Microgrids formed by renewable energy integration into power grids pose electrical protection challenges

Highlights

The paper presents MATLAB model of a microgrid formed by renewable-energy sources.

Different protection relays, used in a microgrid, are modelled in MATLAB/Simulink.

Conventional protection techniques are analysed for microgrids’ application.

Conventional protection schemes do not work correctly in a microgrid.

A protection scheme has been designed that works satisfactorily in microgrids.

Abstract

System parameters of a microgrid change in its two operating modes primarily due to output current limitation of PWM based inverters connected with renewable energy sources. The unavailability of an appropriate protection scheme, which must be compatible with both modes of a microgrid operation, is a major problem in the implementation of a microgrid. Two important properties of the microgrid components are peer-to-peer, and plug-and-play. It means that there is no component like a master controller which is critical for the operation of a system, and a distributed-generation unit can be installed at any location in a microgrid. These properties further complicate the protection of a microgrid. This paper reports the MATLAB/SIMULINK model of a microgrid along with the models of the conventional protection schemes and renewable energy distributed-generation resources. Malfunctioning in the conventional protection schemes in islanding mode is identified and models of newly proposed protection schemes are developed. Different types of faults are simulated in all the protection zones of the system and the system parameters are analysed to identify the possible fault detection methods. Based on the simulation results, a protection scheme is recommended that can meet the protection standards such as selectivity, co-ordination and reliability.

Keywords

  • Islanding-mode protection;
  • Microgrid;
  • Microgrid modeling;
  • Microgrid protection;
  • Relay models

1. Introduction

Increased awareness about the climate changes and the demand for green energy have caused mushrooming of renewable energy generation units in power systems. The demand for better system reliability requires the utilities to plug in these generation sources close to the loads [1]. One major problem with sources such as solar, wind turbines, fuel cells and micro-hydel turbines is their integration into an existing power grid, without major redesign of the system [2], [3]. An efficient method of resolving it is to integrate these units into a microgrid. This is the first step in the development of smart grids.

Microgrids are defined as medium- or low-voltage networks that have distributed generation sources together with local storage devices and loads (both critical and non-critical) [4]. Their total generation capacity varies between a few hundred kilowatts to a few megawatts. In normal operation a microgrid operates while remaining connected to a distribution network (grid connected mode), but in case of a grid fault, it is disconnected by a static switch (isolation or islanding mode). This ensures that supply to critical loads is not interrupted. Once the fault is cleared, the microgrid is resynchronized and reconnected with the utility grid.

In order to have all the microgrid functionalities without major system redesign and to have flexibility in the placement of new resources in a power system, a microgrid must have two main properties; peer-to-peer, and plug-and-play [5]. The peer-to-peer functionality demands that there are no components, like master controller or main communication hub, necessary for the operation of a microgrid. This ensures that the operation of a microgrid is not affected by the loss of any system component or generator. The plug-and-play functionally requires that a distribution generation (DG) unit can be placed at any point in a power system without the need of re-design of a protection scheme. This eliminates the chances of engineering errors and also gives a lot of flexibility in the installation of new units.

However, there are some challenges associated with incorporation of microgrids in a power system. The control and protection are major problems in the implementation of a microgrid [6]. In recent years a lot of work is being carried out on the control of microgrids. One area which needs more attention is the protection of a microgrid, especially when it is in islanding mode [7].

The key protection issue related with microgrids is that in islanding mode, the fault currents are much lower than those in the grid-connected mode [6]. This is mainly because of the output current limitation of most PWM converters which are required to interface renewable resources such as micro-turbines, photovoltaic cells, fuel cells, solar panels and wind turbines with a power system. Therefore, in the islanding mode, faults have to be cleared with techniques that do not rely on the detection of high fault currents.

A protection scheme is required to possess features such as selectivity, security and coordination. If selectivity is compromised, the reliability of critical loads in a microgrid is affected. The plug-and-play and peer-to-peer functionalities of a microgrid require that its protection schemes are adaptive, i.e. they should not rely on the location of the DG units, and there is no centralized protection device or master controller for the relays.

Some techniques have been proposed by researchers to detect the faults in a microgrid in islanding mode. It was proposed that the protection relay settings (fault current pickup values and operating times) can be modified by using communication signals on run time based on system operating states [8][9], or based on the value of differential current at different buses [10]. However, by using these techniques, the plug-and-play functionality of a microgrid is compromised and there is an additional cost of communication between different protection relays. Differential current protection can be used to detect shunt faults [11]. The problem with this technique is that any relay downstream will not detect the fault current. Some other techniques make the use of optimal current pickup values between the load current and the fault currents [12], [13]. They can only work if the fault currents are higher than the peak load currents. With a high penetration of PWM inverters, this can not be assured.

Under-voltage detection can be utilized for the detection of L-G faults but the problem is that under voltages travel very rapidly in a microgrid due to its small area. Therefore, the under-voltage tripping will result in shutting down of the whole microgrid in case of a fault in any region. Thus, the property of selectivity is lost. Similarly, the problems arise while detecting line-to-line faults [7].

Negative sequence current detection can be a possible solution [14]. But in this case the protection device must be able to differentiate between negative-sequence currents caused by normal, abnormal and faulty system conditions. Some renewable generation units inject single-phase power into a microgrid, e.g. small photovoltaic systems. It further complicates the sequence currents based fault detection.

In this paper a microgrid is modeled in MATLAB/SIMULINK. The system is analysed to determine parameters such as currents, voltages, line impedances, and active and reactive powers, to develop protection algorithms. The proposed protection schemes by other researchers such as symmetrical-current protection [14], and time-graded voltage protection techniques [15] are also analysed. From this study a general protection scheme is recommended which should protect a microgrid under all fault conditions. The proposed protection scheme is secure and selective. It does not require any communication between the protection devices.

2. System analysed

The single-line diagram of a microgrid is shown in Fig. 1. A utility grid can consist of a large number of generating sources, transmission and distribution lines, transformers, and loads. The power level and hence the short-circuit currents of a utility grid are usually very high as compared to the power level of a microgrid. Feeders A, B and C form the microgrid. The microgrid is connected with the utility grid by a fast static switch. DG renewable sources are connected with feeders A, B and C. The total generation of these sources should be equal to the total load connected in the microgrid; otherwise non-critical loads are shut off in islanding mode.

Single-line diagram of a microgrid.

Single-line diagram of a microgrid.

Fig. 1. 

Single-line diagram of a microgrid.

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