Field Discharging Methods

Requirement of field discharging
The excitation system must withstand faults or abnormal system operating conditions that are caused by transients induced into the generator field. These conditions occur when major system short circuits (faults), having little impedance between the fault and the generator, occur or when a synchronous machine pulls out-of step.

A generator pulls out-of-step, or loses synchronism, when insufficient excitation is available for the amount of Megawatts being delivered into the power system. Most voltage transients are less than 150 microseconds in duration and are clamped by a low power device(s) such as a metal oxide varistor(s). Voltage transients caused by major system faults, however, require larger power handling devices to dissipate the energy being induced into the field without producing potentially damaging field over voltages. Excessively high voltage transients can damage power semiconductors in the power rectifier bridge because of that field discharge is needed.

Types of Field discharge method
Three types of field discharging method.
1) DC Field Breaker 2) Free Wheel Diode 3) Crowbar circuit

DC Field Breaker
The continuity circuit with a dc field breaker is made of a DC
field contactor or breaker. This is a specific DC contactor
specifically designed for disconnecting inductive loads. The
contactor includes 2 normally open poles and 1 normally
closed pole. During the opening/closing transition of the
contactor, there is an overlapping zone, while all poles are
closed, in order to never stop the field current loop.

Freewheel Diode
This type of discharge method includes for half wave
rectification and, in this case, the continuity circuit is made
by using a free wheel diode placed in parallel with the
excitation circuit. As the excitation voltage is always
positive, the diode only begins conducting when the power
is turned off. The free wheel diode handles the current until it
is brought down to zero.

Crowbar Circuit
The continuity circuit is made of a crowbar. 2 SCRs are
mounted in anti-parallel, and controlled by a firing circuit.
This system has a dual function: it realizes the continuity
circuit, and it also protects the field during transient
overvoltages positive or negative (such as during a pole
slip). When a transient that is caused by a short circuit is
induced into the field, a large negative voltage and positive
current results. During this condition, the peak current from
the fault will combine with the rectifier output and overload
the rectifier bridge. A directional voltage sensitive crowbar circuit detects a specific overvoltage level that is negative and gates on SCR. SCR shunts the positive current from the field through the discharge resistor and simultaneously disables the 6 SCR rectifier bridge to prevent overload. The MOV clamps the voltage for the initial 200 microseconds until the crowbar takes over.

Crowbar circuit method is superior over DC field breaker and free wheel diode method because of following advantages 
  • Predictable overvoltage turn-on of crowbar SCRs.
  • 50 times faster then a DC Breaker.
  • No contacts in the field circuit.
  • No maintenance (compared to DC Breaker).
  • Low cost, reliable, short lead time.
  • Discharging Methods and options available.
  • Response time less than 200 microseconds.

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