1, Solar Power Plant Overview

This is a solar power plant in China, The total installed capacity of this plant is 50 MW;

The fixed capacity is 500 kW, the box transformer is a double-split transformer.

Fixed capacity 1000 kVA, 1 double split box transformer with 2 sets Chinese style inverter.

A total of 50 box-type transformers in the whole station, which are connected to the 35kV bus of the 110kV PHOTOVOLTAIC power station through buried cables in 5 rounds of 35kV lines. The 35kV bus adopts the single-bus connection mode.

2, Accident

When the operator at the solar power plant found a bus box emitting near the Inverter # 1 in Area 71 in the main control video surveillance machine smoking, with small fire.

On-site inspection found that there was an internal fire in the No. 1 busbar of inverter in Area 71. The fire point was located at the positive busbar of the upper outlet of the DC output circuit breaker.

After the combiner box caught fire, the internal gas accumulated. The door of the combiner box sprang open, and the burner fell down and ignited the hay on the ground. The fire burned along the backboard of the assembly and burned eastward for nearly 30 MR.

Under the influence of optical abandon and power limit, only 2 inverters in the whole plant are in normal operation when the accident occurs, and the other inverters are under the unified Control of Automatic Generation Control (AGC) platform for dispatching and controlling the power limit.

The # 1 inverter in area 71 operates with limited power. The dc side input circuit breaker on the inverter and the grid-connected the AC side circuit breaker are all in a closed state.

The inverter is connected downward to 7 bus boxes. The dc output circuit breaker switch of the bus box is in a closed state. The connection system between the inverter and bus box is shown in Figure 1.

Will area 71  box-type transformer, inverter and 1 with 7 junction box column from the system solution, ordinal for photovoltaic modules, junction box, inverter, box-type transformer for inspection, found that the rest of the junction box power module 6 sets all have different rate of damage, main show is input capacitance bulge leakage ceramic gas discharge tube is damaged, the data acquisition module.

During the inspection, it was also found that the inverter AC side V phase output filter capacitor was burned down, and the metal oxide arrester connected to the V phase bus of the low-voltage circuit breaker on the low-voltage side of the box transformer was burned down.

At the same time, it was found that the serial line DC fast blown fuse and anti-diode were not damaged, and the DC output circuit breaker did not jump.

The # 1 combiner box was measured and the 4 branches were short-circuited to earth.

Dig the cable trench, take out the cable, found 4 photovoltaic cables sintered in one.

One of the cables burned down.

Traces of heating were found on the fuse holder.

Check the photovoltaic incoming cable in other confluence boxes, and see no abnormality.

3. Failure analysis

3.1 Confirmation of short circuit point and cause of short circuit

3.1.1 Operation mode of the system after short-circuit grounding of two groups

(1), Photovoltaic set of string into the wire cable grounding short circuit, short circuit point, form a low impedance point in the other groups of current converge to short-circuit point, short circuit set list of defence of the diode to the from state transition to reverse saturation state, form a clamping, short-circuit point only bear string of their own set of short circuit current (as shown in figure 2 a), due to the short circuit current is very small, do not cause large arc of fire, but will cause photovoltaic cable heating sintering together.

(2) If the anti-anti-diode bears a large reverse voltage during the turn-off process, resulting in the anti-anti-diode reverse breakdown, short circuit branches form a large current loop.

A bus box has a total of 16 photovoltaic group strings. At this point, the short-circuit point will bear the short-circuit current of the other 15 photovoltaic group strings (see Figure 2B). The current value is far greater than the operating current of the branch fuse, and the dc fuse of the short-circuit branch will fuse to remove the line grounding.

According to the above two short-circuit grounding operation modes, combined with the on-site accident, determine the bus bar of the dc output circuit breaker of the combiner box as the fault point of short-circuit grounding.

Not finished, The rest content please refer to:

Solar energy power plant failure analysis-2