Gas Fired Back-up Power - Back-up power for wind

Utilities are increasingly faced with the challenge of incorporating intermittent generation such as wind and solar into the grid. It is a challenge that is particularly significant in the USA, which installed around 6000 MW of new wind power generation in 2009

This growth in wind power is resulting in an increasing number of flexible generating plants based on reciprocating gas engines, which are able to provide support or ‘wind firming’ and other ancillary services in addition to the traditional intermediate or peak power production.

WIND FIRMING

Xcel Energy is a USA utility that operates across eight states. The utility has 3.4 million electricity customers supplied by a mix of generating assets. According to the American Wind Energy Association (AWEA), the utility is the largest investor-owned wind power provider in the US, with more than 3000 MW of wind power generating capacity within its eight-state service territory – a figure it is planning to increase to 7000 MW by 2020.

Such a massive amount of wind has seen the utility employ flexible generating plants. In Colorado, for example, Xcel Energy has more than 50 power generating sites operated by its subsidiary Public Service of Colorado (PSCo), but it still had a need for more flexible sources of generation in its system.

Cogentrix, a large US-based independent power producer, therefore commissioned a power plant in Arvada to deliver power and ancillary services under a long-term contract with PSCo. Known as Plains End I, the 111 MW plant began commercial operation in 2002.

The value of this plant was confirmed when a similar order for a second plant was awarded in 2006. Plains End II is located adjacent to Plains End I. This 115.6 MW plant, which uses 14 Wärtsilä 20V34SG engines, began commercial operation just over 12 months ago.

PSCo does not operate in a deregulated market, but it does have a control area in which it needs to balance load and supply. This control area has to have sufficient spinning reserve. When PSCo needs power quickly, the Plains End plants are started and operated until a baseload plant can be brought online. Since their start-up, Plains End I and II have proved their worth. It may be hard for some utilities to justify investing in a reciprocating engine plant purely for the purpose of wind firming.

Plants like Plains End I and II are therefore purchased with the aim of providing a combination of services. They are used to provide peaking or intermediate generation, as well as system support and firming of any form of intermittent generation. Plains End I and II can be operated according to the wind output. As a flexible, quick and generating assets in the PSCo system, Plains End I is used to balance the power dispatched as wind power production varies.

When the load reduces in the evening and at night, flexible assets are dropped off in favour of less flexible baseload assets such as coal. However, the wind typically blows at these times, essentially hitting at the worst time of the day in terms of system stability. This is where flexible reciprocating engine-based plants can be useful.

These flexible gas engine plants can start-up in ten minutes to provide non-spinning reserve, and could in the future be tuned to start up in as fast as five minutes, if the requirements for this product change.

In addition to the ten minute non-spinning reserve, there is also a need for plants that react immediately to varying market demand. Therefore some plants have to be kept running at partial load to provide spinning reserve. If, for example, a plant is kept ‘spinning’ at 50 per cent load, this power can be sold. But the capability to quickly ramp-up to provide an additional, for example, 30 per cent capacity can be sold in the ancillary service market. Plains End II was primarily installed to provide generating capacity and black start capabilities to the local grid and has the capability to provide all of these generation requirements.


There is a great deal of work being undertaken by the North American Electric Reliability Corporation (NERC) and other groups in the USA on how to integrate variable generation into the grid.

The flexibility of reciprocating engine-based plants with multiple units is extremely useful for wind integration. Plants have been sold recently in areas of the USA where there is massive wind power build. Texas, for example, currently has around 8-9 GW of wind power capacity.

In addition to being variable, the best wind resources are often far away from the load. This means long transmission lines have to be built from the wind farms to the urban areas in central and eastern Texas.

This is not cost-effective for a wind farm that has a relatively low load factor and will not utilize the transmission system fully. A wind firming plant can improve cost-effectiveness by using this transmission capacity when the wind plants are not running.

When integrating wind into a grid, the first thing to do is to aggregate it over the largest possible area. In ERCOT for example, the entire system is operated as a single entity where the wind can be aggregated over a large area. This makes balancing the system much easier because the net variability is reduced.

However, there are still occasional large ramps especially at times of minimum load at nights when large baseload plants are running. Since these large plants can only be turned down to 50 per cent, at best, of their maximum output, there is the possibility that any excess power from wind would have to be wasted. A flexible asset, which can be turned on and off in response to these ramps, can be very useful in this scenario in enabling the full utilization of the wind energy.

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