1.  The figure below shows a protection system for a transmission line, consisting of a CT, PT, a relay and its associated circuit breaker.

2.  We consider relay as a black box with current and voltage as its input, and closure of the normally open contacts as its output.  This output of the relay is normally wired to a circuit breaker.  The relay has another user settable input, which is the setting of the relay.  The relay compares the values from the CT and PT to the setting values and then takes the trip/no trip decision

3.  The figure below shows a conceptual diagram of a relay.

4.  A protective system should possess the following properties ◦ Sensitivity ◦ Selectivity ◦ Speed ◦ Reliability and Dependability

5.  Sensitivity: the protective system should be sensitive enough or alive enough to the presence of the smallest fault current.  The smaller the fault current it can detect, the more sensitive it is.  Selectivity: the protective system should detect and isolate the faulty equipment only.  Speed: the protective system should be fast in detecting the fault and isolating the faulty equipment because the longer the fault persists in the system higher the chances of instability.

6.  Reliability and Dependability: a protective system is useless if its not reliable. Generally simple systems are more reliable.  We try to make the system reliable but sometimes primary protection system fails. So we use backup protection as well.

7.  It has two functions: ◦ It steps down the current to such levels that can be easily handled by relay current coil. ◦ It isolates the relay circuitry from the high voltage of the EHV system.  Standard step down current ratings of a CT are 1 A, 5A.  These standard values help in standardizing the relay design irrespective of the primary current.  Connected in series with the line in which current is to be measured.

8.  The figure below shows a CT connected to a line:

9.  A metering CT is designed in a way that in case of faults or when the current increase 1.2 times the full load current it will saturate and protect the connected measuring instrument from damage.  Where as a protective CT is required to transform the primary current throughout its range from normal load current to short circuit current.

10.  It steps down the line’s high voltage to a level safe enough for the relaying system and personnel to handle.  The standard secondary voltage is 110V, this helps in standardizing the design of the relay irrespective of the value of the primary EHV.  A PT is connected in parallel at the point where a measurement is desired.

11.  The figure below shows a PT

12.  The figure below shows a CT and PT connected to a supply, load and relay.

13.  Circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit.  It is operated by the output of the associated relay.  It is closed in the normal operating conditions and open in case of overload or fault.

14.  A system is divided into different protection zones. If a fault is detected within a zone all its allied circuit breakers should trip.  Faults within the zone are termed as internal faults where as faults outside the zone are called external faults.  A relay looking after the protection zone should operate only for internal faults.  The farthest point from the relay location which is still inside the zone is called the reach point

15.  The distance between the reach point and the relay location is called the reach of the relay.

16.  The figure on the side shows various zones of protection for a power system  It can be seen that the adjacent zones overlap, otherwise there could be some portion which is left out and remains unprotected.

17.  If the fault occurs in the overlapped portion, more than minimum number of circuit breakers will trip.

18.  Primary protection may fail due to failure of CT/PT, relay or circuit breaker.  So it is a normal practice to provide another zone of protection which should operate and isolate the faulty element in case the primary protection fails.

19.  It should not have anything in common with the primary system  It should also be preferably be located at a place different from where the primary protection is located.  Back up system must wait for the primary system to operate or the operating time of back up must be delayed compared to primary.

20.  The figure below shows a power system

21.  Relay B and circuit breaker CB B are providing protection to line section B-C. relay A with circuit breaker CB A provides back up protection to the section B-C.  Consider a fault in section B-C.  When fault takes place both the primary relay R B and the back up relay R A start operating simultaneously.  If the primary protection operates successfully the line B-C gets deenergized but the loads on buses A and B remain unaffecetd. Therefore, the back up protection resets without issuing a trip command.

22.  If the primary protection fails to operate, the back up which is already monitoring the fault, waits for the tie in which the primary would have cleared the fault and then issues the trip command to its allied circuit breakers.  When the back up operates the time for which the fault persists is longer and disruption to the loads also lasts longer

23.  Most common types of protection principles are shown in the table below:

24.  Question 1:In the diagram shown below T B =0.2 sec. if the circuit breaker operating time is 0.5sec, determine the operating time of relay R A so that there is no loss of selectivity.

25.  Question 2: Assume the operating time of relay RB =0.3 sec, operating time of relay RA=0.6 sec. The circuit breaker operating time is 0.5sec. Find whether there will be any loss of selectivity between the primary and the back up protection