Hydro-Wind Synergies

1. Introduction

Nearly one third of the world’s population has no access to electricity and by 2025 approximately 50 % of people will be facing some form of water scarcity.  The elimination of the energy-poverty nexus is essential to sustainable development and fundamental to achieving the United Nations Millennium Development Goals.  To meet this need the International Energy Agency (IEA) has estimated that energy demand will grow by an average of 1.7% annually until 2030. Meeting this demand with fossil fuels will only increase the problem of global warming – the world’s most pressing environmental problem.

Against this background the World Summit on Sustainable Development (WSSD) in Johannesburg in 2002, and Renewables 2004 in Bonn, have called for an increase in the amount of energy supplied from renewable resources. This presents an opportunity for developing countries to take advantage of rapidly evolving renewable energy technologies and “technologically leapfrog” the developed countries.

Hydro and wind power are two commercially viable renewable energies that can assist the developing world in this industrialisation leapfrog process.  Hydropower already supplies 13% 1of the world’s electricity, and the global wind power is doubling every two years.  Renewable energy can significantly improve local standards of living by reducing air pollution, creating employment, reducing dependence on fossil fuel imports and its associated price risk, and therefore increasing domestic energy security.

In addition to local benefits, the global benefits of renewable energy and its resultant emissions reductions, have, and will result in new income streams for companies and countries. Participation in Certified Emission Reduction (CER) schemes and other opportunities available under Kyoto’s Clean Development Mechanisms (CDM), are now beginning to demonstrate these win-win scenarios.

This paper will focus on the opportunities associated with hydropower and wind power which when used in combination can maximise wind penetration. A Tasmanian Case Study is used to explore the synergies between these technologies in order to provide tangible evidence of the opportunities available to other regions of the world with wind and hydro potential.

2. Technical Challenges with Large Scale Wind Development

A number of technical challenges are faced when integrating wind into electricity grids. The key issues include:

In Tasmania recent work has demonstrated that there are significant synergies provided by wind and hydro-electric development that can provide solutions to technical constraints on wind development.

2.1. Tasmanian Wind Power Development

In 2000 the Australian Government introduced the Renewable Energy (Electricity) Act 2000 (known as the Mandatory Renewable Energy Target or MRET). The legislation requires electricity retailers to source an additional 9500 GWh of renewable energy, or 2% of the electricity supplied to the national energy market, by 2010. MRET has provided significant stimulus to renewable energy development in Australia. In combination with the Basslink2 development and the abundant wind resource present in the “Roaring Forties” (Latitude 40o South) Hydro Tasmania’s has embarked upon an ambitious 450 MW wind development program in Tasmania. Key project in the state include the:

Hydro Tasmania completed in 2005 the second stage of the Woolnorth Wind Farm which now consists of 37 x 1.75 MW Vestas V66 wind turbines (64.75 MW installed).

Figure 1: Hydro Tasmania’s Planned Tasmanian Wind Farms

Tasmania provides an excellent case study for hydro/wind synergies and early indications are that significant benefits can be achieved.

Figure 2: Woolnorth Wind Farm

2.2. Limits to Wind Development in Tasmania

In addressing the technical challenges described in section 2, two threshold levels became apparent.

The first issue that had to be overcome was the need for large wind farms to continue to operate during transmission faults. This requirement was met by purchasing wind turbines with fault ride through capability.

The second issue identified, was compliance with Tasmanian frequency standards. As with the fault ride through requirement, this requirement was met through wind turbine capability improvement.

After a comprehensive review of technical constraints on wind development in Tasmania it was found that approximately 140of development (the full Woolnorth Wind Farm) could be achieved as long as location specific, transmission connection issues were addressed.

Beyond this 140 MW level, wind development must be complemented by dispatch of additional inertia and additional voltage support (fault level contribution) under some load/generation scenarios (described in greater detail in section 2.2 & 2.3).

It was found that there are ways to provide the additional inertia and fault level contribution to allow the planned 450 MW of Tasmanian wind development even under significant testing load/generation scenarios. The most extreme testing scenario was found to be:

Under this extreme scenario wind penetration will be 50% of system load but 75% of local generation.

2.3. The Tasmanian Example - Fault Ride Through

Many modern wind turbines now utilise variable speed technology and partial or full duty power electronics. This technology was developed with the objective of maximising energy production and was not initially developed with fault ride through capability. Whilst small wind development is possible even without fault ride through capability, for large scale wind development to be achieved, fault ride through capability is critical. This capability has now been developed by a number of wind turbine manufacturers. This capability does not draw on the synergy between hydro and wind power.

2.4 The Tasmanian Example - System Frequency

Wind turbines are typically designed to operate within the 48 – 52 Hz range but recent work has proven lower frequency operation. As will be seen later, loss of generation at high frequency is a positive contributor to frequency management as long as it is controlled or staged to some degree.

Power system frequency is managed through a number of actions:

Whilst high levels of wind penetration do not impact on load shedding capability, it can have an impact on system inertia and generator governing response.

Short Term Inertial Response. Wind turbine variable speed technology generally isolates the inertia of the rotating mass of the wind turbine from the power system. This means that for each MW of wind operating on the system, there is a corresponding reduction in the system inertia due to the displacement of synchronous generation.

In the Tasmanian system, studies have shown that if wind turbines are not able to contribute to system inertia, large scale wind penetration will, at times, require the dispatch of synchronous hydro generators either as spinning reserve (part loaded generators, with fractional water flow) or as synchronous condensers (generators “motoring” on the network but only supplying reactive power). It was found that under some relatively infrequent circumstances, for each MW of wind power operating an equivalent amount of hydro installed capacity must be operating unloaded. The circumstances that are most testing, are periods of low Tasmanian demand, when Basslink is operating in import and wind production is high. Basslink is not expected to contribute any inertia.

There is a low cost associated with operating hydro generators as synchronous condensers during these infrequent system circumstances.

Studies of the power system have shown that there is a relationship between the Tasmanian load and the minimum amount of hydro plant that must be operating to maintain system inertia.

Figure 3 - Minimum Hydro Operation vs Tasmanian Load
(to meet minimum acceptable system inertia levels)

It can be seen from the above graph that with low Tasmanian load, the operation of 450of wind will result in lower than acceptable levels of hydro generation. For example, if load is 900and 450of wind is operating, whilst only 450 MW of hydro production is needed to meet demand, 700 MW of hydro machines are required to at least spin to provide inertia.

In a report by the Large-Scale Wind Integration Working Group (a group of specialist interests commissioned to study the integration of wind into the Tasmanian grid) it was estimated that the Tasmanian system had adequate inertia to replace approximately 150 MW of hydro generation by the wind generation (using current technology) with minimum effect on the grid3. System simulations can be undertaken to determine how frequently these requirements occur.

There is a limit to how much a hydro plant can be dispatched as spinning reserve due to minimum acceptable loading levels (~50% loading). On this basis in Tasmania, spinning reserve limitations would restrict wind production to 300 MW under the most testing operating conditions (low load, and high Basslink import).

However, the operation of hydro generators as synchronous condensers overcomes this constraint, and enables the full integration of the planned 450 MW of wind.

The overall cost of dispatching hydro machines as synchronous condensers to compensate for the lack of inertia can then be readily estimated based on this data and was found to be quite low in the case of Tasmania. This primary cost associated with synchronous condenser operation is the power required to motor the generator at grid frequency.

Load Shedding – Load shedding arrangements are typically triggered by system frequency dropping to a threshold level. These arrangements are not expected to substantially change with the introduction of large scale wind development but some tuning of settings may be needed.

Governing Response – Reducing generation output in the event of a sudden reduction in demand is readily achieved with conventional hydro machines and can be achieved with recent developments in wind turbines controllers. This governor capability is typically referred to as the “Lower” Frequency Control Ancillary Service. The term ancillary service reflects the way these services are viewed as ancillary to the energy market.

In order to increase generator output, partially loaded spinning plant must be available. This spinning reserve is ideally provided by hydro plant which can operate at part load with minor loss of efficiency. Part loaded hydro generators are ideally suited to responding rapidly to the need for increased production. This capability is typically referred to as the “Raise” Frequency Control Ancillary Service.

Whilst it is technically possible with modern wind turbines to provide the raise service, it can only be achieved by deliberately operating the wind turbine at reduced output unless called upon to increase or “Raise” production. Providing this service from wind turbines would be wasteful as energy is lost in the process.

Hydro plants have the unique ability to operate at less than full capacity while using a minimal amount of fuel (water) and a small loss of efficiency. In comparison, thermal generation cannot sustain operation at low output, and even small reduction of output is associated with significant loss of efficiency.

Historically the flexibility of hydropower has been utilised to provide regulating reserves to meet the fluctuating gap between supply and demand.

Wind turbine production is characterised by significant variation in production as wind speeds vary. The term “intermittency” is now commonly used to describe this characteristic. Whilst this effect is reduced with large wind developments due to the diversity of the output of many machines and several wind development locations the large scale development of wind power will increase the need for both Raise and Lower Frequency Control Ancillary Services.

The following diagram shows predicted wind production (a Danish example). Predicted variation can be managed through scheduled operation of generators on the system whereas spinning reserve needs to be dispatched to cover the uncertainty in production. Hydro generation is ideally suited to the role of spinning reserve, providing both Raise and Lower Frequency Control Ancillary Services.

Figure 4 – Predicted Variation vs Production Uncertainty
"Zephyr" wind power forecasting system - Risø National Laboratory

Both wind power and run of river hydro plant are intermittent generation, although run of river hydro varies less in the very short term. Ideally hydro systems have a combination of run of river hydro generators and hydro generators with large reservoirs. Hydro generators with a storage reservoir can provide a balancing role for wind in the same way they do for run of river hydro.

Installed hydro capacity typically exceeds average production by a factor of around two. This enables hydro plant to be operated at high output to either meet peak load or to capture available water without excessive spill.

By reducing hydro output when wind production is high, energy can effectively be stored in hydro reservoirs.

2.5. The Tasmanian Example - Management of power system voltage

The National Electricity Code (NEC) requires management of transmission voltage across the power system and stable system operation. This results in direct requirements being placed on generators connecting the system but also creates a need for whole of system consideration of voltage management.

In the Tasmanian situation the introduction of the DC interconnection (Basslink) will result in frequent switching of components within the Basslink AC/DC converter station. This switching will result in frequent voltage disturbances that must be managed by generators contributing reactive power to smooth out voltage fluctuations to within acceptable levels.

Current wind turbine designs typically have limited capability to provide voltage support in these circumstances. As the amount of wind installed on the power system increases, the need for additional voltage support correspondingly increases.

In the case of Tasmania it was found that the solution to the inertia issue described earlier also addresses the voltage support requirement. In supporting the system voltage however, it is important that the synchronous condensers that are dispatched are located in areas of the system that require voltage support.

2.6. Relevance of Tasmanian Case Study

The Tasmanian case study is relevant to other areas in the world where hydro and wind power can be developed together. The synergy between wind and hydro power is strong, with hydro generators being able to provide important support to wind generation. In Tasmania, the synergy enables a much greater wind penetration than would otherwise be possible. Hydro generators operating as spinning reserve or synchronous condensers can play a key role in maximising wind penetration.

3. International Opportunities

Following the WSSD and the Bonn Declaration a review of international hydro and wind potential has shown that there is vast potential for both hydro development and wind power development. Currently there are ~300 GWof new hydro capacity planned for development across the globe. These plans are only a fraction of the 8,180,000 GWh/year (nearly 2,000 GW of economically feasible hydro development identified worldwide (Hydropower & Dams, World Atlas 2004).

An estimate of the potential of wind identified a global onshore wind potential of 18,010,000 GWh/year (almost 7,000 GW at less than 5 c/kWh(US) by 20204. The Greenpeace Wind Force 12 paper identifies the potential for global wind development to meet 12% of global power (1,200 GW) by 2020.

There is clearly a great potential for synergies between hydro and wind power to be utilised on a very wide basis across the globe as the level of wind penetration picks up in different regions of the world.

The fact that substantial hydro facilities are already in place will greatly assist with the rapid adoption of wind power in some regions. Where less hydro is installed, an optimal approach may well be to simultaneously develop wind and hydro capacity.

4. Conclusion

Hydro and wind power represent a significant opportunity for addressing the shortfall in global energy supply. Substantial community benefits can be expected from such development including, improved quality of life through energy availability, reduced local air pollution, local employment associated with local manufacture construction and economic activity that is made possible through access to electricity.

The work to date in Tasmania has shown that high levels of wind penetration are possible in association with hydro power. The inherent flexibility of hydro plant clearly complements the intermittent nature of and limited ancillary service contribution from wind power.

The scale of global hydro and wind power development is immense and can provide a sizable contribution to meeting future energy needs in a manner that is globally more equitable than is the case with fossil fuel resources.

Mark Kelleher
Roaring 40s, Hobart, Tasmania, Australia

In November 2005, Hydro Tasmania and China Light and Power (CLP) Asia formed a joint venture company, Roaring 40s, to pursue renewable energy opportunities throughout Asia and Oceania.

Roaring 40s is now the largest wind energy developer in Australia, and the leading foreign renewable energy company in China, with wind energy projects under construction in the Jilin and Shandong provinces. Roaring 40s has plans to install 1000 MW of wind in China by 2010 and is pursuing other significant development opportunities throughout Asia.

For more information: www.roaring40s.com.au

1 International Energy Agency (2004). Renewable Energy-Market Policy Trends in IEA Countries. Pg. 48.
2 Basslink is a direct current (DC) transmission connection under construction between Tasmania and mainland Australia. Basslink is due to be completed in November 2005 and will have a 600 MW export capacity and 300 MW import capacity.
3 Transend, 2004, Integration of Large-Scale Wind Generation: A report by the Large-Scale Wind Integration Working Group to the NEM Entry Coordination Group.
4 "The Potential of Wind Energy to Reduce CO2 Emissions" - Garrad Hassan.