Connection of embedded wind generation

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Distribution utilities have an obligation to operate the electrical distribution networks in such a way as to provide power to their customers at an agreed quality. At present, power quality requirements are based on national standards although a common European position is emerging described in British Standard (1995b). However, it should be noted that this document describes minimum standards of supply which a customer may expect and is not directly applicable to the connection of embedded generation.

The main parameters relevant to the connection of wind turbines are:

slow (or steady-state) voltage variations, rapid voltage changes (leading to flicker), waveform distortion (i.e., harmonics), voltage unbalance (i.e., negative phase sequence voltage), and transient voltage variations (i.e., dips and sags).

Therefore consideration of whether a wind generation scheme may be connected to a distribution circuit is based on its impact on other users of the network. Similar considerations apply to the connection of any load. The steady-state voltage variations are generally considered assuming conditions of minimum network load with maximum generation and maximum network load with minimum generation. These are rather onerous and conservative assumptions to ensure that steady-state voltage limits will never be violated. In some cases, local agreements have been reached between the network and generator operators so that some wind turbines are constrained off at times of low network load or abnormal network conditions. This then allows a larger wind farm to be connected than would otherwise be the case. Considerations of power quality (i.e., maximum instantaneous real and apparent power, reactive power, voltage flicker and harmonics) were considered recently by an IEC Working Party and a draft standard has now been issued (IEC, 2000b).

For wind farms of relatively large capacity connected to weak distribution networks the calculations required can be involved. Hence some countries have adopted an assessment approach to allowing connection based on the ratio of wind farm capacity (in MW) to symmetrical short-circuit level, without the wind farm connected (in MVA). This is sometimes known as the short-circuit ratio. Typical values chosen range from 2 percent-5 percent based on studies and on experience (Gardner, 1996). However, such simple rules can be too restrictive and lead to refusal of permission to connect or to excessive reinforcement of the distribution system. Table 10.5 gives data on two large wind farms in successful commercial operation in the UK with much higher ratios of wind farm capacity to short-circuit level. Both wind farms have operated successfully for some years but it should be noted that the number of turbines on each site is large and so the impact of any individual machine is small. Also, both sites are connected to 33 kV circuits from which no other customers are supplied directly.

Figure 10.11 shows the calculated variation of voltage with wind farm output at one site. The generator voltage of the turbines was 690 V and each turbine had a local 690/33 kV transformer. The 33 kV curve refers to the voltage at the point of connection of the wind farm to the public distribution network. With zero windfarm output the system voltage is close to nominal as the distribution network is lightly loaded. As the wind farm output increases the voltage rises but, once rated power is reached, then the reactive power drawn by the generators increases rapidly and so the voltage drops. At 140 percent output power the load flow calculation fails to converge indicating that the network voltage is likely to collapse.

Table 10.5 Ratio of Site Capacity to Connection Short-circuit Level for Two Large Wind Farms

Site

Number of

Voltage of

Short circuit level

Ratio of site capacity

capacity

turbines

connection (kV)

of connection

to short-circuit

(MW)

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Renewable Energy 101

Renewable Energy 101

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable. The usage of renewable energy sources is very important when considering the sustainability of the existing energy usage of the world. While there is currently an abundance of non-renewable energy sources, such as nuclear fuels, these energy sources are depleting. In addition to being a non-renewable supply, the non-renewable energy sources release emissions into the air, which has an adverse effect on the environment.

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