Studies On Control Strategies Of The Thyristor Controlled Series Capacitor

TCSC (Thyristor Controlled Series Compensation) is one type of FACTS (Flexible AC Transmission System) device. It can realize the large-scale smooth regulation of the TCSC impedance through changing the trigger angle of thyristor, which is of significance for electric power system. TCSC can be used to improve the transmission capacity and voltage distribution of some transmission aisle. In the netlike power network, TCSC can be used to control power flow based on the system operation condition, therefore the network loss can be reduced and the distribution of power flow can be optimized. If located on appropriate transmission lines, TCSC can make the transmission line appear electrically shorter which results in increasing the synchronizing torques between the generators and thus improves the transient stability margin of the power system. TCSC can also be used to ameliorate the weak damp or negative damp status between inter-network or local network, accordingly improve the dynamic stability. Through a certain trigger rule, TCSC has the capability to control the energy conversion between the series capacitor and the turbine-generator shaft, restrain the subsynchronous heft, and thus, avoid the risk of potential subsynchronous resonance problems.After introducing and analyzing the development and research status of TCSC, this dissertation focuses on two technological problems which are encountered during the domestic design of TCSC: one is relation curve of trigger angle and fundamental frequency impedance considering thyristor conduction characteristic and resistance of the reactor; the other is mode switch control policy, which will be the emphases to be studied in this dissertation.After a discrete digital simulation model is constituted in this paper, an in-depth analysis of hierarchical control system is made and a series practical control methods are derived. The main research tasks of this dissertation include the following four aspects: 1. Bottom Layer Control of TCSCThe bottom control is the basis and prerequisites for TCSC applications. The keyproblem of the bottom control is to establish the relation curve between fundamental-frequency impedance and trigger angle. To explain distinct difference between the theoretic impedance and the practical impedance in low voltage simulation and dynamic simulation, this dissertation thoroughly discusses the influence of thyristor conduction characteristic and the resistance of the reactor in reactor branch of TCSC on the bottom control, and proposes the following methods to deal with this influence: using the constant resistance in the little capacity mode simulation and the dynamic simulation, and using ignoring, constant resistance and constant resistance plus constant voltage (same direction with the thyristor current) in the engineering reality according to the load and the inductance of reactor.Adapting to the little capacity and dynamic simulation experiment in the development stage of TCSC, this dissertation regards the thyristor conduction characteristic, resistance of the reactor and other contacting resistances as constant resistance. Through rigorous mathematic deducing, the relation curve of fundamental-frequency impedance and trigger angle is obtained. It is the first time to put forward the dual solution theory of fundamental-frequency impedance in this study, which has significant meaning to the engineering reality. The conclusion includes:1) Through deducing of the fundamental-frequency impedance, it is found out that there is one inductive solution and a capacitive solution corresponding to the same trigger angle in most of the steady operation area of TCSC, so there are two branches in fundamental-frequency impedance curve of TCSC. When the equivalent impedance of reactor branch is large enough, the impedance and slop of these two branch curves will not be too large. If the voltage redundancy of TCSC is good enough, both solutions can stay in steady state. Which steady solution will be chosen is determined based on the phase relationship between capacitor voltage and line current before trigger regulation.2) Using simulation approach, this dissertation study has approved for the first time that the region stepping is possible to happen in TCSC under large disturbance when the quality factor is lower, which also has important meaning to design and experiment of TCSC. This dissertation also points out that TCSC can just movealong one branch of fundamental-frequency impedance curve and will not step to the other one if only the trigger angle is changed. Therefore, the mode switch cannot be simply realized through adjusting trigger angle, but must be realized through a certain control strategy.3) This fundamental-frequency impedance curve has been validated precisely by digital simulation results. The quality factor of the reactor branch in TCSC dynamic simulation system is deduced to be 1.9 around by fitting the result data of dynamic simulation experiment with the impedance curve put forward in this paper. Then the digital simulation of open-loop impedance control is accomplished, using the impedance curve with this quality factor. The results comparison between the digital simulation and the dynamic simulation experiment proves the advantage of the impedance curve introduced by this dissertation. II. Mode Switch Control of TCSCMode switch of TCSC belongs to the bottom control, which is the key issue with TCSC whether it can work in the stabilization control of power system or not. At the same time, it can protect MOV (Metal Oxide Varistor) through bypassing MOV when it is overloaded. This dissertation discusses the mode switch process deeply and obtains a whole practical set of mode switch method. The research outcome of this aspect includes:1) After simulation waveforms are compared when TCSC is switched to Block (thyristor blocked) mode from different impedance values, this dissertation presents a switching method in which TCSC is switched from high impedance value to a low value first, and then switched to Block mode, so the direct current float is avoided, which is brought by the discharging of the capacitor.2) Moreover, after comparing the influence of the quality factor of reactor branch on switch from capacitive vernier mode or Block mode to Bypass (thyristor full conduction) mode, this paper discovers the limitation of traditional hardware bypass method and proposes a more practical switch scheme from other mode to Bypass mode. By this policy TCSC can be switched from one mode to the other mode quickly, with smooth transition, so it can fulfill the reality requirement. Inthis method, the trigger pulse is demanded to be given out at the zero crossing instant of line current, so the common method to judge zero crossing is not applicable before. This paper puts forward a method to predict the zero crossing instant of line current, which can meet the demand very well.3) In this dissertation, an improved main connection structure of TCSC is also brought forward in which a small resistance is in series with the reactor in the reactor branch and a pair of thyristor is in parallel with it. At usual time, all of the pairs trigger and break synchronously which will have no influence on TCSC; while Bypass mode is required, this pair of thyristor is opened, then the switching can be realized using hardware bypass method. This method is simple in logic and takes full consideration of voltage resistance requirement of the thyristor, so it has certain practicability.4) An improved compel-current-synchronization method is also introduced in this dissertation to realize switch from the capacitive vernier mode or Block mode to the inductive vernier mode.HI. Impedance Control of TCSCThe impedance control belongs to middle-layer control. It can accept target impedance (the impedance which is demanded for TCSC to take on) from superstratum control and correct ordering impedance according to impedance regulation error or directly correct trigger delay time. So the close loop control is realized and TCSC can operate better under the ordering impedance. It is an intermediate link bridging the preceding course and the following course in TCSC hierarchical control. And it is the key problem in the realization of the impedance control.This dissertation proposes a kind of PID method on impedance control using impedance error to correct the trigger time instead of the order impedance to avoid checking the impedance form frequently. The influences on the results of impedance control by the synchronization manner through line current and capacitor voltage are compared by digital simulation, and the advantages of synchronization manner through line current is proved.This dissertation also discusses the reason why the robustness of the traditional PID...