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Utility Capacitor Bank Switching Studies


Utility capacitor bank switching can have negative impacts on power quality, especially for customer power systems. AC and dc drives, along with other electronic equipment, can be very sensitive to transient voltages. Utility capacitor bank switching transients can be magnified at low voltage capacitor locations on customer power systems, causing drives to trip and production and other processes to stop. In a time when customers are being allowed to choose their power provider, utilities cannot afford to be seen as the cause of customer power problems.

Studying, analyzing, and preventing utility capacitor bank switching events often requires the use of sophisticated computer simulation tools. Simulations provide a convenient way to characterize transient events, determine resulting power quality problems, and evaluate possible solutions. Frequently, simulations are performed in conjunction with power system monitoring for verification of models and identification of important power quality concerns. Application considerations include capacitor bank configurations, insulation withstand levels, switchgear capabilities, grounding, overcurrent protection, overvoltage protection, energy duties of protective devices, and unbalance detection.

There are a number of important transient-related concerns when transmission voltage level capacitor banks are applied. Transmission system concerns include insulation withstand level, switchgear capabilities, energy duties of protective devices, and system harmonic considerations. These considerations must be extended to also include distribution systems and customer facilities.

Capacitor Bank Energizing Transient
Capacitor Bank Energizing Transient

The capacitor bank energizing transient is important because it is one of the most frequent utility system switching operations. It can produce high phase-to-phase overvoltages on a terminating transformer, excite circuit resonances resulting in transient voltage magnification in secondary voltage networks, or cause problems with sensitive electronic equipment in customer facilities. Power quality symptoms related to utility capacitor bank switching include: customer equipment damage or failure (due to excessive overvoltage); nuisance tripping of adjustable-speed drives or other process equipment shutdown (due to dc bus overvoltage); transient voltage surge suppressors (TVSS) failure; and computer network problems (e.g., UPS cycling).

Electrotek's Capacitor Bank Application Study

Electrotek has developed a comprehensive approach for computer simulation and analysis of utility capacitor bank switching events. We use PSCAD to simulate electromagnetic, electromechanical, and control systems transients in multi-phase power systems.

Analytical methods provide the framework for evaluating a variety of power quality phenomena, including the impact of utility capacitor bank switching on the customer system. Typically, we develop a model of the customer system and relevant parts of the utility system to conduct transient switching surge analysis. This model can be used for simulations to predict power quality problems and evaluate possible solutions. Working with the customer, we collect and compile data for the model into a database for reference during the study.

We use computer simulations to analyze potential switching problems associated with capacitor bank applications. Analysis results include solution recommendations for possible switching problems. Analysis of various methods for controlling transient overvoltages is based on economic, control, and technical considerations. We also determine specifications for required equipment including switching devices, current limiting reactors, surge arresters, and customer surge control devices.

Primary concerns generally evaluated for a capacitor bank application study include

34.5kV Multiple Restrike Event
34.5kV Multiple Restrike Event

Study results lead to recommendations regarding the following:

Magnified Transient at Low Voltage Bus
Magnified Transient at Low Voltage Bus

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