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64R Voltage Injection Rotor Earth Fault Protection
Built with 32-bit high-performance DSP and high-speed Ethernet, this device provides Voltage Injection Rotor Earth Fault Protection for all generators except brushless excitation types. Based on the Wheatstone bridge circuit, it detects single and double-point rotor earth faults. It records fault positions for single-point faults and identifies double-point faults via changes in calculated grounding coordinates.
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Description
This device is designed based on 32-bit high-performance DSP (Digital Signal Processor) and high-speed Ethernet communication technology, and is applicable to Voltage Injection Rotor Earth Fault Protection for generators of all capacities (excluding brushless excitation generators).
Adopting the Wheatstone bridge principle, the device implements single-point rotor earth fault protection and double-point rotor earth fault protection for generators.
When a single-point rotor earth fault occurs, the Voltage Injection Rotor Earth Fault Protection device can record the fault location.
If a second earth point subsequently appears (i.e., a double-point rotor earth fault), the calculated grounding position will change, which enables identification of double-point rotor earth faults.
Main Functions
(1) Relay Protection Functions:
- Single-point Rotor Earth Fault Protection
- Double-point Rotor Earth Fault Protection
(2) 10 digital switching inputs, 1 DC analog input (excitation voltage), and 6 digital switching outputs.
(3) Device power-loss alarm function.
(4) Time synchronization function: time synchronization via host software and GPS pulse timing.
(5) The device can store the latest 40 alarm records after power failure.
(6) Equipped with one RS485 bus communication port and one Ethernet communication port to support dual-network communication.
Protection Principle of Rotor Earth Fault Relay
Single-point Rotor Earth Fault Protection
According to the field lead-out mode of the rotor winding, either the double-end injection or single-end injection principle of Voltage Injection Rotor Earth Fault Protection can be selected. The injection power supply is injected between the positive and negative terminals (or only the negative terminal) of the rotor winding and the main shaft.
The switching cycle of the injection power supply can be adjusted based on the capacitance-to-ground of the rotor winding.
The device calculates the single-point rotor grounding resistance in real time, and the protection responds to the drop in insulation resistance between the generator rotor and the main shaft.
— measuring loop resistance; — high-power injection resistance; — injection power supply module; — insulation resistance between rotor winding and main shaft.
Double-point Rotor Earth Fault Protection
If the single-point rotor earth fault protection is configured for alarm operation:
When the rotor grounding resistance Rg falls below the setting value of the normal stage and the single-point rotor earth fault protection of Voltage Injection Rotor Earth Fault Protection operates, the double-point rotor earth fault protection will be automatically enabled after a preset time delay.
If the grounding position changes by more than a specified threshold, a double-point rotor earth fault is identified, and the protection will trip the generator.
Differences Between Injection-Type and Ping-Pong Rotor Earth Fault Protection
Injection-type and ping-pong schemes are two mainstream technical solutions for generator rotor earth fault protection. Their core differences lie in whether an external power supply is required, as well as the resulting disparities in operating range and anti-interference performance.
Ping-Pong Type Generator Rotor Earth Fault Protection
Core Principle
Self-powered with alternating sampling. This scheme requires no external power supply and uses the generator’s own excitation voltage as the signal source. Two high-speed electronic switches alternately close and open (in a ping-pong switching manner) to switch measurement between the positive and negative terminals. The currents flowing to the grounding point are measured separately to calculate the grounding resistance and fault location.
Main Limitation
It cannot operate when the generator is shut down. Since the protection relies on excitation voltage, it loses functionality during generator shutdown or barring operation (no excitation voltage available), creating a blind zone for insulation monitoring.
Performance Features
The calculation adopts conductance ratio algorithm. Theoretically, measurement accuracy remains high and immune to fluctuations of excitation voltage and winding capacitance-to-ground.
Voltage Injection Rotor Earth Fault Protection (Injection Type)
Core Principle
Active injection and independent operation. This scheme actively injects an external voltage signal into the rotor circuit (typically a 1–3 Hz low-frequency square wave or 20 Hz AC signal), then calculates the grounding resistance by measuring the resultant current.
Main Advantage
Full operating condition coverage. Equipped with an independent built-in signal source, it enables continuous insulation monitoring regardless of whether the generator is running, starting, stopping or stationary, realizing genuine full-condition protection.
Performance Features
High Sensitivity：Especially with 1–3 Hz low-frequency square wave injection, it can effectively filter out high-order harmonic interference from the excitation system. The alarm stage setting sensitivity can reach 40 kΩ to 80 kΩ, far superior to traditional schemes.
Strong Anti-interference Capacity：Low-frequency signals greatly suppress the disturbance caused by rotor winding capacitance-to-ground for more accurate measurement.
Comprehensive Comparison & Summary
| Comparison Dimension | Ping-Pong Type | Injection Type |
|---|---|---|
| Signal Source | Generator’s own excitation voltage | Dedicated external injection power supply |
| Operating Range | Only available in operating state (with excitation voltage present) | Full operating conditions (including shutdown, startup and shutdown processes) |
| Sensitivity | Relatively high | Extremely high (up to 80kΩ maximum) |
| Anti-interference Performance | Theoretically immune to capacitance-to-ground interference | Low-frequency injection scheme has strong resistance to harmonic and capacitance-to-ground interference |
FAQ
Q1: What is injection-type rotor earth fault protection?
A: It is a full-condition rotor insulation monitoring technology that adopts an independent external low-frequency signal source (1–3 Hz square wave or 20 Hz AC). The relay actively injects voltage signals into the generator rotor circuit, calculates rotor grounding resistance and fault position by sampling loop current, and realizes accurate judgment of rotor single-point and double-point earth faults.
Q2: What is the core advantage compared with ping-pong type protection?
A: The biggest advantage is full operating condition coverage. The ping-pong type relies on the generator’s own excitation voltage and only works when the unit is running. In contrast, the injection type uses a self-contained signal source, supporting continuous monitoring during shutdown, barring, startup, operation and shutdown transition without monitoring blind zones.
Q3: Why is the injection type with 1–3 Hz low-frequency square wave more accurate?
A: Low-frequency injection can effectively filter high-order harmonic interference generated by the excitation system and suppress measurement errors caused by rotor winding capacitance-to-ground. Its alarm sensitivity can reach 40kΩ–80kΩ, much higher than traditional ping-pong schemes, enabling early warning of subtle rotor insulation aging.
Q4: Does injection protection have any technical limitations?
A: Compared with the ping-pong type, it requires an independent injection power module, resulting in slightly higher hardware cost and more compact circuit design. However, its full-condition monitoring, ultra-high sensitivity and strong anti-interference performance fully compensate for this, bringing higher operational safety for generator units.
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Jack –
Compared with the ping-pong type protection, this device is equipped with a built-in signal generation module, resulting in slightly higher hardware costs.