21, rue d’Artois, F-75008 PARIS C1-108 CIGRE 2012 http : //www.cigre.org Te Uku Wind Farm – Planning and Operation of a Deeply Embedded Power Plant with Advanced Ancillary Services RAY W BROWN HAYDEN N SCOTT-DYE Meridian Energy Ltd Meridian Energy Ltd New Zealand New Zealand SUMMARY Meridian completed commissioning of the 64.4 MW Te Uku Wind Power Plant (WPP) in 2011. The WPP is embedded within a rural 33 kV distribution network and located approximately 25 km from the 220 kV Grid Exit Point (GXP). Before the WPP was developed, the 220 kV GXP voltage could move out of Grid Code requirements during transmission outages. Planning studies have shown that the WPP is able to reliably control voltage levels across the 33 kV distribution network and maintain the 220 kV GXP 220 kV voltage within Grid Code requirements during contingencies. The WPP is the largest deeply embedded wind farm in New Zealand. Integrating it with the 33 kV distribution network provided a number of challenges and these are discussed. New Zealand does not have subsidies for renewable energy power plants. It has a lightly regulated competitive electricity market with relatively low electricity prices. Environmental approvals allowed for a larger wind farm however the optimal commercial development of the WPP was required in order for its owner to achieve adequate returns in the electricity market. Various technical performance requirements were also placed on the WPP. Transmission integration options to address economic and technical requirements are discussed in the paper. The methodology and simulation studies performed to arrive at the optimal solution are described. The WPP uses full converter output wind turbine generators. The converters provide isolation between the power system and the turbine generator resulting in exceptional grid integration performance. The WPP can provide voltage support even when there is no wind, and modelling has shown that this can solve Grid Code non-compliant 220 kV voltage issues. The WPP is therefore able to provide voltage support ancillary services similar to a STATCON or a synchronous generator operating in synchronous condenser mode. New Zealand is an island system that has high penetration of renewables with over 70% of electricity supply coming from renewable sources. NZ’s energy strategy is to generate 90% of [email protected] NZ’s electricity from renewable energy by 2025. It is conceivable that within ten years, North Island wind generation levels may exceed 50% of North Island load during light load periods. As wind penetration grows, wind power plants will need to provide ancillary services that have traditionally been provided from other sources. Frequency related services have historically been provided from grid connected plant. The Te Uku WPP has governor systems and the performance and integration of these will be discussed. Instantaneous reserve performance from the wind farm in particular appears to be superior to conventional plant. The use of the wind farm to provide frequency support ancillary services is explored in the paper. In summary, this paper provides an overview of the electrical and grid integration design of Te Uku WPP, and also explores the ability of the WPP to provide ancillary services. Practical examples from simulations and commissioning tests are also shown. KEYWORDS Ancillary-Services, DER, Renewable-Energy-Resources, Wind-Power, Optimisation, Frequency- Response [email protected] 1 THE NEW ZEALAND POWER SYSTEM AND ANCILLARY SERVICES The NZ power system consists of two island power systems (the North Island and the South Island) connected by an HVDC (High Voltage Direct Current) link. The North Island’s electricity is supplied by many fuel sources, and approximately 1500 GWh p.a. is transferred north over the HVDC link during years when water is plentiful in the South Island. The South Island’s electricity generation is 100% renewable, coming from hydro generation and small WPPs (Wind Power Plants). Demand Demand Peak Generation Wind Capacity Capacity North Island 24,400 GWh 4,500 MW 5,652 MW 428 MW South Island 14,600 GWh 2,300 MW 3,408 MW 58 MW Total NZ 39,000 GWh 6,500 MW 9,060 MW 486 MW Table 1 - NZ Load and Generation 2009/2010 [1] Due to the low amounts of load and generation, the frequency can change quickly when generation or the HVDC link trips. The frequency range in the North Island is 47 Hz to 52 Hz. NZ’s wind energy resource is extensive because the country lies across the “Roaring 40s”. NZ WPPs generally operate at an average of 42% of maximum capacity, making them commercially competitive with other forms of electricity generation in NZ without subsidies. Wind energy is in its infancy in NZ with annual energy penetration having reached only approximately 4 %. With a NZ light load of approximately 3000 MW in summer, wind generation can reach levels up to approximately 20 % during brief periods in summer. The governments’ energy strategic goals have been to generate 90 % of NZ’s electricity from renewable energy by 2025. In 2010 the government introduced an Emissions Trading Scheme that will favour renewables over greenhouse gas-emitting power stations over time [2]. It is foreseeable that within 10 years, North Island wind generation levels may exceed 50 % of North Island load during light load, high wind periods. It is anticipated that during these periods some WPPs that are unable to provide ancillary services such as frequency and voltage support will not be dispatched and conventional gas, hydro or coal plants that are able to provide ancillary services will be dispatched instead. The revenue for owners of these undispatched WPPs will be reduced as wind penetration grows. In order to enable high wind penetration, WPPs will need to provide instantaneous reserves (IR) and frequency keeping services. IR and frequency keeping services earn revenue in the ancillary services market [3]. Governors have been installed in recent NZ WPPs [4] to enable them to provide IR and frequency keeping services. When it becomes economically optimal to use WPPs for frequency support services, market rules and mechanisms will be developed to enable WPPs to offer these services into the ancillary service market. The structure of the electricity market is a point of difference between NZ and many jurisdictions. NZ has full nodal pricing, security-constrained economic dispatch. There is a reserves market and commercial arrangements for ancillary services such as voltage support. 3 This paper provides an overview of the electrical and grid integration design of Te Uku WPP, a deeply embedded wind farm that has been designed to provide both voltage and frequency ancillary services to the transmission grid. 2 EXISTING TRANSMISSION SYSTEM DESCRIPTION The Te Uku WPP site is located approximately 20 km west of Hamilton in the North Island of New Zealand between the Raglan and Aotea Harbours. The WPP Te Uku covers an area close to 56 km 2 The Distribution Network Operator (DNO) in the region, obtained planning permits for up to 28 Wind Turbine Generators (WTGs) on the generation site which could have a capacity up to 3 MW each, providing for a potential wind farm capacity up to 84 MW. The site wind resource is Class IIB with a high average wind speed of 8.24 m/s at hub height. This provided potential for a generation capacity factor in the order of 40 %. Figure 1 – Te Uku Geographical Overview The centre of the site is approximately 20 km from the Te Kowhai 220 kV / 33 kV substation that supplies a large proportion of Waikato’s West Hamilton region through a single circuit 33 kV line which extended as far as the town of Raglan. The Te Kowhai offtake has generally averaged around 20 MW however it has peaked up to 90 MW during times of low existing embedded generation output (Te Rapa cogeneration) and the switching of load from other parts of the Hamilton distribution network. The nearest 33 kV line was approximately 8 km from the centre of the site and this single circuit line was the primary supply to Raglan. The Te Kowhai substation has two 100 MVA 220 kV / 33 kV transformers. It is connected to one circuit of the 220 kV double circuit Stratford – Taumarunui – Huntly line (see figure 2). This is a long line that has capacitance charging issues when the connection to Huntly is lost. In this case the 220 kV voltage at Te Kowhai can rise to approximately 1.125 pu, outside of Grid Code requirements, particularly when the offtake at Te Kowhai is light. The challenge presented to Meridian and the DNO was to complete an optimised design for the site that could be commercialised. After much detailed analysis, 28 WTGs with 2.3 MW generators and 101 m diameter turbines and a 33 kV transmission solution was adopted. This provides an estimated output of 248 GWh p.a. from the wind farm’s 64.4 MW capacity operating at a capacity factor of 43.9 % 3 PERFORMANCE REQUIREMENTS The initial phase of the transmission solution design process required the establishment of appropriate power quality standards for a large embedded wind farm. This required interpretation of the Grid Codes applicable for the development, and negotiation of standards with the DNO and the Transmission System Operator (TSO). 4 Key Te Uku Figure 2 – Waikato Transmission Network Overview The DNO was most interested in the power quality experienced by customers distributed around its network. The DNO’s requirements were power quality performance based. The TSO was more prescriptive with its targets requiring fault ride through and adequate power system response following specific transmission faults. At the time of design, New Zealand did not have generation fault ride through code requirements.