Field Discharging Methods

Field discharging methods are techniques used to safely and efficiently discharge electrical or stored energy in various fields, like power systems or electronic devices. Here are some common methods:

1. Grounding: Connecting the device or system to the ground to safely dissipate electrical charge. This is often used in power systems and electrical equipment.

2. Bleeder Resistors: High-resistance resistors connected across capacitors or other energy storage devices to slowly discharge the stored energy over time.

3. Short-Circuiting: Directly connecting the terminals of a device to create a short circuit and release stored energy quickly. This method is used with caution due to potential risks.

4. Discharge Circuits: Special circuits designed to control and safely discharge stored energy. These circuits might include components like resistors, transistors, and diodes.

5. Capacitor Discharge Tools: Devices specifically designed to discharge capacitors safely, often used in electronics repair and maintenance.

6. Controlled Discharge: Using electronic controls or software to manage the rate and timing of energy discharge, often seen in sophisticated systems and machinery.

Different methods are suited to different applications, so the choice of method depends on the specific requirements and safety considerations of the situation.

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 contactorspecifically designed for disconnecting inductive loads. The contactor includes 2 normally open poles and 1 normallyclosed 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.

Summary:

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.