BALAJI K DC Breaking Capacity of Circuit Breaker Explanation of DC fault current ➢ At the initiation of fault the current in an inductive circuit (considering for worst case) current cannot change from its instant to its steady state fault value, to compensate this DC component is introduced. Considering a purely inductive circuit in which the current lags voltage by an angle of 90 deg the magnitude of the DC component higher when the fault occurs at voltage zero. ➢ The magnitude of the DC current is almost equal to the peak value of the symmetrical AC current at the time of fault inception but of opposite polarity. ➢ The DC component is periodic in nature and decays exponentially with respect to the 𝑋 𝑋 time constant which is proportional to the ratio of the circuit. Higher the ratio higher 𝑅 𝑅 the time constant and longer time to decay which results in additional stress to the circuit breaker. ➢ In this case the circuit breaker has to be chosen wisely in order to withstand and break the DC component without destroying its arc quenching medium which can result in restrike. ➢ The standard time constant used for circuit breakers is 𝜏 = 45𝑚𝑠 according to IEC 62271-100. ➢ The time constant 45ms is adequate for most of the actual cases but special time constants related to rated voltage of the circuit breaker are used in some cases where the 𝑋 𝑋 circuit ratio is high. For example in case of GCB the breaker should withstand the 𝑅 𝑅 ratio of the Generator, the time constant used for GCB is 133ms. ➢ The time constant is given by 𝜏 = 𝐿 𝑅 which can be derived from 𝑋 𝑅 ➢ For a self tripping, breaker the percentage DC component shall correspond to a time interval equal to minimum opening time of the first opening pole Top of the circuit breaker. Time Tr is considered as 0. %𝐝𝐜 = 𝟏𝟎𝟎 ∗ 𝐞−(𝐓𝐨𝐩+𝐓𝐫)/𝛕 ➢ For a circuit breaker which is tripped by any form of auxiliary power the percentage DC component shall correspond to a time interval equal to minimum opening time of the first opening pole Top of the circuit breaker plus one half cycle of rated frequency (Tr). • Top - minimum opening time of the first pole of circuit breaker. BALAJI K • Tr - minimum possible relay delay time. 10ms as per IEC-62271-100, can vary with type of Relay used • τ - Standard time constant 45ms. Manual calculation: Considering the following input data, 1. Relay delay time (Tr) - 10ms 2. CB opening time (Top) - 30ms 3. τ - 45ms %𝐝𝐜 = 𝟏𝟎𝟎 ∗ 𝐞−(𝐓𝐨𝐩+𝐓𝐫)/𝛕 %dc = 41.11 Validating with ETAP: * Min. Delay (ETAP CB rating tab) - (Top + Tr) also known as contact parting time. BALAJI K Conclusion: ✓ The DC breaking capacity of the circuit breaker is dependent on the contact parting time of the circuit breaker since the time constant is fixed as 45ms for normal cases. ✓ The least possible minimum time is the circuit breaker opening time which is defined as the time interval between energization of the trip coil and contact interrupting time of circuit breaker. ✓ It is found that the DC breaking current increases with decrease in time delay, the Min.Delay can be increased so that it doesn’t cause an restrike. 𝑋 ✓ The ratio of the circuit breaker can be simply defined as the X/R ratio of the 𝑅 equipment situated next to it. References: IEC-62271-100 – High voltage switchgear and control gear – High voltage AC Circuit breakers.
