HVDC Circuit Breaker:
In a.c circuit breakers, arc extinction is achieved at the natural current zero of the a.c. waveform used.But in d.c circuit breakers, natural zero of voltage and current is not available as both are continuously available.Thus for extinction of an arc, artificial current zero is required to be introduced.In this, we will discuss HVDC circuit breaker Working and construction.
Such an artificial current zero is possible by connecting LC circuit in parallel with the circuit breaker.The LC circuit is responsible to produce oscillatory arc currents having many artificial current zeros.At one of the artificial current zero arc gets extinguished.
Such an artificial current zero is possible by connecting LC circuit in parallel with the circuit breaker.The LC circuit is responsible to produce oscillatory arc currents having many artificial current zeros.At one of the artificial current zero arc gets extinguished.
HVDC Circuit Breaker Working Principle:
The below figure shows the principle of HVDC circuit breaker.The HVDC circuit breaker is nothing but a vacuum circuit breaker.The LC circuit along with a switch is connected in parallel with this circuit breaker.
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The capacitor C used is pre-charged capacitor with the polarities as shown in the above figure.When the HVDC circuit breaker starts opening, the switch in the LC circuit gets closed.Due to the charged capacitor, the discharging current starts flowing in opposite direction to that of load current carried by the circuit breaker.Due to this, arcing current starts oscillating producing many natural zeros.Thus artificial commutation results and arc extinction is achieved.
The large transient recovery voltage is the main constraint in HVDC circuit breaker and circuit breaker must able to withstand it.For successful operation of HVDC circuit breaker, the switch in LC circuit must be a high-speed switch with a very fast response.Such systems are complex and are costly as they require costly protection system and control systems.
Selection of Circuit Breaker:
The following parameters are required to be known for selecting proper rating circuit breaker at a given location on a power system:
i)The maximum fault current which is to be interrupted by the breaker
ii) The maximum current to be carried momentarily.
As it is discussed previously the fault current consists of both a.c and d.c components and its correct calculation is very complex.A simplified method is recommended by IEEE committee is given below:
1)To determine firstly the required interrupting capacity of circuit breaker the highest value of initial rms alternating current for any type and location of fault. It can be considered as three phase fault as it carries maximum fault current except in some cases.
This current can be obtained by using sub-transient reactance for generators and transient reactance for synchronous motors while induction motors are to be neglected.Following multiplying, factors can be applied to take into account the d.c components of currents and decrements of both a.c. and d.c components.
This current can be obtained by using sub-transient reactance for generators and transient reactance for synchronous motors while induction motors are to be neglected.Following multiplying, factors can be applied to take into account the d.c components of currents and decrements of both a.c. and d.c components.
8 cycle or slow breaker 1.0
5 cycle breaker 1.1
3 cycle breaker 1.2
2 cycle breaker 1.4
Before applying multiplying factor, all the given factors are increased by 0.1 for the breakers on the generator bus where 3 phase short circuit KVA exceed 500,000.
2)To determine the required rated momentary current with time 1 sec or less of a breaker.The calculation of the highest value of initial rms a.c current can be done as given in the step(1) except using sub-transient reactances of all the machines including induction motors.Multiply the value of current obtained by 1.6.Momentary rated r.m.s current of a circuit breaker = 1.5 x V/X″d
Conclusion:
Now today we have learnt about HVDC Circuit Breaker Working Principle & Construction.You can download this article as pdf, ppt.
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