Weigh the financial effects of power quality variations on
your system against the costs of implementing improvements
The costs associated with power outages at commercial facilities like banks, data centers, and customer service centers can be tremendous, ranging from thousands to millions of dollars for a single interruption. The costs to manufacturing facilities can be just as high, if not higher. And manufacturing facilities can be sensitive to a wider range of power quality disturbances than just outages that are counted in traditional reliability statistics. Voltage dips that last less than 100 milliseconds can have the same effect on an industrial process as an outage that lasts several minutes.
Determining the optimum supply system and electric system characteristics for industrial facilities requires an evaluation of many alternatives. Power quality can be improved by adding power conditioning for selected equipment or raising the bar for specifications and equipment design on either the utility or end-user side of the meter. But as you might imagine, all of these alternatives have different costs and associated benefits.
Basically, the evaluation of power quality improvement alternatives is an exercise in economics. You must evaluate the economic effects of the power quality variations against the costs of improving performance for the different alternatives. The best choice will depend on the costs of the problem and the total operating costs of the various corrective measures.
The economic evaluation procedure. The various technologies for improving power quality must be evaluated in terms of cost and the expected performance improvements they can provide. The improved performance is translated into economic benefits, based on the expected costs associated with the power quality variations. With the costs of the different technologies and the expected benefits, you can compare the different technologies to determine which one will yield the best return on investment, which is defined by the lowest total costs, including the costs of the power quality problems plus the costs of the investment required to improve the quality of power.
This process consists of the following four steps, which can also be demonstrated by the flow chart in Fig. 1 above:
Step 1: Characterize expected performance. The first step in the process is trying to understand the kinds of disturbances that occur on the system and how frequently they occur. For most industrial facilities, the most important disturbances will be voltage sags and momentary interruptions, even though electric utility reliability statistics don't include them.
You can characterize the expected performance using the magnitude and duration data for voltage sags that occur at the facility. Fig. 2 is an example graph of magnitude and duration characteristics for power quality events at a plastics manufacturing facility, plotted along with the equipment ride-through levels set by the standard for semiconductor manufacturing equipment, SEMI F47. This standard was developed to help improve the ride-through characteristics of equipment purchased for semiconductor processes. You can use it as a guide for many other industries. The circled data points represent those events that resulted in a process interruption. Obviously, the equipment at this facility does not yet comply with the SEMI F47 standard.
Step 2: Determine disruption costs. Costs associated with sag events can vary significantly, from almost nothing to several million dollars per event. They'll vary not only among different industry types and individual facilities but also with market conditions. For example, you can expect higher costs if the end product is in short supply and there is limited ability to make up for the lost production. Not all costs are easily quantified or truly reflect the urgency of avoiding the consequences of a voltage sag event.
You can classify the cost of a power quality disturbance as belonging to one of three major categories:
Product-related losses, such as loss of product/materials, lost production capacity, disposal charges, and increased inventory requirements.
Labor-related losses, such as idled employees, overtime, cleanup, and repair.
Ancillary costs, such as damaged equipment, lost opportunity cost, and penalties due to shipping delays.
Focusing on these three categories will help you develop a detailed list of all costs associated with a disturbance.
For the economic analysis, you can often start with the cost of a momentary interruption. This almost always causes a disruption to the process, unless the facility owner has already made specific investments in power conditioning equipment. Table 1 summarizes example costs for different industries.
You can then express the costs of voltage sags using weighting factors for different sag characteristics. For example, you might assign a weighting factor of 1.0 to an interruption or an event in which the voltage level drops below 10% nominal. Or you might assign a factor of 0.6 to a voltage sag of 70% and 80%.
You can use this information in combination with the expected number of events at the specific facility to determine the annual costs associated with voltage sags and momentary interruptions.
Step 3: Evaluate possible improvement options. A wide range of potential solutions, with varying degrees of cost and effectiveness, are available to improve the process performance during voltage sags. You can apply the solutions at different levels or locations within the electrical system.
The four major options are:
Supply system modifications (premium power)
Service entrance technologies that protect the entire plant
Power conditioning at equipment locations within a facility
Equipment solutions (specifications, design, local power conditioning)
In general, the cost of these solutions increases as the power level of the load that must be protected does. This means you can usually save even more money if you can isolate (and individually protect) sensitive equipment or controls from equipment that doesn't need protection.
Ideally, the appropriate ride-through will be part of the equipment design. That was the motivation for the SEMI F-47 equipment standard. However, it's often not a practical option when trying to improve the operation of an existing facility. Original equipment manufacturers may also be reluctant to incorporate “voltage sag ride-through” capabilities in their equipment because the added costs may not translate into an appropriate perceived value for many of their customers. Manufacturers are more inclined to offer a “voltage sag ride-through” option that could be purchased by those customers with the need.
You need to characterize each solution technology in terms of cost and effectiveness. In broad terms, the solution cost should include initial procurement and installation expenses, operating and maintenance expenses, and any disposal and/or salvage value considerations. A thorough evaluation would include less obvious costs like “real estate” or “space-related” expenses and tax considerations. Table 2 provides an example of initial costs and annual operating costs for some general technologies used to improve performance for voltage sags and interruptions. These costs are constantly changing, so you shouldn't consider them indicative of any particular product.
Programmable logic controllers (PLCs) are often some of the components most vulnerable to voltage sags. In some cases, constant voltage transformers (CVTs), which have excellent voltage sag ride-through characteristics, are used to protect the PLCs. However, the I/O circuits, which provide the signals to the PLCs, are often left unprotected against voltage sags. In addition, you may also find the control circuits for each individual don't protect against voltage sags. Test results have shown that the drive controls and the PLC I/O circuits could be dominating the voltage sag sensitivity of the entire process line.
Protection of the controls and PLC, including the I/O circuits, typically doesn't need to come in the form of a UPS system. The rest of the process isn't protected for actual interruptions so there's really no reason to protect the controls. Therefore, either constant voltage transformers (CVTs) or dynamic sag correctors (DySCs) are the optimum solution when trying to protect these components.
These two options have similar costs and effectiveness when you take into account the installation costs and annual maintenance costs. They're treated as one option in the economic analysis.
Step 4: Find the best solution. The process of comparing the different alternatives for improving performance involves determining the total annual cost for each alternative, including both the costs associated with the power quality variations (remember, the solutions don't typically eliminate these costs completely) and the annualized costs of implementing the solution. The objective is to minimize these annual costs, which are the combined power quality costs and solution costs.
Comparing the different power quality improvement alternatives in terms of their total annual costs can help you zero in on the options that warrant more detailed investigations. You generally include the “do nothing” solution in the comparative analysis, and you typically identify it as the “base case.” The “do nothing” solution has a zero annual power quality solution cost but has the highest annual power quality costs.
Many of the costs (power quality, overhead, and maintenance) are, by their nature, annual costs. The costs associated with purchasing and installing various solution technologies are one-time up-front costs that you can annualize using an appropriate interest rate and assumed lifetime or evaluation period.
You calculate the power quality costs associated with the base case and the solution cases by determining the expected number of events that will affect the plant in each case. For each solution, you calculate the expected improvement in performance, and the power quality costs are reduced accordingly. Electrical power quality disturbances can have significant economic consequences for many different types of facilities. A wide variety of solution technologies exist for mitigating the consequences of such disturbances, and an economic evaluation of the range of alternatives can help identify the best option for a facility.
McGranaghan is vice president of consulting services, Stephens is engineering manager, and Roettger is a senior consultant, all for EPRI Solutions in Knoxville, Tenn.