Introduction to Integrating Renewable Technologies

Cables also have typical ratings for the current that they can carry. For three phase cables in air some typical ratings are:

Cross sectional area (A)                     Max Current Level (I_max)

2.5mm²                                     25A

10 mm²                                     60A

50 mm²                                     159A

So by increasing the voltage level of transmission, the current level is reduced and hence reduces the size of cable required, hence reduces the losses occurred in transmission.

Transmitting at such high voltage levels is only viable for transmitting large quantities of power over large distances and that is why there is a hierarchical system for power distribution. As the voltage levels are reduced, the power levels come down as we a sending the power to consumers who do not require large quantities of power.

The figure above shows the layout of the ‘Super grid’ or Transmission Network in Scotland. Within the system there are many distribution lines of lower voltages but these are too numerous to be shown on the map.

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Integrating Renewables 

All power that is received into residential homes has to have a good power quality.

“Good power quality in the UK is 230V +10%-6% @ 50Hz ± 0.5Hz with negligible harmonics, no voltage dips or spikes and no power cuts/outages.”

British Standard BS EN50160  ‘Voltage Characteristics of Electricity Supplied by Public Distribution Systems’.

 In order for renewable energy technologies to be connected to the National Grid they must be constrained to these governing rules of electricity transmission. When connecting any new energy source to the network the ‘strength’ of the network near to the location of the generation plant must be known. One way of defining the ‘strength’ of a network is by finding the fault level of the network. The fault level of a network is a measure of the flow of current that will occur when a fault develops on the network. A network with a high fault level is generally an interconnected network in an area like a city centre or large industrial area whereas a network with a low fault level is generally a long electrical circuit. In general the higher the value of the voltage transmission level the stronger the system.  

Most networks that are ‘weak’ or have a low fault level are the ones that have low voltage transmission lines. These low voltage lines (33kV and 11kV) are most abundant in rural or isolated areas – the location for most renewable energy sources.  

Integrating renewable energy sources such as hydro, biomass, EFW or coppicing are less of a problem than wind, solar, tidal or wave. Such technologies are not much different to that of traditional power stations where a constant output can be maintained and regulated. It is relatively straightforward to control the power output by controlling the quantities of fuel used to generate the electricity.  

The unreliability and intermittency of other renewables such as wind, solar can cause problems with  the power output from them variable and unpredictable (tidal and wave are predictable to a certain extent). So the problems occurring from them have to be considered. 

The electrical network was primarily designed to transmit power in a certain direction, from high voltage (400kV) lines down through the system to lower voltage (6.6kV) systems. If however a generation source is connected to a low voltage network, voltage problems may arise for to consumers near on the network. To ensure that the disturbance to consumers on the network is minimal the local public electricity supplier will require to be informed of the maximum continuous output from the source of generation. 

As mentioned above the power output from some renewables is constantly varying and unpredictable. The voltage variations caused by this continuous varying power output on the network is commonly known as ‘flicker’. Flicker is only usually a problem when the local network is weak and the voltage change is large. If a group of 50 wind turbines all producing 2.5MW at 0.69kV then the total power output would be 12.5MW so the ‘flicker’ disturbance would only be a small percentage of the output and then not really be a problem. On the other hand, one wind turbine would mean the flicker is a large percentage of the output and will be more of a problem. 

Harmonics can be created by generation of renewable energy. Variable speed wind turbines can cause harmonic voltage disturbances to appear on the network. Harmonics on a network can create problems, as they will disturb consumers by causing some connected equipment to overheat or malfunction. 

As mentioned before there are different values of voltages for different transmission and distribution lines and they all have their own rating. So each line has a maximum power they can safely transport. This value must be found from the local public electricity supplier before much work on a renewable energy project is done. The value will determine the maximum power output from the renewable source that can be directly connected without having to ‘strengthen’ the network. 

All these factors must be investigated when considering connecting a renewable energy source to the national grid. The public electricity supplier should be consulted at an early stage as they can provide valuable information about the feasibility and cost of such a project. In order to connect to the grid there is a six-stage process. 

• Notify – The public electricity supplier should be notified about any plans and should be given an outline of the proposition. This is needed to help identify the feasibility of the project.

• Technical Submission – The submission of a more detailed technical information is required by the Public Electricity Supplier (PES) who may conduct further design studies before a decision can be made to proceed with the project. 

• Connection, Planning Quotations - Once details have been agreed the PES will make a formal connection quotation. Similarly if a new connection is required, planning permission and rights of way must be obtained. Again this is usually done by the PES, sometimes with assistance from the developer. The project proposers must, of course, also obtain planning permission for the wind turbines, site roads and buildings and other structures. 

• Connection, Supply and Meter Operator Agreements – Agreements must be made between the PES and the project operator, and include all aspects of the interface between the wind farm and the grid, including protection, earthing and metering. These agreements outline the limits of responsibility of both parties, and charges for exporting and importing electricity. For very large projects, a generation licence is required, which is obtained from the Office of Electricity Regulation (OFFER), it is best to seek further information from them to understand the costs and benefits of this option. 

• Costs – The cost of a project must be given careful consideration and can vary from scheme to scheme. Variations can arise due to the required voltage level for connection and also for the equipment required at the point of connection to the transmission network. There are many costs involved for PES equipment, and the switchgear, cables, transformers and other equipment within system. On large wind farms consideration may also be needed for reinforcing the network at remote locations. Other costs include capitalised charges to cover future O&M (operation and maintenance) of the PES equipment provided specifically for the project.  

• Design - The design of the connection to the electricity network requires an experienced electrical engineer, which could be provided by the wind turbine manufacturer, an electrical contractor, or an independent consultant. These professionals can design the electrical connections required, both within the system and the grid while also negotiate technical decisions with the PES, for example, on the appropriate connection voltage. Considerations must be given to connection voltage, as this can significantly effect on the cost of the connection. All design decisions have to be made with the aim of reducing electrical losses and capital cost, while ensuring that the system is safe and reliable. It is often worth spending more money initially on an efficient electrical design which, by reducing losses, will increase the total energy output.

Although the details may appear complex, the process of connecting wind farms, and other renewables, to the grid is now well established. The growing numbers of grid-connected renewables in the Scotland, and the UK, are proof of the benefits of harnessing nature’s energy. 

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Distributed Generation (Embedded Generation)

Distributed generation is an electricity generation source that is connected to the distribution network rather than the high voltage transmission network. Distributed generation projects are generally smaller scale generation projects and renewable energy generation. Obviously large-scale renewable generation occurs but commonly the renewable generation is on a smaller scale. 

As previously mentioned the network of power lines throughout Scotland operate so that the power is transferred down from the high voltage transmission network through the distribution network to the consumers.
 

Distributed generation alters the topology of the network as we are introducing generation capacity into the distribution network.

In this case distribution lines now must deal with flow both to and from the distribution network. Obviously wind farms and tidal generation stations are constructed to produce electrical flow into the network but they must also be able to receive power flow. The industrial and domestic customers must also be able to send and receive from the network as more and more people are generating their own electricity using PV facades or wind turbines, where any excess generation can be sold back to their local public electricity supplier.

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 Benefits of Distributed Generation

As distributed generation is connected to the distribution network rather than the transmission network it will be located closer to the consumers than large scale generation connected to the transmission network. Therefore the electricity is delivered in a more direct fashion than through the hierarchy of the network. This direct delivery implies that the power has to travel a shorter distance and so it will incur fewer losses through transmission. The power is also transmitted at a closer voltage level than on the transmission network so the cost of large transformers and equipment is gone. By generation closer to the consumers can also effect the environment because the higher percentage of power transmitted (and not ’lost’ through transmission) will reduce the overall amount generated electricity. In the event of a power cut in either the transmission network or distribution network, distributed generation could effectively supply the power demand for the local customers on the network. 

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Types of Generating Systems

If an individual user or small company decides to use renewable energy to provide some of their power there are some choices of what system to deploy. Typically most individual users will use either a PV system or wind turbine or combination of both to generate their power. Nowadays though farmers and people in agriculture are beginning to use biomass, EFW and coppicing to generate power. 

DC System - This type of system is generally not deployed unless for specific small supplies. PV systems provide a DC electricity output so can be connected easily, but most wind turbines use AC generators. The AC output from the turbine will have to put through a rectifier to convert it to DC. (A rectifier is a power electronics component used to convert AC into DC.) The typical use of this system is to store the power in a battery or battery bank. In order to do this a charge controller is required. This device controls the charge into the battery so that it does not overload and at worst ‘explode’. Once the devices are connected to the battery the DC devices that require the power can be connected and the system is complete.

In this case distribution lines now must deal with flow both to and from the distribution network. Obviously wind farms and tidal generation stations are constructed to produce electrical flow into the network but they must also be able to receive power flow. The industrial and domestic customers must also be able to send and receive from the network as more and more people are generating their own electricity using PV facades or wind turbines, where any excess generation can be sold back to their local public electricity supplier.

Benefits of Distributed Generation
As distributed generation is connected to the distribution network rather than the transmission network it will be located closer to the consumers than large scale generation connected to the transmission network. Therefore the electricity is delivered in a more direct fashion than through the hierarchy of the network. This direct delivery implies that the power has to travel a shorter distance and so it will incur fewer losses through transmission. The power is also transmitted at a closer voltage level than on the transmission network so the cost of large transformers and equipment is gone. By generation closer to the consumers can also effect the environment because the higher percentage of power transmitted (and not ’lost’ through transmission) will reduce the overall amount generated electricity. In the event of a power cut in either the transmission network or distribution network, distributed generation could effectively supply the power demand for the local customers on the network.
 

AC / DC System - This type of system employs the same technology as the DC system but also has the advantage of being able to power AC devices also. There are two outputs from the battery bank in this system, the DC output and the AC output. The AC output is initially a DC output but it is then put through an inverter to convert it into AC. (An inverter is a power electronics component that converts DC into AC – there are many different types of inverter available but an inverter that provides an output of 230V @ 50Hz is required) Once the DC output is inverted to AC it is ready to supply any devices.

AC System - This type of system is the same as the AC / DC system except we remove the DC output from the battery bank and supply everything through the inverted output.

These types of system can be considered as ‘stand alone’ systems as they are the only source of power for their applications. If these do not provide enough power there is not enough power available. 

Grid Interfaced System - This is an incremental step forward from the AC system. If the AC system is used to supply the power to all or part of a consumers demand it could become unreliable. Under certain conditions there may be a power deficiency from the AC system. By having a grid interface system the power supply can be switched from the AC system to the grid.

Grid Connected Systems - In this system the power form the grid is available at all times. Instead of using a battery bank to store the generated power the excess power generated is ‘sold’ back to the grid. In this case the public electricity supplier will connect some metering arrangement to measure the quantity of power that is sent into the network. Generally the price paid for power generated by a consumer will be a lot less than the price the consumer will pay for power supplied. Typically the public electricity supplier will use a two-meter system. One meter will measure power imported and the other meter will measure power exported. Alternatively this can be done on a single meter with two dials. One dial measure imported power and the other measures the exported power.

Generally companies do not employ net metering, where one meter is used to measure both imported and exported power. If power is imported the meter will count up and if power is exported the meter will count down. This is generally not employed in the UK as the public electricity supplier is effectively paying the same price as the consumer for power. 

The output from the wind turbine is still rectified to DC and then inverted to AC as the inverter will ensure that the output is 230V @ 50 Hz. The inverter acts as a device for controlling the power quality of the system. 

If a variable speed wind turbine is used the power output and frequency of that output are completely dependant upon the wind speed. So the frequency can vary constantly which is not suitable for the power network. This is where the rectifier and inverter come into play where the output is controlled to provide 230 V @50Hz. 

Engineering recommendations exist that have the governing rules that must be employed when using distributed generation. 

Engineering Recommendation G59 – recommendations for the connection of embedded generating plant to the regional electricity companies’ distribution system.

Engineering Recommendation G77 – UK technical guidelines for inverter connected single phase photovoltaic (PV) generators up to 5kVa.

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