Protection in Sub Station

Feeders /equipments protection

Protection is an insurance against loss of power supply. The intention of protection is to protect ,save the source/other electrical gadgets by isolating the fault. It is achieved by using protective relays.

1.      Incoming power cable from TPS	-O/C +E/F (CDG31) -Fdr differential (HORM-4)
2.      Busbars	-Differential protection (CAG34)
3.      Transformers feeders	-Differential protection (DTH31), -O/C +E/F (CDAG 51) -Buchholz protn,WTI Trip and OTI trip
4.      Incoming feeders from transformer to 6.6/.415 KV switchboard	- O/C +E/F (CDG31) -Back up E/F  (CDG11) -REF (CAG14)
5.      HT/LT Motor (induction)	-CTMM And MM30
6.      Synchronous motor	-MOPN, MFOS,MCAG, -Under/Over Excitation
7.      Outgoing feeders from 11KV SWBD	-          differential (HORM-4) -          O/C +E/F (CDAG 51)
8.      Outgoing feeders from 6.6KV SWBD	-          differential protection(HORM-4) -          O/C +E/F (CDG31)
9.      Outgoing feeders from PCC(to MCC)	- O/C +E/F (CDG34)
10.  LINE PT	- VAGM 22 (40-80%)
11.  BUS PT	- VAGM 22 (40-80%)

Protection system for electrical equipments and switchgear in substation.

Introduction to protection system

What is protection and why it is required?

It is an   insurance again loss of supply –mainly intended to protect /save the source /other electrical equipments by isolating the fault. It is achieved with the help of protective gear.

 What are its components?

Protective gear is a collective term, which defines all equipments necessary for recognizing locating and initiating the removal of fault or abnormal condition from the power system. These are relays, current transformer, potential transformer, fuses, control wires etc.

Protective relays are the devices that detect abnormal condition in electrical circuits. By constantly   measuring the electrical quantities which are different under normal and fault condition. The basic electrical quantities which may change during fault condition are voltage, current, phase angle (direction) and frequency. Having detecting the fault, the relay operates to complete the trip circuits. Which   results in operating the circuits. Breakers   and therefore, in the disconnection of the faulty circuits. A typical relay circuit.  Is shown which shows one phase for simplicity.

Basic requirements of protective relay.

1.   Speed: protective relay should disconnect faulty circuits as quickly as possible to improve the system stability and decrease amount of damaged increased i.e. 0.07 to 0.1 second.

2.    Selectivity:  it is the ability of the system to determine the point at which the fault occurs and select the nearest of the circuit breaker tripping of which will lead to clearing of fault with minimum or no damage to the system.

3.  Sensitivity:

       The system should be sufficient sensitive to operate even at the minimum fault

        Condition for a fault with its zone while remaining stable under through faults.

       and max. load condition

  4.  Reliability:

The system should be ready to function, reliable & correct in operation at all time under any kind of fault and abnormal condition of the power system for which it has been made.

5     Simplicity :

Simplicity of the construction and good quality of the relay, correctness of the design and installation, qualified maintenance and supervision etc. are the main factors which influence protective relaying reliability.

6    Economy :

As with all good engineering design, economy plays a major role.

OVER CURRENT PROTECTION.

When the magnitude of current exceeds the pick up level. The element is an O/C  relay. This protection is used in motors. Transformers, line/feeders etc.

Instantaneous relays are those which have no intentional time lag and which operate in less than 0.1 second, usually less than 0.08 sec.

The relays which are not instantaneous are called time delayed relays such a relays are provided with the delaying means.

An inverse curve is one in which the operating time becomes less as the magnitude of the actuating quantity is increased. However for higher magnitude of actuating quantity time is constant.

Definite time curve is one in which operating time is little affected by magnitude of current. However even definite time delay as a characteristic which is slightly inverse.

The characteristic with definite minimum time and of inverse type is also called

IDMT i.e. INVERSE DEFINITE MINIMUM TIME.   

Method of CT connection over current protection –

EARTH FAULT PROTECTION

When fault current flows through earth return path, the fault is called earth fault. More sensitive protection against earth fault can be obtained by using a relay , which responds only to the residual current of the system, since a residual components exists only when fault current flows  to earth. The relay is therefore, completely unaffected by load currents , whether balanced or not. But for lower setting, since unbalance leakage  or capacitance currents to earth may produce a residual quantity of this order ,therefore this statement  should be taken with reservation for lower settings.

Connection of CT for earth fault connection:

In the absence of earth fault the vector sum of three lines current is zero. Hence vector sum of three secondary currents is zero( Ias+ Ibs+ Ics =0). This sum is called residual current Irs. In the presence of earth fault the condition is disturbed. and Ias+ Ibs+ Ics is no more zero. Hence Irs flows through the relay . if Irs is above its pickup value, the earth fault relay operates.

RESSTRICTED EARTH FAULT RELAY 

 Earth fault relays connected in residual circuit of line CTS give protection against earth faults on the delta or unearthed star connected winding of transformer. Earth fault on secondary side are not reflected on primary side, when the primary winding is delta connected. In such cases, an earth fault relay connected in residual circuit of 3 CTS on primary side operates on internal faults in primary winding only. Because earth faults on secondary side do not produce zero sequence currents on primary side. Restricted earth fault protection may then be used for high speed tripping for faults on star connected earthed secondary winding of power transformer.

COMBINED EARTH FAULT AND PHASE FAULT PROTECTION.

CT connection for combined application is shown below-

Increase in current of phases causes corresponding increase in respective secondary current. The secondary currents follow through respective relay unit. The E/F relay is connected residually as explained earlier

DIFFERENTIAL PROTECTION

When there is no internal fault the current entering in X is equal in phase and magnitude to current leaving X. the CTs are of  such a ratio that during normal conditions are for external faults, the secondary current of the CT s are equal. These currents say I₁ and I_{2 circulate} in the pilot wires. The polarity connections are such that the currents I₁ and I₂ are in same directions in pilot wires, during normal conditions or external faults. Relay operating coil is connected at middle of pilot wires. Relay is of O/C type.

During normal conditions and external faults the protection system is balanced and CT ratios are such that the secondary currents are equal (I1-I2 =0). This balance is disturbed for internal faults. When fault occurs in protected zones, the current entering in the protected winding is no more equal to the current leaving the winding because some currents flow to the fault. The differential I1-I2 flows through the relay operating coil and the relay operates if the operating torque is more than restraing torque.   

Limitation of differential  protection

-Pilot wire resistance.

-CT ratio error

-Saturation of CT magnetic ckt during short ckt.

- Magnetizing   current inrush in transformer while switching in and

 –Tape changing.

PILOT WIRE PROTECTION OR DISTANCE PROTECTION

The differential circulating current protection principal can be readily applied to feeder protection. Two CTs are connected in each protected line one at each end. Under healthy external fault conditions, the secondary currents are equal and circulate in pilot wires. The relays are connected between equipotential points in pilot wires. For external fault and normal condition, the differential current (I1-I2) of two CTs is zero and relay does not operate. During internal faults the balance is disturbed and differential current flows through the relay operating coil.

In line protection relaying point falls in the middle of the line. Therefore, the circuit is modified as by providing two relays one at each end.

 

Pilot wire relaying with split connection using 3-core cable for pilot connections.