 Power System Fundamentals

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Power System Fundamentals
EE 317 Lecture 8 25 October 2010

Chapter 5 Synchronous Machine Construction Speed of Rotation
Voltage of a Synchronous Generator Phasor Diagrams of a Synchronous Generator Synchronous Generator Operating Alone Synchronous Motors

Synchronous Machines Motors and generators whose magnetic field current for the rotor is supplied by a separate DC power source Synchronous generators are used to produce nearly all the electric power produced in the world

Construction S S N N STATOR or Armature Windings ROTOR or
Field Windings S N N

Field windings… Salient pole: constructed in a manner that it protrudes from the surface of rotor

Field windings… Nonsalient pole: constructed flush with the surface of the rotor (see Figure 5-1, p. 193)

How we create the DC current for the Rotor Magnetic field…
External Source: DC currents created by slip rings and brushes (lead to higher maintenance and power/voltage drop across brushes) Brushless Exciter: small AC generator with field circuit mounted on stator and the armature mounted on the rotor creating 3- AC currents. A 3- rectifier changes AC to DC for the main field

Completely independent
Pilot Exciter – uses permanent magnets on rotor to induce 3- AC currents in the armature which then produce exciter fields in armature leading to 3- AC currents in the rotor…etc. Redundancy – many synchronous generators that use brushless exciters also have slip rings and brushes so that an auxiliary method for making DC is available in emergencies

Exciter circuit diagrams
See board

What is relationship? Of electrical frequency and speed of the mechanical (prime mover) device? Where: fe = electrical frequency in Hz nm = mechanical speed of field in rpm (rotor speed) P = number of magnetic poles

Voltage of a Sync. Gen. From chapter 4:
Simplifying for what is controllable during operation:

Equivalent circuit of a sync. gen.
The internal voltage EA is not usually equal to the output voltage V of a synchronous generator due to 4 factors: Armature reaction Self-inductance of armature coils Resistance of armature coils Effects of salient pole rotor shapes The revised equation for output voltage V :

Phasor Diagrams of Sync. Machines

Power and Torque in sync. gen.

Measuring sync. gen. parameters
The model equation for sync gen output voltage V : To model the overall sync gen we need to know: Relationship between field current and flux Synchronous reactance (XS) of the generator Armature resistance (RA)

Measuring the model parameters
Open-circuit test: loads are disconnected (terminals are open), field current is zero, construct plot of EA = V vs. field current IF this determines air-gap line and overall OCC Short-circuit test: loads are disconnected terminals are shorted, field current is zero, construct plot of IA vs. field current IF this determines the overall SCC

Modeling sync. gen. parameters
The model equation for sync gen armature current IA : Since R is much smaller than X we can approximate X at any given point by the following process: Get EA from OCC at given field current Find short circuit current flow (IA) from SCC at field current Calculate Synchronous reactance (XS) of the generator

Equations

Limitations This approximation only is accurate up to the knee in the saturation curve of the OCC, its value as a true approximation of X reduces as saturation increases

Sync. gen. Operating alone
What happens as load (of constant power factor) is increased on generator? (a) lagging power factor (inductive loads) (b) unity power factor (c) leading power factor (capacitive loads)

Generator response (b) slight decrease in V and VT
(a) V and VT decrease significantly (b) slight decrease in V and VT (c) a rise in V and VT

Voltage regulation Normally desirable to keep the voltage out of a generator constant even when loads are varying. How can this be done? Since EA = K which one do you think we can most easily vary? Why? and How?

Changing the Field Resistor FR
1. Decreasing field resistance increases field current 2. Increases in field current increase flux 3. Increase in flux increases EA 4. An increase in EA leads to increase in V and VT PROCESS IS REVERSIBLE

Comparing voltage regulation
the model equation for voltage regulation is defined as: Since R is much smaller than X we can approximate X at any given point by the following process: Get EA from OCC at given field current Find short circuit current flow (IA) from SCC at field current Calculate Synchronous reactance (XS) of the generator

Synchronous motors Same as generators All the same equations apply
Only differences are in phasor diagrams Also when maximum torque is exceeded rotor will start to slip

Chapter 6 Rationale for paralleling Conditions for paralleling
Procedure for paralleling Characteristics of a Synchronous Generator Operation with an Infinite Bus Operating with another of similar size

Paralleling generators
Why? Higher loads Increased reliability under failure Maintenance More efficient operation of the fleet

Conditions for paralleling
Rms line voltages must be equal Same phase sequence Phase angles must be equal Frequency of new generator (oncoming unit) must be slightly higher the frequency of the running system

procedure First – verify terminal voltage of oncoming generator equals line voltage of system Second – verify that the phase sequence of the oncoming generator is the same as the phase sequence of the running system (motor, bulbs, synchroscope) Third – adjust the frequency of the oncoming unit to be slightly higher than the frequency of the running system

Ch. 7 - Induction Machines
Motors and generators whose magnetic field current is supplied by magnetic induction (transformer action) into the field windings of the rotor (a DC power source is not required) Although induction machines can be motors or generators they have many disadvantages as generators. Thus, they are referred to typically as induction motors. Most popular type of AC motor