UETTDRTS01B MAINTAIN NETWORK PROTECTION & CONTROL SYSTEMS

3. Interdependent Protection and their Applications

TOPICS Inter-tripping schemes Bus-bar protection including breaker failure backup and blocking schemes Pilot wire including phase comparison Distance protection scheme Reclosing and check-synchronising Tie-line protection and load shedding

Inter-Tripping Schemes Inter-tripping is the controlled tripping of a circuit breaker The main use of this scheme is to Isolate both sides of the faulty circuit This Schemes are used during the following circumstances –A feeder with a weak in feed at one end, insufficient to operate the protection for all faults –Feeder protection applied to transformer –feeder circuits –Faults between the CB and feeder protection CT’s, when these are located on the feeder side of the CB –For high reliability EHV protection schemes, inter-tripping may be used to give back-up

Inter-Tripping Schemes (Cont…) Direct tripping –In direct tripping applications, inter-trip signals are sent directly to the master trip relay Permissive tripping –Permissive trip commands are always monitored by a protection relay

Inter-Tripping Schemes (Cont…) Blocking scheme –Blocking commands are initiated by a protection element that detects faults external to the protected zone Purpose of Inter-tripping in transformer –In order to ensure that both the high and low voltage circuit breakers operate for faults within the transformer and feeder

Inter-Tripping Schemes (Cont…) Application of protection signalling and its relationship to other systems using communication

Inter-Tripping Schemes (Cont…) Certain types of fault produce insufficient current to operate the protection associated with one of the circuit breakers. These faults are –Faults in the transformer that operate the Buchholz relay and trip the local low voltage circuit breaker –Earth faults on the star winding of the transformer –Earth faults on the feeder or high voltage delta connected winding

Bus-bar Protection Bus-bars have often been left without specific protection, for one or more of the following reasons –The bus-bars and switchgear have a high degree of reliability, to the point of being regarded as intrinsically safe –It was feared that accidental operation of bus-bar protection might cause widespread dislocation of the power system –It was hoped that system protection or back-up protection would provide sufficient bus protection if needed

Bus-bar Protection (Cont…) Bus-bar faults –The majority of bus faults involve one phase and earth The special features of bus-bar protection are as follows Speed: Bus-bar protection is primarily concerned with –Limitation of consequential damage –Removal of bus-bar faults in less time than could be achieved by back-up line protection

Bus-bar Protection (Cont…) Stability –The stability of bus protection is of paramount importance –Dangers exist in practice for a number of reasons. These are Interruption of the secondary circuit of a current transformer will produce an unbalance, which might cause tripping on load A mechanical shock of sufficient severity may cause operation, although the likelihood of this occurring with modern numerical schemes is reduced Accidental interference with the relay, arising from a mistake during maintenance testing, may lead to operation

Bus-bar Protection (Cont…) A number of bus-bar protection systems have been devised –System protection used to cover bus-bars –Frame-earth protection –Differential protection –Phase comparison protection –Directional blocking protection

Bus-bar Protection (Cont…) Busbar blocking system –Advantages Very low or no cost system Simple Faster than faults cleared by back-up relays Covers phase and ground faults Adequate sensitivity–independent of no. of circuits No additional CTs Commissioning is simple–no primary current stability tests

Bus-bar Protection (Cont…) –Disadvantages Only suitable for simple busbars Additional relays and control wiring for complex busbars Beware of motor in- feeds to busbar faults Sensitivity limited by load current Busbar blocking system

Bus-bar Protection (Cont…) System protection schemes –System protection includes over-current or distance systems –It will inherently give protection cover to the bus- bars Frame-earth protection (HOWARD PROTECTION) –Frame leakage protection has been extensively used in the past in many different situations. –There are several variations of frame leakage schemes available, providing bus-bar protection schemes with different capabilities

Bus-bar Protection (Cont…) Single zone frame-earth protectionCurrent distribution for external fault

Bus-bar Protection (Cont…) Three zone frame earth scheme Frame-earth scheme: Bus section breaker insulated on one side only

Bus-bar Protection (Cont…) For satisfactory operation of supervisory protection schemes audible and visual alarms are used. These alarms are used for –Bus-bar faults –Bus-bar protection in service –Bus-bar protection out of service –Tripping supply healthy –Alarm supply healthy

Bus-bar Protection (Cont…) Numerical Bus-bar protection schemes Differential protection requires sectionalizing the busbars into different zones –High impedance bus zone –Advantages Relays relatively cheap – offset by expensive CTs Simple and well proven Fast–15…45 m secs Stability and sensitivity calculations easy, provided data is available Stability can be guaranteed

Bus-bar Protection (Cont…) –Disadvantages Very dependent on CT performance CT saturation could give false tripping on through faults Sensitivity must be decreased DC offset of CTs unequal–use filters Expensive class X CTs–same ratio–Vknp = 2 times relay setting Primary effective setting ( %) Limited by number of circuits Z grounded system difficult for ground fault Duplicate systems–decreased reliability Require exact CT data Vknp, Rsec, imag, Vsetting High voltages in CT circuits (+/− 2.8 kV) limited by volt dependent resistors Biased medium impedance differential

Bus-bar Protection (Cont…) –Advantages High speed 8…13 m secs Fault sensitivity +/− 20% Excellent stability for external faults Normal CTs can be used with minimal requirements Other protection can be connected to same CTs No limit to number of circuits Secondary voltages low (medium impedance) Well proven systems worldwide Any busbar configuration No need for duplicate systems Retrofitting easy No work on primary CTs Biasing may prevent possibility of achieving a sensitive enough ground fault setting of Z grounded systems

Bus-bar Protection (Cont…) –Disadvantages Relays relatively expensive Offset by minimal CT requirements Relays with auxiliary CTs require a separate panel

Bus-bar Protection (Cont…) Available protection functions are: –backup over-current protection –breaker failure –dead zone protection

Bus-bar Protection (Cont…) Architecture for numerical protection scheme

Pilot wire Including Phase Comparison Following are the reasons for not using current differential relaying –The likelihood of improper operation owing to CT inaccuracies under the heavy loadings that would be involved –The effect of charging current between the pilot wires –The large voltage drops in the pilot wires requiring better insulation –The pilot currents and voltages would be excessive for pilot circuits rented from a telephone company

Pilot wire Including Phase Comparison (Cont…) Purpose of a pilot Transmission-line sections for illustrating the purpose of a pilot

Pilot wire Including Phase Comparison (Cont…) D-C wire-pilot relaying Schematic illustration of a d-c wire-pilot relaying equipment D = voltage-restrained directional (mho) relay O = over-current relay T = auxiliary tripping relay S = auxiliary supervising relay PW = pilot wire

Pilot wire Including Phase Comparison (Cont…) Schematic illustration of a d-c wire-pilot scheme where information is transmitted over the pilot D = voltage-restrained directional (mho) relay B = auxiliary blocking relay O = over-current relay TC = trip coil PW = pilot wire

Pilot wire Including Phase Comparison (Cont…) A-C wire-pilot relaying Schematic illustration of the circulating-current principle of a a-c wire- pilot relaying

Pilot wire Including Phase Comparison (Cont…) Schematic illustration of the opposed-voltage principle of a-c wire- pilot relaying

Pilot wire Including Phase Comparison (Cont…) Effect of Shorts Effect of Open Circuits Opposed voltage Cause trippingBlock tripping Circulating current Block trippingCause tripping

Pilot wire Including Phase Comparison (Cont…) Schematic connections of a circulating-current a-c wire-pilot relaying equipment

Pilot wire Including Phase Comparison (Cont…) Schematic connections of an opposed-voltage a- c wire-pilot relaying equipment P = current polarizing coil R = voltage restraining coil O = current operating coil

Distance Protection Schemes The most important and versatile family of relays is the distance-relay group It includes the following types –Impedance relays –Reactance relays –Mho relays –Angle impedance relays –Quadrilateral relays –Elliptical and other conic section relays

Distance Protection Schemes (Cont…) Basic principle of operation

Distance Protection Schemes (Cont…) Balanced beam principle Bridge comparatorPlain impedance characteristic

Distance Protection Schemes (Cont…) Tripping characteristics MHO characteristicOffset MHO characteristic

Distance Protection Schemes (Cont…) Application onto a power line Zone 1 MHO characteristic3 Zone MHO characteristics

Distance Protection Schemes (Cont…) Effect of load current Load encroachment Effect of arc resistance

Distance Protection Schemes (Cont…) Different shaped characteristics Lenticular characteristic

Distance Protection Schemes (Cont…) Figure-of-eight characteristic Trapezoidal characteristic

Reclosing & Check Synchronising Reclosers –Adjacent reclosers can be coordinated more closely since there are no inherent errors Self contained pole mounted ARC –The following are the system service conditions which are suitable for using AR on non-effectively earthed and effectively earthed networks Nominal system voltage : 11 kV Maximum system voltage : 15 kV Continuous current rating : 630 A Short circuit-breaking capacity: 12.5 kA

Reclosing & Check Synchronising (Cont…) Rated power frequency withstand voltage : 50kVrms Impulse withstand voltage : 125 kVp System frequency : 50 Hz; Number of phases : 3 Insulation medium : Solid dielectric Operating mechanism :Magnetic actuator Ambient temperature (minimum) : 5 °C Ambient temperature (maximum) : 50 °C Max. Relative humidity. : 100%

Reclosing & Check Synchronising (Cont…) Check Synchronising –Faults on overhead lines fall into one of three categories: Transient Semi-permanent Permanent

Reclosing & Check Synchronising (Cont…) Single-shot auto-reclose scheme operation for a transient fault

Reclosing & Check Synchronising (Cont…) Operation of single-shot auto-reclose scheme on a permanent fault

Reclosing & Check Synchronising (Cont…) Application of auto-reclosing –The most important parameters of an auto-reclose scheme are Dead time Reclaim time Single or multi-shot Advantages of using Auto-reclosing on HV distribution Networks –Reduction to a minimum of the interruptions of supply to the consumer –Instantaneous fault clearance can be introduced, with the accompanying benefits of shorter fault duration and fewer permanent faults –Improved supply continuity –Reduction of substation visits

Reclosing & Check Synchronising (Cont…) Several factors affect the selection of system dead time as follows –System stability and synchronism –Type of load,CB characteristics –Fault path de-ionisation time –Protection reset time

Reclosing & Check Synchronising (Cont…) Factors affecting the setting of the reclaim time are –Type of protection –Spring winding time

Reclosing & Check Synchronising (Cont…) Auto-reclosing on EHV transmission lines Effect of high-speed three-phase auto-reclosing on system stability for a weak system

Reclosing & Check Synchronising (Cont…) De-Ionisation of Fault Arc –The de-ionisation time of an uncontrolled arc, in free air depends on The circuit voltage Conductor spacing Fault currents Fault duration Wind speed Capacitive coupling from adjacent conductors Line voltage (kV) Minimum de- energisati on time (seconds) Fault-arc de-ionisation times

Reclosing & Check Synchronising (Cont…) The advantages of single-phase auto- reclosing are –The maintenance of system integrity –On multiple earth systems, negligible interference with the transmission of load

Reclosing & Check Synchronising (Cont…) Typical three zone distance scheme

Reclosing & Check Synchronising (Cont…) Delayed auto- reclose scheme logic

Reclosing & Check Synchronising (Cont…) The synchronism check relay element commonly provides a three-fold check –Phase angle difference –Voltage –Frequency difference Auto-close scheme Standby transformer with auto-closing

Tie-Line Protection and Load Shedding Special protection schemes –System protection schemes are protection strategies designed to detect a particular system condition that may cause unusual stress to the power system The most common types of system protection schemes are as follows –Generator rejection –Load rejection –Under frequency load shedding –System separation –Turbine valve control

Tie-Line Protection and Load Shedding (Cont…) –Load and generator rejection –Stabilizers –HVDC controls –Out of step relaying –Discrete excitation control –Dynamic braking –Generator runback –VAR compensation

Tie-Line Protection and Load Shedding (Cont…) The main objectives of using system protection schemes are –Operation of power systems closer to their limits –Increase power system security particularly during extreme contingencies –Improve power system operation

Tie-Line Protection and Load Shedding (Cont…) Estimation of rate of change of frequency (df/dt) H = System Inertia Constant on system base (seconds) f o = Frequency at the time of disturbance (Hz) df/dt =Rate of change of frequency (Hz/Sec) Δ P =( PL-PG)/PG, Power change (per unit in system load base) PL =Load prior to generation Loss in MW PG =System Generation after Loss in MW D =Power/frequency characteristic of the system in pu/Hz Δf =Change in frequency

Tie-Line Protection and Load Shedding (Cont…) The overload may be differentiated by the method that is used to detect and respond to condition –Dispersed frequency monitoring –Dispersed voltage monitoring –Utility Scale SCADA System Configuration Monitoring –Industrial Scale System Configuration Monitoring –Local equipment overload monitoring

Tie-Line Protection and Load Shedding (Cont…) Methods of overload detection and load shedding –By Frequency Monitoring –By Voltage Monitoring –By SCADA System Monitoring Application to Utility Scale Systems Application to Smaller Industrial Facilities –By Current and/or Power Monitoring Drawbacks of breaker interlock load shedding –Load shedding based on worst-case scenario –Only one stage of load shedding –Almost always, more load is shed than required –Modifications to the system are costly

Tie-Line Protection and Load Shedding (Cont…) Pre-disturbance operating conditions –Total system load demand –Total system power exchange to the grid –Generation of each on-site unit –Spinning reserve for each on-site unit –Control settings for each running unit –System configurations

Tie-Line Protection and Load Shedding (Cont…) Post-disturbance operating conditions –New system load demand –Remaining generation from on-site generation –Spinning reserve for each remaining unit –Time duration to bring up the spinning reserve –New system configurations –Status, settings and loading conditions of the remaining major rotating machines

Tie-Line Protection and Load Shedding (Cont…) Nature and duration of the disturbance –Electrical and/or Mechanical faults –Complete or partial loss of power grid connection –Complete or partial loss of on-site generation –Load addition (impact) –Location of disturbance –Duration of disturbance and its termination –Subsequent system disturbances

Tie-Line Protection and Load Shedding (Cont…) System transient response to a disturbance –System frequency response –System voltage response –Rotor angle stability of each remaining unit –Operation of protective devices

Tie-Line Protection and Load Shedding (Cont…) Load shedding system can be designed to meet the following objectives –Map a complex, highly nonlinear, nonparametric, load shedding problem, to a finite space with a limited number of data collection points –Automatic recall of system configuration, operating condition, and system response to disturbances –Pattern recognition capability to predict system response to disturbances –Systems knowledgebase trainable by user defined cases –Self-learning capability to new system changes –Make prompt decisions regarding which loads to shed based on the online status of sheddable loads –Shed the minimum amount of load to maintain system stability

Tie-Line Protection and Load Shedding (Cont…) Load Shedding scheme function Block diagram