When you hear the term “sensitive loads,” do you automatically think of data centers? If so, it may be time to broaden that perception. While the continuity of power to computers is critical, experience has shown that the switch-mode power supplies used in most computers are quite robust and able to operate smoothly even with moderately deformed voltage waveforms and varying frequencies. Truly sensitive electrical loads react negatively when the normal rms values of voltage waveforms change quickly from the expected nominals. Perhaps the best examples are semiconductor microchip fabrication tools and electronic variable frequency drives for motors. Voltage sags of a few cycles can create havoc with these devices, which in turn disrupt crucial processes and sensitive control systems. In this article, we'll discuss some of the major technologies currently available that mitigate power quality disturbances originating from utilities. We'll also review the current trends by manufacturers to make low- and medium-voltage devices more reliable and easier to apply.
Much has been written about the types of power disturbances that can affect sensitive devices. The vast majority of PQ-related events are caused by utility voltage sags, transients, and momentary and complete outages .
Other types of disturbances, such as voltage and current harmonics, generally stem from the interaction of devices within a facility's power system.
In a well-designed facility, transient voltage surge suppression (TVSS) systems are used to help mitigate high-speed voltage events. In addition, step-down transformers (delta-wye) and, in extreme cases, filters aid in harmonic suppression . In rare instances, some electrical loads produce major disturbances on the incoming utility line that are reflected back into the entire utility distribution line.
Some of the newest alternatives for mitigating power problems are systems designed to protect a facility's entire utility service entrance. Typically, critical loads are deployed in buildings or plants where needed and powered from the closest electrical panels. Unless facilities are designed with separate electrical distributions for critical loads, it's generally impractical to isolate voltage-sensitive systems in one area. Critical processes often require precise coordination of mechanical systems and electronic devices to function properly. These systems can require high power levels, making voltage stability to the entire load a challenge.
Power quality system manufacturers are making significant financial investments at the medium-voltage level. In recent years, semiconductor wafer fabrication plants have led the way in developing newer and better solutions to protect large (typically 10 MVA to 30 MVA) critical loads at voltage levels up to 25kV. Because of these load sizes, the typical customer has a utility source directly tied to or near the transmission system and tends to experience only voltage sags — as opposed to sags and outages. In addition, these loads typically have at least two utility feeds. These conditions offer more solution options with a broader range of costs. Let's turn our attention to some of those solutions now.
Dynamic voltage restorer (DVR) systems
Developed in the early 1990s, DVR systems improve power quality by injecting voltages of controlled phase and magnitude on the secondary of an injection transformer connected in series with a circuit feeding a critical load (see Fig. 1, on page 22). A capacitor bank stores a small amount of energy, which is the source of the injection voltage. Since most sags are the result of momentary distribution or transmission faults, a total sag event is generally less than 200 ms with magnitudes as low as 50% of nominal.
DVRs are generally rated for the amount of compensation being injected. For example, a DVR rated at 2 MVA could be installed on a 4 MVA load and provide sag mitigation on all three phases down to 50% of nominal. Fig. 2, on page 22, shows a typical utility sag down to 50% of nominal and an overlay of the corrected voltage after being boosted by a DVR.
Since most sags are not symmetrical on each phase, the DVR's sensing system must be capable of providing the appropriate amount of compensation needed on each phase. The degree of voltage boost can be adjusted as a function of the load. At lower load levels relative to the DVR rating, deeper sags can be corrected. If sags occur on only one phase of the utility, greater compensation can be achieved as well.
Photo 1 shows a 10 MVA, 12.47kV DVR at a semiconductor wafer fabrication plant in Virginia. The facility is protected for sags down to 50% for a duration of 30 cycles. DVR technology, which is generally designed to protect large-scale loads, has been scaled down to protect loads as low as 600kVA.
In cases where it's practical to isolate critical loads, low-voltage devices, such as the dynamic sag corrector (DySC), are available to help correct voltage sags on loads of 300kVA or smaller. The technology used in these low-voltage devices varies somewhat from DVRs in that there is typically no separate energy storage unit. They do not employ a transformer in the series voltage-injection device and are capable of boosting line voltage by using the remaining voltage and current available on either unsagged or sagged phases (in the case of 3-phase voltage sags).
Medium-voltage UPS systems
Large-scale critical load sites that experience deep voltage sags (50% or greater) coupled with occasional outages may require a mitigation system with a large amount of stored energy. Offline, medium-voltage UPS systems have been developed to ride-through complete outages for up to 60 sec. An offline design shrinks the system footprint and increases overall operating efficienty to 99%, resulting in lower operating costs. The stored energy allows enough time for backup generators to start and transfer loads to emergency power should long-term outages occur.
One of the largest installations featuring this medium-voltage UPS design is located at a semiconductor plant in the southwest United States (see Photo 2, on page 24). The site is fed directly by two independent transmission lines that are reduced to a distribution voltage of 12.47kV. Because there are no other customers between the plant and the transmission system, lightning strikes and utility equipment faults cause the majority of disturbances that impact the plant.
The plant has a total critical load consisting of mechanical systems and a large number of sensitive tools in the clean rooms. Utility records revealed that the majority of events were voltage sags down to 50% to 60% of nominal for periods up to 500 ms. The plant's owners wanted to install a protection system with total outage capability to ensure mitigation of short outages as well as sags.
In addition, because the utility had two independent sources, the customer felt it was prudent to install dual 10 MVA UPS systems with static source transfer, providing a double redundant capability. A large diesel-generator plant backs up both systems to provide total protection.
Dual source transfer switches (STSs)
As stated earlier, critical facilities of 10 MVA or greater most commonly have at least two incoming utility feeders. Depending upon the degree of independence between the two feeders, a cost-effective solution to sags and outages may be a high-speed STS. There are power-electronic switch devices available that allow the construction of STSs up to 1200A (continuous duty at 15kV) and as large as 600A at 25kV. These systems have the ability to switch large blocks of load between feeders with a typical sense-and-transfer time of 4 ms or ¼-cycle response. Ideally, the two sources should be from separate substations; at a minimum, they should be different buses in the same substation.
Opting for this solution requires an analysis to determine how many power quality events occur at the distribution level as opposed to the transmission level. If the majority of sags occur on the transmission system, the total effectiveness of an STS may be limited. Rapid switching of large blocks of load needs to be closely coordinated with the utility to ensure proper operation. In cost analyses for most critical processes, overall effectiveness in mitigating power quality events should be greater than 90%, as compared to other medium-voltage alternatives.
Superconducting magnetic energy storage (SMES)
Another way to provide deep sag protection is through the use of SMES systems. These systems store real power (approximately 1 sec) in a superconducting magnet and, when called on, supply energy to support loads of up to several megawatts. Most recently, SMES devices have been applied to help stabilize utility transmission during faults. These systems provide var support for power factor control and inject real energy into lines during faults to help restore transmission voltages.
Low-Voltage UPS Growth
Low-voltage UPS systems sized for an entire critical load and installed at the service entrance have gained acceptance in the industry, particularly by utilities and mission-critical customers. Facility managers desiring 100% mitigation of disturbances can opt for protecting the entire load, particularly in retrofit applications where downtime must be kept to a minimum. Low-voltage systems can help protect loads up to 2500kVA economically. High-power battery systems (30 sec to 60 sec) are most commonly used in larger units for energy storage, while flywheels are used in smaller systems.
Moving a UPS system outdoors is beneficial in many municipalities as officials enact stricter rules governing battery rooms. The shorter ride-through time helps reduce the overall space needed for the battery and greatly simplifies the total installation.
Going offline also improves battery management in two key areas: it eliminates DC ripple and yields true equalize charging. An offline system has no rectifier. Therefore, the battery is not exposed to ripple on the DC bus. The net result is an increase in battery reliability.
Likewise, the battery string “floats” at its open-circuit voltage. Photo 3 shows a 313kVA UPS retrofitted to protect an entire call center. The existing 300kW generator building appears in the background.
Dealing with Hostile Loads
Not all power quality disturbances are caused by problems with utility distribution or transmission systems. Some customers, including rock-crushing plants, saw mills, or arc-welding facilities draw “pulses” of current because of the mechanical or electrical characteristics caused by their processes.
For example, when rocks enter a crusher, the drive motors can experience near-lock rotor conditions that cause voltage sags, which are commonly referred to as voltage flicker. Repeated dips in excess of 5% can cause annoying pulsing in lighting systems and trigger customer complaints to the local utility. In some cases, this flicker can reach 10% to 20%, which can cause other customers' sensitive loads connected to the same utility feeder to malfunction.
Dynamic static compensator systems (DSTATCOMS) use high-speed power electronics inverters coupled with a control system. The control system senses these rapid voltage changes (flicker) and injects compensating voltages on a cycle-by-cycle basis to reduce or eliminate flicker conditions.
Photo 4, on page 25, shows a typical DSTATCOM installation at a rock-crushing site. This system is at the end of an 11-mile, 12.47kV feeder shared with 1000 customers. The site has numerous horsepower (200 hp to 250 hp) motors with a running load of 3 MVA. Installation of the DSTATCOM reduced an 8% to 12% voltage flicker problem to less than 4% of nominal.
Static VAR compensators (SVCs) are similar to DSTATCOMs and are used to balance capacitive and reactive power. SVCs typically consist of arrays of reactors and capacitors both switched via solid-state devices. An AC filtering array absorbs harmonics created by switching the reactors. The criteria to consider when choosing between a SVC or DSTATCOM are based on the speed of response required and the amount of flicker correction desired.
As technological advances continue to occur, the number and types of facilities with sensitive electrical loads continues to grow each year. Improvements in utility distribution schemes are allowing an increase in power reliability while lowering outage downtimes for a greater number of customers. Voltage sags and momentary outages have proven to be the most difficult to solve. Fortunately, there are new PQ solutions available to meet this challenge. Instead of conventional UPS systems distributed throughout a facility, more and more facility owners are installing premium-power solutions that protect whole buildings on the entire utility service entry.
Tim Wood is a senior applications engineer at the power electronics division of S&C Electric Co. in East Troy, Wis. You can reach him at [email protected].
Brad Roberts is director of marketing at the power electronics division of S&C Electric Co. in East Troy, Wis. You can reach him at [email protected].
Electric Power Research Institute (EPRI). EPRI TR-106294-V2, “An Assessment of Distribution System Power Quality, Volume 2.” May, 1996.
R. Dugan, M. McGranaghan, and H. Beaty. Electrical Power Systems Quality. McGraw-Hill, 1996.