NEED OF GRADING CAPACITORS IN CIRCUIT BREAKERS IN EHV SYSTEM

The Grading capacitors are the capacitors (Typical value of around 500 Pico farads) which are positioned parallel with the interrupting units of the circuit breakers.

Before judging the necessity of grading capacitors, let us discuss the importance of these capacitor units across interrupting unit of the circuit breaker and what are the consequences of neglecting them.

What is the purpose of Grading Capacitors?

Let's examine a 400 kv Transmission line linking two substations via a circuit breaker at each end. The purpose of this transmission line is to facilitate the bidirectional power transfer between Busbars of these substations. Suppose if there is a grounded fault on this line (May be a single phase or two phase or three phase fault) somewhere between the two substations.The relays situated at both ends of the line will detect this fault and will trip the breakers accordingly.Now,with the breaker open,One end of the circuit breaker is grounded via the transmission line (due to fault),while the other end is connected to the bus via an isolator,as illustrated in the figure below.

 Now the scenario is Breaker is in open condition,One end of the Circuit breaker is grounded via transmission line (due to fault) and the other end of the circuit breaker is connected to the bus via an isolator as shown in the fig below.

Currently there exists a potential difference of 400 kv across the open contacts of the circuit breaker as the bus is charged also by the other incoming lines.

Essentially, three capacitances are formed within the circuit breaker:

i) The capacitance of Interrupter-1 (which is grounded via the transmission line due to the fault), denoted as C1.

ii) The capacitance of Interrupter-2 (which is connected to the 400 kV bus via an isolator), denoted as C2.

iii) The capacitance of the pole column (which is grounded via the circuit breaker metal structure), denoted as Cp.

The capacitance of the two interrupting units is equal due to symmetry, whereas the capacitance of the pole column is approximately double that of the interrupting units, considering the length of the pole column is approximately twice that of the interrupting unit. The capacitance connections are illustrated as shown below.

In this configuration,the pole column and interrupter unit-1 (Which is grounded via the transmission line due to the fault) are connected in parallel. This parallel connection results in the addition of their capacitances, leading to a resultant capacitance of 3C. Now, the resultant capacitance connection will be a series combination of 3C and C, with a bus voltage of 400 KV,as depicted in the below figure.

It is clear from the derivation that there is an imbalance of voltage across two interrupter units which is three fourth of Bus voltage across Interrupter-2 and one fourth of voltage across Interrupter-1 Unit.Now,this creates the problem.The effective voltage applied across the  interrupting units of the breaker should be same.

The solution to avoide this condition is by placing a grading capacitors of comparatively large value  across two interrupting units due to which net capacitance of the two interrupting units will be approximately equal (Because the capacitance of interrupting units of the breaker were comparitively very small compared to the Grading capacitors across it) and results in equal distribution of Bus voltage across two interrupting units of the circuit breaker.

So basically necessity of Grading capacitor arises as the Open the contacts of the Circuit breaker (Two interrupting units ) will be applied with an voltage equal to Bus voltage across it via a bus isolator .Here the bus isolator is the culprit which connecting the Sub station bus to the Circuit breaker end.

What if we are able to disconnect that Bus connection from  the  Circuit breaker Interrupter-1 (By opening the Bus isolator) immediately after the  tripping of the circuit breaker due to fault.?

If we do so, there will not be any voltage across the open contacts of the circuit breaker and the there is no need of provision of  Grading capacitors .

We can automate this opening of isolator Open coil after the tripping of Circuit breaker due to fault by using the Auxillary switch contacts of the circuit breaker.

Connection of three auxillary switch contacts (Of different phases ) in series will avoide Unnecessary opening of isolator due to single phase faults there by ensuring smooth operation of the system during auto reclose operations.

It is better to provide a delay of few milli seconds for the operation of isolator coil as immediate opening of the isolator may creates huge sparking at the terminals of the isolator.

It is highly suggestable to initiate the pulse for isolator after fully opening of circuit breaker contacts (Arc quenching will be completed in breaker itself).

By

B.S.N.Uday Kumar,

Engineer,NTPC Ltd.