An analytical literature review of the available techniques for the protection of micro-grids

Electrical Power and Energy Systems 58 (2014) 300–306

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Electrical Power and Energy Systems journal homepage: www.elsevier.com/locate/ijepes

An analytical literature review of the available techniques for the protection of micro-grids Sohrab Mirsaeidi a, Dalila Mat Said a,⇑, Mohd. Wazir Mustafa a, Mohd. Hafiz Habibuddin a, Kimia Ghaffari b a b

Centre of Electrical Energy Systems (CEES), Faculty of Electrical Engineering (FKE), Universiti Teknologi Malaysia (UTM), 81310 Skudai, Johor, Malaysia Young Researchers and Elites Club, Saveh Branch, Islamic Azad University, Saveh, Iran

a r t i c l e

i n f o

Article history: Received 8 April 2013 Received in revised form 16 December 2013 Accepted 11 January 2014

Keywords: Micro-grid Protection schemes Grid-connected mode Islanded mode DG units

a b s t r a c t During the last decade, besides the rapid increase in the penetration level of Distributed Generation (DG) units of micro-grids, the connection of micro-grids as a developed technology to the existing distribution networks has also attracted much attention. One of the major challenges associated with the protection of micro-grids is to devise a proper protection strategy that is effective in the grid-connected as well as the islanded mode of operation. In order to deal with the challenge, many researchers have recently proposed various techniques. The purpose of the current study is to provide a comprehensive review of the available protection techniques that are applied to address micro-grid protection issues in both grid-connected and islanded mode. The most up to date relevant options are described and categorized into specific clusters. A comparative analysis is carried out in which the advantages and disadvantages to each technique are assessed. Lastly, after the appraisement of the existing protection techniques, some conclusions and suggestions are put forward for the protection of micro-grids in the future. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Increasing concerns regarding global warming caused by greenhouse gases, which are substantially generated by conventional energy resources, e.g., fossil fuels have created significant interest in the research and development in the field of renewable energies [1,2]. Such interests are also intensified by the finitude availability of conventional energy resources. To take full benefit of renewableenergy resources, e.g., wind and solar energy, interfacing power electronics devices are essential, which together with the energy resources form DG units [3–6]. The increasing proliferation of DG units, such as wind turbines, micro-gas turbines, photovoltaic generators and fuel cells is anticipated, and this inevitably challenges the traditional operating principles of the power networks [7–11]. An emerging philosophy of operation to alleviate the technical issues with regard to high penetration of DG units, and to offer additional values is to designate relatively small areas of a distribution network that embed DG units and loads, and to operate them in a deliberate and controlled way. Such sub-networks, referred to as micro-grids [12]. The structure of a typical micro-grid is depicted in Fig. 1. The most important benefit of micro-grids is to provide highreliability and high-quality power for the consumers who require ⇑ Corresponding author. Tel.: +60 129732985; fax: +60 75557005. E-mail address: [email protected] (D. Mat Said). http://dx.doi.org/10.1016/j.ijepes.2014.01.032 0142-0615/Ó 2014 Elsevier Ltd. All rights reserved.

uninterruptible power supplies [13,14]. Furthermore, micro-grids bring significantly economic benefits with the utilization of combined heat and power technology. In fact, micro-grids have the potential to generate the electrical and useful thermal energy simultaneously (hot, cold, or both) to optimize the consumed energy efficiency by applying cogeneration or tri-generation systems [15]. Micro-grids have the ability to operate independently or in conjunction with the rest of the distribution network. The philosophy of the micro-grid’s operation is that under normal conditions the micro-grid operates in the grid-connected mode but when the utility damages or has a power failure; it expeditiously disconnects from the utility by the static switch at the Point of Common Coupling (PCC) and then operates in isolation from the rest of the network [16–22]. In spite of numerous advantages provided by micro-grids, there are some technical challenges, which require to be met for the engineers and one of them such as micro-grid protection and its entities. Since the protection devices of the existing distribution systems are designed according to the large fault currents, they do not have the ability to protect micro-grids. This is because when a fault takes place in the micro-grid with the widespread proliferation of electronically-coupled DG units, operating in autonomous mode, the DGs are not able to contribute adequate currents towards the total fault current. It is due to the inverters have a low thermal overload capability, limiting their maximum output

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Fig. 1. The structure of a typical micro-grid.

current to about 2–3 times the rated current [23]. On the other hand, despite the traditional distribution networks, power flow within micro-grids can be bidirectional owing to DG connections at the different locations. Accordingly, in order to protect microgrids in both grid-connected and islanded mode of operation, novel protection schemes should be employed [24,25]. This paper aims to present a brief analysis of various protection schemes based on the published papers in attempting to provide an appropriate protection strategy which is capable of protecting micro-grids in both grid-connected and autonomous mode of operation. The organization of this paper is as follows: Section 2 discusses the available techniques for the protection of micro-grids, and Section 3 analyzes the proposed techniques as well as putting forward some suggestions for the protection of micro-grids in the future and finally; Section 4 concludes the paper. 2. Available techniques for the protection of micro-grids An appropriate technique for the protection of micro-grid should have the ability to respond to both utility grid and microgrid fault incidents. In other words, if a fault occurs on the utility grid, the desired response is to isolate the micro-grid from the rest of the network. This leads to an autonomous operation of microgrid, and if a fault takes place within the micro-grid, the protection system should remove the smallest possible faulted area of microgrid to clear the fault. In recent years, various techniques have been proposed to present an adequate protection strategy for microgrids. These techniques are precisely illustrated in the following subsections. 2.1. Adaptive protection: available techniques and their challenges Adaptive protection schemes have the ability to solve the problems associated with the protection of micro-grids in both gridconnected and islanded mode of operation. In such protection schemes, there is an automatic readjustment of relay settings when the micro-grid alters from grid-connected mode to islanded mode and vice versa. In fact, adaptive protection is an online system that modifies the preferred protective response to change in system circumstances or requirements in a timely manner through

external generated signals or control actions. Numerical directional over-current relays, which have the potential of using several settings groups, are employed in the practical implementation of adaptive protection systems. In order to provide more effective protection, a communication system can be applied such that individual relays can communicate and exchange information with a central computer or between different individual relays. The work by Tumilty et al. [26], suggested an adaptive protection strategy without the need of the communication system. The authors simulated the voltage response for both short-circuit and overload events. The results of simulations indicated that the voltage magnitude has a reduction in both events. Nevertheless, the magnitude of this reduction was such that these two events could be differentiated. In fact, the voltage drop resulting from short-circuits were significantly greater than that of overloads. Accordingly, they employed a voltage based fault detection method to discriminate the voltage drop in short-circuit and over-load incidents. Based on centralized architecture, Oudalov and Fidigatti [27] presented a novel adaptive protection scheme using digital relaying and advanced communication technique. In the scheme, the protection settings were updated periodically by the micro-grid central controller with regards to the micro-grid operating states. The scheme was realized using numerical directional relay with the directional interlock capability to act selectively to protect the micro-grid. In the following year, Han et al. [28] analyzed the fault behavior of an inverter-based micro-grid, and proposed an adaptive fault current protection algorithm. They deployed the voltage and current fault components at the installation of protection to determine the system impedance. Afterwards, the current instantaneous protection adjusted the settings automatically by comparing with the utility grid and micro-grid impedances. In another study conducted by Dang et al. [29], they proposed an adaptive strategy using Energy Storage (ES) and isolation transformers to protect low-voltage micro-grids in the islanded mode as well as the grid-connected mode of operation. Firstly, in order to recognize the micro-grid’s mode of operation, the over-current protection and dq0 voltage detection were utilized for the gridconnected mode and islanded mode, respectively. Then, the different protection zones could be discriminated by comparing the zero sequence current and a threshold value.

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However, Ustun et al. [1] suggested an additional adaptive protection technique that made use of extensive communication for monitoring and updating settings of relays in accordance with different micro-grid’s operation mode. In the proposed scheme, micro-grid was equipped with a Central Protection Unit (CPU) which communicated with the relays to update their operating currents and with DGs to store their status as ON/OFF. The work of Khederzadeh [30] used the numerical relays in the micro-grid to adapt the settings of the relays to the status of the micro-grid. Different settings for the over-current relays were calculated off-line and saved in the relays. Whenever the micro-grid was disconnected from the grid, the relay settings were automatically changed to the associated group of settings. The main challenges associated with the implementation of the above-mentioned adaptive protection techniques are as follows: – The need for updating or upgrading the protection devices which are currently utilized in the power networks. – The necessity to know all possible configurations of microgrid before the implementation of these schemes. – Establishment of a communication infrastructure can be costly. – Short-circuit calculations will be complicated for a microgrid with different operating modes. 2.2. Differential protection: available techniques and their challenges Differential protection is applied to protect many elements within a power system. The structure of the differential protection is shown in Fig. 2. The protected element might be a length of the circuit conductor, a bus section, etc. From Fig. 2 it can be seen that differential relaying is a basic application of Kirchhoff’s Current Law (KCL). The relay operates on the sum of the currents flowing in the CT secondaries, I1 + I2. For through current conditions, such as load or an external fault, the currents in the two CT’s will be equal in magnitude and opposite in phase (assuming the CT’s have the same ratio and are properly connected), and there will be no current flow in the relay operate coil [31]. In the event of a short-circuit occurrence within the protected section between the two CT’s, current will flow through the operate circuit causing the relay to issue a trip output. Nikkhajoei and Lasseter [32] presented a combined technique to protect micro-grids by differential protection and symmetrical component calculations against Single Line-to-Ground (SLG) and Line-to-Line (LL) faults. In this method, a micro-grid was divided into several protection zones with relays. The differential current components were deployed to detect faults that occur in the up-stream zone of protection, whereas the symmetrical current components (zero and negative sequence current) were used to detect SLG fault in the downstream zone of protection and LL faults in

all zones of protection. Simulations were performed at different location of faults for inverter-based DG micro-grid and the results indicated that the scheme has the ability to protect the micro-grids in islanded mode of operation. Zeineldin and his co-workers [33] discussed the future of microgrids and regarded two major challenges ahead, including voltage/ frequency control and protection. In the developed strategy, they made use of differential relays at both ends of each line. These relays designed to operate in 50 ms could protect the micro-grid in both grid-connected and autonomous operation modes. The work reported by Conti et al. [34], utilized a differential protection scheme in a test micro-grid containing synchronousbased and inverter-based DGs. They described three protection strategies to detect phase-to-ground faults in isolated neutral micro-grids. Additionally, three more protection schemes were presented for three-phase faults in micro-grids with synchronous-based and inverter-based DGs. The progress of differential protection was further established by Sortomme et al. [35]. They offered a novel protection scheme based on some of the principles of synchronized phasor measurements and microprocessor relays to recognize all kinds of faults, including High Impedance Faults (HIFs). In this scheme, the primary protection for each feeder relies on the instantaneous differential protection. If absolute values of the two samples were found to be above the trip pre-determined threshold, a tripping signal was sent to the switching device. Prasai et al. [36], suggested a multi-level approach would provide the most effective form of network protection of a meshed micro-grid, while ensuring a high level of reliability and power quality by expeditiously and automatically identifying faulted points in the system, and actively isolating them. One of the advantages of this method was that the proposed protection scheme made used of the power line itself (power line carrier technology). Therefore, the communication architecture was highly reliable. Last but not least, a novel methodology based on differential protection was put forward by Dewadasa et al. [37] in 2011. The methodology, which includes all the protection challenges such as bidirectional power flow and reduction of the fault current levels in islanded operation mode were taken into account. It could protect micro-grids in both grid-connected and islanded modes of operation. In the method, the authors not only concentrated on feeder protection, but they also proposed some solutions to protect buses and DG sources within the micro-grid. One of the most significant benefits of the implementation of differential protection approaches is that they are not sensitive to bidirectional power flow and reduction of fault current level of islanded micro-grids. However, some drawbacks associated with them can be summarized as follows: – As the communication system may fail, providing a secondary protection scheme is necessary. – Establishing a communication infrastructure is relatively expensive. – Unbalanced systems or loads may result in some difficulties in the above-mentioned protection schemes. – Transients during connection and disconnection of DGs may bring about some problems. 2.3. Distance protection: available techniques and their challenges

Fig. 2. The structure of the differential protection.

Since the impedance of a line is proportional to its length, for distance measurement, it is appropriate to use a relay capable of measuring the impedance of a line up to a predetermined point (the reach point). Such a relay is described as a distance relay and is designed to operate only for faults occurring between the relay location and the selected reach point.

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The main strategy for this group was developed by Dewadasa [38,39]. It was based on an admittance relay with inverse time tripping characteristics. The relay could distinguish and isolate the faults in both grid-connected and autonomous micro-grids. Distance relays having Mho characteristic with two zones of protection were employed in the study. Zone settings were chosen such that Zone-1 covers 80% of the protected line and Zone-2 covers the whole protected line, plus 50% of the adjacent line. In the strategy, the fault currents in the faulted phases were restricted by reducing the converter output voltage. Afterwards, by analyzing fault characteristics, the sequence currents and voltages at the relay locations were calculated. Simulations were done for gridconnected and autonomous modes of operation for different types of faults at different locations with changes in fault resistance and load conditions. Nevertheless, the effectiveness of this scheme is still not validated. Some challenges associated with the application of these types of relays are as follows: – Harmonics and transient behavior of current may result in some problems with the accuracy of extracting fundamentals. – Fault resistance may make some errors in the measured admittance. – The measurement of the admittance for short lines in distribution networks is challenging. 2.4. Voltage-based protection: available techniques and their challenges Voltage-based protection techniques substantially make use of voltage measurements to protect micro-grids against different kinds of faults. The main approach in this field was confidently proposed by Al-Nasseri and Redfern [40] in 2006. The scheme, in which output voltages of DG sources were monitored and then transformed into dc quantities using the d-q reference frame, had the ability to protect micro-grids against in-zone and out-of-zone faults. Moreover, a communication link was deployed to discriminate in-zone and out-of-zone faults. Subsequently, Hou and Hu [41] proposed a new fault judgment method based on detecting the positive sequence component of the fundamental voltage such that it could provide reliable and fast detection for different types of faults within the micro-grid. In this method, the waveforms of the three-phase voltages and the voltage magnitudes under symmetrical and unsymmetrical fault conditions were transformed into the d-q reference frame and compared to the amplitude of the fundamental positive sequence voltages in the d-q coordinate system. Within the same year 2009, Loix et al. [24] proposed a novel protection technique. This technique which was based on the effect of different fault types on Park components of the voltage was capable of protecting micro-grids against three phase, two phase and one phase-to-earth faults. The protection methodology did not rely on the communication system during its operation, but it could be optimized through communication links between different detection modules. The salient feature of this scheme compared to the one in [40] was that the proposed strategy was not only designed for a certain micro-grid but could also be used to protect different micro-grids with various configurations. The recent report by Wang et al. [42] introduced an additional protection strategy based on busbar fault direction to protect micro-grids in both grid-connected and autonomous operation modes. Furthermore, the authors designed the relay protection hardware and software using Industrial Personal Computers (IPCs). The main challenges in regard to possible implementation of voltage-based protection strategies are:

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– Any voltage drop within the micro-grid may lead to misoperation of protection devices. – HIFs cannot be identified using above-mentioned methodologies. – Most of these techniques are designed and tested for specific micro-grids. In fact, they are strongly dependent on the micro-grid configuration and on the definition of the protection zone. Therefore, they may not be convenient for micro-grids with different structures. – Less sensitivity in the grid-connected mode of operation. 2.5. Protection techniques with the deployment of external devices and their challenges As mentioned earlier in the introduction, the fault current levels are significantly different between grid-connected and the autonomous operation modes. Therefore, the design of an adequate protection system, which performs in both modes of operation, can be a real challenge. In this regard, there is a possibility of employing a different approach which effectively modifies the fault current level when the micro-grid alters from grid-connected to the autonomous operation mode and vice versa, through specific externally installed devices. These devices can either increase or decrease the fault level. The main options are as follows: – To decrease the aggregated contribution of many DG sources, which can change the fault current level enough to exceed the design limit of various equipment components, as well as to guarantee an adequate coordination in spite of the feeding effect of DG to fault current, Fault Current Limiters (FCL) can be used [43]. This effect is particularly evident with synchronous machine based DG. – To level the fault current level in both grid-connected and islanded modes of operation, owing to the limited fault contribution by inverter-interfaced DG sources. This can be achieved in two different ways: (a) By applying the energy storage devices (flywheels, batteries, etc.) in the micro-grid will increase the fault current to a desired level, and allowing over-current protection to operate in a traditional way [44,45]. (b) By installing specific devices between the main grid and the micro-grid, to reduce the contribution of fault current from the utility grid [46,47]. The main problems associated with the deployment of these types of devices embedded in the micro-grid are as follows: – Storage devices require large investment. – The deployment of schemes based on a FCL technology is only possible up to a certain amount of DGs connected. For very high levels of DGs, it can be difficult to determine the impedance value of the FCL, due to the mutual influence of the DGs. – Sources with high short-circuit capability (flywheels, etc.) require significant investments, and their safe operation is dependent on the correct maintenance of the unit. – The methods based on an additional current source are highly dependent on the technology of islanding detection and the proper operation of the current source. 2.6. Protection techniques based on over-current and symmetrical components and their challenges These protection techniques which are mainly based on the analysis of current symmetrical components, attempt to improve

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Table 1 Comparison of the available micro-grid protection techniques. Protection technique

Type of relay used

Micro-grid operation mode

Type of DG connected

Availability of the communication link

Cost

Adaptive protection

Distance relay

Rotating-based and inverter-based Rotating-based and inverter-based Inverter-based

Depending on the technique used Yes

Distance protection

No

Reasonable

Voltage-based protection Deployment of external devices Over-current and symmetrical components

– Over-current relay

Grid-connected and islanded Grid-connected and islanded Grid-connected and islanded Islanded Islanded

Reasonable

Differential protection

Voltage restrained over-current relay/numerical directional over-current relay Digital relay

Inverter-based Inverter-based

Yes No

Grid-connected and islanded

Rotating-based and inverter-based

Depending on the technique used

Reasonable Very expensive Reasonable

Digital relay

Expensive

the performance of traditional over-current protection and provide a robust protection system for micro-grids. The main proposal in this field was developed by Nikkhajoei and Lasseter [32] in 2006. They presented a possible solution to recognize the fault in autonomous micro-grids based on the measurements of current symmetrical components. To be precise, the authors proposed to use zero-sequence current detection in the event of an upstream single line-to-ground fault (coordinated with unbalanced loads) and negative sequence current for line-to-line faults. Two years later, Best et al. [48] proposed a three-stage communication assisted selectivity scheme. In the scheme, stage-one recognized the fault event in accordance with the local measurements. Stage-two deployed inter-breaker communications, and stage-three adapted the settings of the relays via a supervisory controller. Subsequently, Zamani et al. [12] designed a microprocessorbased relay with directional element, in conjunction with the fault detection module. The relays were discriminated based on the definite-time scheme, starting from the load side of secondary main and ending at the micro-grid interface point. This caused a longer fault clearance time from the generation side of the grading path but not damaging the micro-grid equipment. The upper limits for the definite time delays at the generating side were determined based on some limitations such as the sensitivity of the critical loads to voltage disturbances, the duration over which electronically-coupled DGs can contribute to the fault current, and the stability of the rotating-machine based DGs. The salient feature of the proposed scheme was that it had no need of communication links between the relays. The main disadvantage of the majority of the above-mentioned protection techniques is related to the necessity of communication systems. In such techniques, the protection coordination may be endangered in case of communication system failure.

them. Based on the analysis of the wide range of technical publications proposed in the previous section, some conclusions and suggestions for the micro-grid protection in the future are as follows:

3. Comparative analysis and suggestions for the protection of micro-grids in the future

Micro-grids have been designed to meet the reliability and power quality needs of customers. Nevertheless, the emergence of micro-grids has been accompanied with significant challenges. Micro-grid protection and its entities is one of them. In recent decades, numerous approaches have been put forward to present an adequate protection technique for micro-grids. A robust protection technique should be able to protect the micro-grid against different types of faults and ensure its safety and secure operation in both grid-connected and autonomous mode. The goal of the current study was to provide a comprehensive review of the existing proposals to deal with the micro-grid protection issues. Additionally, an attempt was made to classify these proposals into specific groups and finally; some conclusions and practical suggestions were derived from the analyzed references.

According to the recent publication [49], the attributes of a protection technique are reliability, selectivity, speed, cost as well as simplicity. Nevertheless, it is impossible to have all the attributes in a single protection technique owing to many contributing factors, like topology change, bi-directionality as well as relay characteristics. Consequently, each protection technique is designed only for a specific test system and DG Technology. Table 1 shows the comparison of the available protection techniques based on the type of relay used, micro-grid operation mode, type of DG connected to the micro-grid, availability of the communication link and cost. Realization of future micro-grids requires that all technical issues are solved. Micro-grid protection and its entity is one of

– In spite of many efforts which have been performed in the field of micro-grid protection, there are still limited numbers of publications. Furthermore, the information given to the micro-grid structures and the proposed techniques are too brief or incomplete. The present observation realizes that most of the published works are generally a form to an idea than a practical proposal. – The majority of the presented techniques to the area of micro-grid protection are strongly dependent on the network architecture. In fact, the techniques do not have the ability to protect different micro-grids with various configurations. – A reliable protection technique must be able to distinguish high impedance faults, which may have current magnitudes similar to those of normal loads, such that it will not be causing any noticeable voltage drop. However, only a small number of publications from the references have considered such a kind of fault. – Most of the proposed techniques are designed only for the micro-grids with radial feeders and are not capable of protecting micro-grids containing looped feeders. – Regardless of the protection technique, it is highly likely that some kind of communication system is going to be necessary, either centrally operated or distributed. – In order to possess an optimal protection system for microgrids, a combined action of different protection techniques will be necessary. 4. Conclusion

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