Factors Limiting Integration
Given the current trends in wind turbine generator technology noted above, it can be seen that some of the key factors thought to be limiting wind energy integration are already being addressed. However, there are still a variety of issues that are relevant in quantifying the limitations on wind generated electricity integration. These are as follows (each being discussed in further detail below):
Frequency Management
The electricity supply system normally operates at a more or less constant frequency of 50 Hz. If frequency moves outside normal safe limits then damage may occur to some generators or loads, and hence any event that causes frequency changes must be quickly corrected.
Frequency management takes place at three levels, each with its own requirements and issues. Frequency changes occur when the difference between generation and demand changes. So if demand is constant and generation increases then frequency will increase, and vice versa.
In each of the North and South Islands, there is currently one generating station that is designated in each half hour as the "frequency regulating station". It constantly regulates its output at a rapid rate to maintain the frequency within the limits of 49.8 Hz and 50.2 Hz. High levels of wind integration potentially cause large, rapid swings in generation which in turn can create rapid frequency changes, possibly to the point of being too much for the regulating station to cope with. As wind integration increases the performance of the frequency regulating stations will be monitored closely.
Large, rapid and potentially damaging drops in frequency can occur, though infrequently, when a large amount of generation is suddenly lost to the electricity supply system, e.g. a large generating station malfunctions and disconnects from the grid. This possibility requires the SO to ensure that prudent levels of reserve generation are always connected to the grid and able to increase their output instantaneously.4
How far the frequency falls when generation is lost is highly dependent on the physical characteristics of all generators connected and running at that time. While some modern wind farms are capable of providing so-called "instantaneous reserves" while the wind blows, a more important consideration is how individual wind turbine generators assist in slowing the rate at which frequency falls. Not all wind turbine generators are capable of contributing to the restoration of frequency to 50 Hz in the same way that conventional generators are. Therefore, high levels of wind integration will require a detailed understanding of how wind turbine generators influence the frequency after generation is unexpectedly lost.
Short Term Variation in Wind Farm Output
Large swings in wind farm output over periods ranging from minutes to hours require commensurate changes in the output of other generation to ensure that demand is always met. This other plant must be available and be able to make large output changes, either individually or in aggregate. We are fortunate in New Zealand to have a large amount of hydro-electric generating plant which is ideally suited to this role.
Large swings may also increase the frequency of occasions when wires on the grid reach or exceed their safe operating limits, requiring greater efforts on the part of the SO, or other measures, to manage the security of the grid.
Generation Scheduling
In addition to very short term uncertainty over wind farm output, there is even greater uncertainty about wind farm output extending from about 3 hours ahead out to the next day In New Zealand we have large coal and gas fired generating stations that can take several hours to start from cold. The potential of large scale wind energy to increase the uncertainty around the need to start these stations is considerable.
Planning ahead to ensure capacity is available from real-time out to the next day, therefore, requires the development of suitable methods of forecasting wind farm output to assist in this process.
Clustering of Wind Farms
When the wind blows it does not blow equally at all locations across the country. However, clusters of wind farms within small geographical regions on the grid will tend to increase or decrease their output together, potentially creating large swings in their collective generation.
A significant cluster of wind farms is developing in the Manawatu and currently includes the Tararua wind farm and the Te Apiti wind farm. In the next few years it will also probably include the Te Rere Hau wind farm and an expansion of the Tararua wind farm. The evidence to date is that this wind region is already creating large swings in generation output which are causing the System Operator, Transpower, to review its operating procedures and the capacity of the nearby lines on the grid.5
On the other hand, if wind farms are geographically dispersed in future then swings in the combined output of all wind farms will be minimised and the limits on wind energy integration will potentially be higher than they would be if wind generation develops in one or two clusters in each island.
As wind turbine technology develops, control functions that can also limit the magnitude of swings in wind farm output will increasingly become standard issue.
Development of Standards
Looking ahead there is a high likelihood that there will be continued investment in wind-generated electricity in New Zealand - it is already the fastest growing sector of the generation market. In anticipation of this, the adoption or development of appropriate standards for wind farms connecting to the grid is a matter of increasing importance. Progress has already been made in New Zealand in updating the Electricity Governance Rules (EGRs), by which electricity sector participants must abide, to accommodate the particular characteristics of wind farms into the wholesale electricity market.
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