Case Studies

Off Grid Generator Issue

The PQPro™ with PVII™ software are the ideal tools for capturing and analyzing detailed power system information. In this case study a new generator was installed in an off grid power system that has solar panels and batteries with a charger/inverter.

During commissioning of the generator, it was noticed that there was a relatively large voltage transient during the generator shutdown when the load was transferred from the generator to the inverter. In one instance half the breakers in the breaker panel tripped! In order to identify what the issue was a PQPro™ was connected to collect data. The PQPro™ was configured for Continuous Waveform storage so that a complete analysis of the data could be done – no trigger levels were needed as every waveform for every cycle and RMS data with 200 millisecond resolution was recorded!

The starting and stopping of the generator is controlled by the battery bank charger/inverter Automatic Generator Start (AGS) module. When the battery state of charge (SOC) reaches a low setting, a signal is sent to the generator to start. When the battery SOC reaches a high setting the AGS shuts off the generator and the inverter takes over supplying the load.

Figure 1: Generator Start Process

V1 (Red): one leg of inverter output
V2 (Yellow): one leg of generator output
V3 (Blue): Battery bank voltage

Sequence of events during generator start:

1. AGS start signal sent to generator
2. Inverter backs off voltage to allow generator to carry load
3. Batteries start charging from the generator

Figure 2: Generator Stop Process with Controlled Cooldown

Sequence of events during generator stop:

4. The inverter raises the voltage to take over the load from the generator and the AGS starts the generator cool down period which was set for 1 minute. Batteries start discharging as they are now supplying power to the load.
5. AGS cool down finished, stop signal sent to generator. Generator starts its own fixed 5-minute cool down.
6. Generator takes over supplying power to the house and batteries start charging again. The inverter assumes that the generator is in Manual Run mode, but generator is in its own 5-minute factory set cool down period.
7. Generator 5-minute cool down ends and generator stops abruptly. Inverter takes over supplying power to the house and batteries start discharging again. There is a large AC voltage transient (up to 158V) when the generator stops

The large voltage transient during stopping is due to the generator assuming that it has been disconnected from the load and is spinning down for the cool down period. While this would not be a problem if this was a grid connected system where the generator is connected through a transfer switch, it is a problem when the generator output is connected in parallel with the inverter output. There are a few ways to solve this problem. The easiest and most cost-effective solution is to disable the generator cool down period which allows the generator to stop as soon as it receives the stop command from the AGS. This was done and now the handover from the generator to the inverter is seamless.

Figure 3: Generator Stop Process Without Controlled Cooldown

Sequence of events during generator stop without a generator-controlled cooldown:

8. The inverter raises the voltage to take over the load from the generator and the AGS starts the generator cool down period which was set for 1 minute. Batteries start discharging as they are now supplying power to the load.
9. AGS cool down finished, stop signal sent to generator. Generator immediately shuts down and transfer is seamless!