08-11-2016, 02:41 PM
1466883777-PerformanceAnalysisofHybridPowerFiltertoimprovePowerQuality.docx (Size: 603.83 KB / Downloads: 5)
Abstract—This paper presents performance analysis of a three-phase hybrid power filter in detail. The proposed strategy in this paper improves the behaviour of the shunt passive filter which is a component of hybrid power filter and it is observed that the overall performance of hybrid power filter is better than shunt active power filter. This is verified using MATLAB SIMULINK. The simulations are performed with shunt passive filter alone and hybrid power filter under different conditions: i)balanced and unbalanced loads ii)variation in the source impedance iii)variation in load. Power factor is also calculated at each step to observe the improvement in electric power quality under different situations. Comparative analysis of hybrid power filter and shunt active power filter is also studied to prove the superiority of hybrid power filter.
Keywords—Active power filters, hybrid power filters, harmonics, power quality, dual p-q theory.
I. INTRODUCTION
The increased use of non-linear loads has led to the number of power quality problems. One of the serious problems is harmonics. Due to the source impedance, any current (or voltage) harmonic will result in the generation and propagation of voltage (or current) harmonics at the point of common coupling (PCC) and this affects the entire power system. Some harmful effects of harmonics are: mal-operation of control devices, overheating of capacitors, transformers and rotating machines, additional noise from motors and other equipments, telephone interference etc[1],[2].
Using higher pulse rectification, passive, active and hybrid filters are the recommended solutions to reduce and control harmonics. Usually, a passive filter is used to compensate harmonics. Although it is relatively inexpensive, it has few drawbacks, because of which it cannot give complete compensation. Passive filters can eliminate only one or two harmonics(e.g. 5th and 7th harmonic). It creates parallel resonance circuits with system impedance. It is bulky and large in size. It has unsatisfactory performance under varying load conditions. These disadvantages are overcome by active power filters and that is why they have become more popular[1].
There are different configurations of passive filters and active power filters[5],[8]. Here, hybrid power filter is considered which is the combination of shunt passive and series active power filter. The topology of this hybrid power filter is shown below in Fig 1, where Ls is the source impedance and Crf, Lrf forms ripple filter. Ripple filter is needed to suppress unwanted noise signals. The series active power filter injects the voltage in the system of reverse polarity to that of voltage harmonics[4]. The function of this hybrid power filter is not to compensate the voltage harmonics directly, but to enhance the shunt passive filter characteristics by providing harmonic isolation. To analyse this behaviour, simulations are carried out under different conditions. The control scheme used is dual p-q theory.
Focusing on Hybrid Power Filter, this paper is presented in five sections. Section I gives brief introduction about harmonics and different methods to mitigate it. Section II discusses concisely the performance of shunt active power filter to mitigate harmonics. Section III describes the control strategy of hybrid power filter i.e. vectorial dual p-q theory. The control strategy is discussed with the help of flowchart. Section IV consists of MATLAB simulations without filter, with shunt passive filter alone and hybrid power filter under different conditions: i)balanced and unbalanced loads ii)variation in the source impedance iii)variation in load. This is done to analyse the performance of hybrid power filter. The current waveforms, percentage current THD, FFT analysis of the current and power factor of the system are observed and analysed for each simulation. Section V presents the results, followed by conclusion.
MATLAB simulations for shunt active power filter were carried out using the above control strategies. According to the simulation results, the percentage current THD was reduced from 19.02% to near about 5% for balanced loads and 7% for unbalanced loads. This can be further reduced to less than 2% using hybrid power filter with balanced and unbalanced loads. The performance of hybrid power filter is better than shunt active power filter. Dual p-q theory is the control strategy used for the hybrid power filter and is discussed in Section III below.
The measured loss is smaller in hybrid power filters and thus, it is more efficient than the shunt active power filters. The initial cost of shunt active power filter is high. The hybrid power filter is a very practical and economical way to filter harmonic currents. It not only gives better filtering characteristics, but also has low initial costs. So, use of hybrid power filter is recommended over shunt active power filter.
III. CONTROL STRATEGY OF HYBRID POWER FILTER: DUAL P-Q THEORY
The calculation of reference voltages of series active power filter used in hybrid power filter is done using the dual p-q theory. In this theory, it is considered that the currents, the real and imaginary powers are known and the voltage components are to be calculated[3]. Fig 3 shows the flowchart for the dual p-q theory.
VI. CONCLUSION
The control scheme for a hybrid power filter which is combination of series active power filter and shunt passive filter is presented. Hence, the results are verified for the hybrid power filter. The following statements can be concluded:
- It is observed that the unlike passive filter, compensation characteristics of the hybrid power filter does not depend on the system impedance of the system.
- The non-linear load behaves like a resistive load when connected with the hybrid power filter.
- Passive filter is used to mitigate lower order harmonics, while the series active filter is used to mitigate higher order harmonics.
- The hybrid power filter is not affected by the changing load conditions.
- The hybrid power filter improves the compensation characteristics of passive filter as well as the power factor of the load, which consequently enhances the power quality of the system.
Thus, power quality becomes better using hybrid power filters as compared to shunt active power filters, and therefore, hybrid power filters must also be commercially available and must be largely manufactured as they are most economical.