Sync diesel generator with solar grid inverters - during Islanding condition

Abstract: As mentioned in my previous blog on solar grid interactive inverters, there are various issues involved with solar grid inverter for Indian conditions. Now in this blog I will be discussing more about the issues and methods to integrate/synchronize a grid connected inverter with diesel generator during a power cut/grid failure. In states like Tamil Nadu we have power-cuts every day, so the solar rooftop owner will try to consume all the power generated from solar and reduce its dependence on diesel generators.

The inverters do not start without the reference voltage and frequency, so during this islanding condition the rooftops owners cannot use their solar power generated. The owners usually consider solar as a backup for power cuts and also consider payback by selling excess generated. Now the only solution in the market is the hybrid inverters, but these are very expensive compared to normal grid inverters. But what about installations with existing grid inverters? Most of the rooftop owners are not aware of these islanding conditions and the EPC vendors just install existing grid inverters without any alteration proposed below.

What I propose here is, the grid inverters needs to be connected to a common bus bar. In the common Bus-bar all the sources and loads are connected. During the islanding condition the DG switch ON to generate reference voltage and frequency to start up the solar grid inverters (we fool the inverters here). Once the inverter is ON the load will be supplied by both inverter and DG sets (at low RPM). Is it practically possible to synchronize a grid tie inverter with a diesel generator (DG sets) and make it work? ie. during a power failure, when grid power is not available.

I feel its possible only under the following condition's 

• We have a stable/good quality generator which gives good stable voltage and frequency. 
• The other condition is the generator should not sag, dip or create any surge current when the inverter turns ON. 

Lets now consider that the inverter has turned ON using the DG sets reference voltage and current. I see two possible conditions now,

1. The load is always more than the PV generator supply
2. Possibility that load can be less than PV generator supply

During the condition 1, i do not see any issues if everything works fine. 

But during the condition 2, the power generated from PV will be greater than the load requirements and hence there is a possibility that this current will be fed to the ac line of of the bus bar / DG sets (stator of DG sets). What will happen during this condition? We can expect a rise in AC voltage and frequency- above critical level. If this happens, this condition can trip the inverter due to over-voltage/frequency protection circuits. Now the rooftop owner cannot consume power from the solar and therefore he is forced to operate the DG sets. This condition can also damage the vital parts of the generator, here the protection of generators become very important. 

How to custom design the distribution panel board to use the solar generator during the condition 2 and also protect the DG sets? I also foresee very high requirement of a effective control system, which can handle all the above and below conditions

- Using a dump load to remove the excess power generated. 
- Effective control of loads  (To increase or decrease the load as per requirements using PLC based controls) 

I can also see that there will be no commercial benefit in running a generator just for reference voltage and current. This is because when we operate the DG sets running at lower efficiency (20-30%) or less than its rated value can consume more fuel than during rated power (ie. 85-100% efficiency). This can be solved by using a smaller DG sets (Just for the emergency load) than the required full load size, but the disadvantage is during power failure in the evenings or night.