Conclusions
Future scope
6.1 CONCLUSIONS
In this research work, our main objective being compensation of current harmonics
generated due to the presence of non-linear loads in three-phase four-wire distribution system, the research studies presented in this thesis starts with an introduction to harmonics clearly specifying its description, causes and consequences. The role of passive power filters in harmonics elimination is discussed. But to avoid the inevitable drawbacks of passive filters, we moved towards the use of Active power filters (APFs). After comparing various APF configurations, we chose the three-phase four-wire capacitor midpoint Shunt APF VSI- PWM configuration for the modelling of shunt APF.
We have evaluated the performances of p-q and Id-Iq control strategies by comparing the
THDs in compensated source currents and DC link voltage regulation under balanced, un- balanced and distorted/non-sinusoidal supply conditions. DC-link voltage regulation with the help of PI controller, Type-1 FLC and Type-2 FLC with different Fuzzy MFs (Trapezoidal, Triangular and Gaussian MF) to minimize the power losses occurring inside APF is studied. We had a discussion on various drawbacks encountered in conventional PI controller. The next research concentrated on the implementation of Type-1 FLC with different Fuzzy MFs (Trapezoidal, Triangular and Gaussian MF), this also suffers from several drawbacks resulting in severe deterioration of APF performance. Hence we have developed Type-2 FLC with different Fuzzy MFs; that could overcome the drawbacks observed in the PI and Type-1 FLC. Three-phase reference current waveforms generated by proposed scheme are tracked by the three-phase voltage source converter in a hysteresis band control scheme. The performance of the control strategies has been evaluated in terms of harmonic mitigation and DC link voltage regulation. The simulation (MATLAB) results are validated with real-time implementation on Real-time digital simulator (OPAL-RT).
The findings of the above investigations are summarized in the Figs. 6.1, 6.2, 6.3 and 6.4.
Fig. 6.1, Fig. 6.2, Fig. 6.3 and Fig. 6.4 (line graphs), Clearly illustrates the amount of THD
of source current reduced from one controller to other controller for shunt active filter control strategies under various source conditions using MATLAB and Real-time digital simulator.
Fig. 6.1. The Line Graph indicating the amount of THD reduced for PI controller, Type-1 FLC and
Fig. 6.3. The Line Graph indicating the amount of THD reduced for PI controller, Type-1 FLC and
Type-2 FLC with different Fuzzy MFs using p-q control strategy with Real-time digital simulator
Fig. 6.4. The Line Graph indicating the amount of THD reduced for PI controller, Type-1 FLC and
Type-2 FLC with different Fuzzy MFs using Id-Iq control strategy with Real-time digital simulator
It can be inferred from the simulation and real-time results of Chapter 2 that, p-q control strategy yields inadequate results under un-balanced and/or non-sinusoidal source voltage conditions.
Under unbalanced and/or non-sinusoidal condition, p-q control strategy is not succeeded in compensating harmonic currents, notches are observed in the source current. The main reason behind the notches is that the controller failed to track the current correctly and thereby APF fails to compensate completely. So to avoid the difficulties occur with p-qcontrol strategy, we have considered Id-Iq control strategy.
The Id-Iq scheme is the best APF control scheme for compensation of current
harmonics for a wide variety of supply voltage and loading conditions. The THD in source current can also be lowered down satisfactorily below 5%, thereby satisfying the IEEE-519 standards on harmonic level. Simultaneously, it also compensates for excessive neutral current.
Under un-balanced and/or non-sinusoidal conditions, PI controller is unable to maintain the DC link voltage constant (Vdc is nearer to 780V, but Vdc-ref is 800V) and it is unable to mitigate the harmonics completely and THD is close to 5%. The mitigation of harmonics is poor when the THD of source current is more. But according to IEEE 519-1992 standard, THD must be less than 5%. So to mitigate harmonics effectively, we have considered Type-1 FLC with different Fuzzy MFs. Id-Iq control strategy using Type-1 FLC with different Fuzzy MFs, the SHAF is able to
maintain the DC link voltage constant (Vdc is nearer to 790V, but Vdc-ref is 800V) and it is able to mitigate harmonics (THD is nearly equal to 2.5% to 3.5%) in a better way than that of p-qcontrol strategy using Type-1 FLC with different Fuzzy MFs (THD is nearly equal to 3% to 5%).
Even though, the Id-Iq control strategy using Type-1 FLC with different Fuzzy MFs is
able to mitigate the harmonics but notches are present in the source current. So to avoid the difficulties occur with Type-1 FLC based p-q and Id-Iq control strategies
with different Fuzzy MFs, we have considered Type-2 FLC with different Fuzzy MFs. The proposed Type-2 FLC based SHAF with different Fuzzy MFs is able to eliminate
the uncertainty in the system) and SHAF gains outstanding compensation abilities.
Type-2 FLC is able to maintain the DC link voltage constant (Vdc is nearer to 797V;
which is almost equal to Vdc-ref 800V) and it is able to mitigate harmonics (THD is nearly equal to 1% to 2%) in a superior way than Type-1 FLC (THD is nearly equal to 2.5% to 5%).
While considering the Id-Iq control strategy using Type-1 FLC (Trapezoidal,
Triangular and Gaussian MF) and Type-2 FLC (Trapezoidal, Triangular and Gaussian MF) and the p-q control strategy with Type-1 FLC (Triangular and Gaussian MF) and Type-2 FLC (Trapezoidal, Triangular and Gaussian MF), the SHAF has been found to meet the IEEE 519-1992 standard recommendations on harmonic levels.
PI controller, Type-1 FLC and Type-2 FLC based shunt active filter control strategies (p-q and Id-Iq) with different Fuzzy MFs (Trapezoidal, Triangular and Gaussian) are
verified with Real-time digital simulator (OPAL-RT) hardware to validate the proposed research.
The Simulation and Real-time implementation results showed that even if the supply voltage is un-balanced and/or non-sinusoidal the performance of SHAF using Id-Iq
theory with Type-2 FLC (Gaussian MF) showing better compensation capabilities in terms of THD compared to Id-Iq theory with PI, Type-1 FLC (Trapezoidal, Triangular