TABLE OF CONTENTS The use of a single time frame throughout the simulation is inefficient for studies involving widely varying time constants. A typical example is multimachine transient stability assessment when the system contains HVDC converters. In such cases the stability levels are affected by both the long time constant of the electromechanical response of the generators and the short time constant of the converter's power electronic control.
It is, of course, possible to include the equations of motion of the generators in the electromagnetic transient programs to represent the electromechanical behaviour of multimachine power systems. However, considering the different time constants influencing the electromechanical and electromagnetic behaviour, such approach would be extremely inefficient. Electromagnetic transient simulations use steps of (typically) 50 ?s, whereas the stability programs use steps at least 200 times larger.
To reduce the computational requirements the NETOMAC package [1] has two separate modes. An instantaneous mode is used to model components in three-phase detail with small time steps in a similar way to the EMTP/EMTDC programs [2]. The alternative is a stability mode and uses r.m.s. quantities at fundamental frequency only, with increased time-step lengths. The program can switch between the two modes as required while running. The HVDC converter is either modelled elementally by resistive, inductive and capacitive components, or by quasi-steady-state equations, depending on the simulation mode. In either mode, however, the entire system must be modelled in the same way. When it is necessary to run in the instantaneous mode, a system of...
