Trip and control circuit schemes of signaling between relays and circuit breakers

Relay Power Supply

Modern microprocessor relay has a power supply that transforms the station voltage into a suitable processor and controls voltages for the relay’s internal electronics. The power supplies generally draw only a few volt-amperes of load from the supply. Therefore, they do not add significant load to the station power system.

Thoughtful consideration should be given to the source from which the power supply voltage is to be obtained. Most utilities use the station or plant battery. It is generally designed to supply a reliable source that is resistant to transients and is relatively immune to system disturbances.

Another choice for power supply voltage is to use the station service supply especially if the battery is not available. Due to the fact that these voltages can reflect the system voltages during fault conditions i.e. drop to zero or drop to some value below the power supply threshold, these sources should not be considered for relays performing protective functions.

However, relays that perform strictly control functions have been successfully applied using the station service or voltage transformers as a source i.e. capacitor bank control relays.

Power Supply Circuit Overcurrent Design

When designing the power supply connections from the DC system to a microprocessor relay, attention should be given to protecting the DC system from short circuits in the conductors to the relay or in the relay itself.

The trip circuit design in this regard to microprocessor relays does not appreciably differ from the electromechanical schemes.

Battery Load Creep

Microprocessor relays add load to the station battery due to the use of continuously energized digital inputs and their own energy requirements. This affects the battery design load curve. In substations or power plants where microprocessor relays are replacing electromechanical relays, it should be considered that as more new relays are added, the continuous load on the battery and charger will gradually increase or “creep”.

Battery System Grounding Considerations

Battery systems are generally grounded at a single point through resistors to establish a known reference voltage between positive, negative, and ground. Battery chargers may include this circuitry internally, or external resistors or lamps may be used as shown below in Figure 1.

Whether balanced resistors are used to establish equal voltage above and below ground potential, or if unbalanced resistors are used to offset the ground reference, monitoring the DC rail-to-ground voltage is a common way to detect extraneous battery grounds.

Resistance values are generally chosen to limit the current drain to several milliamps or less.

In Figure 1, capacitors C1 and C2 represent stray capacitance of the DC circuitry, surge capacitance, and power supplies. Under normal conditions, the voltage across each capacitance is one half battery voltage.

The first extraneous ground does not impair the battery system. However, it is important to detect and alarm on the first extraneous ground because the second ground could completely short the battery system.

A shift in these voltages indicates an extraneous battery system ground. A relatively simple technique uses two microprocessor based relay inputs that are rated for the battery system voltage, but will not assert at one half battery voltage or less. This scheme is shown in the partial DC schematic in Figure 2.

In this scheme, the normal DC voltage across each input, IN3 and IN4 is balanced at one half battery voltage. Neither input will be picked up. A ground on the positive DC bus rail will cause the voltage on input IN 4 to collapse to zero, and the voltage on input IN 3 will rise to full battery voltage, causing input IN 3 to assert. Conversely, a ground on the negative DC bus rail will cause input IN 4 to assert.

Logic in the relay can be used to create a DC ground alarm and indicate whether the ground is on the positive or negative DC bus rail. It is important to note that in the microprocessor relay, the current drawn by the relay inputs (Optoisolated inputs) when energized is very low.

Therefore, the input threshold voltage or possibly the setting has to be verified to make sure a DC ground fault in the protection scheme could not trigger the inputs.