1. Introduction
Interconnections between adjacent zones in a power system are erected to ensure economic and reliable operation [
1]. During significant amount of power export from a generation-rich area to its neighboring zone, a loss of interconnection can cause an excessive rise in system frequency in the power-exporting zone [
2]. In conventional power systems where sufficient numbers of synchronous generators are committed, a sudden increase in frequency can be stopped via inertia and governor responses [
3]. However, due to the increased penetration of wind generation, conventional synchronous generators are being replaced in the generation mix [
4,
5]. Modern wind power plants predominantly deploy Type-III (Doubly Fed Induction generator (DFIG)) and Type-IV (Full Scale Converter (FSC)) wind turbine generators (WTGs). These WTGs are interfaced with the grid via power electronic converters, which decouple them from the corresponding network [
6]. Consequently, these machines usually do not offer inertia and governor response to control system frequency [
7]. Therefore, under high wind penetration, maintaining an adequate frequency response is becoming challenging due to the presence of a few synchronous generators in a low-inertia grid.
Following the loss of an interconnection in a low-inertia grid under power export conditions, network frequency can go above a certain threshold [
8]. As a result, over-frequency relays disconnect the online generators [
9]. Consequently, cascading tripping of generators may take place, which eventually can cause a significant amount of generation loss. Therefore, the mitigation of over-frequency is a vital concern to enhance frequency resilience. To this end, a number of strategies are reported in the literature to address this issue. An over-frequency mitigation scheme via the tripping of generators located close to the disturbance is suggested in [
10]. Furthermore, an Over-Frequency Generator Shedding (OFGS) technique is proposed in [
11], where coordination is preserved between the over-speed protection controller and generator shedding relays. Furthermore, an active power reduction scheme of virtual power plants aided by a decision tree strategy is reported in [
12]. Similarly, active power reduction from a distributed generator for over-frequency management is given emphasis for an islanded microgrid in [
13,
14]. In addition, for conventional power systems, grid connected wind farm shedding is found to be effective to arrest frequency rise [
15]. Notably, the rapid response time of the Battery Energy Storage System (BESS) enables it to be used for frequency control as it can supply or absorb active power following a disturbance. However, the deployment of BESS for over-frequency mitigation is overlooked in the current literature.
BESS has been predominantly utilized in power systems for various applications viz. variability and intermittency reduction in renewable sources, power quality and reliability improvement, load leveling, peak shifting, valley filling, increasing spinning reserves and so on [
16,
17]. In addition, BESS can be placed in microgrids with higher renewable penetration for active power sharing [
18]. Siting and sizing strategies of BESS for reducing daily production cost in a renewable integrated power system are investigated in [
19]. Moreover, network congestion reduction is accomplished by the optimal sizing and placement of BESS in [
20]. For providing a primary frequency response in a Mexican transmission network, a BESS allocation strategy is proposed in [
21]. In [
22], a BESS sizing methodology is developed by determining energy and power ratings to provide an inertial response as well as a primary frequency response in a wind-dominated power grid. A BESS sizing methodology is presented in [
23] using grid voltage code violation as a limiting criterion. Likewise, an optimized BESS capacity is determined in [
24] and validated through measurement data. Optimal placement of energy storage is reported in [
25] by using the alternating direction method of multipliers algorithm for providing ancillary service in a power system. Optimal siting and sizing of a distributed energy storage system is specified using bi-level optimization in [
26] for mitigating the voltage impact of solar photovoltaic (PV) in a distribution system. An AC optimal power flow method and linearized DC power flow approximation method are utilized in [
27,
28] to find the most appropriate allocation strategy of BESS.
Conventionally, power systems rely on an OFGS scheme to mitigate over-frequency events. However, this causes a certain amount of generation loss. To avert generator shedding (i.e., to avoid the activation of the OFGS scheme), deployment of BESS can be a prudent choice. However, BESS is expensive; hence, its size needs to be appropriately determined to minimize financial concerns. Furthermore, proper siting of BESS can ensure voltage stability, which is an additional advantage for network operators. In the literature, a significant number of studies are carried out to investigate the optimal siting and sizing of BESS from various perspectives. However, none of the existing works report any strategies for the optimal allocation of BESS to resolve the over-frequency problem in a low-inertia grid. To address this important yet unexplored research gap, this paper aims to make the following contributions.
An optimal sizing approach for frequency-responsive BESS is developed to mitigate the risk of over-frequency and subsequent generator shedding in a low-inertia power system under high wind power penetration.
Frequency deviation is utilized as an objective function in the optimization formulation, whereas Rate of Change of Frequency (ROCOF) and BESS State of Charge (SoC) are considered as constraints in the optimization formulation. Thus, both frequency response parameters and battery charging limits are taken into account while evaluating the optimal BESS size.
A siting strategy of BESS is utilized to take care of voltage stability besides frequency response adequacy. To this end, BESS is placed at the weakest bus using a voltage stability index called reactive power margin.
The proposed methodology is applied to a low-inertia test network, which represents the equivalent high-voltage transmission network of South Australia. The effectiveness of the proposed approach is demonstrated via dynamic simulations following a large contingency in various wind penetration cases. Moreover, the developed technique is validated by comparing the frequency response performances using with BESS and without BESS strategies.
It is to be clarified that the proposed approach is complementary to frequency reserves. To this end, a symmetric primary frequency control reserve (FCR) is considered in this paper. As such, following an over-frequency event, power outputs of synchronous generators decrease via governor action. However, in a low-inertia system, a few synchronous generators are committed in the grid. Therefore, the amount of FCR is not adequate to stop the frequency rise following a large contingency. Consequently, the frequency may go above the OFGS threshold. It could cause the loss of a certain amount of generation. To avert such a situation, BESS is deployed along with FCR to keep the system frequency below the OFGS activation threshold.
The rest of the paper is organized as follows.
Section 2 describes the proposed methodology supported by the necessary theoretical background.
Section 3 contains the simulation network and simulation scenarios.
Section 4 presents comprehensive simulation results, analyses, validation and other aspects of the proposed techniques. Finally,
Section 5 summarizes the key findings to conclude the paper.
5. Conclusions
This paper proposes a methodology for the siting and sizing of BESS to mitigate the over-frequency problem in a low-inertia power system. To this end, a frequency-responsive BESS is placed in the bus with the lowest reactive power margin (i.e., at the weakest bus). By doing so, voltage stability is retained along with frequency stability following a contingency. Then, an optimization model is formulated to determine the most appropriate size of BESS for alleviating over-frequency. The proposed methodology applied to a low-inertia power system consists of a few synchronous generators and high wind penetration. The optimal size of BESS is evaluated for a 450-MW interconnection trip by taking this as a baseline contingency. The optimization model results in a BESS size of 115 MW, which is eventually connected to the most voltage-sensitive bus. Furthermore, extensive simulations are carried out in various inertia levels for different magnitudes of contingencies to explore the effectiveness of the developed method.
It is found that, following the loss of 400-MW and 450-MW interconnections under power export conditions, the optimally sized BESS successfully stops the frequency escalation before the OFGS scheme is activated. Over-frequency generator shedding only occurs when a 500-MW contingency is considered and three generators are online. Furthermore, the ROCOFs are confined to the acceptable limit (i.e., less than 1 Hz/s) in all cases. To validate the proposed methodology, its performance is compared to the condition when no BESS is utilized. It is noted that frequency summits (i.e., frequency peaks) and ROCOFs significantly reduce when BESS is deployed. In addition, the amount of generation shedding considerably decreases due to BESS incorporation. Therefore, the proposed methodology effectively mitigates the over-frequency phenomenon. It also yields better performances compared to its counterpart. Finally, it is worth mentioning that the developed technique can be applied to any power system to manage the over-frequency challenge, especially under high renewable penetration.