Investigation of Active and Passive Islanding Detection Techniques

Transcription

1 Investigation of Active and Passive Islanding Detection Techniques M.A.Abdel Razek, R.A.Swief (1), M. A. Badr (1) (1) Department of Electrical Engineering, Faculty of Engineering, Ain-Shams University, Cairo, Egypt Abstract- This paper presents an investigation of different islanding detection techniques which focus on operating parameters. These parameters are to decide whether the system will be islanded or not. The process is based on measuring the voltage, frequency and phase angle at the point of common coupling. The rate of change must be integrated to distinguish the changes in the system, the rate of change are including the change of voltage, rate of change of frequency (ROCOF), rate of change of phase angle difference (ROCPAD). In non detective zones the need of active techniques are also needed to decide if the system in the permissible range of international standard or not. The study is focusing on the ability of each technique and the ranges which is cable of distinguishing the operation. The proposed technique use different method of islanding detection and different cases of loading and compare them to determine the margin and zones that each method can operate effectively in it and the situation that fails. Keywords- rate of change of voltage, ROCOF, ROCPAD, non Detective Zones (NDZ). I. INTRODUCTION Islanding is a situation occurs when a part of the utility system containing both load and DGs disconnected from the main utility but the independent DGs continue to energize this part of utility that isolated from the main grid. This condition can be dangerous to grid workers who may not realize that the load is still energized even through there is no power from the grid [1]. So, when the main grid disconnected and during the islanding operation, the DG can supply electrical power to the loads but according to the IEEE Std. IEEE1547 group has produced a standard which serve as a guide for practicing an international islanding operation in an electric power system [2]. Although DG systems have a vital rule in the distribution system, critical concern is the islanding detection or preventing islanding. Failure to trip islanded DG can lead to a number of problems for DGs systems and the connected load [3]. So the current industry practice is to disconnect all distributed resources (DRs) immediately aft er the occurrence of islands. And also IEEE standard requires the disconnection of DG once it is islanded [4]. IEEE standard requires islanding to be detected and DG disconnected at most 2 seconds [5]. Similarly, IEC also require islanding detection and DG disconnection at most within 2 seconds [5]. The disconnection performed by a special protection schema called islanding detection relays. The international standards must to be considered during the design of islanding detection technique [5]. Islanding detection technique classified into three main groups: (1) Passive methods, (2) Active methods, and (3) communication-based method. Passive methods depended on measuring and monitoring a main parameter of the system and make a decisions based on these measurements and the pre-set thresholds. The advantage of passive techniques are simple techniques, easy to implement and does not have an impact on the normal operation of the DG system. The main disadvantage of this technique is non detective zone (NDZ). Passive techniques include under/over frequency, under/over voltage [2], rate of change of frequency ( ROCOF) [6], rate of change of voltage [5], rate of change of power signal [10], the phaseshift method [10], the vector surge technique [10], the harmonic impedance estimation technique [10]. Active methods are based on injecting locally signal into the system and then monitor the response to determine an islanding condition. The advantage of this technique that has smaller NDZ but it has drawback on the system, that it can decrease the power quality of the system. Active technique includes impedance measurement, voltage phase jump, phase shift, Slip Mode frequency (SMF) [11], Frequency Bias or Active frequency Shift (AFD) [11], Sandia frequency shift (SFS) [12], frequency jump (FJ) [12], and Sandia voltage Shift (SVS) [12]. The third technique is the communication-based methods (Remote technique) which are telecommunication devices that are designed to trip DG s when islands are formed. These methods have negligible NDZ but are more expensive methods and not very popular due to the need of communication between DG and Grid. These technique classified into three category: a) supervisory and data acquisition (SCADA) method, b) method use power line to carry signals of communications and c) methods that monitor the device used to connect/disconnect from grid [13]. Table 1 illustrate the detection time of the conventional methods. This paper investigates different techniques of islanding detection which uses passive and active technique to incorporate the advantage of two methods and reduce their drawbacks. It uses multiple system parameters to identify and classify any possible islanding operation at a specific target location (location at which the islanding detection is Reference Number: JO-P

2 to be done). The technique use under / over voltage method, under/over frequency, ROCOF, ROCPAD as passive technique and rate of change of voltage and real power shift (RPS) as active technique.the paper also compare the different technique and decide the best technique in each situation of islanding cases. This paper s content is organized as follows, section II explains the proposed islanding detection method and mathematical model, and section III presents the system and DG interface model under study. Section IV presents simulation results. Finally; conclusions are drawn in section V. II. DIFFERENT ISLANDING DETECTION TECHNIAQE The concept of the proposed technique based on monitoring voltage and frequency. The first technique is Voltage and frequency deviation.according to IEEEStd.1547and UL 1741 that provide thresholds on the amount of acceptable voltage and frequency deviation. Thresholds on voltage deviations are in the range of 88% to 110% of nominal voltage value [2]. Any voltage deviation, resulting from an islanding condition, within these limits, would not be detected and the corresponding load would be considered within the NDZ but not within these limits, the relay will detect islanding. Also the permissible frequency is 59.5HZ. If the system frequency falls into less than 59.5 the decision from the point of view of control is made to shed some of load but from the point of view of protection study is to islanded the system and disconnect all DGs [7&8]. The seconded technique is to calculate the rate of change of voltage. In this method, the average rate of voltage change is calculated as passive method. As voltage is measured at PCC every voltage period and calculate rate of change of voltage (dv/dt). When dv/dt >VSMin (where VSMin is a minimum set point to suspect islanding) is detected, the magnitude of average rate of change voltage change during t1 time interval (Avt1) is used to check whether the system has been islanded or not. Avt1 is given by equation (1)[5]: Avt 1 = 1 N 1 dv ( ) N 1 i = 1 dt Where, N1 is the number of dv/dt measurement in t1 time interval. If Avt1 is larger than VSMin (Avt1 > VSMin ), islanding is suspected. Nevertheless this is not islanding condition; in this case the system can be operating with this small influence. But if Avt1 is larger than VSMax (Avt1 > VSMax) as a result of a large mismatch of generation and demand, it is cleared the system is islanded. However, if Avt1 is between VSMin and VSMax, the change in voltage can be result of islanding or any other normal events in distribution system.in such a situation real power shift (RPS) is used and go to the third technique[5]. i (1) TABLE 1 DETECTION TIME OF THE CONVENTIONAL METHODS Detection method Detection time(sec) Over/Under voltage 0.03/0.05 Over/Under frequency 0.02/0.02 PLL based active method Active frequency drift 0.09 Current Injection Current Injection Current Injection 3 1 Current Injection The Third technique is an active technique because it injects (Real Power Signal) RPS. The technique calculate the magnitude of the average rate of change voltage change during t2 (Avt2) after the initiation of the RPS. (Avt2) is used to differentiate islanding from any other event in the distribution system. If Avt2 is larger than VSMaxU (where VSMaxU is set point to detect islanding with RPS),then it is from an islanding condition. Avt2 is given by equation(2)[5]: 1 N 2 dv Avt 2 = ( ) i (2) N 2 i = 1 dt Where, N2 is the number of dv/dt measurement in t2 time interval. The Fourth technique is the rate of change of frequency (ROCOF). Change of frequency is caused by the imbalance between the input mechanical power and load. This imbalance is due to the disconnection from the grid. After disconnection from the grid, the frequency starts to change and therefore the ROCOF algorithm detects this fluctuation in frequency.the model of the relay utilized in this paper implements the rate of change of frequency measurement based on the following by equation (3) [1&6&14]: df dt = f n f T d Where: f n : Nominal frequency f c : current frequency measurement T d : time duration between the nominal frequency and Current frequency The Fifth technique is rate of change of phase angle difference (ROCPAD): As the system measure phase angle, ROCPAD can be calculated by equation (4) [3&10]: ROCPAD Where: δ n : nominal Phase angle δ c : current Phase angle t: time duration between the nominal frequency and current frequency = c ( n c ) t (3) (4) Reference Number: JO-P

3 II. POWER SYSTEM MODEL The system study is shown in fig.1 consists of DG unit (gas turbine generator) connected to the main supply (main grid) through point of common coupling (PCC). The DG unit is placed at a distance of 50 km with distribution lines. The operating voltage of this system is 19.5 kv. This system is heavy loading system. The details of the generator, DG, transformers, distribution lines, and load are mentioned as below. Fig 1 shows the schematic diagram of the test system. DG : 3.3MVA,6.3KVA,60HZ Transformer(TR1):20MVA,60KVA/20KVA,60HZ. Transformer(TR2):3.3MVA,6.3KV/20KV,60HZ. Distribution Lines(DL):50km,60HZ Nominal loading data:l1=20kv,2.6mw,2.6mvar PCC Main Grid TR1 DL Fig1: Microgrid with DG interface The voltage, frequency and phase shift are monitored at the PCC.The complete simulation is carried out using Matlab (SIMULINK) package and the flow chart in fig.2 shows the steps of the proposed algorithm. There are many possible situations of islanding and non islanding conditions, this paper just deal with the situation of sudden change of loading at PCC. The existed load is resistive and in the study load will be changed 0% to 80% of the load at PCC [1]. The time of inspection of sudden load (Disturbance) will be at second 1. The study starts with the first, third and forth technique and then second technique. CB L1 Fig 2: Procedure flow chart TR2 DG IV SIMULATION AND RESULTS A. The first technique is Voltage and frequency deviation with load change at PCC from 0% to 80%MW. The technique is tested on the system shown in fig(1).an islanding condition occur at the time of inspection by opening the circuit breaker of utility. The technique monitors frequency and the voltage and calculates the voltage drop. And compare it with the permissible range of voltage according to IEEE as mentioned. The TABLES (2) and (3) illustrate the values of Voltage drop and frequency respectively. TABLE 2 Values of voltage drop for loading from 0% to 80 % and the decision of Islanding or Non-Islanding Time Duration of Simulation =1.36 sec Time inspection = 1sec Load % Voltage drop % Islanding Non-Islanding No Load % 1% % 2.07% 20% 2.86% 30% 3.68% 40% 4.52% 50% 5.39% 60% 6.26% 70% 7.18% 80% 8.1% TABLE 3 Values of frequency for loading from 0% to 80 % and the decision of islanding or non-islanding Time Duration of Simulation 1.36 sec Time inspection=1 sec Load % Frequency(HZ) Islanding Non-Islanding No Load % % % % % % % % % 59.5 Table (2) show s that the loading from 0% to 50 % is not islanding but above 50% loading the system is in critical situation and there is no accurate decision show this cases is islanding or not although this cases in the permissible range of voltage. Also Table (3) show that all conditions are in permissible range of frequency, But as mentioned previously, from the view of protection the system should be islanded and DG shut down. So in the cases above, we are not sure that the system is islanding or non-islanding with this technique. So we go to next techniques. B. The second and third technique are calculating the rate of change of voltage and initiation of RPS. As auto reclosing to Reference Number: JO-P

4 clear temporary faults widely uses and hence it is essential to detect islanding before the auto-recloser operates to avoid out of phase reclosing. According to Danish distribution networks the typical recloser opening time used during the fast operation is 500ms [5]. So assuming that the relay and circuit breaker take less than 140ms to clear a fault during an instantaneous pickup, and hence t1and t2 are chosen as 140ms and 360ms, respectively. It is assumed that dv/dt is calculated once every voltage period, then N1and N2 are 7 and 18 respectively. so Avt1 and Avt2 are here called Av7 and Av18.In case of large mismatch(av7 >VSMin ) islanding can be detected in almost 140ms.In case of VSMin > Av7 > VSMax,the RPS is initiated and Av18is computed. The values for the RPS, VSMin, VSMax, VSMaxU, are also system specific. So the value for VSMin is set at 40 V/s, VSMax is set at 7200 V/s, VSMaxU is set at 100 V/s and the RPS changes the DG power set point by 2%.The RPS is initiated after t1 time passed. Av18 is calculated during time t2 has passed. Table (4) illustrates different scenarios with different loading condition starting 0% to 80% sudden-load change (Pure Resistive load) at PCC. Table (4) confirms that for no loading condition and loads 1% Disturbance, the system is in Non-islanding condition, but for more than 10% loading we are make sure with this technique that the system is in islanding condition. sudden load change at PCC starting from 0% to 80%.It is obvious from that at high value of load change(active power imbalance at higher end) the magnitude of ROCOF is higher and thus ROCOF easily to identify. And as the value of load change is small (active power imbalance decrease) the magnitude of ROCOF diminishes and it is difficult to detect the islanding situation. If a threshold is 0.5 or 0.2, then it cannot detect islanding for 0% load change. Thus, ROCOF fails to detect islanding for sudden-load change falls below 15%[1]. TABLE 4 Values of Rate of change of voltage for loading from 0% to 80 % and the decision of islanding or non-islanding Time Duration of Simulation 1.36 sec Time inspection=1 sec, RPS decrease set point at DG by 2 % Load % AV7 AV18 Islanding Non-Islanding No Load e % e % e % e % e % e % e % e % e % e Table ( 4) shows that the loading from 0% to 1 % is not islanding case but the other cases where clear islanding according to the set thresholds. C. The Fourth technique is rate of change of frequency (ROCOF): Fig (3) shows the ROCOF for comparison between islanding and non islanding condition with sudden load change at PCC. The event of inspection (islanding or non-islanding) is initiated at 1 sec. It is observed that the islanding conditions can be distinguished from non islanding condition with a set threshold. The active power imbalance (power mismatch-load sudden change) is one of the critical factors which decide the operating zone of ROCOF relays [1]. Fig (4) shows the comparisons of ROCOF for different Fig.3 ROCOF for 0% to80% loading condition D.The Fifth technique is rate of change of phase angle difference (ROCPAD): Fig (5) shows the performance of ROCPAD for islanding and non islanding conditions. It is observed that the magnitude of ROCPAD sharply changes during islanding compared to non islanding condition. Thus a threshold may be 50 degree/sec or 100 degree/sec [1] can be set to issue the tripping signal. The response time of ROCPAD is within 15 ms from the event inspection with 100 degree/s as threshold, which is less than one cycle (20ms) from the islanding inspection. Fig(6) shows the ROCPAD for different sudden-load change ( active power mismatch) stating from 0% to 80%and it is observed that setting one absolute threshold(may be 100 degree/sec) works effectively for islanding detection for wide range of active power imbalance. As shown in fig. (5) That for 0% to 10 % is not islanding. So this technique indicate that for 10 % loading the Reference Number: JO-P

5 system can withstand and operate normally although other previous techniques fails in this case Fig.5 ROCPAD for islanding and non islanding condition Fig.6 ROCPAD for 0% to80% loading condition VI. CONCLUSION An investigated study of different islanding detection techniques has been done showing the leak ability of most of passive detection techniques. The passive techniques which are based on the parameters itself are not fit in islanding detection but may help after the islanding took place. The passive techniques based on rate of changes are more efficient in islanding detections but fails in some cases. Integrating the active signal with the passive signals clarified more fuzzy cases in islanding detection. So, the need to combine more than active and passive technique will lead almost to the best decision as investigated in the paper. REFERENCES [1] Ankita Samui and S.R. Samantaray Assessment of ROCOPAD Relay Islanding Detection in Distributed Generation IEEE Trans. On Smart Grid, Vol.2, No.2, pp , June [2] Hazlie Mokhils,Mahzahr Karimi,Amidaddin Shahriari,Abd Halim Abu Bakar,Javed Ahmed Laghari A New Under-Frequency Load Shedding Scheme for Islanded Distribution Network IEEE,Innovative smart grid technologies(isgt),pp1-6, Feb2013 [3] H.H.Zeineldin, and James L.Kirtley A Simple Technique for Islanding Detection With Negligible Nondetection Zone IEEE Trans. On Power Delivery, Vol.24, No.2, pp April [4] Khalil El-Arroudi,Géza Joós,Innocent Kemwa and Donald T.McGillis Intelligent-Based Approach to Islanding Detection in Distributed Generation IEEE Trans. On Power Delivery, Vol.22, No.2, pp april [4] Adam Dysko,Graem Burt,Rafal Budgal Novel Protection methods for active distribution networks with high penetrations of distribution generation DTI center for distributed generation and sustainable electrical energy,pp.10-11,june2006. [5] Jae-Hyung Kim,Jun-Gu Kim,Young-Hyok Ji,Yong- Chae Jung,and Chung-Youn Won An Islanding Detection method for a Grid-connected System Based on the Goertzel Algorithm IEEE Trans. On Power Electronics, Vol.26, No.4,pp , April 2011 [6] Pukar Mahat Control and Operation of Islanded Distribution System in Proc.2010 Denmark PhD,Alaborg university,pp1-28,sep [7] Refal Bugdal, Adam, Dysko, G.M.Burt, J.R.MCDonald, Performance analysis of ROCOF and Vector Shift methods using a dynamic protection modeling approach PSP 2006,Effect of distributed generation on power system protection,pp ,2006. [8] J.A.Laghari,Hazlie Mokhlis,A.B.Halim Abu Bakar,M.Karimi,A.Shahriari An Intelligent Under Frequency Load Shedding Scheme for Islanded Distribution Network IEEE international Power Engineering and Optimization Conference (PEOCO2012),Melaka,Malaysia, pp.40-45,june [9] David Reigosa,Fernando Briz, Cristian Banco,Pablo Garcia,Juan Manuel Guerrero Active Islanding Detection Using High Frequency Signal Injection IEEE2011,Energy conversion congress & Exposition (ECCE),pp ,Sep2011. [10] Mehrnoosh Vatani,Turaj Amrall & Iman Soltan Comparative of Islanding Detection Passive methods for Distributed Generation Application International Jouurnal of innovation & Scientific Research Vol.8 No.2,pp ,Sep [11] Nattapon Boonyapakdee,Tirasak Sapaklom,and Mongkol Konghirum An Implement of Improved Combine Active Islanding Detection Method on frequency and phase perturbations IEEE,International Conference on Electrical Machines and System,pp ,Oct2013. [12] Ward Bower & Michalel Ropp Evaluation of islanding detection methods for utilty-interactive Inverter in photo voltic systems Sandia National Laboratories,pp1-60,NOV2002 [13] Truptimayee Pujhari Islanding Detection in Distributed Generation Ms.c,National institute of technology Rourkela,pp.1-27,May2009. Reference Number: JO-P