An Active Islanding Detection Method for Grid ...

An Active Islanding Detection Method for Grid-Connected Converters Fangrui Liu, Xinchun Lin, Yong Kang, Yu Zhang and Shanxu Duan College of Electrical and Electronic Engineering Huazhong University of Science and Technology Wuhan 430074, China

Abstract-Islanding detection is a mandatory function for gird-connected converters. The popular slip mode frequency shift and auto phase shift active islanding detection methods are investigated and an improved slip mode frequency shift (IM-SMS) strategy is proposed in this paper. In the proposed method, additional phase shift is introduced to help stimulating the islanding detection function and the algorithm is simplified as well. When the utility grid is disconnected, the algorithm keeps the frequency of the converter output voltage deviating until the frequency protection relay is triggered. The working principle of the method is introduced and the guidance of parameters selection is also provided. The islanding detection performance is evaluated through theoretical analysis, digital simulation and experiment. The IM-SMS method exhibits features of simplicity, easy implementation and high reliability and is expected to be an effective active islanding method.

I.

INTRODUCTION

Due to the increasing energy consumption around the world and the eminent exhaustion of fossil energy resources, more attention can be noticed on the renewable energy resources such as solar power, wind power and fuel cell. They are usually utilized to generate electric power and connected to utility grid through grid-connected converter. And such converter is required to present an effective islanding detection function for protection purpose [1-9]. Islanding phenomena of grid-connected converters refers to their independent operation when the utility is disconnected. The local section is isolated from the power system but still energized by the converters [1]. It causes a number of undesirable effects, such as the danger to utility maintenance personnel and equipment malfunction [2]. An over voltage relay (OVR), an under voltage relay (UVR), an over frequency relay (OFR), and an under frequency relay (UFR) enable the grid-connected converter basic islanding detection capability [1, 5]. However, large none detection zones (NDZ) still exists with such relays. A number of methods have been proposed to reduce NDZ. The passive detection methods including voltage phase jump detection, voltage harmonic detection suffer a trip threshold problem [2]. The active methods which introduce perturbations into the system draw much attention. It requires that the NDZ should be well reduced and the influence on Project Supported by Delta Science & Technology Educational Development Program (DREK200501).

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power quality as low as possible. Among all the active detection techniques, active frequency drift with positive feedback (AFDPF) method [2, 3] is an effective away to detect islanding by forcing the frequency of voltage in the island to drift up or down. However, zero intervals usually exist in the converter output current waveforms, resulting in a lower output power quality. Slip mode frequency shift (SMS) method alleviates such problem by introducing phase shift perturbation [2-4]. If the perturbation at nominal grid frequency operation is pretty small, the islanding may not be detected within the specified time (e.g. IEEE Std 929-2000 [1]) after the utility disconnection. By introducing an initial value in the phase shift perturbation, auto phase shift detection method (APS) well solved this problem [5]. However, several parameters are presented in the phase shift algorithm, contributing to the parameters selection and optimization problems. In this paper, both SMS and APS methods are investigated and an improved slip mode frequency shift method is proposed. The working principle of the strategy is introduced and the parameters selection guidance is also provided. The islanding detection performance is evaluated through theoretical analysis, digital simulation and experiment. The IM-SMS method exhibits features of simplicity, easy implementation and high reliability. II.

ANAYSIS OF SMS AND APS METHODS

In the SMS method, the phase angle of grid-connected converter output current is controlled as the function of a deviation of the point of common coupling (PCC) voltage frequency. The converter current can be expressed as [3], iCON = I sin(2πft + θ SMS ) (1) where f is the PCC voltage frequency and θ SMS is the phase angle for SMS method. This phase angle is set as a sinusoidal function of grid nominal frequency f g ,

θ SMS =

2π π f − fg ) θ m sin( 360 2 fm − f g

(2)

where θ m is the maximum phase angle in degree and f m is the frequency at which θ m occurs. Referred to (2), θ SMS is almost zero when the utility frequency is at its rated value. Once the grid is disconnected, the SMS method is solely stimulated by an uncontrollable,

externally-supplied perturbation caused by noise, measurement inaccuracy and etc. in practice [3]. If such perturbation is small enough, this method may fail to detect islanding within the time specified by IEEE Std 929-2000. APS method solves such problem by introducing an initial value to the phase angle θ APS as (3) [5]. A permanent phase perturbation is therefore existing in the converter’s output current. 1 f [k − 1] − 50 θ APS [k ] = ⋅ ( ) ⋅ 360° + θ 0 [k ] (3) α 50 where α is a constant and f [k − 1] is the measured PCC voltage frequency in the previous cycle. θ 0 [k ] is the additional phase shift and can be expresses as, θ0 [k ] = θ0 [k − 1] + ∆θ ⋅ sign(∆f ) (4) where ∆θ is a constant and sign (∆f ) is determined by the PCC voltage frequency of the previous two cycles as, 1 f [k − 1] > f [k − 2]  (5) sign(∆f ) = 0 f [k − 1] = f [k − 2] − 1 f [k − 1] < f [k − 2]  Due to additional phase shift, the islanding detection speed is accelerated. However, a larger phase shift will deteriorate the converter output power quality. Therefore, the APS algorithm has difficulties to select and optimize the parameters. III.

where Q f and f 0 are the RLC load quality factor and resonant frequency respectively. The aim of IM-SMS algorithm is to ensure no stable operation point inside the frequency threshold once the utility is disconnected. Therefore, the phase angle of the converter should increase faster than the angle of the parallel RLC load with resonant frequency around the utility frequency to make sure that the IM-SMS method could work successfully at such worst case [7-8]. Thus, the following equations has to be guaranteed for f = f o = f g , dθ load df

f = fo

dθ IM −SMS df

(9) f = fg

Neglecting the additional phase shift and substituting (6) and (8) into (9), the following equation can be obtained, 360Q f n≥ (10) πf g For achieving islanding protection for loads with Q f < 2.5 , n can chosen a value of 6. IV.

SIMULATION RESULTS

To illustrate the design feasibility of the proposed IM-SMS islanding method, a grid-connected converter system with the following specifications is chosen: 1) utility grid: 220 V, 60 Hz; 2) dc voltage: 400 V; 3) converter rated power : 2 kW (unity PF operation); 4) frequency threshold: 60.5Hz (upper), 59.3 Hz (lower); 5) IM-SMS parameters: n = 6 , θ 0 = 1° . A MATLAB/SIMULINK model for the grid-connected converter system is developed to perform a digital simulation with diagram shown in Fig.1 and verify the effectiveness of the proposed IM-SMS method.

IM-SMS ISLANDING METHOD

In order to overcome the disadvantages of the SMS and APS methods, the IM-SMS islanding detection strategy with a simplified phase shift is proposed as, θ IM − SMS = n( f − f g ) + sign( f − f g ) ⋅ θ o (6) where n and θ 0 are constants and sign( f − f g ) is defined as the sign of the frequency error, f ≥ fg 1 (7) sign ( f − f g ) =  − 1 f < f g Compared with (2), additional phase shift sign ( f − f g ) ⋅ θ 0 is introduced in the IM-SMS algorithm. When the grid frequency is at its nominal value f g , the additional phase shift still exists and contributes to stimulate the islanding detection. Therefore, the effectiveness of this islanding detection method is improved. As the current noise and harmonics and measure inaccuracy can also contribute to the perturbation in islanding detection, only a small value of coefficient θ 0 can serve this purpose. When the utility is disconnected, the phase difference between the converter output voltage and current is determined by the load. A parallel RLC load is usually employed to investigate the islanding detection [1] and the corresponding phase angle of the current leading the voltage can be expressed as [6-8], f f 1 (8) θload = tan−1[ R(ωC − )] = tan−1[Q f ( − o )] ωL fo f

≤

f pcc Iref Vpcc Iref

Improved SMS Pulses Fault Im

f pcc

+ g +

Current Controller

-

v

Fault VRMS

Frequency & RM S Vdc

A

+

f pcc

Vpcc

i -

VRMS

UVP/OVP UFP/UFP

Breaker

B

Grid-connected Converter

Parallel RLC Load

Grid

Fig. 1. MATLAB/SIMULINK model of the gird-connected converter system

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When the converter output equals to the power demand of a parallel RLC load with quality factor of 2.5 and resonant frequency of 60 Hz, the system typical waveforms with the proposed islanding detection method are shown in Fig. 2. This is the worst case for islanding detection. The utility grid is disconnected at 0.1 s. It can be seen that the voltage frequency exceeds the upper limit (60.5 Hz) at 0.216s and the gating signal is therefore disabled. Under the same working

Fig. 4. The islanded system voltage and current Fig. 2. Converter output voltage, current and frequency with IM-SMS islanding detection method and a local parallel RLC load (Qf=2.5, fo=60 Hz)

Fig. 5. PCC voltage and the converter output current with a local parallel RLC load (Qf=2.62, fo=50.1 Hz)

Fig. 3. Converter output voltage, current and frequency with SMS islanding detection method and a local parallel RLC load (Qf=2.5, fo=60 Hz)

conditions, the system waveforms with the SMS islanding detection method are shown in Fig. 3. Referred to (2), the perturbation is pretty small, resulting in a low detection speed. The frequency changes only 0.1 Hz in 0.2 s, and it may fail to detect the islanding in 2 s. Comparing Fig. 2 with Fig. 3, due to the additional phase shift in IM-SMS method, the islanding can be detected and detection reliability is well improved. The operation of the proposed islanding detection algorithm has been verified by experiment as well. A local parallel RLC load with L = 45mH and C = 220uF was chosen for the islanding detection testing. In order to accurately acquire the load resonant frequency, the active detection function and frequency protection relays are disabled to measure the islanded system frequency. The converter output voltage and current waveforms are shown in Fig.4 after the grid disconnection and the frequency is 50.1Hz. When the converter output current is 6A, the local resistive load is set as 39 Ω and the load quality factor is 2.62. The PCC voltage magnitude in Fig. 5 can be seen increasing a

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little after grid disconnection due to the small active power mismatch, but still within the voltage protection limit. The rated utility frequency is 50 Hz and the corresponding frequency protection thresholds are set as 50.5 Hz (upper) and 49.3 Hz (lower). The proposed method persistently perturbs the converter output current phase angle to drift the system frequency out of the upper limit, and the islanding can be detected in 0.36s. The detection speed can be further by improved by properly choosing a larger θ 0 or n . V. CONCLUSION In this paper, the popular SMS and APS active islanding detection methods are investigated and the improved SMS method is proposed. Additional phase shift is introduced and the algorithm is simplified for the proposed IM-SMS strategy. The working principle of IM-SMS is analyzed and the guidance of parameters selection is provided as well. The islanding detection performance is evaluated through theoretical analysis and digital simulation. The IM-SMS method exhibits features of simplicity, easy implementation and high reliability and is expected to be an effective active islanding method.

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[7]

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