generator earth fault protection
DESCRIPTION
Electrical ProtectionTRANSCRIPT
GENERATOR STATOR EARTH FAULT PROTECTION BY GENERATOR’S OWN HARMONICS. By: Eng. Fayyaz Ali Shah ‐ Executive Director (Technical) ICC (Pvt) Ltd. General : Harmonics are undesirable due to several reasons but mainly due to increased iron losses and circuit resonance resulting in large harmonic current. Advantage is taken of this undesirable component of Power system in the form of Effective Stator earth fault protection. Stator is the most expensive part of Generator. If the winding is damaged there is possibility to repair it in some days but if the fault persists and arcing starts, laminations will also get damaged which may take long time to repair and pass it through routine tests. As such quick and effective protection is required to clear the fault before heavy damage occurs. The most dangerous fault for the machine is low level earth fault in the stator. If an earth fault occurs near the Generator Neutral, it will not be detected by conventional relays and the probability of second fault will be much increased. Second fault will occur as a result of insulation deterioration caused by unstable low continuous current due to arcing phenomena at the first fault position. This current will increase till heavy damage is resulted and protection operates. Therefore 100 % stator earth fault protection is required to detect the fault and isolate it immediately. Stator earth fault protection under discussion is third harmonic injection into the relay and its monitoring by the latter. Conventional relays limitations: Conventional Stator protection, consisting of Differential relays, Stator Earth fault relays etc. are in‐effective to clear the fault unless the fault develops and current reaches operating value. If fault is close to Neutral, Earth fault relay will not initiate at all because there will not be sufficient voltage to drive the current. Differential rely will sense only when the damage is big enough to cause differential current to flow. It will also depend on method of Neutral earthing. Normally the generator neutral is earthed through high resistance to limit the earth fault current. In this case it will be more difficult for the relay to operate. The higher the resistance, the lesser the fault current and more difficult for relay to operate. Thermal relay operates when the winding gets overheated. Subsequent to overheating, winding’s insulation properties are impaired and it is liable to damage any time. Earth Faults in the system stress the generator winding and core. The possibility of undetected Earth faults may cause arcing due to transient over voltages, resulting in damage to insulation and core. Damages have already been recorded worldwide and within the country Harmonics injection Technology provides 100% E/F protection. A stator winding earth fault with low fault resistance will result in increase of fundamental frequency Neutral point voltage, proportional to the distance of the fault from the star point. As such for minimum relay setting of 5%, only 95 % winding will be protected and the rest will not be detected. A.C machines produce odd harmonic voltages, out of which third harmonics are most dominant. Analysis of A.C Generators of different sizes and operating under different conditions revealed that the 3rd harmonic voltage produced in the machines vary from 1‐ to 3% of terminal voltage of these generators. When the machine is running under normal conditions the third harmonics voltages are shared between terminal side and Neutral side. Proportion of Line side third harmonic voltage to Neutral side third harmonic voltage is almost constant. Analysis has shown that about 40‐60 % of harmonic voltages appear across Neutral resistance. The following data will explain how voltages are distributed between stator’s terminal side and Neutral side.
Distribution of capacitances, capacitive reactances and Neutral impedances are shown in figures given below. Operation Principle: Third harmonic voltage induced in the machine stator has same magnitude and angle in all the three phases and there is a closed loop of currents through phase to earth capacitances and earth resistance. As is explained above there is almost fixed ratio of Phase to Neutral 3rd harmonic voltages under normal conditions. When fault occurs anywhere in the machine, this ratio will disturb, causing the relay to sense and operate. If the fault is near terminals, harmonic voltage on that side will reduce and that on Neutral side will increase and the ratio will disturb. Similarly if the fault is on neutral side, harmonic voltage on terminal side will increase and that on Neutral side will reduce, again creating unbalance. As such relay will work perfectly under all load conditions, all operating conditions and even if the fault is at star point. Practical arrangement: Arrangement for V.T’s on Generator phase and Neutral sides are shown in fig. (1) below:
PRINCIPLE OF STATOR E/F RELAY
FIGURE-1
FIGURE-2 Fig (2) shows the distribution of harmonic voltages under healthy conditions.
100% V3 = % PHASE-N VOLTAGE OF MACHINE
FIGURE-3
X – FAULT POSITION
FIGURE-3(a) Fig(3) shows the distribution of voltages for various fault positions referred to Neutral point. It also shows quiescent conditions “Q”. Fig(3a) shows the scalar quantities /VL3/ & /Vn3/ as a percentage of total 3
rd harmonics with respect to fault position X.
This difference is continuously measured by the relay. If it exceeds the set value, called dead band setting K, relay will operate. When the fault is on neutral end /Vn3/ becomes zero and if it is on terminal side /VL3/ becomes zero. In either case the difference in the two magnitudes >>0.
Fig(4) shows Principle Block diagram of relay. Voltages are picked up from terminal V.T’s and Grounding transformer and fed to the relays. In the relay 3rd harmonic voltages are filtered, with high rejection of fundamental frequency and other sub‐ harmonic voltages, leaving only 3rd harmonic voltages. Absolute values of line side voltage, VL3 and Neutral side voltage, VN3 and absolute values of total 3
rd harmonic V3, which is vector sum of Line side and Neutral side harmonic voltages are derived from the filter outputs. The difference [/VN3/ ~/VL3/] is compared with a set proportion of total 3rd harmonic k* [/VL3/ + /VN3/]. K is dead band setting adjusted in the field by means of a switch. Any difference between [/VN3/ and /VL3/is nullified by means of Null detector, during field testing under healthy conditions. This is shown by an LED provided on the relay, which will switch off when the voltage is zero. The [ /Vn3/‐ /VL3/ ] is compared with a preset percentage of total 3rd harmonic voltage in a comparator circuit. Setting is adjusted by a switch. Comparator output is fed to AND gate output of which triggers a timer with adjustable setting. Two other outputs fed to AND gate are fuse failure inhibition circuit and arming circuit. Arming circuit checks the voltage and if it is less than preset percentage of terminal voltage (normally 85‐90%) relay will be inhibited from tripping. Similarly if fuse blows due to circuit problem, relay operation will be inhibited. As explained there will be a point in the winding, for which the relay will be blind. For that purpose Other features: Protection also takes care of Neutral link. If link is left opened; e.g. after meggaring of winding during routine testing, it will be devastating because none of earth fault relays will detect fault.
FIGURE-5
As can be seen from fig. 5 (a) third harmonic voltage will not be picked up by relay from Neutral end and will operate. Similarly if the Neutral is earthed for maintenance purpose and not removed before start of machine, third harmonic voltage will become zero on Neutral side and will cause the relay to operate as can be visualized from fig. 5 (b)
References:
1. Advanced Electrical Engineering by AH Morton 2. Higher Electrical Engineering by Shepherd, Morton & Spence 3. Electrical Transmission and Distribution reference Book by central Station Engineers of Westinghouse. 4. Electrical machines by M. Kostenko, L. Piotrovsky translated from Russian to English by A. Chernukhin 5. ABB Application Guide 64‐18 AG 6. ALSTHOM Protective Relays application guide. 7. Practical experience of writer in installation, testing & commissioning of Generator Protection.