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A deterioration of the power quality is a major concern due to harmonics injected by the non-linearity of the consumer loads which are specially power electronic devices (Rectifier Load). A single phase Diode Bridge Rectifier (DBR) draws non-linear current thus degrading the power quality at the PCC. This paper focuses on the single phase series active compensator to alleviate the voltage and current harmonics under non-linear load condition.
A DC link capacitor is shared between VSI and single phase DBR so as to reduce the component requirement and to resist the wide variation of the AC mains voltage.
The compensator behavior has been analyzed graphically with the single phase DBR as a non-linear load. To validate the efficacy of the suggested system an extensive digital simulation has been carried out in MATLAB/Simulink environment.
The proliferation of power electronic devices in low voltage distribution network leads to the deterioration of the power quality of the system. Despite of the benefits provided by the use of power electronic devices in power system, harmonics injected by them is a major concern nowadays.
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It becomes challenging to maintain power balance between supply and load due to the nonlinear and electronically switched loads as they have tendency to distort the voltage and current waveforms. Alleviating voltage and current waveform distortions to acceptable levels has been a trouble in power system.
Overall electrical system performance and power quality [1] is affected by the introduction of harmonics, such as Overheating of Transformers, Capacitors and Motors, Mal-operation Relays and Circuit Breakers, Communication Interference Problems, Unreliable operation of Electronic Equipment etc.
There are a number of voltage quality problems in the AC mains nowadays, such as harmonics, sag, dip, flicker, swell,
fluctuations, and imbalance, and these problems increase losses in many loads and sometimes trip the sensitive loads causing loss of production. Various custom power devices are becoming popular solution in order to mitigate such power quality disturbances. The DVRs mainly used for compensating the voltage quality problems such as sag, swell, fluctuations, and imbalance. However, the series active power filter (SAPF) protects the sensitive loads from these distortions (especially harmonics) in the voltage of the AC mains. A series active filter filters the voltage harmonics appearing in the supply side so that the loads are supplied with clean sinusoidal supply voltage.
A diode rectifier with a large DC bus capacitor filter can be considered as a voltage-fed type nonlinear load and are used to realize the DC voltage source with the DC capacitor. Such types of loads are normally used these days for feeding the voltage source inverter (VSI) in numerous applications. These voltage-fed nonlinear loads [2] draw peaky and discontinuous current and inject a large amount of harmonic currents into the AC mains. In such situations, series APFs are quite effective for harmonic current compensation with moderate rating.
This paper mainly deals with an application of single phase SAPF comprising of single phase H-bridge VSI sharing a common DC link capacitor with voltage fed non –linear load in low voltage distribution system [3]-[6]. This suggested system results into reduced component requirement thus providing cost effective solution and restricts AC mains voltage variations. Because of SAPF efficient capabilities, they are attaining remarkable significance in series applications. In addition to this, an important aspect of SAPF is the reference current generation algorithm. A variety of control algorithms are illustrated in literature [7],[8]. One of the effective control schemes is the unit vector based control algorithm.
Therefore, in this paper, in conjunction with single phase bridge VSI based SAPF; a unit vector generation control algorithm is presented. This approach indicates a very effective way of eliminating distortions in the supply voltage and supply currents.
Fig.1. shows the uncompensated system. The system consists of a single phase AC supply with series impedance which is connected across a nonlinear load consisting of a resistive load along with the diode bridge rectifier. The rectifier circuit provides a DC output at the load. The DC link capacitor is connected across the load to give a constant DC output.
The Diode bridge rectifier consists of four diodes D1, D2, D3, D4 connected in the two legs. The two diodes operate simultaneously for the half cycle of the AC supply.
As the output obtained at the load side is not exact a sinusoidal but it is flattened at the top due to the presence of the harmonic contents. The nonlinear load i.e. bridge rectifier injects harmonics in the circuit which makes the supply side voltage and current completely distorted. As the entire system is a closed loop structure, the harmonic affected voltage is then feed as the input in the supply and the system is affected.
The performance of the system is simulated in the MATLAB software with the Simulink toolbox. The discrete step Solver and the time step of 1e-6s are used for the system simulation.
The performance of the system is shown in Fig 2. The waveforms for the supply Voltage (Vs), Source Current (Is), Load Voltage (Vl) are shown in Fig.2. From Fig. 2 it can be seen that, due to voltage fed type harmonic load the source currents and supply voltage becomes erroneous in nature. It is observed that the source voltage is flattening at the top because of the presence of harmonics in the system that arises as a result of DBR. The load voltage appears similar to the supply voltage. The Harmonic spectra for the voltage harmonics and the current harmonics are shown in Fig.3(a) and Fig. 3 (b), respectively.
The Harmonic Spectra for the voltage and current waveform shows the %THD (Total Harmonic Distortion) contents of about 10.83% and 85.72%, respectively.
Fig.2. Uncompensated System; (a) Source Current, (b) Source Voltage, (c) Load Voltage
In order to mitigate the harmonic distortion, the system is compensated with the help of a single phase series active compensator.
The Single phase series active compensator employing a DBR and VSI along with the series injected transformer is utilized for the system configuration. DBR’s with a DC bus filter capacitor at the front ends is used to feed small three phase or single phase VSIs.
These VSIs are used to control the speed of AC motors in various applications such as air conditioners, compressors, blowers, induced draft fans, etc. The DBR draws highly nonlinear current causing deterioration of power quality (PQ) at the point of common coupling. The basic configuration of the proposed system is shown in Fig.4.
The series active compensator is connected in series with the AC mains and the load. The DC link of the series active compensator and diode rectifier of the load are forming a common DC bus. An injecting transformer (Tr) is connected in series with the line and it injects the suitable voltage in series to balance and regulate the terminal voltage of the load or line.
The performance of the compensated system with SAPF is shown in Fig. 5. The waveforms for the supply Voltage (Vs), Load Voltage (Vl), injected voltage (Vinj) and the Source Current (Is) are shown, respectively.
Fig. 5. Compensated system; (a) Supply Voltage, (b) Load Voltage, (c) Injected Voltage, (d) Supply Current
By the application of SAPF in low voltage distribution system, it is observed that the SAPF is able to make supply side source currents as well as supply voltage waveform distortion free as depicted in Fig.5 (a) and Fig.5 (d) by injecting the voltage via injecting transformer. Fig. 6 (a) and Fig.6 (b) shows the Harmonic spectra for the voltage harmonics and the current harmonics.
Design Consideration and Control Algorithm
The design of single phase series active power filter consists of various components such as, ripple filter. There rating estimations are given in detail,
Here fr = fs/2, Where fs (20kHz) is the switching frequency. Considering Xcr = 3 Ω, XLr = 100 Ω. These values of ripple filter maintain the minimum ripple at the output voltage of the SAPF.
The above (Fig. 6(a) and Fig. 6(b)) shows the %THD of source voltage and source current and it is found that %THD is reduced due to the effective working of SAPF. Comparative analysis of compensated and uncompensated system based on %THD is shown in Table 1. Component specifications required to simulate the system with SAPF in MATLAB environment is given in Table 2.
Comparative Analysis Of Uncompensated And Compensated System
| System | %THD | Isa | |
|---|---|---|---|
| Uncompensated System | 85.72% | 10.83% | |
| Compensated System | 5.00% | 1.50% |
The Table 1 above gives the comparative analysis of uncompensated and system on the basis of %THD. As it can be seen that the values of %THD for uncompensated system for source current and source voltage are 85.72% and 10.83%, respectively, which is more than the desired value (not as per IEEE standard). Whereas the %THD for the current and voltage in the compensated system is 1.50% and 5%, respectively, which are almost nearer to desired value.
System Parameters
TABLE 2
| Parameters | Ratings |
|---|---|
| AC Supply Voltage (Vsa) | 220V (RMS), single phase, 50Hz |
| Line Impedance (Ls and Rs) | 1.1mH and 0.12 ohms |
| DC Link Voltage (Vdc) | 300 volts |
| DC Link Capacitor (C) | 2500 µF |
| DC PI Controller gains | Kp = 1.1, ki = 0.29 |
| Current PI Controller gains | Kp = 1.5, ki = 0.32 |
| Ripple Filter | Lr = 2mh, Cr = 5µF |
A single phase series active compensator has been introduced which has common DC link capacitor between SAPF and voltage fed type Harmonic load. To mitigate voltage and current harmonics caused due to voltage fed type harmonic load, the series active compensation technique (SAPF) is found to be one of the effective method for maintaining the voltage and the current of the supply system balanced and distortion free. Also, the common DC link of the Series compensator and the load provides cost effective solution. The VSI acting as the Series active Compensator along with common DC link Filter Capacitor reduces the notches and ripples that are obtained due to the harmonics and makes the supply side stable which increases the Power Quality. In addition to that, the %THD of the supply side voltage and current is reduced and reaches nearer to the desired value (as per IEEE standard).
Performance Analysis of Single Phase Series Active Compensator Using Control Algorithm Based on Unit Vector Generation. (2024, Feb 13). Retrieved from https://studymoose.com/document/performance-analysis-of-single-phase-series-active-compensator-using-control-algorithm-based-on-unit-vector-generation
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