Longitudinal Power System Research Articles

Stable region for autoparametric resonance in longitudinal power systems

In longitudinal power systems, there is the possibility that a low-frequency oscillation mode may become unstable because of autoparametric resonance. The resonance occurs through interaction between two oscillation modes. In this paper, we calculate the stable region for the resonance by considering the interaction of the modes. First, we calculate steady-state solutions by the harmonic balance method. The steady-state solutions are stable or unstable. If we decrease the amplitude of one mode in an unstable solution, the modes decay. Conversely, if we increase the amplitude, the modes diverge. Namely, the unstable solution is located on the boundary of the stable solution. The amplitudes of the modes are rarely the same as those of the steady-state solution. However, the amplitudes approach a steady state after some transients. If the steady state is in the stable region, the system is stable. If it is in the unstable region, it diverges. Lastly, we estimate the amount of damping torques necessary to stabilize the system with the newly calculated stable region. We have obtained results similar to those derived from the Mathieu diagram, for example, a certain amount of damping torques can stabilize the system irrespective of its size. AVRs can substantially reduce the amount of damping torques and other quantities. © 2000 Scripta Technica, Electr Eng Jpn, 132(2): 29–38, 2000

SVC補償くし形系統における電圧振動モードの安定性に関する検討

SVC補償くし形系統における電圧振動モードの安定性に関する検討

Studies on steady-state stability of looped power systems and stabilization by SMES

With the increasing number of dispersed power sources interconnected to the power supply system, power systems can take a form similar to that of a longitudinal system. Also, they can become looped, with both ends of the longitudinal system connected to each other. In the present paper, the steady-state stability of longitudinal power systems and looped power systems is analyzed by means of the mode analysis method via eigenvalue and eigenvector calculation, and power system stabilization by SMES (Superconducting Magnetic Energy Storage) for looped systems is examined relative to that for longitudinal systems. © 2000 Scripta Technica, Electr Eng Jpn, 133(3): 48–54, 2000

On the possibility of SSR in longitudinal power systems

This paper presents a theoretical consideration on the possibility of subsynchronous resonance (SSR) in longitudinal power systems. Shunt capacitors are used for reactive power compensation in our country, but series capacitors are not used in general. The possibility of SSR is therefore small. However, if power transmission increases, and accordingly, if shunt compensation increases in amount, there is no guarantee that SSR will never occur. First, we investigate network impedance viewed from a generator. Its resonance frequencies become lower with increasing transmission power. One of them gets subsynchronous if the power exceeds a certain value. In this area, there is some possibility of SSR, which is confirmed with the damping property of the generator. The admittance matrix of the load buses is singular at the resonance frequencies. Their number is equal to the dimension of the matrix. The frequencies are common to all generators but not limited to one particular generator. One of them becomes equal to 60 Hz as we increase transmission power. We regard this power as a limit for SSR. However, steady-state stability limit is lower than this limit, and steady operation is not possible at the limit. Therefore, it is impossible to enter the area of SSR. Thus, we conclude that SSR does not occur in shunt compensated systems. However, this property is easily lost if some series compensation is introduced. © 1999 Scripta Technica, Electr Eng Jpn, 130(3): 30–38, 2000

Application of thyristor-controlled series compensators to enhance oscillatory stability and transmission capability of a longitudinal power system

Power transfer capability between the Northern area and the Central area of the Taiwan power system is severely limited by the fact that the power flows over the two parallel corridors connecting the two areas are rather disproportionate to their thermal limits due to different conductor sizes. Placing series capacitors on the corridor which is lightly loaded seems to be a reasonable means to enhance the overall transfer capability of the parallel corridors without overloading the other corridor which is already close to its thermal limit. A joint research project is conducted by the researchers at National Taiwan University and the engineers at Taiwan Power Company (TPC) in order to examine the feasibility of installing a combination of conventional series capacitors and thyristor-controlled series compensators (TCSCs) at TPC's 345 kV transmission system. Extensive power flow studies are conducted to investigate how the power flows over the two corridors are affected by different locations and capacities of the series capacitors. In addition to steady-state power transfer levels over the two corridors, the eigenvalues for the inter-area oscillation modes are also computed for the system with various compensation plans. A proper compensation plan is determined based on requirements on both steady-state power transfer level and inter-area mode damping. To improve system dynamic performance, a supplementary TCSC damping controller is designed. It is concluded from the results of this work that good damping characteristic can be achieved by coordinated application of the designed TCSC damping controller and the power system stabilizers.

An investigation on the use of power system stabilizers for damping inter-area oscillations in longitudinal power systems

This paper presents the progress of an investigation on the use of power system stabilizers (PSSs) to enhance the damping of low-frequency inter-area modes in longitudinal power systems. A dynamic model of the Mexican Interconnected System (MIS) is adopted to evaluate the potential for small signal stability improvement by means of PSSs. Power oscillation flow studies, transfer function residues and controllability and observability characteristics are used to understand the nature of oscillation patterns, mode identification and PSSs siting. It is shown that PSSs can significantly enhance the damping of modal oscillations, especially under high stress conditions.

長距離くし形系統における内部共振の安定領域

In longitudinal power systems, there is the possibility that a low-frequency oscillation mode may become unstable because of autoparametric resonance. The resonance occurs through interaction between two oscillation modes. In this paper, we calculate the stable region for the resonance by considering the interaction of the modes. First, we calculate steady-state solutions by the harmonic balance method. The steady-state solutions are stable or unstable. If we decrease the amplitude of one mode in an unstable solution, the modes decay. Conversely, if we increase the amplitude, the modes diverge. Namely, the unstable solution is located on the boundary of the stable solution. The amplitudes of the modes are rarely the same as those of the steady-state solution. However, the amplitudes approach a steady state after some transients. If the steady state is in the stable region, the system is stable. If it is in the unstable region, it diverges. Lastly, we estimate the amount of damping torques necessary to stabilize the system with the newly calculated stable region. We have obtained results similar to those derived from the Mathieu diagram, for example, a certain amount of damping torques can stabilize the system irrespective of its size. AVRs can substantially reduce the amount of damping torques and other quantities. © 2000 Scripta Technica, Electr Eng Jpn, 132(2): 29–38, 2000

Impact of high-speed railway loads on a longitudinal power system

The impact of the upcoming high-speed railway loads on the characteristics of the Taiwan Power System (Taipower) is investigated. A newly developed three-phase Newton optimal power flow method is adopted as an analytical tool. The voltage unbalance ratio, under-load-tap-changer adjustment, generator voltages, voltages of high-speed railway stations, and the static voltage stability problem are examined. It is found that the Taipower system still has good characteristics when single-phase high-speed loads are imposed to the system.

長距離くし形系統の短絡容量に関する理論的考察

長距離くし形系統の短絡容量に関する理論的考察

Transmission capacity of longitudinal power systems and SVC voltage control

Power transfer between systems is important due to uneven distribution of generating plants. This paper investigates the relation between the transfer capacity of a longitudinal power system and voltage control of static var compensators. The transfer capacity is basically limited by the thermal capacity of transmission lines. However, the practical systems, it is much restricted by stability and the power transfer level is considerably lower than the thermal capacity. In this paper, we consider a basic case in which SVCs are applied to all buses except generator terminals. In this case, it is possible to transfer power up to the reciprocal of the transmission reactance. Two modifications are then applied to the basic case. One is removal of SVCs on the high-voltage sides of the generator transformers. In this case, generator damping torques deteriorate, and the local oscillation mode becomes unstable. The other is removal of SVCs at intermediate buses on the trunk system. In this case, the shapes of the oscillation modes change greatly, and the global mode becomes unstable. The voltage control of SVCs maintains the generator damping torques and prevents deformation of mode shapes. By investigating different system sizes and transmission circuits, we show that the system transfer capacity is determined by the capacities of the individual transmission lines. © 1997 Scripta Technica, Inc. Electr Eng Jpn, 119(3): 49–60, 1997

Analysis of inter-area damping enhancement by static var compensators in longitudinal power systems

The work reported in this paper synthesizes the main findings of an eigen-analysis-based research programme, aimed at assessing the potential for dynamic stability improvement by means of static VAR compensators. The results of the first phase of this research include the determination of modes that are more controllable by existent SVCs, and the use of modulation controls to damp low-frequency inter-area modes of oscillation in longitudinal power systems.

A decentralized control system for stabilizing a longitudinal power system using tieline power flow measurements

A decentralized control system for stabilizing a longitudinal power system is studied. The system is designed by a design methodology proposed earlier for decentralized control. In the process of the design, it is found that only the control information of phase angles from other areas is indispensable for maintaining the system stability. New measurements of the tieline power flow are substituted for the indispensable information. Simulation studies demonstrate that the resulting decentralized control system using tieline power flow measurements has as excellent performance in enhancing the stability of a longitudinal power system as a centralized control system.

長距離くし形系統の輸送能力に関する理論的考察

長距離くし形系統の輸送能力に関する理論的考察

Constrained multiobjective global optimisation of longitudinal interconnected power system by genetic algorithm

A genetic-based algorithm is proposed to solve the multiobjective global optimisation problem of a longitudinal interconnected power system, which incorporates various constraints during the optimisation of dispatch solution with two objectives in mind: economic and environmental requirements. Its effectiveness is verified through a three-area test system with multiple fuels and multiple pollutants. Consequently, the results of the sample system are analysed to demonstrate the internal relationships among various constraints and objectives.

Transient stability assessment in longitudinal power systems using artificial neural networks

Results of the application of artificial neural networks to the problem of transient stability assessment are presented. This technique is applied to a real longitudinal power system that includes discrete supplementary controls. Different representations of the training space patterns and neural networks architectures are investigated. Input variables include topological changes, load and generation levels and contingencies. A special organization of training patterns with a separation by type of contingency is proposed to reduce classification errors. A graphical presentation of results is power system suggested as an aid to help system operators to select preventive control actions.

A decentralized control system for stabilizing multimachine power systems

AbstractA decentralized control system is studied for stabilizing multimachine power systems. A longitudinal power system with three areas, each having one machine, is considered in this study. A decentralized control design method is proposed, which is based on the optimal regulator theory. First a centralized control system is designed without any consideration on whether state variables are all available or not. Second a pseudo‐decentralized control system is designed by omitting control gains corresponding to state variables which give hardly any effects on the power system stability. It is found that only one variable of phase angle of each machine is absolutely necessary for the pseudo‐decentralized control system. This leads to an idea based on power system engineering, that is to say, new variables of tieline power flow are introduced in the decentralized control system design to substitute for the phase angle of each machine. Thus a decentralized control system for power system stability can be designed using the new variables of tieline power flow. It is demonstrated from simulation studies that the decentralized control system improves even longitudinal power system stability as well as the centralized control system.

Improvement of transient stability using unified power flow controller

The aim of the paper is to analyze the effect of a unified power flow controller (UPFC) on transient stability margin enhancement of a longitudinal power system. To utilize the UPFC possibilities fully, the three controllable UPFC parameters were determined during the digital simulation process performed by the NETOMAC simulation program. The basis for determination of the suitable damping strategy and for determination of the optimal UPFC parameters is a mathematical model, which describes the interdependence between longitudinal transmission system parameters, operating conditions and UPFC parameters in the form of analytical equations. On the basis of the mathematical model, the theoretical UPFC limits were also detected, and their appearance explained.

長距離くし形系統の輸送能力とSVCの電圧制御

長距離くし形系統の輸送能力とSVCの電圧制御

Analysis of Inter-Area Damping Enhancement by Static Var Compensators in Longitudinal Power Systems

The work reported in this paper synthesizes the main findings of an eigen analysis based research aimed at assessing the potential for dynamic stability improvement by means of static VAR compensators. The results of the first phase of this research include the determination of modes that are more controllable by existent SVCs and the use of modulation controls to damp low frequency inter-area modes of oscillation in longitudinal power systems

On-Line Transient Stabilily Assessment Using Artificial Neural Networks

In this paper an Artificial Neural Networks methodology is proposed for the On-Line Transient Stability Assessment of longitudinal electric power systems. The objective of the system designed is to help power system operators in control centers in decisions involving generation changes and/or activation of discrete supplementary controls to improve power system stability. Main steps followed to develop the artificial neural networks are presented and test results using the Northeast Area of the Mexican Interconnected System are reported.