If you're in charge of power quality at your facility, the very thought of a power outage — not to mention the slew of problems that accompany one — should make you shudder. But don't underestimate the damage voltage sags can impose on your systems. Typically affecting equipment and process operation much more frequently than actual outages, these disturbances occur whenever there is a fault on the power supply system, regardless of whether or not the fault actually causes an outage.
For instance, a short circuit in a transmission line rarely causes an outage (due to the network nature of the transmission system), but it can cause many customers to experience a short-duration voltage sag (typically 3 cycles to 12 cycles) that may affect many processes. This makes it even more important for you to understand the sensitivity of your equipment to voltage sags — and the options available for protecting it.
How many voltage sags can you expect to see? The number of voltage sags that can occur at your facility depends on where you're located, the characteristics of your utility's distribution system (underground vs. overhead, lengths of the distribution feeder circuits, and number of feeders), lightning level in the area, number of trees adjacent to the power lines, and several other factors.
From 1993 to 1995, EPRI conducted a major benchmarking project for utility distribution systems across the United States (An Assessment of Distribution System Power Quality, Volumes 1-3, TR-106926, V1-V3, EPRI, Palo Alto, Calif., 1996). Let's call this study DPQ1. Based on the results of this project, distribution utility customers could expect an average of about 18 events per year in which the minimum voltage would go below 70% (referred to as SARFI-70). This is much higher than the average number of outages a customer could expect, which is about 1.3.
EPRI performed a follow-up study (Distribution System Power Quality Assessment: Phase II, TR-1001678, EPRI, Palo Alto, CA, 2003) using results from the permanent monitoring systems that many utilities have in place. Let's call this study DPQ2. The particular focus of this study was on understanding voltage sag performance and the effect of various system factors on this performance. The results of this second study were very consistent with those from the EPRI DPQ1 study.
How sensitive is your equipment? The main factor that determines whether voltage sags will affect your facility is the sensitivity of the equipment in your process. The sensitivity curve developed by the Information Technology Institute Council (ITIC) shows that computer equipment should be able to ride through short-duration voltage sags, if the voltage doesn't go below 70% (Fig. 1). For sags of longer duration, voltages below 80% could affect the equipment.
The semiconductor industry developed a recommended equipment voltage sag tolerance curve that specifies improved ride-through for the first 200 milliseconds, which provides substantial benefit for many voltage sags. Figure 2 shows example voltage sags plotted along with the ITIC and SEMI F47 tolerance curves. (SEMI F47 refers to the semiconductor industry standard, Semiconductor Equipment and Materials International, 1999.)
The Table summarizes the expected voltage sag performance for different types of systems, comparing the performance you can expect if your equipment has ride-through characteristics specified by either ITIC or SEMI F47.
Unfortunately, you often don't know the actual sensitivity of the equipment within a facility; therefore, you don't know whether or not voltage sags will affect them. However, there is reference information you can use to judge equipment susceptibility to voltage sags. According to the book Electrical Power Systems Quality (ISBN 0-07-138622-X), there are three categories of equipment sensitivity:
Equipment sensitive only to the magnitude of a voltage sag. Here, the important characteristic is the sensitivity to the minimum (or maximum) voltage magnitude experienced during a sag (or swell), with the duration of the disturbance usually being of secondary importance. Devices in this category include undervoltage relays, process controls, motor drive controls, and many types of automated machines, such as semiconductor manufacturing equipment.
Equipment sensitive to both magnitude and duration of voltage sag. The important characteristic of this group is the sensitivity to the duration of which the rms voltage is below a specified threshold where the equipment trips. This group includes almost all equipment using electronic power supplies.
Equipment sensitive to characteristics other than magnitude and duration. Some devices are affected by other sag characteristics, such as phase unbalance during a sag event, the point-in-the-wave at which the sag is initiated, or any transient oscillations occurring during the disturbance.
These characteristics are more subtle than magnitude and duration, and their impacts are much more difficult to generalize.
Where's the best place to protect equipment? There are many options for protection of equipment sensitive to voltage sags. You can apply protection at levels ranging from the entire plant all the way down to individual components of a specific tool. If a significant portion of the equipment in the plant is sensitive, it might be worthwhile to consider protection at the plant level. However, it's unusual for all the equipment in your facility to require support during voltage sags. In fact, many types of equipment can ride through these short-duration events without a problem.
Adjustable-speed drive (ASD) manufacturers offer options for riding through voltage sags. Most motor loads can ride through voltage sags without affecting the process due to the inertia of the motor and the load — unless the motors drop out because of the sensitivity of contactors or relays protecting the motor.
Before you spend a lot of money on power conditioning equipment to protect your entire process, make sure the process controls themselves aren't causing the whole process to shut down during voltage sags. Programmable logic controllers (PLCs), relays, and contactors are often the most sensitive equipment. Contactor holding coils and relays can drop out during a voltage sag, resulting in the entire process shutting down. In these cases, it may be possible to improve the performance of the whole by protecting the circuit.
There are many options for protecting individual loads or groups of equipment within a facility. Although you can use traditional UPS systems, this may not be the best solution. If most of the events affecting the process are voltage sags and not interruptions, there are probably many more economical alternatives. Even if you need protection for short-duration interruptions, new options with flywheels, superconducting magnets, or capacitors for energy storage might be preferable because they're much smaller and don't require battery maintenance.
Other devices you can use at the equipment level include:
Constant voltage transformers (ferroresonant transformers), which are 1:1 single-phase devices that are excited at the high point on their saturation curves, provide an output voltage that is not significantly affected by input voltage variations.
Magnetic synthesizers, which are 3-phase devices that take advantage of their 3-phase magnetics to provide improved voltage sag support and regulation.
Active series compensators (1kVA to 5kVA, single-phase), which boost voltage by injecting a voltage in series with the remaining voltage during a voltage sag. These devices are also referred to as dynamic voltage restorers (DVRs), dynamic sag correctors (DySCs), or automatic voltage conditioners (AVCs).
For ASDs, you may be able to purchase an option for ride-through from the manufacturer. If not, one vendor offers a device to support the voltage in the DC link of the drive during voltage sag conditions, allowing the inverter to continue operating and supplying voltage to the motor until the input voltage returns to normal.
Of course, adopting equipment specifications that specify voltage sag ride-through levels, such as SEMI F47 (ride-through capability during short-duration voltage sags of 50% of nominal), can help assure a minimum capability at the equipment level. (See Sidebar below)
For most industrial facilities, you can use medium-voltage-rated active series compensators to boost the voltage during voltage sags. This type of protection can be much more economical and require less maintenance than UPS systems because little or no energy storage is needed.
It's also possible to improve performance through supply system modifications, such as static transfer switches and fast transfer switches. Static transfer switches, available in medium-voltage ratings, use power electronic switches to make the transfer within about a quarter of an electrical cycle. Fast transfer switches that use vacuum breaker technology can transfer in about two electrical cycles, which may be fast enough to protect many sensitive loads.
Finding the best option. Finding the optimum investment in technologies to improve voltage sag compatibility depends on the number of voltage sags you can expect, the cost of disruptions to your process, and the characteristics of your equipment.
Deciding on the best alternative for improving voltage sag ride-through performance at your facility is a problem that comes down to simple economics. First, you have to understand the sensitivity of the equipment and how much it costs every time a voltage sag negatively affects the equipment. Then you need information from your electric utility so you can estimate the number of voltage sags on your system per year. With this information in hand, you can then determine your costs associated with voltage sags. The optimum solution will minimize the combined costs of the ride-through solution and the resulting losses from the events not solved by the specific solution — the cost of the solution plus the cost of the disturbances.
The solution costs are lower as you focus on the particular equipment and controls that are sensitive. However, this approach may have additional costs associated with characterizing the sensitivity of the process components and installation. Understanding the sensitivity of all parts of your process is usually very worthwhile in coming up with the best solution.
McGranaghan is vice president of consulting services at EPRI Solutions in Knoxville, Tenn.
Editor's note: This piece is an adaptation of an article that first appeared in the September 2003 issue of EC&M. This version includes new information from a recent EPRI study, “Distribution System Power Quality Assessment: Phase II,” a listing of voltage sag performance levels, and voltage sag protection.
Sidebar: Improving Equipment Ride-Through Characteristics
Ultimately, better equipment design is the best long-term solution for voltage sag problems. If manufacturers offered options for improved ride-through, it might be more economical to purchase these options than to install external devices for protection.
The semiconductor industry, in cooperation with EPRI and electric utilities, recognized this and developed a set of standards to provide better compatibility between the equipment characteristics and the characteristics of the utility supply. The SEMI F47 standard specifies an improved voltage sag ride-through for process tools. It requires a ride-through down to 50% voltage for 200 milliseconds, which will significantly reduce the number of voltage sags that may cause process disruptions in semiconductor plants.
Many other industries can use this as a model to improve compatibility. In fact, EPRI Solutions has been working with utilities and industries such as the automotive industry and the food processing industry to help develop more widespread guidelines for equipment voltage sag performance.