Research Article  Open Access
TianTian Qian, ShiHong Miao, "DiscreteTime Nonlinear Control of VSCHVDC System", Mathematical Problems in Engineering, vol. 2015, Article ID 929467, 11 pages, 2015. https://doi.org/10.1155/2015/929467
DiscreteTime Nonlinear Control of VSCHVDC System
Abstract
Because VSCHVDC is a kind of strong nonlinear, coupling, and multiinput multioutput (MIMO) system, its control problem is always attracting much attention from scholars. And a lot of papers have done research on its control strategy in the continuoustime domain. But the control system is implemented through the computer discrete sampling in practical engineering. It is necessary to study the mathematical model and control algorithm in the discretetime domain. The discrete mathematical model based on output feedback linearization and discrete sliding mode control algorithm is proposed in this paper. And to ensure the effectiveness of the control system in the quasi sliding mode state, the fast output sampling method is used in the output feedback. The results from simulation experiment in MATLAB/SIMULINK prove that the proposed discrete control algorithm can make the VSCHVDC system have good static, dynamic, and robust characteristics in discretetime domain.
1. Introduction
Voltage source converter based high voltage direct current (VSCHVDC) technology which uses advanced (insulated gate bipolar transistor) IGBT device and pulse width modulation (PWM) method overcomes the disadvantage of traditional HVDC. It can not only adjust the active and reactive power independently, but also supply power to passive network system. Therefore the VSCHVDC technology has a broad application prospect in the fields of distributed generation, asynchronous AC network interconnection, remotearea power supply, and so forth [1–5].
Because VSCHVDC is a multiple input multiple output, strong coupling nonlinear system, its control problem is always attracting much attention from the scholars. In chronological order, its research process can be classified into the following two phases.(1)The steady state mathematical model and basic control strategy of VSCHVDC system: [6] developed the steady state model and proposed control strategy which combined an inverse model controller with a PI controller. Reference [7] presented an equivalent continuoustime state space model of VSCHVDC in the synchronous  reference frame and proposed a decoupled PI control strategy using the feedforward compensation method. Reference [8] presented the elements of VSCHVDC and proposed a feedforward decoupled current control strategy. In this phase, the study is mainly based on the traditional PI controllers under the linear decoupling control strategy. The design method of this kind of control strategy is easy. And it shows good adjustment ability. But the parameters of PI controllers are fixed, and their adjustment ability is limited. When system suffers from large disturbance, they show weak robustness and dynamic characteristics. Some severe cases can lead to sustained oscillation of the system. Therefore some scholars started the second phase study.(2)Optimizing and nonlinear control strategy of the VSCHVDC system: [9] proposed an adaptive control design to improve dynamic performances of VSCHVDC systems. The adaptive controllers designed for nonlinear characteristics of VSCHVDC systems, which were based on back stepping method, considered parameters uncertainties. Reference [10] presented a robust nonlinear controller for VSCHVDC transmission link using inputoutput linearization and sliding mode control strategy. The feedback linearization was used to cancel nonlinearity and the sliding mode control offered invariant stability to modeling uncertainties. Reference [11] proposed the controller design based on control methodologies to deal with the nonlinearities introduced by requirements to power flow and line voltage. Reference [12] addressed two different robust nonlinear control methodologies based on sliding mode control for the VSCHVDC transmission system’s performance enhancement and stability improvement. In this phase, the studies are based on the nonlinear control, robust control, and optimizing control of the VSCHVDC system. The experimental results from research papers show these control strategies can improve static, dynamic, and robust performance of VSCHVDC system.However, the referred literatures are based on the continuoustime state. In practice, nowadays most controllers are implemented in discrete time. It is known that the realization of a controller using digital elements and complex programmable logic devices can achieve maximum reproducibility at minimum cost. So it is necessary to do research on the discrete model and control strategy. In [13], the discrete PI controllers of VSCHVDC system were established. Reference [14] studied the discrete PI control algorithm of VSCHVDC system which supplied power to the passive network.
But so far, the discretetime mathematic model and control strategy based on nonlinear control methods are seldom studied by scholars. Through proper feedback linearization control strategy, complex nonlinear system synthesis problems can be transformed to linear system synthesis problems. As a kind of robust control method, sliding mode control is versatile to linear and nonlinear system. It is easy to be designed and carried out. Because of its complete robustness to the parameters change and external disturbances which meet the match condition, it gets extensive attention from scholars and engineers [15].
Therefore, the discrete mathematical model of the VSCHVDC system based on nonlinear feedback linearization and discrete sliding mode control algorithm are proposed in this paper. And the fast output sampling (FOS) technique is adopted in the output feedback, which ensures the stability of the closed loop system in the condition of quasi sliding mode control. The simulation results performed in MATLAB/SIMULINK show that the proposed discrete control strategy can make the VSCHVDC system have good operation performance.
2. The Mathematical Model of VSCHVDC
2.1. ContinuousTime State Space Model
The structure diagram of VSCHVDC is shown in Figure 1. The more widely used continuoustime mathematical models based on the synchronous reference coordinates are employed in this paper, which are shown as (1) and (7).
2.1.1. Rectifier Side
Choose state variables , controlled variables , and output variables . The mathematical model of the rectifier side is shown aswhere and are the  axis currents and voltages on the rectifier side, respectively. and are, respectively, the  axis control inputs on the rectifier side. and are the corresponding equivalent resistance and the inductance on the rectifier side. is the AC system frequency on the rectifier side.
Here, the output variables on the rectifier side should be . and are, respectively, the output values of active and reactive power on the rectifier side. For convenience, define as the line voltage effective value of the rectifier power supply. Define the axis in the synchronization reference frame coincidence with axis in the threephase reference coordinate, so , , . Therefore the output variables can be chosen as .
DefineTherefore (1) can be reorganized byThen, do exact feedback linearization on system equation (3). By calculation, the relative degree of is denoted by . The relative degree of is denoted by . Based on the feedback linearization theory [16], the exact feedback linearization state equations of the rectifier side are obtained asBecause , is nonsingular.
The controlled variables and can be denoted by virtual control variables and : To make it convenient for using FOS (fast output sampling) technique in Section 3.1 [17], substitute (5) into (3); then the system equation of rectifier side can be denoted by
2.1.2. Inverter Side
Choose state variables , controlled variables , and output variables . The mathematical model of inverter side is shown as where and are the  axis currents and voltages on the inverter side, respectively. and are, respectively, the control inputs on the inverter side. and are the corresponding equivalent resistance and the inductance on the inverter side. and are, respectively, the DC voltages on the rectifier side and inverter side. and are, respectively, the converter station capacitance and DC line resistance. is the AC system frequency on the inverter side.
Here, the output variables on the inverter side should be . is the output value of reactive power on the inverter side. For convenience, define as the line voltage effective value of the inverter side power source. Define the axis in the synchronization reference frame that coincides with axis in the threephase reference coordinate, so , , and . Therefore the output variables can be chosen as .
DefineTherefore (7) can be reorganized byThen, do exact feedback linearization on system (9). By calculation, the relative degree of is denoted by . The relative degree of is denoted by . Based upon the feedback linearization theory [16], the exact feedback linearization state equations of inverter side are obtained aswhere
Because , is nonsingular.
The controlled variables and can be denoted by virtual control variables and :To make it convenient for using FOS (fast output sampling) technology in Section 3.1 [17], substitute , , , and (12) into (7); then the system equation of inverter side can be denoted byHere, should be . Since, during normal operation, is approximately equal to , for convenient calculation, is simplified into .
2.2. DiscreteTime State Space Model
Discretize the virtual control variables , , , and with sampling time :Discretize system equations (6), (13) with sampling time , shown as Rearrange (16), and then get whereAssume that the pairs , are controllable and the pairs , are observable through properly sampling output variables.
3. Discrete Sliding Mode Control of VSCHVDC System
In ideal continuoustime case, the SMC (sliding mode control) switches at infinite frequency and forces the states to slide on the socalled switching hyperplane. In practical applications, direct implementation of continuoustime SMC schemes using digital elements, which are considered as the device for imperfect switching, will inevitably induce chattering phenomenon and deteriorate performance or even induce instability. Chattering will cause serious harmonics which is undesirable in VSCHVDC systems. Hence, the controller design using the discretetime SMC (DSMC) algorithm is desirable for a successful implementation of the VSCHVDC control systems. And due to the finite sampling frequency, the controller inputs are calculated once per sampling period and held constant during that interval. Under such a circumstance, the trajectories of the system states of interest are unable to precisely move along the sliding surface, which will lead to a quasi sliding mode motion only [18]. Therefore, only using static output feedback technology has not effectively ensured the control effect of discrete sliding mode control. The fast output sampling technology should be used [17, 19].
3.1. Fast Output Sampling (FOS) Technology
Compared with the static output feedback technology, FOS not only keeps its advantage, but also can randomly configure the system poles and always make the closed loop system stable. And then FOS can ensure the effectiveness of the discrete sliding mode control. In the FOS, every sampling period is divided into subintervals. Here and is equal to or greater than the observable index of system . The output variables are measured at time instants , . Consider the discretetime system having be at time ; the fast output samples are obtained as [17, 19]Then the rectifier side can be expressed as whereAnd assume that and are invertible through appropriate choice. is the system parameter matrix with sampling rate , .
Define . This means is sampled once and is sampled once in each sampling period .
The inverter side can be expressed as whereAnd assume that and are invertible through appropriate choice. is the system parameter matrix with sampling rate , .
Define . This means is sampled twice and is sampled once in each sampling period .
According to (20) and (22), state vectors and can be deduced as
3.2. The Rectifier Side Control
The differences between output and reference variables are denoted by (26). Because the relative degrees of and are, respectively, equal to 1, 1, the sliding mode surfaces are defined as (27). Considerwhere and are the reference values. Their calculation processes are shown in Figure 2. , , , and are, respectively, the output value and reference value of active and reactive power. ConsiderDesired state trajectories of the discrete variable structure system shown as in (28) can be obtained by control law based on reaching law method:where , , , and are all greater than zero. And , .
(a) The reference value of axis current
(b) The reference value of axis current
Now replacing (26) with (27) and then with (28), the virtual control variables can be denoted byThe control variables shown as in (30) can be deduced by (14), (24), and (29):where and are the first and second row of in Section 3.1. , can be expressed by through FOS.
3.3. The Inverter Side Control
The differences between output and reference variables are denoted by (31). Because the relative degrees of and are, respectively, equal to 2, 1, the sliding mode surfaces are defined as (32). Considerwhere and are the reference values. Usually is known. The calculation process of is shown in Figure 3. and are, respectively, the output and reference value of reactive power:Similar to the rectifier side, desired state trajectories of the discrete variable structure system shown as in (33) can be obtained by control law based on reaching law method:where , , , and are all greater than zero. And , .
Now replacing (31) with (32) and then with (33), the virtual control variables can be denoted byThe control variables shown as in (35) can be deduced by (15), (25), and (34):where , , and are the first, second, and third row of in Section 3.1. and can be expressed by through FOS.
4. Simulation Results
The typical VSCHVDC system composed of two converter stations is taken as example and the detailed parameters are shown in Table 1. The simulation experiment is performed in MATLAB/SIMULINK. In the perunit value system, the based power is 200 MW, the based voltage at AC side is 81.65 KV, and the based voltage at DC side is 100 KV. The sampling period . Also and . The discrete SMC controllers parameters are , , , , and .

4.1. The Steady State Operation
In steady state operation, , , , and are, respectively, 1 pu, 0 pu, 1 pu, and 0 pu. As shown in Figure 4, the active, reactive power and DC voltage can track their reference values effectively. The proposed mathematical model and control strategy can be proved to make the system operate well in steady state condition.
(a) The active and reactive power at rectifier side
(b) The active and reactive power at inverter side
(c) The DC voltage
4.2. Reversion and Step Changes
As shown in Figure 5, the change process of is as follows: the keeps 1 pu from 0 s to 1.5 s. At 1.5 s, it steps to −1 pu, and then keeps this value until 3.0 s. At last, it steps to 1 pu at 3.0 s. From the experimental results, the referred changes least affect the reactive power on the rectifier side and reactive power on the inverter side. As shown in Figure 6, the change process of is as follows: the keeps 0 pu from 0 s to 1.5 s. And then steps to 0.1 pu at 1.5 s. The change process of is as follows: the keeps 0 pu from 0 s to 2.5 s. And then steps to −0.1 pu at 2.5 s. From the experimental results, the referred changes least affect the active power on the rectifier side and the active power on the inverter side. These can prove that the proposed discrete SMC strategy can make the active and reactive power decoupled and independent.
(a) The active and reactive power at rectifier side
(b) The active and reactive power at inverter side
(a) The active and reactive power at rectifier side
(b) The active and reactive power at inverter side
4.3. Robustness Test
The equivalent resistance and inductance at the converter stations both reduce 20%. The experimental results are shown in Figure 7. The reference values are the same as those in Section 4.1. The active, reactive power and DC voltage can track their reference values smoothly and quickly. This can prove that the proposed control strategy has good robustness.
(a) The active and reactive power at rectifier side
(b) The active and reactive power at inverter side
(c) The DC voltage
5. Conclusions
This paper deduces the discrete mathematical model of VSCHVDC system by nonlinear inputoutput feedback linearization method. Based on this, the discrete sliding mode robust controllers are designed. And to ensure the effectiveness of the controllers in quasi sliding mode condition, the FOS technology is used in output feedback. The results from simulation experiment in MATLAB/SIMULINK prove the effectiveness of proposed discrete mathematical model and control strategy. Since the actual computer control system is discrete sampling and SMC has a promising prospect, the proposed discrete mathematical model and control strategy have certain practical application prospect.
Conflict of Interests
The authors declare that there is no conflict of interests regarding the publication of this paper.
Acknowledgment
This work is supported by the Natural Science Foundation of China (51377068).
References
 N. Flourentzou, V. G. Agelidis, and G. D. Demetriades, “VSCbased HVDC power transmission systems: an overview,” IEEE Transactions on Power Electronics, vol. 24, no. 3, pp. 592–602, 2009. View at: Publisher Site  Google Scholar
 Y. Wei, Q. He, Y. Sun, and C. Ji, “Improved power flow algorithm for VSCHVDC system based on highorder newtontype method,” Mathematical Problems in Engineering, vol. 2013, Article ID 235316, 10 pages, 2013. View at: Publisher Site  Google Scholar
 L. Zhang, L. Harnefors, and H.P. Nee, “Interconnection of two very weak AC systems by VSCHVDC links using powersynchronization control,” IEEE Transactions on Power Systems, vol. 26, no. 1, pp. 344–355, 2011. View at: Publisher Site  Google Scholar
 L. Zhang, L. Harnefors, and H.P. Nee, “Modeling and control of VSCHVDC links connected to island systems,” IEEE Transactions on Power Systems, vol. 26, no. 2, pp. 783–793, 2011. View at: Publisher Site  Google Scholar
 F. Xu and Z. Xu, “A modular multilevel power flow controller for meshed HVDC grids,” Science China Technological Sciences, vol. 57, no. 9, pp. 1773–1784, 2014. View at: Publisher Site  Google Scholar
 G. Zhang and Z. Xu, “Steadystate model for VSC based HVDC and its controller design,” in Proceedings of the IEEE Power Engineering Society Winter Meeting, vol. 3, pp. 1085–1090, February 2001. View at: Google Scholar
 M. Yin, G.Y. Li, T.Y. Niu, G.K. Li, H.F. Liang, and M. Zhou, “Continuoustime statespace model of VSCHVDC and its control strategy,” Proceedings of the Chinese Society of Electrical Engineering, vol. 25, no. 18, pp. 34–39, 2005. View at: Google Scholar
 R. Song, C. Zheng, R. Li, and Z. Xiaoxin, “VSCs based HVDC and its control strategy,” in Proceedings of the IEEE/PES Transmission and Distribution Conference and Exhibition: Asia and Pacific, pp. 1–6, August 2005. View at: Publisher Site  Google Scholar
 S.Y. Ruan, G.J. Li, X.H. Jiao, Y.Z. Sun, and T. T. Lie, “Adaptive control design for VSCHVDC systems based on backstepping method,” Electric Power Systems Research, vol. 77, no. 56, pp. 559–565, 2007. View at: Publisher Site  Google Scholar
 A. Moharana and P. K. Dash, “Inputoutput linearization and robust slidingmode controller for the VSCHVDC transmission link,” IEEE Transactions on Power Delivery, vol. 25, no. 3, pp. 1952–1961, 2010. View at: Publisher Site  Google Scholar
 N. Nayak, S. K. Routray, and P. K. Rout, “State feedback robust ${H}_{\infty}$ controller for transient stability enhancement of VscHvdc transmission systems,” Procedia Technology, vol. 4, pp. 652–660, 2012, Proceedings of the 2nd International Conference on Computer, Communication, Control and Information Technology (C3IT '12) on February 2526, 2012. View at: Google Scholar
 H. S. Ramadan, H. Siguerdidjane, M. Petit, and R. Kaczmarek, “Performance enhancement and robustness assessment of VSC–HVDC transmission systems controllers under uncertainties,” International Journal of Electrical Power & Energy Systems, vol. 35, no. 1, pp. 34–46, 2012. View at: Publisher Site  Google Scholar
 X.G. Wei, G.F. Tang, and J.C. Zheng, “Study of VSCHVDC discrete model and its control strategies,” Proceedings of the Chinese Society of Electrical Engineering, vol. 27, no. 28, pp. 6–11, 2007. View at: Google Scholar
 H. Yang, N. Zhang, and M. J. Ye, “Study of VSCHVDC connected to passive network discrete model and its control strategies,” Power System Protection and Control, vol. 40, no. 4, pp. 37–42, 2012. View at: Google Scholar
 V. I. Utkin and H.C. Chang, “Sliding mode control on electromechanical systems,” Mathematical Problems in Engineering, vol. 8, no. 45, pp. 451–473, 2002. View at: Publisher Site  Google Scholar  MathSciNet
 H. K. Khalil and J. W. Grizzle, Nonlinear Systems, Prentice hall, Upper Saddle River, NJ, USA, 1996.
 H. Werner, “Multimodel robust control by fast output sampling—an lmi approach,” Automatica, vol. 34, no. 12, pp. 1625–1630, 1998. View at: Publisher Site  Google Scholar
 T.L. Tai and J.S. Chen, “UPS inverter design using discretetime slidingmode control scheme,” IEEE Transactions on Industrial Electronics, vol. 49, no. 1, pp. 67–75, 2002. View at: Publisher Site  Google Scholar
 M. C. Saaj, B. Bandyopadhyay, and H. Unbehauen, “A new algorithm for discretetime slidingmode control using fast output sampling feedback,” IEEE Transactions on Industrial Electronics, vol. 49, no. 3, pp. 518–523, 2002. View at: Publisher Site  Google Scholar
Copyright
Copyright © 2015 TianTian Qian and ShiHong Miao. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.