IEEE 421.6:2017 pdf free download – IEEE Recommended Practicefor the Specification and Designof Field Discharge Equipment for Synchronous Machines
4. Field discharge equipment
The field discharge equipment plays a vital role in protecting the synchronous machine and excitation system.In case of a severe electrical fault in the vicinity of the synchronous machine or excitation system, the functionof the field discharge equipment is to provide a quick machine rotorflux reduction in order to limit the damagescaused by the very high energy released during the fault. The field discharge equipment also has the functionto limit the overvoltages between the positive and negative terminals of the synchronous machine field coil.The field discharge equipment can physically disconnect the voltage source from the field in case of severeexcitation system faults and allow discharge via the field discharge resistor. During normal shutdown, multiplestrategies are common for field discharge including implementing two power Silicon Controlled Rectifiers(SCRs) connected in anti-parallel (crowbar) with a series discharge resistor across the field.At shutdown, thecrowbar SCRs are gated(triggered) on simultaneous to the field breaker trip command in order to dischargethe field energy through the discharge resistor. In case of static excitation, the SCR converter bridge may havethe triggering pulses blocked or forced to the full inversion initiated simultaneously with the field breaker tripcommand.
During normal shutdown of the synchronous machine,in some implementations the ac or dc breaker is notopened. In this case the excitation system needs a conducting path to allow the circulation of the field currentduring shutdown (e.g., freewheel diode or the two last conducting SCRs of the converter bridge).
Another method includes “inverter operation”or “full inversion,” where the 6-SCR bridge is forced to gatenegative to quickly remove the field energy based upon the available power potential transformer secondarysupply voltage.Any of these shutdown methods may be used in case of mechanical faults to achieve fasterremoval of electrical energy stored in the field of the synchronous machine.The generator decay time usingthis method is compromised if 100% of the power potential transformer supply voltage is not available. It is acommon practice to trip the ac or dc field breaker and enable current flow through the discharge resistor onlyin the case of electrical faults. By adopting this philosophy, the number of operations of the field dischargeequipment may be drastically reduced and its (their) lifetime is increased significantly.
The field suppression that trips the field breaker and discharges the field to the field suppression resistor isneeded only to limit the damage in case of electrical faults on the generator side.Further, the field suppressionneeds to be initiated in case of certain faults occurring within the excitation system up to generator slip rings.The basic functions of the equipment are the elimination of the excitation source that drives the fault currentand the fast discharge of the field encrgy.
lt is uncontested that fast field discharge has a significant impact on reducing the amount of damage caused byelectrical faults.Repair costs may be higher if the field discharge takes a longer time to discharge the electricalenergy. The ratio between the equipment costs and the effectiveness of the field discharge function needs beconsidered during the selection and design of the excitation system.Figure 1 shows sampie results of the statorvoltage decay time with and without a field discharge resistor. Notice the decay time to reach 30% of initialvoltage is 8.5 s without the field discharge resistor and only 3.7 s with the discharge resistor. However, therelationship between the discharge time and the amount of damage, as well as the question whether the repaircosts are really lowered with a fast field discharge, is difficult to demonstrate and quantify.