This full bridge DC-DC convenor with IGBTs uses zero voltage switching (ZVS) for one leg of the bridge and zero current switching (ZCS) for the other. It is shown that an active snubber greatly improves the performance over previous methods. Experimental results are shown for a 6 kW circuit switching at 20 kHz.

In a previous article, protection of electrical apparatus by flameproof enclosures was considered. Protection in this context means that, due to its method of enclosure, the apparatus will not cause ignition of a surrounding explosive atmosphere. However, with very low power apparatus such as for instrumentation and process data management, the type of protection known as intrinsic safety is usually more appropriate. Intrinsic safety is defined as a protection technique based on the restriction of electrical energy to a level which is too low to be able to ignite an explosive atmosphere. The following article describes some of the principles involved

The introduction in the mid-1970s of high-voltage highcurrent silicon thyristors for electrical power conversion created a need for extremely reliable but economical liquid cooling systems in which purified water-based coolants could be circulated. Research and development over 2 1/2 years on XLPE tube systems and compatible nonmetallic connectors has resulted in a single-circuit purified glycol/water fluid system cooling a 150 MW back-to-back HVDC link in Canada

Short circuits applied close to substations cause large transient peak fault currents, but remote faults are difficult to detect because the fault current may be less than a normal train load. It is important to be able to predict the form of both closeup and remote fault currents for design of new protection systems and to assess the suitability of particular protection arrangements for specific railways sites. The model of the substation rectifier is an essential part of any means of predicting the current profiles. A detailed rectifier model based on a piecewise linear digital simulation, particularly suitable for prediction of fault current profiles owing to closeup faults is presented. The accuracy is demonstrated by comparison with a case study of a practically measured fault current profile. It is shown that commonly used models based on simpler equivalent circuits can not match the accuracy of the detailed model. It is also shown that a simple DC power source model of a rectifier is appropriate for remote fault calculations because the profile is not significantly affected by the switching action of the rectifier. The validity of the simpler model is demonstrated by comparing the simple model with the more detailed one.

The experimental modelling of large nonlinear loads (such as arc furnaces), their supply systems and reactive shunt compensation is described. The laboratory model can be programmed to draw a load current which is a scaled copy of a system load or any other waveform that may be required. The modelling of the compensator is based on a thyristor controlled shunt reactor (TCR), using a fast, but very simple, integral of voltage controlling algorithm. Using a variety of amplitude modulated and synthesised load current waveforms it is shown how the frequency response of the compensator can be determined, indicating that there is a range of modulating frequencies over which the compensator is effective in reducing the voltage fluctuations, whereas for higher modulation frequencies its effect is to increase these fluctuations. The results of the laboratory model are compared with those from a mathematical model, with good agreement. The ability of the equipment to model a 56 MVA electric arc furnace is described and it is shown that to model such a load mathematically, a degree of randomness has to be introduced into the synthesised current waveform. Spectral estimation techniques are used to judge both the laboratory and mathematical models' performances and the reader's attention is drawn to some of the problems associated with the use of such techniques.

The paper examines the virtues of representing the magnetising curves of a switched reluctance motor by customised cubic splines and storing its coefficients in a new data base. It is shown that cubic splines are capable of handling the highly nonlinear characteristics. Furthermore they are found to be well suited for computer efficient generation of the flux linkage/current, torque/angle and current/flux linkage functions as required for drive simulation purposes. A case study demonstrating the use of the new data base as well as its compilation is discussed.

A numerical computation of squirrel cage induction machine parameters is presented using the finite element method (FEM). The effects of main field saturation are studied and the different parameters can be calculated for any magnetising current value. To take into account magnetic state variations, a ‘dynamic’ mutual inductance is introduced into the saturated machine model. All these parameter values are used in the field-oriented control system realisation, and experimental results are described for constant and weakened flux operating points. Special attention is given to the adaptation of the control law during flux-weakening, to take into account desaturation effect. The results obtained are quite satisfactory.

Parameter variation in the load of a servoposition control drive requires a robust controller. The sliding mode control, which constrains the system motion to a state trajectory, provides for greater robustness than classical control schemes. The design and implementation of a brushless DC servomotor drive using sliding mode position control is described. The design procedure is outlined, and the features of practical sampling rate and supply source limitation are incorporated. The implementation of the system, based on a Motorola 68000 processor, is described, and both simulated and practical responses are presented and evaluated.

Electricity is often featured in the media for its role in causing pollution: acid rain and the greenhouse effect are two side effects of power generation.

The steady state response of a resonant convertor can be represented graphically by the state plane diagram. On using the perturbation method on the steady state trajectory, a discrete small signal model for the convertor can be derived from the input voltage, controlled switching frequency and state variable perturbations. The conventional series resonant convertor is used as an example to demonstrate an analytical approach. However, this method of analysis can be applied to resonant convertors in general. From the small signal model derived, the frequency response of the convertor can be calculated.