Management of medium voltage retrofitted circuit breakers in Nuclear Power Plant

The switch choice is generally based on the following criteria:
1) Deep knowledge of Overvoltage generation problem
2) Technology to avoid overvoltage
3) Test and Certification

Over Voltage generation problem
In the modern power plant a critical aspect is the transient of the switching. In the switching phase the shape of the voltage sinusoidal wave changes in base to the load inductive and capacitive applied to the medium voltage terminals. The over voltage implies a distortion of the fundamental wave with peaks and disturbs. These disturbs even present in the frequency dominion with specific frequency with are multiplicative of the fundamental. A specific frequency disturb can break capacitive load as capacitor bank, or create disturbs to the rest of the network. Typically, these high frequency disturbs flickers in illumination, or suddenly opening of other circuit breaker in the network after the sensor and relè revealed a false short circuit or even problems to electric motors.

After tripping of a breaker, the interrupter contacts separate, and an arc is drawn between the contacts conducting the current as long as it is maintained above the chopping current of the breaker. “The electric energy remains trapped in the inductance and capacitance of that part of the system disconnected by the breaker. This energy may - depending on the type and size of the system parameters - start to oscillate resulting in voltage build-up across the contacts”. (Dullni, Liljestrand, Kropf, & Piasecki, 2013)

Each time the recovery voltage us (t) exceeds the dielectric withstand of the gap, a breakdown takes place and an arc is re-established between the contacts. Since the dielectric withstand of the fully open gap is designed to withstand the rated lightning impulse voltage (BIL), the crest value of overvoltage generated during these repetitive – multiple – re-ignition processes can reach values up to 5 times the rated voltage of the breaker. (Dullni, Liljestrand, Kropf, & Piasecki, 2013) It’s relevant to make to notice five time the nominal current is higher than the insultaion level test with is tested at the double of the nominal voltage of insulation. For example :

° Nominal Volte level = 20 kV
° Insulation Nominal level class = 24 kV
° Frequency industrial test = 48 kV
° Crest value of overvoltage= 100 kV
° BIL = 125 kV

The crest value consequentially can reach such high value to be a source of danger as well a root cause of arc flash. It’s a fundamental parameter to be considered in reliability of production power plant.