A static var compensator (SVC) is a shunt-connected electrical device for providing fast-acting reactive power support on high-voltage electrical transmission networks. SVCs are part of the flexible ac transmission system (FACTS) device family, regulating voltage and stabilizing the power system. Unlike synchronous condensers, which are rotating electrical machines, SVCs have no significant moving parts other than internal switchgear. Prior to the development of the SVC, power factor compensation was achieved using large rotating machines (e.g. synchronous condensers) or switched capacitor banks.
SVCs are gaining popularity because they are based on power electronic devices with a well-proven robustness to supply dynamic reactive power with faster response times and lower maintenance costs. A SVC typically includes a thyristor controlled reactor (TCR), thyristor-switched capacitors (TSCs), harmonic filters, and other associated equipment. The harmonic filters, which are used to control the harmonics produced by the TCR, are capacitive at the fundamental frequency. A dedicated transformer is generally used, meaning that the compensator equipment is connected to a medium voltage bus.
A SVC design includes a number of engineering studies to determine equipment ratings and potential problems for the surrounding power system, including dynamic performance, harmonic performance, electromagnetic transients, insulation coordination, grounding, and protection coordination. The harmonic impedance and filter equipment performance evaluations may require additional utility background harmonic measurements for completing the necessary calculations and simulations.
Harmonic currents will be generated by SVCs that include a TCR. A portion of the harmonic current will flow back into the utility transmission system, thereby causing possible equipment problems, such as component overheating, and protection system malfunction. Therefore, a SVC system with a TCR will also generally include shunt fixed harmonic filters to limit the harmonic currents injected into the transmission system. The design and specification of the harmonic filters and the selection of their component ratings often depends upon detailed harmonic studies. Guidelines for completing the harmonic performance and component rating studies are summarized in IEEE Std. 1031 (IEEE Guide for the Functional Specification of Transmission Static Var Compensators).
The harmonic studies determine the maximum harmonic levels at the SVC point-of-common-coupling (PCC) and the resulting maximum stresses on all SVC components. The maximum permissible current and voltage distortion planning levels for utility transmission systems are summarized in various international standards (e.g., IEEE Std. 519, IEC 61000-3-6). The measurements which are used during completion of the studies are collected using power analyzers equipped to trend power, harmonics, negative sequence voltage unbalance, flicker, and other power quality parameters to the latest industry standards (e.g., IEEE Std. 1159). The monitors can also provide fault recording of voltage sags and other events.
Harmonic Statistical Summary
To provide meaningful analysis of predicted SVC performance, the data collected should include monitoring of power quality and system disturbances as well as high-resolution trends. Many utility SCADA and fault recording systems can collect vast amounts of data; however, there is technology available now that can provide a greater level of detail in a user friendly, readily accessible format.
We use the Dranetz Encore Series™ system for the power quality monitoring. Co-developed by Dranetz, and Electrotek, this web-based monitoring platform is ideal for capturing the required data and making it available to interested parties using standard web browsers. The Encore Series can detect a variety of power system disturbances and offers accurate time stamping of data as well as optional GPS to aid in fault location and analysis.
We place monitoring equipment at the planned SVC substation, most specifically at the points of connection with the utility transmission system. Through our powermonitoring.comSM service service, we download data collected from the instruments and make it available via a password-protected website. Anyone with access privileges can view the data, along with analysis and summaries (via PQWeb®).
The instrument used for the background harmonic measurements is the Dranetz Encore Series. The instrument samples voltage at 256 points-per-cycle, current at 128 point-per-cycle, and follows the IEC 61000-4-3 method for characterizing harmonic measurement data. This involves analysis of continuous 200msec samples and storing aggregated 10-minute minimum, average, and maximum trend data up to the 50th harmonic.
Harmonic Distortion Histogram
Section B.5.3.3 of IEEE Std. 1031 includes guidelines for determining the effects of background harmonics and recommendations for gathering the background harmonic data. The guide suggests using harmonic voltages that are measured at the PCC over a sufficient period to include the effects from working days, holidays, changes in generation pattern, system outages, etc., or other seasonal effects. A typical monitoring period is one-to-six months. In addition, harmonics of order 2 to 21 may be analyzed. The measured harmonic voltages are commonly used behind an equivalent impedance to consider the effects on harmonic stresses for the SVC components.
A detailed monitoring report includes an analysis of the steady-state parameters enabled for trending during the monitoring period. Example quantities include rms phase-to-neutral (Van) and phase-to-phase (Vab) voltages, rms phase currents, frequency, rms harmonic voltages, total harmonic distortion (VTHD), negative sequence unbalance, short-term flicker (Pst), long-term flicker (Plt), interharmonic voltages and currents, telephone influence factor (TIF), as well as various digital quantities (e.g., circuit breaker open/close status). The power quality meter can also be enabled to record system transient and rms disturbances.
Harmonic Distortion Trend
The report includes background and site information, transducer ratings, system events, and the required measurement and statistical analysis, which is completed using the PQView® program (www.pqview.com).
Click here to download Electrotek's SVC harmonic monitoring brochure.