Control and protection

The control equations are solved separately from the power system equations though still using the EMTP philosophy, thereby maintaining the symmetry of the conductance matrix. The main facilities developed to segment the control, as well as devices or phenomena which cannot be directly modelled by the basic network components, are TACS and MODELS (in the original EMTP package) and a CMSF library (in the PSCAD/EMTDC package). The separate solution of control and power system introduces a time-step delay, however with the sample and hold used in digital control this is becoming less of an issue. Modern digital controls, with multiple time steps, are more the norm and can be adequately represented in EMT programs. The use of a modular approach to build up a control system, although it gives greater flexibility, introduces time-step delays in data paths, which can have a detrimental effect on the simulation results. The use of the z-domain for analysing the difference equations either generated using NIS, with and without time-step delay, or the root-matching technique, has been demonstrated. Interpolation is important for modelling controls as well as for the non-linear surge arrester, if numerical errors and possible instabilities are to be avoided. A description of the present state of protective system implementation has been given, indicating the difficulty of modelling individual devices in detail. Instead, the emphasis is on the use of real-time digital simulators interfaced with the actual protection hardware via digital-to-analogue conversion.