Rotating Diode Failure Relay Working Principle

Rotating Diode Failure Relay Working Principle?

The diode failure relay is used to protect the alternator’s automatic voltage regulator from severe damage against the rotating diodes failure. Alternator field coils is excited with the DC supply. Here, In brushless excitation system, the RRA assembly (Rotating rectifier assembly) is used to convert the exciter AC voltage to DC voltage. The output of the RRA is connected with generator field coils. In such a way the rotor receives DC supply from AVR. The RRA consist of diodes. The input of the exciter voltage is controlled by the AVR Automatic Voltage Regulator. Therefore, the output current of the AVR should be monitored properly. The diode failure relay is used to monitor the AVR output current.

Principle of Diode Failure Relay:

It works under principle of ohms’ law. When the current through the resistor increases, the voltage drop across the resistor increases simultaneously.

Note: For larger alternator, the resistor is connected in AVR output, For small alternator the resistor is connected in series with the exciter coil.

diode failure relay
Diode failure relay

Normally, the relay detects two types of fault, one is open diode and another one is short diodes.

Open diode fault:

Open diode fault means the one or more diode gets open circuit. In this case the field current increases a little but the machine can continue to run quite normally. On this type of failure, the generator is not at any immediate risk so operation during limited time is still possible. But, if two or more diode failure means the relay trip the alternator.

Diode failure relay open diode
Diode failure relay open diode

Short diode Fault:

In this case, the exciter field current takes high as to maintain the generator voltage at rated. This fault increases largely with the risk of severe damage to the Automatic Voltage Regulator and to the exciter; the relay has then to trip and to shut the machine down.

diode failure relay short diode
Diode failure relay short diode

Relay acted:

  • Diode failured relay opted in annunciator
  • AVR trip
  • 86 M generator master trip opted.

After tripping of your alternator on diode failure protection, you restart the machine, but if the fault persists then contact electrical maintenance staff to check the RRA.




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