Description

Table of Contents

Overview of Generator Protective Relay

Generator Protective Relay belongs to generator protective relays, which is suitable for differential and backup protection of small generators, as well as protection for hydro-generators up to 2 MW and turbine generators up to 10 MW.

Features and Specifications

Differential Instantaneous Trip （Trip）
Percentage Differential Protection （Trip）
Excessive Differential Current （Alarm）
Three-stage Overcurrent Protection （Trip）
Inverse-time Overcurrent （Trip; Normal / Severe / Extreme mode optional）
Overload Protection （Alarm / Trip optional）
Two-stage Negative Sequence Overcurrent Protection （Stage I: Trip; Stage II: Alarm / Trip optional）
Inverse-time Negative Sequence Overcurrent （Trip）
Three-stage Zero-sequence Overcurrent （Stage I/II: Trip; Stage III: Alarm / Trip optional）
Loss of Excitation Protection （Trip）
Reverse Power Protection （Trip）
Overfrequency Protection （Trip）
Underfrequency Protection （Trip）
Overvoltage Protection （Trip）
Undervoltage Protection （Alarm / Trip optional, with current blocking）
Zero-sequence Overvoltage Protection （Alarm / Trip optional）
PT Circuit Disconnection （Alarm）
CT Circuit Disconnection （Alarm）
Control Circuit Disconnection （Alarm）
Non-electrical Quantity Protection （Alarm / Trip optional）
PT Voltage Loss （Alarm）
System Power Loss （Alarm / Trip optional）

Generator Protection Relay Numbers(ANSI):

87/87G/50/51/49/46/50N/51N/40/32/81O/81U/59/27/59N/60/63/27

Measurement and Control Functions

Busbar voltage: Ua, Ub, Uc, Uab, Ubc, Uca;
Measured current: Ia, Ic;
Power: P, Q, COSφ;
Frequency: f;
Electric energy: EP+, EP-, EQ+, EQ-;
Supports 21-channel active binary inputs (AC/DC 220V, DC 100V or DC 48V; voltage level shall be specified when ordering);
Equipped with 11 protection output channels, 2 signal output channels, 1 closing position signal output channel, and 1 device power-loss output channel.

Communication Functions

Item	Description
Communication Interfaces	2 × Ethernet ports 2 × RS485 ports (2nd port shared with time sync, selectable via configuration)
Communication Protocols	IEC-103, IEC-61850, Modbus
Time Synchronization	Communication message, SNTP, IRIG-B
Printing Function	Compatible with serial dot-matrix printers; prints settings, event logs and fault waveforms

Generator Protective Relay Settings

No.	Setting Name	Setting Range	Unit	Default Value	Remarks
1	Differential Starting Current	0.1–100	A	1
2	Differential Instantaneous Setting	0.1–100	A	5
3	Inflection Point Setting	0.1–100	A	1
4	Ratio Restraint Coefficient	0.1–0.9	—	0.5
5	Generator Rated Current	0.1–100	A	5
6	Overcurrent Stage I Setting	0.1–100	A	8
7	Overcurrent Stage II Setting	0.1–100	A	7
8	Overcurrent Stage II Time Delay	0–100	S	0.5
9	Overcurrent Stage III Setting	0.1–100	A	5
10	Overcurrent Stage III Time Delay	0–100	S	1
11	Undervoltage Blocking Current Setting	1–400	V	70
12	Negative Voltage Blocking Current Setting	1–400	V	10
13	Inverse-time Overcurrent Setting	0.1–100	A	5
14	Inverse-time Overcurrent Time Delay	0–100	S	1
15	Inverse-time Overcurrent Type	0–3	—	1	1=Normal, 2=Severe, 3=Extreme
16	Overload Setting	0.1–100	A	5
17	Overload Time Delay	0–100	S	2
18	Overload Type	0–2	—	1	0=Exit, 1=Trip, 2=Alarm
19	Negative Sequence Overcurrent Stage I Setting	0.1–100	A	2
20	Negative Sequence Overcurrent Stage I Time Delay	0–100	S	1
21	Negative Sequence Overcurrent Stage II Setting	0.1–100	A	1
22	Negative Sequence Overcurrent Stage II Time Delay	0–100	S	2
23	Negative Sequence Overcurrent Stage II Type	0–2	—	1	0=Exit, 1=Trip, 2=Alarm
24	Negative Sequence Inverse-time Overcurrent Setting	0.1–100	A	5
25	Negative Sequence Inverse-time Overcurrent Time Delay	0–100	S	1
26	Negative Sequence Inverse-time Overcurrent Type	0–3	—	1	1=Normal, 2=Severe, 3=Extreme
27	Zero-sequence Overcurrent Stage I Setting	0.1–100	A	2
28	Zero-sequence Overcurrent Stage I Time Delay	0–100	S	0.2
29	Zero-sequence Overcurrent Stage II Setting	0.1–100	A	2
30	Zero-sequence Overcurrent Stage II Time Delay	0–100	S	1
31	Zero-sequence Overcurrent Stage III Setting	0.1–100	A	2
32	Zero-sequence Overcurrent Stage III Time Delay	0–100	S	2
33	Zero-sequence Overcurrent Stage III Type	0–2	—	1	0=Exit, 1=Trip, 2=Alarm
34	Zero-voltage Blocking Zero-sequence Setting	1–400	V	5
35	Loss of Excitation Impedance Setting A	0–100	—	1
36	Loss of Excitation Impedance Setting B	0.2–200	—	20
37	Loss of Excitation Protection Time Delay	0–100	S	1
38	Undervoltage Blocking Loss of Excitation Setting	1–400	V	10
39	Reverse Power Protection Setting	0–600	W	50
40	Reverse Power Time Delay	0–100	S	1
41	Overvoltage Setting	1–600	V	120
42	Overvoltage Time Delay	0–100	S	0.5
43	Undervoltage Setting	1–400	V	80
44	Undervoltage Time Delay	0–100	S	1
45	Zero-sequence Overvoltage Setting	1–400	V	30
46	Zero-sequence Overvoltage Time Delay	0–100	S	0.5
47	Zero-sequence Overvoltage Type	0–2	—	1	0=Exit, 1=Trip, 2=Alarm
48	Overfrequency Protection Setting	35–65	Hz	51
49	Overfrequency Protection Time Delay	0–100	S	0.5
50	Underfrequency Protection Setting	35–60	Hz	49
51	Underfrequency Protection Time Delay	0–100	S	0.5
52	Undervoltage Blocking Frequency Setting	1–400	V	20
53	Slip Blocking Frequency Setting	1–30	Hz/S	3
54	PT Disconnection Time Delay	0–100	S	5
55	PT Voltage Loss Time Delay	0–100	S	5
56	Control Circuit Disconnection Time Delay	0–100	S	10
57	System Power Loss Type	0–2	—	1	0=Exit, 1=Trip, 2=Alarm
58	Thermal/Hydraulic Accident Type	0–2	—	1
59	Thermal/Hydraulic Accident Time Delay	0–100	S	0.1
60	Excitation Fault Type	0–2	—	1
61	Excitation Fault Time Delay	0–100	S	0.1
62	Electrical Accident Type	0–2	—	1
63	Electrical Accident Time Delay	0–100	S	0.1

Note: Refer to the manual for generator protection relay setting calculation instructions.

Outline and Installation Dimensions of Generator Protective Relay

FAQ

Q：which relay is used for generator protection relay？

A:Generator out of step protection relays, reverse power relay for generator protection, three-stage generator current protection, generator loss-of-field protection, generator rotor protection relay,generator differential protection, etc.

Q：which relay is used for Gengenerator Protective Relay？

A:The Gengenerator Protective Relay price is generally around 6,000 RMB, with higher-specification models costing more.

Q What is a generator protective relay?

A dedicated numerical IED for generator units, monitoring electrical faults and abnormal operating conditions, sending trip or alarm signals to protect the generator stator, rotor and prime mover.

Q What core ANSI protection functions are included?

87G Stator Differential, 50/51 Overcurrent, 59 Overvoltage, 27 Undervoltage, 46 Negative Sequence Current, 32 Reverse Power, 64R Rotor Earth Fault, 81 Frequency Protection, Out-of-Step Protection.

Q Which equipment does it protect?

Synchronous generators for hydropower, thermal power, diesel gensets and distributed grid-connected generators.

Q What does 87G stator differential protection guard against?

Detects stator winding internal short circuits; compares current at generator neutral side and terminal side, trips immediately once differential current appears.

Q Why configure reverse power protection (32)?

Prevents the generator from motoring mode: when the prime mover loses power, the generator draws electricity from the grid, causing turbine blade damage.

Q Function of rotor earth fault protection (64R)

Monitors insulation breakdown of rotor winding and excitation loop. Single-point grounding alarms; two-point grounding triggers trip to avoid rotor magnetic circuit burnout.

Q Purpose of negative sequence protection (46)

Targets three-phase unbalance, single-phase grounding and phase loss. Negative sequence current generates eddy heat on the rotor and avoids rotor overheating damage.

Q When will out-of-step protection operate?

The generator loses synchronization with the power grid; it separates the unit from the grid to prevent system oscillation and unit mechanical damage.

Q Linkage with synchronization panel

The relay blocks the closing circuit if the unit has faults during paralleling, forbids out-of-phase synchronization.

Q Coordination with excitation & governor systems

It sends interlock signals to cut excitation and shut down the prime mover when severe faults occur.

Q Relay fails to trip during internal winding fault

CT polarity error, CT secondary open circuit, protection setting value too high, or trip circuit disconnected.

Q Unnecessary tripping of negative sequence protection

External grid unbalance, wrong CT wiring, or unreasonable action threshold setting.

Q Main commissioning tests

Secondary current injection test, CT polarity verification, logic interlock test, fault simulation and tripping loop check.

Q Daily maintenance requirements

Regularly check fault records and communication status; disable auto-tripping protection during generator maintenance with locking measures.

2 reviews for 87G/50/51 Generator Protective Relay

xiao zhang – June 3, 2026

Provides full-range motor protection including differential and backup protection, rapidly isolating faults to ensure motor safe and stable operation.
Rated 5 out of 5

Jack – June 4, 2026

Works seamlessly with SCADA systems — no communication dropouts.

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