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Recently, the penetration of Distributed Generation (DG) at medium and low voltages in utility networks is increasing in developed countries and takes special place for them worldwide. Due to the DGs advantages, including use of Renewable Energy which do not pollute environment and has endless nature, the application of DGs can potentially reduce the need for traditional system expansion, controlling a potentially huge number of DGs and creates a daunting new challenge for operating and controlling the network safely and efficiently. One problem that was encountered when using DGs, is an unwanted Islanding phenomenon. In this paper, an active technique for inverter based distributed generations using positive feedback of rate of change of negative voltage sequence of voltage in order to detect islanding conditions has been present. The simulation results performed in MATLAB, clearly show improved operation of this method. 
 
 Key words: Distributed Generation-Islanding, switching, active technique, rate of change of voltage positive sequences.


INTRODUCTION
During the last years, growing power demand and increasing concern about the use of fossil fuels in conventional power plants, the new paradigm of distributed generation is gaining greater commercial and technical importance worldwide (Maleh et al., 2013).Renewable distributed generation involves the interconnection of small-scale, on-site distributed energy resources (DERs) with the main power utility at distribution voltage level (Katiraei et al., 2005).DERs mainly combined from renewable energy sources like solar PV, wind turbines, fuel cells, small-scale hydro, tidal and wave generators, micro-turbines, combined heat power (CHP) systems, etc.
Bulk of distributed generations in power system is from renewable energy.Depending on the distributed generators, their production can be AC or DC.But in all, most of these products are connected through electronic power converter to the network (Shahabi et al., 2009).
Most of inverters of DGs that produce DC voltage, usually operate with current control system in order to control the output power of DG, however, DGs will have effects in the network.One of these effects has become an islanding phenomenon.
An essential requirement of the grid interconnected DG system is the capability of islanding detection (Trafdar Hagh et al., 2011).Islanding state happens when one or more DGs without connecting to the network are connected and supplied local loads.In most cases this phenomenon can occur unwanted.The islanding operation of DG may cause potential hazards to linemaintenance personnel, equipment damage due to instability in user voltage and frequency and risk the DG in being damaged by out of phase reconnection to the grid.The majority of utilities require that DG should be disconnected from the grid as soon as the islanding occurs.Therefore, according to IEEE1547 standard, E-mail: Noradin.ghadimi@gmail.com.Tel: +989147028949.(Ghadimi et al., 2013).So far many methods to detect Islanding condition have been proposed.These methods can be classified in two broad categories of active and passive classifications (Chowdhury et al., 2009).In active techniques, disturbances are injected locally into the system and responses of these disturbances are used to detect islanding conditions.Including the active method we can point to the followings: Impedance measurement method (Chowdhury et al., 2009), Sandia Frequency Shift (SFS) and Sandia Voltage Shift (SVS) (Trafdar Hagh et al., 2011), Frequency domain analysis (Trafdar Hagh et al., 2011), Changing voltage amplitude and reactive power method (Kim and Hwang, 2000), the mid-harmonic method (Jou et al., 2007), Reactive power export error detection (Redfern et al., 1993), and the passive techniques make decisions based on the local measurements of voltage and current signals.Passive techniques include: Under/over voltage and under/over frequency (Trafdar Hagh et al., 2011), Rate of change of frequency relay (df/dt) (Lu et al., 2006), Phase displacement monitoring (Chowdhury et al., 2009), Output power speed changing (Lu et al., 2006), Comparison of rate of change of frequency (COROCOF) (Bright, 2001), Unbalanced voltage and Total current (or voltage) harmonic distortion (THD) (El-Arroudi et al., 2007).
In this paper, one proposed algorithm has been proposed that can be detecting the islanding condition based on Rate of Change of Negative Voltage Sequence (RoCoNVS) injection.Most of the multiple DER units control is based on dq-current control strategy according to drop characteristics of reactive-power/voltage and active-power/frequency in grid connected mode.For islanding detection in this paper, RoCoNVS of load voltage add to q-component of dq-current control that can change the output voltage when islanding is happen.

STUDY SYSTEM
Figure 1 show the proposed algorithm applied to this study system.As illustrated, the DG unit is represented by a dc voltage source, a VSC that control with conventional dq-current controller, a series filter (Lf and Cf), and a step-up transformer.The local load is represented by a three-phase parallel RLC network at the PCC.A step-up transformer is located between DGs local loads and utility grid.The utility grid was simulated with ideal source and Rs, Ls.Connection between utility grid and DG is done with Circuit Breaker (CB).In order to convert the DC voltage to AC voltage, as can be seen, inverter with IGBT switches was used.The parameters of study system are given in Table 1.
When the CB as shown in the figure is closed, in this mode, DG together with local load is connected to power grids and power produced by DG is injected.But when the CB is opened, in this mode, islanding state occurs, and DG along with a local load constitute an islanding state which creates an independent power grid in which just DG supplies loads demand power.In these conditions, that should a case be detected islanding mode and power production is entirely disconnected from the power grid and after reconnection it starts to produce power again.
To detect the islanding condition, a measurement system installed at the head of a local load and output of the mentioned system ended in a central processor in which measured signals are being processed and in the Islanding state a fast decision made and command for disconnecting system will be exported.

CONTROL STRATEGY AND PROPOSED ALGORITHM
DG systems are connected to the distribution system through an inverter as shown in Figure 1.The inverter performs two main functions: i) Controlling the active power output of the DG and, in some cases, injecting a suitable amount of reactive power to mitigate a power quality problem.ii) According to the IEEE Standard 1547, the DG should be equipped with an anti-islanding detection algorithm, which could be performed using the inverter interface control (active methods).
In this paper these variables are controlled using the dq frame of reference (Schauder and Mehta, 1993).The VSC controls its output real and reactive power components.Figure 2 depicts the dqcurrent control strategy in order to control active and reactive power of DG's output.
Under the test configuration of Figure 1, the RLC tank draws the fundamental current component supplied by the VSC, at the unity power factor.If a disturbance signal is injected into the system through the converter current controllers, the corresponding disturbance flows into the low-impedance path offered by the utility, as shown in Figure 3.
The proposed islanding detection method in this paper is based on injecting an appropriate disturbance signal through the Id controller (Figure 3), and illustrates the proposed algorithm that inject the derivative of negative voltage sequence of load voltage into the controller.When DG works in grid connected mode the value of RoCoNVS is small and does not have great influence in output power, but when DG become isolated from utility and islanding occurred, the amount of RoCoNVS increase and can affect output current.This algorithm act as positive feedback and

Nominal load
In this case, the active and reactive power of local load is set to Table 1 value.At first the CB is closed and system utilized in grid connected mode.At t=1.4 s, CB is opened and DG together with local load is separated from power grid and islanding condition occurs.Figure 4c shows the RoCoNVS of DG voltage.It is obvious from the figure that after islanding the amount of RoCoNVS of voltage increased.Figure 4a   (p.u).Prior to the obvious island condition, the three phase voltage is in balanced condition but when islanding occurred the voltage and current of Distributed Generation become unbalanced resulting in detection of the island condition.Therefore islanding condition is detected very fast after islanding happen.

Load condition 2
In the second condition, the active and reactive power of   condition can be detected.Figure 5c shows the RoCoNVS signal.It is obvious from the figure that after islanding the amount of RoCoNVS increased.

Switch of capacitor bank
Here, performance of the algorithm is studied for capacitor bank switching in grid connected mode to be shown that the proposed algorithm not mistaken in capacitor bank switching, and detection properly the islanding state from capacitor bank switching conditions (Figure 6a and 6b).Initially the system works in nominal load condition that depicted in Table 1.At t = 1 s a capacitor bank with 30 KVAr reactive power switching and connect to system. Figure 6c, illustrate RoCoNVS of DG.From of this figure can be seen the RoCoNVS of load voltage doesn't have mostly changing, therefore can be said that islanding does not occurred and system can be work without any problem.

Motor starting
One of the other switching condition that algorithm may be mistake is motor starting.Initially the system works in a connecting to the network mode with nominal load situation.At t = 1 s an induction motor with P = 50 kW and Q=75 kVAr started to work. Figure 7a and 7b, shows the three phase load voltage and current respectively.
Figure 7c shows that for RoCoNVS, the value of this signal does often change and the system continues working without mistake.

Conclusion
This paper proposed a new active technique based on injection of RoCoNVS signal in order to detect DGs islanding conditions.The results indicate that the performance of the proposed method is desirable because this proposed technique easily detect islanding conditions from other switching situations without any mistake and making appropriate decision to disconnect the system.Simulation results are taken in MATLAB software and the results were shown for various loads.The results specified that this algorithm works well properly.

Figure 1 .
Figure 1.Studied power system including DG and power grid.

Figure 2 .Figure 3 .
Figure 2. dq-current controller in order to control of active and reactive power of DG's output.

Figure 4 .
Figure 4.The output result of load voltage for nominal load a) three phase voltage b) three phase output current (p.u) c) RoCoNVS signal of DG.
and b shows instantaneous values of three phase load voltage and output current of DG in

Figure 5 .
Figure 5.The output result of load voltage for load condition 2 a) three phase voltage b) three phase output current (p.u) c) RoCoNVS signal of DG.
inductance and capacitor considers P=75 KW, Q L =65 KVAr and Q c = 55 KVAr respectively.At the grid connected mode CB is closed.At t=1.4 s, CB is opened and DG together with local load is separated from power grid and islanding condition occurred again.
Figure 5a and b depicted instantaneous three phase voltage of load in (p.u).As can be seen, after islanding situation the three phase voltage become unbalanced while, before islanding condition the three phase voltage of Distributed Generation was balanced.From this voltage island

Figure 6 .
Figure 6.The output result of load voltage for capacitor switching a) three phase voltage b) three phase output current (p.u) c) RoCoNVS signal of DG.

Figure 7 .
Figure 7.The output result of load voltage for motor starting a) three phase voltage b) three phase output current (p.u) c) RoCoNVS signal of DG.

Table 1 .
Characteristic of studied power system.