Observability and Controllability in Highly Compensated Systems

September 2007

Pterra Consulting

Shunt compensation, in the form of capacitor banks and static var devices (SVD), are commonly used to provide voltage support in heavily loaded systems.  Shunt devices offer a relatively cheap and easy-to-implement solution to providing reactive power to load pockets or remote load areas of the grid.  In concept, one can add a combination of switched and controlled shunt compensation to increase import capacity up to the thermal limit of the transmission system.  The savings from deferred investment in new transmission or congestion costs can justify the implementation of large shunt devices.  (The largest existing SVDs are a +500/-150 MVAR behemoth in the Allegheny Power service territory in Pennsylvania, USA, and the Chamouchouane SVC (actually two SVCs at one site) rated +330/-330 MVAR in Quebec, Canada.)

But, the impact ...

The impact of static shunt devices on system operations can be a concern.  One such impact is a high operating voltage that may show only a small dip before the demand reaches a voltage collapse condition.  This effect can be viewed from the perspective of the P-V curve.  The P-V curve is a plot of voltage at a monitored bus as load or transfer is increased.  A typical curve is shown below.  The curve has a "knee" shape and voltage collapse eventually occurs at a level of load or transfer referred to as the MW or voltage collapse limit.

When shunt compensation is added to the system, the shape of the P-V curve changes, flattening the portion at lower load or transfer levels and extending the P-V curve such as to obtain a higher MW voltage collapse limit.  Shunt compensation also raises the voltage such that voltage collapse may occur at near or above normal operational levels.  (See figure at right.)  Hence, shunt compensation reduces observability by masking the onset of voltage collapse from the normally monitored parameter of system voltage.

Definitions  

Observability is the aspect of the power system that provides operators with indicators to anticipate potential problems, in this case, voltage instability. The indicators need to be of a form that operators can observe and monitor in order to have sufficient time to respond to a system disturbance. For voltage stability, the traditional observed quantities are voltages at various nodes of the power system. However, when operating a system with high compensation levels, the pre-contingency voltage can be high, and voltage may not be a sufficient monitored parameter to observe the voltage stability of the grid.

Controllability is the aspect of the power system that provides operators with sufficient control and response capability to maintain reliability, or in this specific case, voltage stability. The controls available to the operators of today include dispatch, voltage schedules and switching of capacitors and lines, and load adjustments. At high levels of shunt compensation, the sensitivity of voltage to the controls is quite high, i.e., small shifts in dispatch and voltage schedules may result in significant shifts in the voltage stability of the system. The primary causes of this sensitivity are shunt compensation devices in SVCs and capacitor banks whose reactive output vary as the square of the terminal voltage. As voltage shifts take place, the total reactive output from the static devices changes, and needs to be balanced by the reactive output from rotating devices.

Solutions

To enhance observability, additional monitored parameters are required. One of the most commonly used indicators is the reactive reserve. This is a measure of the level of reactive output of generators and static devices. Although this measure is typically determined from Q-V curves, dynamic simulation offers a better alternative in identifying optimal levels of reactive output from various reactive sources. Other indicators that have been used in Europe and the United States include

• the rate of voltage change per MW change in load,
• the rate of reactive output change per MW change in load, and
• online dynamic security assessment tools.

To improve controllability, additional supervisory and supplementary controls may be specified.  these my take the form of wide-area monitoring or control, or a centralized reactive power controller.

Conclusion

The result of high levels of shunt compensation added to a power system is a loss of observability and controllability.  But there is a certain economic incentive in adding shunt compensation in place of other alternatives, including constructing new transmission.  In concept, switched and controlled shunt devices can be added to increase transmission capacity up to the exiting thermal limit.  Hence, some very large SVCs, in the rating size above 500 MVA, have been implemented, are under construction or are in the planning stages.

To compensate for loss of observability through the classical means of monitoring system voltages, operators need additional observed parameters and/or an online security assessment tool.  To address the increase in difficulty of controlling the power system through the classical methods such as switching and adjusting voltage setpoint, operators need further supplementary controls such wide-area monitoring and control to be able to continue to maintain a reliable, voltage-stable system. 

References

1. Pterra Consulting, "South Island Grid Upgrade Project Dynamic Voltage Stability Study Final Report,", September, 2007.
2. A Berizzi, S. Corse, D. Dosi, P. Finazzi, P. Marannino, and S. Corsi, “First and second order methods for voltage collapse assessment and security enhancement,” IEEE Trans. Power Syst., vol. 13, pp. 543–551, May 1998.
3. C. Canizares, F. Alvarado, C. L. DeMarco, I. Dobson, and W. F. Long, “Point of collapse and continuation methods for large ac/dc systems,” IEEE Trans. Power Syst., vol. 8, pp. 1–8, Feb. 1993.
4. Zima, M.; Larsson, M.; Korba, P.; Rehtanz, C.; Andersson, G., "Design aspects for wide-area monitoring and control systems," Proceedings of the IEEE, Volume 93, Issue 5, May 2005 .

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