AC Supply System Research Articles

Incorporation of a SMES coil into a superconducting LVdc transmission system

The performance of a superconducting low voltage direct current (LVdc) transmission system can be significantly enhanced through the addition of a superconducting magnetic energy storage (SMES) coil. The power conversion equipment necessary for connecting a SMES coil to a dc system is much simpler and easier to control than the interface needed to connect a SMES coil to an ac system. There is also added flexibility if the LVdc system is mesh connected. The LVdc system already has the power conditioning necessary for connection to the ac load and supply systems. The SMES coil can be connected to the LVdc mesh either through a dedicated dc-dc converter or it can be added directly into the mesh and controlled using the converters already present on the mesh. The rectifier voltage regulation schemes control the charging and discharging of the coil. If one of the rectifiers is supplied by a unit-connected generator, a simpler rectifier scheme can be used, and the generator field excitation control can be used to control the SMES coil. The SMES coil could then be used both to level generator loading and provide added reliability in case of disturbances on the ac supply system.

Harmonic worst-case identification and filter optimal design of MRT systems using genetic algorithms

A mass rapid transit (MRT) operation results in harmonic distortions of AC supply systems. To estimate the effects of harmonics in the system, the worst case should be identified in order to determine whether the harmonic level is beyond the tolerable limit. Levels of voltage and current harmonic distortions in MRT supply systems are affected by train operating modes and system configurations. Harmonic worst-case identification can be treated as an optimisation problem in terms of train separations and traffic conditions. In the paper an approach based on a genetic algorithm (GA) is proposed for solving such problems. The worst-case information is then used in another GA search to place and size the corrective filters when required. Afterwards, the search for the worst case after correction is repeated. The approach uses approximate train movement and consumption models, and the AC/DC harmonic load flow for evaluating the overall harmonic distortion. Simulation results demonstrate the robustness of the approach, which is capable of solving highly nonlinear problems beyond the capability of conventional techniques.

Analysis and design of a series voltage unbalance compensator based on a three-phase VSI operating with unbalanced switching functions

Voltage unbalance in AC supply systems is typically corrected by means of a shunt connected thyristor-controlled static VAr compensator. This approach has the disadvantage of slow response, harmonic injection into the AC system, and the requirement for large passive components. The proposed system consists of a three-phase PWM voltage source inverter, connected in series with the line through a three-phase transformer. The unbalance compensation is achieved by canceling the negative sequence component of the line-to-line voltages of the source. It is also shown that by having the inverter operate with unbalanced switching functions, it is possible to balance the load voltage and to control the amplitude of the positive sequence component in order to perform load voltage regulation. A complete mathematical description of the method is presented, demonstrating that the compensation can be achieved with low kVA inverters and low harmonic injection. Implementation procedures, design equations, and a design example are also included in order to illustrate the proposed method. Experimental results of a 1.5 kVA laboratory prototype system confirm the feasibility of the technique. >

Control of parallel connected inverters in standalone AC supply systems

A scheme for controlling parallel-connected inverters in a standalone AC supply system is presented. This scheme is suitable for control of inverters in distributed source environments such as in isolated AC systems, large and distributed uninterruptible power supply (UPS) systems, photovoltaic systems connected to AC grids, and low-voltage DC power transmission meshes. A key feature of the control scheme is that it uses feedback of only those variables that can be measured locally at the inverter and does not need communication of control signals between the inverters. This is essential for the operation of large AC systems, where distances between inverters make communication impractical. It is also important in high-reliability UPS systems where system operation can be maintained in the face of a communication breakdown. Real and reactive power sharing between inverters can be achieved by controlling two independent quantities: the power angle and the fundamental inverter voltage magnitude. Simulation results are presented. >

A digitally based HVDC firing-pulse synchronization control-description and model development

The design and basic operation of a digitally based DC firing-pulse synchronization control is described. A discrete-time model, suitable for small-signal analysis is also presented. The basic dynamic performance of the loop indicates good regulation capability for changes in firing angle order from the primary regulator and for changes in the AC supply frequency or phase angle. The potential for steady-state error during rapid AC supply system acceleration was identified, and improved performance was achieved by modifying the frequency input portion to the firing-angle correction path. By developing a small-signal model of the synchronizing function, the authors lay the foundation for further control system stability analysis which examines the impact that the AC and DC systems have on the synchronizing loop and vice versa. >

Analysis and design of a series voltage compensator for three-phase unbalanced sources

Voltage unbalance typically present in three-phase AC supply systems adversely affects power system components, static converters, drive systems, electric machines, etc., connected to the system. A method to eliminate this unbalance by means of a voltage compensator connected in series with the supply through transformers is described. The technique is based on extracting the negative sequence voltage component of the supply and canceling it in order to obtain balanced voltages. The positive sequence component is then adjusted to achieve voltage regulation. It is shown that the compensation can be achieved with low kilovolt-ampere inverters and that harmonic injection is reduced to a minimum. The authors include implementation principles, design equations, and a design example. Simulated and experimental results confirm the theoretical concept and feasibility of the proposed system.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Practical approaches for AC system harmonic impedance measurements

AC supply system harmonic impedance measurements are discussed. On the basis of a given and known switchable shunt impedance and/or nonlinear load and site measurements, three methods are presented. The first approach assumes a no-load distorted busbar and uses basic circuit theory to calculate the required harmonic impedance. The second method disregards any previous busbar distortion and the harmonic injection but it takes into account any busbar distortion prior to the nonlinear load connection. Using a laboratory model and by comparing harmonic impedance results it is shown that both no-load busbar distortions and the correct measurement of harmonic phase angles may strongly affect the accuracy of calculation.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Developments in electric traction

Railways are in a competitive business sector and need to keep up with the latest developments in technology. This article reviews recent developments of traction equipment for HV AC supply systems and DC supplied railways, and finally looks at inverter-fed induction motor drives which are increasingly being used on both AC and DC supplies

Steady-state analysis of three-phase bridge converter supplying an active load

This paper describes the analysis of the steady-state performance of a three phase six pulse thyristor controlled converter feeding a dc back emf load in series with a resistance and a smoothing dc link inductance. A complete current and voltage solution for the balanced three phase converter system is derived together with the harmonic spectrum analysis for both voltage and current on both dc and ac side.A novel method is used in deriving the complete results. The method utilizes the symmetrical property of three-phase ac supply systems in saving computing time by a factor of m which is the pulse number of the converter. Digital computer programs are developed to simulate the steady state performance of such a system. The losses and forward voltage drops of the conducting thyristors are taken into consideration by employing a first order thyristor model. The proposed method can be extended to the analysis of a more generalized system which considers ac system imbalances both in structure and in supply voltages and takes the filters both on dc and ac side into account.

Simulation study of DC transit systems with inverting substations

The increasing use of chopper-controlled regeneratively braked stock in rapid transit and suburban railways has brought into focus the problems of limited energy receptivity in the DC supply network. While the use of on-board rheostatic braking or mechanical braking are established methods of dealing with non-receptivity, an alternative arrangement using inverting equipment at the DC substation has been introduced at some locations in Japan, Germany and Brazil. This paper describes a simulation-based study of a rapid-transit system with substation inverters, which was undertaken to establish operating limits and optimum design criteria which could be generally applicable. The paper deals with the choice of installed inverter capacity, inverter location, current commutation limits and inverter control. In addition the energy-saving consequences of using inverter equipment are discussed in detail. The implications of fitting inverters, with respect to the bulk AC supply system, in terms of voltage distortion, voltage regulation and reactive power demand, are considered for a range of typical values of short-circuit level.

Analysis of a symmetrically stabilized three-dimensional oscillator and some implications

Abstract A recently suggested symmetrically constructed three-dimensional oscillator model is analysed. It is proved by using Lyapunov's stability theory that the system possesses two main steady states. In one of the steady states the solutions are three sinusoids arranged symmetrically (like the voltages of a three-phase electric AC supply system). The second steady state is such that all three solutions approach the same constant value when time increases. The system can be viewed as consisting of two parts : a conservative oscillator model, and non-linear terms which introduce damping forces responsible for stabilizing the waveforms in steady state. The dynamic behaviour of the system can be interpreted in view of its structural properties and symmetrically cyclic topology. The features of the system structure and the properties of its dynamic behaviour suggest that it can be employed as a simplified model for representing synchronous generators in largo power systems simulations. Another application of the system is as a model for constructing a precise electronic function generator.

Uninterruptible Power Supply for Critical AC Loads -- A New Approach

An uninterruptible ac supply system consisting of two power sources (static inverter and ac line) and a thyristor switch is presented. The thyristor switch is operated in such a way that the equalization current flow between the sources is prevented. Consequently continuous parallel operation of the sources may be applied. Faults in one of the sources do not affect the voltage supplied to the load. In order to obtain these features the trigger mode of the switch must meet certain requirements. The analysis of these requirements is given and an implementation of the system is presented. Test results are illustrated by oscillograms. The advantages of the system are that 1) load transfer is performed in an instantaneous and smooth way, 2) the system is able to deliver large inrush currents required by the load and to blow fuses in branch circuits of the load, and 3) no voltage monitors are required.

Future High Voltage DC Power Supplies of the Shielded Design

A large number of research laboratories are now using stabilized 600 kV dc power supplies of the entirely shielded design, with electronic ac supply and feed-back control systems.1 Recently, a new 600 kV power supply with a rated current of 20 mA has been marketed. At the present time, 1 MV dc power supplies which will operate in aluminum vessels under pressurized sulphur hexafluoride gas insulation are being constructed. The standard models have an overall stability including ripple voltage and noise of 1 part in 1000, both for continuous and pulsed operation.