N.C. Pahalawaththa

Maximum likelihood estimation of fault location on transmission lines using travelling waves

An improved method for fault location on transmission lines using the maximum likelihood estimate of the arrival times of reflected travelling waves is presented. The method is compared with previously proposed correlation based methods using results determined by ATP. The effectiveness of the new method for small fault angles and close in faults has been detailed. >

Analytical modelling of HVDC-HVAC systems

A new HVDC-HVAC analytical model is presented in this paper. The model comprises three subsystems: AC system, PLL and DC system. The model is structured in such a manner to enable small signal analysis of HVDC-HVAC interactions and possible problems arising from these interactions, CIGRE HVDC Benchmark model is used as a test system. Model verification, performed using PSCAD/EMTDC simulations, showed good response matching for all DC and AC system variables. As an example of application of this model the influence PLL dynamics on the system stability is studied offering the important rules for tuning of PLL gains.

SVC dynamic analytical model

This paper presents a linear state-space model of a static var compensator (SVC). The model consists of three individual subsystem models: an ac system, an SVC model, and a controller model, linked together through d-q transformation. The issue of nonlinear susceptance-voltage term and coupling with a static frame of reference is resolved using an artificial rotating susceptance and linearizing its dependence on firing angle. The model is implemented in MATLAB and verified against PSCAD/EMTDC in the time and frequency domains. The verification demonstrates very good system gain accuracy in a wide frequency rangef <150 Hz, whereas the phase angle shows somewhat inferior matching above 25 Hz. It is concluded that the model is sufficiently accurate for many control design applications and practical stability issues. The models use is demonstrated by analyzing the dynamic influence of the PLL gains, where the eigenvalue movement shows that reductions in gains deteriorate system stability.

Small signal analysis of HVDC-HVAC interactions

This paper presents a small signal analysis of interactions between HVAC and HVDC systems. The eigenvalue analysis, along with analysis of participation factors, is firstly described. The eigenvalue sensitivity analysis shows how the changes of AC system parameters influence the stability of the considered HVDC-HVAC system. The influence of short circuit ratio (SCR) changes on both the rectifier and the inverter AC system, is studied by examining the relative movement of the system eigenvalues. The most important conclusions about AC-DC interactions are obtained by investigating the nature of inherent feedback loops between the systems. By examining the changes of all interaction variables, it is determined which of the interaction variables should be controlled and which are better left uncontrolled.

Investigation of the use of inverter control strategy instead of synchronous condensers at inverter terminal of an HVDC system

This paper presents a comparison of performance of traditionally used synchronous condensers (SCs) at inverter side HVDC terminal and a novel inverter control method. The novel controller has earlier demonstrated good responses with very weak inverter AC systems and widely changing AC system parameters. The aim of this research is to study if it is possible to offer adequate replacement for SC at weak HVDC-HVAC interconnection point. The simulation results on an actual HVDC system show excellent small signal responses, but the main obstacle for the application of the new control method could be increased probability of commutation failure. Subsequent tests show that in order to keep the same commutation failure probability, half of the SC must be maintained in normal operation. Further SC can also be disconnected, when the new control method is employed, however the nominal operating angle has to be somewhat increased. The cost benefit study is performed to analyze the financial benefits of the proposed project. It is concluded that, although the benefits are evident for the new HVDC links, the proposed replacement of SC may not be justifiable for the existing links.

Novel current controller design for elimination of dominant oscillatory mode on an HVDC line

A procedure for designing a novel HVDC current controller is presented. The proposed controller makes HVDC system performance less prone to fundamental frequency oscillations on a DC line, and thus less susceptible to the second harmonic instability. The design is based on a linearised AC/DC system model and on the pole-placement technique. Controller performance is tested against the CIGRE HVDC Benchmark model using the EMTDC/PSCAD simulation package. It is demonstrated that this controller eliminates the dominant oscillatory mode on the DC side, and that it gives a significant improvement in system response even for low SCR AC systems.

A comparison of frequency measurement methods for underfrequency load shedding

A comparison of frequency measurement methods for the purpose of underfrequency load shedding is presented in this paper. Comparison is made under the assumptions of the presence of DC offset, high order harmonics and noise in the measured signal. The major constraints have been the accuracy of frequency measurement, time needed to measure the frequency and complexity of implementation in terms of computational requirements. A method for efficient, low-cost implementation of frequency relay which can be used at domestic consumer installations is proposed and its implementation is described.

Stabilization of a multi-machine power system using nonlinear robust variable structure control

Abstract Design of a robust nonlinear variable structure power system stabilizer which can be implemented using only local measurements is presented in this paper. The controller design procedure using a nonlinear variable transformation technique and variable structure control (VSC) theory is discussed. A method of eliminating chattering encountered in variable structure controllers is also presented. Performance of the proposed controller is evaluated by using computer simulation of a multi-machine power system which exhibits multi-modal oscillations. Results show that a significant improvement in the system performance can be obtained by the proposed control scheme.

Frequency regulation of rectifier side AC system of an HVDC scheme using coordinated fuzzy logic control

This paper presents a fuzzy logic controller for improving the frequency regulation of the rectifier side AC system of an HVDC scheme interconnecting two power systems. The controller coordinates the modulation of, the current in the DC scheme and the generator excitation of the rectifier side AC system, in response to rectifier side AC system frequency deviation. The fuzzy logic controller is designed without using any precise system model. Only the qualitative knowledge about the system is used in designing the controller. An iterative technique using steepest descent gradient search method is proposed in this paper for further tuning of the output gains of the controller to optimise a chosen objective function. Simulation results reveal that, a fuzzy logic controller can be successfully implemented to realise significant improvements in frequency regulation by coordinating the modulation of the DC current and generator excitation of the rectifier side AC system.

ac — Small power dc hybrid transmission for improving power system stability

Abstract The paper presents a novel FACTS concept to improve the dynamic stability of an ac power system. The concept of parallel ac/dc transmission between two ac systems has been exploited in the scheme presented, using one ac line for both the ac and the dc transmission. A small unit of power is extracted from the system, converted to dc and injected into the ac line through the transformer, neutrals at the two ends to enhance the dynamic stability. The objective of the paper is to introduce the new concept and prove its feasibility. To demonstrate the validity of the proposed method, computer simulated dynamic responses of an ac power system, with and without the controlled dc injection are presented. It has been shown that by using about 2% of ac power transfer, as modulated dc power, substantial system of damping could be achieved.