Power quality personnel at utilities and industrial facilities worldwide are familiar with the potential negative consequences of voltage sags. However, more than 80% of all power disturbances last less than 2 seconds, which means the overwhelming majority can be addressed by power quality solutions that regulate the voltage to ride through those events. Two of those solutions — and the subjects of this article — are American Superconductor's D-VAR system™ (for utilities) and PQ-IVR™ system (for industrial users). Such systems provide voltage regulation at the point on the grid where VAR support is needed most.
The D-VAR and PQ-IVR systems operate on microprocessor-based power electronic inverters. The inverters feature insulated gate bipolar transistor (IGBT) technology in a pulse-width-modulated (PWM) control scheme. IGBTs offer higher current densities and superior output characteristics — an advancement over older technologies like high-voltage power MOSFETs or power bipolar transistors. The inverters, which are capable of four-quadrant operation, control both sags and swells.
Each inverter module within a D-VAR or PQ-IVR system is rated at 250kVA (based on 480VAC), with a total system support of up to 8 MVAR of continuous output. In addition, both systems operate at up to 2.3 times their continuous rating for short durations. Therefore, more boost is provided for a protected load during momentary voltage sags.
The performance of both systems is completely dynamic (not stepped across a range), with response times in the subcycle range. In typical installations, recovery times occur within one cycle after a sag begins.
Each system is self-contained in a single semitrailer (see the photo), complete with controller, PLC, 480V output breakers, and control equipment. For utilities with voltage regulation needs that vary in time across their network, the trailer can be moved to where it's needed.
Voltage sags that degrade power on the utility grid can stem from a number of sources at the utility. These sources may include trips, recloses, fault depressions, commutations, and inductive electrical loads. Industrial loads that involve electric arc furnaces, large motor starts and stalls, and large capacitor-bank switching also can disrupt voltage on utility networks. Fortunately, D-VAR and PQ-IVR systems are suitable for a wide range of industrial and utility applications where voltage sags are known to occur.
As an example, consider the case of a system fault. Motors with 500 hp or more usually cannot regain speed and synchronize with the system on fault clearance. They may either stall, drawing over three times their rated current in the process, or trip. A PQ-IVR system located at an industrial facility can eliminate any chance of motor stalling. On one utility's 10MW load, voltage recovery took place in less than 1.5 cycles because of the PQ-IVR's injection (see Fig. 1).
In another example, a local utility successfully provided carryover protection to a semiconductor manufacturing facility that was experiencing a series of events on a 12kV line to the plant. Five faults took place during a 91-minute period, with the first three occurring over a span of just 24 seconds. A D-VAR system at the utility responded to those sags (of 15%, 24%, and 25%, respectively) and subsequently performed a correction to a sag that started as a 2-phase fault of 18% and ended as a 3-phase fault (see Fig. 2).
While it may seem counterintuitive, intentionally increasing supply system impedance can actually create more boost to counter larger voltage sags. In other words, added impedance can help leverage voltage sag correction on stronger utility supply systems.
A test with an 8 MVA PQ-IVR unit on a large-scale power system to restore load voltage confirmed the speed of voltage correction to be 1 cycle to 1 ½ cycles. Minimum voltage specified for the load was 93% of normal. In this test, the normal supply voltage experienced a sag to 85% of normal voltage and was followed 0.3 seconds later by a deeper sag to 61% of normal voltage (see Fig. 3, on page 36).
Wind farms also demonstrate the advantage of a D-VAR system over a conventional reactive system because of their varying energy output. Large wind farms connected to utility transmission grids must manage voltage fluctuations that arise from variations in the power source. Traditionally, wind farms use several banks of capacitors to compensate for these voltage variations. Capacitor-bank switching, however, produces excessive torque in the wind generators' gearboxes, which can lead to premature failure in the mechanical components. In addition, using capacitors alone to manage output voltage means a high volume of switching — a scenario that could cause premature failure of the capacitors and capacitor switches.
At the Wyoming Wind Energy Project (co-owned by PacifiCorp and the Eugene Water and Electric Board), a D-VAR system regulates the voltage and integrates it into the western utility grid. SeaWest Wind Power Inc. operates the 183-turbine project, which is connected to PacifiCorp's Foote Creek Rim substation in south central Wyoming. The wind farm currently produces 135MW of power, or enough electricity to meet the needs of 25,000 homes. Annual average winds run approximately 25 mph at Foote Creek Rim, making it ideal for wind energy projects. Most other projects rely on sustained winds in the 15 mph to 20 mph range.
The D-VAR's dynamic nature system makes it well suited for voltage regulation under these conditions. It can meet large reactive demands with a combination of leading continuous VAR output and switched capacitor banks. Voltage sags are mitigated through instantaneous VAR injections. The system also controls capacitor-bank switching events and eliminates harmful step voltage changes that can damage the wind turbine's gearbox.
In many cases where utility and industrial systems suffer from degraded power quality, devices that feature dynamic voltage regulation may provide the necessary means to achieve cleaner power and ensure better power flows throughout the grid.
Conventional technologies designed to regulate voltage fluctuations are series-connected devices; they can create their own threats to grid reliability. If they fail, they could take down the system they were designed to protect. In addition, other voltage regulation systems that use capacitive switching can introduce harmonic variances into the supply voltage, which, for many users, can be as undesirable as the original voltage fluctuations.
PQ-IVR systems, which employ shunt-connected inverters and stored energy, use the impedance of the transmission grid itself to help correct voltage sags at industrial loads. This makes for a worthwhile solution, especially when it also helps avoid the costly effects of voltage disruptions.
Charles W. Stankiewicz is the general manager of American Superconductor's power electronics systems business unit. You can reach him at 608-828-9150 or [email protected].