Bus Differential Protection 87B

Description

Bus Differential Protection Overview

• The bus protection with differential relay device is suitable for busbars in conventional substations rated at 110 kV and below regardless of wiring configuration; it supports up to nine connected units including the bus tie, perfectly fitting application scenarios of conventional substations.
• Busbar protection devices feature multiple protection functions and adopt flexible 87B bus differential protection schemes configurable to match various busbar wiring requirements.

What Is an 87B Relay?

• An 87B relay is a busbar differential protection relay identified by ANSI device number 87B. It compares the currents entering and leaving a defined busbar protection zone and detects internal bus faults when the current balance is disturbed.

Comparison of Common ANSI Protection Device Numbers

How Does an 87B Relay Work?

• When the vector sum of the currents entering and leaving the protected busbar zone is not balanced, the 87B relay identifies the condition as an internal bus fault and sends trip commands to the associated circuit breakers.

Protection features

• Busbar Differential Protection
• Bus Tie Charging Protection
• Bus Tie Circuit Breaker Failure and Dead Zone Protection
• Circuit Breaker Failure Protection
• CT Broken Line Detection
• PT Fuse Failure Detection

Product Features – Differential Current Protection of Busbar

• The software is designed using modular principles, allowing for flexible configuration of protection functions to meet the needs of different users.
• This differential current protection device adopts the ratio-braking differential protection principle, integrating large-difference and small-difference functions for each busbar section.
• The large-difference function takes the entire double-busbar system as the protected unit to detect internal busbar faults, while the small-difference functions regard each individual busbar section as the protected unit to accurately locate the faulty busbar section.
• This differential electrical protection device is equipped with 3 Ethernet ports and supports communication with monitoring systems via the IEC 61850-MMS protocol.
• The differential current protection of busbars supports multiple time synchronization methods, including SNTP, Code B and GPS pulse.
• This current differential protection of busbar adopts an instantaneous differential current algorithm, ensuring fast protection response with a total operating time within 25 ms.
• Bus protection with differential relay and bus differential protection systems can automatically adapt to various busbar operating modes.
• During switching operations in double-busbar configurations, no protection deactivation is required. By detecting changes in isolating switch positions, the system automatically identifies the operating mode through software, adjusts the calculation logic for each busbar section’s differential protection, and switches feeder trip commands accordingly.

Busbar Differential Protection Scheme

Protection startup criteria

• This current differential protection relay is equipped with two trip functions to adapt to power systems of different capacities and various fault types: fast trip based on differential current variation and delayed trip based on differential current integration. The fast trip enables rapid and precise fault detection, while the delayed trip guarantees reliable operating sensitivity.

Subfunctions of Differential Protection for 87B Differential Protection Relay

• Section I Busbar Ratio Restrained Minor Differential Protection Function
• Section II Busbar Ratio Restrained Minor Differential Protection Function
• Ratio Restrained Major Differential Protection Function
• CT Broken Line Blocking and Alarm Function
• Composite Voltage Blocking Function

Small-difference differential protection for Section I busbars

Section II Busbar Ratio Differential Protection Function

Ratio-based differential protection function

High-Impedance vs Low-Impedance 87B Protection

87B Relay Applications

87B Relay Settings

Appearance of a Busbar Differential Protection Device

FAQ

Q：What is the bus differential protection configuration?

A:

The differential scheme for busbar protection adopts phase-separated ratio differential protection as the core configuration of busbar differential protection systems, which includes a large-difference circuit to distinguish in-zone and out-of-zone faults, and a small-difference circuit to identify and select the faulty busbar section.

The system is supplemented by comprehensive auxiliary functions, such as bus tie charging protection, bus tie failure protection, bus tie dead-zone protection, bus tie overcurrent protection, circuit breaker failure protection, composite voltage interlocking, and CT open-circuit interlocking.

For power systems with a rated voltage of 220 kV and above, two independent sets of protection equipment are configured strictly in accordance with the redundancy design principle.

Q: What is ANSI 87B Bus Differential Protection?

A：87B is the standard ANSI device number for Busbar Differential Protection. It forms a dedicated protection zone for substation switchgear busbars, comparing vector sum of secondary currents from all CTs connected to the bus. It trips all incoming & outgoing breakers instantly for internal bus faults, and stays stable during normal load and external out-of-zone faults.

Q: What is the fundamental working logic of 87B?

A：Formula: Differential Current Idiff​=∑Iin​−∑Iout​

1. Normal operation / external fault: Incoming and outgoing currents balance, Idiff​≈0, relay blocks tripping.
2. Bus internal short circuit: Current balance breaks, obvious differential current generates; relay activates trip command to isolate the entire faulty bus rapidly.

Q: Can 87B be used for ring bus or breaker-and-a-half bus configuration?

A：Yes. Need to correctly divide protection zones and configure CT polarity & breaker position auxiliary contacts. For breaker-and-a-half layout, each bus section sets independent 87B zone, and tie breaker CT signals are assigned to corresponding bus zones via logic switching.

Q: What is the effect of excessive wiring resistance on high-impedance 87B?

A：Overlong cable or poor terminal contact increases loop resistance, reduces differential voltage during internal faults, may cause 87B refusal to operate. Construction requires minimizing CT secondary cable length and tightening all terminals to control loop impedance within design range.

Q: Core setting items for high-impedance 87B relay?

A：Pickup voltage threshold (main setting value, calculated based on maximum external fault CT saturation residual voltage);Varistor clamping voltage: limits overvoltage to protect relay internal components during severe faults;Optional CT circuit abnormal alarm threshold for residual differential current monitoring.

Q: What is percentage restraint slope in numerical 87B?

A：Restraint current is taken from maximum through current of all branches. When external fault current rises and CT saturates, differential current increases proportionally; slope curve raises protection action threshold following restraint current, fundamentally preventing maloperation from CT saturation. Common slope setting: 0.3~0.5.

Q: Why residual differential current exists even under no-load normal status?

A：Minor unbalance from CT excitation current, manufacturing error and cable resistance difference. Allowable static differential current is usually below 5~20mA. If the value rises sharply (exceed 20% historical data), inspect CT insulation, terminal oxidation and water ingress in terminal boxes immediately.