Modern Analytical Tools Are Changing the Way Transmission Planning Is Done

Exeoutive Summary Modern computation tools, such as the Optimal Power Flow (OPF) and Automatic Contingency Analysis (ACA), reduce significantly the time required to develop reliable and economical transmission plans. The traditional use of multiple power flows for planning large power systems with complex interconnections is very labor intensive and time consuming and limits the number of options that can be considered. Typical Study A flowchart for a power flow-based transmission planning methodology using OPF and ACA is shown in Figure 1. This methodology promotes efficient planning of transmission systems given a generation plan, a load development plan, possible transmission additions and transmission reliability criteria. It comprises five stages as follows: Stage 1: DC ContingencvAnalvsis and Interactive Planning Here the initial power flow is a model of the initial transmission network with pre-planned generation dispatch and load, deferring for the moment consideration of voltage and reactive requirements. DCbased analysis identifies transmission configurations that satisfy thermal capacity reliability criteria. This step is performed interactively such that the planner is able to select or dismiss candidate transmission additions, perform ACA, and evaluate results for alternative plans. Further, considerable time can be saved using sensitivity techniques to rank candidate transmission additions based on their potential impact on thermal loading for normal and contingency operation, as in PTI's TPLAN program and its implementation of interactive transmission planning'. The interactive process makes it . possible to identify the most useful and least cost transmission additions. Thus, the transmission planner is able to formulate alternative plans quickly and efficiently. Stage 2 : Optimal Power Flow The OPF identifies requirements for voltage control, such as shunt compensation, transformer tap-changing and generator reactive capability for each transmission plan. The OPF formulation' ensures that the voltage profile and other operating limits meet reliability criteria. One common application is the allocation of shunt capacitor compensation to fix the peak load voltage profile of a planned system. The optimal shunt compensation scheme from OPF allocates the minimum amount of shunt capacitors at the most effective sites. Also, efficient use is made of existing voltage control devices before adding new capacitors. Shunt reactors can be treated similarly for light load conditions. The OPF formulation thus implicitly minimizes new investment for voltage control. It would be daunting to attempt to solve this capacitor allocation problem with a conventional power flow. Further, with the OPF formulation in PTI's PSS/OPF program, the planned networks have the added characteristic of not having voltage collapse conditions'.