Review of Islanding Detection Methods for Distributed ...

DRPT2008 6-9 April 2008 Nanjing China

Review of Islanding Detection Methods for Distributed Generation Pukar Mahat, Zhe Chen and Birgitte Bak-Jensen

Abstract-- This paper presents an overview of power system islanding and islanding detection techniques. Islanding detection techniques, for a distribution system with distributed generation (DG), can broadly be divided into remote and local techniques. A remote islanding detection technique is associated with islanding detection on the utility side, whereas a local technique is associated with islanding detection on the DG side. Local techniques can further be divided into passive techniques, active techniques and hybrid techniques. These islanding detection techniques for DG are described and analyzed. Index Terms-- Distributed generation, islanding detection.

I. INTRODUCTION

T

ODAY’S power system operates through the synchronized operation of large power plants producing bulk power and transmitting it through long transmission lines at high voltage before reducing the voltage for consumption in customer premises. This is mainly due to the cost of production of bulk quantities of electricity being much lower than the cost of producing many small quantities of electricity. But the advancement in technologies like fuel cells, gas turbines, micro-hydro, wind turbines and photovoltaic, new innovation in power electronics, electricity market deregulation, customer’s demand for better power quality and reliability, and above all environment concern are forcing the power industry for yet another shift and this time back to the distributed and dispersed generation. Distributed or dispersed generation may be defined as generating resources, other than central generating stations, that is placed close to load being served, usually at customer site. The number of DG in distribution system is rising as DG can avoid transmission and distribution (T&D) capacity upgrades, reduce transmission and distribution line losses, improve power quality, improve voltage profile of the system, etc [1]. In fact, many utilities around the world already have a significant penetration of DG in their system. But there are many issues to be taken into account with the DG and one of the main issues is islanding.

P. Mahat is with the Institute of Energy Technology, Aalborg University, Pontoppidanstræde 101, Aalborg, DK-9220, Denmark (e-mail: [email protected]). Z. Chen is with the Institute of Energy Technology, Aalborg University, Pontoppidanstræde 101, Aalborg, DK-9220, Denmark (e-mail: [email protected]). B. Bak-Jensen is with the Institute of Energy Technology, Aalborg University, Pontoppidanstræde 101, Aalborg, DK-9220, Denmark (e-mail: [email protected]).

978-7-900714-13-8/08/ ©2008DRPT

Islanding is the situation in which a distribution system becomes electrically isolated from the remainder of the power system, yet continues to be energized by DG connected to it. Traditionally, a distribution system doesn’t have any active power generating source in it and it doesn’t get power in case of a fault in transmission line upstream but with DG, this presumption is no longer valid. Current practice is that almost all utilities require DG to be disconnected from the grid as soon as possible in case of islanding. IEEE 929-1988 standard [2] requires the disconnection of DG once it is islanded and IEEE 1547-2003 standard [3] stipulates a maximum delay of 2 seconds for detection of an unintentional island and all DGs ceasing to energize the distribution system, as there are various issues with unintentional islanding. II. ISSUES WITH ISLANDING Although there are some benefits of islanding operation, there are some drawbacks as well. Some of them are as follows: • Line worker safety can be threatened by DG sources feeding a system after primary sources have been opened and tagged out. • The voltage and frequency may not be maintained within a standard permissible level. • The islanded system may be inadequately grounded by the DG interconnection. • Instantaneous reclosing could result in out of phase reclosing of DG. As a result of which large mechanical torques and currents are created that can damage the generators or prime movers [4]. Also, transients are created, which are potentially damaging to utility and other customer equipment. Out of phase reclosing, if occurs at a voltage peak, will generate a very severe capacitive switching transient and in a lightly damped system, the crest over-voltage can approach three times rated voltage [5]. Due to these reasons, it is very important to detect the islanding quickly and accurately. III. ISLANDING DETECTION The main philosophy of detecting an islanding situation is to monitor the DG output parameters and/or system parameters and decide whether or not an islanding situation has occurred from change in these parameters. Islanding detection techniques can be divided into remote and local techniques and local techniques can further be divided into

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passive, active and hybrid techniques as shown in Figure 1.

Fig. 1. Islanding detection techniques

A. Remote islanding detection techniques Remote islanding detection techniques are based on communication between utilities and DGs. Although these techniques may have better reliability than local techniques, they are expensive to implement and hence uneconomical. Some of the remote islanding detection techniques are as follows: Transfer trip scheme: The basic idea of transfer trip scheme is to monitor the status of all the circuit breakers and reclosers that could island a distribution system. Supervisory Control and Data Acquisition (SCADA) systems can be used for that [6]. This method requires a better interaction between the utility and DGs and this often increases the costs for both the utility and DG owners. Power line signaling scheme: Here, a signal generator at the transmission system continuously broadcasts a signal to the distribution feeders using the power line as the signal path. DGs are equipped with signal receivers. If the receiver does not sense the signal (caused by the opening of breakers between the transmission and distribution systems), there is an island condition [7-9]. Figure 2 shows a power line signaling scheme. This scheme can be effectively used in multi DG system.

Fig. 2. A power line signaling scheme

B. Local detection techniques It is based on the measurement of system parameters at the DG site, like voltage, frequency, etc. It is further classified as: 1. Passive detection techniques Passive methods work on measuring system parameters such as variations in voltage, frequency, harmonic distortion, etc. These parameters vary greatly when the system is islanded. Differentiation between an islanding and grid

connected condition is based upon the thresholds set for these parameters. Special care should be taken while setting the threshold value so as to differentiate islanding from other disturbances in the system. Passive techniques are fast and they don’t introduce disturbance in the system but they have a large non detectable zone (NDZ) where they fail to detect the islanding condition. There are various passive islanding detection techniques and some of them are as follows: Rate of change of output power: The rate of change of output power, dP / dt , at the DG side, once it is islanded, will be much greater than that of the rate of change of output power before the DG is islanded for the same rate of load change [6]. It has been found that this method is much more effective when the distribution system with DG has unbalanced load rather than balanced load [10]. Rate of change of frequency: The rate of change of frequency, df / dt , will be very high when the DG is islanded. The rate of change of frequency (ROCOF) can be given by [11] df ∆P ROCOF , = f (1) dt 2 HG Where, ∆P is power mismatch at the DG side H is the moment of inertia for DG/system G is the rated generation capacity of the DG/system Large systems have large H and G where as small systems have small H and G giving larger value for df / dt . ROCOF relay monitors the voltage waveform and will operate if ROCOF is higher than setting for certain duration of time. The setting has to be chosen in such a way that the relay will trigger for island condition but not for load changes. This method is highly reliable when there is large mismatch in power but it fails to operate if DG’s capacity matches with its local loads. However, an advantage of this method along with the rate of change of power algorithm is that, even they fail to operate when load matches DG’s generation, any subsequent local load change would generally lead to islanding being detected as a result of load and generation mismatch in the islanded system. Rate of change of frequency over power: df / dP in a small generation system is larger than that of the power system with larger capacity. Rate of change of frequency over power utilize this concept to determine islanding condition. Furthermore, test results have shown that for a small power mismatch between the DG and local loads, rate of change of frequency over power is much more sensitive than rate of change of frequency over time [12]. Change of impedance: The utility impedance is considerably smaller than the impedance of a power island. The impedance of a section of network will increase when that section becomes disconnected from the utility [13-14]. Continuous monitoring the source impedance will give the idea of whether the system is islanded or not. Voltage unbalance: Once the islanding occurs, DG has to take charge of the loads in the island. If the change in loading is large, then islanding conditions are easily detected by monitoring several parameters: voltage magnitude, phase

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DRPT2008 6-9 April 2008 Nanjing China

displacement, and frequency change. However, these methods may not be effective if the changes are small. As the distribution networks generally include single-phase loads, it is highly possible that the islanding will change the load balance of DG. Furthermore, even though the change in DG loads is small, voltage unbalance will occur due to the change in network condition [15-16]. Harmonic distortion: Change in the amount and configuration of load might result in different harmonic currents in the network, especially when the system has inverter based DGs [15]. One approach to detect islanding is to monitor the change of total harmonic distortion (THD) of the terminal voltage at the DG before and after the island is formed [17]. The change in the third harmonic of the DG’s voltage also gives a good picture of when the DG is islanded [18]. 2. Active detection techniques With active methods, islanding can be detected even under the perfect match of generation and load, which is not possible in case of the passive detection schemes. Active methods directly interact with the power system operation by introducing perturbations. The idea of an active detection method is that this small perturbation will result in a significant change in system parameters when the DG is islanded, whereas the change will be negligible when the DG is connected to the grid. Some of the active detection techniques are as follows: Reactive power export error detection: In this scheme, DG generates a level of reactive power flow at the point of common coupling (PCC) between the DG site and grid [11] or at the point where the Reed relay is connected [14]. This power flow can only be maintained when the grid is connected. Islanding can be detected if the level of reactive power flow is not maintained at the set value. For the synchronous generator based DG, islanding can be detected by increasing the internal induced voltage of DG by a small amount from time to time and monitoring the change in voltage and reactive power at the terminal where DG is connected to the distribution system. A large change in the terminal voltage, with the reactive power remaining almost unchanged, indicates islanding [19]. The major drawbacks of this method are it is slow and it can not be used in the system where DG has to generate power at unity power factor. Impedance measurement method: The main philosophy is the same as that of the passive technique that the impedance of the system changes with islanding. In an active direct method, a shunt inductor is briefly connected across the supply voltage time to time and the short circuit current and supply voltage reduction is used to calculate the power system source impedance [11]. However, in an indirect method, a high frequency signal is injected on the DG terminal through a voltage divider. This high frequency signal becomes more significant after the grid is disconnected [13]. Phase (or frequency) shift methods: Measurement of the relative phase shift can give a good idea of when the inverterbased DG is islanded. A small perturbation is introduced in form of phase shift. When the DG is grid connected, the

frequency will be stabilized. When the system is islanded, the perturbation will result in significant change in frequency. The Slip-Mode Frequency Shift Algorithm (SMS) [20] uses positive feedback which changes phase angle of the current of the inverter with respect to the deviation of frequency at the PCC. A SMS curve is given by the equation:  π f ( k −1) − f n   (2) θ = θ m sin   ( ) 2 f f − m n   Where θ m is the maximum phase shift that occurs at

(

)

frequency f m . f n is the nominal frequency and f ( k −1) is the frequency at previous cycle. A SMS curve is designed in such a way that its slope is greater than that of the phase of the load in the unstable region. A SMS curve, with θ m =10° and f m = 53 Hz, is shown in Figure 3. When the utility is disconnected, operation will move through the unstable region towards a stable operating point (denoted by black dots in Figure 3). Islanding is detected when the inverter frequency exceeds the setting.

Fig. 3. Phase response of DG and local load

This detection scheme can be used in a system with more than one inverter based DG. The drawback of this method is that the islanding can go undetected if the slope of the phase of the load is higher than that of the SMS line, as there can be stable operating points within the unstable zone [21]. Active Frequency Drift (AFD) is implemented by adding a short period of zero time in the output current of the inverter based DG, as shown Figure 4. TI and TV are the time period for a half cycle of DG’s output current and utility voltage, respectively. TZ , dead or zero time, is the time for which the DG’s output current remains zero. When such distorted waveform is applied and there is no grid connection, the voltage frequency of the islanded system will shift up or down continuously as the inverter operating at unity power factor tends to seek the resonant frequency of the local load. This method is very effective for purely resistive loads but it may fail for other loads [22].

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 Tav − T ( k −1)   ( k −1)   T 

k) θ (Adb = π 

(7)

Where, Tav =

Fig. 4. Distorted current waveform for AFD

Active Frequency Drift with Positive Feedback Method (AFDPF) increases the dead time of the inverter current corresponding to the increase in deviation of the frequency away from nominal [22]. The problem with the AFDPF is that the phase angle of a parallel RLC load depends on the operating frequency and this sometimes may result in islanding not being detected. The Automatic Phase-Shift Method (APS), a modified form of SMS, can get rid of this (k ) ) of the inverter [23]. With APS, only the starting angle ( θ APS output current is changed according to frequency of the previous voltage cycle ( f ( k −1) ) as (k ) θ APS

1  f ( k −1) − f n  360° + θ 0( k ) =   α  fn 

(3)

An additional phase shift ( θ0k ) is introduced each time the frequency of the terminal voltage stabilizes before the trip points, which changes as (4) θ 0(k ) = θ 0(k −1) + ∆θ sys(∆f ss ) Where ∆θ is a constant ∆f ss is the change in the steady state frequency

θ 0(k ) =0; ∀k ≤ 0

(5)

if ∆f ss > 0 1  sys(∆f ss ) = 0 if ∆f ss = 0 (6) − 1 if ∆f < 0 ss  It keeps the frequency of the inverter terminal voltage deviating until the trip point is reached when the system is islanded. Since the additional phase shift is only added at each possible stable operating point, the APS algorithm sometimes acts slowly and fails to detect islanding during certain load conditions. An Adaptive Logic Phase Shift (ALPS) algorithm regulates the additional phase shift and evaluates the effect of every phase shift [24]. This algorithm will produce a small phase shift when the inverter based DG is grid connected, whereas islanding would result in a quick phase shift. The phase shift between the inverter voltage and output current can be given by

( )

1  k −1 (i )  ∑T N  (i=k −1− N )

(8)  The period of the output current is chosen assuming the constant period of grid voltage in case of the APS, whereas, in case of the ALPS, it is chosen as the average of the previous N/2 voltage periods, which can represent the actual variation of frequency in the real power system [23]. If the DG site has been islanded from the grid then, for i=k-1, k, - - -, k-1+N, it is likely that if θ Ad > 0 then ∆T =T av−T ( i+1) > 0  (9) or , if θ Ad < 0 then ∆T =T av −T ( i+1) < 0  Where, (k ) (k ) θ Ad = θ Adb + ∆θ 0( k )

(10)

The additional phase shift, ∆θ 0( k ) (given by equations (4) and (5)), is applied when the probability of cause and effect (PCE) which satisfies the equation (9) is greater than 0.5 at the end of N voltage cycles. Average value of N/2 voltage periods is calculated. If the difference between two consecutive N/2 periods is small or the PCE is less than 0.5, indicating the grid is available, then the additional phase shift is disabled by setting ∆θ 0( k ) = 0. 3. Hybrid detection schemes Hybrid methods employ both the active and passive detection techniques. The active technique is implemented only when the islanding is suspected by the passive technique. Some of the hybrid techniques are discussed as follows: Technique based on positive feedback (PF) and voltage imbalance (VU): This islanding detection technique uses the PF (active technique) and VU (passive technique) [25]. The main idea is to monitor the three-phase voltages continuously to determinate VU which is given as V+ Sq VU = (11) V− Sq V + Sq

and V−Sq

are the positive and negative sequence

voltages, respectively. Voltage spikes will be observed for load change, islanding, switching action, etc. Whenever a VU spike is above the set value, frequency set point of the DG is changed. The system frequency will change if the DG has been islanded. Technique based on voltage and reactive power shift: In this technique voltage variation over a time is measured to get a covariance value (passive) which is used to initiate an active islanding detection technique, adaptive reactive power shift (ARPS) algorithm [26]. (12) Co-variance ( Tav ' , Tv ) = E Tav(n') − U av Tv( n ) − U v Where,

(

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)(

)

DRPT2008 6-9 April 2008 Nanjing China

Tav ' is the average of the previous four voltage periods. U av is the mean of Tav ' Tv is the voltage periods U v is the mean of Tv The ARPS uses the same mechanism as ALPS, except it uses the d-axis current shift instead of current phase shift. The d-axis current shift, i dk , or reactive power shift is given as idk = k d (

Tav' − Tv(k ) Tv(k )

)

(13)

k d is chosen such that the d-axis current variation is less than 1 percent of q-axis current in inverter's normal operation. The additional d-axis current, after the suspicion of island, would accelerates the phase shift action, which leads to a fast frequency shift when the DG is islanded.

which can be broadly classified into remote and local techniques. Local techniques are further divided into passive, active and hybrid techniques. Each technique has its own advantage and limitation. Table 1 summarizes the islanding detection techniques, their advantage and disadvantage, and examples. There is no single islanding detection technique which will work satisfactorily for all systems under all situations. The choice of the islanding detection technique will largely depend on the type of the DG and system characteristics. Recently, hybrid detection techniques have been proposed and it seems that the hybrid detection technique is the way to go with passive technique detecting the islanding when change in system parameter is large and initiating the active technique when the change in system parameter is not so large for the passive technique to have an absolute discrimination.

IV. COMPARISON AND DISCUSSION Many islanding detection techniques have been proposed, TABLE I COMPARISON OF ISLANDING DETECTION TECHNIQUES

Islanding Detection Techniques 1 Remote Techniques

Advantages • Highly reliable

Disadvantages • Expensive to implement especially for small systems.

• Transfer trip scheme • Power line signaling scheme

• Difficult to detect islanding when the load and generation in the islanded system closely match • Special care has to be taken while setting the thresholds • If the setting is too aggressive then it could result in nuisance tripping • Introduce perturbation in the system • Detection time is slow as a result of extra time needed to see the system response for perturbation • Perturbation often degrades the power quantity and if significant enough, it may degrade the system stability even when connected to the grid

• Rate of change of output power scheme • Rate of change of frequency scheme • Rate of change of frequency over power scheme • Change of impedance scheme • Voltage unbalance scheme • Harmonic distortion scheme

2 Local Techniques

a. Passive Techniques

• Short detection time • Do not perturb the system • Accurate when there is a large mismatch in generation and demand in the islanded system

b. Active Techniques

• Can detect islanding even in a perfect match between generation and demand in the islanded system (Small NDZ)

c. Hybrid Techniques

• Have small NDZ. • Perturbation is introduced only when islanding is suspected.

Examples

• Islanding detection time is prolonged as both passive and active technique is implemented

• Reactive power export error detection scheme • Impedance measurement scheme • Phase (or frequency) shift schemes (like SMS, AFD, AFDPF and ALPS)

• Technique based on positive feedback and voltage imbalance • Technique based on voltage and reactive power shift

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V. CONCLUSION

[19]

This paper describes and compares different islanding detection techniques. Fast and accurate detection of islanding is one of the major challenges in today’s power system with many distribution systems already having significant penetration of DG as there are few issues yet to be resolved with islanding. Islanding detection is also important as islanding operation of distributed system is seen a viable option in the future to improve the reliability and quality of the supply.

[2] [3] [4] [5] [6] [7] [8]

[9]

[10]

[11] [12] [13]

[14] [15]

[16]

[17] [18]

[21]

[22]

[23]

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VII. BIOGRAPHIES Pukar Mahat received B.Eng and M.End degree from Kathmandu University, Nepal and Asian Institute of Technology, Thailand, respectively. Currently, he is a PhD student at Institute of Energy Technology, Aalborg University, Denmark. His research interest includes Distributed Generation, Wind Power Generation and Distribution System Planning. Zhe Chen (M’95, SM’98) received the B.Eng. and M.Sc. degrees from Northeast China Institute of Electric Power Engineering, Jilin City, China, and the Ph.D. degree from University of Durham, U.K. He was a Lecturer and then a Senior Lecturer with De Montfort University, U.K. Since 2002, Dr. Chen became a Research Professor and is now a Professor with the Institute of Energy Technology, Aalborg University, Denmark. His background areas are power systems, power electronics and electric machines; and his main current research areas are renewable energy and modern power systems. Dr Chen has more than 140 publications in his technical field. He is an Associate Editor of the IEEE Transactions on Power Electronics, a Member of the Institution of Engineering and Technology (London, U.K.), and a Chartered Engineer in the U.K. Birgitte Bak-Jensen (M’88) received her M.Sc. degree in Electrical Engineering in 1986 and a Ph.D. degree in “Modelling of High Voltage Components” in 1992, both degrees from Institute of Energy Technology, Aalborg University, Denmark. From 1986-1988 she was with Electrolux Elmotor A/S, Aalborg, Denmark as an Electrical Design Engineer. She is an Associate Professor in the Institute of Energy Technology, Aalborg University, where she has worked since August 1988. Her fields of interest are modelling and diagnosis of electrical components, power quality and stability in power systems. During the last years, integration of dispersed generation to the network grid has become one of her main fields, where she has participated in many projects concerning wind turbines and their connection to the grid.

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