Commercial solar grid protection

Wednesday, October 28, 2020

Step by step

In this blog we will be looking at the grid protection requirements for commercial solar systems systems over 30 kW on the AC side. We will also ask what is Primary Protection? What is Secondary Protection and when is Secondary Protection required? 

What is Primary Protection?

This is the first line of defence and is the protection settings programmed to each individual inverter in a system over 30 kW and is mandated by AS/NZ 4777.1 (Australia). Other countries will have their own applicable regulations concerning the protection. Even though this is applicable Australia wide, each DNSP (Distribution Network Service Provider) may have additional requirements.

What is Secondary Protection?

So we have looked at the first line of defence so now the backup. Secondary Protection is a term used to describe the specific protection between the solar inverter(s) and the grid.

Effectively, this is a “Plan B” after the Primary Protection offered by the inverters, and consists of various components inside an enclosure designed to protect the grid.


When is Secondary Protection required?

Since July 2017, all systems with a combined inverter capacity of over 30 kW* need secondary protection and this applies Australia wide, to all DNSP’s.

*Note, the protection is for the AC side, NOT the DC side; thus solar capacity can exceed 30 kW with no Secondary Protection required in some circumstances. 

What does Secondary Protection actually do?

It shuts down the flow of energy from the inverters to the grid and the main component is a relay which sends a signal to open a contactor, hence interrupting the flow of energy to the grid.

The relay responds to pre-programmed settings that are triggered by certain conditions.


What components are inside?

A basic secondary protection enclosure contains the following major components:

  • Inverter circuit breakers
  • AC contactor and main switch or motorised circuit breaker
  • Protection relay
  • Power supply for the relay
  • Cabling
  • Test links

Inverter circuit breakers

As an example, let's say the system has been designed with three Fronius Eco 27 kW inverters.

We will need three corresponding circuit breakers in the Secondary Protection Board, say 50A each and as the designer/installer you select these, and they are included in the whole package. 

AC contactor/motorised CB

Continuing our hypothetical system with three Fronius Eco 27 kW inverters:

The AC contactor is sized according to the combined inverter capacity on the AC side as it is the main switch.

On bigger systems the AC contactor and the main switch is replaced with a motorised CB, all-in-one.

For example, a system with 2 x 50 kW inverters would require a motorised CB to cope with the 144A/phase of the output of the inverters in question.


The relay is the the brains behind the protection board. Commonly used brands include: 

  • MainsPro
  • Tele
  • Woodward 

Some distributors only accept certain brands and we use all three depending on the customer’s requirements.

Triggers for the relay

Now the relay needs to respond to certain triggers that have been stipulated primarily through AS4777, though the respective DNSP may demand additional triggers. 

The triggers include:

  • Sustained over voltage 
  • Over voltage
  • Under voltage
  • Over frequency
  • Under frequency

For example, in addition to the above, Ausgrid stipulates Rate of Change of Frequency (ROCOF), and Vector Shift (VS), also be included.


Where are they installed?

The Secondary Protection units are installed between the inverter(s) and the Main Switchboard, between the inverter(s) and a Distribution Board

Note: some DNSP’s only accept connection to the MSB (Main Switch Board).


Commercial solar systems over 30 kW on the AC side, Australia wide, need secondary protection which is effectively plan B after the inverter’s primary protection.

Depending on the combined AC capacity of the system in question, a motorised circuit breaker can be used instead of a contactor and main switch. The relay is the brains of the secondary protection and responds to certain triggers to stop the flow of energy from the inverter(s) to the grid. The secondary protection is usually installed between the inverters and the main switch board.

Greenwood Solutions manufactures off the shelf and custom boards and is happy to discuss your next project’s needs.

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