Yingduo Han

Mitigation of Multimodal SSR Using SEDC in the Shangdu Series-Compensated Power System

The Shangdu power plant has four 600-MW turbine-generators connected to the North-China Grid through two 500-kV transmissions, which are compensated with 45% fixed series capacitors. Extensive studies conducted on the system model indicate that the system suffers from multimodal subsynchronous resonance (SSR). To solve the problem, a countermeasure is developed by combining the supplementary excitation damping control (SEDC) and the torsional stress relay (TSR). In this paper, the characteristics of the SSR problem are investigated. Then the developed SEDC is presented. To validate the effectiveness of the proposed SEDC as well as the results of model studies, field tests were conducted under various operating conditions. The tests fully exposed the realistic threat of SSR in the system. Meanwhile, it is demonstrated that the developed SEDC can improve torsional damping significantly, and thus solve the multimodal SSR problem effectively. This is the first time in China that practical SEDCs have been developed and their ability to mitigate multimodal SSR has been verified in a real series-compensated system.

A Novel Modal Decomposition Control and Its Application to PSS Design for Damping Interarea Oscillations in Power Systems

The residue method has been widely used for tuning power system stabilizers (PSSs) in large power systems to improve the damping of interarea oscillations. However, an additional PSS installation may affect the performance of existing PSSs due to interactions among different modes. When contending with several interarea oscillations, compromise among different modes becomes necessary. In this paper, a novel method based on modal decomposition is proposed for tuning PSSs for damping of the concerned interarea mode, while minimizing its effect on other modes by weakening the interactions among different modes. Design considerations, PSS structure and tuning procedure are formulated. The performance of the proposed method has been validated based on a two-area four-machine system and an actual large power system, China Southern Grid.

Implementations and experiences of wide-area HVDC damping control in China Southern Power Grid

The weakly damped low frequency oscillation deteriorates the stability of some interconnected power systems. In order to suppress the swings more efficiently, the global dynamics of different areas are required, and then the synchrophasor measurement techniques are becoming necessary for real time and continuous control. In this paper, the design, implementation and commission of a coordinated wide-area adaptive damping control (WADC) system through the modulations of multiple HVDC links in China Southern Power Grid (CSG) are presented. Based on the calculations of observability and controllability, six phasor measurement units (PMU) and three HVDC links were chosen to be included in this system. The controllers were coordinated to damp two dominant inter-area oscillation modes. Another key issue for the wide-area control system is the random time delay and a solution using low-pass filter was proposed. Based on the improved online Prony identification, the controller parameters can be adapted according to the changes of the oscillation frequency. The above WADC were implemented and tested in the CSG real time digital simulations (RTDS) platform, which is composed of more than 10 racks of RTDS, real HVDC control and protection cubicles. After field debugging and trial operations, the WADC system was further validated in the practical CSG through artificial block and de-block of three different HVDC links and tripping a 500kV AC tie-line in the years of 2008 and 2009. The field test results show that the commission of WADC system can increase the damping ratio of the dominant modes more than 10%.

Practical Wide Area Damping Controller Design Based on Ambient Signal Analysis

Frequency response and residue are the two popular methods of designing damping controller. However, due to practical limitations, the former is limited to local controller design, which is not preferable for damping inter-area modes. Although the latter benefits wide area control, it strongly relies on the open loop dynamic model of power system which, for large systems, is hard to obtain. This paper, therefore, combines the two methods to design a wide area damping controller which obviates the need for identifying the open loop dynamic model. The method includes feedback loop selection, residue phase calculation and power system stabilizer (PSS) tuning using modal decomposition control (MDC). The core issue, residue phase calculation, has been validated in a two-area four-machine system and the whole procedure for wide area damping controller design was performed in a real power system, China Southern Grid. Results demonstrate that the proposed method is feasible and effective in real power systems.

Mathematical Expectation Modeling of Wide-Area Controlled Power Systems With Stochastic Time Delay

Summary form only given: Communication network brings new challenges such as time delay and data loss to wide-area damping control of power systems. The uncertainty caused by stochastic time delay of network communication increases the difficulty of wide-area control performance analysis. This paper proposes a mathematical expectation modeling approach to model and analyze wide-area controlled power systems considering the effect of stochastic time delay. In the proposed method, mathematical expectation of stochastic time delay distribution is derived to accurately model the impact of communication network on wide-area control performance. The theoretical frame of the proposed modeling approach is presented and the effectiveness of the method is verified through the simulation of a power system with 8 generators and 36 nodes.

Residue and identification based wide-area damping controller design in large-scale power system

Low frequency inter-area oscillations are poorly damped in large-scale power systems, although hundreds of local Power System Stabilizers (PSSs) have been installed. Wide-area PSS (WAPSS) with global feedback signals is a novel technique to solve this problem. Because of the global properties, conventional PSS design methods based on detailed mathematical models which are not available for the complicated system are no longer feasible to WAPSS. In this work, an innovative procedure of Wide-area PSS design in large-scale power systems was presented. Feedback signals and locations of WAPSS were first selected, and followed by the identification of the reduced model of the controlled system. Based on the identified model, a residue-based design method of WAPSS was proposed. This approach has been applied to the design of WAPSS in China Southern Power Grid. Results demonstrated the effectiveness of the proposed method.

SEDC's Ability to Stabilize SSR: A Case Study on a Practical Series-Compensated Power System

Supplementary excitation damping controller (SEDC) has been widely acknowledged as one of the effective measures to stabilize subsynchronous resonance (SSR). However, how to quantify its stabilizing ability is still an open issue raising widespread concerns. There are few in-depth studies or conclusions in this aspect yet. This paper presents a quantitative measure of SEDCs ability to damp SSR, namely stabilizing ability index (SAI). It is defined as the percentage margin of the effective gain relative to the critical stability gain (CSG). The CSG is the minimum gain of SEDC to guarantee SSR stability, while the effective gain is the control gain of SEDC actually functioning following system disturbances. A practical system is used to investigate the various factors determining the effective gain, the CSG and thus the SAI, based on eigenvalue analysis as well as time-domain simulation. The dominant factors identified include: the total equivalent series-compensation degree of system, the parameters and operating status of the generator, the response time and ceiling voltage of the exciter, and the severity of the disturbance. Why and how these factors affect the control capability of SEDC is examined and thus a practical method is offered for the assessment of SEDCs ability to stabilize SSR.

Field experiments of wide area damping controllers for multiple HVDC links

Wide-area measurement system (WAMS) provides a platform for more efficient control, and some discontinuous types of wide-area control projects have been reported in commission in the recent years. In this paper, a continuous type of wide-area damping control system (WADCS) is introduced, which is used to suppress the inter-area oscillations in a complicated real power grid by modulating multiple HVDC links. The background of WADCS project is described. The design fundamentals and the implementation framework of WADCS are briefly summarized. The details of the field experiment items and some important test results are presented, which show that some new problems in wide-are control are successfully solved and WADCS can operate correctly in different conditions.

Combined Application of SEDC and GTSDC for SSR Mitigation and Its Field Tests

The subsynchronous resonance (SSR) problem may become more serious with the continuous evolvement of a series-compensated power system. Consequently, the existing SSR mitigation scheme might no longer keep the system stable. This issue has been encountered by Shangdu Power Plant, where the deployed supplementary excitation damping controllers (SEDCs) that well stabilized the system before cannot maintain torsional stability after the recent change occurred in the power system. Therefore, a combined mitigation scheme of SEDC and generator terminal subsynchronous damping controller (GTSDC) is proposed in this paper to regain system stability. SEDC provides electrical damping by modulating the excitation voltage at the rotor side. Meanwhile, GTSDC can damp SSR via injecting super-synchronous and subsynchronous currents into the generator stator. Field tests have proven the effectiveness of this combined scheme in addressing the deteriorated SSR issue. Eigenvalue analysis indicates that the combined scheme can provide enough positive damping for the system under any operating condition. Thanks to the low investment of SEDC and the flexibility of GTSDC, this combined scheme provides an economical and scalable solution for SSR mitigation, especially when the target system changes constantly.

Ambient signal based load model parameter identification using optimization method

Load modelling is a crucial part in modelling a power system. The complex, nonlinear and stochastic characteristics of power load increase the difficulty of modelling. In this paper, a new approach of ambient signal based load model parameter identification method is proposed to solve this problem. With the proposed method, load model parameter can be identified in any time regardless of the existence of fault. First, Z+M model is simplified to reduce the number of parameters to be identified. Then, parameters of Z+M model are identified with an optimization method, the objective function of which is calculated from WAMS measured power and voltage data. An iterative process is proposed to solve the problem of initial value sensitivity in optimization problems. Finally, the effectiveness of this identification method is validated through the simulation results on WSCC three machines nine nodes system under different operation situation including both small disturbance and large disturbance.