Electric vehicles that can transfer power in reverse are commonly known as vehicle-to-grid (V2G) or vehicle-to-building (V2B) systems. These configurations enable power from the EV’s battery to be transferred back into the grid or into a building. In Australia, for such systems to be legally installed and utilized, they need to meet the standards of AS/NZS 4777.2:2020 as inverter energy systems (IES). In essence, a V2G or V2B arrangement is an inverter of a special kind and must be accredited and registered with the Clean Energy Council (CEC) under one of three categories: supplementary supply, alternative supply, or independent supply. The principal user of energy, here, is the electric vehicle.

With the publication of AS/NZS 4777.2:2020 and a revision of it in 2024, there has been a significant clarification. Electric Vehicle Supply Equipment (EVSE) for reverse power transfer is no longer classified as a battery energy storage system. Therefore, the equipment does not require compliance with the battery-specific safety standards as set out in AS/NZS 5139:2019. This adjustment is indicative of the special nature of EV-based systems and offers a clearer regulatory framework under which they can be integrated into electrical networks. For additional clarification, individuals who work with single-phase inverters on multi-phase connections are referred to FAQ 11.1, which provides information regarding phase balancing requirements.

Defining Supply Types in AS/NZS 4777.1:2024

The revised AS/NZS 4777.1 standard has brought about four different types of inverter supply: supplementary, alternative, substitute, and independent supply. These categories assist electrical professionals in knowing when inverters are regarded as grid-connected and when they need to comply with certain compliance and approval requirements, such as those of Distributed Network Service Providers (DNSPs). All four types contribute to safe inverter operation, particularly when inverters are feeding circuits that are not necessarily connected to the grid.

A supplementary supply is an inverter that is typically grid-connected and will supply electricity to an installation in addition to grid power. Most residential solar installations are set up this way. A different supply, however, is intended to supply electricity only if in isolation from the grid. It exists in two forms: one is grid-connected but only powers up via a special port when the grid is down; the other is always isolated from the grid and manually switched when required.

A standby supply is different from the other two in that it supplies power through a special socket outlet that is electrically isolated from the main installation. This type will only have an upper current rating of 15 amps and should not be expected to supply the entire installation. Lastly, a separate supply is an inverter that is connected to the grid but complies with a set of special provisions under Clause 3.4.4 and Appendix M of AS/NZS 4777.2:2020. Such inverters do not have the capacity to support basic grid functions like voltage regulation or power quality control, i.e., they are unable to supply electricity back into the grid.

DNSPs need advance approval for the majority of connections of inverters that are in the supplementary, alternative, or independent supply type. As of the present, no independent supply inverters have been approved for connection on the Energex or Ergon Energy networks. Any future effort to connect such a device must follow a negotiated application.

Understanding Inverter Power Sharing Devices (IPSDs)

An Inverter Power Sharing Device (IPSD) is a device that enables one or more inverters to share their ancillary supply with several electrical installations. This is especially convenient in environments where a cluster of buildings or units want to share power that comes from an in-common inverter system. In Queensland, the DNSPs—Energex and Ergon Energy—enable IPSDs to be conducted under one embedded generation agreement.

It is only the principal retail customer who may apply for an IPSD connection. Nevertheless, they should initially obtain consent from all the other parties that are going to be sharing the inverter’s supply. Additionally, all applications under IPSD have to be done by a Registered Professional Engineer of Queensland (RPEQ) and accompanied by a Design Compliance Report (DCR) and Commissioning Report (CR), as advised by FAQ 5.

In the physical installation of an IPSD, there are various technical conditions that need to be satisfied. Any IPSD will have to meet the fundamental installation safety requirements as outlined in AS/NZS 4777.1. This is inclusive of compulsory interface protection if the total connected inverter capacity is greater than 30 kVA. Significantly, IPSDs are not allowed to be combined with alternative, substitute, or independent supply types. Design and commissioning by an RPEQ are required for all installations to achieve safety and standards compliance with local laws.

Interface Protection for Embedded Networks

Embedded networks that have a low-voltage connection to the DNSP and are bulk metered, like multi-dwelling units, have further interface protection rules. For residential properties that fall within Class 1a of the National Construction Code, interface protection is not required when installing an IES. But if the aggregate connected capacity of all IES units at the site is equal to or greater than 200 kVA, then a single interface protection device must be fitted. The device must meet DNSP standards and be fitted downstream of the site main switch but upstream of all inverters. The same applies to installations with rotating electrical machines.

Applying Generation Limit Control in V2G and V2B Systems

Generation limit control is an inverter feature enabling it to limit the level of active power delivered to ensure it remains below a defined threshold. It has no influence on the inverter nameplate rating nor the inverter maximum apparent power handling. It will only govern how much power can be transferred into the grid or into an installation at a specified moment in time.

This form of control is particularly useful in situations where technical limitations or regulatory boundaries call for precise management of energy. For instance, it can facilitate a single-phase V2B or V2G inverter system, which is rated below 50 kVA, in fulfilling phase-balancing requirements in a multi-phase connection. It is also utilized in certain high-voltage connections, or under special circumstances such as remote villages supplied by Ergon Energy’s isolated power stations, where stability of the grid is paramount.

While generation limit control can be applied in other situations, note that it should not be employed as a method of downgrading the rated capacity of the inverter. That is, the utilization of this feature does not decrease the technical rating of the inverter as far as the DNSPs are concerned. All inverters will still need to be tested and certified to AS/NZS 4777.2 compliance by a testing laboratory authorized by the CEC, which is registered on the CEC, and installed in accordance with AS/NZS 4777.1.

Voltage Rise Calculations and Compliance

Voltage rise is an important factor in any distributed energy system, especially when connected to the grid. Clause 3.3.3 of AS/NZS 4777.1:2024 dictates how voltage rise is to be calculated during IES installation design. There are currently three acceptable methods: full nameplate rating, export limited, and generation limited.

The full nameplate rating approach must be employed if the DNSP has approved full export from the system. In dynamic systems, this approach is also mandatory unless the DNSP issues specific alternative instructions. Export-limited calculations can be applied only where the system has been assigned a fixed, non-dynamic export limit. Designers should remember that the DNSP can change this later, and therefore future-proofing the calculation is advisable.

The generation-limited approach is used in systems where the DNSP has authorized the application of generation limit control, for example, in some V2G connections. This approach guarantees that voltage rise is within safe and authorized levels even when the inverter does not export at its rated capacity.