Differential Relay Protection 87: Principles and Applications in Relay Protection

In power system protection of relay, Differential Relay Protection represented by differential current protection is a familiar concept to those in the power industry. When a fault occurs within power equipment, differential protection can quickly and accurately isolate it, thereby ensuring the safe and stable operation of the power grid.

So, exactly what kind of protection mechanism is this? And how does it detect faults?

What is differential relay protection?

Many people who are new to relay protection often wonder what a differential protection relay is and what differential protection is. As the name suggests, it is a type of protection that detects faults based on the “difference” in current between the two ends of the protected equipment.

Differential protection relay working principle

The principle is that during normal operation or in the event of an external fault, the current flowing into the device should equal the current flowing out of it; however, when a fault occurs within the device, this balance is disrupted, resulting in a noticeable difference in current.

Current differential protection uses this current difference as the basis for initiating the protective action.

Differential protection diagram

Taking transformer differential protection as an example

What are the technical requirements for differential protection CTs?

Accuracy Class and Type

For equipment of 110kV and below, differential protection adopts 5P20 and 5P30 protective CTs with a composite error less than 5% under 20 or 30 times rated current. For high-voltage equipment of 220kV and above, generators and main transformers, TPY and TPE transient CTs are required to suppress DC saturation with a remanence coefficient below 10%. 0.2S and 0.5S measuring CTs are prohibited to prevent protection maloperation.

Ratio and Polarity Requirements

For the 87 protection relay, CT polarity for differential protection requires uniform ratio and polarity for all CTs on each side of the differential protection system. Mismatched CT ratios can be calibrated through software adjustment or intermediate converters to eliminate unbalanced current. The secondary rated current is normally set to 5A, whereas 1A is applied in long cable scenarios to effectively reduce circuit voltage drop.

Saturation Characteristics and Limit Requirements

Protective CTs shall have an accuracy limit factor (ALF) of no less than 20 and comply with the 10% error curve criterion to avoid maloperation caused by CT saturation during external faults. TP-class CTs adopt air-gap iron cores for excellent transient performance, while 5P-class CTs must be of the same model and consistent characteristics.

Secondary Load and Wiring Specifications

The differential protection relay 87 requires the CT secondary circuit to be dedicated solely to differential protection with no external loads connected, ensuring symmetrical and light burden conditions. The cable length discrepancy between the two sides must not exceed 50 meters, and single-point grounding is implemented for the secondary circuit to prevent electromagnetic interference and phase shift errors.

Matching and Consistency Requirements

CTs in the same differential loop shall be identical in model, batch and accuracy class with matched volt-ampere characteristics and a matching error within 0.5%. The secondary windings are independently dedicated to differential protection without sharing metering or measuring functions.

Ansi code for differential protection

87

How Does a Differential Relay Operate?

• CTs Measure Current
• Relay Compares Currents
• Differential Current Is Calculated
• Relay Checks the Operating
• Threshold Relay Trips for an Internal Fault

The differential current is defined by the formula:Idiff = |I₁ − I₂|

Under normal load conditions, I₁ is approximately equal to I₂, resulting in a small differential current and no relay tripping. During external faults, the currents from the two CTs stay largely balanced, so the relay remains stable without operation. When an internal fault occurs, I₁ and I₂ become unbalanced. The differential current rises above the operating threshold, triggering relay operation and opening the circuit breaker.

Differential Relay Operation Under Different Fault Conditions

Operating Condition	Current Relationship	Differential Current	Relay Operation
Normal load	Balanced	Very low	No trip
External fault	Approximately balanced	Low/controlled	No trip
Internal fault	Unbalanced	High	Trip
CT saturation	May become unbalanced	Can increase	Restraint/stability required
CT wiring error	Incorrect current comparison	Abnormal	Possible misoperation

Operating Current and Restraining Current in Differential Relays

Operating current (Iₒₚ) :It determines whether the differential current is sufficient to trigger relay operation.

Restraining current (Iᵣₑₛₜ) It helps the relay remain stable during heavy external faults, CT measurement errors and CT saturation.

Simplified formulas:

Iop = |I1 − I2|

Irest ≈ (|I1| + |I2|) / 2

A differential relay operates when the operating current exceeds the relay’s characteristic or pickup requirement relative to the restraint current.

Differential Relay Operating Characteristic

Differential protection relies on key parameters including pickup current and bias/restraint slope to define the operating region and restraint region on its characteristic curve. With these settings, the relay can accurately distinguish between internal faults, which trigger tripping, and external faults, where stable restraint is maintained to prevent misoperation.

How CT Saturation Affects Differential Relay Operation

During an external fault, a high through-current leads to CT saturation. This distorts secondary currents and creates secondary current mismatch, generating apparent differential current. Without restraint characteristics or properly selected CTs, this phenomenon may result in unwanted differential relay operation, also known as differential relay misoperation or false operation. This explains why differential relay operation during external faults is closely linked to CT saturation in differential protection systems.

Differential Relay Operation Diagram

Differential Relay Operation for Different Applications

Differential Relay	Protected Equipment	Operation Principle
87T	Transformer	Compare transformer-side currents
87G	Generator	Compare terminal and neutral currents
87L	Transmission line	Compare currents at line terminals
87B	Busbar	Compare currents entering/leaving bus zone

Differential Relay Operation vs Overcurrent Relay Operation

Feature	Differential Relay	Overcurrent Relay
Main principle	Current comparison	Current magnitude
Protection zone	Defined by CT locations	Usually broader/system coordinated
Main fault type	Internal faults	Overcurrent/short circuit
Selectivity	Very high	Depends on coordination
Typical ANSI	87	50/51

CT Differential Protection and Its Applications in Various Power Equipment

Differential protection of power transformer

Differential protection for transformers — critical equipment in power systems — acts as their main primary protection.

Transformer Differential Protection ansi

87T

Transformer differential protection challenges and solutions

Issues Encountered with Transformer Differential Protection

Transformer differential protection relay for transformer differential protection faces two major challenges: Differences in voltage levels and connection configurations (e.g., Y/Δ-11) on both sides of the transformer result in discrepancies in the magnitude and phase of the secondary currents of the current transformers on both sides.

Furthermore, the inrush current during transformer energization can reach 6 to 8 times the rated current and contains a significant amount of non-sinusoidal components, making it highly prone to causing protection misoperation.

87 Differential Protection Relay scheme

The protection device compensates for current amplitude and phase deviations through software calibration; it employs second-harmonic braking and waveform interruption angle detection to counteract excitation inrush currents. The transformer differential protection can sensitively detect internal faults while reliably mitigating the effects of external faults and inrush currents.

Typical slope settings for transformer differential protection 87t

Core Concept

The slope (percentage restraint slope) is designed to prevent misoperation caused by CT saturation during external through faults. Modern digital differential relays adopt two-stage slope curves as the mainstream standard worldwide for power substations.

Standard Recommended Slope Values (IEC / ANSI Global Common Settings)

1. First Segment Slope (Low current zone)

• Slope 1: 15% ~ 25% Most universal default value: 20% Applicable range: Operating current below the breakpoint current

2. Second Segment Slope (High current zone, heavy external faults with severe CT saturation)

• Slope 2: 50% ~ 75% Most widely adopted engineering value: 60%

3. Breakpoint Current (Knee Point)

Fixed threshold separating two slopes:

Usually set to 1.0 × In (transformer rated current)

Complete Typical Default Parameter Group (Used by Siemens, ABB, GE domestic & overseas projects)

1. Slope 1: 20%
2. Slope 2: 60%
3. Breakpoint Current: 1.0 Iₙ
4. Minimum differential pickup current: 0.2~0.3 Iₙ
5. 2nd harmonic restraint ratio: 15%~20% (to block inrush current)

Application Scenario Adjustments

1. Small distribution transformers (<10MVA): Slope 1=15%，Slope 2=50% (gentler slope for high sensitivity)
2. Large power station main transformers (>100MVA): Slope 1=25%，Slope 2=75% (steeper slope, stronger anti-saturation capability)

Differential protection of transmission line

Transmission line differential protection typically uses optical fiber as the communication channel to exchange real-time CT current data at both ends of the line.

Current differential protection for transmission lines, also referred to as line differential protection, treats the entire line as the protected object. During normal operation, the vector sum of the currents at both ends is zero; in the event of an internal line fault, a differential current is generated, causing the protection to operate instantaneously.

This protection offers absolute selectivity and is unaffected by system oscillations or unbalanced operation. It is suitable for short lines, series-compensated lines and other complex scenarios, and serves as the main protection for ultra-high-voltage and extra-high-voltage transmission lines.

Electric motor protection relay

What is motor protection relay?

Motor protection is a relay protection system designed to monitor in real time and quickly isolate various faults that may occur during motor operation—such as overloads, short circuits, phase loss, ground faults, stalled rotors, and unbalanced conditions—in order to prevent motor burnout and damage.

For motor protection which relay is used?

Motors with a rated power exceeding 2000 kW require differential protection, while those rated at 2000 kW or less generally use standard motor protection. In special cases, a flexible approach should be taken.

Motor differential protection calculation

Motor protection setting calculation is the process of precisely setting the operating currents and times for each protection function in accordance with regulations, based on the motor’s rated parameters and operating conditions, to ensure that the protection neither trips prematurely nor fails to trip when necessary. Refer to the motor protection technical manual.

Differential protection of generator

Generator protection relay diagram

What is generator protection, and what role does generator differential protection play in it?

As a key generator protective relay, generator differential protection primarily protects against phase-to-phase short circuits in the stator windings and lead wires. It employs a series differential connection with current transformers (CTs) installed on both sides.

As an important part of protective relaying for power generation systems, it is unaffected by external short circuits, load fluctuations, or fluctuations in excitation current. In the event of an internal fault, it can quickly trip to de-energize the field and prevent the fault from propagating; inter-turn short circuits must be detected by cross-differential protection.

Generator differential protection relay setting

The calculation of generator differential protection settings involves using the generator’s rated current and CT parameters to set the minimum operating current (approximately 0.2–0.3 Ie), the braking inflection point current (approximately 0.8–1.2 Ie), the ratio-to-inrush current slope (approximately 0.3–0.5), and the instantaneous differential current (approximately 4–6 Ie), so that the protection operates sensitively for in-zone faults and reliably inrushes for out-of-zone faults.

Bus differential protection relay

Bus differential protection scheme

Busbar differential protection（differential protection of busbar）：This comprehensive technical solution involves measuring the current in all connected components on the busbar and using Kirchhoff’s current law to compare the difference between incoming and outgoing currents. When the differential current exceeds the set value, the system instantly clears the busbar fault.

Differential busbar protection instantly trips busbar faults once the differential current exceeds the setting value. It adopts ratio braking and CT saturation detection to avoid misoperation from external faults and CT saturation, realizing fast and selective busbar protection.

Bus differential protection schematic

Conclusion on Differential Relay Protection

Regardless of the type of differential protection, they all share one common feature: they calculate the difference between the incoming and outgoing currents detected by the equipment. When the calculated differential current exceeds the protection setting and the predetermined protection operating time is reached, the differential protection relay operates.

FAQ

Q1: What is ANSI 87 Differential Relay Protection?

A1: ANSI 87 refers to differential protection, a main protection that compares the incoming and outgoing currents of protected equipment. When the vector difference between input and output current exceeds the preset threshold, the relay judges an internal fault and immediately sends a trip signal to cut off the power supply. It is designated as 87 in the ANSI standard relay protection coding system.

Q2: What are the common sub-types of 87 differential protection and their ANSI codes?

A2:

• 87T: Transformer Differential Protection
• 87G: Generator Differential Protection
• 87L: Transmission Line Differential Protection
• 87B: Busbar Differential Protection
• 87M: Motor Differential Protection

Q3: What core working principle does differential relay protection rely on?

A3: Under normal operation or external short-circuit faults of the protected equipment, the inflow current is equal to the outflow current, and the differential current is nearly zero; once an internal short circuit, winding breakdown or inter-turn fault occurs inside the equipment, the balance of inflow and outflow current is broken, and the differential current rises sharply, triggering the 87 relay to act and trip.

Q4: What equipment is 87 differential protection mainly applied to?

A4: It is used as primary main protection for key power assets: power transformers, generators, large industrial motors, high-voltage busbars, and important long-distance transmission lines. It acts faster than overcurrent backup protection and can quickly isolate internal faults.

Q5: Why is CT (Current Transformer) wiring the most frequent fault point for differential protection 87?

A5: Differential protection depends entirely on CT sampling signals. Wrong polarity, reversed wiring, loose terminals, CT open circuit or inconsistent CT ratio will create false differential current. This causes two typical faults: nuisance tripping during external faults or failure to trip when internal faults happen.

Q6: What is the difference between 87 differential protection and 50/51 overcurrent protection?

A6:

1. 87 Differential Protection: Main protection, targets internal equipment faults, extremely fast tripping speed, only responds to faults inside the protected zone;
2. 50/51 Overcurrent Protection: Backup protection, acts for overload and external short-circuit faults, with configurable time delay, used as backup protection when main protection fails.

Q7: How to test and verify the function of 87 differential relay on site?

A7: Adopt a relay protection tester for secondary injection test: inject analog current into the two groups of CT secondary loops respectively, simulate normal load current and internal fault differential current, check pickup value, operating time, restraint characteristic curve and trip output function of the differential protection.

Q8: What is the restraint current in transformer 87T differential protection?

A8: Restraint current is introduced to avoid false operation caused by magnetizing inrush current when the transformer is energized or external short-circuit saturation of CT. The restraint loop suppresses the differential action signal during inrush and external faults, ensuring the relay only trips for real internal faults.

Q9: What will happen if the 87 differential protection fails to operate during an internal transformer fault?

A9: The fault current cannot be cut off instantly, which will severely burn transformer windings, lead to insulation collapse, oil leakage and even explosion of the transformer body. Only the delayed backup overcurrent protection can act later, which will cause irreversible damage to the main equipment.

Q10: Can 87 busbar differential protection replace feeder protection on substation busbars?

A10: No. 87B busbar differential protection is dedicated to faults on the busbar itself. Each outgoing feeder still needs independent 50/51 overcurrent protection. The bus differential trips the total incoming switch when the bus fails, while feeder protection isolates faulty single outgoing lines.

Q11: What common settings need to be configured for differential relay protection?

A11: Main parameters include CT transformation ratio matching, differential current pickup threshold, restraint coefficient, second harmonic blocking ratio (for transformer inrush prevention), trip delay, and interlocking logic with circuit breakers.

Q12: Is numerical differential relay different from traditional electromagnetic differential relay?

A12: Traditional electromagnetic differential relays rely on the magnetic superposition of coils to judge differential current; modern numerical 87 differential relays calculate current vectors through sampling, add harmonic blocking, CT disconnection alarm, fault waveform recording and remote communication functions, with higher anti-interference and reliability.

Q13: Why does transformer differential protection adopt second harmonic braking?

A13: A large amount of second harmonic component exists in transformer magnetizing inrush current during switching on. The second harmonic braking link locks the differential protection temporarily to prevent misoperation when no internal fault exists.

Q14: What alarm signals will a faulty 87 differential protection relay generate?

A14: Typical alarms include CT circuit break alarm, differential current over-limit alarm, protection self-check fault, communication interruption, restraint loop abnormal alarm, etc.

Q15: How often is periodic maintenance and testing required for ANSI 87 differential protection devices?

A15: For medium and high voltage main equipment protection, the on-site secondary injection test and setting verification are carried out every 1~5 years in accordance with power grid specifications; CT wiring and loop insulation inspection shall be done during each major equipment overhaul.

Q16:where can i buy protection relays that offer overcurrent and differential protection with iec 61850 communication?

A16:Available in China. Kindly furnish other key technical specs apart from listed requirements; contact us anytime as required.

Q17:Why Does a Differential Relay Operate?

For an external fault outside the protected zone, the current entering and leaving the protected equipment should remain substantially balanced. The differential current therefore remains below the relay’s operating characteristic, so the relay remains stable.