The function of a network protector is to prevent power from flowing in a reverse direction through it. It will open when reverse power flow occurs and will not close under conditions where reverse power flow would happen as soon as the contacts close. It identifies reverse power from a closed condition by comparing the phase angle of the current signal from the Current Transformers to the phase angle of the voltage on the lines. It identifies the potential for reverse power in an open condition by comparing the phase angles of the voltages on either side of the open contacts and the amplitude of both voltages.
If you have something on the load side which is capable of putting energy back into the system for a short period of time, and the "normal" load current is too small, the burst of output power can drive the current/power through the protector negative for long enough to cause it to trip. I think that this condition on just one phase will be enough to trip it.
Some of the things which could supply energy into the circuit from the load side are:
1. PV grid tied inverters.
2. Large motors forced by their loads to rotate faster than synchronous speed.
3. VFDs which have Active Front End / Regenerative breaking circuitry, when the controlled motor is shut down and braked.
4. Possibly large UPSs under certain strange conditions. No idea about that, but if they are the main loads, I would have to suspect them.
Your steps to mitigate this, once you identify the source of the reverse power are to either increase the static load to more than the reverse power or to reduce the reverse power available from whatever source you have identified.
