Zexiang Cai

Study on the dynamic performance characteristics of HVDC control and protections for the HVDC line fault

The travelling wave protection and under voltage protection which are initiated by the rate of change of voltage can not detect the high impedance fault. The operation time of differential protection is so long that it can not play the role of back-up protection of the DC line fault. Whats more, the HVDC control system has great influence on the performance characteristic of the DC line protection, especially in case of the low load level. The differential protection will miss tripping because of the current fluctuation caused by the control system. Based on the EMTDC simulation including the actual HVDC line parameters, control and protection models, the currently used DC line protection and the influence of the control system are evaluated in this paper, which is analyzed by combination of the fault records and the simulation results. The improvement scheme is also proposed for the actual system.

Modeling and Harmonic Analysis of TCSC with Dynamic Phasors

This paper develops systemic dynamic models of the thyristor-controlled series capacitor (TCSC). This large-signal, time-invariant model can easily consider the nonlinear dynamic and harmonic characteristics. The proposed models are expressed in terms of dynamic phasors including fundamental, third and fifth harmonics. The simulation results demonstrate that in the capacitive operating region the proposed models trace the envelope and make an excellent agreement with the detailed time-domain model, the most important harmonic voltages are the third and fifth harmonics, and the capacitor voltage can be described as dynamic fundamental phasor as delay angle increasing. In the inductive region the third and fifth harmonics are not dominant harmonics. The dynamical phasor models are well suited for fast numerical simulation and harmonic analysis directly

A method for frequency dynamics analysis and load shedding assessment based on the trajectory of power system simulation

A method for frequency dynamics analysis and load shedding assessment is proposed in this paper, which maps the simulation trajectories in the multi-machine space to a single-machine space by using the center-of-inertia transform. This method not only takes advantage of numerical integrationl method, which is not limited by the model, but also uses the concept of uniform frequency of the single-machine model to compensate the shortage of numerical integration method effectively. Therefore, the coefficients calculated based on the method can represent the influencing factors to the frequency dynamics, such as operation modes, network configurations and model parameters of generators and loads, etc. Furthermore, an under frequency load shedding scheme based on the method is proposed for Guangdong power system. The simulation results show that the new scheme has much more adaptability than the traditional UFLS scheme.

Analytical Modeling of Traffic Flow in the Substation Communication Network

A custom-designed and well-monitored substation communication network (SCN) can maintain fast and reliable information transmission and lead to improved operation and management of a substation automation system (SAS). In order to achieve this goal, a traffic-flow model, including a port connection model, a traffic-flow source model, and a traffic-flow service model of an SCN is developed in this paper. Based on the traffic-flow model, a traffic-flow calculation algorithm is designed to obtain the distribution of traffic load and maximum message delay. In order to verify the accuracy of the proposed method, the SCN of a simplified substation is established in the laboratory. And the distribution of traffic load and maximum message delay calculated using the proposed method are compared to that measured by a network analyzer. Furthermore, possible applications, such as network device selection, network performance analysis, and sensitivity analysis of the proposed method are demonstrated based on a typical 220-kV substation.

Multi-infeed HVDC/AC power system modeling and analysis with dynamic phasor application

In this paper, dynamic phasor method has been applied to HVDC transmission system and FACTS device modeling. A hybrid-model simulation algorithm is suggested for transient stability analysis with HVDC transmission system and FACTS devices in dynamic phasor models and other AC system elements in electromechanical models. Computer test results show clearly that the proposed hybrid-model simulation algorithm can be applied for transient stability analysis effectively and accurately. A multi-infeed HVDC/AC (MIDC/AC) system is studied using the suggested hybrid-model transient stability program. The dynamic behaviors of MIDC/AC system under different circumstances are discussed. The characteristics of HVDC system recovery and recovery strategy for MIDC system are considered in the stability analysis. Furthermore, the impacts of advanced FACTS controllers such as STATCOM on system dynamic are studied for enhance system operation security

Nonlinear backstepping design of robust adaptive modulation controller for TCSC

In this paper, a nonlinear robust adaptive modulation controller (RAMC) for thyristor controlled series capacitor (TCSC) in interconnected power systems is developed. The controller drives centers of inertial of different areas (COIs) to maintain synchronous operation based on the dynamic COI signals obtained from the wide area measurement system (WAMS). The overall system model is established using COI coordinates. The control law of the RAMC is then derived using the backstepping method, and not only is it adaptive to unknown parameters such as damping coefficients of generators, but also robust to uncertain disturbances and model errors. Computer simulation results illustrate that the proposed controllers can improve system dynamic more effectively when compared with conventional linear controllers

Modeling and Analysis of Large Scale Power System with Multi-infeed HVDC by EMTDC

This paper establishes a simulation platform of large scale power grid with multi-infeed DC using EMTDC, in order to analyze the interaction of AC/DC systems and its influence on protective relaying. The principles and methods of the models adoption, the scale of power system, network simplification and data processing are dealt with to balance the simulation demands between precision and efficiency. Particularly, this paper presentes the reasonable treatment scheme to Tianguang HVDC systems equivalence access of 500 kV mainframe grid. It also concludes with a comparative study in the form of simulation between the existing power grid (Guangdong power grid in China) and the platform constructed by analyzing different fault and system conditions. It demonstrates the possible applications of the platform and means to study different cases of these problems.

A novel controller design for STATCOM based on the concept of area center of inertia

This paper proposes a novel controller for STATCOM to enhance the inter-area synchronous stability. The main idea of this controller comes from the concept of area-COI (center of inertia). The potential of area-COI as a unified signal for synchronous in large scale interconnected power systems is explored. This new controller uses areas-COI information and keeps them synchronous. Dynamic model based on area-COI- coordinate is derived first. Then the STATCOM control strategy is derived using back-stepping method. At last one simple 2-area- 4-geneartor interconnected power system is used for performance tests. The simulation results show that the suggested the designed STATCOM controller based on area-COI- coordinate model works very well with superior performance compared with the conventional controller.

Influence of HVDC commutation failure on directional comparison pilot protection of AC system

Several incidents of transient power converse due to the interaction of AC/DC system have been reported recently, and the incidents result in the malfunction of directional comparison pilot protection (DCPP) and the miss-trip of vital AC transmission lines. Taking Jingling-Echeng HVDC and its connected inverter side AC system into account, the corresponding electromagnetic transient simulation model is established in PSCAD/EMTDC based on the operating environment of a mal-operation incident. The response characteristics of HVDC system and the fault features of AC system are studied in this paper while faults occur at the inverter side AC system on the basis of the analysis of the real recorded fault data. Results show that the rapid response of HVDC results in the malfunction of DCPP while AC fault occurs. The mechanism of transient power converse and the variations of AC fault features are analyzed. The corresponding measures are also proposed.

The Study of Nonlinear DC Power Modulation Control in Multi-infeed HVDC Power Systems

A novel nonlinear DC power modulation control is proposed for multi-infeed HVDC in this paper. The idea is that the inter-area oscillations can be damped out by multi-infeed HVDC systems with COIs signals which means individual areas are driven to the stable equilibrium point. A three-area AC/DC power system is used as a sample to show the model derivation in COI coordinates. The back-stepping method is then applied to work out the nonlinear robust adaptive control law for DC power modulation in order to damp low frequency oscillations. Computer simulation results show that the proposed controller is superior to conventional DC power modulation controllers and can damp inter-area oscillations very well.