A review of the islanding detection methods in grid-connected PV inverters

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Abstract

Islanding is undesired because it may impair the safety of maintenance service workers and/or damage load equipment through unsynchronized re-closure. In principle, islanding detection is the monitoring of islanding—indicating changes in inverter output parameters or other system parameters. This paper aims to aid design efforts through its comprehensive review of islanding detection methods (comparing their non-detection zones and detection speeds) and anti-islanding standards. As a result, this paper shall provide a handful information and clearer vision for researchers to determine the best method for their product.

Introduction

Renewable energy has been intensively developed over the past decade. It produces lower pollution than do fossil fuels and nuclear generation systems [1]. The new paradigm of distributed generation (DG) thus increases in technical importance and increases profits globally [2]. In principle, DG is a small-scale generation unit installed to the load and connected to the grid, for selling or buying of energy. Its most important consideration is islanding [1], which, as specified in [3], is the condition when “a portion of utility system that contains both load and distributed resources remains energized while it is isolated from the remainder of the utility system”. Such an undesired event could be due to circuit tripping, accidental disconnection of the utility through equipment failure, human error, disconnection for maintenance services, or network reconfiguration (which is uncommon) [1], [2]. Integrating DG into utility is a major concern. One problem is that DG may accidentally continue to supply the local load demand when the networks are already isolated from the main system. Successful detection of the islanding is an ongoing challenge to many researchers because existing methods are still not entirely satisfactory [4].

Methods of islanding prevention have been studied. Fig. 1 is a graph of IEEE conferences and journals published on anti-islanding, between 1989 and 2012. From 2002 onwards, it shows that there are an increased of interest on islanding detection.

The 1989–2012 conferences and journals present two types of anti islanding methods: local, or remote. The local methods are either passive or active as shown in Fig. 2.

Passive islanding detection relies on changes to electrical parameters to determine whether islanding had occurred [5]. Its methods were the first to be developed. As technology progressed, more papers discussed active methods, whose development aimed to overcome the limits of passive methods. Remote methods are more reliable but are neither more cost-effective nor simpler to implement than passive or active methods. Section 2 [6] gives examples of passive, active, and remote detection methods. They each have advantages and drawbacks when applied. High demand for the “perfect” method led to much research on wavelet-based islanding detection. Wavelet-based method detects islanding through local measurements of PCC voltage and current signals, just as in passive methods. It is able to evaluate the high-frequency components injected by inverter, just as in active methods [7]. Its main advantage is its sensitivity to signal irregularities such as those in islanding [8].

Two factors aid understanding of islanding: the established standards for grid-connected systems (which address issues of islanding and the procedure for testing and qualifying a DG system) [5], and NDZ (the zone in which an islanding detection method would fail to operate on time, and an evaluating criterium for islanding detection methods).

Section 2 of this paper gives examples of passive, active, and remote methods of islanding detection. The anti-islanding standards are compared in Section 3. Section 4 discusses NDZ in general. Section 5 discusses the primary objective of this paper, presenting past reported principles of detection methods, whose effectiveness are compared before drawing the conclusions.

Section snippets

Islanding detection methods

In grid-connected PV inverters, the methods of islanding detection fall into 3 categories: passive islanding, active islanding, and remote islanding.

Anti-islanding standards

Among the popular reference standards for islanding include IEEE 929-2000, IEC 62116, IEE 1547, VDE 0126-1-1, and AS 4777.3-2005. These standards have been fully utilized to help the researchers in designing their product.

Table 2 shows all the standards as having their own Q value, islanding disconnection time, frequency and voltage operation range [1]. According to IEEE929-2000 standard, Q is:Q=tan(arccosine[pf])

The selected Q of 2.5 equals to 0.37 power factor. As power factor increases, Q

NDZ

NDZ enables determination of the best method, which is the one with the smallest NDZ area [29]. [9] proved that passive islanding has larger NDZ than does active islanding. NDZ can be detected/analyzed 2 ways, either by load parameter space (LPS) or by power mismatch space (PMS) [1].

  • LPS is suitable for islanding detection that is based on frequency drifting [30]. LPS or RLC load space, predicts NDZ through zero phase error (called phase criteria) between the PV output current and the terminal

Research review: principle of detection method

A good islanding detection method ensures reliable detection. This section elaborates on the principle of the islanding detection methods commonly used now: detection of THD or harmonics, frequency of PCC voltage, changes to the grid impedance, power variation, voltage at PCC, wavelet detection, even combinations of several detection methods into one. The followings are the summary of the works done based on the above mentioned ways of islanding detection:

Conclusion

This paper comprehensively reviewed the research done on islanding detection. Anti-islanding techniques can be classified into two groups based on their location in the DG system: local or remote. In local, the detection algorithm is on the inverter side, whereas in remote, the detection is on the grid side. Local techniques, consisting of passive and active methods, have been discussed. Much research has been on active methods. This article also compared related anti-islanding standards with

Acknowledgment

This work was supported by the UMPEDAC (University of Malaya Power Energy Dedicated Advanced Centre), a HICoE (Higher Institution Centre of Excellence) of the Ministry of Higher Education Malaysia and Campus Network Smart Grid System for Energy Security (account number: H-16001-00-D000032).

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