Adoption of Renewable Energy to Provide Ancillary Services

By Claude Ziad El-Bayeh and Khaled Alzaareer

The power demand of the customers fluctuates unexpectedly both in real-time and over the course of hours, days and months. Furthermore, unexpected power outages may occur in the generation and transmission network and may lead to instability. Therefore, system operators need efficient tools to keep the system to operate in a stable and reliable manner in real-time and over several hours, days, months and years. There is a necessity for the system operators to have flexible and fast responsive technology and equipment to meet the challenges of intermittent resources such as solar irradiance and wind speed. On the other hand, there is a need to meet the fluctuating load demand for electricity customers. Therefore, the integration of Renewable Energy Sources (RES) creates new challenges to the system operator in which the grid stability and reliability are threatened. Despite the disadvantages of integrating RES into the power systems, they play a central role in providing ancillary services by supporting the grid and meet the demand of the electricity customers. Hence, advanced technologies and control techniques are required to reduce the fluctuation and provide ancillary services to the grid such as voltage, frequency, and power control.

Statement of the Challenge and Opportunity: Gaps, Opportunities, and Drivers

Over the last years, Renewable Energy Sources (RES) and Distributed Generations (DG) have drawn the attention of researchers and industries for many reasons. Actually, wind, solar and water are natural sources and freely available in nature. They do not produce waste nor pollutant materials and do not participate in climate change. These natural free-sources can be harvested and converted into electricity. RES can replace other conventional pollutant sources such as fuel, diesel, coal, and natural gas, in which their quantity is limited and will not last for more than a few decades. Consequently, the current research is mainly focusing on how to increase the efficiency of harvesting and converting these resources into electricity. For this reason, many technologies such as photovoltaic, wind turbine and Concentrated Solar Power Tower, are used and others are under development. RES are considered variable and intermittent sources, in which their integration into the power grid can yield voltage, power and frequency fluctuations and may lead to instability on the network. In fact, solar irradiance and wind speed are highly fluctuating and may vary drastically in a fraction of second leading to a stochastic power generation and may unbalance the supply-to-demand ratio. Therefore, the power and distribution grid could be negatively affected by this high fluctuation. From this point, there is a need to control the power and energy flow of these variable sources in order to mitigate their integration into the electricity network.

There is a big opportunity to improve the integration of RES into the power grid. Thanks to the development of new technologies of converters and control techniques, in which we are able to control the active and reactive power flow. These technologies help the grid to improve its stability even when a high penetration level of RES is deployed on the network. Moreover, RES helps the system operator to meet the demand with the generation.

The deployment of RES and DG should be associated with Energy Storage Systems (ESS), in which a lack of power generation from RES can be supported by ESS. On the other hand, if RES generates an excess of energy, ESS is used to store this energy. Therefore, the supply-to-demand ratio is always stable and close to unity.

Different Types of Ancillary Services

Ancillary services are defined as the ability to maintain the reliability and stability of the power and distribution systems over a very short period up to one day. They provide a flexible and fast response to match the variation between the demand and the supply while maintaining the frequency and the voltage within the recommended limits.

Renewable energy sources and energy storage systems (RES/ESS) are considered an excellent combination in providing ancillary services to the network. Six different kinds of ancillary services are identified as follows:

Scheduling and dispatch

• Improve the generation dispatch
• Faster response time for ancillary services compared to the conventional power plants
• Shift the hourly generation portfolio
• Generate revenue from ancillary services
• Reduce the operation cost of the power system by turning off some generators when the RES/ESS is able to supply the power demand

Reactive power and voltage control

• Voltage regulation
• Frequency regulation
• Control the active power flow
• Control the reactive power flow

Loss compensation

• Reduce the total harmonic distortion
• Reduce the energy losses on the network
• Reduce line and transformer losses
• Reduce financial losses

Load following

• Peak Shaving
• Shift the excess of energy demand to valley hours

System protection

• Improve grid stability
• Improve grid efficiency
• Improve grid reliability
• Improve the power quality

Energy imbalance

• RES/ESS is used as spinning reserve
• Reduce network congestion
• Improve the load factor

Technological Innovation and Advances

Some of the ancillary services can be freely provided by large generators such as hydro dams, where the characteristics of electrical generators (such as synchronous generators) help in controlling the active and reactive power flow. Other ancillary services are provided by power electronic converters and inverters in which the control of the phase angle can change the flow of the active and reactive power; therefore, voltage and frequency are regulated. Many technological innovations and advances already exist in the market and others are under development. Some of these technologies used for ancillary services can be classified as follows:

• Active filters (e.g., shunt, series, hybrid)
• Capacitor banks, super-capacitors
• Electrical generators (e.g., synchronous, asynchronous, “DFIG”, “PMSG”, “SCIG”, etc.)
• PV farm • Wind turbine farm
• Battery Storage systems
• Electric Vehicles
• Energy management systems
• Tap-changer transformers
• Solid-State Transformers

Conclusion

Distributed Generations (DG) and Renewable Energy Sources (RES) have been recently deployed to support the grid by injecting or absorbing power depending on the network’s needs. Their integration yields a challenge to the power system because of their variable nature. Their integration into the power system should be associated with sophisticated tools and techniques to mitigate their impact. In addition, the combination of RES and Energy Storage Systems (ESS) increases the success of these technologies.