As the global automotive industry moves forward with commercializing plug-in electric vehicles (PEVs), the issue of how to ensure recharging will take place in an efficient manner is becoming more of a challenge. The concern is that PEV owners in a specific location might all decide to recharge their vehicles at the same time.

This scenario is quite plausible because there is the expectation that PEVs will be recharged overnight by their owners so they will be ready for use the next morning. In a previous TLT article,¹ a researcher developed a model to determine how the electric grid will handle 200 residential households in a specific location who charge their PEVs at the same time. While there might not be much effect on the aggregate power grid, the researcher concluded that the local power grid will be stressed.

Local control of power generation appears to be a critical factor in ensuring that users can recharge PEVs. While electric grids cover a wide area, they are supplemented by localized microgrids. Yuri Rodrigues, doctoral student in electrical engineering at the University of British Columbia Okanagan in Kelowna, British Columbia, Canada, says, “Microgrids are defined as electrical regions comprising a group of loads and distributed energy resources (small hydro power plants, gas turbines, renewable energy sources and energy storage systems) with welldefined electric boundaries having local controllability and capable of operating connected
to the main grid and/or in islanded mode.”

Rodrigues further explains how microgrids operate in islanded mode. He says, “These regions are able to significantly improve the power system’s reliability through locally performing controls previously held at the transmission level during abnormal circumstances such as failures in the main grid, scheduled maintenance and other unpredicted events that could otherwise lead to the interruption of the power supply.”

In effect, microgrids can operate autonomously from the main power grid and enable power to be available in a specific region. As part of the process of maintaining a stable and reliable electric grid, frequency regulation is required. Rodrigues says, “Frequency regulation is a part of power system control that ensures electrical generation by networks matches variations in the system demand, without compromising the system frequency operating level.”

In North America, power systems operate in alternate current with regulated voltage and frequency levels (e.g., 110 Volts at 60 Hertz). Rodrigues says, “In case of a load increase, the system frequency naturally decreases if control actions are not taken. An analogy is the need to inject more fuel into an automobile starting up a hill to keep it moving at a constant velocity. Frequency regulation ensures that power systems operate with a constant frequency.”

The importance of maintaining the frequency has led to the utilization of primary and secondary regulation tools to maintain islanded microgrid. Rodrigues says, “This regulation can be represented as a two-stage process, namely, primary and secondary control. Primary control provides fast stabilizing actions to ensure generation/demand balance. These actions are performed locally using frequency information, thus leading to a steady-state frequency error. In this sense, secondary regulation is performed by selected generating units seeking to resume the network operation at the reference frequency level.”

Past efforts to provide secondary regulation to power systems have utilized automatic generation control. Rodrigues says, “Traditional frequency controllers have their goal limited to resuming power system operation to the 60 Hertz reference level.”

A new, more sustainable approach has now been devised that allows for greater flexibility in secondary control of frequency regulation. 

Conservation frequency reduction
Rodrigues, working with Morad Abdelaziz, assistant professor of electrical engineering at the University of British Columbia Okanagan (see Figure 1), and their colleagues have developed the concept of conservation frequency reduction (CFR) that seeks to adaptatively adjust the microgrid frequency controller parameters based on the availability of local energy resources and expected reconnection time. This enables the determination of the most adequate frequency of operation for an islanded microgrid to take advantage of their frequency dependency. Rodrigues says, “Adequately controlling the microgrid operating frequency is possible through the use of CFR to significantly improve microgrids islanded operating time in environments where the local generation capacity is restricted to the available stored energy.”


Figure 1. The researchers shown have developed the concept of conservation frequency reduction that better enables localized microgrids to maintain power in a specific region. Figure courtesy of the University of British Columbia Okanagan.

The researchers validated the proposed approach by using dynamic analysis for small and large microgrids composed with synchronous generators, renewable resources and energy storage systems. To evaluate the effectiveness of CFR, simulations were carried out for different islanded network perspectives where a failure in the main grid led to these microgrids islanding, and the network reconnection was not possible before local resources were completely depleted.

Using the automobile analogy again, Rodrigues feels that CFR will enable an automobile to operate in an eco mode versus a sports mode. He says, “The microgrid can distribute power at a slightly diluted level that will not negatively impact electronics while allowing power to flow for longer periods without running out.”

This means that a microgrid can conserve power so any shortfall can be better handled by the microgrid itself. On the matter of the possibility of CFR assisting microgrids with better handling power demands placed in recharging electric vehicles, Rodrigues says, “There are several approaches in the literature dealing with controlled charging of electric vehicles to avoid overcurrent and under voltage problems. CFR application can take advantage of electric vehicles storage capacity to support the system, supplying in vessel to grid (V2G) mode during microgrids operation in islanded mode.”

In V2G, PEVs communicate with the microgrid to either return electricity or charge at a reduced rate.

The researchers in the future will evaluate the combination of CFR, with conservation voltage reduction (CVR), to simultaneously capitalize on loads frequency and voltage dependency to reduce demand consumption. Rodrigues says, “Our objective is to increase the autonomous capacity of islanded networks with limited local energy resources.”

Additional information can be found in a recent article² or by contacting Rodrigues at yuri.rodrigues@ubc.ca.

REFERENCES
1. Canter, N. (2018), “How will the growing use of plug-in electric vehicles affect the power grid?” TLT, 74 (4), pp. 12-13.
2. Rodrigues, Y., Monteiro, M., Abdelaziz, M., Wang, L., de Souza, A. and Ribeiro, P. (2020), “Improving the Autonomy of Islanded Microgrids Through Frequency Regulation,” International Journal of Electrical Power and Energy Systems, 115, Article Number: 105499.