Diesel Generator (DG) Protection

During power outages and in remote areas diesel generators are often used to provide backup power. Unless proper generation control is implemented, solar power plants are usually switched off to prevent backfeeding.

How DG Power Limiting Works

Diesel generators have a rated capacity in kW and KVA.  Unlike with zero feed in solutions where solar power can match building loads, diesel generators have a minimul loading that needs to be respected at all times - this is typically about 30% of the rated capacity.
DG Protection refers to the algorithm that ensures that the genset runs at minimal load at all times if possible, thus ensuring minimal fuel usage and maximal solar generation.

Figure showing diesel generator, energy meter and inverter, and power flow directions.

Wattmon requires an energy meter to be installed at the output of the genset, which is used to determine the optimal solar generation required to ensure minimal loading of the genset.


Let's take an example to illustrate this:

A genset of 100kW is installed in a factory, along with a 100kW solar plant with a single inverter.
During power outages, a minimum of 30% - or 30kW needs to be supplied from the genset.

  1. The genset is switched on.  Initially the solar inverter will not be producing power.
  2. If the building load is 50kW then a command would be sent to the inverter to generate 20% of its rated power. So 30kW would come from the genset and 20kW from solar (the inverter would be set at 20%).
  3. If the load changed to 100kW then initially the generator would be supplying 80kW would come from the genset and 20kW from the inverter (it's current 20% setting).  The power control algorithm would change the set point to 70% to generate 70kW of solar, thus maintaining 30kW on the genset.
  4. If the load now reduces to 80kW then the genset would be generating 10kW and the solar 70kW.  Wattmon would quickly change the inverter percent to 50% so the genset would once again produce 30kW and the inverter 50kW.