ANSI 87 Differential Protection: Types, Working Principle and Applications

IANSI 87 differential protection is a protection function that compares electrical quantities at the boundaries of a defined protection zone to detect internal faults. It is widely used for transformers, generators, transmission lines, motors and busbars because it can provide fast and selective protection for faults within the protected zone.

The ANSI device number 87 identifies differential protection, while additional letters are commonly used to describe the protected equipment, such as 87T for transformers, 87L for transmission lines, 87G for generators, 87B for busbars and 87M for motors.

This guide explains how differential relay protection works, covering the main ANSI 87 protection types, CT requirements, typical applications and the differences between 87T, 87L, 87G, 87B and 87M.

What Is ANSI 87 Differential Protection?

ANSI 87 differential protection is a protection method that compares current quantities at different points in a power system to detect faults within a defined zone. It is commonly used to protect transformers, generators, busbars, and transmission lines.

What Does ANSI 87 Mean?

The ANSI device number 87 refers to differential protection. It is part of the ANSI/IEC standard device numbering system for protection relays. The number 87 is followed by a letter to indicate the specific application:

• 87T – Transformer differential protection
• 87L – Line differential protection
• 87G – Generator differential protection
• 87B – Busbar differential protection
• 87M – Motor differential protection

How Does Differential Protection Work?

The relay measures current from CTs installed at the boundaries of the protected zone. Under normal conditions and external faults, the current entering the zone is equal to the current leaving the zone (within CT error limits), so the differential current is near zero. When a fault occurs inside the zone, the balance is disrupted and the differential current becomes significant, causing the relay to trip.

What Is the Protection Zone?

The protection zone of an 87 relay is defined by the locations of the CTs. The zone is the area between the CTs. Any fault occurring inside this zone is detected by the relay. Faults outside the zone are not detected, making the protection inherently selective.

Why Is Differential Protection Selective?

Selectivity is achieved because the relay only responds to currents that originate within its defined zone. Faults outside the zone are cleared by other protection devices, while the 87 relay remains stable. This zone-based principle provides fast, secure, and precise fault discrimination without unnecessary tripping of healthy equipment.

ANSI 87 Differential Protection Types

ANSI Code	Protection Type	Typical Application
87T	Transformer Differential	Power transformers
87L	Line Differential	Transmission/distribution lines
87G	Generator Differential	Generator stator protection
87B	Busbar Differential	Substation busbars
87M	Motor Differential	Large motors
87S	Selective / special differential applications	Scheme dependent

ANSI 87 Differential Protection Working Principle

Normal Load Condition

Under normal load, currents entering and leaving the zone are balanced. The differential current is near zero and the relay remains stable.

External Fault

During an external fault, fault current flows through the zone but the fault is outside. Incoming and outgoing currents remain balanced. The relay remains stable and does not trip.

Internal Fault

When a fault occurs inside the zone, the current balance is disrupted. The differential current increases and, when it exceeds the operating threshold, the relay trips.

Key Principle

The relay trips only when the differential current exceeds the threshold, providing fast, selective protection for faults within the defined zone.

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 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

Main Applications of ANSI 87 Differential Protection

87T — Transformer Differential Protection

87T differential protection is used to protect power transformers against internal winding faults. The protection zone is defined by the CTs installed on the HV and LV sides. The relay compares currents entering and leaving the transformer, applying ratio compensation to account for CT differences, and vector group compensation to correct phase displacement. Percentage bias ensures stability during external faults, while harmonic restraint prevents false tripping during transformer energization.

For a complete explanation, read our Transformer Differential Protection Working Principle guide.

87L — Line Differential Protection

87L differential protection is used for overhead lines and cables. It compares currents measured by CTs at both ends of the protected line, using a communication channel (typically fiber optic) to exchange current data between the line ends. The relay calculates the differential current and determines whether a fault is internal or external to the protected line.

For detailed application guidance, see our Line Differential Protection page.

87G — Generator Differential Protection

87G differential protection is used to detect phase-to-phase and phase-to-ground faults within the generator stator winding. CTs are installed at the generator neutral and terminal sides, defining the protection zone. The relay compares currents from both sides and trips when a differential current indicates an internal stator fault.

87B — Busbar Differential Protection

87B differential protection is used to protect busbars by comparing currents from all connected feeder CTs. The busbar defines the protection zone. During normal and external fault conditions, the sum of currents entering and leaving the bus is near zero. When a bus fault occurs, the current balance is disrupted and the relay trips to clear the fault quickly. Stability is ensured through CT saturation detection and security algorithms.

87M — Motor Differential Protection

87M differential protection is used to protect large or high-value motors against stator winding faults and internal phase faults. CTs are installed at both ends of the motor winding, defining the protection zone. The relay compares currents and trips when an internal fault is detected.

Differential protection is commonly considered for large or critical motors where the required sensitivity and selectivity justify a dedicated differential scheme.

ANSI 87 Differential Protection vs 50/51 Overcurrent Protection

Feature	ANSI 87	ANSI 50/51
Principle	Current comparison	Overcurrent
Main target	Internal faults	Overcurrent / backup faults
Selectivity	Protection-zone based	Coordination based
Speed	Generally fast	Instantaneous or time delayed
CT arrangement	Multiple boundary CTs	Usually one measurement point
Communication	Required for some schemes such as 87L	Usually not required
Typical role	Main protection	Main or backup depending on application

ANSI 87 differential protection compares currents at multiple points to detect faults within a defined zone, providing fast and selective protection. ANSI 50/51 overcurrent protection responds to excessive current magnitude and can be applied as instantaneous (50) or time-delayed (51) protection. It does not require communication and is generally simpler to implement.

The selection between 87 and 50/51 depends on the application, system configuration, and protection requirements. 50/51 may serve as primary or backup protection depending on the application.

How ANSI 87 Distinguishes Internal and External Faults

Condition	Differential Current	Expected Action
Normal load	Low	No trip
External fault	Ideally low, but CT errors may increase it	Restraint / stability
Internal fault	High	Trip
CT saturation during external fault	May increase apparent differential current	Bias / stabilization

Testing ANSI 87 Differential Protection

Testing of ANSI 87 differential protection schemes should verify correct configuration, CT performance, relay logic, and overall scheme operation. The following test items are typically included:

• Secondary injection – Verifies relay pickup, slope characteristic, and operating time against the relay settings
• CT polarity – Confirms correct phase and polarity alignment of all CT inputs
• Differential characteristic – Tests the bias slope and differential pickup across the characteristic curve
• Trip logic – Confirms trip output, alarm signals, and blocking logic operate as configured
• Binary I/O – Verifies correct mapping and response of binary inputs and outputs
• Communication (where applicable) – Checks protocol operation and data exchange with SCADA or substation automation systems

For 87T transformer differential protection, see our Transformer Differential Protection Testing guide. For 87L line differential protection, refer to the dedicated Line Differential Testing resources.

ANSI 87 Differential Protection – Q&A

Q1. What does ANSI 87 mean in relay protection?

ANSI 87 is the standard device number for differential protection. It refers to protection schemes that compare currents entering and leaving a defined zone to detect faults within that zone.

Q2. What is the working principle of ANSI 87 differential protection?

The relay compares currents at the boundaries of a protection zone. Under normal and external fault conditions, the currents balance and differential current is near zero. When an internal fault occurs, the balance is disrupted and the relay trips.

Q3. What are the main types of 87 differential protection?

The main types include 87T (transformer), 87L (line), 87G (generator), 87B (busbar), and 87M (motor). The letter following 87 indicates the application.

Q4. What is the difference between 87T, 87L, 87G, 87B and 87M?

The difference lies in the protected equipment:

• 87T – Transformer
• 87L – Transmission line or cable
• 87G – Generator stator
• 87B – Busbar
• 87M – Motor

Each applies the same differential principle to a different protection zone.

Q5. Why are CTs important in differential protection?

CTs provide the current measurements that the relay uses for comparison. CT ratio accuracy, polarity, and saturation performance directly affect the relay’s ability to detect faults and remain stable during external faults.

Q6. What is percentage-restrained differential protection?

Percentage-restrained differential protection uses a bias characteristic that raises the operating threshold as through-current increases. This ensures stability during external faults while maintaining sensitivity for internal faults. It may be implemented as single-slope or dual-slope.

Q7. What is the difference between ANSI 87 and 50/51 protection?

ANSI 87 uses current comparison across multiple CTs to detect faults within a defined zone. ANSI 50/51 overcurrent protection responds to current magnitude and does not require comparison. 87 provides zone-based selectivity; 50/51 provides coordination-based protection and may serve as primary or backup protection depending on the application.

Q8. Why can CT saturation cause differential protection instability?

CT saturation distorts secondary current waveforms, introducing measurement errors that can produce apparent differential current during external faults. Percentage bias and saturation detection algorithms help maintain stability under saturated conditions.

Q9. How is ANSI 87 differential protection tested?

Testing typically includes secondary injection tests for pickup and slope, CT polarity verification, trip logic checks, binary I/O verification, and communication testing where applicable. For complete procedures, refer to dedicated testing guides for each application.

Q10. Can numerical relays integrate multiple 87 protection functions?

Yes. Many numerical relays support multiple differential protection functions in a single device, allowing protection of transformers, lines, generators, busbars, or motors with appropriate configuration and CT inputs.