#37 Protection System in Power System

Protection System in Power System

This portion of our website covers almost everything related to protection system in power system including standard lead and device numbers, mode of connections at terminal strips, color codes in multi-core cables, Dos and Don’ts in execution. It also covers principles of various power system protection relays and schemes including special power system protection schemes like differential relays, restricted earth fault protection, directional relays and distance relays etc. The details of transformer protection, generator protection, transmission line protection and protection of capacitor banks are also given. It covers almost everything about protection of power system.
The switchgear testing, instrument transformers like current transformer testing voltage or potential transformer testing and associated protection relay are explained in detail.

The close and trip, indication and alarm circuits different of circuit breakers are also included and explain.

Objective of Power System Protection

The objective of power system protection is to isolate a faulty section of electrical powersystem from rest of the live system so that the rest portion can function satisfactorily without any severer damage due to fault current.

Actually circuit breaker isolates the faulty system from rest of the healthy system and this circuit breakers automatically open during fault condition due to its trip signal comes from protection relay. The main philosophy about protection is that no protection of power system can prevent the flow of fault current through the system, it only can prevent the continuation of flowing of fault current by quickly disconnect the short circuit path from the system. For satisfying this quick disconnection the protection relays should have following functional requirements.

Protection System in Power System

Let’s have a discussion on basic concept of protection system in power system and coordination of protection relays.In the picture the basic connection of protection relay has been shown. It is quite simple. The secondary of current transformer is connected to the current coil of relay. And secondary of voltage transformer is connected to the voltage coil of the relay. Whenever any fault occurs in the feeder circuit, proportionate secondary current of the CT will flow through the current coil of the relay due to which mmf of that coil is increased. This increased mmf is sufficient to mechanically close the normally open contact of the relay. This relay contact actually closes and completes the DC trip coil circuit and hence the trip coil is energized. The mmf of the trip coil initiates the mechanical movement of the tripping mechanism of the circuit breaker and ultimately the circuit breaker is tripped to isolate the fault.

Functional Requirements of Protection Relay

Reliability

The most important requisite of protective relay is reliability. They remain inoperative for a long time before a fault occurs; but if a fault occurs, the relays must respond instantly and correctly.

Selectivity

The relay must be operated in only those conditions for which relays are commissioned in the electrical power system. There may be some typical condition during fault for which some relays should not be operated or operated after some definite time delay hence protection relay must be sufficiently capable to select appropriate condition for which it would be operated.

Sensitivity

The relaying equipment must be sufficiently sensitive so that it can be operated reliably when level of fault condition just crosses the predefined limit.

Speed

The protective relays must operate at the required speed. There must be a correct coordination provided in various power system protection relays in such a way that for fault at one portion of the system should not disturb other healthy portion. Fault current may flow through a part of healthy portion since they are electrically connected but relays associated with that healthy portion should not be operated faster than the relays of faulty portion otherwise undesired interruption of healthy system may occur. Again if relay associated with faulty portion is not operated in proper time due to any defect in it or other reason, then only the next relay associated with the healthy portion of the system must be operated to isolate the fault. Hence it should neither be too slow which may result in damage to the equipment nor should it be too fast which may result in undesired operation.

Important Elements for Power System Protection

Switchgear

Consists of mainly bulk oil circuit breaker, minimum oil circuit breaker, SF₆ circuit breaker, air blast circuit breaker and vacuum circuit breaker etc. Different operating mechanisms such as solenoid, spring, pneumatic, hydraulic etc. are employed in the circuit breaker. Circuit breaker is the main part of protection system in power system and it automatically isolate the faulty portion of the system by opening its contacts.

Protective Gear

Consists of mainly power system protection relays like current relays, voltage relays, impedance relays, power relays, frequency relays, etc. based on operating parameter, definite time relays, inverse time relays, stepped relays etc. as per operating characteristic, logic wise such as differential relays, over fluxing relays etc. During fault the protection relay gives trip signal to the associated circuit breaker for opening its contacts.

Station Battery

All the circuit breakers of electrical power system are DC (Direct Current) operated. Because DC power can be stored in battery and if situation comes when total failure of incoming power occurs, still the circuit breakers can be operated for restoring the situation by the power of storage station battery. Hence, the battery is another essential item of the power system. Some time it is referred as the heart of the electrical substation. An electrical substation battery or simply a station battery containing a number of cells accumulate energy during the period of availability of AC supply and discharge at the time when relays operate so that relevant circuit breaker is tripped at the time failure of incoming AC power.

There are different contacts connected in series along a trip circuit of an electrical circuit breaker. There must be some situation when the circuit breaker should not trip even a faulty current passes through its power contacts. Such situations are low gas pressure in SF₆ circuit breaker, low air pressure in pneumatic operated circuit breaker etc. In this situation the trip coil of the CB must not be energized to trip the CB. So there must be NO contacts associated with gas pressure and air pressure relays, connected in series with breaker trip coil. Another scheme of trip coil is that it should not be re energized once the circuit breaker is opened. That is done by providing one NO contact of breaker auxiliary switch in series with trip coil. In addition to that the trip circuit of a CB has to pass through considerable numbers of intermediate terminal contacts in relay, control panel and circuit breaker kiosk. So if any of the intermediate contacts is detached, the circuit breaker fails to trip. Not only that, if dc supply to the trip circuit fails, the CB will not trip.

To overcome this abnormal situation, trip circuit supervision becomes very necessary. The figure below shows the simplest form of trip circuit healthy scheme. Here one series combination of one lamp, one push bottom and one resistor is connected across the protective relay contact as shown. In healthy situation all the contacts except protective relay contact are in close position. Now if push bottom (PB) is pressed, the trip circuit supervision network is completed and lamp glows indicating that the breaker is ready for tripping.The above scheme is for supervision while circuit breaker is closed. This scheme is called post close supervision.

There is another supervision scheme which is called pre and post close supervision. This trip circuit supervision scheme is also quite simple. The only difference is that here in this scheme, one NC contact of same auxiliary switch is connected across the auxiliary NO contact of the trip circuit. The auxiliary NO contact is closed when CB is closed and auxiliary NC contact is closed when CB is open and vice versa. Hence, as shown in the figure below when the circuit breaker is closed the trip circuit supervision network is completed via auxiliary NO contact but when the circuit breaker is open the same supervision network is completed via NC contact. The resistor is used series with the lamp for preventing unwanted tripping of circuit breaker due to internal short circuit caused by failure of the lamp.So far whatever we have discussed it is only for local controlled installation but for a distance control installation, relay system is necessary. The figure below shows the trip circuit supervision scheme wherever a remote signal is required.When trip circuit is healthy and circuit breaker is closed, relay A is energized which closes the NO contact A₁ and hence relay C is energized. Energized relay C keeps NC contact in open position. Now if the circuit breaker is open, relay B is energized which closes No contact B₁hence relay C is energized. As C is energized, it keeps the NC contact C₁ in open position. While CB is closed, if there is any discontinuity in the trip circuit relay A is de-energized which opens contact A₁ and consequently relay C is de-energized and which make the NC contact C₁ in close position and hence alarm circuit is actuated. Trip circuit supervision is experienced by relay B with the circuit breaker is open in a similar manner as relay A with the circuit breaker is closed. Relays A and C are time-delayed by copper slugs to prevent spurious alarms during tripping or closing operations. The resistors are mounted separately from the relays and their values are chosen such that if any one component is inadvertently short-circuited, a tripping operation will not take place.
The alarm circuit supply should be separated from main trip supply so that the alarm can be actuated even the trip supply failed.

Any internal fault inside the stator winding is cleared by mainly differential protection scheme of the generator or alternator.
The differential protection is provided in the generator by using longitudinal differential relay.Generally instantaneous attracted armature type relays are used for this purpose because all they have high speed operation and also they are free from being affected by any AC transient of the power circuit.

There are two sets of current transformers one CT is connected to the line side of the generator and other is connected to the neutral side of the generator in each phase. It is needless to say that the characteristics of all current transformers installed against each phase must be matched. If there is any major mismatched in the current transformer’s characteristics of both sides of the generator, there may be high chance of malfunctioning of differential relay during the fault external to the stator winding and also may be during normal operating conditions of the generator.

To ensure that the relay does not operate for the faults external to the operated zone of the protection scheme, a stabilizing resistor is fitted in series with the relay operating oil. It also ensures that if one set of CT has been saturated, there will be no possibility of malfunctioning of the differential relay.It is always preferable to use dedicated current transformers for differential protection purpose because common current transformers may cause unequal secondary loading for other functionalities imposed on them. It is also always preferable to use all current transformers for differential protection of generators or alternators should be of same characteristics. But practically there may be some difference in characteristics of the current transformers installed at line side to those installed in neutral side of the generator. These mismatches cause spill current to flow through the relay operating coil. To avoid the effect of spill current, percentage biasing is introduced in differential relay.The percentage biased differential relay comprises two restraint coils and one operating coil per phase. In the relay, the torque produced by operating coil tends to close the relay contacts for instantaneous tripping of circuit breakers but at the same time the torque produced by the restraint coils prevents to close the relay contacts as restraint coils torque is directed opposite of the operating coil torque. Hence during through fault the differential relay would not be operated because the setting of the relay is increased by restraint coils and also it prevents malfunctioning of relay due to spill current. But during internal fault in the winding of the stator, the torque produced by restraint coils is ineffective and the relay closes its contact when setting current flows through the operating coil.

Differential current pickup setting/bias setting of the relay is adopted based on the maximum percentage of allowable mismatch adding some safety margin.
The spill current level for the relay is to just operate it; is experienced as a percentage of the through fault current causing it. This percentage is defined as bias setting of the relay.

Now days, a major portion of electrical and electronics industry have switched upon industrial automation system. In electrical system, all time attention is given to the processes and plants are very much essential, irrespective of the voltage levels. In electrical and electronics systems, the word Annunciator literally means the device which announces the mishaps or unusual activities, coming from the system or process associated with it.

What is Alarm Annunciator?

It is basically an audio visual warning system, which highlights the fault or mishap which is going on, or even before it happens. This is very necessary for safety concern also, and sometimes the warning comes before improper procedure which warns the operator to avoid unwanted accident etc. This is the basic concept of Alarm Annunciator, and the alarm annunciation system. Let us look at the operation of a typical alarm annunciator device.

Operation of Alarm Annunciator

Alarm Annunciation System

In order to understand the fundamental operation and connections of alarm Annunciator, we have to understand the basic concept of alarming system in process monitoring. Suppose, an electromagnetic coil is energized by power supply and acts as an electromagnet for certain application. Now, because of over voltage a portion of the coil has been burnt. As a consequence, the entire process associated with it gets hampered. So, when finding the very cause of this mishap, you have to check each and every part of the system in order to find and recognize the actual fault. Now think you have 50 such coils, which you have to monitor. In this case finding the actual faulty coil becomes very difficult and time consuming too.

But if you connect a bulb in series with the power supply of each coil, it glows if and only if the coil is energized and healthy. In this way, for 50 such electromagnetic coils you need to use 50 bulbs each of them connected in series with each of the individual coil through which you can monitor the processes by viewing the glow status of those bulbs. This is the basic and easiest model of process monitoring.
Alarm Annunciator is a centralized model, which gives audio visual signals for the faulty processes. Latest models of annunciators are based upon microprocessor or microcontroller circuitry, which ensures the maximum reliability as well as enhanced wide ranges of features and functionalities.

Connection of Alarm Annunciator

There are two types of connections for each annunciation system; they are input fault contacts and output relay changeover contacts. Input fault contacts are simple connection normally open (or NC Selectable) with respect to a common C contact. Usually these input fault contacts are potential free contacts. The logic is, if any fault contacts and the common contact C becomes short circuited by any means, the respective fascia or fault window will start blinking, and the output relay contact will changeover instantly.Suppose, you are using 8 windows annunciation system, which means you are monitoring 8 operations at a time, by the annunciation system. Let us think your fault 1 (F1) is assigned as over voltage alarm of motor 1 and your fault 2 (F2) is assigned as overheating of a motor 2 armature. You will connect an over voltage relay with motor 1 and a PTC thermistor relay with Motor 2, and the respective outputs (Normally open output, changes to close when faulty) of those relays will be connected across F1 (fault input) and C (common), and F2 (fault input) and C (common) of the annunciator system. Therefore, if the voltage of motor 1, is increased beyond the predefined safe level, the over voltage relay will operate and will make a closed loop between F1 and Common. So, the F1 window will start blinking which indicates the motor 1 is getting over voltage. At the same time, the annunciator relay will changeover, and if you connect a hooter previously with its output contacts, then the hooter will start alarming.

Similarly, if the armature temperature of motor 2 is increased beyond the predefined safe level, then the PTC thermistor relay will changeover and will make a loop path between F2 and Common C of annunciation system. So, the F2 window will start blinking which indicates the motor 2 is getting over heated. At the same time, the annunciator relay will changeover, and the hooter connected with its contacts, it will start alarming. Basically, the annunciator output relay changeover is common, irrespective of any faults. A single hooter is used for all fault windows. An auxiliary AC/DC supply is necessary to operate the annunciator and in modern annunciators, there is also a window and connection provided for monitoring its own auxiliary supply.

Modern Alarm Annunciators consist of a power supply unit SMPS, a programming unit CPU and other connections including fault contacts and facial display units. The blinking windows are generally acrylics, which are enlightened by LED with very low power consumption. Typically, annunciation effectively starts from 4 faults that is 4 windows, if the number of faults to be monitored is more than 64, it is preferable to install the programming unit CPU, power supply unit PSU and the display facial unit individually, which ensures the maximum accuracy and effectiveness.