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Transient Studies

Background

Transient overcurrents and overvoltages caused by switching operations or lightning strikes to electric facilities have significant potential to damage equipment or disrupt operation. High frequency transients have been recognized for some time as a threat (wide range of failures and misoperations) to electronic loads. Low and medium frequency oscillatory transients, such as utility capacitor bank switching, are usually of modest magnitudes but contain substantial energy, so their effects can be felt quite far electrically from the point of origin. Low frequency transients have also been strongly correlated with nuisance tripping of customer power-electronic equipment, such as some types of adjustable-speed drives.

Transient voltages and currents are a result of sudden changes within the electric power system. Opening or closing of a switch or circuit breaker causes a change in circuit configuration and the associated voltages and currents. A finite amount of time is required before a new stable operating point is reached. Lightning strokes to exposed transmission or distribution circuits inject a large amount of energy into the power system in a very short time, causing deviations in voltages and currents which persist until the excess energy is absorbed by dissipative elements (e.g., arresters). Both of these events cause a temporary departure of system voltage and current from their normal steady-state sinusoidal waveforms. All transients are caused by either connection or disconnection of elements within the electric circuit or injection of energy due to a direct or indirect lightning stroke or static discharge.

Types of Transient Studies

We use computer simulations to analyze potential switching problems associated with a variety of utility and customer equipment operations. Analysis results include recommendations for optimum solutions to possible switching problems. Analysis of various methods for controlling transient overvoltages is determined based on economic, control, and technical considerations. Specifications are determined for equipment required, including switching devices, current-limiting reactors, surge arresters, and customer surge control devices (TVSS). Representative transient studies previously completed include:

Analytical Capabilities and Simulation Tools

Computer simulations provide a convenient means to characterize power quality problems, predict disturbance characteristics, and evaluate possible solutions to problems. They should be performed in conjunction with monitoring efforts and measurements for verification of models and identification of important power quality problems. The models required for these simulations depend on the system characteristics and the power quality variations being evaluated. Transient simulations are generally performed using a time-domain analysis program (e.g., PSCAD). These programs are used to simulate electromagnetic, electromechanical, and control system transients in multiphase power systems. These are valuable tools for analysis of circuit switching operations, capacitor bank switching, lightning transients, and transients associated with power-electronic equipment operation.

Vacuum Switch Pre-strike
Vacuum Switch Pre-Strike Waveform

Transient studies involving use of these programs have objectives that fall in two general categories. One is design, which includes insulation coordination, equipment ratings, protective device specification, control systems design, etc. The other is solving operating problems such as unexplained outages or equipment failures.

Power conditioning for transient protection requires knowledge of the type of transient that is occurring. Generally, short term monitoring is all that is required to characterize switching transients, since these tend to occur frequently. If the problem is lightning, the time of equipment failure and misoperation is also important. Monitoring will often indicate lightning induced problems, and technology that can locate lightning strikes is now available. Lightning transient protection often requires a review of wiring and grounding practices as well, since this is important in determining how the transient propagates. High-frequency impulsive and lightning induced transients can generally be solved by applying surge suppression devices. However, these types of devices often do not provide adequate protection for low-to-medium frequency switching transients, such as those caused by utility capacitor bank energizing.

ASD Motor Transient Voltage
ASD Motor Transient Voltage Waveforms

We utilize state-of-the-art computer simulation and analysis tools. We have expertise in a wide range of technical evaluations including electrical transient analysis.

We use the following simulation and analysis tools:

These tools are used throughout the power industry, and allow us to work with a wide range of model data formats.


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