Loss of excitation protection function (40Z)

Loss of excitation protection function (40Z) Budapest, April 2016

Loss of excitation protection function The loss of excitation protection function can be applied mainly for synchronous generators. On loss of excitation, the flux decreases and the reactive current demand increases relatively slowly. At the end, high reactive current flows from the power system into the machine. To protect the stator coils from the harmful effects of the high currents and to protect the rotor from damages caused by the induced slip-frequency current, a disconnection is required. The loss of excitation (loss-of-field) protection function is designed for this purpose. When the excitation is lost then a relatively high inductive current flows into the generator. With the positive direction from the generator to the network, the calculated impedance based on this current and on the phase voltage is a negative reactive value. As the internal e.m.f. collapses, the locus of the impedance on the impedance plane travels to this negative reactive value. With an appropriate characteristic curve on the impedance plane, the loss of excitation state can be detected. The applied characteristic line is a closed offset circle, the radius and the center of which is defined by parameter setting (see Figure on the following page). If the calculated impedance gets into the offset circle then the function generates a trip command. The loss of excitation protection function provides two stages, where the parameters of the circles and additionally the delay times can be set independently. The main features of the loss of excitation protection function are as follows: A full-scheme system provides continuous measurement of impedances separately in three independent phase-to-phase measuring loops. Impedance calculation is conditional on the values of phase currents being sufficient. The operate decision is based on offset circle characteristics. Two independent stages. Input signals: o The Fourier components of three phase voltages. o The Fourier components of three phase currents. o Parameters. o Binary input signals: Blocking/enabling. VT failure signal. Binary output signals: o Starting signals for the two stages. o Tripping signals for the two stages. The loss of excitation protection supplied by PROTECTA Ltd. continuously measures the impedances in the three line-to-line measuring loops. The calculation is performed in the phase-to-phase loops based on the line-to-line voltages and the difference of the affected phase currents. The result of this calculation is the positive sequence impedance of the measuring loops. The numerical processes apply the simple R-L model. For the equivalent impedance elements of the measuring loop, the following differential equation can be written: u Ri L di dt VERSION 1.1 2/5 2016-04-21 Péter Erdős

If current and voltage values sampled at two separate sampling points in time are substituted in this equation, two equations are derived with the two unknown values R and L, so they can be calculated. The operate decision is based on offset circle characteristics. The calculated R 1 and X 1=L 1 co-ordinate values of the three measuring loops define three points on the complex impedance plane. These impedances are the positive sequence impedances in the measuring loops. The protection compares these points with the offset circle characteristics of the loss of excitation protection, shown for stage 1 in Figure below. For stage 2 the characteristic is the same with independent parameters. Parameter settings decide the size and the position of the circle. The center of the circle can be on the positive R and negative X quadrant of the impedance plane. The R offset and X offset values are defined to be positive in this quadrant. jx Compound circle Stage1 R offset R Stage1 X offset Stage1 Z If a measured impedance point is inside the circle, the algorithm generates the true value of the related output binary signal. The calculated impedance values are compared one by one with the setting values of the offset circle characteristics. The procedure is processed for each line-to-line loop. The result is the binary setting of three status variables. This indicates that the calculated impedance is within the processed offset circle characteristics. The impedance protection function can operate only if the current is sufficient for impedance calculation. The current is considered to be sufficient for impedance calculation if it is above the level set by parameter IPh Base Sens. VERSION 1.1 3/5 2016-04-21 Péter Erdős

Technical data Function Range Accuracy Rated current In 1/5A, parameter setting Rated Voltage Un 100/200V, parameter setting Current effective range 35 2000% of In ±1% of In Voltage effective range 3-110 % of Un ±1% of Un Impedance effective range In=1A In=5A 0.1 200 Ohm ±5% 0.1 40 Ohm Zone static accuracy 48 Hz 52 Hz ±10% 49.5 Hz 50.5 Hz ±5% Zone angular accuracy ±3 Operate time Typically 50 ms ±3 ms Minimum operate time <60 ms Reset time 30 60 ms Reset ratio 1.1 Parameters Enumerated parameters Parameter name Title Selection range Default Parameter for disabling stage 1 UEX_40Z_Op1_EPar_ Stage1 Operation Off, On Off Parameter for disabling stage 1 UEX_40Z_Op2_EPar_ Stage2 Operation Off, On Off Boolean parameters Parameter name Title Default Explanation Boolean parameter to disable the trip command for stage 1 Set 0 value to generate also an UEX_40Z_StOnly1_BPar_ Stage1 Start Only 0 operate signal for stage 1 Boolean parameter to disable the trip command for stage 2 Set 0 value to generate also an UEX_40Z_StOnly2_BPar_ Stage2 Start Only 0 operate signal for stage 2 Integer parameter Parameter name Title Unit Min Max Step Default Definition of minimal current enabling impedance calculation: UEX_40Z_Imin_IPar_ IPh Base Sens % 10 30 1 20 Float point parameters Parameter name Title Unit Min Max Digits Default Raduis of the circle of stage 1 UEX_40Z_Z_1_FPar_ Stage1 Z ohm 0.1 250 2 10.0 X offset of the circle of stage 1 UEX_40Z_Z1_1_FPar_ Stage1 X offset ohm 0.1 250 2 10.0 R offset of the circle of stage 1 UEX_40Z_Z1_2_FPar_ Stage1 R offset ohm 0.0 100 2 0.0 Raduis of the circle of stage 2 UEX_40Z_Z_2_FPar_ Stage2 Z ohm 0.1 250 2 10.0 X offset of the circle of stage 2 UEX_40Z_Z2_1_FPar_ Stage2 X offset ohm 0.1 250 2 10.0 R offset of the circle of stage 2 VERSION 1.1 4/5 2016-04-21 Péter Erdős

UEX_40Z_Z2_2_FPar_ Stage2 R offset ohm 0.0 100 2 0.0 Timer parameters Parameter name Title Unit Min Max Step Default Time delay for stage 1 UEX_40Z_Del1_TPar_ Stage1 Delay msec 50 60000 1 500 Time delay for stage 2 UEX_40Z_Del2_TPar_ Stage2 Delay msec 50 60000 1 500 Binary output status signals Binary status signal Title Explanation UEX_40Z_GenSt1_GrI_ General Start1 General start signal of the first stage UEX_40Z_GenTr1_GrI_ General Trip1 General trip signal of the first stage UEX_40Z_GenSt2_GrI_ General Start2 General start signal of the second stage UEX_40Z_GenTr2_GrI_ General Trip2 General trip signal of the second stage Binary input status signals The conditions of the binary input status signals are defined by the user, applying the graphic equation editor. Binary status signal Title Explanation UEX_40Z_Blk_GrO_ Block Blocking of the loss of excitation function UEX_40Z_VTSBlk_GrO_ Block from VTS Blocking of the loss of excitation function from the VT supervision VERSION 1.1 5/5 2016-04-21 Péter Erdős