If the overvoltage relay is connected to the wrong ground beforehand, as described in Inverter Operation Failure, when a leakage fault occurs on the grid side (short circuit to ground), the operator who finds the location of the fault and prevents the fault is electrocuted. The grounding over-voltage relay (OVGR) set on the grid-connected equipment is responsible for shutting down the inverter (PCS) after detecting the fault and stopping the power supply between the photovoltaic power station and the grid.
The grounding over-voltage relay is installed on the step-up grid-connected device. Its operation output contact and the external input signal terminal of the inverter are connected by wires. When installing multiple inverters in a photovoltaic power plant, read the instruction manual carefully to confirm the connection method.
This article describes that because the inverter manufacturer's instructions were not followed, other methods were used to connect the grounding overvoltage relay's operation output contacts to the inverter's external input signal terminals, resulting in incorrect operation.
When we performed a pre-operational inspection of a megawatt solar power plant with three inverters and 1.5MW output power, we found that the operation output contact of the grounding overvoltage relay and the external of each inverter The input signal terminals are connected by a transition line (Figure 1).
Fig. 1: Operation of the grounding overvoltage relay The output contact and the external input signal terminal of each inverter are connected by a transition line.
Since the connection of the transition line of the external input signal of the inverter was seen in each photovoltaic power station, the inspection was ended without any doubt about this.
However, when the boosted grid-connected equipment receives power in the future and the power supply network is connected to the grid, one of the three inverters shows an operation error code of the grounding over-voltage relay and stops.
However, the grounding overvoltage relay set on the boost-up grid-connected device did not operate. After the problem occurred, we reconfirmed the wiring, but no abnormalities were found.
Connecting with a transition line causes misuse
Therefore, we returned to the original point and reread the instruction manual. As a result, it was found that, as a precaution for setting up such multiple inverters, the words “in order to prevent the control power supply from other devices from being mixed, the external input signal must be provided for each inverter separately†is marked.
To be on the safe side, we also confirmed it to the manufacturer. Sure enough, the answer was: "The inverter's external signal input terminal has been mishandled during the transitional connection, so be sure to provide the external input signal separately."
That is, this time the inverter may have an erroneous operation as shown in FIG. 1 in the state of being connected with a transition line, resulting in an improper shutdown of the inverter.
In order to relieve this condition, one contact must be set for each inverter.
We introduced the situation to the construction party. An auxiliary relay was set between the grounding overvoltage relay operation output contact and the inverter external input signal terminal, and each inverter was wired separately (Figure 2).
Figure 2: Connect each inverter individually to a grounded overvoltage relay to eliminate misoperation
In this case, because the problem was discovered during the on-grid connection after the completion of the inspection, it was fortunate that it did not cause a disaster. If negligence occurred, the power generation would be officially started and the inverter would be shut down when the inverter malfunctioned. The consequences of long-term power generation stoppages have resulted in loss of electricity sales to power generation operators.
This case made us feel the importance of sharing the experience and accumulating knowledge with the peers engaged in the inspection.
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