ANSI Relay Protection Schemes and Distance Protection

ANSI 21

- Distance Relay, Voltage + Current - Used in transmission line protection

ANSI 24

- Overexcitation, Voltage or V/Hz - Protects against high voltage-to-frequency ratio

ANSI 25

- Synchronism Check, Voltage + Frequency - Ensures generators are synchronized before closing

ANSI 27

- Undervoltage, Voltage - Trips if voltage drops below a threshold

ANSI 32

- Power (Directional), Voltage + Current - Reverse power or directional power protection

ANSI 37

- Undercurrent, Current - Detects loss-of-load conditions

ANSI 46

- Negative Sequence Overcurrent, Current (sequence) - Detects phase imbalance or faults

ANSI 47

- Negative Sequence Voltage, Voltage (sequence) - Detects unbalanced voltage or open-phase

ANSI 49

- Thermal Overload, Current or RTD - I²t-based overload or direct temperature input

ANSI 50

- Instantaneous Overcurrent, Current - Immediate tripping on high current

ANSI 51

- Time Overcurrent, Current - Time-delayed tripping based on overcurrent

ANSI 51G -

Ground Time Overcurrent, Current - Ground fault time delay protection

ANSI 52

- Circuit Breaker, Control - Represents a breaker, not a measurement

ANSI 59

- Overvoltage, Voltage - Trips on high voltage

ANSI 60

- Voltage Balance, Voltage - Detects imbalance in three-phase systems

ANSI 62

- Time Delay, Control/Logic - Timing operations in protection logic

ANSI 67

- Directional Overcurrent, Voltage + Current - Directional overcurrent protection

ANSI 79

- Reclosing Relay, Logic/Sequence - Automatic reclosing after temporary faults

ANSI 81

- Frequency Protection, Frequency - Under/overfrequency relay

ANSI 87

- Differential Protection, Current - Compares currents entering and leaving a zone

ANSI 87G

- Generator Differential, Current - Protects generator winding faults

ANSI 87T

- Transformer Differential, Current - Protects transformer winding zones

ANSI 87B

- Bus Differential, Current - Busbar internal fault protection

Generator Protection - Common ANSI Elements

, 87G (differential), 51/50 (overcurrent), 32 (reverse power), 40 (loss of excitation), 27/59 (voltage), 81 (frequency), 49 (thermal), 24 (overexcitation), 78 (out-of-step), 25 (synchronism)

Transformer Protection - Common ANSI Elements

87T (differential), 51/50 (overcurrent), 49 (thermal), 50/51G (ground), 63 (gas), 64R (restricted earth), 27/59 (voltage), 24 (V/Hz), 46 (negative seq)

Motor Protection - Common ANSI Elements

49 (thermal), 51/50 (overcurrent), 51LR (locked rotor), 66 (start limit), 46 (unbalance), 27/59 (voltage), 14 (underspeed), 38 (bearing temp), 48 (incomplete sequence)

Bus Protection - Common ANSI Elements and Schemes

87B (differential), 51/50 (backup), 27/59 (voltage), 86 (lockout), 74 (alarm); Schemes: Zone interlocking, high/low impedance diff, breaker failure, segmentation

Common Bus Configurations in Substations

Single Bus, Main & Transfer Bus, Double Bus-Single Breaker, Double Bus-Double Breaker, Breaker-and-a-Half, Ring Bus, Mesh Bus - Each offers different levels of redundancy, cost, and flexibility

Single Bus

All feeders connect to one bus; simple and low-cost but offers no redundancy. A fault disables the entire system.

Main and Transfer Bus

Two buses: one main, one transfer. Transfer bus is used for maintenance; requires switches to shift loads manually.

Double Bus, Single Breaker

Each feeder is connected to one breaker but can switch between two buses. Offers operational flexibility with moderate cost.

Double Bus, Double Breaker

Each feeder has two breakers and can connect to both buses simultaneously.

Double Bus, Double Breaker

Each feeder has two breakers and can connect to both buses simultaneously. High reliability and redundancy; expensive and space-intensive.

Breaker-and-a-Half

Three breakers protect two circuits; allows any breaker to be taken out of service without loss of circuit. Popular in EHV substations.

Ring Bus

Breakers are arranged in a ring; each section connects two breakers and a circuit. Highly reliable and expandable.

Mesh Bus

Network of breakers forming a grid. Offers maximum flexibility and reliability but is the most complex and difficult to maintain.

Pilot Protection

A protection scheme that uses communication channels between relays at both ends of a transmission line to detect faults faster and more accurately. It compares voltage and/or current conditions at both ends.

Pilot Wire

Metallic wire connection (short distances only)

Power Line Carrier (PLC)

Uses high-frequency signals over the power line

Microwave

Radio-based high-speed communication

Fiber Optic

Modern, fast, and immune to interference

Radio (UHF/VHF)

Wireless option for remote locations

POTT Scheme

Permissive Overreaching Transfer Trip. Both ends detect fault directionally; trip only if both agree and receive permissive signal from the other end. Uses overreaching elements to cover entire line.

PUTT Scheme

Permissive Underreaching Transfer Trip. Each relay uses an underreaching zone (less than full line length); sends permissive signal only if a fault is within that zone. Faster than POTT for close-in faults.

Blocking Scheme

A directional element detects a reverse fault and sends a blocking signal to prevent tripping at the far end. If no block is received, tripping is allowed. Often used with PLC communication.

Unblocking Scheme

Also known as Unblocking Transfer Trip. Under normal conditions, relays are blocked from tripping. Upon detecting a fault, relays remove the block signal to allow tripping. Similar goal to permissive schemes but operates in reverse logic.

Distance Protection

A protection scheme that measures impedance between the relay location and the fault. Since impedance is proportional to distance on a transmission line, it can determine the fault's location and trip accordingly.

Why is Distance Protection Used?

It operates faster than time-overcurrent for transmission lines and doesn't require communication. It provides selective, fast protection with zone-based operation.

Zone 1 in Distance Protection

Instantaneous protection covering 80-90% of the line length from the relay location. No intentional delay; trips immediately for faults in this zone.

Zone 2 in Distance Protection

Overreaching zone, typically set to 120-150% of line length. Includes remote bus and some part of the next line; operates with a time delay (~0.3s) to allow coordination.

Zone 3 in Distance Protection

Backup zone, often set to 200-250% of protected line. Covers remote buses and lines in case primary protection fails. Has a longer time delay (~1s) to coordinate with downstream relays.

Why are Zones Delayed in Distance Protection?

To allow primary relays time to operate first. Only if the primary zone doesn't clear the fault does the next zone act as backup with intentional time delays.

Reach Settings in Distance Protection

Settings that define how far each zone of the distance relay extends based on line impedance (Z). Typically set using % of protected line length and impedance values.