Power Quality Monitoring

Power quality monitoring is the process of gathering, analyzing, and interpreting raw measurement data into useful information. The process of gathering data is usually carried out by continuous measurement of voltage and current over an extended period. The process of analysis and interpretation has been traditionally performed manually, but recent advances in signal processing and artificial intelligence fields have made it possible to design and implement intelligent systems to automatically analyze and interpret raw data into useful information with minimum human intervention.

Power quality monitoring programs are often driven by the demand for improving the systemwide power quality performance. Many industrial and commercial customers have equipment that is sensitive to power disturbances, and, therefore, it is more important to understand the quality of power being provided. Examples of these facilities include computer networking and telecommunication facilities, semi conductor and electronics manufacturing facilities, biotechnology and pharmaceutical laboratories, and financial data-processing centers Hence, in the last decade many utility companies have implemented extensive power quality monitoring programs.

Before embarking on any power quality monitoring effort, one should clearly define the monitoring objectives. The monitoring objectives often determine the choice of monitoring equipment, triggering thresholds, methods for data acquisition and storage, and analysis and interpretation requirements. Several common objectives of power quality monitoring are summarized here.

Monitoring to characterize system performance. 

This is the most general requirement. A power producer may find this objective important if it has the need to understand its system performance and then match that system performance with the needs of customers. System characterization is a  proactive approach to power quality monitoring. By understanding the normal power quality performance of a system, a provider can quickly identify problems and can offer information to its customers to help them match their sensitive equipment’s characteristics with realistic power quality characteristics.

Monitoring to characterize specific problems. 

Many power quality service departments or plant managers solve problems by performing short-term monitoring at specific customer sites or at difficult loads. This is a reactive mode of power quality monitoring, but it frequently identifies the cause of equipment incompatibility, which is the first step to a solution.

Monitoring as part of an enhanced power quality service. 

Many power producers are currently considering additional services to offer customers. One of these services would be to offer differentiated levels of power quality to match the needs of specific customers. A provider and customer can together achieve this goal by modifying the power system or by installing equipment within the customer’s premises. In either case, monitoring becomes essential to establish the benchmarks for the differentiated service and to verify that the utility achieves contracted levels of power quality.

Monitoring as part of predictive or just-in-time maintenance. 

Power quality data gathered over time can be analyzed to provide information relating to specific equipment performance. For example, a repetitive arcing fault from an underground cable may signify impending cable failure, or repetitive capacitor-switching restrikes may signify impending failure on the capacitor-switching device. Equipment maintenance can be quickly ordered to avoid catastrophic failure, thus preventing major power quality disturbances which ultimately will impact overall power quality performance.

Benchmarking Process

Electric utilities throughout the world are embracing the concept of benchmarking service quality. Utilities realize that they must understand the levels of service quality provided throughout their distribution systems and determine if the levels provided are appropriate. 

This is certainly becoming more prevalent as more utilities contract with specific customers to provide a specified quality of service over some period of time. The typical steps in the power quality benchmarking process are

1. Select benchmarking metrics. The EPRI RBM project defined several performance indices for evaluating the electric service quality. A select group are described here in more detail.

2. Collect power quality data. This involves the placement of power quality monitors on the system and characterization of the performance of the system. A variety of instruments and monitoring systems have been recently developed to assist with this labor-intensive process. 

3. Select the benchmark. This could be based on past performance, a standard adopted by similar utilities, or a standard established by a professional or standards organization such as the IEEE, IEC, ANSI, or NEMA.

4. Determine target performance levels. These are targets that are appropriate and economically feasible. Target levels may be limited to specific customers or customer groups and may exceed the benchmark values.

The benchmarking process begins with selection of the metrics to be used for benchmarking and evaluating service quality. The metrics could simply be estimated from historical data such as average number of faults per mile of line and assuming the fault resulted in a certain number of sags and interruptions. However, electricity providers and consumers are increasingly interested in metrics that describe the actual performance for a given time period. The indices developed as part of the EPRI RBM project are calculated from data measured on the system by specialized instrumentation. 

Electric utilities throughout the world are deploying power quality monitoring infrastructures that provide the data required for accurate benchmarking of the service quality provided to consumers. These are permanent monitoring systems due to the time needed to obtain accurate data and the importance of power quality to the end users where these systems are being installed. For most utilities and consumers, the most important power quality variation is the voltage sag due to short-circuit faults. Although these events are not necessarily the most frequent, they have a tremendous economic impact on end users. The process of benchmarking voltage sag levels generally requires 2 to 3 years of sampling. These data can then be quantified to relate voltage sag performance with standardized indices that are understandable by both utilities and customers.

Finally, after the appropriate data have been acquired, the service provider must determine what levels of quality are appropriate and economically feasible. Increasingly, utilities are making these decisions in conjunction with individual customers or regulatory agencies. The economic law of diminishing returns applies to increasing the quality of electricity as it applies to most quality assurance programs. Electric utilities note that nearly any level of service quality can be achieved through alternate feeders, standby generators, UPS systems, energy storage, etc. However, at some point the costs cannot be economically justified and must be balanced with the needs of end users and the value of service to them.

Most utilities have been benchmarking  reliability for several decades. In the context of this book, reliability deals with sustained interruptions. IEEE Standard 1366-1998 was established to define the benchmarking metrics for this area of power quality. The metrics are defined in terms of system average or customer average indices regarding such things as the number of interruptions and the duration of interruption (SAIDI, SAIFI, etc.). However, the reliability indices do not capture the impact of loads tripping off-line for 70 percent voltage sags nor the loss of efficiency and premature equipment failure due to excessive harmonic distortion.

Interest in expanding the service quality benchmarking into areas other than traditional reliability increased markedly in the late 1980s. This was largely prompted by experiences with power electronic loads that produced significant harmonic currents and were much more sensitive to voltage sags than previous generations of electromechanical loads. In 1989, the EPRI initiated the EPRI Distribution Power Quality (DPQ) Project, RP 3098-1, to collect power quality data for distribution systems across the United States. Monitors were placed at nearly 300 locations on 100 distribution feeders, and data were collected for 27 months. The DPQ database contains over 30 gigabytes of power quality data and has served as the basis for standards efforts and many studies. The results were made available to EPRI member utilities in 1996. 

Upon completion of the DPQ project in 1995, it became apparent that there was no uniform way of benchmarking the performance of specific service quality measurements against these data. In 1996, the EPRI completed the RBM project, which provided the power quality indices to allow service quality to be defined in a consistent manner from one utility to another. The indices were patterned after the traditional reliability indices with which utility engineers had already become comfortable. Indices were defined for

1. Short-duration rms voltage variations. These are voltage sags, swells, and interruptions of less than 1 min.

2. Harmonic distortion.

3. Transient overvoltages. This category is largely capacitor-switching transients, but could also include lightning-induced transients.

4. Steady-state voltage variations such as voltage regulation and phase balance. 

Power Quality Terms

So that you will be better able to understand the material in this book, we have included the definitions of many common power quality terms that are relevant to the material in this book. For the most part, these definitions coincide with current industry efforts to define power quality terms. We have also included other terms relevant to the material in this book. Active filter Any of a number of sophisticated power electronic devices for eliminating harmonic distortion. See passive filter. CBEMA curve A set of curves representing the withstand capabilities of computers in terms of the magnitude and duration of the voltage disturbance.

Developed by the Computer Business Equipment Manufacturers Association (CBEMA), it had become the de facto standard for measuring the performance of all types of equipment and power systems and is commonly referred to by this name. CBEMA has been replaced by the Information Technology IndustryCouncil (ITI), and a new curve has been developed that is commonly referred to as the ITI curve. See ITI curve. 

common mode voltage The noise voltage that appears equally from current-carrying conductor to ground.

coupling A circuit element, or elements, or a network that may be considered common to the input mesh and the output mesh and through which energy may be transferred from one to another.

crest factor A value reported by many power quality monitoring instruments representing the ratio of the crest value of the measured waveform to the root mean square of the fundamental. For example, the crest factor of a sinusoidal wave is 1.414.

critical load Devices and equipment whose failure to operate satisfactorily jeopardizes the health or safety of personnel, and/or results in loss of function, financial loss, or damage to property deemed critical by the user.

current distortion Distortion in the ac line current. See distortion.

differential mode voltage The voltage between any two of a specified set of active conductors. dip See sag.

distortion Any deviation from the normal sine wave for an ac quantity.

fast tripping Refers to the common utility protective relaying practice in which the circuit breaker or line recloser operates faster than a fuse can blow. Also called fuse saving. Effective for clearing transient faults without a sustained interruption, but is somewhat controversial because industrial loads are subjected to a momentary or temporary interruption.

fault Generally refers to a short circuit on the power system. fault, transient A short circuit on the power system usually induced by lightning, tree branches, or animals, which can be cleared by momentarily interrupting the current.

ferroresonance An irregular, often chaotic type of resonance that involves the nonlinear characteristic of iron-core (ferrous) inductors. It is nearly always undesirable when it occurs in the power delivery system, but it is exploited in technologies such as constant-voltage transformers to improve the power quality.

flicker An impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time.

frequency deviation An increase or decrease in the power frequency. The duration of a frequency deviation can be from several cycles to several hours. 

frequency response In power quality usage, generally refers to the variation of impedance of the system, or a metering transducer, as a function of frequency.

fundamental (component) The component of order 1 (50 to 60 Hz) of the Fourier series of a periodic quantity.

ground A conducting connection, whether intentional or accidental, by which an electric circuit or electrical equipment is connected to the earth, or to some conducting body of relatively large extent that serves in place of the earth. Note: It is used for establishing and maintaining the potential of the earth (or of the conducting body) or approximately that potential, on conductors connected to it, and for conducting ground currents to and from earth (or the conducting body).

ground electrode Aconductor or group of conductors in intimate contact with the earth for the purpose of providing a connection with the ground.

ground grid A system of interconnected bare conductors arranged in a pattern over a specified area and on or buried below the surface of the earth. The primary purpose of the ground grid is to provide safety for workers by limiting potential differences within its perimeter to safe levels in case of high currents that could flow if the circuit being worked became energized for any reason or if an adjacent energized circuit faulted. Metallic surface mats and gratings are sometimes utilized for the same purpose. This is not necessarily the same as a signal reference grid.

ground loop A potentially detrimental loop formed when two or more points in an electrical system that are nominally at ground potential are connected by a conducting path such that either or both points are not at the same ground potential.

 

ground window The area through which all grounding conductors, including metallic raceways, enter a specific area. It is often used in communications systems through which the building grounding system is connected to an area that would otherwise have no grounding connection.

harmonic (component) A component of order greater than 1 of the Fourier series of a periodic quantity.

harmonic content The quantity obtained by subtracting the fundamental component from an alternating quantity.

harmonic distortion Periodic distortion of the sine wave. See distortion and total harmonic distortion (THD).

harmonic filter On power systems, a device for filtering one or more harmonics from the power system. Most are passive combinations of inductance, capacitance, and resistance. Newer technologies include active filters that can also address reactive power needs.

harmonic number The integral number given by the ratio of the frequency of a harmonic to the fundamental frequency.

harmonic resonance A condition in which the power system is resonating near one of the major harmonics being produced by nonlinear elements in the system, thus exacerbating the harmonic distortion. 

impulse A pulse that, for a given application, approximates a unit pulse or a Dirac function. When used in relation to monitoring power quality, it is preferable to use the term impulsive transient in place of impulse.

impulsive transient A sudden, nonpower frequency change in the steady-state condition of voltage or current that is unidirectional in polarity (primarily either positive or negative).

instantaneous When used to quantify the duration of a short-duration variation as a modifier, this term refers to a time range from one-half cycle to 30 cycles of the power frequency.

instantaneous reclosing A term commonly applied to reclosing of a utility breaker as quickly as possible after an interrupting fault current. Typical times are 18 to 30 cycles.

interharmonic (component) A frequency component of a periodic quantity that is not an integer multiple of the frequency at which the supply system is designed to operate (e.g., 50 or 60 Hz).

interruption, momentary (electrical power systems) An interruption of a duration limited to the period required to restore service by automatic or supervisory-controlled switching operations or by manual switching at locations where an operator is immediately available. Note: Such switching operations must be completed in a specified time not to exceed 5 min.

Short-Duration Voltage Variations

This category encompasses the IEC category of voltage dips and short interruptions. Each type of variation can be designated as  instantaneous, momentary, or temporary, depending on its duration as defined in Table 2.2.

Short-duration voltage variations are caused by fault conditions, the energization of large loads which require high starting currents, or intermittent loose connections in power wiring. Depending on the fault location and the system conditions, the fault can cause either tempo rary voltage drops (sags), voltage rises (swells), or a complete loss of voltage (interruptions). The fault condition can be close to or remote from the point of interest. In either case, the impact on the voltage during the actual fault condition is of the short-duration variation until protective devices operate to clear the fault.

Interruption
 

An interruption occurs when the supply voltage or load current decreases to less than 0.1 pu for a period of time not exceeding 1 min. Interruptions can be the result of power system faults, equipment failures, and control malfunctions. The interruptions are measured by their duration since the voltage magnitude is always less than 10 percent of nominal. The duration of an interruption due to a fault on the utility system is determined by the operating time of utility protective devices. Instantaneous reclosing generally will limit the interruption caused by a nonpermanent fault to less than 30 cycles. Delayed reclosing of the protective device may cause a momentary or temporary interruption. The duration of an interruption due to equipment malfunctions or loose connections can be irregular.

Some interruptions may be preceded by a voltage sag when these interruptions are due to faults on the source system. The voltage sag occurs between the time a fault initiates and the protective device operates. Figure 2.5 shows such a momentary interruption during which voltage on one phase sags to about 20 percent for about 3 cycles and then drops to zero for about 1.8 s until the recloser closes back in.

  

Sags (dips)

A sag is a decrease to between 0.1 and 0.9 pu in rms voltage or current at the power frequency for durations from 0.5 cycle to 1 min.

The power quality community has used the term sag for many years to describe a short-duration voltage decrease. Although the term has not been formally defined, it has been increasingly accepted and used by utilities, manufacturers, and end users. The IEC definition for this phenomenon is dip. The two terms are considered interchangeable, with sag being the preferred synonym in the U.S. power quality community.

  

Terminology used to describe the magnitude of a voltage sag is often confusing. A “20 percent sag” can refer to a sag which results in a voltage of 0.8 or 0.2 pu. The preferred terminology would be one that leaves no doubt as to the resulting voltage level: “a sag to 0.8 pu” or “a sagwhose magnitude was 20 percent.” When not specified otherwise, a 20 percent sag will be considered an event during which the rms voltage decreased by 20 percent to 0.8 pu. The nominal, or base, voltage level should also be specified.

Voltage sags are usually associated with system faults but can also be caused by energization of heavy loads or starting of large motors. Figure 2.6 shows a typical voltage sag that can be associated with a single-line-to-ground (SLG) fault on another feeder from the same substation. An 80 percent sag exists for about 3 cycles until the substation breaker is able to interrupt the fault current. Typical fault clearing times range from 3 to 30 cycles, depending on the fault current magnitude and the type of overcurrent protection. Figure 2.7 illustrates the effect of a large motor starting. An induction motor will draw 6 to 10 times its full load current during start-up. If the current magnitude is large relative to the available fault current in the system at that point, the resulting voltage sag can be significant. In this case, the voltage sags immediately to 80 percent and then gradually returns to normal in about 3 s. Note the difference in time frame between this and sags due to utility system faults. 

  

Until recent efforts, the duration of sag events has not been clearly defined. Typical sag duration is defined in some publications as ranging from 2 ms (about one-tenth of a cycle) to a couple of minutes. Undervoltages that last less than one-half cycle cannot be characterized effectively by a change in the rms value of the fundamental frequency value. Therefore, these events are considered  transients. Undervoltages that last longer than 1 min can typically be controlled by voltage regulation equipment and may be associated with causes other than system faults. Therefore, these are classified as long-duration variations.

Sag durations are subdivided here into three categories—instantaneous, momentary, and temporary—which coincide with the three categories of interruptions and swells. These durations are intended to correspond to typical utility protective device operation times as well as duration divisions recommended by international technical organizations.