How to solve Abnormal Grid Voltage alarm

Introduction

This article describes troubleshooting the Abnormal Grid Voltage alarm (High or Low voltage in the grid).

Diagnostic tools

To solve the issue you may need the following instruments:

• AC voltmeter

• pyrometer or thermographic camera
  

According to the manual

ID	Name	Severity	Cause	Solution
301	Abnormal Grid Voltage	Major	The grid voltage is beyond the acceptable range. The possible causes are as follows: Cause ID = 4 The grid voltage is below the specified lower threshold.	Cause ID = 4 1. If the alarm occurs accidentally, the power grid may be abnormal temporarily. The SUN2000 automatically recovers after detecting that the power grid becomes normal. 2. If the alarm occurs frequently, check whether the grid voltage is within the acceptable range. If no, contact the local power operator. If yes, log in to the SUN2000 app, SmartLogger, or NMS to modify the grid overvoltage and undervoltage protection thresholds with the consent of the local power operator. 3. If the fault persists for a long time, check the AC switch and AC output power cable.

 

Problem Description

During the inspection of a 20 MW PV plant, an inverter was found to report an alarm indicating that the power grid voltage was abnormal and no power was generated. Log in to the SUN2000 app using a mobile phone onsite and find that the AC side phase B-C voltage is obviously lower than the power grid voltage. The alarm ID is 301, the fault cause ID is 4, and the AC side phase B-C voltage is lower than the allowed range.

The possible causes are as follows

1. The grid voltage monitored by the SUN2000 is unbalanced because the AC power cable on the grid side is short-circuited or disconnected. The voltage is below the lower threshold(measured);

2. The internal sampling board of the inverter may be faulty（Disconnect the AC power cable from the inverter and measure the power grid voltage on the combiner box side.

Troubleshooting Process and Fault Location

Step 1: Measure the three-phase voltage from the power grid side of the inverter onsite. It is found that the three-phase voltage is unbalanced.  A-B phase to phase voltage is 466 V, A-C phase to phase voltage is 221 V, and B-C phase to phase voltage is 197 V. This indicates that the phase-to-phase voltage measured by the inverter sampling board is consistent with the actual value. Therefore, the combiner box on the power grid side is faulty.

A-B phase to phase voltage: 466 V	A-C phase to phase voltage: 221 V	B-C phase to phase voltage:  197 V

Step 2: Measure the three voltages. The voltage of A to ground is 269 V, the voltage of B to ground is 271 V, and the voltage of C to ground is 48 V. It can be determined that phase C of the combiner box is faulty. Further, check the combiner box.

A: 269V relative to ground	B: 271V relative to ground	C: 48V relative to ground

Step 3: When you open the combiner box and turn off the circuit breaker to measure the voltage from the box-type transformer, the phase C cable of the circuit breaker is blown.

Therefore, it can be determined that the phase C of the circuit breaker is blown, which causes the inverter to generate an alarm indicating that the power grid voltage is abnormal. The phase C of the circuit breaker corresponding to the inverter for which the alarm is generated is blown.

Suggestions on onsite O&M

1. During preventive maintenance, use a pyrometer or thermographic camera to measure the temperature at the AC wiring terminal and DC wiring terminal, and compare the temperature between the wiring terminal and the DC wiring terminal. Wiring posts and DC terminals with high temperature may be burnt out. It is recommended that terminals be replaced.

Example:

2. If the power grid is faulty, check the power grid in the following sequence: inverter, combiner box, box-type transformer, or box-type transformer-combiner box, inverter. Locate the fault point step by step.

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