The model description of islanding detection through wavelet transform are as follows.
Doubly fed induction generator based wind turbines are used here, each of 9 MW and they are attached to a 120KV grid and it has been possible through a 25 KV feeder of 25 km length.
A 0.9 MVAR filter and 500 kw resistive load attached to 575v bus. PWM converter is AC/DC/AC IGBT based where switching frequency choosen as 1620 hz. DFIG is used for extraction of maximum possible energy when wind speed is less and wind speed is fixed at 10 m/s say. Sample time used for the model to discretize it is 5µs and that of control system is taken as 10 µs.
The details of the equipment like transformer, generator, inductor, resistor are described here.
For generator, Vbase
=
120kv,
Vrated = 120kv and short circuit rated power= 1000MVA. In case of transformer T1,rate power=25MVA, frequency(f)=50hz, 120/25kv stepdown, X1=0.1 pu, R1=0.00375 pu and Rm and Xm
both
500pu. For other transformer, rated power 10 MVA, frequency=50hz, 575v/25kv step-up, base voltage=25kv, X1=0.1 and R1=0.00375 in pu, Rm and Xm same as before.
Distributed generator: In this case doubly-fed induction generator of 9 MW in used attached to 25 kv power export through a 25 kv feeder of length 30 km to 120 kv grid.
Distribution lines, in simulation diagram(indicated as L) are of 30 km, 25 kv, and 20 MVA, base voltage being 25kv and load magnitudes are as given,
R1=0.413 Ώ, L1=3.32e-3 H, L0=1.05e-3 H, C0=11.33e-009 F and X1=5.01e-009 F and all these parameters are measured per km.
34
Finally the normal loading information is, line 1 of 15 MW and 5 MVAR and for other line it is 8MW and 3 MVAR.
According to this proposed method the voltage and current signal are extracted from the output of the generator on which we are experimenting about islanding and only the negative sequence components are passed through wavelet transform. DG-1 is considered here for and the situation about which islanding and non-islanding condition are described below. Using these data and various loading conditions as minimum, normal and maximum loads. Simulation is done at 1.6khz and signals are extracted from both the generator where islanding occurs at 0.3s at the lower one after the the loss of mains power. Now the simulation part is done using MATLAB SIMULINK and results are shown after which we shift to the next one.
Discrete THD
Fig 6.1(the circuit model of our simulation as described above)
35
Condition 1: Normal condition where circuit breaker and circuit breaker 1 both are in closed condition, normal 3-phase power flow occurs and sinusoidal curve for both voltage and current are shown in figure.
Condition 2: Now the three phase circuit breaker is opened, there will be loss of mains power and load changes occurs keeping the circuit breaker1 closed. The distributed generator continues to supply power to the connected loads and this one is an islanding condition where sudden trip signal is need to be sent to disconnect the DG to avoid further fault conditions.
Condition 3: in DG generator line trip condition the main circuit breaker is closed and circuit breaker-1 opens for which the simulation result will be shown.
Condition 4: there will be load change up to 50 percentage suddenly which may not be a islanding condition.
For above four conditions simulation programme is run at period of sampling 5e-006 and 32 samples/cycle.
After islanding voltage may increase or decrease depending upon load change.
Fig 6.2(change in voltage after islanding)
36
Fig 6.3( after extraction of negative sequence component its wavelet transform is done where we get the d1 coefficient)
Where negative sequence components are found by formulas,
vn = 1
3(va+a2vb+avc) and similarly for current where va,, vb and vc the voltages of the three phases respectively and vn the negative sequence voltage .
a = 1∟120 degree
the d1 coefficient localizes the corresponding islanding event.
37 a) Normal condition:
Fig 6.4(showing the output negative sequence voltage and d-1 coefficient of this) Sample(time) is shown and voltage as pu.
b) Islanding condition:
Fig 6.5(-ve sequence voltage and corresponding d-1 coefficient when islanding occurs)
38 c) Line trip condition:
Fig 6.6(negative sequence voltage and respective d-1 coefficient after line trip)
d) Sudden load change:
Fig 6.7(negative sequence voltage and corresponding d-1 coefficient when sudden load change is done)
39
Another useful method described in this thesis is the hybrid technology, where we test the method first in active methodology and then measuring the parameter through passive technology and concluding about the islanding condition. This method is best for inverter based distributed generator. As external disturbances can be given through the inverter to the distributed generator easily.
Figure below represents simulation diagram of the whole scheme of detecting islanding through passive method.
Fig 6.8(hybrid anti-islanding controller based grid inverter connection)
The data we have used for this simulation are as follows.
Grid: voltage=220v, frequency=50hz.
Distributed generator: Rated power=8 KVA, Rated voltage=220V and DC link voltage=300V and with load rated power 8kw.
40
The simulation diagram of the full cotroller(both active and passive is shown below.
Fig 6.9(full controller diagram of hybrid technology)
The simulations are plotted and the results are obtained to compare with the wavelet transform method which is the basis of this thesis.
41
Anti-islanding technology after the MATLAB SIMULATION previously done.
Fig.6.10(inverter voltage, inverter frequency and the circuit breaker signal)
This result is an output for inverter voltage, inverter frequency and circuit breaker signal before the islanding has occurred.
But in the absence of grid current which is a case for islanding condition both the grid current and inverter current will change. In our obtained result in fig.6 load current has decreased from 6 amp to 4.4 amp where as the inverter current has changed from 2.8 amp to 4.4 amp. Inverter current will increase after the islanding. In the mean time load current increases. Results of this islanding condition shown in fig 6. In all this simulation the non detective zone(NDZ) can be shown where the voltage and frequency at the connection point are located between 0.8pu<Vpcc<1.1pu and frequency between 49.3hz<fpcc<50.5hz .
42
Fig.6.11(load current, inverter current, grid current and circuit breaker signal after islanding)
Fig.6.112(passive monitoring parameter(v, f ,active power, reactive power ,THD, circuit breaker signal with controller). It has not varied much with or without controller in our simulation)
43
The result is an output for inverter voltage, inverter frequency and circuit breaker signal before the islanding has occurred.
But in the absence of grid current which is a case for islanding condition both the grid current and inverter current will change. In our obtained result in fig.6 load current has decreased from 6 amp to 4.4 amp where as the inverter current has changed from 2.8 amp to 4.4 amp. Inverter current will increase after the islanding. In the mean time load current increases. Results of this islanding condition shown in fig 6. In all this simulation the non detective zone(NDZ) can be shown where the voltage and frequency at the connection point are located between 0.8pu<Vpcc<1.1pu and frequency between 49.3hz<Fpcc<50.5hz . For wavelet transform method we have obtained our simulation for 4 condition as shown below.
For each condition we have obtained the negative sequence component of voltage and corresponding d-1 coefficient and the same is done for negative sequence current and plotted through simulation.
We have obtained our result both for negative sequence component based islanding detection and hybrid detection and observed that both the method serves as good basis for islanding detection where one is for wind turbine generator and another is best for inverter based islanding detection. We can say wavelet method is a passive method where as other one is combination of two and both method has a very small non-detective zone.
Conclusion:
We have verified the results of our work through MATLAB SIMULATION and also shown that the proposed method is suitable for islanding state determination although it has a very small non detective zone. Hybrid method while using both active and passive method is able to detect islanding when the voltage and frequency doesn’t lie in the region between 0.8pu<Vpcc<1.1pu and 49.3hz<Fpcc<50.5hz respectively and wavelet transform method is able to detect islanding universally except in certain cases when there is sudden load change(approximately 50 percentage or above). Hybrid technology is the best for islanding detection when distributed generator is inverter based as it becomes easier to inject external disturbance to the output of the generator to destabilize the parameter and push the parameter outside the limit so that islanding can be detected even if there is a close mismatch between generation and load. Also in case of wavelet transform we can do time space representation of signal and localize the corresponding islanding event where the result shows considerable fluctuation outside the threshold to denote the islanding condition.
44
References:
[1] A. M. Massoud , K. H. Ahmed , S. J. Finney and B. W. Williams "Harmonic distortion-based island detection technique for inverter-based distributed generation", IET Renew. Power Generation, vol. 3, no. 4, pp.493 -507 2009 [2] Jenkings, N.; Allan, R.; Crossley, P.; Kirschen, D.;and Strbac, G., “ Embedded Generation ” IEE Power and Energy Series 31, IEE 2000.
[3] M. N. Abdul Kadir, S. Mekhilef, and H. W. Ping “Dual Vector Control Strategy for a Three-Stage Hybrid Cascaded Multilevel Inverter” Journal of Power Electronics, vol. 10, no. 2, pp.155-164, 2010
[4] M. E. Ropp, M. Begovic, A. Rohatgi, "Determining the Relative Effectiveness of Islanding Prevention Techniques Using Phase Criteria and Non-detection Zones", IEEE Transactions on Energy Conversion Vol. 15 no. 3, p. 290-296, September 2000.
[5] R. A. Walling, and N. W. Miller, “Distributed generation islanding implications on power system dynamic performance,” IEEE Power Engineering Society Summer Meeting, vol.1, pp.
92-96, 2002.
[6] S. Jang, and K. Ho Kim, “An Islanding Detection Method for Distributed Generations Using Voltage Unbalance and Total Harmonic Distortion of Current” IEEE Transactions on Power Delivery, Vol. 19, No. 2, April 2004.
[7]M. A. Kashem and Gerard Ledwich. Multiple distributed generators for distribution feeder voltage support. IEEE Transactions On Energy Conversion, 20:676–684, 2005.
[8] P. L. Villenueve. Concerns generated by islanding [electric power generation]. Power and Energy Magazine, IEEE, 2(3):49– 53, 2004.
[9] Manisa Pipattanasomporn, Michael Willingham, and Saifur Rahma. Implications of on-site distributed generation for commercial/industrial facilities. IEEE Transactions of Power Systems, 20(4):206–212, 2005.
[10] T. Ackermann, G. Andersson, and L. Soder, “Electricity Market Regulations and their Impact on Distributed Generatiron,” IEEE, 0–7803–5902-X/00, 2000.
[11] A. Safari and S. Mekhilef “ Simulation and Hardware Implementation of Incremental conductance MPPT with Direct control method using Cuk converter” IEEE Transactions on industrial electronic, DOI: 10.1109/TIE.2010.2048834, in press.
[12] M. N. A. Kadir, S. Mekhilef, and H. W. Ping “Voltage Vector Control of a Hybrid Three-Stage Eighteen-Level Inverter by Vector decomposition” IET Transaction on Power Electronics, Vol. 3, Issue 4, pp.601- 611, 2010.
45