Grid connection

Authors: Craig Memery, Brett Dutton

Contributors: Alicia Webb, Alex Pollock

Investigating and designing the grid connection for a wind farm is an expensive process. It can also take a long time, so it's worth prioritising this along with wind monitoring and energy resource assessment. This article discusses some of the major milestones and technical hurdles you'll encounter trying to connect your wind farm into the electrical grid.

The electrical grid

When we talk about electricity networks, there are two main categories — transmission networks and distribution networks.

The transmission network

Transmission networks are designed to convey power over long distances and typically operate at very high voltages such as 132 kV, 225 kV or 500 kV. Major power generators supply energy into these networks so it can be sent over long distances to lower voltage distribution networks. Transmission networks are controlled by statewide network operators such as SP Ausnet, Electranet or Synergy. They are often referred to as Transmission Network Service Providers (TNSPs), and they penetrate most areas of Australia. The energy on transmission networks is not 'ready to use' by the ordinary consumer because it's at such a high voltage.

The distribution network

Distribution networks are local networks that take power from transmission networks and transform it to a lower voltage so it can be more easily used. They penetrate deeper into the countryside or suburbs than the higher voltage network.

Distribution networks typically operate at 66 kV, 33 kV or 22 kV and are controlled by Distribution Network Service Providers (DNSP's) such as Powercor, CitiPower, SP AusNet, Jemena and United Energy (in Victoria). By the time power gets to your home from a major generation facility (like a coal-fired power station), it's been through transmission networks, distribution networks and then local networks. The voltage is reduced at each stage, until it reaches you at the familiar 240 V (having been reduced 1,000 to 2,000 times from its original transmission voltage).

Connecting to the network

If you intend to build and operate a community-scale renewable energy generation plant, you're likely to connect your facility to a distribution network rather than a transmission network. This is because you'll generate comparatively small amounts of power (5 to10 MW) as opposed to utility scale generation (100 to 500 MW).

Both the TNSPs and the DNSPs are bound by regulation to provide a certain quality of power to consumers. In order to ensure this, TNSPs and DNSPs have strict requirements which they impose on generators connecting to their networks. These requirements are often referred to as Technical Performance Obligations (TPOs), and their intent is to control new generator connection to their network, so the quality of supply to customers is not adversely affected.

Compliance to TPOs is mandatory. You will not be allowed to connect your wind farm unless you can show it will not adversely affect the quality of the power supply to consumers. This involves a highly specialised analysis, which needs to be done by a reputable consultancy. The analysis is sometimes referred to as 'Grid studies' or 'Connection assessment'.

Grid studies

Grid studies involve mathematical modeling of the effect your wind farm will have on the electrical network under different generating conditions. It's very important that the modeled analysis closely represents the real-life results. The DNSP relies on your modeling to ensure it will not compromise network quality or security. So you can expect that, as part of your development process, the DNSP will ask you to provide evidence that the actual performance of the finished project closely represents the modeled performance.

Milestones

Below are some of the major grid connection milestones. Before you even begin to work through them, you need to find out which distribution network service provider (DNSP) operates in your area. This should be straightforward to find online.

Once you know which company you're dealing with, you can start the discussions and investigations in the table below.

Grid connection milestone	Project stage
Initial discussions with distribution business	Pre-feasibility
Obtain estimate of grid connection cost	Feasibility
Technical investigation of grid connection and preparation of application to connect	During detailed assessment, planning and approvals
Obtain development approval for any new transmission line	During detailed assessment, planning and approvals
Submit application to connect, including payment of fee for detailed design (if required)	During planning and approvals
Receive Offer to Connect	During planning and approvals
Negotiate or appeal Offer to Connect (if required)	During planning and approvals
Receive amended Offer to Connect or determination by regulator (if negotiating or appealing offer to connect)	During planning and approvals
Decision to accept Offer to Connect	At final decision to proceed to build wind farm
Accept Offer to Connect	As soon as required to satisfy the needs and conditions of wind farm construction

Technical considerations

The technical issues you're most likely to have to consider or evaluate during your grid connection application, include:

Connection capacity of the network

You can usually find the rated capacity of substations in the appendices of reports, freely available on distributors' websites. These alone may help to indicate the upper limit of the capacity at a point of the network, which will, at best, only help to indicate the maximum geographical extent of network augmentation required to build a new wind farm.

Embedded generation capacity is awarded on a first-come, first-served basis. While this is a significant issue for larger scale wind farms which connect to the subtransmission or transmission networks, it's not likely to be an issue for community scale wind farms, which usually connect to the network at distribution voltages of up to 33 kV (22 kV in Victoria).

Point of connection

The connection point is where the distribution network (which the distributor or DNSP is responsible for building and operating) meets a generation installation (in this case, the wind farm). A connection for a community scale wind farm is most likely to be:

• at a distribution voltage of between 11 and 33 kV (22 kV in Victoria)
• located at or near a substation on the wind farm property
• near the distribution meter for the wind farm.

The connection point is the reference point for many power quality measurements.

Shallow and deep network augmentation

As well as the cost of all electrical works on the wind farm, you'll be responsible for some or all of the cost of building and extending the distribution network to bring the connection to the wind farm — even though the network infrastructure will effectively remain the property of the DNSP.

Shallow works are any new network components that are built solely to provide a point of connection for the new wind farm. They are dedicated, in that they are not shared with other customers at the time of construction.

Deep works are those carried out on any existing part of the network that's shared with other customers. The need for deep works is difficult to foresee, and you won't know the cost until you receive an 'Offer to Connect'.

Grid connection equipment

While individual projects will vary, most require similar equipment to connect to the grid.

Wind turbine transformer and switchgear

At the wind turbine, there is a transformer that steps up the voltage from the wind turbine generator (usually on the order of 690 to 1000 volts) to a "medium voltage" (MV) to carry power within the project. In a large wind farm, this medium voltage is usually 33kV. In a community project, this medium voltage would probably be matched to the voltage of the distribution system that is being connected into.

On the medium voltage side of the wind turbine transformer, there is usually a switchgear unit. This unit provides a means to disconnect the wind turbine from the electrical network and acts as a circuit breaker, opening if there is a short circuit somewhere. The wind turbine transformer and switchgear are sometimes supplied by the wind turbine manufacturer and sometimes by the project developer.

Collection system

From the wind turbine transformer, power is usually carried by underground cables, referred to as either the collection system or reticulation. (The collection system can also be run on overhead lines.) In a large wind farm, the underground cables will bring power from all the wind turbines to a common substation. For a community wind farm with only a few turbines, the collection system will be much simpler and might lead directly to the point of connection.

Interface equipment at the point of connection

The equipment at the interface between the wind farm and the electrical grid can vary widely depending on the project size, DNSP requirements, etc. There will be a clear delineation of responsibility between the DNSP's system and the wind farm's equipment.

In the simplest case, the underground cables would emerge from the ground, go up an electrical pole, and connect to the distribution network with fuses and a disconnect switch. Metering equipment will also go here. More complicated arrangements might require a small substation with switchgear, another transformer (to change to the distribution voltage), and communications equipment to communicate signals with the DNSP.

Loss factors

When power is transmitted over a signficant distance, some is lost due to electrical resistance and heating in the power lines. Over the whole eastern power grid, these losses are around 10%. In practical terms, this means that more electricity must be generated than is required at the load to allow for this loss during transportation.

If a community renewable energy facility generates power near a load (like a town) then that generation will offset the power that would have otherwise been produced at a large power station, as well as the power that would have been lost in transmission and distribution.

These avoided losses can increase the value of the power generated, so it's worth investigating the loss factors that will apply at your site.

Loss factors come in two types, Marginal Loss Factor or MLF which refers to losses in the transmission network, and Distribution Loss Factor or DLF which refers to losses in the distribution network.

More information on these can be found on the AEMO website.