Controller hardware in the loop testing of microgrid secondary frequency control schemes

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Abstract

This paper describes a controller hardware-in-the loop (C-HIL) approach for testing centralized and distributed secondary frequency control schemes of AC microgrids operating in islanded mode. We describe the formulation of the secondary frequency control problem and the theory behind the centralized and distributed implementations of the control schemes. Then, we describe the testbed utilized for C-HIL testing activities. Finally, we provide the testing results that compare the performance (in terms of the system response time), and resilience (in terms of withstanding the failure of a control device), of both schemes.

Introduction

The past decade has seen a sharp rise in the deployment of distributed energy resources (DERs) in electric power grids across the globe [1]. Along with this continuing trend came the microgrid concept, which has been shown to be a promising approach for efficient integration and management of DERs [2], [3], [4]. Loosely speaking, a microgrid is a group of interconnected loads and DERs, within a small geographical footprint with clearly defined electrical boundaries, that act as a single controllable entity with respect to the external grid to which it is connected [5]. A microgrid can operate in both grid-connected and islanded modes. In islanded mode, frequency control is a major problem; this is due to the intermittent nature of renewable-based DERs, e.g., PV installations, and the utilization of power electronic inverters to interface DERs to the microgrid, which leads to low or no rotating inertia [6]. Among the various frequency control objectives, a key one is secondary frequency control [7], which entails ensuring that, following a change in operating point of the microgrid, the system-wide frequency returns to its nominal value.

Over the years, several coordination and control schemes for microgrid secondary frequency control have been proposed in the literature [8], [9]. These schemes have primarily utilized centralized and decentralized decision-making approaches, which have several limitations. For example, the centralized decision-making approach is susceptible to a single point of failure, while the decentralized decision-making approach typically lacks the flexibility that is necessary for a seamless integration of additional resources. An alternative, the distributed decision-making approach, has gained some popularity among researchers in the last decade [10], [11], [12], [13]. In theory, coordination and control schemes based on the distributed decision-making approach should overcome the limitations of its other counterparts. However, to the best of our knowledge, there is no study that quantifies and compares the performance of microgrid controls based on the distributed decision-making approach with those based on centralized or decentralized decision-making approaches, especially in the context of performing secondary frequency control. Hence, there is a need to test, validate, and compare the performance of these schemes so as to understand which one is best suited for microgrid frequency control.

Controller hardware-in-the Loop (C-HIL) testing is an effective way to test microgrid controls. In this paper, we describe such testing for two microgrid frequency control schemes. The first scheme is based on a centralized decision-making approach, while the second one is based on a distributed decision-making approach. The setup for testing the centralized control scheme comprises a National Instruments (NI) compact rio (cRIO) device, a centralized entity, that carries out secondary frequency control of an islanded AC microgrid whose components, i.e., the electrical network and its connected DERs and loads, are simulated using a Typhoon HIL real-time emulator (see [14], for details on a microgrid implementation using Typhoon HIL simulator). The setup for testing the distributed control scheme comprises the same emulated microgrid, but instead of using the NI cRIO device for centralized monitoring and control, several interconnected Arduino devices are used to implement our distributed algorithms for microgrid secondary frequency control (see [11], [13], [15], for details on these algorithms). Each Arduino device utilizes the information acquired, e.g., from measurements and other information obtained through exchanges with other nearby arduino devices, to perform successive computations and adjust the set-points of each controllable entity in the emulated microgrid, so as to achieve the secondary frequency control objective. We provide experimental results obtained from the C-HIL testing of both schemes, and utilize well defined metrics, e.g., the system response time and system resilience to a control device failure, to qualitatively and quantitatively compare them.

The remainder of this paper is organized as follows. In Section 2, we describe the secondary frequency control problem for an islanded AC microgrid with inverter-interfaced DERs. In Section 3, we provide a description of the C-HIL setup for testing the centralized and distributed coordination and control schemes. In Section 4, we present the C-HIL testing results for the two aforementioned frequency control schemes and compare their performance. Finally, in Section 5, we provide concluding remarks.

Section snippets

Secondary frequency control of islanded AC microgrids

In this section, we first describe the microgrid model adopted in this work, and provide an overview of the frequency control problem (see [15] for details). Afterwards, we describe two schemes that solve the secondary frequency control problem; one of them is based on a centralized decision-making approach, whereas the other one is based on a distributed decision-making approach.

C-HIL testing of centralized and distributed frequency control schemes

In this section, we describe two C-HIL testbed setups, one for testing the centralized frequency control scheme, and the other for testing the distributed control scheme. The C-HIL testbed is comprised of two layers, the physical layer and the cyber layer. In both setups, the physical layer comprises a real-time emulation of an islanded AC microgrid network and the loads and DERs connected to it. In the centralized setting, the cyber layer comprises a single control node on which the

C-HIL testing results

We start out by describing the active power profiles that were used for testing both schemes. Afterwards, we present results depicting the load change, the system frequency response, and the DER set-point changes. For comparison, we make use of two performance objectives, namely, response time and resilience, both of which highlight the effects of each secondary frequency control scheme.

Concluding remarks

In this paper, we provide a comparison of centralized vs. distributed schemes for microgrid secondary frequency control. We made use of two key performance objectives, namely response time and resilience, to identify the merits of the two schemes tested. We found that the response time of the centralized frequency control scheme is better than that of the distributed one. However, in terms of resilience, the distributed scheme outperforms the centralized scheme.

There is scope for improvements

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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