Adaptive Var Compensator (AVC) See Also: Static Var Compensator (STATCOM) The following information was provided by Trench Electric, so it may be biased toward them and/or their products. THE PROBLEM: Poor Power Quality, Voltage Instability, Flickering Lights, Low Electric Efficiency Utility Distribution systems supply commercial and industrial electrical loads which include Induction driven equipment such as elevators, Pumps, and conveyors. Due to their abrupt and rapidly varying mechanical Work cycles, corresponding Reactive Power surges are felt in the Power supply. As a result, voltage fluctuations of high magnitude and short duration (voltage flicker) occur, leading to poor power quality and adversely affecting equipment performance and operational costs. The total power (kVA) delivered by an electric utility consists of the real power (kW), which actually produces work, and the reactive power (kvar). A measure of the effectiveness of an electrical installation is given by the ratio of the real power to the total power, normally referred to as power factor. Therefore as the reactive power increases the system effectiveness decreases for the same real power load. The reactive power not only causes voltage drop but it also displaces transmission capacity, increasing system loss (see Losses). Most electric utility rate schedules (see Rates - Residential) incorporate a power factor penalty for those customers operating below a minimum average power factor value. Reactive power compensation, normally in the form of Capacitors, can bring the average power factor to the desired level. However, mechanical Switching of capacitors, is not fast enough to cope with rapid and frequent variations of reactive power. The capacitor switching mechanism also generates high Transient Currents. The resulting poor power quality can and does play havoc with Microprocessor based manufacturing processes and creates a multitude of problems for Computer based environments. Under certain conditions switching of fixed capacitance could give rise to self-excitation and/or damaging transient Torque on high Inertia Motors. In addition they may lead the network into a resonance condition. THE SOLUTION: The Adaptive Var Compensator (AVC) The AVC is a solid state (see Solid-State Physics) switched capacitor bank which can successfully compensate, within one cycle, with variable increments of reactive power, for any fast changes in reactive demand. Under development since 1981, by the University of Washington, and sponsored by Bonneville Power Administration (BPA) and Southern California Edison (SCE), the AVC has successfully been installed on various industrial and utilities systems since 1986. Trench Electric has attained the technology and manufacturing know-how to produce the AVC for the worldwide market. The AVC technology recognizes the drawbacks associated with mechanically switched capacitors and provides an unparalleled solution. The AVC is a stand alone, indoor or outdoor, power quality device which may be economically connected directly to any distribution level industrial, commercial or utility power system. Using patented electronic circuitry (see Electric Circuit), optic isolation and light activated SCR's, the AVC is much more than an automatic power factor controller. The AVC provides reactive compensation by switching preCharged capacitors, at the natural zero crossing of the current (see Alternating Current (AC) ) and with essentially zero voltage across the solid state switches, within one cycle of the power Frequency. The amount of compensation is adjusted on each cycle, on each phase independently, thereby adapting to the load's reactive demand. The AVC operates without introducing any transients or harmonics onto the electric system. The AVC monitors the voltage and current on each phase and independently determines the appropriate capacitance to compensate for the inductive load connected to the respective phase. The AVC can selectively be operated on a manual mode in which a local or remote operator determines the required compensation level or it can be operated in automatic mode in which the AVC determines, on a cycle by cycle basis, the appropriate amount of capacitance that must be switched on to achieve the following: 1. Provide reactive compensation adjusted on a cycle basis. 2. Maintain a predetermined voltage level on each phase, independently or on a time schedule basis. 3.Maintain a predetermined power factor on each phase. An optional Modem can be provided which will allow remote control or allow the AVC to be connected directly to Trench Electric for analysis of the device's operation. Advantages To Electric Utilities Optimized and fast reactive compensation Enhancement of voltage regulation Reduced reactive power demand Maximum utilization of installed generation (see Generator) and distribution capacity Reduced Transformer loading Reduced system losses Advantages To Industrial/ Commercial Users Enhancement of voltage regulation Elimination of voltage flickering Reduction of system losses Overall improvement in the power quality, resulting in enhanced equipment life Reduction of electric bill, ensuring rapid pay-back Increase of system capacity Potential Applications Wind generation Utility distribution feeders Rock crushing plants Saw mills Electric rail applications Steel mills Arc furnaces Office buildings (elevators) Pumps Mining applications Adjustable Speed Drive OPERATION OF THE AVC The AVC monitors the voltage and current flowing through each phase and, using signals proportional to the phase currents, when the voltage Waveform crosses zero (ie. the peak magnitude of the reactive current), determines the amount of compensating capacitance that must be connected to each phase to maintain the desired operating mode. Independent phase sensing of the reactive current allows capacitors of each phase to be switched independently, making the AVC suitable for unbalanced power systems. Any harmonic present on the line voltage or current is filtered out from the respective signals used by the AVC so that the determination of the required amount of compensation is unaffected by waveform distortion. The capacitors in each phase are charged to the negative peak of the supply voltage and remain so until a triggering signal is applied to the solid state switches (SCR's). The triggering signals are timed so as to set the switches into conducting mode only when the system voltage reaches its negative peak. At this point the potential across the switch is essentially zero and it coincides with the natural zero crossing of the capacitor current. In this fashion no harmonics are generated and no transients are created by the AVC operation. The switches are light activated and optically isolated from the electronic circuitry thereby eliminating the effects of Electromagnetic Field(s) , allowing the AVC to be connected to power lines at high voltages. The switching circuit of each phase consists of several binary ratio capacitors, connected in Y and in series with an SCR. Additionally, snubbing circuits and small series inductances provide transient protection to the SCR's. A microprocessor updates the switching pattern of the capacitors every cycle, to minimize the magnitude of the line reactive current. This output is a decision code which determines the proper value of capacitors to be switched on. Should a failure occur in the switching circuit resulting from power line transients or Lightning surges, a failure detection circuit will provide the best possible compensation for the given status of the switching circuit. The AVC has a built-in feature which identifies the nature of the malfunction. The AVC is enabled only when an interlock circuit detects currents from both sides of the device. This feature prevents any over-voltage, self-excitation, or resonant condition. Main Features Of The Avc Reactive compensation within one cycle. No harmonic distortion or transients are generated due to the AVC's fast and smart controller, in spite of line frequency shifts. Each phase is compensated independently, making the AVC suitable for improving unbalanced conditions. The voltage, the reactive power flow or the power factor is maintained at a specified value for all reactive load variations. The optimum amount of compensation required is determined and switched in or out on a cycle-by-cycle basis. The capacitors are on a binary ratio to provide optimum range of compensation. No additional transformer is required for the AVC, unlike other reactive power compensation devices. Resonance is inherently suppressed within a cycle. The AVC can be monitored and its settings changed via local Personal Computer (PC) or remotely via modem access. Failure of key components is automatically diagnosed and the controls are adjusted to still provide an optimum compensation for the given status of the remaining components. In case of failure, the AVC will fail safe. The AVC's construction is modular for ease of maintenance. The AVC is self-starting and does not require any special power source. Custom design allows for integration with other automatic warning/control systems.