500 amps Ensures power to Load 2 1 2 75A Fuses Figure: Selective Tripping ELO 1.1

Circuit Protection Example
Power Plant Electrical System Isolates faults to prevent damage Maintains as much of the system energized as possible, maximizes plant reliability Functions as selective tripping Four Condensate Booster Pumps Motor Control Center (MCC) 1 supplies two pumps through individual breakers Motor Control Center (MCC) 2 supplies two pumps through individual breakers Ability to isolate faults at individual pump or power supply levels Draw simple booster pump line-up on whiteboard. 2 MCCs with 2 pumps on each. Use for discussion of this slide and next ELO 1.1

Circuit Protection Example
Fault on Individual Pump Motor Isolate the pump to prevent damage Maintain three pumps available and the plant online Maintain full electrical system availability Fault at Power Supply Level Isolate the power supply at the MCC Maintain other MCC available Maintain two pumps available and possibly the plant online ELO 1.1

Circuit Interrupting Devices
ELO 1.2 – Describe the protection provided by each of the following: fuses, protective relays, circuit breakers, and overload devices. Fuses Device containing fusible link that protects electrical circuit from overcurrent condition only Fusible link directly heated and destroyed by excessive current passing through it Element sized so heat generated by normal current flow does not melt element Overcurrent or short-circuit current flows through fuse Fusible link melts to open circuit (blown fuse) May be time-delayed Vital to replace safety-related fuses with correct type K1.05 Function of thermal overload protection device ELO 1.2

Types of Fuses Plug Fuse Consists of zinc or alloy strip
Fusible element enclosed in porcelain or Pyrex™ housing Screw base Normally used on circuits rated at 125V or less to ground Figure: Typical Fuses ELO 1.2

Types of Fuses Cartridge Fuse
Constructed with zinc or alloy fusible element Enclosed in cylindrical tube Element ends attached to metallic contact piece at ends of tube Normally used on circuits rated between 250 volts and 600 volts Maximum continuous current- carrying capacity of 600 amps Figure: Typical Fuses ELO 1.2

Circuit Breaker Protective Relays
Varied types of protective relays detect fault conditions Send signals to trip one or more circuit breakers to isolate the fault Protect equipment from damage and personnel from injury Different parameters are monitored to Provide prompt response to a fault condition Also avoid unnecessary system interruptions ELO 1.2

Overload Relay Devices (also known as overcurrent) Breakers usually provided with three overcurrent tripping devices Provides breaker with long-time, short-time, and instantaneous tripping capabilities Long-time delay trip (also known as 51) Device reacts to light overloads and trips breaker after a time delay Trips breaker on overload condition slightly higher than normal full load Example: gradual bearing failure ELO 1.2

Overload Relay Devices Short-time delay trip (also known as 51) Device reacts to slightly higher current and trips breaker in a shorter period of time Allows for motor starting currents without tripping breaker unless current level has not decayed within certain time frame after motor start Example: Locked/seized rotor Instantaneous trip (also known as 50) Device reacts quickly to trip breaker due to high currents Example: Short-circuit ELO 1.2

Undervoltage Relay (also known as 27) Used in large power systems with many induction motors Induction motors draw more current when voltage drops Challenges entire power system Isolating the cause of undervoltage protects the system Underfrequency Relay (also known as 81) Trips breaker when frequency drops below a preset value Protects loads on a system that cannot tolerate a significant change in frequency Westinghouse Reactor Protection Systems use class 1E UV and UF relays to protect the Rx core from low flow. ELO 1.2

Lockout (also known as 86) Fault should be isolated by breaker actuation Appears as if fault cleared, could allow breaker to reclose on fault Lockout relays prevent automatic reclosure Ensures the system is not re-energized before the fault is isolated Extreme caution is needed when reclosing a breaker that was opened by a protective relay: “closing back into a fault” can cause very high ground currents and explosive damage to equipment already degraded by the first short-circuit. ELO 1.2

Reverse-Power Relay (also known as 32) Senses a change in normal direction of current, indicating an abnormal condition A change in direction of power flow through breaker Power flowing into source versus power flowing out of source Usually used to protect electrical generator from damage Due to motoring Trips generator output breaker ELO 1.2

Thermal Overloads Heat sensitive element and an overload heater connected in series with motor load circuit When motor current is excessive and sustained Heat from heater causes heat sensitive element to open the motor breaker or motor line contacts Figure: Three-Phase Magnetic Controller With Thermal Overloads ELO 1.2

Molded case breakers with larger current ratings also have magnetic trip element to supplement thermal trip element Magnetic unit utilizes magnetic force surrounding conductor to operate circuit breaker tripping linkage After activation, must manually reset an overload device to resume motor operation Can reset magnetic overload devices immediately ELO 1.2

Resetting Overload Devices
An activated overload device must be reset for motor operation NOTE: Thermal overloads must cool before they can be reset Manual Reset Located in controller enclosure which contains overload device Usually has a hand-operated rod, lever, or button that returns device tripping mechanism to its original position and resets interlocks Automatic Reset Usually uses a spring or gravity operated device to reset overload device without operator action Only after condition causing overload has cleared Electrical Reset Actuated by an electromagnet controlled by a push button Used when desired to reset an overload device remotely ELO 1.2

Knowledge Check – NRC Exam Bank Which one of the following breaker trip signals will trip the associated motor breaker if a motor bearing seizes while the motor is running? Undervoltage Underfrequency Time-delayed overcurrent Instantaneous overcurrent Correct answer is C. Correct answer is C. NRC Bank Question – P2927 of – Motors and Generators Analysis: A. WRONG. Undervoltage trips are not commonly used on pump motors as they consume power, not provide power; undervoltage trips are commonly employed on incoming bus breakers or diesel generator output breakers. B. WRONG. Underfrequency trips are not commonly used on pump motors as they consume power, not provide power; underfrequency trips are commonly employed on incoming bus breakers or diesel generator output breakers. C. CORRECT. Gradual motor bearing damage will result in a higher steady state motor current, however, not high enough to reach the instantaneous overcurrent setpoint. The higher steady state motor current can result in the breakdown (thermal) of winding insulation. In order to secure power to the pump motor, the thermal overload devices, installed on the breaker, will heat up (I2R) and open contacts to secure power to the motor. Keep in mind that a motor bearing normally “gradually” seizes causing current to slowly go higher. This is why motor bearing failure normally results in some of short-term or long-term overcurrent protective device to actuate to protect the motor. D. WRONG. Although the breaker will trip on overcurrent, instantaneous overcurrent trip will only be actuated when a short or ground exists to cause the current to rise to a significantly higher value. ELO 1.2

Transfer and Disconnect Switches
ELO 1.3 – Describe the function of the following types of switches: disconnect switch, automatic transfer switch, and manual transfer switch. Provide flexibility within an electrical distribution system Used to change lineup of system or power source for loads within system Provide direct visual indication that a circuit is broken High-voltage disconnects used in switchyard operation K1.10 The function, control, and precautions associated with disconnects ELO 1.3

Disconnect Switches Two-position switches used for isolation of power supplies from one or more loads or motor control centers May be used in pairs to transfer power supplies from one source to another Figure: Typical Disconnect Switch ELO 1.3

Disconnect Switch Design & Operation
Disconnects differ from breakers Operated manually Not designed to be opened under load Design does not include arc chutes or any other means to extinguish arc drawn when disconnect opened Cannot be opened under load When energizing circuit with disconnect switch: Close disconnect switch first, then close breaker When de-energizing circuit with disconnect switch: Open breaker first, then open disconnect ELO 1.3

Disconnects may contain fuses, which provide overcurrent protection for loads supplied by disconnect If not equipped with fuses, provide isolation for circuit only Separate fuses or breakers would be required elsewhere in circuit to provide protection for loads ELO 1.3

Safety Switches Low voltage (less than or equal to 600VAC) switches that are enclosed May be locked in OFF Used in isolation points for electrical maintenance Special precautions required operating disconnects to protect personnel from potential arcs Leather gloves and safety glass should be worn Operator should stand to side of disconnect and look away during operation Protects operator's eyes and face from arc that may occur during switch operation ELO 1.3

Before disconnect can be opened, all electrical loads fed by disconnect must be verified off, or not operating Opening disconnect under load can result in damage to disconnect and injury to personnel ELO 1.3

Transfer Switches Used to make and break electrical circuits in order to provide smooth power transfer from one source of power to another Manual Transfer Switch Similar to disconnect switches Except, have three positions to allow power supply for an electrical component to be transferred from one source to another Manual transfer switches may contain internal fuse protection ELO 1.3

Knowledge Check What is an advantage of using high-voltage disconnect switches instead of breakers to isolate main power transformers? Disconnect switches can be operated either locally or remotely. Disconnect switches provide direct visual indication that the circuit is broken. Disconnect switches are cheaper and provide the same automatic protection as a breaker. Disconnect switches are capable of interrupting a higher current flow with less heating than a breaker. Correct answer is B. Correct answer is B. NRC Bank Question – P943 Analysis: A. WRONG. Like circuit breakers, motor-operated disconnects can be operated locally or remotely. B. CORRECT. Since high-voltage disconnects are not opened under load, they do not need any sort of arc suppressant. As a result, their contacts can do not have to be housed and can be visually seen to ensure the contacts are broken. Usually disconnects lift up or swing out to separate the line from the load side. C. WRONG. They may be cheaper, but they do NOT provide the same automatic protection as a breaker because they are opened unloaded (and usually by manual crank). D. WRONG. Unlike circuit breakers, disconnects do not provide the ability to interrupt current under load. A severe arc would occur. ELO 1.3

Knowledge Check A 480 volt AC motor control center supplies a load through a breaker and a manual disconnect. Which one of the following sequences will provide the greatest level of personnel safety when de-energizing the load for maintenance and when re-energizing the load after the maintenance? Open breaker first (de-energizing); shut disconnect first (reenergizing) Open disconnect first (de-energizing); shut breaker first (reenergizing) Open breaker first (de-energizing); shut breaker first (reenergizing) Open disconnect first (de-energizing); shut disconnect first (reenergizing) Correct answer is A. Correct answer is A. NRC Bank Question – P2944 (modified order) Analysis: Disconnects are used to manually isolate equipment electrically and that are NOT under load. If a disconnect were opened under load, an arc across the disconnect would occur. This is because unlike circuit breakers, a disconnect is simply a conductor; there is nothing to quench the arc. The interlock shown above between the feeder breakers (located downstream of the transformers) and the remote-operated disconnect (upstream of the transformer) prevents damage to the disconnect. This is accomplished two ways: Ensuring the breaker is open before the disconnect is open. This prevents the disconnect from being opened under load, which would result in arcing. 2. Ensuring the breaker is open before the disconnect is closed. This prevents attempting to shut the disconnect with a direct path for current downstream. If this was done, an arc would occur as the strong electrostatic field collapses the air across the disconnect as it was shut. ELO 1.3

Safety and Equipment Protection
ELO 1.4 – Describe the personnel safety and equipment protection procedures and precautions associated with circuit interrupting devices and relays. Personnel should always observe electrical safety precautions and PPE requirements: Do not open a disconnect switch under load Disconnects should not be used to start and stop equipment Follow all precautions for working on energized equipment when checking voltages on breakers, relays, and switches with test equipment K1.07 Safety procedures and precautions associated with breakers, including MCC bus breakers, high, medium and low voltage breakers, relays and disconnects K1.08 Effects of closing breakers with current out of phase, different frequencies, high voltage differential, low current, or too much load ELO 1.4

Do not remove or replace any fuse under load Never replace a fuse with one that has a different voltage or current rating than that of the intended circuit Perform the following before racking out circuit breakers: Ensure circuit breaker is open Ensure control power is removed when applicable Tag or lockout applicable electrical sources Utilize Personal Protective Equipment as specified for the voltage and current involved Always strip loads prior to reenergizing a dead bus ELO 1.4

Precautions The following electrical safety precautions are good work practices: Have a person stand by to deenergize the equipment in the event of an emergency Stand on insulating rubber material to increase the electrical resistance of the body to ground Cover exposed energized circuits with insulating material to prevent inadvertent contact Use insulated tools to prevent inadvertent contact with adjacent equipment ELO 1.4

Knowledge Check Which one of the following is an unsafe practice if performed when working on or near energized electrical equipment? Use insulated tools to prevent inadvertent contact with adjacent equipment. Cover exposed energized circuits with insulating material to prevent inadvertent contact. Attach a metal strap from your body to a nearby neutral ground to ensure that you are grounded. Have a person standing by with the ability to remove you from the equipment in the event of an emergency. Correct answer is C. Correct answer is C. NRC Bank Number is P2940 Analysis: A. WRONG. Using insulated tools will prevent a path for current flow through the tool to the body when the tool contacts an energized conductor. This is a safe practice. B. WRONG. Covering exposed energized circuits with an insulating material helps prevent inadvertent contact which could lead to electrocution. This is a safe practice. C. CORRECT. See discussion above. D. WRONG. When working on energized equipment, a person standing by to remove the person from the equipment; if a person contacts a conductor, muscles contract and it can be impossible to let go. This is a safe practice. ELO 1.4

Electrical Drawings ELO 1.5 – Interpret symbols for breakers, relays, and disconnects in a simple one-line diagram, and explain the operation of the control circuit. Common symbols include contacts, fuses, breakers, indicating lights, trip coil, and closing coil Breaker Indicating Lights Closing Coil, Trip Coil Fuse K1.06 Interpretation of symbols for breakers, relays and disconnects in a one-line diagram Overloads Open Contact Closed Contact ELO 1.5

Electrical Drawings Common Symbols
Overloads, relays, switches, rectifier bridge, and transformer Relays Switch Transformer Switches Open Switch Closed Switch ELO 1.5

Electrical Drawings Usually a legend on first sheet of drawing series
Different suppliers have differences in their conventions, so operator should review drawing legend when in doubt "a" contacts are open when the relay controlling it is de-energized, and closed when it is energized “b" contacts are closed when the relay controlling it is de-energized, and open when it is energized Drawing representation shows contacts in their de-energized state ELO 1.5

Circuit Breaker Control
To operate circuit breakers from remote location Electrical control circuit must be incorporated Control power supplied by AC source rectified to DC Some vital power sources use DC as control power Figure: Breaker Control Circuit ELO 1.5

Major Components: Rectifier unit Closing relay Closing coil Tripping coil Auxiliary contacts Circuit breaker control switch Only UF and UV trips shown, most breakers would also have 50/51 overcurrent trips Figure: Breaker Control Circuit ELO 1.5

Control circuit can be designed with protective features: Overcurrent Underfrequency Undervoltage Fault conditions cause associated contact to close Energize tripping coil Trip circuit breaker Figure: Breaker Control Circuit ELO 1.5

Closing a Remotely Operated Circuit Breaker
To close circuit breaker control switch, turn to CLOSE position Provides path to energize closing relay (CR) Energized closing relay shuts an auxiliary contact Energizes closing coil (CC) Closing coil shuts circuit breaker Figure: Breaker Control Circuit ELO 1.5

Once breaker is shut It is latched in the closed position "b" contact associated with the closing replay opens de- energizing Closing relay Closing coil (anti-pumping feature) Figure: Breaker Control Circuit ELO 1.5

When breaker closes "a" contact closes Enables trip circuit Circuit breaker control switch returns to neutral position when released Figure: Breaker Control Circuit ELO 1.5

Opening a Remotely Operated Circuit Breaker
Control switch turned to TRIP position Provides path to energize trip coil (TC) Releases latching mechanism Circuit breaker will open "a" contact will open deenergized tripping coil "b" contact will close Sets up next remote closure Control switch may be released Figure: Breaker Control Circuit ELO 1.5

Opening a Remotely Operated Circuit Breaker
Circuit breaker is equipped with Underfrequency trip Undervoltage trip Both/Either energize the breaker's trip coil opening breaker ELO 1.5

Analyzing Valve Control Circuits
Valve Fully Open Valve open limit switch contact is shut A1 relay energized A1(a) contact shut; red, open lamp energized Valve shut limit switch open A2 relay deenergized and A2(b) contact is shut Both A1(b) and A2(a) contacts open; green shut light out ELO 1.5 Figure: Valve Fully Open Circuit ELO 1.5

Analyzing Valve Control Circuit
Valve in Intermediate Position Both valve open and valve closed limit switches open Both relays deenergized; (b) contacts closed, (a) contacts open Both lamps energized ELO 1.5 Correct terminology would be “Limit Switch closed when valve FULLY shut”. Figure: Valve Intermediate Position

Analyzing Valve Control Circuits
Valve Closed Valve shut limit switch contact closed A2 relay energized; A2(a) contact closed, green shut lamp energized Valve open limit switch is open; A1 relay deenergized, A1 (b) contact is shut Both A2(b) and A1(a) contacts open; red open light out Correct terminology would be “Limit Switch closed when valve FULLY shut”. Figure: Valve Closed Circuit ELO 1.5

Electrical Drawings Example 1
Opening and Closing the Valve As you can see, energizing K1 relay opens valve, deenergizing K1 relay closes valve Refer to this drawing for the next seven slides. This is a drawing used in several NRC Bank Questions. Figure: Valve Control Circuit ELO 1.5

To manually open the valve Push PB2 Energizes K3 contact, which in turn energizes K1 relay There is no other means to open valve through control circuit After valve is opened, K1 relay has a seal-in K1 contact which maintains power to K1 relay and keeps valve open Figure: Valve Control Circuit ELO 1.5

To close the valve Push PB1 Interrupts power to K1 relay Causes the K1 seal-in contact to open when K1 relay is deenergized Valve closes and stays closed There is no other means to close valve through control circuitry Manually open and close valve with no automatic features Figure: Valve Control Circuit ELO 1.5

Valve response to loss of control power K1 relay must remain energized for valve to stay open If control circuit loses power, effect is the same as pushing PB1; power to K1 relay is interrupted and valve closes K1 seal in contact opens; when power is restored, valve stays closed Figure: Valve Control Circuit ELO 1.5

Alarm Light Function As you can see from the drawing, the alarm lights when K2 time delay relay picks up and closes K2 contact Occurs after K2 relay has been energized for a 10-second period If deenergized before time delay completes, they reset to zero Figure: Valve Control Circuit ELO 1.5

K2 time delay relay is energized if the following sequence occurs: PB2 is pushed to open valve energizes K3 relay closes K3 contact, energizing K1 relay and closing K1 seal in contact Power supplied to K2 time delay relay only if LS1 (limit switch) is made up, which occurs when valve is fully closed Figure: Valve Control Circuit ELO 1.5

Circumstances required for alarm to light are: Button is pushed to open the valve and 10 seconds later, with an open signal still applied, valve is fully closed If valve is partially or fully open, LS1 will be open and alarm will not sound If operator depresses PB1 to close the valve, K1 relay will deenergize and its seal in contact will open, alarm will not sound Figure: Valve Control Circuit ELO 1.5

Simple Motor Control Circuit Control power is taken from termination L1 Returns at termination L2 Requires START pushbutton to be depressed Energizes MAIN coil Closes maintaining contacts Starting resistors in circuit Until Time Delay coil energizes Control power is taken from one phase of line current at termination L1, returns to a different phase at termination L2 Following control power loop from termination L1, the first component that can interrupt control power is the stop pushbutton If the stop pushbutton is depressed, the maintain contact is deenergized and line contacts deenergize, stopping the motor. Continuing through the maintain contact, the next components that could deenergize motor are overloads. Senses current on two phases, if either phase senses overcurrent condition, the overload relay opens the line contacts, deenergizing the motor. This is a drawing used in several NRC Bank Questions. Figure: Control Power Circuit ELO 1.5

Time Delay energizes Accelerating Coil Closes contacts to bypass resistors Motor up to speed and starting current drops off When motor stopped with STOP coil (or overload) MAIN coil deenergizes Time Delay coil deenergizes Accelerating coil deenergizes Starting resistors ready for next motor start Figure: Control Power Circuit ELO 1.5

Electrical Drawings Knowledge Check - NRC Question
Refer to the drawing of a typical valve control circuit. What is the purpose of depressing the S1 pushbutton? To de-energize the K3 relay after the initiating condition has cleared. To prevent energizing the K3 relay when the initiating condition occurs. To manually energize the K3 relay in the absence of the initiating condition. To maintain the K3 relay energized after the initiating condition has cleared. Correct answer is A. NRC Bank Question – P540 Analysis: Initially, assume that the initiating condition caused the K3 relay to energize, which shut contact #3. Then, after the initiating condition cleared, contact #3 acts as a “seal in” to keep K3 energized (keeps valve open). If the S1 pushbutton is pressed with the initiating condition clear (#1 and #2 contacts open), there is no path for current flow through the K3 relay. Therefore, the K3 relay will be de-energized and the valve will close. Correct answer is A. ELO 1.5

Electrical Drawings Knowledge Check – NRC Question
Refer to the drawing of a valve control circuit below. Note that limit switch (LS) contacts are shown open regardless of valve position, but relay contacts are shown open/closed according to the standard convention for control circuit drawings. ELO 1.5

Electrical Drawings Knowledge Check (continued)
Which one of the following describes the purpose of the alarm? Alert the operator when the valve motor circuit has been energized for 10 seconds after pushbutton PB2 is depressed. Alert the operator when the valve has not moved off its closed seat within 10 seconds of depressing pushbutton PB2. Alert the operator that the valve is opening by sounding the alarm for 10 seconds after PB2 is depressed. Alert the operator if the valve has not reached full open within 10 seconds of depressing pushbutton PB2. Correct answer is B. NRC Bank Question – P1540 Analysis: When PB2 is pressed, the K3 relay will be energized, causing the #3 contact to close. Therefore, the K1 relay is energized, which shuts the #1 contact (seal in) and should begin to open the valve. The K2 relay will pickup after 10 seconds if LS1 does not indicate the valve is off its closed seat. LS1 and LS3 still closed indicates the valve is full closed. Assuming a failure of the valve actuator, with the valve still closed after 10 seconds (assuming PB2 is no longer pushed), the #1 contact will still provide a path for current flow to the K2 relay, which will become energized. This will shut the #2 contact to actuate the alarm. As long as the valve comes off its closed seat within 10 seconds, relay K2 never energizes and the alarm never sounds. Correct answer is B. ELO 1.5

Automatic Transfer Switches
ELO 1.6 – Explain the purpose and function of normal and power seeking automatic transfer switches. Used in electrical distribution systems to quickly disconnect a deenergized electrical load from one power supply and connect it to a backup power supply such as an emergency diesel generator Ensure source of power available to essential electrical loads at all times No related KA’s ELO 1.6

Grouped into two categories based on operation Power-seeking Auto to alternate power supply Manual back to normal supply Normal-seeking Auto back to normal power supply ELO 1.6

Many normal-seeking ATSs equipped with time delay prevents them from shifting back to normal source of power until it has been restored for a pre-set period of time (e.g. five seconds) Ensures ATS will not shift its electrical loads back to an unreliable source of power when normal source restored Power-seeking ATSs do not make a distinction between power sources It will stay connected to new source of power until that source of power is lost, or until the ATS is manually shifted back to original source of power ELO 1.6

Motor Controllers ELO 1.7 – Describe the functions, operation, and protective features of motor controllers. Range from a simple toggle switch to a complex system using solenoids, relays, and timers Basic function to control and protect the operation of a motor includes Starting and stopping motor Protecting motor from overcurrent, undervoltage, and overheating conditions that would cause motor damage Two basic categories of motor controllers Manual controller Magnetic controller No related KA’s ELO 1.7

Manual Motor Controllers
Operated by hand Provided with thermal and direct acting overload units Protect motor from overload conditions ON-OFF switch with overload conditions Could be push buttons Controller whose contact assembly is operated by mechanical linkage from toggle-type handle or pushbutton arrangement ELO 1.7

Manual Motor Controllers
Used on small loads such as Machine tools Fans/blowers Pumps Compressors Simple design and operation Provide quiet operation ELO 1.7

Magnetic Motor Controller
Master switch frequently operated automatically Float switch Pressure switch Thermostat Manually operated master switches for these types of controllers include Push buttons Drum switches Knife switches Figure: Three-Phase Magnetic Controller With Thermal Overloads ELO 1.7

Magnetic Motor Contactor
Operated by electromagnet Electromagnet and movable iron armature on which movable and stationary contacts are mounted When no current flowing through electromagnetic coil armature held away by spring When coil energized electromagnet attracts armature and closes electrical contacts Figure: Magnetic Contactor Assembly ELO 1.7

Motor Controller Types and Operation
Three major types of AC across-the-line controllers: Low-voltage protection (LVP) Low-voltage release (LVR) Low-voltage release effect (LVRE) ELO 1.7

Low Voltage Protection
Main purpose is to Deenergize motor during low voltage Prevent restarting automatically upon return of normal voltage Figure: Low-Voltage Protection Controller ELO 1.7

Low-Voltage Protection Controller
When START button pushed Contactor M coil is energized Closing M and Ma contacts When START button released M “a” contact remains closed, completing circuit Figure: Low-Voltage Protection Controller ELO 1.7

When low voltage condition occurs M coil will drop out at some pre-determined value of voltage Usually 70-80% of rated voltage M and Ma contacts open To restart motor, START button must be pushed Figure: Low-Voltage Protection Controller ELO 1.7

Depressing STOP button deenergizes M coil Opens M and Ma contacts Stops motor Figure: Low-Voltage Protection Controller ELO 1.7

Low-Voltage Release Controller
General purpose Deenergize motor in low voltage condition Restart motor when normal voltage is restored Used primarily on small and/or critical loads Cooling water pumps required for safety related equipment Figure: Low-Voltage Release Controller ELO 1.7

Placing START switch in run Energizes M coil Closing M contacts Starting motor When low-voltage condition occurs M coil drops out Opening M contacts Deenergizing motor Figure: Low-Voltage Release Controller ELO 1.7

When normal voltage restored M coil is reenergized Closing M contacts Restarting motor Figure: Low-Voltage Release Controller ELO 1.7

Low-Voltage Release Effect
Maintains motor circuit at all times Manual controller found mostly on small loads that must start automatically upon restoration of voltage May contain overloads If overloads are used, they are placed in lines to load Figure: Low-Voltage Release Effect Controller ELO 1.7

NRC Bank Question – P742 Correct answer is D.
Knowledge Check Knowledge Check Refer to the drawing of a typical valve control circuit for a 480 VAC motor-operated valve below. The valve is currently open with the contact configuration as shown. If the S1 pushbutton is depressed, the valve will ____________ and when the S1 pushbutton is subsequently released, the valve will ____________. remain open; close remain open; remain open close; open close; remain closed Correct answer is D. NRC Bank Question – P742 Correct answer is D. This is an example of a valve that gets an OPEN signal during something like an SI signal. However, when SI is reset the valve stays in the OPEN condition until pushbutton S1 is depressed. For example, you wouldn’t want several valves to close at the same time that an SI signal is reset. You want to be in control of when these valves close. ELO 1.7

Circuit Breakers TLO 2 – Explain the construction, operation, and indications for electrical circuit breakers. 2.1 Explain the construction and functions of circuit breakers, the different types of circuit breakers and their applications, and the protective features incorporated into circuit breakers. 2.2 Describe the following associated with racking out circuit breakers: purpose for racking out circuit breakers, effect of racking out breakers on control and indicating circuits, removal of control power on breaker operation. 2.3 Describe the indications provided for each of the following: local circuit breaker position indications, control room circuit breaker status indications, circuit breaker and protective relay trip indications. 2.4 Describe the effects of losing circuit breaker control power (to include circuit breaker indicator lights and the ability to open and close a circuit breaker remotely). TLO 2

Circuit Breakers ELO 2.1 – Explain the construction, functions, and operation of circuit breakers, the different types of circuit breakers and their applications, and the protective features incorporated into circuit breakers. Circuit breakers used to Isolate circuits Circuit protection in the event of faults Switching during normal operation Circuit breakers do not sense faults Relays or overload devices that sense faults often contained in same cabinet as circuit breaker they signal to trip K1.04 Operation of various push buttons, switches and handles and the resulting action on breakers K1.05 Function of thermal overload protection device Each facility has specific types and models of circuit breakers that are unique to their location; plant specific vendor information, operating procedures, and in some cases task qualification are required to operate and rack-out these breakers. Since they are NOT generic in nature, the NRC doesn’t test any specific type of breaker. A few different types/models are presented here as examples. ELO 2.1

Circuit Breakers Breaker Classifications
High voltage: above 15,000 volts Intermediate or medium voltage: ,000 volts Low voltage: less than 600 volts ELO 2.1

Figure: Molded Case Circuit Breaker
Circuit Breakers Low-Voltage Air Circuit Breaker For circuits rated at 600 volts or lower Applications: Molded case breakers Small motor control center (MCC) breakers Figure: Molded Case Circuit Breaker ELO 2.1

Figure: Cutaway View of Molded Case
Circuit Breakers Molded Case Breaker Operation Turning ON or OFF position will connect or disconnect a circuit All breakers, except very small ones, have a linkage that allows for a quick make (quick break) contact action If circuit breaker opens under fault condition, handle goes to "trip-free" position A breaker in the “trip free” condition should not be quickly reclosed without checking out the associated motor for overheating, charring, or other indications of degradation/shorts. Plant procedures will determine if Electrical Maintenance tests like “meggars” or SRO permission is required before reclosing a tripped breaker. Figure: Cutaway View of Molded Case Circuit Breaker ELO 2.1

Figure: Cutaway View of Molded Case
Circuit Breakers Automatic Tripping Circuit breaker automatically trips when current through it exceeds a pre-determined value In lower current ratings, thermal tripping devices provide the means of automatic tripping Figure: Cutaway View of Molded Case Circuit Breaker ELO 2.1

Thermal Tripping Elements
Consists of bimetallic element calibrated so normal current heat does not cause deflection High current will cause element to deflect and trip linkage that holds circuit breaker shut Short circuit Overload Circuit breaker opened by spring action Bimetallic strip consists of two dissimilar metals that heat up at different rates. When current increases (gradual bearing failure, for example), the bimetallic strip will bend causing contact opening. Figure: Thermal Tripping Element ELO 2.1

Thermal Tripping Elements
Bimetallic element is responsive to heat produced by current flowing through it Inverse-time characteristic If an extremely high current is developed, circuit breaker will be tripped very rapidly For moderate overload currents, it will operate more slowly Figure: Thermal Tripping Element ELO 2.1

Magnetic Tripping Elements
Arc Chutes When the separable contacts of an air circuit breaker open, an arc develops between contacts Different designs and arrangements of contacts and their surrounding chambers Most common design places moving contacts inside an arc chute Construction allows this arc chute to magnetically draw arc formed as contacts open ELO 2.1

Large Air Circuit Breaker
Large distribution systems require much larger air circuit breakers Breakers have current ratings as high as 4,000 amps, and interrupting ratings as high as 150,000 amps Requires stronger mechanism to “make” and “break” contact Figure: Large Air Breaker Front ELO 2.1

Operation Closing device, stored energy mechanism Uses large compressed coil springs Springs compressed Manually Small charging motor Figure: Large Air Circuit Breaker ELO 2.1

Closing breakers compresses tripping spring Tripping spring and trip latch trip open breaker Trip latch operation Manually Remotely by trip coil Electrically-operated circuit breakers used when Operated at frequent intervals Remote operation required Figure: Large Air Circuit Breaker ELO 2.1

When electrically operated breaker tripped Closing spring recharged by spring charging motor Makes breaker ready for next closing operation Manually-operated circuit breaker closing springs normally compressed by hand crank just prior to operation of breaker Figure: Large Air Circuit Breaker ELO 2.1

Closing springs are compressed by pulling downward on large operating handle on front of breaker Closing springs may also be charged by an electric motor Motor powered from breaker control power Will charge springs after breaker closing attempt If control power lost, can charge motor manually using a lever or hand crank Figure: Large Air Breaker Front ELO 2.1

High-Voltage Breaker Classifications
Circuits with voltage ratings higher than 600 volts use high-voltage circuit breakers (including breakers rated at intermediate voltage) Standard voltage ratings are from 4,160 to 765,000 volts with three- phase interrupting ratings of 50,000 to 50,000,000 kVA Early design of high-voltage circuit breakers were oil circuit breakers Newer designs are magnetic or compressed air ELO 2.1

Magnetic Air Circuit Breakers Rated up to 750,000 kVA at 13,800 volts, interrupts air with aid of magnetic blowout coils When contacts separate during a fault condition (see figure), magnets draw arc out horizontally and transfer it to a set of arcing contacts Blowout coil provides a magnetic field to draw arc upward into arc chutes Arc accelerates upward into arc chute where it is elongated and divided into many small segments Figure: Magnetic Air Circuit Breaker ELO 2.1

Compressed Air Circuit Breakers Uses compressed air stream directed toward separable contacts to interrupt arc Air-blast circuit breakers developed for extra high-voltage applications Figure: Compressed Air Arc Chute ELO 2.1

Oil Circuit Breakers Have contacts immersed in oil, arc cooled and quenched Oil tanks in oil circuit breakers are sealed Electrical connections between contacts and external circuits through porcelain bushings The oil is stored in accumulators to rotate the contacts hydraulically and also helps quench the arc created when the breaker opens. Oil and SF6 breakers must not be opened dry, as the arc will destroy the breaker without a dielectric to quench it. Figure: Oil Circuit Breaker ELO 2.1

GE Magne-Blast Breaker
GE Magne-Blast breaker is a medium-voltage breaker (see figure) with wide use in power plant switchgear application Early designs were an air circuit breaker, with a solenoid operated mechanism Latest designs outfitted with vacuum type contacts Reference: NRC Medium Voltage Circuit Breaker Training , Chapter 6 Figure: GE Magne-Blast Breaker ELO 2.1

GE Magne-Blast Breaker
Chassis contains primary contact assembly and bushings, interlocks, and ground strap Primary contact assembly is the main current carrying part of the breaker Assembly consists of all barriers, arc chutes, and air puffer system Some breaker designs include a manual bar to charge closing springs locally at breaker Reference: NRC Medium Voltage Circuit Breaker Training , Chapter 6 Figure: GE Magne-Blast Breaker ELO 2.1

Charging Motor Normally, charging motor charges closing spring(s)
Located below operating mechanism on front left side and connected to a drive fitting and levers to ratchet wheel Spring discharge interlock Discharges the closing springs when the breaker is rolled in or out of switchgear cubicle Reference: NRC Medium Voltage Circuit Breaker Training , Chapter 6 ELO 2.1

Local Circuit Breaker Operation
Some circuit breakers are designed for local rather than remote operation Locally operated breakers may be designed to be opened and closed electrically or manually Figure: Circuit Breaker ELO 2.1

Electrically operated Provided with a local breaker control switch Uses breaker control power to electrically operate circuit breaker Normally located in associated switchgear ELO 2.1

Control switch has three positions TRIP, breaker opens Midposition, normal "at rest" position CLOSE, breaker closes if interlocks met Figure: Breaker Control Switch ELO 2.1

Logic circuitry checks breaker interlocks are satisfied before allowing closing Loss of control power breaker fails as is Breaker control switch may have a PULL OUT (Pull-To-Lock) position that prevents automatic closing Standard with this type of control switch and does not imply breaker has automatic closing features ELO 2.1

Electrically operated breakers may also have two different position indicators Circuit breaker position indicating flags Red flag indicates breaker was last positioned closed Green flag indicates breaker was last positioned open “Flag mismatch” if breaker positions differently from switch Amber or white light Circuit breaker position indicating lights Illuminated red indicates closed Illuminated green indicates open ELO 2.1

Manually Operated Breakers Some MCC or load center circuit breakers do not have capability of being operated electrically Manually operated through use of a "TRIP" and a "CLOSE" push button Do not have: Circuit breaker control switches Position indicating lights Logic circuits to check for interlocks Must be operated per procedure ELO 2.1

Local-Remote Transfer Switch
Some components provided with REMOTE/LOCAL transfer switch DG output breaker Remote Shutdown Panel components When REMOTE selected: control power available for trips and breaker can be operated remotely from control room When LOCAL selected: control power available for trips, but breaker can only be operated locally Appendix R required local operation switches in case a fire in the control room destroyed or shorted out the normal control wiring. ELO 2.1

Circuit breaker control circuit can be designed so that any one of a number of protective features may be incorporated If conditions exists while circuit breaker is closed Relay will close its associated contact Energize breaker tripping coil Circuit breaker trips open Various protective relays presented earlier (for example) Undervoltage, underfrequency, reverse power Long-term, short-term, or instantaneous overcurrent ELO 2.1

Circuit Breakers Knowledge Check – NRC Question
A typical 120 VAC manual circuit breaker has tripped due to overload. To close this circuit breaker, the breaker handle must be moved from the... OFF position directly to the ON position; trip latch reset is not required. midposition directly to the ON position; trip latch reset is not required. OFF position to the midposition to reset the trip latch, and then to the ON position. midposition to the OFF position to reset the trip latch, and then to the ON position. Correct answer is D. Correct answer is D. NRC Bank Question – P840 Analysis: A. WRONG. Note that the tripped position is in midposition, and typically quite close to the ON position, but will not be completely aligned with other breaker handles in the ON position. B. WRONG. Note that the tripped position is in midposition, and typically quite close to the ON position, but will not be completely aligned with other breaker handles in the ON position. C. WRONG. To reset the circuit breaker, the operator must move the breaker handle from midposition to the OFF position. This action resets the trip latch. With the trip latch reset, the operator can then move the breaker handle to the ON position to close the breaker. The breaker will go from the tripped position to the ON position. D. CORRECT. When a circuit breaker trips on overload, the breaker handle will move from the ON position to the “tripped-free” (mid) position. Note that the tripped position is in midposition, and typically quite close to the ON position, but will not be completely aligned with other breaker handles in the ON position. Normally a “tripped-free” breaker has some “wiggle” while in the midposition. To reset the circuit breaker, the operator must move the breaker handle from midposition to the OFF position. This action resets the trip latch. With the trip latch reset, the operator can then move the breaker handle to the ON position to close the breaker. Of course, this is not normally done until the overload condition has cleared. ELO 2.1

Circuit Breakers Knowledge Check – NRC Question
How is typical breaker operation affected when the associated breaker control power transfer switch is placed in the LOCAL position? Control power will be available to provide protective trips, and the breaker can be electrically operated only from the control room. Control power will be removed from both the open and close circuits, and the breaker can be electrically operated only from the control room. Control power will be available to provide protective trips, and the breaker can be electrically operated only from the breaker cabinet. Control power will be removed from both the open and close circuits, and the breaker can be electrically operated only from the breaker cabinet. Correct answer is C. Correct Answer is C. NRC Bank Question – P639 Analysis: Some components have a control power transfer switch that allows transferring control power to ensure electrical isolation from its normal control power. These were typically installed post-Browns Ferry for Appendix R reasons. In the local position, control power is available to provide protective trips, and the breaker can be electrically operated only from the breaker cabinet (safe shutdown panel, for example). ELO 2.1

Racking Circuit Breakers
ELO 2.2 – Describe the following associated with racking out circuit breakers: purpose for racking out circuit breakers, effect of racking out breakers on control and indicating circuits, and removal of control power on breaker operation. Breakers may be racked to three positions Connect (racked in): normal position, breaker supplies power to load Test: breaker will not connect to load, but control power is supplied to breaker Disconnect (racked out): deenergizes the load for maintenance (control power must be removed to completely deenergize) K1.01 Purpose of racking out breakers (de-energize components and associated control and indication circuits) ELO 2.2

Large ACB Racking Operation
To completely deenergize an electrical component and its associated control and indication circuits breaker should be racked out with control power fuses removed There are many types of ACB circuit breakers Method of rackout will be specific to type of breaker A few types are presented in upcoming slides. ELO 2.2

Draw out circuit breakers disconnected by moving breakers physically away from bus Accomplished with racking wrench Figure: Draw out Breaker ELO 2.2

Breaker moves from CONNECT position to TEST position to DISCONNECT position Some plants have breaker racking tools that rack the breaker with the operator at a distance Interlock prevents circuit breaker operation at any point during racking operation Figure: Breaker Racking Tool ELO 2.2

Remote rackout device minimizes danger associated with potential arc blast as breaker is racked out Ensure plant procedures and required PPE are used during breaker racking operation Accidents when racking breakers can be fatal! Figure: Racking Arc Blast ELO 2.2

CONNECT position allows energizing/ deenergizing load TEST position allows operation of breaker with control power while not connecting load to bus DISCONNECT allows removal of breaker from switchgear Figure: Breaker Racked Out ELO 2.2

To perform a verification of breaker position and operability, following must be ensured: Indication on floor of circuit breaker housing corresponds to markings on the circuit breaker Racking release lever is fully in CONNECT position (extreme counterclockwise position) Closing spring motor toggle switch is in ON position and closing spring is charged (or fuses installed) Control power breaker is closed and/or control power fuses in place (not blown), ensuring power is available ELO 2.2

Racking Circuit Breakers
Knowledge Check To completely deenergize an electrical component and its associated control and indication circuits, the component breaker should be… open with the control switch in Pull-To-Lock. open with the control switch tagged in the open position. racked out and tagged in racked-out position. racked out with control power fuses removed. Correct answer is D. Correct answer is D. NRC Bank Question – P37 Analysis: A. WRONG. If the breaker is just opened, then power is secured to the electrical component, but it is capable of being restored unless the breaker is racked out. With the breaker racked out, power is secured to the component, but an extra step must be taken to ensure associated control and indication circuits are deenergized: the control power fuses must be removed. Control power provides control and indication circuits; unless control power fuses are removed, the control and indication circuits will remain powered. B. WRONG. See ‘A’. C. WRONG. Control power provides control and indication circuits; unless control power fuses are removed, the control and indication circuits will remain powered. D. CORRECT. To completely de-energize an electrical component and its control and indication circuits, the breaker must be racked out and have its control power fuses removed. If the breaker is just opened, then power is secured to the electrical component, but it is capable of being restored unless the breaker is racked out. With the breaker racked out, power is secured to the ELO 2.2

Circuit Breaker Position Verification
ELO 2.3 – Describe the indications provided for each of the following: local circuit breaker position indications, control room circuit breaker status indications, and circuit breaker and protective relay trip indications. Circuit breaker status can be determined by a number of means Breaker's OPEN/CLOSE mechanical indicators Breaker position indication lights (control power) Load-side voltage/current Physical breaker position All can be used to positively determine breaker position K1.02 Local indication that breaker is open, closed or tripped K1.09 Effect of racking out breakers on control and indicating circuits and removal of control power on breaker operation K1.11 Control room indication of a breaker status K1.12 Trip indicators for circuit breakers and protective relays ELO 2.3

Control indications include: Red (energized/closed) indicating light Green (deenergized/open) indicating light Amber (mismatch) indicating light On some breakers Breaker control switch position Load ammeter ELO 2.3

Circuit Breaker Position and Control Switch Mismatch
Amber “mismatch” light indication When light is ON Physical breaker position (open or closed) does not match current control switch position Provides additional control room indication of potential problem ELO 2.3

Circuit Breaker Trip Flags
Automatic circuit breaker trip may be determined by local device protection flag indication Once protective trip occurs, the trip coil energizes and the circuit breaker opens Indicated by local mechanical flag If any protective devices are actuated local mechanical trip flags must be manually reset once the condition clears However, relay reset not required to close breaker Unless it trips an 86 lockout device ELO 2.3

Use all available indications to determine circuit breaker condition Best indications are those solely dependent on breaker position Load side voltmeter readings Local OPEN/CLOSE mechanical flags Protective devices (relays) not good indicators Breaker may indicate tripped by a protective trip mechanical flag but may actually be closed Circuit breaker could actually be closed but the trip flag may still indicate tripped because protective device was not reset (manually) from previous trip ELO 2.3

Circuit breaker position indicating lights are not necessarily a good indicator of actual breaker position Without breaker control power, breaker may indicate open (red light is not illuminated) when actually closed Indicating bulbs may also be burned out Load side ammeter readings cannot be counted on to give an accurate indication of breaker position Unless breaker directly Starts/Stops a motor ELO 2.3

Knowledge Check Breaker local overcurrent trip flag indicators, when actuated, indicate that… a breaker trip will occur unless current is reduced. a breaker overcurrent condition is responsible for a breaker trip. an overcurrent condition has cleared and the breaker can be closed. the associated breaker has failed to trip open during an overcurrent condition. Correct answer is B. Correct answer is B. NRC Bank Question – P1444 Analysis: A. WRONG. A breaker overcurrent trip flag will actuate when the breaker has tripped on overcurrent. B. CORRECT. A breaker overcurrent trip flag will actuate when the breaker has tripped on overcurrent. The overcurrent trip flag indicator can only be manually reset when the overcurrent condition has cleared; if the operator attempted to manually reset the flag with the overcurrent condition present, the flag would not reset because the condition is still present. C. WRONG. A breaker overcurrent trip flag will actuate when the breaker has tripped on overcurrent. The overcurrent trip flag indicator can only be manually reset when the overcurrent condition has cleared. D. WRONG. A breaker overcurrent trip flag will actuate when the breaker has tripped on overcurrent. ELO 2.3

Knowledge Check While remotely investigating the condition of a normally-open 480 VAC motor control center (MCC) feeder breaker, an operator observes the following indications: Green breaker position indicating light is out. Red breaker position indicating light is lit. MCC voltmeter indicates 480 VAC. MCC ammeter indicates zero amperes. Based on these indications, the operator should report that the feeder breaker is __________ and racked __________. open; in closed; in open; to the TEST position closed; to the TEST position Correct answer is B. Correct answer is B. NRC Bank Question – P1932 Analysis: As stated in the stem, the red breaker position indicating light is lit, and the green light is extinguished. These are indications of a closed circuit breaker. Also, the MCC voltmeter indicates that the MCC voltage is normal (typically 480 V). In order to achieve normal voltage on the MCC, the breaker must be closed (also previously determined using the indicating lights) and racked in (to connect). Note that although the MCC ammeter reads zero amps, this simply indicates that no loads are currently drawing current on the bus (all load breakers are open). ELO 2.3

Knowledge Check While remotely investigating the condition of a typical normally-open motor control center (MCC) feeder breaker, an operator observes the following indications: Green breaker position indicating light is lit. Red breaker position indicating light is out. MCC voltmeter indicates zero volts. MCC ammeter indicates zero amperes. Based on these indications, the operator can accurately report that the breaker is open and racked to __________ position. the OUT the IN the TEST an unknown Correct answer is D. Correct answer is D. NRC Bank Question – P1838 Analysis: As described in the stem, the red breaker position indicating light is extinguished, and the green light is lit. These are indications of an open circuit breaker. To determine which position the breaker is racked to, the operator must observe the other indications. With the indications provided for voltage and current (zero on both), it is impossible to deduce if the breaker is in the TEST position or the IN position. Since the green light is ON, the breaker can’t be racked out, but could be in TEST or IN based on the other indications. Therefore, “D” is correct because the breaker is in an “unknown” condition based on these indications. ELO 2.3

Circuit Breaker Control Power
ELO 2.4 – Describe the effects of losing circuit breaker control power on breaker operation and indications. To operate breakers using a control switch, remotely or locally, a control circuit must be provided Control power is usually taken from line contacts, rectified, and provided to the control circuit through fuses K1.03 Loss of power supply circuit breaker indicator lights and capability in remotely open and close K1.09 Effect of racking out breakers on control and indicating circuits and removal of control power on breaker operation ELO 2.4

Rectifier unit Closing relay Closing coil Trip coil Auxiliary contacts Breaker control switch The rectifier converts AC line to DC to operate the relays. A somewhat more common arrangement is a control power transformer installed here to operate 120VAC relays. Figure: Breaker Control Circuit ELO 2.4

Protective features can be added to trip circuit breaker Control circuit shown has protective features that will close contacts to energize trip coil Underfrequency Undervoltage Figure: Breaker Control Circuit ELO 2.4

Figure: Breaker Control Circuit
Closing Breaker Control switch to close Energizes closing relay Energizes closing coil Closes breaker Closes "a" contact to enable the trip coil No automatic closures Figure: Breaker Control Circuit ELO 2.4

Figure: Breaker Control Circuit
Opening Breaker Control switch to "trip" Trip coil releases latch, allowing breaker to open "a" contact opens, deenergizing trip coil "b" contact closes, enabling closing relay Automatic opening features Underfrequency Undervoltage When relays actuate, contact energize trip coil Figure: Breaker Control Circuit ELO 2.4

Loss of Control Power Effects of losing circuit breaker control power
Lose local and remote breaker indication lights Lose remote breaker control (open/close) Lose capability to trip open automatically from a protection trip device or electrical fault Lose ability for breaker closing spring to electrically recharge after local closing of breaker (charging motor not energized) Can still OPEN breaker locally/manually ELO 2.4

Knowledge Check Loss of breaker control power will cause... breaker line voltage to indicate zero regardless of actual breaker position. the remote breaker position to indicate open regardless of actual breaker position. inability to operate the breaker locally and remotely. failure of the closing spring to charge following local closing of the breaker. Correct answer is D. Correct answer is D. NRC Bank Question – P40 Analysis: A. WRONG. If the breaker is closed, line voltage will be equal to bus voltage. B. WRONG. Loss of control power results in a loss of all breaker position indication. Therefore, both the open and the closed lights will be extinguished. C. WRONG. The breaker can still be tripped locally without control power. Also, if necessary, the closing springs can be manually charged locally and the breaker shut. Therefore, ability to operate the breaker locally remains. D. CORRECT. When a breaker loses control power, the component that is being powered by the breaker will continue to function. However, all indicating lights at the circuit breaker will be extinguished, and the breaker will not trip on automatic or manual trips (because control power energizes the trip coil). The breaker cannot be actuated remotely, but can be tripped locally. The closing spring will not recharge, but typically a hand-crank can be used to manually charge the closing springs. ELO 2.4

Knowledge Check When a typical 4,160 volt breaker is racked to the TEST position, control power is __________ the breaker; and the breaker is __________ the load. removed from; isolated from removed from; connected to available to; isolated from available to; connected to Correct answer is C. Correct answer is C. NRC Bank Question – P642 Analysis: When a 4160 V breaker is racked to the TEST position, the physical connection of the breaker to the bus is opened. Although the circuit breaker can no longer power its component, control circuit power is maintained, which means the breaker can be operated (opened or closed). The TEST position is typically used for circuit breaker and relay testing. ELO 2.4

Paralleling AC Sources
TLO 3 – Describe the conditions that must be met prior to paralleling two generators including effects of not meeting these conditions. 3.1 Describe the conditions required to properly parallel two AC power sources, including voltage, frequency, and phase. 3.2 Describe the effects of paralleling two AC sources under the following conditions: current out of phase, frequencies not matched, high voltage differential, and low current or too much load. There is a lot of similarity between these ELOs and ELO 4.3 of – Motors and Generators. Duplication is required to ensure the KA’s associated with this chapter are covered. TLO 3

Paralleling AC Sources – Conditions
ELO 3.1 – Describe the conditions required to properly parallel two AC power sources, including voltage, frequency, and phase. Three conditions must be met prior to paralleling or synchronizing AC sources. Terminal voltages almost equal Minimize VAR loading Frequency of incoming machine slightly higher than grid Synchroscope rotating slowly in the FAST (clockwise) direction Ensures incoming machine picks up some load instead of becoming a load (motorizing) Output voltages in phase Minimize current surge through breaker K1.08 Effects of closing breakers with current out of phase, different frequencies, high voltage differential, low current, or too much load ELO 3.1

Paralleling AC Sources
During paralleling operations, voltages of the generators to be paralleled are shown on voltmeters Frequency matching is accomplished through the use of output frequency meters A synchroscope is a device that senses two frequencies indicates phase differences between generators allows phase matching of two generators Once breaker closed, synchroscope locks in at 12 o’clock Most plants have “sync check relays” Doesn’t allow breaker closure out of phase In 95% of the industry, having the incoming generator at a slightly higher frequency results in the needle moving “slowly in the FAST (clockwise) direction”. A few plants have a (user-unfriendly) set of paralleling operations wherein some selections of the “incoming” side reverse this. ELO 3.1

Abnormal Conditions During Paralleling Operations
ELO 3.2 – Describe the effects of paralleling two AC sources under the following conditions: current out of phase, frequencies not matched, high voltage differential, and low current or too much load.. Recall that three conditions must be met prior to paralleling or synchronizing AC sources Following these three minimize current through breaker being paralleled Minimum VAR loading Minimum real load picked up Minimum current from phase differences K1.08 Effects of closing breakers with current out of phase, different frequencies, high voltage differential, low current, or too much load ELO 3.2

Paralleling Terminal Voltages
Voltages of running and incoming should be matched Some plants allow incoming voltage to be slightly higher Voltage indication might be “stepped down” to 120 volts on meters Once generator loaded, voltage setting can be changed to carry desired VAR loading Usually raise voltage in morning to pick up anticipated VARs out Usually lower voltage in evening to lower anticipated VARs out The last bullets assume a lagging power factor, which is most common due to induction motors on the grid. The capacitance of long transmission lines can (rarely) produce excess MVARs and require low excitation to absorb the excess. ELO 3.2

Paralleling Frequencies
Frequency of incoming machine slightly higher Ensures some load picked up by incoming generator Operator usually goes to RAISE on speed control to pick up additional load Frequency of incoming machine slightly lower Synchroscope rotating in SLOW (counterclockwise) direction Known as "motoring or reverse powering“ Breaker could stay closed if speed control immediately taken to RAISE to pick up load If frequencies matched Synchroscope stays motionless at given clock position Could be in-phase or out-of-phase ELO 3.2

Paralleling Output Voltages in Phase
Incoming (generator) and running (grid) “in-phase” as synchroscope passes through 12 o’clock When breaker closed, generator gets “locked” in phase with grid Synchroscope goes to 12 o’clock and stays there If breaker closed “out-of-phase”, current surge could damage breaker or generator Sync check relays might allow closure Between 11 and 1 o’clock Basically between +30 degrees and -30 degrees phase differential Therefore, anything > 30 degrees could be damaging Worst case mismatch would be 180° out of phase, resulting in an opposing voltage between the two generators of twice the output voltage This high voltage can cause damage to distribution system due to extremely high currents and large mechanical torque exerted on both generators The greater the phase mismatch, the greater the damage that will occur to generator output breaker due to excessive arcing when circuit breaker is closed ELO 3.2

Powering a Deenergized Bus
Closing the output breaker of a three-phase generator onto a deenergized bus can produce an overcurrent condition on the generator Occurs if the bus was not first unloaded Due to instantaneous flow of starting current for all loads running when power was lost If DG provided sole power to a vital bus Bus usually “load shed” first (all breakers open) DG comes up to speed DG output breaker closes Sequencer loads bus in timed sequence Allows starting currents to diminish before starting next load ELO 3.2

Paralleling Operations
Knowledge Check A main generator is about to be connected to an infinite power grid. Generator voltage is equal to grid voltage and the synchroscope is rotating slowly in the counterclockwise direction. The generator breaker is closed just prior to the synchroscope pointer reaching the 12 o'clock position. Which one of the following is most likely to occur after the breaker is closed? The breaker will remain closed and the generator will supply only MW to the grid. The breaker will remain closed and the generator will supply both MW and MVAR to the grid. The breaker will open due to overcurrent. The breaker will open due to reverse power. Correct answer is D. Correct answer is D. NRC Bank Question – P2441 Analysis: A. WRONG. If the needle is rotating in the slow (counter-clockwise) direction, this indicates that generator frequency is lower than grid frequency. This would be undesirable because closing the generator output breaker under this condition would make the generator a real load on the grid (reverse power conditions). B. WRONG. As mentioned in distractor “A”, the generator will become a real load on the grid; also, because generator voltage equals grid voltage, there will be no change in reactive power when the breaker is closed. C. WRONG. Because the needle was spinning slowly, the generator will only be a small load on the grid. D. CORRECT. if the needle is rotating in the slow (counter-clockwise) direction, this indicates that generator frequency is lower than grid frequency. This would be undesirable because closing the generator output breaker under this condition would make the generator a real load on the grid (motorizing, or reverse power conditions). This will eventually lead to the breaker opening. ELO 3.2

Paralleling Operations
Knowledge Check If a main generator output breaker is closed when the generator output voltage is 5 degrees out of phase with the power grid voltage, the main generator will experience a __________ stress; if the breaker remains closed and no additional operator action is taken, the main generator voltage will __________ with the grid voltage. minor; remain out of phase minor; become locked into phase potentially damaging; remain out of phase potentially damaging; become locked into phase Correct answer is B. Correct answer is B. NRC Bank Question – P7022 Analysis: A. WRONG (1/2). Although the stress will be minor, once the generator breaker is closed, and remains closed, the generator is connected to the entire grid. Thus, the generator becomes locked in-phase with the grid. B. CORRECT (2/2). The synchroscope at the 12 o’clock position indicates that generator and grid voltages are in-phase. It is desirable to shut the generator output breaker at this position; shutting the breaker with the synchroscope not at 12 o’clock will result in arcing due to the voltages being out of phase. The degree to which the needle is not at the 12 o’clock position indicates how far out-of-phase the main generator is with the grid. A small difference (e.g., 5 degrees) should not result in significant damage to installed plant equipment since this is well within the normal tolerances of most sync check relays. C. WRONG (0/2). The stress will be minor (only a five percent difference between grid and generator frequency), once the generator breaker is closed, and remains closed, the generator is connected to the entire grid. Thus, the generator becomes locked in-phase with the grid. D. WRONG (1/2). The stress will be minor (only a five percent difference between grid and generator frequency). ELO 3.2

NRC KA to ELO Tie KA # KA Statement RO SRO ELO K1.01
Purpose of racking out breakers (de-energize components and associated control and indication circuits) 2.6 2.8 2.2 K1.02 Local indication that breaker is open, closed or tripped 2.9 2.3 K1.03 Loss of power supply circuit breaker indicator lights and capability in remotely open and close 3.1 2.4 K1.04 Operation of various push buttons, switches and handles and the resulting action on breakers 3.0 2.1 K1.05 Function of thermal overload protection device 1.2 K1.06 Interpretation of symbols for breakers, relays and disconnects in a one-line diagram 1.5 K1.07 Safety procedures and precautions associated with breakers, including MCC bus breakers, high, medium and low voltage breakers, relays and disconnects 3.3 1.4 K1.08 Effects of closing breakers with current out of phase, different frequencies, high voltage differential, low current, or too much load 3.5 3.2 K1.09 Effect of racking out breakers on control and indicating circuits and removal of control power on breaker operation 2.3, 2.4 K1.10 Function, control, and precautions associated with disconnects 2.7 1.3 K1.11 Control room indication of a breaker status K1.12 Trip indicators for circuit breakers and protective relays 1.2, 2.3

Operator Generic Fundamentals

Similar presentations

Presentation on theme: "Operator Generic Fundamentals"— Presentation transcript:

Similar presentations

About project

Feedback

Log in

Auth with social network:

Operator Generic Fundamentals

Similar presentations

Presentation on theme: "Operator Generic Fundamentals"— Presentation transcript:

Similar presentations

About project

Feedback