First, the main cause of abnormal voltage output of power transformers
Under normal conditions, the transformer output voltage should be maintained within a certain range. Low or high may be an electrical fault. Finding such a fault can be done from the following aspects.
    Power supply voltage
    The power supply voltage is low or high, so the output voltage must be low or high. In this case, just measure the power supply voltage. If the power supply is high voltage, the measurement can be compared by a voltage transformer .
    2. The tap changer gear is not correct.
    For high voltage power transformers , the tap changer is used for voltage regulation. The 10kV distribution transformer tap changer has 3rd gear, and the voltage ratio of each gear is shown in Table 2-12.
Table 2-12 Voltage ratio corresponding to the 10kV transformer tap changer gear
Gear | High voltage (kV) | Low pressure (V) |
I | 10.5 | 400 |
II | 10 | |
III | 9.5 |
    If the power supply voltage is low and the tap-changer is placed in the I-stop, the output voltage must be low, otherwise the output voltage is high.
    3. Winding inter-turn short circuit
    A short-circuit between the high-voltage or low-voltage windings of the transformer actually changes the turns ratio of the high- and low-voltage windings, that is, changes the voltage ratio.
    (1) If the high-voltage winding is short-circuited between turns, the number of turns N1 on the primary side decreases, the transformer ratio decreases, and the output voltage rises.
    (2) If the low-voltage winding has a turn-to-turn short circuit, the secondary side turns N2 decreases, the transformer voltage ratio increases, and the output voltage decreases. The turn-to-turn short circuit fault can be further searched by measuring the winding DC resistance or transformation ratio.
    4. Core and winding defects
    When the load is applied, if the output voltage drops a lot when it is no-load, it means that the internal voltage of the transformer is reduced too much. This is because the core and the winding have some defects, so that the leakage magnetic impedance increases, and when the load current flows through the impedance, The voltage is much lower.
    5. Three-phase load asymmetry
    If the distribution transformer is supplied with illumination and the welding machine has a single-phase load, these loads are not three-phase symmetrical, then the three-phase current is asymmetrical, which causes the three-phase impedance voltage drop in the transformer to be different, so that the three-phase output voltage is unbalanced. . The three-phase load is asymmetrical, and the most serious case is that only one phase has a rated load and the other two phases are unloaded. At this time, the phase voltage with the load is significantly reduced, and the voltage of the other two phases of the no-load is significantly increased. In severe cases, the phase voltage can be increased by a stone. It is precisely because of this situation that it is often seen that when a phase welding machine is working, the bulbs on the other two phases are obviously brightened or even burned, and the phase in which the welding machine works, the bulb is obviously darkened, and the reason is Here.
    In order to limit the degree of asymmetry of the load, the relevant regulations stipulate that the current on the neutral line of the transformer must not exceed 25% of the rated current of the phase line.
Second, the transformer high-voltage side breaks one phase on the low-voltage side caused by fault finding
    Accident phenomenon
    A factory is going to work at night, suddenly some workshop lights are off, some workshop lights are down, and the motor can not be started after single-phase protection. No abnormalities were found in the factory electrical inspection switches and lines. Check the transformer outdoors and find that the primary side C-phase fuse of the transformer has fallen.
    2. Analysis of the cause of the accident
    The wiring of the transformer is YynO. After the primary side C phase is disconnected, the A and B phase windings are connected in series, and each receiving line voltage Uuv
    Since the C phase is broken, U C ' =0 (the magnetic flux of the A and B phases passes through a part of the C phase, so the C phase induced electromotive force can be omitted).
    The electromotive force on the secondary side of the transformer changes with the primary electromotive force. Therefore, after the primary side C is disconnected, the voltage on the secondary side becomes U C ' =0, U A ' = UB' = 0.866 U Φ2 , that is, the secondary voltage is also Dropped to 0.866 times the normal value. In the event of an accident, the single-phase load operating voltage connected to phases A and B drops by about 14%, so the brightness of the lamp is reduced. Phase C has no voltage, so the light goes out. The three-phase power supply lacks one phase, and the motor operates in two phases, so the protection action cannot be started. If the motor does not have single-phase protection (or the rated current of the fuse is large) there is a danger of burning the motor. Therefore, when the transformer is in operation, either the primary side or the secondary side is disconnected, it must be highly valued.
Third, the high and low voltage side phase discrimination method of transformer
    A phase loss fault is one of the most common faults in power usage. It may be caused by a fuse blown, a broken line, or the like. In addition, due to product quality, maintenance and improper operation, transformer high-voltage side drop fuses (some fuses are also installed in the middle of high-voltage lines) often occur naturally, and will also cause phase loss. So, how to quickly determine whether the phase loss fault is on the high pressure side or on the low pressure side?
    1. High-voltage side phase loss
    As shown in Figure 2-7, the three-phase transformer is set to be disconnected from the high-voltage side B phase. At this time, the A-phase high-voltage winding and the C-phase high-voltage winding share the line voltage u AC .
    (1) When the two-phase load of the secondary side ac is completely symmetrical, the magnetic flux generated by the load in the two core columns of A and C is equal, so the synthetic magnetic flux values ​​in the two core columns are equal, and the direction is one upward, one toward Next, therefore, the magnetic flux in the middle core column is zero. According to the formula E=4.44fwμm, the A, C two-phase windings each bear half of the line voltage u AC . Therefore, the voltages on the low-voltage side a, c two-phase load are equal and both are lower than 200V (when the line voltage is 380V, this voltage is 190V), and the bulb is insufficiently illuminated. The b phase voltage is zero.
    (2) When the load on the secondary side a, c is asymmetrical, such as Ia>Ic, the resultant magnetic flux Φ A of the A core column is smaller than the magnetic flux ΦA in the c core column. Similarly, known from the above equation, A C-phase voltage is less than the phase voltage, lamp brightness differences larger two-phase, can not work and even household appliances. Meanwhile, since ΦA <ΦC the magnetic flux flowing from the intermediate core columns B, thereby generating a voltage value not in the b-phase winding (typically tens of volts to several tens of volts, with the degree of asymmetry of the load is changed) . If phase b light bulb, the filament can be seen red.
    It is worth recalling that when the body is still in contact with phase conductor b is very dangerous.
    2. The low-side phase
    B-phase transformer low voltage side is provided disconnection. Since the magnetic resistance of the zero-sequence magnetic flux of the three-column transformer is large, the neutral point displacement of the primary winding caused by the load asymmetry is not significant, so here the primary side three-phase voltage is not affected by the load asymmetry. local. Therefore, when the b phase is broken, because of the existence of the neutral line, the two phases a and c are self-contained, and do not interfere with each other, and the voltage is basically unchanged.
    3. High- and low-pressure side phase missing method
    In summary, since the general transformer belts are all mixed loads, household appliances, lighting, and power are all, and most of them are load asymmetry. Therefore, according to the method listed in Figure 2-8, it can be judged whether there is a phase loss on the low pressure side or a phase loss on the high pressure side.
Fourth, the fault caused by poor contact of the transformer tap changer
    Fault phenomenon
    Two 315OkVA main transformers are installed in a substation. When two main transformers were operated in parallel, the active power of the main transformer was found to be the same, but the reactive power of the main transformer No. I was about half lower than that of No. 2. When the two main transformers are operated separately, the meter is normal. Check that the metering circuit of the No. 1 main transformer is normal, and therefore it is suspected that there is a problem with the reactive power meter, and no further inspection is performed. The load continues to drop and the main transformer No. I is out of service. After the load increases, the two main transformers run in parallel, and the same problem still occurs, and the operating personnel found that the upper oil temperature of the No. 2 main transformer exceeded the normal oil temperature and rose above 80 °C (the temperature uses the electrical contact signal thermometer) Measurement, the normal temperature is in the range of 40~60 °C, when the temperature reaches above 85 °C, the main transformer temperature is too high). The two main transformers are unpacked , and the meter is normal when operating alone. It is considered that the gears of the two main transformer tap switches are not the same. Check the main transformer tap changer: No. 2 main transformer tap changer is normal, and when the main transformer tap changer of I is shifting, the gear position rotation is out of order, and there is jamming phenomenon. The DC resistance is determined to be unbalanced. The main transformer of No. 1 was checked by the hanging core. It was found that the tap changer had poor contact and burn marks. The static and dynamic contacts had been misaligned. One static and static head fell on the iron core and the tap changer was replaced.
    2. Analysis of the cause of the failure
    In the spring inspection, only the I-resistance of the tap-changer was measured, and the other tap-resistance was measured without rotating the tap-changer. The tap changer of No. 1 main transformer has been running in the first gear for a long time, the contact is poor, and there is a fever phenomenon, which makes the impedance of the main transformer increase and the reactive power decreases, resulting in the increase of the reactive power of the No. 2 main transformer and the temperature is too high. . In addition, the quality of the tap changer of the main transformer No. I is not high. When the inspection is turned, the fault is lost, the position is dislocated, and the contacts fall off, causing the above faults to occur.
Five, transformer tap changer wiring error
    Two transformers in a factory operate in parallel. After several years of operation, the tap-changer is in the II block. Later, due to the high grid voltage, the high-voltage side was around 11.3kV, and the low-voltage side was around 430V. The reactive power compensation cabinet automatically controlled by the computer could not be put into operation, which caused the factory to be fined by the power supply department because of its low power factor. To this end, the electrician will adjust the two transformer tap switches to the I gear at the same time, reducing the operating voltage so that the capacitor cabinet can automatically switch.
    Accident phenomenon
    First disconnect all circuit breakers on the load side, then pull the bus coupler switch and the low voltage side isolation switch of the two transformers apart, and measure the voltage of the low voltage busbars of the two transformers with a multimeter. The voltage of the three-phase line of the No. 1 transformer is 430V, and the voltage of the three-phase line of the No. 2 transformer is 425V. Disconnect the high-voltage side drop switch of No. 2 transformer, turn the tap-changer to the I-stop position, and close the drop switch to send power. Then use a multimeter to measure the low voltage bus voltage of No. 2 transformer, No. 2 transformer line voltage AB phase 410V, BC phase 410V, AC phase 405V; No. 1 transformer AB phase 440V, BC phase 415V, AC phase 440V, extremely unbalanced, not in line with the juxtaposition Claim. If the tap changer number is incorrect, turn the tap changer to the III position and measure the voltage.
    At this time, the AB phase is 430V, the BC phase is 450V, and the AC phase is 430V, which is exactly the opposite of the front and does not meet the parallel requirements. Preliminary analysis believes that there are two reasons for this phenomenon. One is that the tap of a phase tap changer is connected to the wrong line, and the other is that there is a problem with the number of coil taps. Decide to check and find out the crux.
    2. Analysis of the cause of the accident
    Checking the suspension of No. 1 transformer, it is found that the tap changer is connected to the wrong line, and the "I" and "III" taps are mutually adjusted, causing the above serious hidden danger. After correction, the line voltage AB phase is 415V, the BC phase is 415V, and the AC phase is 415V. The two transformers run in parallel. It is understood that the tap changer has never been moved during the overhaul of the transformer. It can be seen that the manufacturer is negligent.
Sixth, the transformer tap changer contact is poor, causing the coil to short circuit
    (1) Accidents
    A transformer was burned out soon after it was put into operation. Appearance inspection, C-phase high-voltage side fuse is blown, low-voltage side fuse is intact, and both sides of high and low voltage have no phase-to-phase short circuit and single-phase grounding.
    (2) Accident analysis
    1. Test the burnt transformer
    (1) The insulation test is qualified.
    (2) The DC resistance test failed.
    (3) The oil has an abnormal smell and the oil color turns black.
    (4) Hanging inspection, the outer phase I block tap wiring of the C phase line package, burned more than 30 åŒ, the bottom insulation damage.
    2. Analysis
    Comparing the DC resistance test data several times, the percentage difference between the lines increases year by year, which indicates that there are internal welding defects, inter-turn short circuits, poor contact of the tap changer, etc., but no attention is paid within the qualified range, so that the fault point Sharply expanded, causing damage to the transformer. According to the test data changes and the hanging check in previous years, it was found that there was a short circuit between the turns at the I-out tap. It is judged that due to poor soldering at the tap, long-term heat is generated and the insulation is damaged, thereby causing a short circuit between turns and a transformer damage.
Seven, transformer short circuit caused by circuit trip
    Accident phenomenon
    An overcurrent trip of a 10kV distribution line occurred in a substation, and a part of the load was pressed off.
    2. Analysis of the cause of the accident
    After the technician arrives at the scene, first measure the insulation resistance of the line, which is 2M. Left and right, and then connected to the voltmeter at the secondary opening triangle of 10kV TV, the measured voltage is about 40V when tested. Inquire that the line does not increase the load before the trip, so the overload overcurrent is excluded, and the overcurrent trip and grounding should not occur under normal conditions. According to the analysis, it is possible that a distribution transformer of the line may have an inter-turn short circuit fault. Then, the substation duty officer is notified to turn the line from operation to standby, and the line is notified. Further inspection revealed that there was one 315kVA distribution transformer in the line and the A phase on the high voltage side was short-circuited. This is the real cause of the trip.
    The following is an analysis of the overcurrent and false grounding caused by the short circuit between the distribution transformers.
    Because of the internal short circuit between distribution transformers, the simplified equivalent circuit is shown in Figure 2-9.
    Provided Z A, Z B, Z C, respectively, for distribution transformers A, B, C-phase impedance. Uo is the neutral point potential, when the three-phase load balancing, Uo = 0; when the three-phase load imbalance, resulting in the neutral point displacement Uo: When a short circuit between the A-phase distribution transformer turns, the impedance value should decrease , then I A will rise. An overcurrent trip occurs when the sum of all A phase currents of I A and other distribution transformers of the line is greater than the relay protection overcurrent action value I dz . When a transformer of the line A phase is short-circuited between turns, the impedance A of the transformer A phase drops, and the voltage of the triangle side of the TV opening rises. When the voltage exceeds the relay setting value, the central signal board of the control room emits a 10kV system ground signal. The above-mentioned distribution transformer is close to the substation lkm, and the distance is relatively close, so a grounding signal occurs.
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