1
Study on the Control Strategies and Dynamic Performance of DC Distribution Network Qing Zhong Lingxue Lin Yao Zhang Zhigang Wu Abstract— Some changes are required to improve the characteristics of power system to implement the developing smart grid technology. For distribution system, DC mode is a very good complementation of AC techniques. The control strategies and dynamic performance of DC distribution system are studied to investigate the feasibility of the DC distribution mode. A novel prototype of distribution system is provided. Three different control strategies are applied in different converters for the demand of corresponding functions. The simulation results show the advantages of DC distribution system: the converters can provide accurate active power transaction and balance reactive power in the stable situation, while provide the ability to mitigate the voltage sags during the faults in AC distribution system and improve the power quality in transient situation. Index Terms— smart grid distribution generation
DC microgrid
power quality
I. INTRODUCTION AC distribution system has been designed for over 100 years since George Westinghouse won the competition with Thomas Edison [1]. But with the enlargement of the scale of AC distribution networks, traditional AC distribution networks face many problems such as the increasing of shortcircuit capacity, load stability and distribution resource interfacing and power quality propagation, and so on. Therefore some novel distribution structure need put forward for the development of the distribution network [2]. With the development of the power electronics technologies, many converters have been used in AC power system to control the power flow, improve the power quality, and enhance the stability. They are also used to control the speed of induction motor, charge the power energy and so on. The converting between AC and DC is feasible and controllable. HVDC transmission systems have been applied to the long distance and huge power transmission. HVDC Light is also used to improve the dynamic performance of large AC system [3]. However, when considering of the distribution system, whether DC power distribution is suitable and when to use DC way to distribute the energy as the supplement of AC distribution system remain uncertainty. Focus on the generation in the coming new power system, lots of distribution resource and energy restoration will access to the distribution network [4]. The wind energy and solar energy, which are connected with grid, are dispersed in the roof of the building, the side of the roads The retainable
energies include both DC supply and AC supply, so the accession of the energy must be flexible [5] [6]. On the other side, the loads of the customer are more multiplicate and complicated. Uninterrupted power supply (UPS) and automatic speed deregulator (ASD) are used in the factories to prevent interruption and improve the efficiency. More and more electric vehicle will be put into market to reduce the carbon emission. All of these untraditional loads need various power supplies [7]. In 21st century, digital economy need higher power quality, because many digital devices will be affected by the power quality events, which can cause huge economic loss [8]. DC distribution network is an effective way to establish the high power quality power supply system [9][10]. Therefore, DC distribution network is a novel field for the power distribution. It can be a very good complementation of traditional AC system. This paper pays attention to the topology and control strategies of DC distribution networks. With consideration of distribution resource accessing the distribution network, a new topology of DC distribution network is given in this study. For the different function of the converters in the network, different control strategies are studied to improve the dynamic performance of DC distribution network. II.
TOPOLOGIES OF DC DISTRIBUTION NETWORK
A. Prototype of DC distribution network According to the idea of DC energy distribution, one DC distribution network is shown in figure 1. The core of the DC network is two DC buses connecting with AC system and loads. DC source and energy restoration can be connected directly with DC buses. AC system, AC source and AC loads are connected with DC buses with converters.
Fig.1 topology of DC distribution network in the study Qing Zhong, is with School of electric power, South China University of Technology, Guangzhou, 510640, China (e-mail:
[email protected])
978-1-4673-2729-9/12/$31.00 ©2012 IEEE
2
B. Advantages of the network The DC distribution network supposed here provides DC way to connect the large power system, distributed energies and loads. With the converters as the interface between AC and DC system, the short-circuit capacity of AC distribution system will not increase any more. The active power and reactive power of the converter can be controlled independently; therefore, the reactive power can be balanced locally. The DC distribution network would never lose the load stability. At the same time, the DC distribution network can act as the voltage support for the AC system. With the capacitor on the DC buses, the power quality propagation can be isolated between AC and DC system. The power quality compensators are mostly used power electronics, thus the DC distribution network can act as high power quality. Voltage sags, harmonics, voltage fluctuation can be mitigated in the network. With two DC buses, the DC distribution network can be operated more reliable. The fault in any converter would not affect the operation of other modules. The replacement of converter modules can recover the power supply immediately. The fault on one bus will not affect the operation of the other bus. Therefore, the reliability of DC distribution network is higher than those with only single DC bus. The DC distribution system needs 3 wires, positive, negative and ground, which can use the already exist three phase AC cable and the lines of DC distribution network. For this reason, the investment of the cable in DC will not be more than that of an AC system. The number of converters is the key factor to decide the investment of the system. The number of converters in both AC and DC system are given in table I. It is clear that the converters in DC system will be less than those in AC system if there are distribution generators with sustainable energy in the distribution network, thus the investment of DC distribution network is also less.
C. Testing model A simulation model is built in PSCAD\EMTDC to test the dynamic performance of the DC distribution network, as shown in figure 2. The model is consisted of four modules. Module I is connected with large AC system shown in figure 3. The energy of DC distribution network is mainly supported by this module. Module II is connected with the other large AC system shown as figure 4, which get some power from DC distribution network in the operation. The different between these two modules is that module II has some loads need the power supply from DC system. But in the emergency situation, this module can also supply power for DC system. Module III is connected with an AC system with distribution generations, which are 3 wind generators here shown as figure 5. The AC system connecting the wind generators is relatively weak. The DC system draws some power from the wind plant. Module IV is connected with an island load without power supply shown as figure 6. Above all, the DC distribution system get power from AC system and distributed generators by module I and module III, while supply power by module II and module IV, just like the arrows shown in figure 3~6.
Module II Module I
Module III
TABLE I COMPARISON OF NUMBER OF CONVERTERS
Item / number of converters Large system Wind energy (induction motor) Wind energy (DFIG) PV Energy restoration (cells) ac motor with ASD Gas plant Hydro electric power ac load (island) UPS Power quality compensators
ac distribution
dc distribution
0
2
2
1
2
1
1
0
1
0
2
1
0
1
0
1
0 2
1 1
2
1
Module IV Fig.2 structure of simulation model of DC distribution network P
Fig.3 structure of simulation model of Module I P
Fig.4 structure of simulation model of Module II
3
IV.
P
Fig.5 structure of simulation model of Module III P
Fig.6 structure of simulation model of Module IV
III.
CONTROL STRATEGIES OF DC NETWORK
For different function of components in the DC distribution system, three control strategies are applied in the converter stations as follows. A. Constant DC voltage control Dc voltage is a very important factor to keep the DC distribution system stable. As consequence, there must be a component to maintain the DC voltage as constant. As the main power supplier of DC system, constant DC voltage control is applied in module I. In DC system, only one converter is used to keep DC voltage constant, which can be looked as the slack bus in AC system. B. Constant PQ control Accurate active power transaction and reactive power balance are two important functions in DC distribution system. The active power and reactive power of converter are independently controllable, so the active power transaction can be controlled as desired. To avoid the losing of load stability caused by the unbalance reactive power, the converter can generate or absorb reactive power to satisfy the system demand.
SIMULATION RESULTS
The testing model is established with PSCAD/EMTDC, the AC voltage is 10kV and transformed to step down to 800V connected with converters in module I, II and III. The nominal AC voltage of module IV is 800V. The AC voltage can be transformed by transformers. The nominal DC voltage of the system is set as 1600V. The simulations study two situations: one is the normal operation; the other is the same power system with a threephase short circuit fault in the AC side of module I. Under the control strategies given above, the simulation results are shown as follows. A. Normal operation Under normal situation, the DC voltage can keep constant with the constant DC voltage control in module I. The set point of DC voltage is 1.6kV shown as figure 7. With the constant DC voltage, the system can operate well.
Fig.7 DC voltage in module I with constant DC voltage control
For module II, the constant PQ control can keep the active power transaction as 1.0pu and keep the reactive power as zero. The converter will not consume reactive power to balance the AC system. The results are given in figure 8.
C. Constant AC voltage control For island AC load, there is no power supply except for the DC system. How to keep the AC voltage stable is very important index for the power quality of power customers. Constant AC voltage control can supply the voltage without fluctuation to improve the power quality. TABLE II CONTROL STRATEGIES OF MODULE IN DC SYSTEM
Component Module I
Control strategy Constant DC voltage control
Module II & module III
Constant PQ control
Module IV
Constant AC voltage control
Object Keep DC voltage of the system constant Keep the accurate active power transaction and balance the reactive power Keep the AC voltage of loads constant
Fig. 8 active power and reactive power of module II with constant PQ control
For module III, there are some wind generators. The outputs of wind generators are stochastic as well known. The output of wind generators is given in figure 9. When the AC system connecting the wind generators is weak, the stochastic output will cause voltage fluctuation, as shown in figure 10. The converter takes respond to achieve accurate active power from wind generator and balance the reactive power of wind generator. The results are shown in figure 11. The voltage fluctuation can be restrained too, as shown in figure 12.
4
is kept 1.0pu. The shape of AC current is sinusoidal with some high order harmonics.
Fig. 9 stochastic active power and reactive power of the wind generator
Fig. 13 AC voltage of module IV with constant AC voltage control
Fig. 10 stochastic voltage fluctuation of the wind generator Fig. 14 AC current of the island loads in module IV
B. Operation under fault Abnormal condition is simulated by setting a three-phase short circuit fault in AC side of module I, the inception time is 2s and the duration is 0.1s. The AC voltage of module I is shown in figure 15. In traditional AC distribution, this fault will cause voltage sags in the system. The retained voltage near the fault is close to zero. While in DC distribution system, because of the existence of DC bus, the effect of voltage sag can be mitigated. The DC voltage in module I is shown in figure 16. The drop of DC voltage is very small (about 0.9 p.u remained.).
Fig. 11 active power and reactive power of the module III with constant PQ control
Fig. 15 AC voltage in module I with three-phase short circuit fault
Fig. 12 Ac voltage of module III with constant AC voltage control
For module IV, the load is isolated with power system. The power is supplied by the converter. The constant AC voltage control is applied in the converter. The AC voltage and current is shown in figure 13 and 14. The RMS of AC voltage
For module III, since the wind generator and AC system connected to the converter, there is no voltage sag here. The AC voltage is given in figure 17.
5
REFERENCES [1]
Fig. 16 DC voltage in module I with three-phase short circuit fault
Fig. 17 AC voltage in module III with three-phase short circuit fault
For module IV, the voltage sags a little, the retained voltage is above 0.98pu, which wouldn’t affective the power customers dramatically. The AC voltage is shown in figure 18.
R. Moran. Executioner’s Current: Thomas Edison, George Westinghouse, and the Invention of the Electric Chair, New York: Random House, 2002. [2] D. Hammerstrom. AC versus DC distribution systems - did we get it right?. IEEE Power and Energy Society General Meeting, Tampa, FL, Jun. 2007. [3] Qing Zhong, Yao Zhang, Lingxue Lin, etc. Study of HVDC light for its enhancement of AC/DC interconnected transmission systems. IEEE Power and Energy Society General Meeting, 2008, Pittsburg, US, p 4596003. [4] W. El-Khattam, M.M.A. Salama. Distributed generation technologies, definitions and benefits. Electric Power Systems Research, 2004(71): 119–128. [5] Thomas F. Garrity. Getting smart. IEEE Power & Energy Magazine, 2008, March/April, pp: 38-45. [6] Y Ito, Y Zhongqing, H Akagi. DC microgrid based distribution power generation system. IEEE Trans. on Power Delivery, 2004, 19(3): 14-16. [7] Ambra Sannino, Giovanna Postiglione, Math H. J. Bollen. Feasibility of a DC network for commercial facilities. IEEE Trans. on. Industry Applications, 2003, 39(5):1499-1570. [8] DOE/NETL. Provide power quality for the digital economy. Oct. 2009, http://www.netl.doe.gov/martgrid/referenceshelf/whitepapers/Provides% 20Power%20Quality_APPROVED_2009_11_02.pdf. [9] Weixing Lu, Boon-Teck Ooi. Premium Quality Power Park Based on Multi-Terminal HVDC. IEEE Trans. on Power Delivery, 2005, 20(2):978-983. [10] Cuiqing Du, Math H.J. Bollen, Evert Agneholm, ect. A New Control Strategy of a VSC-HVDC System for High-Quality Supply of Industrial Plant. IEEE Trans. on Power Delivery, 2007, 22(4): 2386-2394. Qing Zhong, received Ph. D and MSc in South China University of Technology in 2003 and 2000, and BSc in North China University of Technology in 1997, all in Electrical Engineering. He is now teaching in School of electric power, South China University of Technology. His main fields of interest include power quality, HVDC transmission control, and power electronics control.
Fig. 18 AC voltage in module IV with three-phase short circuit fault
Above all, the short circuit fault will not cause voltage sag in the DC system. The power quality is improved evidently. V.
Lingxue Lin was born in Guangdong Province, China, on August, 16th, 1979. She received MSc and BSc in South China University of Technology in 2005 and 2002, all in Electrical Engineering. She is now doing the Ph.D. study in School of electric power, South China University of Technology. Her main fields of interest include HVDC transmission control and voltage stability.
CONCLUSIONS
DC distribution network is an old but still novel research field. Though the power distribution used DC voltage, with power electronics devices, the novel DC distribution network takes more missions than the older one. DC distribution network has some advantages comparing with AC system. For different functions of the converters in the network, three different control strategies: constant DC voltage control, constant PQ control and constant AC voltage control are applied in different modules. A testing model is set up with PSCAD. The DC distribution network can control active power transaction accurately. The reactive power on the AC side of converter can be balanced with constant PQ control. The DC distribution network can supply the island loads well with constant AC voltage control. With constant DC voltage control, the voltage sag can be mitigated.
Yao Zhang, received the Ph.D. Master and Bachelor degree from Tianjin University, in 1993, 1981, 1970, respectively. He is the Senior Member of CSEE and professor of school of electric power, South China University of Technology. His main fields of research interests include the power system stability and control, voltage stability, and electric power market. Zhigang Wu, received the Bachelor of Tech. degree in Electrical Engineering from the Department of Automation, Tiajin University, Tianjin, China, in 1996, the Master of Tech. and PhD degree in Electrical Engineering from the College of Electrical Engineering and Automation, Tianjin, China, in 1999 and 2002. Currently, he is an associate professor in the Electric Power College at the South China University of Technology, Guangzhou, China, and is working as a research associate in the Department of Electrical Engineering, the Hong Kong Polytechnic University, Hong Kong. His present research is in the area of voltage stability analysis and power system simulation.
