The Unified Power Quality Conditioner
Han Yibo
Dept. of Electronic and Computer Engineering
University of Macau
Abstract-This report mainly studies the unified power quality conditioner, a kind of power control system which can improve the electric power quality at distribution levels. The report shows the historical background, objective, functions and control strategies of UPQC. Beside these characteristics, this report discusses the advantages and disadvantages of UPFC, then give a brief discussion of methods to improve the system.
Index terms-Power quality, UPQC, reactive power compensation.
I
I. INTRODUCTION
n most power system, a main challenge to improve the system is to maintain the quality of electric power within the acceptable limits.
In general, poor power quality may result into increased power losses, abnormal and undesirable behavior of equipment, interference with nearby communication lines or something like that [1]. The widespread use of power electronic based systems has further put the burden on power system by generating harmonics in voltages and currents along with increased reactive current. The term active power filter (APF) is a widely used terminology in the area of electric power quality improvement [2]. APFs have made it possible to mitigate some of the major power quality problems effectively. The UPQC is one of the APF family members where shunt and series APF functionalities are combined together to achieve superior control over several power quality problems simultaneously.
UPFC is a power quality system which came after APF. There are two important types of APF, shunt APF and series APF. The shunt APF is the most promising to tackle the current-related problems, whereas, the series APF is the most suitable to overcome the voltage-related problems [6]. Since the modern distribution system demands a better quality of voltage being supplied and current drawn, installation of these APFs has great scope in actual practical implementation. Many researchers and engineers tried to design a power quality system which could solve current-related problems and current-related problems at the same time, however, installing two separate devices to compensate voltage-related power problems and current-related power quality problems independently may cost much.
In 1989, S. Moran described a new power quality condition control system, the system combine both series and shunt APFs. Other two researchers, Fujita and Akagi later tested the practical application of this system with 20 kVA experimental power system and got good experimental results [8]. They named this device as unified power quality conditioner (UPQC), and since then the power quality system came into attention and the name UPQC has been popularly used by majority of researchers. The back to back inverter topology design of UPQC system has been also addressed as series–parallel converter, unified APF (UAPF), universal active power line conditioner, universal power quality conditioning system (UPQS), load compensation active conditioner, universal active filter, and something
like that.
II. FUNCTIONS AND CONTROL
STRATEGIES OF UPQC
UPQC is a power quality control system, it can improve power quality on both current side and voltage side. In construction, a UPQC is similar to a unified power flow controller (UPFC). Both UPQC and UPFC employ two voltage source inverters (VSIs) that are connected to a common DC energy storage element. A UPFC is employed in power transmission system whereas UPQC is employed in a power distribution system, to perform the shunt and series compensation simultaneously. However, a UPFC only needs to provide balance shunt and/or series compensation, since a power transmission system generally operates under a balanced and distortion free environment [5]. On the other hand, a power distribution system may contain DC components, distortion, and unbalance both in voltages and currents. Therefore, a UPQC should operate under this environment while performing shunt and/or series.
The main purpose of a UPQC is to compensate for supply voltage power quality issues, such as, sags, swells, unbalance, flicker, harmonics, and for load current power quality problems, such as, harmonics, unbalance, reactive current, and neutral current.
Fig. 1. UPQC general block diagram representation
The key components of UPQC system are as follows:
1. Two inverters—one connected across the load which acts as a shunt APF and other connected in series with the line as that of series APF.
2. Shunt coupling inductor L is used to interface the shunt inverter to the network. It also helps in smoothing the current wave shape. Sometimes an isolation transformer is utilized to electrically isolate the inverter from the network. 3. A common DC link that can be formed by using a capacitor or an inductor. In Fig. 1, the DC link is realized using a capacitor which interconnects the two inverters and also maintains a constant self-supporting DC bus voltage across it.
4. An LC filter which serves as a passive low-pass filter (LPF) and helps to eliminate high-frequency switching ripples on generated inverter output voltage.
5. Series injection transformer that is used to connect the series inverter in the network. A suitable turn ratio is often considered to reduce the current or voltage rating of the series inverter. In principle, UPQC is an integration of shunt and series APFs with a common self-supporting DC bus [1]. The shunt inverter in UPQC works in a current control mode. The shunt inverter plays an important role in achieving required performance from a UPQC system by maintaining the DC bus voltage at a set reference value.
In fig. 1, we can get a current equation:
𝑖𝑠ℎ(𝑤𝑡)=𝑖∗𝑠(𝑤𝑡)−𝑖𝐿(𝑤𝑡)
In the equation, 𝑖𝑠ℎ(𝑤𝑡)represents the shunt
inverter current, 𝑖∗𝑠(𝑤𝑡) represents the reference
source current, and 𝑖𝐿(𝑤𝑡) represents the load current, respectively.
Similarly, the series inverter of UPQC works in voltage control mode such that it generates a voltage and injects in series with line to achieve a sinusoidal, free from distortion and at the desired magnitude voltage at the load terminal. The basic operation of a series inverter of UPQC can be represented by another similar equation:
𝑣𝑆𝑟(𝑤𝑡)=𝑣𝐿∗(𝑤𝑡)−𝑣𝑆(𝑤𝑡)
In the equation,𝑣𝑆𝑟(𝑤𝑡), 𝑣𝐿∗(𝑤𝑡), 𝑣𝑆(𝑤𝑡),
represent the series inverter injected voltage, reference load voltage, and actual source voltage,
respectively [1]. In the case of a voltage sag condition, VSr will represent the difference between the reference load voltage and reduced supply voltage, the injected voltage by the series inverter to maintain voltage at the load terminal at reference value. In most kind of UPQC, the shunt inverter is operated as controlled current source and the series inverter as controlled voltage source. The control of DC-link voltage plays an important role in achieving the desired UPQC performance [6]. During the system dynamic conditions, for example, sudden load change, voltage sag, the DC-link feedback controller should respond as fast as possible to restore the dc-link voltage at set reference value, with minimum delay as well as lower overshoot.
III. ADVANTAGES AND LIMITATIONS OF
UPQC
The UPQC is the most versatile and complex of the FACTS devices, combining the features of the STATCOM and the SSSC. The main reasons behind the wide spreads of UPQC are: 1. UPQC could achieve comprehensive power quality control by using only one device. It could deal with voltage problems and current problems at one processing time.
2. UPQC is easy to control power by using an injected component power flow. We only need to control the parameters of this injected component power flow to control the whole power system. 3. UPQC can resist the disturbance on the second synchronous resonance or other kinds of disturbance. It also can reduce inner system disturbance effectively.
4. UPQC could increase the system security through raising its stability. In the power flow control system, stability is a significant value, UPQC is effective in error control and loop control, and the system’s control method make it convenient to modify the control procession. 5. The UPQC has ability to pass the real power quality
bi-directionally,
maintaining
well-regulated DC voltage and workability in the vast
range of operating conditions [3].
6. Comparing with other kinds of power quality controllers, UPQC is much more effective than other control facilities in regulating the power quality in real time.
7. The UPQC system can maintain the source currents with constant magnitude even the load currents are varying.
The reason of the UPQC’s popularity is the system construction. There are two voltage source inverters (VSIs) sharing a common DC storage capacitor, and connected to the power system through coupling transformers. One VSI is connected to in shunt to the transmission system via a shunt transformer, while the other one is connected in series through a series transformer. The DC terminals of the two VSCs are coupled and this creates a path for active power exchange between the converters. Therefore, a different range of control options is available compared to STATCOM or SSSC [4]. The technology to develop commercial UPQC system is available today. However, the overall cost and complexity of such a system still imposes some limitations.
The capacity of small and large-scale renewable energy systems based on wind energy, solar energy, installed at distribution as well as transmission levels is increasing significantly [12]. These newly emerging DG systems are imposing new challenges to electrical power industry to accommodate them without violating standard requirements. The switching operation of these systems is contributing as increased harmonic levels both in the grid voltages and currents. The aforementioned power quality issues suggest potential applications of UPQC in renewable-energy-based power systems.
IV. DEVELOPMENT TRENDS In recent years, several UPQC configurations and topologies have been discussed. Among these configurations, UPQC-DG could be the most interesting topology for a renewable-energy-
based power system. This configuration can offer multifunctional options, namely, active power delivery from DG system to grid (normal DG operation), voltage-related and current-related power quality compensation (UPQC operation), and uninterruptible power supply operation. In the future, the UPQC should be developed to UPQC-DG field, and reduce device cost will still be a significant topic.
V. CONCLUSION
Among the FACTS components, Unified Power Quality Conditioner (UPQC) is one of the most efficient. The UPQC provides a flexibility for both current control and voltage control. It possesses a great aptitude to achieve an independent and simultaneous control mode [11]. Different aspects of UPQC and up to date developments in this area of research have been briefly addressed. For future use of the UPQC, it still has space of improvement on device cost and UPQC-DG combination.
VI. REFERENCES
[1] R. C. Dugan, M. F. McGranaghan, and H. W. Beaty, Electrical Power Systems Quality. New York: McGraw-Hill, 1996.
[2] C. Sankaran, Power Quality. Boca Raton, FL: CRC Press, 2002.
[3] FUJITA H., AKAGI H.: ‘The unified power quality conditioner: the integration of series and shunt-active filters’, IEEE Trans. Power Electron., 1998, 13, (2), pp. 315–322.
[4] IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems, IEEE Standard 1547-2003, 2003. [5] N. G. Hingorani and L. Gyugyi, Understanding
FACTS:
Concepts
and
Technology of Flexible AC Transmission Systems. New York: Institute of Electrical and Electronics Engineers, 2000.
[6] V. K. Sood, HVDC and FACTS Controllers—Applications of Static Converters in Power Systems. Boston, MA: Kluwer, 2004.
[7] A. Ghosh and G. Ledwich, Power Quality Enhancement Using Custom Power Devices. Boston, MA: Kluwer, 2002.
[8] H. Akagi, “Trends in active power line conditioners,” IEEE Trans. Power Electron. vol. 9, no. 3, pp. 263–268, May 1994.
[9] B. Singh, K. Al-Haddad, and A. Chandra, “A review of active filters for power quality improvement,” IEEE Trans. Ind. Electron. vol. 46, no. 5, pp. 960–971, Oct. 1999.
[10] M. El-Habrouk, M. K. Darwish, and P. Mehta, “Active power filters: A review,” IEE Electric. Power Apply. vol. 147, no. 5, pp. 403–413, Sep. 2000.
[11] S. Moran, “voltage regulator/conditioner for harmonic-sensitive load isolation,” in Proc. Ind. Appl. Soc. Annu. Meet. Conf. Oct. 1–5, 1989, pp. 947–951.
[12] F. Kamran and T. G. Habetler, “Combined deadbeat control of a series-parallel converter combination used as a universal power filter,” in Proc. Power Electron. Spec. Conf. Jun. 18–22, 1995, pp. 196–201.
因篇幅问题不能全部显示，请点此查看更多更全内容