The two main issues of grid connection are:
- Conflicting technical requirements between a project and the existing infrastructure
- Potential cost penalties that the distribution network service providers (DNSP) may incur for enabling the connection.
Drawing from experience in Europe, grid connection appears to be much more of an issue in Australia than in other parts of the world. For one thing, Australia has a relatively small population that is spread out over massive distances, and as a result Australia’s electricity infrastructure is already pushed to its limits.
In addition, embedded or distributed generation is relatively new territory for Australian DNSP, and people are slightly concerned that this might damage or break the grid. To be frank, it won’t, but caution is still being adopted to avoid serious consequences.Article continues below…
Issues with grid connection
One of the main issues with grid connection is to do with voltage rise. When electricity leaves the source (generally from a transformer on the distribution network), it is at a set voltage, and this voltage fluctuates depending on how much electricity is being used at any one time. Factors such as the size and length of the street asset cables also affect voltage drop.
For example, energy might leave the transformer at 250 volts (V) but only be 230 V by the time it reaches a project site.
This phenomenon means that the further away the project is from the source, the lower the voltage will be at a premises. The system has been designed to accommodate this reduction in voltage, by adding what is known as taps to transformers. Taps will raise or lower the supply voltage depending on how close (or far away) a premise is from the source.
A solar inverter pushes electricity back out to the grid by slightly raising its voltage above the source grid voltage. The result of this is the further away the transformer, the higher the voltage drop and thus the inverter has to increase its voltage to push the energy back out to the grid. This phenomenon is what is called ‘voltage rise’, and is one of the main concerns to distribution companies regarding solar photovoltaic (PV) installations.
Getting large-scale projects grid connected
If DNSP are not supplying electricity at prescribed voltage limits they can be penalised by the energy ombudsman. In the case of a large solar PV system on a long high-voltage distribution feeder, it can push voltages above these prescribed limits causing voltage issues for the other properties also supplied from this feeder. Subsequently, there is a disincentive for DNSP to allow connection to their grid as it will affect the quality of the ‘product’ they supply, a reasonable concern for a commercial operation. This phenomenon is of particular concern with large scale solar PV installations in rural areas that are connecting onto long rural distribution lines.
One solution is to use a ‘mains protection relay’, which provides a single programmable point of protection interface between a solar installation and the grid. This interface protection ensures that if the grid goes out of specification (i.e. over voltage or over frequency) the solar system is isolated from the network. This protects the grid and the large-scale solar investment by ensuring that the system is isolated from electricity that is outside prescribed limits.
Connecting in the UK
Whilst working in the United Kingdom (UK) I was involved in designing and delivering a number of large-scale solar PV plants. The British have a very formulaic and prescribed method for grid connection of any embedded generation over 50 kilowatts (kW). This method relies on a document entitled Engineering Recommendation G59/2. This document applies to all embedded generation types. The standard covers the electrical limits at which the generator should disconnect from the network.
For example, if the grid goes over voltage to a level greater than 253 V, then the generator will disconnect. Furthermore, if the grid goes under frequency to less than 47.5 hertz then the generator will disconnect. There are also appropriate disconnection times and two stages of values. This central guideline has enabled British engineers working within the distribution companies to be confident that if they follow this method the grid will remain healthy.
In the UK, once a large solar installation is finalised it is only connected to the grid after commissioning has been witness tested by a representative of the DNSP. The commissioning involves simulating a loss of mains, over/under voltage, over/under frequency and vector shift signal to the relay and measuring the disconnection time. The DNSP provides the parameters at which the relay should be set to, and these are generally based on the G59/2 values.
Guiding embedded generation
Australia seems to lack an overarching guideline for the connection of embedded generation. There is, however, great promise with the current Energy Networks Association (ENA) document entitled ‘ENA Guideline for the preparation of documentation for connection of Embedded Generation within distribution networks’. This document contains technical requirements and explains the process of getting your installation connected to the network. The DNSP themselves are also getting up to speed with current technologies and are going through a process of developing standard application forms for large-scale solar plants. As a minimum the following should be provided to your DNSP to allow them to process your application as efficiently as possible:
- DNSP–specific application forms with site plan. The site plan should include where you intend to connect and any on-site transformers or substation you intend to have
- Inverter make/model number and any other power conditioning equipment
- Inverter electrical details (including maximum power output, power factor, direct- current injection, prospective fault current)
- Transformer ratings and specifications
- A single line schematic diagram showing all relevant features including protection schemes, circuit protection devices and earthing arrangements.
Contacting the DNSP as soon as possible is essential when tackling issues to do with grid connection. Voltage rise is just one of the key points that DNSP are concerned about. If you are planning a large-scale solar installation, talk to experienced engineers about the full suite of potential issues you may face, in order to ensure a successful project. From an engineering perspective it makes sense for Australia to adopt a G59/2 standard that outlines the grid connection parameters and disconnection requirements.
It is also worth remembering the electricity network is a shared resource, we all rely on the electricity it distributes, and by knowing the issues and working with a solid set of grid connection parameters we can have a healthy grid full of solar PV.