System and customer impact: considerations for series custom power devices

The growing interest in power quality has led to a variety of devices designed for mitigating power disturbances, primarily voltage sags. One class of these devices is the series device. The size of a series device (SD) is usually described as having a certain percent RMS magnitude voltage injection capability. The required RMS magnitude is usually taken from measured data or the results of a circuit model simulation. These results can be graphically illustrated using magnitude versus duration matrices or curves. Due to the one cycle averaging effect of the RMS calculation, waveshape, transients, and phase shift are not adequately described, and thus does not accurately predict the necessary injection capability required of the series device. Careful consideration must also be taken when applying a series device onto a system. The series device acts as an additional energy source on the system. The device must coordinate with other protective devices to prevent damage to other loads on the system, particularly upstream loads. The series device requires a continuous current path to insert energy, so protective devices must be arranged accordingly. However, a load upstream of the series device and downstream of the protective device creates a potential for reverse current flow through the upstream load