1 Introduction STATCOM is a new concept of reactive power compensation that combines technologies such as static var compensator, voltage source inverter, and the manufacturing and control of high-power gate turn-off transistors (GTOs). It is an important member of the FACTS (Flexible AC Transmission Systems) family. China's first ±20Mvar STATCOM will be put into operation soon. Among them, the design of the controller is a very important part of the entire STATCOM device design. This paper attempts to discuss the coordination problem of multiple control objectives encountered in the controller design. In addition to providing voltage support to the system, STATCOM should also contribute to improving the system's transient stability limit and damping the system's power oscillations [1, 2]. This paper shows through theoretical analysis and digital simulation that the above control objectives are often contradictory in the transient process of the system, and it is impossible to make these control objectives simultaneously optimal. Therefore, this paper gives the principle of compromise for these contradictions and designs a coordination controller accordingly. Digital simulations of single-machine and multi-machine systems show that the controller can effectively coordinate the above control objectives. [b]2 Interface between STATCOM and the system[/b] Figures 1-3 show a single-machine-infinite system with a STATCOM connected at the midpoint, its equivalent circuit diagram, and its operating characteristic curves. As can be seen from Figure 3, the STATCOM has current source characteristics. When conducting system-level research, describing the STATCOM with a controllable reactive current source [2] can more accurately describe the characteristics of the STATCOM, and at the same time simplify the interface between the STATCOM and the system. Therefore, in this paper, it is regarded as a controllable current source. In the following analysis and digital simulation, the following assumptions are also used: (1) The mechanical input power of the generator is constant. [img=310,176]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dgjsxb/dgjs99/dgjs9902/image/22-1.gif[/img] Fig.1 A Single-machine Infinite-bus system with STATCOM [img=267,148]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dgjsxb/dgjs99/dgjs9902/image/22-2.gif[/img] Fig.2 The Equivalent circuit of STATCOM [img=250,159]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dgjsxb/dgjs99/dgjs9902/image/22-3.gif[/img] Fig.3 The VI working curve of STATCOM (2) The generator is represented by the classical second-order model, and E′ is constant. (3) The resistance of the generator, transformer, and line is ignored. (4) Iq is positive when it supplies inductive reactive power to the system. [b]3 Multi-objective control problem of STATCOM[/b] STATCOM maintains (or changes) the voltage at its connection point by providing reactive current to the system, thereby regulating the electromagnetic power output of the generator and thus improving the stability of the system. Therefore, the main control objectives faced by STATCOM are at least: (1) Maintaining the system voltage. Maintaining the voltage at a certain point of the system at a set level through rapid reactive power regulation is one of the original intentions of developing parallel static var compensators. Reference [2] points out that STATCOM is the device with the best voltage regulation effect in the FACTS family. (2) Improve the transient stability limit of the system, that is, improve the stability of the first swing. According to the equal area rule, after a large disturbance occurs in the system and the motor enters the power angle swing, if the power angle curve can be raised as much as possible during the first swing (the change in generator speed Δω>0) to increase the deceleration area, the transient stability limit of the system can be effectively improved. For the system shown in Figure 2, let x1＝x2=XL/2, E'=Vc=1, then the electromagnetic power output by the generator after connecting to STATCOM is [img=301,45]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dgjsxb/dgjs99/dgjs9902/image/22-4.gif[/img] It can be seen that if the STATCOM generates the maximum inductive reactive current Iqmax during the first swing, the P-δ curve can be maximized, the deceleration area can be increased, and thus the transient stability limit of the system can be effectively improved. (3) Increase system damping. Increasing system damping by controlling the reactive output of STATCOM to change the active output of the generator is a development of the PSS concept in generator control [3]. Consider the generator rotor motion equation: [img=316,44]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dgjsxb/dgjs99/dgjs9902/image/22-5.gif[/img] Where D——generator damping coefficient 2H ——generator inertial time constant, s Obviously, the only thing that can affect the motor rotor motion law is the controllable ΔPe. If we assume that Iq=0 in steady state, we can get from equation (1) [img=321,46]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dgjsxb/dgjs99/dgjs9902/image/22-6.gif[/img] Therefore, by controlling the output reactive current ΔΙq of STATCOM, we can achieve the purpose of controlling ΔPe. If ΔIq includes a portion proportional to Δω, the damping power coefficient KD will increase, thereby enhancing the system's damping. However, unlike excitation control, for STATCOM (and other FACTS devices such as SVC, TCSC, etc., located far from the generator), Δω cannot be directly measured locally. This paper assumes that Δω can be obtained directly or indirectly. [b]4 Contradictions and Coordination of Multiple Control Objectives[/b] For each of the above control objectives, a corresponding optimal controller can be designed. For example, a PI controller with appropriate parameters can be used to ensure the dynamic quality and steady-state accuracy of voltage control; since the capacity of the STATCOM device is limited, switching control can be used to make the STATCOM generate a certain value of inductive reactive current when Δω＞0 and absorb a certain value of inductive reactive current when Δω＜0, which can greatly enhance the system's damping. Reference [4] points out that this switching control method can dampen the system's oscillation more effectively than continuous control for the control of SVC; to improve the system's transient stability limit, the STATCOM should generate the maximum inductive reactive current during the first swing period of the generator after the disturbance occurs. However, the problem we must solve is that the above control strategies do not lead to the same goal; they are often contradictory. For example: (1) Pure voltage control cannot provide positive damping to the system. The voltage at the STATCOM connection point in system 1 is [img=301,40]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dgjsxb/dgjs99/dgjs9902/image/23-2.gif[/img] Expanding at the equilibrium point, we get [img=319,44]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dgjsxb/dgjs99/dgjs9902/image/23-3.gif[/img] Assuming the device capacity is large enough and the voltage control is strong enough so that Vm is constant, i.e. ΔVm=0, then [img=154,62]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dgjsxb/dgjs99/dgjs9902/image/23-4.gif[/img]（6） Thus, from equation (3), we get [img=146,65]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dgjsxb/dgjs99/dgjs9902/image/23-5.gif[/img]（7） Therefore, pure voltage control cannot increase the damping torque coefficient KD, but can only increase the synchronous power coefficient. (2) To enhance the system's ability to dampen oscillations, voltage quality must be sacrificed. Damped control requires the output current of STATCOM to change with Δω. In the phase plane, Δω leads Δδ by 90°, so emphasizing damped control will inevitably destroy equation (6), thereby damaging the voltage quality. (3) Similarly, the switching control used in the first swing to improve the system's quasi-stability limit is also not conducive to maintaining the voltage. Figure 4 shows the power angle and voltage response curves of system 1 under different control modes after being subjected to a large disturbance. [img=300,196]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dgjsxb/dgjs99/dgjs9902/image/23-6.gif[/img] Figure 4 Comparison of system responses under voltage control and damping control - emphasizing stability control ... emphasizing voltage control Fig.4 System responses underfault with voltage controlled or damping controlled STATCOM The disturbance in the simulation is: three-phase short circuit at point A at 1s, and disconnection of the faulty line at 1.1s. Figure 4 clearly shows the contradiction between voltage control and damping control. On the one hand, if the damping controller is carefully adjusted, the system can be brought into a new steady state with only two switching, which can effectively suppress power oscillations, but the voltage quality is severely damaged. On the other hand, if the voltage control accuracy is emphasized, the voltage at the STATCOM connection point can remain almost constant during the dynamic process, but it cannot provide positive damping for the system. Therefore, finding a suitable compromise, without excessively compromising the achievement of other objectives, to satisfy the main control objectives as much as possible is a problem that the control of STATCOM must solve. As we all know, the stability of the first swing of the system must be guaranteed by all means. Under the premise that the voltage fluctuation does not exceed the allowable limit, the STATCOM should generate the maximum inductive reactive current during this period. That is: (1) In the stage of δ increase after the fault is recovered, if U < Ulim max, then Iq = Iqmax; else Iq = kIqmax, where k is a positive number less than 1. It should be noted that during the system fault process, the STATCOM is generally in a pulse blockade state to protect itself and is immediately put back into operation after the fault is recovered. If the system does not lose stability in the first swing, the new primary goal becomes to quickly quell the power oscillations. The challenge then becomes striking a balance between damping the oscillations and stabilizing the voltage. The goal is to quell the power oscillations as quickly as possible without causing significant voltage fluctuations. Therefore, the voltage control component should be "weakened", that is: [img=221,59]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dgjsxb/dgjs99/dgjs9902/image/24-1.gif[/img][font=SimSun] (8) [/font] Where, ΔIqU=-kUΔU, kU＞0 and [img=393,79]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dgjsxb/dgjs99/dgjs9902/image/24-2.gif[/img] k1＜k2, both are positive numbers, and R is the threshold of fluctuation. [img=357,59]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dgjsxb/dgjs99/dgjs9902/image/24-3.gif[/img] ΔIqmax and ΔIqmin are the maximum and minimum output reactive currents of the STATCOM, corresponding to the maximum inductive reactive power generated and absorbed. k3 and k4 are positive constants, and their determination principles are as described in reference [4]. In summary, the control principle during this period is to fully utilize the allowable voltage fluctuations under dynamic operating conditions to quickly quell power oscillations. When the amplitude of power oscillations is less than a certain value, continuing to use switching control will subject the system to undue shocks. The main control objective after the oscillations have basically subsided is to quickly and accurately adjust the voltage back to the given value. At this time, using a PI controller is an appropriate choice. That is: [img=350,62]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dgjsxb/dgjs99/dgjs9902/image/24-4.gif[/img] In this case, the controller has fixed parameters. Based on the above three principles, a coordinated controller can be constructed. In Figure 5, the solid line is the dynamic response curve of system 1 under the action of this controller under the aforementioned fault. The dashed line is the adjustment effect of the controller composed of fixed-parameter voltage PI control superimposed with speed change starting switch control. [img=345,228]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dgjsxb/dgjs99/dgjs9902/image/24-5.gif[/img] Fig. 5 Effect of coordinated control—coordinated control…uncoordinated control Fig. 5 System response with proposed STATCOM controller Fig. 6 is the simulation result of a 99-machine, 621-node system in four provinces: Henan, Hunan, Hubei, and Jiangxi. Fig. 7 is a simplified part of the Henan backbone network, with the STATCOM installed at Chaoyang station. The simulation examines the winter operation mode of the Henan network, taking a disturbance of 2.5s for a three-phase short circuit at point B and 2.62s for the faulty line to be cleared. Since the STATCOM installation point and short-circuit capacity are approximately 9800Mvar, to fully reflect the effects of different control strategies, the STATCOM capacity is set to ±50Mvar in the simulation. The curves in Figure 6 represent the power angle swing of the Shouyangshan No. 1 generator relative to the system equilibrium node (within Hubei Province) and the 220kV bus voltage of the Chaoyang station, respectively. It can be seen that coordinated control can effectively improve the system's transient stability limit, fully utilize allowable voltage fluctuations to improve system damping, and leverage the smooth adjustment effect of PI control under small disturbances. Simulation results also show that the controller design principle is effective for both single-unit and multi-unit systems. Since STATCOM (and other FACTS devices such as SVC and TCSC) designed to improve system transmission capacity are generally installed at the midpoint of long-distance transmission lines between two regions, references [3] and [5] point out that the damping control of TCSC and SVC can be analyzed by equivalence of the systems on both sides of the device and provide equivalence principles. Under these conditions, the rules obtained from the analysis of single-unit systems above are still applicable to multi-unit systems. Furthermore, although this paper analyzes the contradiction between voltage and damping control through linearization at the equilibrium point and designs a controller accordingly, simulation results show that the controller remains effective after large disturbances. Reference [6] uses the damping torque coefficient analysis to demonstrate that strong voltage control in STATCOM always has an adverse effect on the damping control effect. Numerous calculations of the damping torque coefficient show that this rule holds true at any operating point. Therefore, controllers designed based on this rule always tend to enhance the system's damping. [img=355,223]http://zszl.cepee.com/cepee_kjlw_pic/files/wx/dgjsxb/dgjs99/dgjs9902/image/24-6.gif[/img] Fig.6 Simulation result on the Henan Power Grid — Coordinated control…Uncoordinated control Fig.7 Parts of the simplified Henan Power Grid [b]5 Conclusion[/b] Coordination of multiple control objectives is a problem that STATCOM control must solve. This paper analyzes the contradictions among the various control objectives of STATCOM through theoretical analysis and digital simulation, and proposes principles for coordinating these objectives. The paper points out that the most important control objective should be determined based on the current system state, and that the primary objective should be achieved as much as possible without severely compromising the achievement of other objectives. Digital simulations show that controllers designed according to these principles can effectively solve the multi-objective coordinated control problem of STATCOM. Further work is needed to establish criteria for rapidly estimating the system state under dynamic conditions, and to design multi-objective controllers for STATCOM based on the evaluation results of these criteria. [b]References[/b] [1] Jiang Qirong. Research on Modeling and Control of Novel Static Var Generator: Doctoral Dissertation. Beijing: Department of Electrical Engineering and Applied Electronics, Tsinghua University, 1997. [2] CIGRE TF 38-01-06 on Load Flow Control, Load flow control in high voltage power systems using FACTS controllers. 1996. [3] Larsen EV et al. Concepts for design of FACTS controllers to damp power swings. IEEE T-PWRS, 1995, 10(2): 948～956 [4] Zhou E Z. Application of static var compensator to increase power system damping. IEEE T-PWRS, 1993, 8(2): 655～661 [5] Larsen EV, et al. Control design for SVC's on the Mead-Adelanto and Mead-Phoenix transmission projects, IEEE T-PD, 1996, 11(3): 1498～1506 [6] Li Chun, et al. Design of a rule-based controller for STATCOM. Proceedings of IECON'98, Aachen Germany, 1998.