Zero Crossing Detection - GE P642 Technical Manual

Chapter 10 - CB Fail Protection P64x
3.2

ZERO CROSSING DETECTION

When there is a fault and the circuit breaker interrupts the CT primary current, the flux in the CT core decays to a
residual level. This decaying flux introduces a decaying DC current in the CT secondary circuit known as
subsidence current. The closer the CT is to its saturation point, the higher the subsidence current.
The time constant of this subsidence current depends on the CT secondary circuit time constant and it is generally
long. If the protection clears the fault, the CB Fail function should reset fast to avoid maloperation due to the
subsidence current. To compensate for this the device includes a zero-crossing detection algorithm, which ensures
that the CB Fail re-trip and back-trip signals are not asserted while subsidence current is flowing. If all the samples
within half a cycle are greater than or smaller than 0 A (10 mS for a 50 Hz system), then zero crossing detection is
asserted, thereby blocking the operation of the CB Fail function. The zero-crossing detection algorithm is used
after the circuit breaker in the primary system has opened ensuring that the only current flowing in the AC
secondary circuit is the subsidence current.
This zero-crossing detection algorithm considers the current inputs T1, T2, T3, T4 and T5 on a per phase basis. If
IN< Input is set as measured, the zero crossing detection algorithm considers the current inputs TN1, TN2 and
TN3. If IN< Input is set as derived, the zero crossing detection algorithm considers the current inputs T1, T2, T3,
T4 and T5. If more than 12 consecutive samples are greater than 0A or more than 12 consecutive samples are
smaller than 0A, then a zero crossing detection condition is asserted, which blocks the operation of the circuit
breaker failure function. The zero crossing detection is asserted after the breaker in the primary system has
opened to ensure that the current flowing in the AC secondary circuit is just the subsidence current.
230 P64x-TM-EN-1.3