Load Frequency Control in a Smart Grid with Distributed Energy Storage System
By M Rukonuzzaman
Distributed generation from wind and solar sources is increasing rapidly to reduce dependency on expensive fossil fuel and meet the increasing demand of electricity, while reducing greenhouse gas emission. When distributed power generation systems are connected to the utility grid, there can be a negative impact, due to the unpredictable nature of wind and sun, since those are solely dependent on weather conditions. In case the power generation scale of the intermittent and weather-dependent distributed energy generation systems becomes significantly large, demand and supply mismatch can degrade the power quality causing the voltage and frequency of the utility grid to vary significantly and make the power system unstable.
In order to address this problem of frequency fluctuations (due to the supply-demand mismatch) and make the utility grid system stable, thermal power normally regulates the frequency by adjusting the electricity generation. But there is a limit up to which it can be effective. In recent years, large scale (MW scale) battery energy storage has been used to effectively regulate the frequency of the utility grid and enable it to increase its use of renewable energy. When there is frequency deviation in the grid, load frequency control (LFC) (otherwise known as secondary frequency control, SFC, or automatic generation control, AGC) is employed to automatically recover the utility grid frequency to the standard value within a few minutes using the MW scale energy storage system. By utilizing battery energy storage system, which can act rapidly, the performance of LFC can be improved significantly.
In order to secure a stable utility grid system, a mechanism for balancing the supply and demand of electricity based on distributed Home Energy Storage System will be discussed in this article. Due to availability of Feed in Tariff (FIT) or net metering pricing policy, most of the household that generates power from solar or wind energy either stores the redundant power to the battery or injects it to the utility grid. All of the households with energy storage that are connected to the grid in a particular area will be controlled via internet through a central cloud server. When the demand is higher than supply and load frequency decreases, energy stored in the battery will be discharged to the utility to compensate this frequency fluctuations and make the utility grid stable. On the contrary, when the demand is lower than supply and load frequency increases, excessive power will be stored in home battery storage system. In this way, a cloud server based control system will keep track of the state of charge of battery of every household and can be capable of providing supply-and-demand power balancing solution for a stable utility grid using large number of distributed energy storage system on the consumer side. This technology enables real time control with high efficiency and high stability of large number of storage batteries, the number of which can be increased or decreased as required.
Recently, virtual integration control system, which can control thousands of consumers battery energy storage system to regulate utility grid frequency through a centrally located cloud server has been gaining interest. This system controls the individual consumers battery storage system for a short period, but optimizes the control of the whole group of storage batteries for a long period to make the LFC effective and the grid stable. The long period coordination signals are collected from thousands of individual consumers for a period of about 10 minutes through a central server. The virtual integration control system controls consumer sides storage batteries using long-period coordination signals from the control server together with the reference signals such as frequency of each consumers terminal, the reference information and the LFC signal. While the control distribution of the whole storage battery group is optimized using the coordination signal, individual batteries are controlled for a short period independently from the cycle of the coordination signal, so that a fast and effective control response can be achieved. As the cloud server also collects the state of charge (SOC) information of consumers batteries in real time, it is also possible to control the charge/discharge to protect the batteries from damage, due to excessive actions. Even if communication error occurs during delivery of the coordination signal, the time availability for retransmitting the signal ensures high reliability control. Furthermore, the possibility of identifying the control status of each storage battery also contributes to improving the reliability of the control procedures. As large number of distributed storage batteries can be integrated virtually, thousands of storage batteries can be handled in the same manner as a single and large MW-scale storage battery.
The virtual integration control based demand and supply balancing solution to regulate load frequency can be employed in community based grid where local generation and consumption of energy are possible using distributed power generations from solar and wind and battery based energy storage system. Community grid is capable of supplying power using home energy (battery) storage units or electric vehicles installed in the consumer premises even during emergency when there is power outage in the utility grid.
M Rukonuzzaman completed his Bachelor’s degree in Electrical and Electronic Engineering from BUET (Bangladesh University of Engineering and Technology) and obtained his PhD from Yamaguchi University, Japan. He has more than a decade of industrial experience (design and development) at TDK and Shindengen Electric, Japan in the field of digital control applications of power electronics, utility interactive inverter for home energy storage systems. He is now working as an associate professor in the EEE department at the United International University (UIU), Bangladesh. Current research interests include Home Energy Management System (HEMS), Utility Interactive Inverter, Digital Control in Power Electronics, AI in Power Electronics, and Ancillary service for the stability of smart grid systems.
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