4 Ways of reverse power flow protection in grid-connected PV systems

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 the 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 this is that a 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 exported power (reverse power) and sum them up.

Whereas the net-meter measures the input power from the grid and subtracts the surplus power injected back into the grid from an on-grid solar power plant.

Also if the PV plant owner is using DG (diesel generator) as a reference power source [in place of gird] for the on-grid (grid-tie) inverter. This excess power of the on-gird PV plant will damage the DG set or trip its breaker [if there is any breaker].

Related post: Why generators are rated in kVA

Do you want to learn how to synchronize a DG set with an 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 the PV plant from the grid by means of tripping the breaker or releasing the contactor if there is any reverse power detected.

#2 Use an Export limiter to limit the power generation of the grid-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 means 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 make of grid-tie solar inverters like ABB, SMA, Hitachi, Consul Neowatt, Huawei, Solar Edge, Kaco, Delta, Solis, Kirloskar, polycab, Sungrow, Growatt, Fronius, REFU Sol, Schneider, Zever solar and many more.

Solution#1

Reverse power relay for solar/ Reverse power relay protection for solar

Reverse power relay (RPR) sometimes also called Reverse power protection relay]

Reverse power relay (RPR) for solar is used to eliminate any power reverse back to gird from an on-grid (grid-tie) PV power plant to the grid or to the generator by tripping either on-grid solar inverter or breaker or any contactor depending upon the type of power distribution and a control circuit.

RPR (Reverse Power Relay) is connected at the coupling point of the grid and load [In simple words between energy meter and load].

When RPR senses 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 a solar inverter on your own.

So if the load varies frequently, the solar power plant will be completely shut for most of the daytime. 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 on the investment] time for grid-connected solar power plants. Because it continuously switches on and off the solar power plant.

Solution#2

Export Limiter (a.k.a Zero Export Device)

The Export limiter also is known as a Zero export device or solar export limiter

The 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 the 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

When the load increases, the export limiter will ramp up the solar power production accordingly.

As long as solar power generation is lower than the load demand, export limiters won't affect any change in solar power generation. But it keeps monitoring the system.

Setpoints of the Export limiter can be set between 0% to 100%. Where 5% means that the minimum 5% of the 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%.

Solution#3

Use of PLC as an export limiter

PLC (like Siemens, Rockwell, etc) can also be used as an Export limiter, but the problem is that they are like 3 times more expensive than export limiters and require experience in PLC to handle.

Similarly, the spare cost of PLC is much more expensive than the export limiter.

Solution#4

Use of Mini PLC as RPR

Mini PLCs are less expensive than PLCs 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 the load/grid.

Mini PLC communicates 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 the grid/load.

Closing of the contactor can either be manual or a timer in the program can be used to close the contactor after some time.

Mini PLCs are much more expensive than RPR, but they are slightly more reliable than RPR.

With Mini PLC RIO of the solar power plant will be slightly better than RPR, but you won't get much improvement in overall operation for the plant.

Conclusion:

1. RPR are the cheapest solution, but also the most unreliable solution for reverse power protection in a grid-connected solar power plant.

2. Mini PLC is somewhat better than RPR but still, the ROI of the solar plant will be too much higher than you expected.

3. Since most of the reputed companies didn't make Mini PLC, it's hard to select the best Mini PLC for your PV project.

4. 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.

5. The 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 needs.

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Here are some screenshots of the Remote monitoring system