| As an important part of Flexible AC Transmission System (FACTS) family, Unified Power Flow Controller (UPFC) is the most attractive device because of its strong ability to regulate bus voltage and optimize power flow on the transmission line synchronously. Since it was proposed 10 years ago, it has been widely concerned by the experts in power system operation and control fields and extensive researches have been gone along in its structure design, control strategy and manufacture.The control strategies in common use (such as PI control, linear optimal control and so on) feed back the measure information, such as the bus voltage, active power and reactive power, into the control system directly, which brings the nonlinear and strong couple characteristics of power system into the UPFC control system. Although Intelligent Predictive-Evaluating Control method had taken notice of this problem, but did not give a simple and convenient way to solve it.This paper introduces the important offshoot of intelligent control field --- Human-Simulated Intelligent Control (HSIC) theory to the control system of UPFC. Based on the comparison of the hierarchical architecture of HSIC and the hierarchical architecture of power system automation, it analyses the analogous relation like theory and practice between them. Further this paper proposes the intelligent control system structure based on HSIC, which divides the whole system into two comparatively independent parts: nonlinear subsystem and servo subsystem to solve the nonlinear and strong couple problem in normal control strategies used in UPFC control system.According to the multiple modes control feature of HSIC and its basic and control object oriented control strategy design method and based on analyzing the charge and discharge dynamic process and the control method on dc-link capacitor voltage of common control strategies used in UPFC control system, this paper confirms the dc-link capacitor voltage strong control strategy and proposes the soft control concept on dc-link capacitor voltage control relatively. The soft control strategy utilizes the dc-link capacitor to absorb or release the transient energy during the fault or disturb period, which will benefit the main control object such as power flow and bus voltage. This paper provides a detail realization of soft control strategy and compares it with the linear optimal control strategy by simulating the system response under the condition of three phase symmetry short and dispatching task changed. The multiple mode control structure feature of HISC provides the developingscope for the intelligence in different levels, which make the controller approach the human-being behaves as a whole. In low level intelligence, using the basic research way, this paper proposes the nonlinear PID (NPID) controller design method based on HSIC and the improving way in P-I-D control parts. It also provides two design examples of NPID based on HSIC, proves its effectiveness by simulating and utilizes it to the low level of UPFC intelligent control system.Based on the soft control strategy on dc-link capacitor voltage and the basic theory of HSIC, this paper integrated the soft control strategy and linear optimal control strategy into the UPFC intelligent control system, proposed the soft-strong control strategy and provides the detail mathematic describe of each level scheme in the control system. During the beginning of strong disturb, if the capacitor is not over voltage, the soft-strong control strategy aims at the main control object such as the power flow and bus voltage, and consider the dc-link capacitor voltage control just under the condition that the system is nearly stable or is disturbed lightly, which accelerates the stable process.Additionally, this paper discusses the performance of UPFC intelligent control system based on HSIC under the disturbance (such as delay and white noise) on the information from the generator control unit.
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