How does a loss of excitation relay work?

How does a loss of excitation relay work?

To prevent the rotor from damage during underexcited operation, loss-of-excitation relays are often incorporated in the generator protection package. When the VAR flow into the machine is excessive, impedance type relays are often used to trip the device with a brief time delay. These devices use an electromagnetic mechanism to break the power circuit to the field coil when excess voltage is present. As long as the excess voltage remains above a certain level, the relay will remain closed and allow current to flow through it. Once the voltage drops below this level, the relay opens the circuit and prevents further current from flowing. These types of relays have no memory; that is, they cannot be reset manually. If left open, they will continue to interrupt power until either a new fault occurs or the power source is restored to its normal state.

Impedance type relays use a biologic element instead of an electromagnet to open and close the power circuit. When excess voltage is detected, the relay closes the circuit and allows current to flow through it. The greater the impedance between the power source and the load, the longer it will take for the voltage at the source to drop low enough to trigger the relay.

When to use the reverse power relay and preferential trips?

It is utilized when one utility or generator works in parallel with another. The relay monitors the generator's power supply and, if the output falls below a predetermined value, it instantly triggers the trip and disconnects the generator. This prevents excessive current draw from the main line which could cause overheating and damage to the generator.

A preferential-trip circuit limits the amount of current that can flow through any one conductor to prevent damage to the system. If you have two conductors to a circuit, there can be no more than one ground wire. A third conductor must be used for a hot wire. If you don't follow this rule, you will get a shock if you happen to be touching both ends of the circuit at the same time.

The preferred-trip circuit controls the flow of current through the conductors by using two resistors. The resistance of both parts of the circuit are equalized so that neither segment of the circuit is favored over the other. When the current flowing through one resistor rises to the point where its resistance becomes greater than that of the other, the circuit terminates function of that segment. The first resistor to exceed its maximum rating gets all the current; the second resistor drops out of the picture.

The reverse-power relay operates on the same principle as the conventional one but uses silicon diodes instead of electromechanical devices as its switching elements.

How are protective relays used in transmission lines?

The following are some of the protective relay types used in transmission lines: Protective Relays: Protective relays function to detect the fault and also to initiate the appropriate control signal, such as the tripping signal. The three main types of protective relays are magnetic, electric, and semiconductor.

A magnetic protective relay uses a magnetically sensitive element called a reed switch to open the circuit when the line current exceeds a certain value. The magnetic switch closes again when the current drops below this value, thus completing one cycle of operation. Magnetic switches are available in several different configurations for various applications. They can be found in service today on almost all transmission systems in the United States that were built before 1970.

An electric protective relay uses a electromagnet to open the circuit when the line current exceeds a certain value. The electromagnet is then removed, allowing the circuit to close again when the current falls below this value. Electric switches are more accurate than magnetic switches and can often detect smaller currents than magnets will pull off of the line. This makes them ideal for use with heavy industrial load conditions or long distance transmissions.

A semiconductor protective relay incorporates silicon-controlled rectifiers (SCRs) into its design. These devices can turn on and off the power to the line automatically when needed, which eliminates many problems that can occur with mechanical switches.

What is the purpose of a distance relay?

This type of relay is utilized for transmission and distribution line backup protection, fault protection, phase protection, and main protection. The schematic of the distance relay is shown below. The distance relay is designed as a basic overcurrent relay. It has two terminals: one is used for connecting to the power source and the other is used as an output. When current passes through the primary coil, the magnetic core will close the circuit and connect the output to the source, thereby opening the circuit from the output to ground.

Distance relays are commonly used in panelboard applications. They provide a safe and convenient way to open a circuit when there is a problem with the wiring or equipment on the block-out side of the breaker or fuse that controls it. The operator simply connects another terminal of the relay to a ground rod or bus bar instead of using a meter or special tool. If current continues to flow through the original conductor, the relay's secondary coil will remain closed even after the primary coil is removed from the winding, which connects the output to the source.

These relays use magnetic cores and springs to determine how far they can be activated by current flowing through the primary coil. The maximum allowable separation between the primary and secondary coils is based on the strength of the magnetic field generated by each coil. If the gap is too large, then the relay may not operate properly.

About Article Author

James Butler

James Butler loves the smell of oil in the air. He's been working in the auto industry for over 30 years and knows all about cars and their parts. James has an eye for detail and can tell what's wrong with a car just by looking at it.

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