Updated: Dec 31, 2020
On-gird (gird-tie/gird connected) solar power (PV) plant generates excess power when the connected load is lesser than the power generated by the solar power plant (Power generation > Power required).
This excess power is synchronized with grid power hence it can revere the power flow. In simple words, power will flow from PV plant owner’s home to the connected grid [when Solar power generation > Power required by connected load].
And if the net-meter is not installed at the PV plant owner’s home. This excess power will increase the electricity bill of the PV plant owner.
The reason behind is that normal energy meter measures only the power flow through it, irrespective of the direction of power. So normal energy meter will measure both imported and export power (reverse power) and sum them up.
Whereas net-meter measure the input power form the gird and subtract the surplus power injected back to gird from an on-grid solar power plant.
Also if PV plant owner is using DG (diesel generator) as reference power source [in place of gird] for on-grid (grid-tie) inverter. This excess power of on-gird PV plant will damage the DG set or trip its breaker [if there is any breaker].
Do you want to learn how to synchronize DG set with On-gird solar power plant? Here is my whole page on "DG synchronization with grid-tie PV plant".
4 ways of reverse power flow protection for gird connected (gird tie) PV system
#1 Use RPR (relay power relay) to isolate PV plant from the gird by mean of tripping breaker or releasing contactor, if there is any reverse power detected.
#2 Use Export limiter to limit the power generation of gird tie solar inverter concerning the power required by the load.
#3 Use of PLC as an export limiter.
#4 Use Mini PLC to isolate PV plant from the gird by mean of tripping breaker or releasing contactor, if there is any reverse power detected.
Conclusion: the best solution for reverse power flow protection
These methods of reverse power flow protection for grid-tie solar power plant works with any makes of grid-tie solar inverters like ABB, SMA, Hitachi, Consul Neowatt, Huawei, Solar Edge, Kaco, Detal, Solis, Kirloskar, polycab, Sungrow, Growatt, Fronius, REFU Sol, Schneider, Zever solar and many more.
Reverse power relay for solar/ Reverse power relay protection for solar
Reverse power relay (RPR) sometimes also called as Reverse power protection relay]
Reverse power relay (RPR) for solar is used to eliminate any power reverse back to gird from on-grid (grid-tie) PV power plant to grid or to generator by tripping either on-grid solar inverter or breaker or any contactor depends upon the type of power distribution and a control circuit.
RPR (Reverse Power Relay) is connected at the coupling point of grid and load [In simple words between energy meter and load].
When RPR sense any reverse power it will generate a signal, which can be used to trip any contactor or breaker or gird-tie solar inverter [if there is an option available in solar inverter].
In simple words, the solar power plant will be completely shut down as long as there is a trip signal from RPR (Reverse power relay).
The manual operation will be required to turn on [start] the solar power plant. In simple words, you need to start solar inverter on your own.
So if the load varies frequently, the solar power plant will be completely shut for most of the day time. To use RPR as reveres power protection successfully, you will be required and good manual monitoring on the load demands.
Also, the use of RPS will increase ROI [return of the investment] time for grid-connected solar power plant. Because it continuously switching on and off the solar power plant.
Export Limiter (a.k.a Zero Export Device)
The Export limiter also is known as Zero export device or solar export limiter
Export limiter is a programmable controller with set points, which can detect reverse power flow (from home to the grid) and control the power generation of the solar power plant.
When export limiter detects reveres power [solar power generation > Load demand] it ramps down the power generation of the grid-connected PV plant.
To match, load demand = solar power generation
And when the load increases, the export limiter will ramp up the solar power production accordingly.
As long as solar power generation is lower the load demand, export limiter won't affect any change is solar power generation. But it keeps monitoring the system.
Setpoints of Export limiter can be set between 0% to 100%. Where 5% means that the minimum 5% of load will be served by the grid.
If the frequency of load variation is very high, we need to fix the setpoint above 0%. the suitable setpoints are 5%, 10% and 15%.
Use of PLC as an export limiter
PLC (like Siemens, Rockwell etc) can also be used as Export limiter, but the problem is that they are like 3 times expensive than export limiter and required experience in PLC to handle.
Similarly, spare cost of PLC is much more expensive than the export limiter.
Use of Mini PLC as RPR
Mini PLCs are less expensive than PLC and they can also be used as a reverse power relay. You will require one multi-function meter for the load measurement at the grid connection point. And one contactor for isolation solar power plant from load/grid.
Mini communicate with MFM via LAN cable. And you can program mini PLC in such a way, that if any reverse power flow is detected it [mini PLC] will send a trigger signal contactor to open it.
By opening the contactor, the solar power plant will be isolated from grid/load.
Closing of contactor can either be manual or timer in the program can used to close the contactor after some time.
Mini PLC's are much more expensive than RPR, but they are slightly more reliable than RPR.
With Mini PLC RIO of the solar power plant will slightly better than RPR, but you won't get much improvement in overall operation for the plant.
RPR are the cheapest solution, but also the most unreliable solution for reverse power protection in a grid-connected solar power plant.
Mini PLC is somewhat better than RPR but still, the ROI of solar plant will be too much higher than you expected.
Since most of the reputed companies didn't make Mini PLC, it's hard to select the best Mini PLC for your PV project.
Export limiter and PLC both are reliable solutions for reverse power protection in a grid-connected solar power plant. But PLC's are 3 times expensive than an export limiter.
Export limiter has an inbuilt remote monitoring system, so it also saves the cost of a remote monitoring system for a solar power plant.
If you are looking for reveres power protection for your grid-connected solar power plants. Then Export limiters are the best option available in the solar market.
We make a whole page on “Export limiter (a.k.a Zero Export Device)” go and checkout and it will help you will be able to select the correct solution for your need.
Use our below Export Limiter simulator to understand the function of Export Limiter
The first slider represents the amount of PV panels can generate.
The second slider represents the load demand
The third slider represents the set point of Expoler Limiter
You can vary all these three variables and you will get
Output power at Solar inverter
And Load share by grid
Here are some screenshots of Remote monitoring system