Generator Protection MiCOM P345

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

Setting guideline, examples and commissioning

CONTENTS1 2 33.1

INTRODUCTION............................................................................................... 3 . SETTING GUIDELINES FOR 100% STATOR EARTH FAULT PROTECTION 4 GENERATOR EARTHED VIA PRIMARY RESISTOR IN GENERATOR STARPOINT ..................................................................................................... 7 .Setting example with generator earthed via a primary resistor in generator starpoint ... 9

44.1 4.2

GENERATOR EARTHED VIA EARTHING TRANSFORMER......................... 11Setting example with generator earthed via earthing transformer and secondary resistor at the starpoint of the generator ......................................................................13 Setting example with generator earthed via earthing transformer and secondary resistor at the terminals of the generator .....................................................................15

55.1 5.2

METHODS TO ESTABLISH THE SERIES IMPEDANCE SETTINGS FOR 64S ........................................................................................................................ 17 .By Calculation .............................................................................................................17 By Measurement .........................................................................................................17

66.1 6.2 6.3 6.4 6.5 6.6 6.7 6.8 6.9

COMMISSIONING .......................................................................................... 18 .Connect the test circuit ................................................................................................18 Check the pick-up settings ..........................................................................................18 Perform the timing tests ..............................................................................................19 Perform the 100% stator earth fault supervision test ...................................................21 64S Calibration procedure...........................................................................................22 Angle compensation setting (64S Angle Comp) ..........................................................23 Series resistance setting (64S Series R) .....................................................................23 Calibration at the 64S alarm and trip settings ..............................................................23 Parallel Conductance (64S Parallel G) ........................................................................24

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

Protection & Control place country-regionGermany

1

IntroductionThe 100% stator earth fault protection using a low frequency injection 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 earth typically used on large machines. A second earth fault 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 ( 0.5, 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 the 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 current and voltage vectors the complex 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 generator 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, 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 earth fault protection and is active during the run-up and 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 generator or the low frequency connection.

2

Setting guidelines for 100% stator earth fault protectionThe 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 typical value for the primary fault resistance alarm setting is between 3-8k . 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 P345s 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. 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

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 faults 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 (I64S(P345)) under fault conditions consists of two components, the 20Hz current component from the 20 Hz injection system, (I64S(20)) and the 20Hz current component produced by the neutral displacement voltage, (I64S(G)). At or around 20Hz, the I64S(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 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