The differential relay is the device that protect the important electrical equipments like transformers and generators from the internal faults and short circuits. It works by comparing the current going into the equipment and the current coming out from the equipments. If there is a mismatch indicating the internal fault the differential relay quickly trips or shuts off the equipment to prevent the damage.

However, it does not respond to faults occurring outside the protected equipment. This allows the differential relay to provide quick protection against internal issues while avoiding unnecessary trips due to external problems

What is Differential Relay?

The differential relay is the device that operates ( trips ) when the difference between the two or more electrical quantities like the current exceeds the set limit. Most other relays work when the single quantity goes above the fixed value but differential relays look at the difference between the multiple quantities.

The purpose of the differential relay is to provide fast, sensitive and selective protection. It quickly detects the faults within the equipment it is protecting.

However differential relays may not detect small faults within windings of machines or transformers. This is because the change in current caused by these faults can be too small for the relay to sense and operate on. The relay needs a certain minimum difference in current to trip.

Differential Relay Working Principle

A differential relay works by comparing two electrical quantities, usually currents. It checks if the difference between these two currents is more than a set limit. For example lets look at the power line. The differential relay compares the current going in the line and the current coming out from the line. In Normal conditions these two currents are equal. However if there is a fault or problem on the line, more current will flow into the line than what comes out. This creates a difference between the two currents.

The differential relay detects this difference. If the difference is bigger than the set limit, it knows there is a fault. So it operates and trips (shuts off) the circuit to protect the line. In normal conditions with no faults, the currents going in and out are the same. So the differential relay does not operate. The differential relay circuit has the two current transformers (CTs) connected on the either side of the equipment being protected like the power transformer. One CT measures the current going into the transformer and the other measures the current that coming out.

The relay compares these two currents. If they are not balanced or equal, it means there is a fault inside the transformer. So the differential relay trips to isolate the transformer and prevent damage. Different types of differential relays like current, voltage, or biased relays work in a similar way – by comparing two electrical quantities and operating when the difference exceeds the set limit.

Types of Differential Relay

Current Differential Relay

This relay compares the currents going into and out of the protected equipment, like a transformer. It looks for any difference in these currents which indicates an internal fault. Normally, the currents going in and out are equal, so the relay does not operate. However, during an internal fault, these currents become unequal. The relay detects this current difference and trips the circuit breaker to isolate the faulty equipment.

Voltage Balance Differential Relay

This type compares the voltages on either side of the protected equipment, like a generator winding. It checks if these voltages are different in value or out of phase with each other. The relay has the two current transformers (CTs) connected to the winding. In normal conditions the voltages measured by the both CTs are the same so no current flows through the relays operating coil. But during the fault the voltages becomes different. This voltage difference creates the current in the relay’s operating coil, causing it to trip.

Percentage/Biased Differential Relay

This relay is designed to prevent operation during external faults or transformer inrush currents, which can cause current differences. It has a restraint coil that produces a restraining force proportional to the current magnitude. This prevents tripping during allowable current differences. Only during genuine internal faults does the operating force exceed the restraining force, allowing the relay to trip and isolate the faulty zone.

In summary, current differential relays compare currents, voltage balance relays compare voltages, and percentage relays use restraint coils to handle allowable current differences – but they all operate by detecting significant differences indicating internal faults.

Advantages and Disadvantages of Differential Relay

Advantages

1. Can handle digital signals using microprocessors for accurate operation.
2. Provide very important protection for power systems.
3. Highly accurate measurements due to precise analog-to-digital conversion.
4. Easily adaptable to different alarm and substation systems.
5. Respond quickly to faults, cannot distinguish between minor faults and heavy loads.
6. Help prevent malfunctions in the network.

Disadvantages

1. Accuracy of current differential relays can be affected by cable capacitance during heavy current flow.
2. Current transformers may have different characteristics or ratings due to cable impedances and construction errors, causing incorrect operation.
3. Voltage balance type relays are complex to construct to achieve perfect CT balance.
4. Protection is effectively provided only for shorter length lines.

Applications of differential relay

1. They are commonly used to protect important equipment like generators and transformers from internal faults or localized faults within that equipment.
2. Specific types like Merz-Price differential relays are used to protect the stator windings of generators from internal faults.
3. Differential relays provide winding protection for transformers as well.
4. They are suitable for protecting compact equipment as well as various power system components like busbars, generators, reactors, transmission lines, transformers, and feeders from internal faults.

The differential relays are mostly applied to the safeguard critical electrical equipment mainly in the generators and the transformers by quickly detecting and isolating any internal faults or the faults that are located within the protected zone.

Conclusion

A differential relay is the protective device that quickly detects and isolates the internal faults inside the important electrical equipments like the transformers and generators. It works by comparing the electrical quantities like the currents going into and out of the equipment. If these quantities are significantly different, indicating an internal fault, the differential relay trips to shut off the faulty equipment and prevent damage. While simple in operation, differential relays provide critical protection against localized faults for valuable power system components.

Frequently Asked Questions on Differential Relay – FAQ’s

How does a differential relay know there is a fault inside the equipment it is protecting?

A differential relay compares the current going into the equipment and the current coming out. If these two currents are significantly different, it means there is a fault or problem inside the equipment causing this mismatch. The relay then trips the circuit to isolate the faulty equipment.

Can a differential relay detect faults outside the equipment it is protecting?

No, differential relays are designed to respond only to internal faults within the specific equipment they are installed to protect, like a transformer or generator. If the fault occurs outside this protected zone, the currents going in and out will still be balanced, and the differential relay will not operate.

What are the different types of differential relays?

The three main types are current differential relays (which compare currents), voltage balance differential relays (which compare voltages), and percentage or biased differential relays (which use a restraint coil to prevent operation during allowable current mismatches like inrush currents).

Why are differential relays important in power systems?

Differential relays provides the critical protection for expensive and vital equipment like the large generators and transformers. They can quickly isolate internal faults before they cause the extensive damage preventing costly repairs and replacements and ensuring the reliable power supply.