Over the past decade, the country’s energy consumption has grown significantly and to fulfil the increasing power demand, the sector is exploring renewable energy sources as an alternative to conventional sources. Renewable energy is unpredictable and intermittent due to the varying nature of natural resources. The biggest challenge for the sector is to accommodate the excess generation of renewable energy in the existing power system without affecting the power quality. While conventional power plants provide good quality power at a constant voltage and frequency, and with minimum disturbances and low harmonics, that is not the case with renewable energy. The power quality and stability of the power system gets affected due to the greater penetration of renewable energy. This increases the load on transmission lines resulting in small disturbances. Renewable integration into the power grid poses significant technical challenges such as voltage regulation, flickers, harmonic distortion and instability.
The power quality problem is manifested in voltage, current and frequency deviations, resulting in the failure or misoperation of consumer equipment. The Institute of Electrical and Electronics Engineers Standard Association defines power quality as the ability of a system or equipment to function in its electromagnetic environment without introducing intolerable disturbances. Power quality mainly deals with the continuity of supply and the quality of voltage. Therefore, the key power quality concerns with respect to renewable integration are voltage and frequency fluctuations, and harmonic resonance. Voltage and frequency fluctuations are caused by the uncontrollable variability of renewable energy resources due to ever-changing weather conditions, which lead to fluctuations. Harmonics are introduced by power electronic devices utilised in renewable energy generation. When the penetration level of renewable energy is high, the influence of harmonics could be significant.
Power quality issues with renewable energy
Wind and solar photo voltaic (PV) power generation experiences intermittency due to a combination of uncontrollable factors and unpredictability of wind and solar resources. A wind turbine needs wind to generate electricity, and a solar PV system requires sunlight to operate. The output of wind and solar power plants depends on wind speeds and the availability of sunlight. The variability could result in voltage and frequency fluctuations in the transmission system.
Thus, renewable energy generation varies depending on day-to-day weather conditions, making it difficult to predict the output with a high degree of accuracy. These fluctuations further lead to voltage sags and swells, frequency deviations, flickers and fluctuations, and voltage spikes in the power system. In case of fluctuations, additional energy is required to balance the supply and demand in the power grid on an instantaneous basis, along with frequency regulation and voltage support.
The integration of large-scale renewable energy sources into the grid introduces current and voltage harmonics. Maintaining harmonics balance in the line currents of renewable energy integrated power systems is one of the biggest challenges today. Inverters connected with renewable energy sources, non-linear customer loads and power electronics devices introduce harmonics in the distribution network. This causes overheating of transformers and tripping of circuit breakers, and reduces the life of the connected equipment. Harmonics in the system can reduce the capacity of distribution equipment such as cables, conductors, transformers and switchgear. Therefore, to maintain the power quality of the network, the harmonics needs to be kept at a minimum level. In the case of high PV penetration, utilities may have to regulate their feeder voltage more frequently, leading to greater maintenance requirements. Simply adding distributed generation sources to the system without preparing for consequences can escalate the issues associated with feeders.
Solutions
Given the increasing number of renewable energy sources and distributed generators, there is a need for new strategies for the operation and management of the electricity grid in order to maintain or improve power supply reliability and quality. The resolution of the power quality problem may take place at different levels of the power system – at power plants, transmission lines and stations, primary and secondary distribution networks, service equipment and at customer premises. There are a number of measures at the utilities’ disposal to address power quality issues caused by high PV penetration. Some examples of conventional measures are reconductoring, upgrading transformer sizes, and adding tap changing voltage regulators or capacitor banks. A number of new technologies that have communication and control capabilities are available to facilitate greater control of the grid. The basic technologies deployed to address power quality issues include distribution static synchronous compensator (D-STATCOM), unified power quality conditioner (UPQC), UPS, and shunt active power filters (SAPFs).
UPQC with its series circuit provides a solution to voltage sags/swells, flickers, voltage imbalances, and harmonics. With its shunt circuit, it provides a solution to power factor lags, harmonic currents, and load imbalances. D-STATCOM improves the power factor and current harmonics. It also works as a filter, a voltage regulator at the distribution bus, and a load balancer. A UPS is mainly used to provide emergency backup power when the main power fails. Its primary use is in telecommunication devices and computers, where data loss is a major concern. Power shortage in case of an emergency can be handled by a UPS. For systems with a large amount of installed solar PV and wind capacity, the frequency can be controlled using a dead band. SAPFs provide advantages of high speed response and flexibility in operations as they produces a harmonic current of equal magnitude and opposite polarity to that of the harmonics produced in a renewables-integrated system.
In addition, to mitigate the power quality issues arising out of renewable energy inegration, steps need to be taken by developers, transmission planners and utilities. Developers and transmission utilities need to conduct studies to assess the harmonic impact of wind and solar projects on the power grid and vice versa. Further, proper mitigation devices must be used to maintain the desired level of power quality.
Static var compensator (SVC) frequency stabilisers can be a technological solution. With renewables integration, the grid frequency gets impacted due to the reduced number of rotating machines. Thus, grid operators are faced with the challenge of providing sufficient system inertia of synchronous generators with high rotating mass to stabilise the grid. An SVC frequency stabiliser can address this challenge as it is able to emulate system inertia by feeding high active power into the grid when needed. In addition, it offers voltage support by means of reactive power compensation. In addition, the application of synchronous condensers in the grid can help meet reactive power needs, increase short-circuit strength and thus system inertia, and assure better dynamic voltage recovery after severe system faults.
Conclusion
Apart from the above measures, integrating energy storage with renewables can help improve power quality and reliability by reducing flickers and harmonics. Going forward, the growing share of renewable energy sources in the energy mix will ensure a sustainable future.
However, as a growing number of renewable energy plants get connected to the grid, maintaining a robust, reliable and smart electrical power transmission and distribution system becomes a challenge. Thus, there is a need to choose the right solutions and devices after taking into account their pros and cons as well as other factors such as severity of the condition, budget, operational and topological changes in the circuit. This can help utilities overcome the power quality challenges associated with greater renewable penetration.