Voltage control using demand-side resources in active distribution networks

Abstract

The demand for electricity is projected to increase over the next few years, driven by economics, population growth and electrification. The need for alternative energy resources, especially at distribution level, will find greater traction in the future, based on this demand, a rise in electricity pricing and a decreased carbon footprint may result. It is projected that renewable energy will account for 38% of energy demand by 2050 in South Africa. This increasing demand for renewable energy, especially at distribution level, will change the traditional passive grid into an active grid and create a challenge for optimal voltage regulation in real-time. Given the above, it is projected that the distribution grid will have a more active role, driven by advancement in power electronics and distributed energy technologies. The control of voltage is an important aspect in modern power systems. The current technology deployed in local distribution networks will not be able to respond to technical challenges that will be imposed by the future grid. A lack of adequate voltage control will limit the widespread adoption of distributed energy resources (DER) in the future grid if not addressed properly. A revision of the control strategy is required to integrate DER into the grid, considering South Africa’s commitment to reduce CO2 emissions by 2035 to 350-420 million metric tons. A substantial level of grid intelligence is required to meet the energy demand increase, projected by 2050. A control strategy must be implemented that takes advantage of available technologies, control techniques, monitoring and communication that mitigates grid stability problems. The South African electrical power industry is at an exciting crossroad of transformation, with the imminent unbundling of the generation and supply authority and the possibility of deregulation, that might provide a way out, for the cash-strapped parastatal. The work in this thesis focused on control strategies that could be implemented in the future distribution network, to deal with the voltage control problem. These control strategies used the integration of distributed energy resource (DER), to mitigate the voltage control problem. The work can broadly be categorized into five parts: Firstly, existing voltage control strategies based on control techniques, strategies and optimization methods were reviewed. The literature showed that an increase in DER penetration causes dynamic and transient voltages to exceed the prescribed limits, which results in damage to electrical equipment. Conventional voltage control devices are unable to respond fast and effectively to bidirectional power flow caused by DER. New control strategies, based on intelligent control are needed to deal with the voltage control problem in distribution networks. Secondly, specific gaps and opportunities in the current control strategy in the South African distribution grid were identified. It was shown that the South African transmission and distribution system is characterised by a centralized unidirectional, demand driven control with no intelligence at the distribution level. Voltage quality parameters are covered by NRS (National Rationalised Specifications) 048 in South Africa. The aim of these standards is to set and provide acceptable voltage quality limits at the point of common coupling (PCC). The current specifications, which this study was based on, places restrictions on control functions for DER with a power output of less than 100kVA. This limits the participation of domestic and light commercial rooftop solar PV for voltage regulation. Opportunities exist for intelligent control using the reactive power capability of DER for voltage regulation in the future distribution grid. Thirdly, a test network was developed, which included DER. Phasor models were developed to test and implement specific voltage control methods. The results showed that an increase in DER penetration causes dynamic and transient voltages to exceed the prescribed limits at low penetration levels, which is greater than 25% but less than 50%. At higher penetration levels i.e., 50% and greater, the violations are severe. It was shown that power factor control of inverter-based DER based on load demand can be used to mitigate these voltage violations. Injecting or absorbing reactive power during a 24hour cycle based on the load demand proved an effective way to limit voltage violations and decrease the number of tap operations of the On-load Tap Changing (OLTC) transformer. Lastly, a discrete model comprising of a 6.5kW residential solar PV system was developed using reactive and active power control. This model was tested on a developed test feeder using OPAL-RT, a real-time simulation software package. The software uses a hardware-in-the-loop concept that incorporates digital real-time simulators. The results showed that reactive power of the solar PV inverter can be used for local voltage control.