Customized and Retrofit Solution as per Available Space
Crowbar Circuit for Linear Field Discharge & Field Overvoltage
Critical to the operation of the crowbar power switch is reliable control of the trigger circuits turn the crowbar power switch ON. The crowbar power switch thyristors must be triggered such that sustained over voltage of the machine field is prevented, yet triggering must not occur under any condition where the six- SCR bridge is expected to continue operating and not shut down. It must be ensured that the current path to the power switch is the only one available to field current during excitation system shutdown.
The crowbar consists of two anti parallel thyristor TH1 and TH2 connected across the excitation field circuit of alternator. Each crowbar control triggers the associated SCR under the following conditions. Only one section is triggered at any given time, dependenting upon the field voltage polarity at that time. Other section triggers the SCR when SCR anode-to-cathode voltage (equal to field voltage prior to triggering) is positive and exceeds a factory-preset level (set by the trip level Adjust for that section). The forward section can be enabled to trigger at any SCR anode-to-cathode voltage level sufficient to fully energize the power supply for that section.
The forward section is enabled by closing the external contact connected to the Crowbar External Trigger input. This results in triggering at a much lower voltage than the trip level. This method of triggering is used during normal shutdown of the excitation system, providing as alternate, resistive path for field current to freewheel and dissipate the stored energy in the machine field.
The total response time from the detection of fault to firing of crowbar thyristor varies from 50 μs to 150 μs depending upon the instant at which gate driver circuit fires in clock synchronization of timer circuit IC U13.
Benefits of Crowbar Circuit:
• The Crowbar has fastest response than DC field breaker – “25 times faster than DC field breaker.”
• The field discharge through Crowbar is linear while in DC Field breaker it is nonlinear.
• The maintenance is not required in Crowbar while in DC Field breaker due mechanical operations regular maintenance required.
• Due to Wear & tear by mechanical operations the life of DC field breaker will reduce.
• The DC Field breaker is not manufactured in India and it is brought out item so spares and technical support is difficult to get timely while Crowbar circuit is Amtech’s proprietary item and manufactured in India so easy technical support can be available.
• Cost & size of system will reduce duo to Crowbar circuit.
