P345 20Hz Stator Earth Fault Application Guide

Generator  Protection MiCOM P345

Stator-Earth Fault Protection via 20 Hz low frequency injection method

Setting guideline, examples and commissioning

CONTENTS 1

INTRODUCTION......... INTRODUCTION........................... ................................. .............................. ................................. ............................... .................... ....... .3

2

SETT SETTIN ING G GUI GUIDE DELI LINE NES S FOR FOR 100% 100% STAT STATOR OR EARTH EARTH FAULT FAULT PROTE PROTECT CTION ION 4

3

GENE GENERA RATO TOR R EART EARTHE HED D VIA VIA PRI PRIMA MARY RY RES RESIS ISTO TOR R IN GENE GENERA RATO TOR R STARPOINT ................................... .................................................... .................................. ................................. ................................ ................ .7

3.1

Setting Setting example example with with generator generator earthe earthed d via a primary primary resisto resistorr in generato generatorr starpoint starpoint ... 9

4

GENERA ERATOR EAR EARTHED VIA EAR EARTHIN THING G TRANSFORMER......................... 11

4.1

Setting Setting example example with with genera generator tor earthe earthed d via earthin earthing g transforme transformerr and secon secondary dary resistor at the starpoint of the generator ......................................................................13

4.2

Setting Setting example example with with genera generator tor earthe earthed d via earthin earthing g transforme transformerr and secon secondary dary resistor at the terminals of the generator .....................................................................15

5

METH METHOD ODS S TO TO EST ESTAB ABLI LISH SH THE THE SER SERIE IES S IMP IMPED EDAN ANCE CE SETT SETTIN INGS GS FOR FOR 64S 64S ...................................... ................................................... ............................... ................................. .............................. ................................. ..................... ... . 17

5.1

By Calculati Calculation on ........................ ...................................... ............................ ............................ ............................. ............................. ............................ ..............17 17

5.2

By Measurem Measurement ent ........................ ...................................... ........................... ........................... ............................ ............................. ..........................1 ...........17 7

6

COMMISSIONING COMMISSIONING .................................. ................................................... ............................... .............................. ......................... .......... 18

6.1

Connec Connectt the the test circuit circuit ........................ ...................................... ............................ ............................ ............................. ............................. ...............18 .18

6.2

Check Check the pick-up pick-up settings settings ......................... ...................................... ........................... ............................ .............................. ........................18 ........18

6.3

Perform Perform the timing timing tests ......................... ....................................... ............................ ............................ ............................. ...........................1 ............19 9

6.4

Perform Perform the the 100% 100% stator stator earth earth fault fault supervi supervision sion test ............ ......................... ............................ ..........................2 ...........21 1

6.5

64S Calibrat Calibration ion procedur procedure................... e............................... ............................ ............................. ........................... ............................ .................22 ...22

6.6

Angle Angle compe compensatio nsation n setting setting (64S Angle Angle Comp) Comp) ......................... ....................................... ............................ ...................23 .....23

6.7

Series Series resista resistance nce settin setting g (64S (64S Series Series R) R) ................. ............................... ............................ ........................... ........................23 ...........23

6.8

Calibrati Calibration on at at the 64S 64S alarm alarm and and trip trip settings settings ...................... .................................... ............................. ..........................2 ...........23 3

6.9

Parallel Parallel Conducta Conductance nce (64S Parallel Parallel G) ................ ............................ .......................... ............................. ............................. ...............24 .24

6.10 Checking with other resistance values................... .................... .......... ..................... ...................... ..................... ............24 ..24 6.11 Testing the 100% stator stator earth fault protection on the the generator ........................ ............ ...................... .......... 24 6.12 Start-up tests............... tests... ........................ ...................... .................... ..................... ..................... ...................... ............ ............ ............ .......25

Protection & Control place country-regionGermany country-regionGermany

1

Introduction The 100% stator stator earth fault protection protection using using a low frequency frequency injection injection technique technique detects earth faults in the entire winding, including the generator neutral point. If an earth fault in the generator starpoint or close to the starpoint is not detected the generator is effectively running with a low impedance earth bypassing the high impedance impedance earth typically used on large machines. A second earth f ault can then cause a very high current to flow which can cause a lot of damage to the machine. This is why 100% stator earth fault protection is a common requirement for large machines. The low frequency injection technique can be used to provide protection for 100% of  rd the stator winding compared to only 20-30% of the winding using the 3 harmonic technique. Also, the low frequency injection technique provides protection when the machine is stopped and running and also when the machine is running up and down. rd The 3 harmonic technique has to be blocked or is not operational when the machine is stopped and when the machine is running up and down. Also, some machines only rd rd produce a low level of 3 harmonic voltage ( 0.5Ω where possible to minimise measurement errors) a high secondary voltage, such as 500V, should be chosen for the neutral or  earthing transformer. It is important that the earthing transformer never becomes saturated otherwise ferroresonance may occur. It is sufficient that the transformer  knee point voltage be equal to the generator rated line voltage. The low frequency voltage is fed to t he relay via a voltage divider and the low frequency measuring current is fed via a miniature current transformer. All interference deviating from the nominal low frequency signal is filtered out. The 100% stator earth fault protection can also be applied with a primary loading resistor. The 20Hz voltage is connected via a voltage transformer and the neutral starpoint current is directly measured via a CT. From the measured measured current current and voltage vectors the complex complex impedance impedance can be calculated and from this the ohmic resistance is determined. This eliminates disturbances caused by the stator earth capacitance and ensures high sensitivity. ,

The relay algorithm can take into account a transfer resistance 64S Series R, that may be present at the neutral or earthing voltage transformer.  An example of the series resistance is the total leakage resistance of the earthing or  neutral transformer, through which the injected voltage is applied to the g enerator  neutral. The algorithm can also account for parallel resistance, 64S Parallel G (G = 1/R), such as an additional earthing transformer connected on the LV side of the step-up transformer. Other error factors can be taken into account by the angle error  compensation, compensation, 64S Angle Comp. The relay includes a 20Hz overcurrent element which can be used as a back-up to the 20Hz under resistance protection. The overcurrent element is not as sensitive as the under resistance elements as it does not include any transfer resistance compensation or any compensation for capacitance affects. In addition to the determination of the earth resistance, the relay also includes 95% stator earth fault protection as a back-up to the 100% stator earth fault protection. The neutral voltage protection from the measured earthing/neutral transformer or  calculated neutral voltage from the 3 phase voltage input can be used to provide 95% stator stator earth fault protection protection and is active during the run-up run-up and run-down run-down of the generator. The 100% stator earth fault protection includes 2 stages of under resistance protection for alarm and trip and an overcurrent protection stage, with each stage having a definite time delay setting. The protection includes a supervision element to evaluate a failure of the low frequency f requency generator or the low frequency connection.

2

Setting Setting guidel guideline ines s for for 100% 100% stato statorr earth earth fault fault prot protecti ection on The 100% stator earth fault protection element can be selected by setting the 64S 100%St EF cell to Enabled. The 64S R Factor is set as described in section 3 and 4. The under resistance alarm threshold, 64S Alarm Set, must be set below the level of  resistance present under normal conditions. This resistance can be determined by viewing the 64S R cell in the MEASUREMENTS 3 menu. A t ypical value for the primary fault resistance alarm setting is between 3-8kΩ . The under resistance trip threshold, 64S R1 Trip Set, must be set above the 20Hz level of  current present under normal conditions. This secondary current can be determined by viewing the 64S I Magnitude cell in the MEASUREMENTS 3 menu. The P345’s 64S protection has a very powerful band pass filter tuned to 20Hz. The band pass filter is designed with an attenuation of at least -80db for frequencies less than 15Hz and greater than 25Hz. -80db is equivalent to a noise rejection capability with a noise-to-signal ratio of 10000 to 1. However, it is not possible for the filter to reject all the ‘noises’ around 20Hz. When the power system frequency is at 20Hz, the relay will not be able to distinguish the power system frequency signal and the injected signal. Under no fault conditions, the influence of the 20Hz power system components is practically negligible. So there is no risk of relay mal-operation under system frequency conditions, from 0Hz to 70Hz. The current measured will effectively be the capacitive current plus the current through the parallel resistance. resistance. The 64S I>1 should be set higher than this quiescent current. For earth faults occurring 0 - 15Hz and 25 - 70Hz at any point on the stator windings both the under resistance (64S R) work correctly under these power system frequency conditions due to the relay filtering. The power 

Protection & Control place country-regionGermany country-regionGermany

system frequency components will be removed by the band pass filter and will have no influence on the protection measurements. The influence of the power system signals depend on the position of the fault. At the star point, the influence is negligible. Therefore, both the under resistance (64S R) work correctly under the complete range of power  system frequencies from 0 to 70Hz when the f aults occur at the star-point. For faults not at the star point where the power system frequency signals are at or  around 20Hz the power system 20Hz signals become more and more dominant as the fault position moves towards the terminal of the generator. For these faults there is a possibility the R< elements can overreach. In most cases the current is 180 out of phase with the voltage. The 64S current (I 64S(P345) ) under fault conditions consists of two components, the 20Hz current component from the 20 Hz injection system, (I 64S(20)) and the 20Hz current component produced by the neutral displacement voltage, (I 64S(G)). At or  around 20Hz, the I 64S(G) cannot be filtered off and thus contributes in magnitude to the I64S(P345), which improves the fault detection capability of the 64S I>1 protection function. Thus, the 64S I> element can be used to t o provide back-up protection for faults that occur when the machine is running at 20Hz. The I64S I>1 Trip can be set as a backup element 15-25Hz to the 64S R