QuickStab: Calculates Maximum Transmission Load and Stability Margin

(Two years ago, the developer of this program was referred to UFTO by DOE, and an UFTO Note was done at that time. Since then, the need has become even greater; there have been improvements to the code; and it has been implemented by a major utility.)

With deregulation, privatization and open access transmission, energy transactions across large electrical distances become commonplace, and can cause major wheeling power flows which, in turn may bring the networks near their limits of stability and loadability.

However, power systems cannot be operated safely near the state of maximum loadability. A much lower load level needs to be defined such that all thermal, voltage and stability constraints are met. NERC calls it the Total Transmission Capability (TTC) and recommends to further reduce it by a Transmission Reliability Margin (TRM) to account for uncertainties. This value must be further adjusted by the Capacity Benefit Margin (CBM) to finally determine the system loading that guarantees full operating security.

The safe operating limit, however, is highly dependent upon topology, voltages, number and location of generators, and other system conditions. For dependable on-line decision making, this limit must be reassessed in real-time for every single change of the operating state.

Critical states occur at or close to the TTC. This limit is not constant. It depends on the generation, customer demand and transmission network conditions, and must be computed from the real-time conditions of the transmission network. Such a capability is needed both in real-time and for postulated conditions, but detailed stability methods are time consuming and require data that may not be readily available.

QuickStabTM answers this need. It identifies both the point of maximum power transfer and the distance to it without computing load-flows. In other words, it predicts the maximum loadability from any operating state, even if far from the limit conditions. This feature is one of the most particular aspects of the short-circuit currents nodal analysis and cannot be found in other algorithms.

Starting from a power-flow or state estimator base case, it computes the system’s MW loading limit such that voltage collapse and steady-state instability do not occur. Then it shows which units and tie-line injections are most likely to cause instability; provides information that can help develop a remedial action strategy; and identifies the P-V points of successively degraded states towards instability. Quickstab also determines the system MW loading for a user-defined security margin.

Results are displayed in a unique, easy-to-understand graphical format. The computations are extremely fast. For example, the solution of a 300 bus case on an inexpensive PC takes just 0.44 sec. Most recently, the QuickStabTM computational modules were enhanced and recompiled with Microsoft C/C++ version 6.0 in a Windows NT 32 bit native environment. The program also runs under Windows 95/98. It is Y2K compliant.

QuickStab’s solution technique is based on the short-circuit currents nodal analysis method, which was perfected in Europe in 1961 and became a “classic” in the voltage-stability circles in 1980. Two studies sponsored by EPRI and Southern Company Services (Southern) in 1990-1993 demonstrated the speed and validity of the approach.

It is now field-proven. In 1998, QuickStab was adopted by Companhia Energetica de Sao Paulo CESP, in Brazil, and by Oficina de Operacion del Sistema Interconectado OPSIS, in Venezuela. CESP uses the program both off-line, on PCs under Windows 98, and in real-time on Digital Alpha processors under Digital Unix. OPSIS uses QuickStab for real-time only. These two EMS implementations of QuickStab were provided by ABB-Bailey Network Management as part of its RANGER baseline.

QuickStab offers significant benefits. It can help increase revenues from wheeling power while meeting higher MW demand and reliability requirements. It can be used on-line, embedded in or as an add-on to an existing EMS. And with its modular design and ANSI C compliant code implementation it can be easily integrated with third-party load-flow, contingency analysis and security assessment programs.
QuickStab is available now to utilities, consultants and universities, under a range of license options.

For additional information, or to make arrangements for a presentation, contact:
Dr. Savu Savulescu
SCS Computer Consulting, Fresh Meadows NY
718-264-7563, savu.scs@worldnet.att.net

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