1 K. Obulesu , 2 A.Bhaskar
Abstract— This paper proposes a three-phase grid impedance detection method based on dual-frequency harmonic current injection for islanding detection. Grid impedance detection based on single harmonic current injection is reliable in three-phase impedance balanced grid. However, under impedance unbalanced condition, the harmonic voltage caused by the injected symmetric harmonic current is asymmetric, which affects the calculation of grid impedances and even leads to failed detection. The method based on dual-frequency harmonic current injection is injecting two non-characteristic symmetric harmonic currents, and then according to the different harmonic voltages caused by different frequency harmonic currents, all three-phase impedances can be calculated accurately. The implementing algorithm of the presented method is derived and its performance is analyzed in detail. Simulation and experiments are carried out under grid impedance balanced and unbalanced conditions. Theoretical analysis and experiment results proved that the proposed method is feasible .
Index Terms—Anti-islanding detection, distributed generation
(DG), harmonic current injection, impedance detection, threephase grid-connected inverter.
I. I NTRODUCTION
T he Global increase in energy consumption is an assignable problem in the current power supply system, which leads the development of renewable energy sources to arise. In the distributed power system, distributed power generation systems such as wind turbines, photovoltaic (PV) systems have the largest utilization nowadays. However, if these systems are not properly controlled, their connection to the utility grid can lead to grid instability or even failure Islanding detection, the distributed resource (DR) units are required to be disconnected, which is known as anti-islanding.
1 K.Obulesu, M.Tech Student, Department of EEE (Electrical Power
Systems),/Narayana college of engineering and Technology,
JNTUA,Anantapuramu,Nellore district,AndhraPradesh,India,(e-mail: kethineniobulesu@gmail.com
).
2 A.Bhaskar, Associate Professor, Department of EEE, Narayana college of engineering and technology ,JNTUA, Anantapuramu,
Nellore district, AndhraPradesh, India ( phd.bhaskar@gmail.com
)
Besides. In addition, a novel method Besides, in order to accommodate more and more DGs and strengthen the power grid, micro grid which can keep working without the utility grid is put forward and researched recently. If autonomous operation of an island is permitted fast islanding detection is required for appropriate decision making to manage autonomous operation of island. Thus, in either case, islanding detection is a requirement for utilization of DR units. when the grid is not present to maintain the frequency or voltage. Grid impedance detection uses the grid impedance to judge the grid state. The European standard EN503 301-1 sets the utility fail-safe protection interface for the PV linecommutated converters, whose goal is to isolate the supply within 5 s after an impedance change of 0.5 . In order to detect the three-phase grid impedances and achieve the anti-islanding goal, several methods have been proposed in
Considering that the positive voltage would obstruct the positive sequence impedances detection, the literature
[presented an impedance detection method based on negative sequence current injection. In, by intermittently injecting the positive sequence current and quantifying the voltage variation of the point of common coupling (PCC), the grid impedances could be estimated with the recurrence based on
LCL filter which uses the resonance characteristic and needs no extra devices was proposed recently. While DGs have benefited from some of these active methods, such active methods have not considered the grid impedance unbalanced condition in detail, which limits these approaches’ applicability. On the occasion that the renewable sources or local loads on the three grid phases are not the same, the grid impedances would be unbalanced probably. Moreover, when the power system failures on the three-phase grid are not uniform, such as single-phase break, the equivalent impedances are non-uniform as well. If power system fault happens and the islanding situation is not detected rapidly, the protection device may not behave and the grid voltage and frequency would be labile. Furthermore, the power line maintenance workers’ safety cannot be guaranteed. The unbalanced islanding conditions ought to be considered particularly. On the unbalanced condition, the harmonic voltage due to the balanced harmonic current is unbalanced, which may affect the calculation precision and lead to the islanding detection failure.

Therefore, a new active method is needed that would work reliably under both the grid impedances balanced and unbalanced conditions. There are two approaches to do islanding detection. One is using power converter itself to inject the harmonic current. This method is mainly applied in the low-power and medium-power converters. The another approach is using a special small-power islanding detection device to inject harmonic current and act in concert with power converters. This approach is commonly employed in the large-power photovoltaic power stations or large-power converters. In large-power applications, the switching frequency of large-power converters is low. Their sampling and control precision is insufficient for islanding detection.
Given the above, a small-power islanding detection device is practical and easy to implement.
This paper presents an active islanding detection method for DR units. The presented method is based on injecting two non characteristic harmonic currents through the current controller and quantifying the corresponding harmonic voltage at PCC of the DR units. Detection and quantification of the
PCC harmonic voltage is implemented by a signal processing approach which provides high degree of immunity to noise and thus enable small harmonic current injection for islanding detection. Performance of the proposed islanding detection method is evaluated based on time-domain simulation in the
MATLAB/SIMULINK and experiments under the grid balance and imbalance condition.
In the experimental process, an individual low-power three-phase inverter is applied to inject the harmonic current intently and the islanding detection algorithm is operated in the microcontroller of the inverter. Compared with the grid and DR units, the injected harmonic current is very small and the power level of the detection device is tiny enough.
Considering that the injected current is inter-harmonic, the total harmonic distortion (THD) doesn’t increase and the impact caused by the converter is able to be negligible. Using the presented method based on two non-characteristic harmonic currents injection, most of islanding states can be detected quickly under all kinds of realistic scenarios. But it’s also worth pointing out that the impedance detection accuracy is relative to the injected current quality and the measurement precision. The study results indicate that the proposed method is feasible.
The rest of this paper is organized as follows.Section II presents the equivalent model of three-phase grid and the performance of the grid impedance detection method based on single harmonic injection. In addition, the reason of detection failure under the grid impedance unbalanced condition is argued in detail. The author proposed a modified grid impedance detection method based on dual-frequency harmonic current injection in Section III. The computational process is introduced and the performance is analyzed as well.
Simulations and experiment results in Section IV show the performance comparisons between the modified method and the conventional one. Finally, some conclusions are drawn in
Section V.
Fig. 1. Detection method of three-phase grid impedances.
II. P ERFORMANCE A NALYSIS OF I MPEDANCE D ETECTION
M ETHOD U NDER G RID I MPEDANCE U NBALANCED
C ONDITION
In general, the load connected to three phases of the grid is different, which leads the grid to be unbalanced. What’s more, nowadays many distributed generators have become connected with the grid. This would make the impedance characteristic more complex, and might as well aggravate the grid asymmetry. Even though the grid voltages are balanced, the impedances’ asymmetry cannot be ignored. In single phase grid, only one impedance needs to be measured. While in three-phase grid, there are three impedances to be measured, which make the computation more complex.
A. Equivalent Model of Impedance Detection System
The three-phase grid impedance detection technique is a steady-state technique that injects a steady harmonic current into the grid and record the voltage change response of PCC.
Results are processed by means of a Fourier analysis at the particular injected harmonic. In this way, the method has the entire control of the injected current and the recovery of the harmonic current and voltage. The calculation resumes only at the specific Fourier terms that give the final results.
Considering that there is scarcely any non-characteristic harmonic component in PCC voltage, injecting the noncharacteristic current and detecting the impedance on the noncharacteristic frequency is a reasonable method. The detection method of three-phase impedances is shown in Fig. 1. The injected harmonic currents can be equivalent with three current sources, . and are the grid impedances values. Module is used to measure the grid impedances. According to the different frequencies, the detection circuit is able to be divided into fundamental equivalent circuit and non-characteristic equivalent circuit. The equivalent circuit on the noncharacteristic frequency is shown in Fig. 2, where n and are the grid impedances values and are injected three-phase balanced current sources. The non-characteristic components of the grid voltages and currents could be obtained with DFT

(Discrete Fourier Transformer). The DFT operation provides harmonic voltages and
Fig. 2. Three-phase grid equivalent model of impedance on on-characteristic frequency. currents’ amplitude and phase with which the projecting component of harmonic voltage on the direction of harmonic current and that on the vertical direction can be calculated. At last, compute resistor and inductor value with the components above. The principle and process is introduced in paper [27].
B. Detection Performance Based on Single Harmonic
Injection Under the Unbalanced Condition
In three-phase three-wire grid, the three-phase currents and voltages are in relation with each other. Only when the voltages and currents are both symmetric can they be separated. However,if the impedances are unbalanced, the currents and voltages are not able to keep symmetric at the same time. Besides the detection complexity, the asymmetry may lead to detection failure in all probability. When the grid impedances are balanced, the harmonic voltages caused by the harmonic currents are balanced as well. Under the circumstances, the sum of the three-phase harmonic voltages is always zero, thus, the phase voltages transformed from line voltages are correct. The three-phase impedances can be calculated accurately by . In Fig. 3(a), the harmonic currents injected into the grid by three-phase inverter are symmetrical, which is accomplished with grid-connected current controller.
Under the condition that three-phase grid impedances are the same, the harmonic voltages are symmetric as well.
Therefore, the detection results of impedances and islanding are all correct, which means that the conventional method based on single harmonic injection is effective under the impedances balanced condition As there is no neutral wire in three-phase three-line grid, each phase voltage is not able to be measured separately, instead, only line voltages could be measured, and then, three phase voltages are obtained by the two-to-three reduction formula. Unfortunately, when the grid impedances are unbalanced, the harmonic voltages caused by harmonic current are not balanced any more,which makes the reduction formula inapplicable. As a result, on this occasion the measured converted phase voltages deviate from the practical ones, which leads to the detection error of grid impedances and even detection failure, thus, the conventional grid impedance detection based on single harmonic injection is infeasible under this grid impedance unbalanced condition.
On the non-characteristic frequency, the vector diagram is shown in Fig. 3(b). As shown in Fig. 3(b), the three harmonic voltages are asymmetric because of the impedances’ asymmetry. Therefore, the sum of three harmonic voltages is not zero any more, . The result is that the equivalent neutrality point shifts from point to point on the non-characteristic frequency.
The components R ′ a i a and 𝑤L ′ a i a obtained with DFT are not equal with the real components RɑIɑ and wLɑIɑ .
According to the relationship of harmonic voltage and current, the system of equations is written by
= R a a i a i a
+ jwL a
+ jwL a i a i a
− R b
− (R b i i b a
− jwL jwL b i a b
)e i b
2 π /3
= (R b i
⃗⃗⃗⃗ = R a b
+ jwL b i b i a
+ jwL b
)e
2 π
3 i b
− (R c
− R i a c i c
− jwL
+ jwL c i a c
)e i c
−
2 π
3
Ua ⃗⃗⃗⃗⃗ + Uc (1)
The relationship between the impedance detection value and the real value of phase a is verified by
R
′ a
L
′ a
=
=
2Ra
3
2La
3
+
+
1
2
R b
+
1
2
R c
+
√3
2 wL b
−
√3
2 wL c
1
2 Lb+
3
1
2 Lc+
√3
2 wRb−
√3
2 wRc (2)
3 where Ra Rb , Rc , La , Lb and Lc and are the real impedances values, and are the detection values. Similarly, the other detected values R ′ b
, R ′ c
, L ′ b
and L ′ c can be obtained by analogy. From (2), the absolute error of the impedance value
(phase a) is derived by
∆R a
=
1
2
(R a
−
R a
+ R 𝐛
3
+ R c
) +
√3w
6
(L c
− L b
)
∆L a
=
1
2
(L a
−
L a
+L 𝐛
+L c
3
) +
√3
6w
(R c
− R b
) (3)
The analysis above demonstrates that the impedance detection value is not only relevant to the real value of certain phase, but also relevant to those of other phases. When that is to say the conventional impedance detection method based on single harmonic current injection is feasible. When Ra=Rb=Rc and
La=Lb=Lc , (2) is simplified to R
′ a
= Ra and L
′ a
= La , that is to say the conventional impedance detection method based on single harmonic current injection is feasible. But when
Ra ≠ Rb ≠ Rc and La ≠ Lb ≠ Lc , the detection values are not equal with the real values. In this case, the conventional grid impedance detection method is inapplicable.
Assuming that the three-phase grid impedances are
R a
=0.20
Ω , R b
= 0.40 Ω , R c
= 0.50
Ω and L a
= 0.6mH, L b
=
1.4mH, L c
= 0.8mH
. According equation (2) to the detection values of the three grid impedances are calculated as
R a
=0.39Ω R b
0.89mH, L c
= 0.36
= 0.89mH
.
Ω , R c
= 0.3
and L a
= 0.93mH, L b
=

On this occasion, the real impedances of phase b and c are out of normal range, whichindicates the islanding state. But the detected impedance values are all in the normal range, that means the islanding detection failure. The impedance values are shown in Fig.3.4. The square dots indicate the real impedance values and the circle dots the detected values. The dotted line expresses the threshold value of islanding
III. G RID I MPEDANCE D ETECTION M ETHOD B ASED ON
D UAL -F REQUENCY H ARMONIC C URRENT I NJECTION
Since the conventional detection method based on single harmonic injection is unfeasible in the impedance unbalanced condition, a modified method is essential to give consideration to both impedance balanced and unbalanced conditions.
Considering that by injecting one harmonic current, the harmonic voltages’ asymmetry is in cognizable. Introducing one more harmonic current and comparing the asymmetry of the two different harmonic voltages caused by two symmetric harmonic currents, the harmonic impedances’ asymmetry might be able to be measured.
Fig. 3. Vector diagram of three-phase grid equivalent model:
(a) impedance balanced condition; (b) impedance unbalanced condition.
Fig. 4.Vector diagram of three-phase real values and detected values of grid impedance.
In Fig. 4. The square dots indicate the real impedance values and the circle dots the detected values. The dotted line expresses the threshold value of islanding. This phenomenon is easy to be explained from (4) because that the detected values are approximately weighted values of all three phase impedances.
Fig. 5.Vector diagram of three-phase grid equivalent model of impedance.
A.
Detection Method
According to Fig. 2, under the non-characteristic frequency, the vector diagram of the three-phase grid equivalent model of grid impedance is shown in Fig. 5. Supposing that three phase grid impedances are R a
+ jwL a
, R b
+ jwL b
, R c
+ jwL c respectively, thus, the component of Ua b on the direction of Ia consists of three parts, R a i a
, R b i b
, wL b i b
, . The relation of that is represented by shown in below equation(6).
√3
2 wL b i b
1
+
2
R b i b
+ R a i a
= M ab−a
(4)
Similarly the component of Ua b on the vertical direction of Ia also consists of three parts, R b i b
, wL b i b
, and wL a i a
. The relation of that is represented by wL a i a
−
√3
2 wR b i b
+
1
2 wL b i b
= T ab−a
(5)
where T ab−a is projection component of on the vertical direction of i a
.
On another non-characteristic frequency, two similar formulas can be obtained as well. Assume that the frequencies of two non-characteristic harmonic currents are and . According to the four formulas, the impedance value is able to be calculated. The equations is written by shown in below equation (6)

√3
2 w
1
L b i b
+
1
2
R b i b
+ R a i a
= M ab−a−w
1
w
1
√3
2
L a i a
− w
2
L b i
′ b
w
2
L a i
′ a
√3
2
R b i b
+
−
+
2
√3
2
1
R b i
′
R b i
′ b a
2
+ R
+
1
1
2 w
1
L b i b a w i
′
2 a
M ab−a−w
2
L b i
′ b
= T ab−a−w
1
(6)
= T ab−a−w
2
Supposing that the effective values of the two noncharacteristic harmonic currents are equal, which means that i a
= i b
= i
′ a
= i
′ b
the calculative process can be simplified. The grid impedance of phase b is derived by
R b
L b
=
=
W
2
T ab−a−w1
−W
1
T ab−a−w2
√3
2
(w1−w2)ib
M ab−a−w2
−M ab−a−w1
(7)
√3
2
(w2−w1)ib
From (7), impedance of phase has business with line voltage , phase current and two harmonic frequencies, which means each impedance can be detected independently without consideration of impedance characteristic. Firstly, DFT operation provides the component of line voltage on the direction of , and the other component of line voltage on the vertical direction of , . And then, the grid impedance of phase b can be calculated with (7). This detection method doesn’t need detecting phase voltage but line voltage which can be measured directly. Assuming that the injected currents follow in the tracks of the current commands, the Fourier component of the grid voltage can be obtained by DFT between the grid voltage and the current command. But in order to enhance the detection accuracy, it is recommended to detect phase currents and pick up the harmonic voltages by DFT between the grid voltage and the injected current. Even so, no extra detector is needed for the grid impedance detection. Similarly to the impedance detection method of phase b, grid impedances of phase a and c are able to be calculated with , , and . By analogy analysis, the computational algorithm of those is summarized by
L
R
L c
R a a c
=
=
=
=
M ca−c−w2
−M ca−c−w1
√3
2
(w2−w1)ia w
2
T ca−c−w
1
− w
1
T ca−c−w
2
√3
2
(w
1
−w
2
)i a
M bc−b−w
2
− M bc−c−w
1 (10)
√3
2
(w
2
−w
1
)i c w
2
T bc−c−w
1
− w
1
T bc−b−w
2
√3
2
(w
1
−w
2
)i c
Equations (7) and (8) indicate that with the detection method based on dual-frequency harmonic current injection, all grid impedances of three phases can be detected accurately. In addition, the computational process is concise and easy to bring about. In the real system, the frequencies of the two injected harmonic are chosen 75 Hz and 125 Hz. These are close to fundamental frequency (50 Hz) and the results of the impedances are also close. Since that the criteria of islanding is the value of grid impedance, the impedance value of every phase is derived by . Considering that , the grid impedance is obtained as (9), shown at the bottom of the page.
B.
Performance Analysis
As analyzed above, most of the islanding conditions including the impedance balanced states and impedance unbalanced ones can be detected by the proposed approach.
But the grid impedance detection accuracy is relative to the injected current quality and the measurement precision.
Comparing with the conventional impedance detection method, the modified method applies another harmonic current whose injecting way is similar with single harmonic.
No extra measurement devices are needed using the modified method. On the other hand, in order to eliminate the distraction of the fundamental voltage to DFT operation, the minimum detection period of the conventional method in which 75 Hz harmonic current is injected is 0.04 s.
Meanwhile, the modified method is able to accomplish 75 Hz and 125 Hz harmonic currents injection and DFT operation in
0.04 s as well. Therefore, the detection speeds of two methods are the same. Considering the phase lock time and injected current regulation time when the grid impedances change, the detection period of both the two methods would increase to
0.06 s.As that the transformation from line voltage to phase voltage is omitted, the time complexity of the proposed method is , which is approximately the same with that of the original method. The difference between the two methods is that the modified method can detect all grid impedances of three phases, which is suitable for grid impedance unbalanced condition, although each of the injected currents decreases and the detecting precision has to enhance. In short, the grid impedance detection method based on dual-frequency harmonic current injection is a feasible islanding detection method used in three-phase three-line grid-connected inverters.
IV. S IMULATION AND E XPERIMENT R ESULTS
A 500-VA three-phase full-bridge inverter is employed to inject the harmonic currents. Both the conventional method and modified one are realized with experiments. The laboratory prototype is shown in Fig. 6. Three impedances are series connected with voltage sources to simulate the grid conditions. The utility three phase grid is not suitable for clarifying the proposed method because the grid-connected sources and loads are uncertain and the grid impedances changes randomly [19]. Three L filters are utilized and the inductor values are all 50 mH. The DC bus voltage is chosen
700 V and the capacitor in the DC bus is 340 uF to keep the inverter operate smoothly.

method. In addition, the algorithm process is shown in Fig. 8.
The left procedure is used to inject the harmonic current and the right one is to measure the grid impedances and judge the islanding state
B.
Experimental Results
A full-bridge inverter that comprised of six IRA1110 power MOSFETs and three 50-mH output filter inductors was built to verify the performance of the proposed method. The experiment test setup is shown in Fig. 10. In the experiment, the inverter is used to inject harmonic current to the utility grid. The employed control framework and the impedance calculation operation with DFT are accomplished in SCM
XE162FM whose
Fig. 6.Structure of a three-phase Three-wire grid-interfacing
The sampling frequency and the discrete control frequency are both chosen 18-kHz which is much higher than the fundamental frequency50-Hz and the injected harmonic currents frequencies 75-Hz, 125-Hz, which insured the output current quality and the accuracy of DFT operation. What’s more, the switching frequency of the MOSFETs in the inverter is also set to 18-kHz so that small
|Z b
| = √L b
2
W
0
2
+ R b
2
=
2
√3 i b
√(M ab−a−w
2
− M ab−a−w
1
) 2 + (2.5T
ab−a−w
1
− 1.5T
ab−a−w
2
)
2
(9)
Parameters
DC link voltage,Vdc
Phase voltage,Vac
Phase frequency, 𝑓 𝑎𝑐
Switching frequency, 𝑓 𝑠
Ferrite material of T
1−6
DC link capacitor,c
Values
700v
220v(rms)
50Hz
18kHz
IRA1100
340uf
Choke inductance,L
Choke Resistance, R 𝑠
50mH
100mΩ
A .
Simulations
The single current closed-loop control strategy is sed in this experiment system. As the frequency of the injected current is not the same with that of the fundamental voltage, the grid voltage feed-forward control is drawn into the control diagram to remove the interference of the grid voltage [28].
The control block is shown in Fig. 7. Set the effective value of the harmonic current to be 0.7 A when employing the conventional detection method and that of the two harmonic currents both to be 0.35 A when employing the modified
Fig.8. The algorithm process of the islanding detection device.
Fig.9.Simulation results for three phase unbalanced grid waveforms

clock frequency was set to 80-MHz. Six PWM modules were used to control the inverter and set to 18-kHz frequency addition, six 10-bit analogy-to-digital converters (ADC) channels were used to measure grid voltages and gridconnected currents, which were triggered at the instant that the
PWM counter is reset. The discrete controller and DFT were executed at the same frequency. Both the conventional method and the modified one are used to detect the grid impedances.
Fig. 11. Experiment results of injected currents of islanding detection method based on single- and dual-frequency harmonic current injection: (a) injected current based on single harmonic injection; (b) injected current based on dual frequency harmonic current injection.
Under these two conditions, the grid impedances are symmetric and the grid is in islanding state. To imitate impedance-asymmetric situation, choose the three-phase grid impedances in series and Under these two conditions, the grid is also in islanding state but the impedances are asymmetric.
The injected single harmonic current of the conventional method is 75 Hz, and the two harmonic currents of the modified method are 75 Hz and 125 Hz. In order to enhance the detection precision, the effective value of injected current of the normal method is set 0.7 A, and those of the modified method are set 0.35 A respectively. Fig. 11 shows the injected currents based on single harmonic and two harmonics injection. These is only 75 Hz component in the current in Fig.
11(a), meanwhile, these are 75 Hz and 125 HZ components in the current in Fig. 11(b).
Fig. 12. Experiment results of detected impedances based on single- and dualfrequency harmonic current injection: (a) detected impedances based on single harmonic injection; (b) detected impedances based on dual-frequency harmonic current injection.
The three-phase grid impedance detection results with both the conventional and modified methods are shown in Fig. 12.
When the three phase grid impedances are 1.2 , the impedance balanced condition, the grid impedances detection values based on single harmonic injection and that based on dual-frequency harmonic current injection are both 1.2 , which equals the realistic grid impedances. But when the grid impedances and , the unbalanced condition, the grid impedances detection values by the convention method are and , apparently at variance with the realistic ones. As contrast, the grid impedances detection values based on two harmonics injection are all 1.2 , which approach the realistic grid impedances. The proposed detection method is practicable under the impedance unbalanced condition. In the experiments, set 1 as the grid impedance threshold value to judge normal condition or islanding condition. When the impedance value becomes large than 1 ,
TABLE II
C OMPARISON OF THE G RID I MPEDANCE D ETECTION R ESULTS
Ra, Rb, Rc/Ω
La, Lb, Lc/mH
Conventional Modified method method
𝐙 ′ 𝐚
/Ω 𝐙 ′ 𝐛
/Ω 𝐙 ′ 𝐜
/Ω 𝐙 ′ 𝐚
/Ω 𝐙 ′ 𝐛
/Ω 𝐙 ′ 𝐜
/Ω
Ra=Rb=Rc=0
La=Lb=Lc=0.1
Ra=1.2,Rb=Rc=0
La=Lb=Lc=0.1
Normal Normal
0.03 0.03 0.02 0.03 0.04 0.03
Normal Islanding
0.81 0.18 0.23 1.21 0.08 0.05
Ra=Rb=Rc=0
La=3.8,Lb=Lc=0.1
Normal Islanding
0.82 0.22 0.24 1.24 0.09 0.04
Ra=Rb=Rc=1.2
La=Lb=Lc=0.1
Islanding Islanding
1.21 1.19 1.17 1.22 1.21 1.18

Islanding condition. Otherwise, the grid is under the normal condition. Change the impedances series to the grid to simulate the different condition of grid.After that, compare the detection results with the two different methods. The experiment results show that the detected impedance values by the modified method always draw near the real impedance values, which means the detection method based on dualfrequency harmonic current injection is applicative under the impedance unbalanced condition, and the detected impedance values by the conventional method are far away from the real impedance values, which verifies the limit of the method based on single harmonic injection. The comparison of the two methods under other conditions are shown in Table II.
Table II demonstrates that under the impedance balanced condition, both the islanding detection method based on single harmonic current injection and the modified one based on dual-frequency harmonic current injection can discover the islanding status. But under the impedance unbalanced condition, the islanding status can be detected accurately with the modified method.
V. CONCLUSION
Reasoned opinion An impedance discovery way based on dual-frequency harmonic current injection for islanding discovery has been presented in this work. The analysis of the islanding discovery way based on single harmonic injection makes clear by reasoning that under the impedance unbalanced condition, the harmonic voltage caused by the put harmonic current are asymmetric, which leads to the impedance discovery wrongness and even the unsuccessful person of islanding. The made an adjustment move near based on dual-frequency harmonic current injection is injecting 75 Hz and 125 Hz like in size harmonic currents with current controller and Computing the grid impedances with the harmonic current and voltage information achieved by DFT from the put currents and grid voltage . Although the harmonic measured in volts are still asymmetric, the asymmetry of grid impedance is able to be represented by opposite in comparison the different frequency harmonic measured in volts.
All three-phase grid impedances can be sensed accurately by this made an adjustment way whenever the grid impedances are balanced or not, which means that the made an adjustment way is right for anti-islanding discovery by the photovoltaic generators or power stations in the use grid . In addition, the doing a play and putting into effect steps are introduced in detail. The impedance discovery accuracy is in comparison with to the put current quality and the measurement precision . Despite of a little increscent thing needed of the discovery precision , making a comparison with the limited by agreement way, no in addition discovery apparatuses are need in the made an adjustment way.
The computational complex conditions of the two methods are similar and the discovery times are the same. At last, the simulation and experiment results proved that the made an offer way is an effective way right for three-phase gridconnected changers.
This Thesis has presented a novel control of an existing grid interfacing inverter to improve the quality of power at
PCC for a 3-phase 4-wireDGsystem. It has been shown that the grid-interfacing inverter can be effectively utilized for power conditioning without affecting its normal operation of real power transfer .The grid-interfacing inverter with the proposed approach can be utilized to i) Inject real power generated from RES to the grid, and/or,ii) Operate as a shunt
Active Power Filter (APF). This approach thus eliminates the need for additional power conditioning equipment to improve the quality of power at PCC. Extensive MATLAB/Simulink simulation results have validated the proposed approach and have shown that the grid-interfacing inverter can be utilized as a multi-function device.
It is further demonstrated that the PQ enhancement can be achieved under three different scenarios: 1) P
RES
=0, 2)
P
RES
< P
Load,
and 3) P
RES
> P
Load
. The current unbalance, current harmonics and load reactive power, due to unbalanced and non-linear load connected to the PCC, are compensated effectively such that the grid side currents are always maintained as balanced and sinusoidal at unity power factor.
Moreover, the load neutral current is prevented from flowing into the grid side by compensating it locally from the fourth leg of inverter. When the power generated from RES is more than the total load power demand, the grid-interfacing inverter with the proposed control approach not only fulfills the total load active and reactive power demand (with harmonic compensation) but also delivers the excess generated sinusoidal active power to the grid at unity power factor.
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Author’s Profile:
K.Obulesu
has Received B.Tech in Electrical and Electronics from Vaishnavi Institute of Technology(VITT) ,Thanapalli,
Tirupati, and Andhrapradesh, India Affiliated the Jawaharlal
Nehru technological university Ananthapur, in 2012, and pursing M. Tech in Electrical Power Systems from the
Narayana College of Engineering and Technology Affiliated to the Jawaharlal Nehru technological university Anantapur,
Andhrapradesh, India in 2014, respectively. EMail.Id: kethineniobulesu@gmail.com.
A. Bhaskar, ME has Received B.Tech in Electrical and Electronics
Engineering from Visvodaya Inistitute of Tecnology &Science,
Kavali, Affliated JNTUA in 2003, and M.Tech in Power Electronics and Industrial Drives Engineering from SATYABAMA University,
CHENNAI, in 2008.He is dedicated to teaching field from the last 10 years. He Published FIVE International Journals. At present he is working as Associate Professor in NARAYANA Engineering
College, Nellore, and Andhra Pradesh, India. E- Mail.id: phd.bhaskar@gmail.com