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A Solar Energy Power Plant Accident Analysis

3.1.2  Short circuit reasons

After the short circuit occurs at the upper positive bus of the DC circuit breaker, No short circuit point form short circuit.

The current flowing through the short circuit point is close to 864 A (the sum of the instantaneous output current of the six combiner boxes), while the combiner box DC circuit breaker is GM-250P PHOTOVOLTAIC special circuit breaker, rated current In=

250 A, the instantaneous action value is 10In, namely 2500 A, so the DC circuit breaker of the combiner box does not jump.

The bus at the short circuit point passes through a large current in a short time, which causes DC arc tension and leads to equipment fire.

At the same time, the 16-way photovoltaic group series in the short-circuit junction box enter the short-circuit state, and the anti-diode forms the clamping protection. All branches are in the internal short-circuit state, and some branches generate serious heat, leading to cable sintering.

The operation mode of the system is shown in Figure 3.

3.2 The capacitor on the input side of the power module of the combiner box is burnt out

The input power of the power module of the combiner box is taken from the DC bus, and the positive and negative lead of the upper mouth of the DC output circuit breaker of the combiner box is connected to the input side of the power module, which is the input voltage of 800 V and the output voltage 24 V buck circuit.

Power module input side capacitance withstand voltage 1200 V. In the event of an accident, short-circuit in the input stage will increase the short-time transient current, and a peak voltage will be generated on the line to cause the capacitor to overvoltage breakdown, resulting in a secondary short circuit and the capacitor on the input side of the power module will be burnt out.

Confirmatory test after stator bar repair.

(1) Test insulation resistance, absorption ratio and polarization index of U-phase stator winding.

(2) Local DC voltage withstand test was carried out on the repaired part. A layer of aluminum foil was applied on the surface of the damaged insulation repair, and 1.7U (N 45.9kV) DC high voltage was applied on the copper conductor of the U-phase stator winding, and the withstand time was 1min.

(3) The DC withstand voltage and leakage current test is carried out in accordance with DL/T 596-1996 standard. The maximum test voltage is 2.5U_N, i.e. 67.5kV, and the maximum test voltage for V and W phases is 2.0Un  (54KV)  current withstand voltage and leakage current test.

(4) U phase power frequency AC voltage withstand test

Apply the test voltage of machining frequency to 1.0U (N 27 kV), observe whether there is obvious heating or discharge between the repaired part and other parts, and whether there is corona phenomenon at the end of the generator;

The test voltage of machining frequency is up to 1.3U (N 35.1kV), and the voltage withstand time is 1 min, according to DL/T 596-1996 standard.

(5) U-phase insulation resistance test shall be conducted after power-frequency AC voltage withstand test.

On December 18, the stator bar of No. 1 generator was repaired.

The modal test of the generator end is carried out, and the test results are basically consistent with the previous one.

Verification test after repair was carried out again, and all of them passed.

On December 24th, # 1 generator was connected to the grid and put into service again. So far, the unit has been operating normally.

3.3 Inverter output filter Filter Filter capacitance burnt out

When the accident occurs, the inverter is controlled by AGC and operates at limited power.

The output power of the single inverter is small, but the connection with the power grid is not disconnected. The inverter as a whole works in the state of high resistance, and its system circuit is shown in Figure 4.

The photovoltaic power station inverter adopts two-stage topology structure, the front stage adopts DC/DC converter to realize the maximum power point tracking of photovoltaic array, and the rear stage adopts DC/AC grid-connected inverter to feed the direct current energy to the power grid.

In the limited power state, the inverter reference power limit value, three-phase inverter circuit conduction Angle is small.

At this point, the system operates at the right side of the MPPT, and the MPPT stops dc voltage regulation and basically keeps the constant voltage output.

In the accident, the V-phase filter capacitance of the INVERTER AC output SIDE LC filter and the V-phase arrester of the low-voltage side of the box transformer were all burnt out. It was preliminarily judged that the V-phase circuit was in the conducting state when the fault occurred.

Due to the delay of the control signal of the system and the inconsistent action of the switch element, when the system is not protected, the short-circuit instantaneous current is connected from the DC end to the output side of the inverter, while the short-circuit current does not exceed the withstand current of the inverter circuit switch device, and the duration is short, so the inverter power device is not damaged.

Instantaneous short circuit can be equivalent to output filter circuit working in short circuit state [6], and the equivalent circuit is shown in Figure 5.

In the short circuit state of LC filter, a large peak voltage is formed on the inductance side, which causes the output filter capacitance to burn out.

3.4 The upper V phase arrester of the low-voltage side circuit breaker of the transformer is burnt out

The short-circuit current is connected to the output side of the inverter in The V phase, and the over-current causes the low-voltage side lightning arrester of the box transformer connected to the inverter to act. The short-circuit current is greater than the nominal discharge current of the arrester, and the arrester burns out seriously due to its heat.

4. Existing problems and preventive action

4.1 Existing problems

(1) In the case of AGC control state and depth limit, the operation state is power limit, that is, the depth adjustment of inverter switch device maintains the grid-connected state.

In this case, if there is a grounding fault on the DC side, the short-circuit current on the output side of the inverter will easily extend the fault to the line side, resulting in line trip.

(2) The circuit breaker on the input side and output side of the inverter has a large breaking capacity. After the output DC cable of the combiner box is short-circuited and grounded, the circuit breaker cannot operate.

The breaking capacity of each switch of the combiner box and inverter is shown in Table 1.

(3) In winter, weeds in PV area are not cleaned in time, causing fire hazard.

(4) The positive and negative DC cables of photovoltaic modules are not wired separately. When the pipes are buried, the positive and negative DC cables of the DC cables are threaded together, resulting in heating and aging of the lines, which may easily lead to short circuit.

4.2 Preventive Action

(1), Carry out an inspection on the equipment of the combiner box to check whether the wiring of the DC switch of the combiner box is fastened and whether there is any broken skin, and whether the filter capacitor input by the power module of the combiner box is bulging and leaking liquid;

Check the turn-on characteristics of anti-anti-diode, as well as the fastening of the photovoltaic group in the junction box;

(2), Install a fast blown fuse on the input side of the inverter to improve the fault isolation capability.

(3), Technical communication with the inverter manufacturer and AGC control manufacturer, change the power limit operation of the inverter to the power limit shutdown operation mode under the condition of deep power limit.

(4), Deeply study of the operating principle and control principle of the inverter, clarify the control situation of the inverter under different working states, identify the unreasonable operation situation.

(5), Clean the electrical corrosion marks on the surface of the insulation bracket, brush the surface with a layer of room-temperature cured epoxy adhesive, and dry it at room temperature for 24 hours.

(This project will be carried out at the same time as the B21 wire rod repair work).

(6), Complete cleaning of # 1 generator stator bar.

5, Conclusion

DC withstand voltage and leakage current test is an important test in generator handover and preventive test.

In this paper, a series of cause analysis and defect search for excessive leakage current during the preventive test of # 1 generator in a plant were carried out, and in-situ repair in the bore was carried out.

The defects of stator windings are eliminated and the safe and stable operation of generator set is ensured effectively, which can be used for reference for similar fault treatment