STATOR EARTH FAULT PROTECTION FOR GENERATORS BY 20HZ INJECTION METHOD

stator earth fault protection - SANDS

The 100% Stator Earth Fault Protection system is based on a sub-harmonic injection principle. To implement a 100% Stator Earth Fault Protection system a 20Hz current is injected into the generator star point. The 20Hz generator SGEN-1001 is used in combination with the Band Pass Filter SBPF-1001 in the Stator Earth fault protection. The injection can be performed by connecting the SGEN-1001 and the SBPF-1001 to the load resistor of an earthing or neutral transformer. If an earth fault occurs in the generator star point, the 20Hz current flows through the fault resistance. From the driving voltage and the fault current, the protective relay determines the fault resistance.

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Features
Application
Specification
  • 100% stator earth fault protection based on subharmonic injection
  • Detects stator earth faults when the generator is offline
  • Provides a wide range of fault resistance coverage
  • Low-cost unit with high performance
  • Plug and play system
  • Generators
Power Supply
Supply Voltage 85 to 260V AC, 1φ, (47Hz to 63Hz) or 120V to 370V DC
System Status
LED Power & Signal
Alarm PFC contact
Output Signal
Output Voltage 50V Peak to peak +10%, Rectangular
Output Current 5A continuously
Output Frequency 20Hz +5%
Efficiency 85%
Environment
Operating Temp 0°to 55°C
Humidity 93% Non-Condensing
Cooling Passively Cooled
Mechanical
Dimension 482mm x 173mm x 245mm +5mm
Enclosure Panel/Table Top Mountable
Protection Class IP20
Test
Burn-in Test IEC 60068-2-78
Vibration Test IEC 60068-2-6
Shock Test IEC 60068-2-27
Seismic Test IEC 60068-3-3
  

20Hz BANDPASS FILTER SPECIFICATION
Input Signal
Frequency 20Hz +5%
Voltage 50V +10%, Rectangular
Output Signal
Output Voltage Min 25V RMS without Load
Output Current 5A continuously
Output Frequency 20Hz +5%
Withstand voltage 245V AC at 50Hz (NGT V)
The impedance at the centre frequency <5Ω @ 20Hz
The impedance at System frequency >50Ω @ 50Hz
Environment
Operating Temp 0°to 55°C
Humidity 93% Non-Condensing
Cooling Passively Cooled
Mechanical
Dimension 482mm x 173mm x 245mm +5mm
Enclosure Panel/Table Top Mountable
Protection Class IP20
Test
Burn-in Test IEC 60068-2-78
Vibration Test IEC 60068-2-6
Shock Test IEC 60068-2-27
Seismic Test IEC 60068-3-3

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FAQ

There are two types of stator protection systems a) 95% scheme which covers the 95% of the winding excluding 5% from the neutral, which is based on the third harmonic measurement, b) 100% stator protection system covers the full range including the 5% closer to the neutral, and this is based on sub-harmonic injection method. Besides, in this 100% scheme, the faults can be detected even in generator shutdown condition, which is not possible in the 95% scheme.
A 20Hz voltage generator, injects 20Hz voltage into the neutral point of the generator via the neutral ground transformer (NGT), and this 20Hz signal is continuously available in the stator, under both idle and running condition of the generator. In case of any earth fault due to insulation breakdown or any other cause, the 20Hz current from the generator increases, thus triggering the protection systems. It is also observed that the 20Hz fault current is proportional to the extent of the fault/damage. It has been observed that most of the earth fault occurs during startup or energizing the generator after major overhauling. The 20Hz injection method is well suited for such conditions too.
Yes, SANDS 20Hz injection kit is compatible with all makes of numerical relays that work sub-harmonic stator protection system, commonly referred as 64S series of relays.
The 20Hz injection kit consists of two components a) 20Hz generator and b) 20Hz Band-pass filter, also called coupling filter. The 20Hz generator generates bi-polar square wave of 20Hz of +/- 30Volts. This signal is passed through a series LCR tuned band-pass filter to generate sinusoidal 20Hz voltage, which is injected/coupled to the neutral ground transformer (NGT). The band-pass filter has very high attenuation at 50Hz; thus protects the generator from 50 Hz voltage across NGT.
The 20Hz injection kit consists of 20Hz generator and 20Hz high-Q band-pass filter. These two are matched and tuned at the factory. Thus, 20Hz injection kit should be used as a matched pair, and one should not interchange. Interchanging of the generator and the band-pass filter from two different sets may result in failure of the generator, due to impedance mismatch.
In general, the numerical relays work on the principle of Go/No-Go principle, by which the system is functional if the fault current is less than the threshold, and the system is turned off if the fault current exceeds the threshold. In practice, the insulation resistance of stator winding is very high under normal conditions, while it is low under fault conditions. The transition phase can be better observed by monitoring the 20Hz and 50Hz current continuously in the NGT path, by using the 20Hz monitoring system.
Yes, the 20Hz monitoring system can transmit the 20Hz and 50Hz current at the NGT or close to the 64S relay point, to any remote location within the plant using RS-485/Ethernet or outside to a cloud using GPRS/broadband connectivity.
The 20Hz monitoring system is 100% non-intrusive and does not require any changes in the wiring or disconnection. A clamp-on CT is used to monitor the signal, and this can be connected/disconnected anytime without disturbing the existing protection system.
Yes, the 20Hz monitoring system is deployed in many power plants of smaller and large capacities, and has been in successful operation for more than two years.