Dynamic harmonic domain modeling of flexible alternating current transmission system controllers

Flexible alternating current transmission system (FACTS) and multi-line FACTS controllers play an important role in electrical power transmission systems by improving power quality and increasing power transmission capacity. These controllers are nonlinear and highly complex when compared to mechanical switches. Consequently, during transient conditions, it is very difficult to use conventional time and frequency domain techniques alone to determine the precise dynamic behavior of the harmonics introduced into the system by these controllers. In particular, the time-varying nature of the harmonic components is not captured by these techniques. The contribution of this work to the state of power systems analysis is the development of new models for seven important and widely-used FACTS controllers (static synchronous series compensator (SSSC), unified power flow controller (UPFC), fixed capacitor-thyristor controlled reactor (FC-TCR), thyristor controlled switched capacitor (TCSC), generalized unified power flow controller (GUPFC), interline power flow controller (IPFC), and generalized interline power flow controller (GIPFC)) using a technique called the dynamic harmonic domain method. These models are more accurate than existing models and aid the power systems engineer in designing improved control systems. The models were simulated in the presence of disturbances to show the evolution in time of the harmonic coefficients and power quality indices. The results of these simulations show the dynamic harmonic response of these FACTS controllers under transient conditions in much more detail than can be obtained from time-domain simulations, and they can also be used to analyze system performance under steady-state conditions. Some of the FACTS controllers' models discussed in this work have a common DC link, but for proper operation, the DC side voltage must be held constant. The dynamic harmonic domain method was applied to the FACTS devices to design feedback controllers, which help in maintaining constant DC side voltage. It allows us to see the effect of the control circuit on power quality indices, thus giving more insight into how the controllers react to the control circuit. This detailed information about dynamic harmonic response is useful for power quality assessment and can be used as a tool for analyzing the system under the steady state and transient conditions and designing better control circuits.