Draft 0.01

February 2, 1998

Copyright ©1998 by the Institute of Electrical and Electronic Engineers, Inc.
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All rights reserved.

This is an unapproved draft of a proposed IEEE Standard, subject to change. Permission is hereby granted for IEEE Standards Committee participants to reproduce this document for purposes of IEEE standardization activities. Permission is also granted for member bodies and technical committees of ISO and IEC to reproduce this document for purposes of developing a national position. Other entities seeking permission to reproduce this document for these or other uses must contact the IEEE Standards Department for the appropriate license. Use of information contained in this unapproved draft is at your own risk.

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Table of contents

1. Overview *

2. References: *

3. Definitions *

4. Instrumentation classes *

5. Application classes *

6. Input voltage and current requirements *

7. Performance test and calibration procedures *

8. Bibliography *

Tutorial discussion on Transient assessment *

Tutorial discussion on long duration variations assessment *

Tutorial discussion on Voltage Imbalance assessment *

Instantaneous assessment and observation windows *

Observation intervals *

Disturbance level measurements adjusted in terms of equipment heating *

Survey duration *

Tutorial on voltage unbalance assessment requirements *

Tutorial on measured intervals *

Tutorial on Waveform distortion assessment *

1. Overview

This guide establishes the data acquisition attributes necessary to characterize the electromagnetic phenomena listed in table 2 of IEEE Std. 1159.

The objective of this guide is to describe the technical measurement requirements for each type of disturbance defined in 24 categories of typical characteristics of power system electromagnetic phenomena. Each category is discussed in several standards in terms of emission limits, severity levels, planning levels or immunity levels.

2. References:

(to be accumulated by each chapter chair)

(Definitions given in this section have not been harmonized with definitions made in other IEEE standards. Several terms need to be defined, however, and IEEE 1159.1 seeks volunteers for the harmonizing of definitions.)

Before scheduling the power quality measurement, the user must select the instrument which meets his needs. This section addresses the language and the assessment specification which need to be understood to communicate with the instrumentation manufacturer. Also, the user needs to define his need clearly.

Mean (): The mean of a number of measurements performed during a period or an interval of a given quantity is defined by

rms ()

The rms of a number of measurements performed during a period or an interval of a given quantity is defined by

Standard Deviation (s)

The standard deviation (s) is a measure of dispersion of a number of measurements performed during a period or an interval. An estimate of this value is

Standard error ()

The standard error indicates the disagreement between averages of the mean of measurements performed during several intervals when a fixed input level is applied.

Systematic Error (E_s)

Systematic error, or bias, is the difference between the mean measured value () and the actual value. This is also referred to as "constant error" but often the error is constant only through a particular set of measurements of observation. For example: a 5% systematic error over 100 measurements.

Random Error (E_r)

The random error is one which causes the measured value to occur randomly around an average value.

Accuracy

Accuracy is the degree of agreement between measured values and the true values (i.e. closeness to the reality).

Precision

Precision is an indication of the likelihood that a measurement will differ from the same measurement at a different, but not correlated, time. The assessment interval specification is also needed to assess precision values. For example, the precision could be assessed using measurements every 3-s interval.

Traceability

Traceability is an attribute of some measurements. Measurements have traceability to the designated standards if an only if scientifically rigorous evidence is produced on a continuing basis to show that the measurement process is producing measurement results (data) for which the total measurement uncertainty, relative to national or other designated standards is quantified.

Acquisition requirements should be well adapted to power quality measurement purposes, this justifies defining the application classes as related to usage. This guide addresses four application classes: benchmarking, trouble shooting, legal and laboratory.

5.1 Benchmarking class

Measurement for benchmarking aims to assess the "Quality Factors" and "Quality Indices" of an entire distribution system. The Quality Factors are the level of deformation of the voltage supply that has an active role in upsetting or damaging customers’ electronic or electrical equipment. The Quality Index is the ratio of total number of times per year, per customer in the survey, that the related Quality Factors exceeded the established planning levels. The planning levels are quality factor levels used for power system design and serve as reference for the power quality indices. These levels can be exceeded occasionally. It is intended for the coordination of the compatibility between power systems at different rated voltages.

Field surveys for benchmarking requests should be carried out to measure each Quality Factor at least once at a specific moment during one specific period. Unfortunately, the appropriate moment to perform this measurement is unknown since the event sought occurs when the disturbance reaches a level not exceeded during a percentage of the time. Therefore, the benchmarking application requires continuous recordings so as to assess the real cumulative function, since very low disturbance levels are now included in power quality assessments of distribution systems.

Steady-state disturbances such as power frequency, flicker (voltage fluctuations), voltage harmonics, voltage imbalance, and long duration voltage variations are reported in term of average or rms levels over a given interval. The sampling of each disturbance should be performed continuously and grouped to assess the rms value occurred during each contiguous interval. For example, a 10-min interval was used for several benchmarking surveys. The sought value for benchmarking should match a level not exceeded by a percentage of measurements cumulated during a given period. Several surveys for benchmarking the power system in Europe and one benchmarking survey performed in Canada reported that the 10-min rms level had not exceeded 95% of the measurements performed during one week per site. The value obtained is therefore valid for the specific week during which the measurement was performed. For benchmarking the power system, several sites should be measured during the 12 consecutive months of the survey. Each benchmarking survey addresses only one user type at a time. For example, the user type can be residential, commercial or industrial. The instrumentation may be moved from site to site during the survey period to supply the Quality Indices in terms of the number of times per year, per customer that recorded factors exceeded the planning level.

5.2 Trouble shooting class

The most common objective to perform measurement involves the preliminary investigation to identify loss sources such as production loss, high maintenance cost or premature damage to equipment. Accurate measurement during the preliminary investigation of a trouble shooting application may be not required until the measurement gives some indication that a particular distortion can cause the problem. Portable hand size meters that are available from several manufacturers can be used. Although, the assessment specifications for these meters are difficult to obtain, they are very convenient at this stage since they are easy to move around for measuring disturbance levels at several points of a plant. However, the user should select the measuring period during which the highest harmonic levels are expected to occur and several readings should be performed during that period.

Therefore, field testing for trouble shooting consists of recording the specific phenomenon that can be associated with a specific power quality problem. As opposed to the benchmarking class, the sought phenomenon can be provoked in several cases. For example, the user can measure the voltage disturbances during an induced event such as motor starting or arc-welder operation. Measurement for a trouble-shooting application class helps in the understanding of the phenomenon but may not necessarily be required to identify the problem. For example, one specific problem can be observed when inducing a motor starting and no instrumentation is needed to reach this conclusion. However, several phenomena such as transients, sag or resonance can occur during the induced event and only measurement with the appropriate instrumentation can indicate the difference between these phenomena. The induced event may not cause the problem each time. For example, the severity of a transient provoked by switching a capacitor bank onto the power system is a function of the operation sequence in the cycle. Capacitor switching transients may be negligible when they occur close to the normal zero crossing of voltage.

Contiguous measurement during long periods may not be necessary if the approach of inducing an event is possible and succeeds in the identification of the problem. However, continuous measurement may be useful. In this case, strip-chart recording and event trigger approaches may be appropriate for the trouble-shooting class.

5.3 Field test for legal issues

Measurements can be performed for contractual purposes to assess the disturbance emission from a particular load or to assess the voltage quality supplied by the utility. If an induced event is possible for the emission test, a few measurements may give the appropriate assessment of the emission level but the user needs to repeat the measurements several times so as to ensure that no other load or power-system configuration affects the result. Unless a single measurement demonstrates clearly a specific legal issue, continuous measurement during at least one week may be preferable.

5.4 Laboratory emission and immunity tests

Repetitive tests during one week are not normally useful if the operating condition and the test environment are well defined. In such a case measurement can be performed in a laboratory. Repetitive tests could be useful to demonstrate the repetitively of the result.

Instrumentation that completely and permanently meets specification is normally a dream. Any instrument may be out of the specification at any time. Drift problems may be accelerated by the environmental conditions. Therefore, the measurement integrity concept included in this guide helps to avoid data corruption during possible abnormal instrumentation operation due to unexpected environmental or random failure. The measurement integrity concept aims to avoid corrupted data influence on each assessment performed during the measurement period and is very useful for application classes requiring long period surveys.

Some manufacturers may specify the period during which the instrument can meet the specification. However, the manufacturer’s specification assumes specific use conditions which can be controlled during an application in laboratory. As other application classes require moving the instrument into uncontrolled environments, the accuracy period specified by the manufacturer should not be used to qualify the integrity during the measurement period.

Instrumentation designed for the benchmarking and legal classes should include features to ensure that the specification is met for the longest portion of the measurement period (>95% of the time for example). A good design provides a means to check the specification during the measurement period.

Specifications of products reflect the variability of components, disturbing processes and use environments. These "contributors" to the specification determine the specification limits that must be tested by the manufacturer. It should be kept in mind that the factory quality control conditions cannot simulate all environmental conditions and all types of disturbances to be recorded.

The variation of any "contributor" is subject to the following stresses:

1. Vibration during travel which affects the instrument specification performance,
2. Environmental conditions (EMC, temperature, humidity …)
3. Disturbances interfering with the assessment objective (phase shift and phase lock loop, transient and harmonic)
4. Aliasing condition.

6.3 Transients

The requirement for the data acquisition attributes necessary for the recording of PQ Events depends upon what type of events from Table 2 of IEEE 1159 are intended to be acquired. This can also vary within a category of event types, as for example voltage transients. These categories and the typical event characteristics are given in the table 6.1 below:

Table 6.1: Transient category and typical event characteristic

A. Attributes

1. B. Performance Criteria

 

1. Amplitude accuracy versus duration and point-on-wave of transient, including positive and negative transients.

2. Point-on-wave accuracy versus magnitude and duration

3. Duration accuracy versus magnitude and point-on-wave

4. Amplitude and frequency accuracy of oscillatory or ringing frequency.

5. Cross-channel coupling.

C. Test Waveforms

1. Lightning : 1.2 u x 50 µs

2. Surge Withstand Capability : 2 MHz exponentially decaying

3. PF Correction Cap switching In/Out : 2 pu, ½ cycle ring.

4. ANSI/IEEE C62.41-1991

The frequency of the power system is established by the angular velocity of mechanically connected prime movers and associated electrical generators. In large, networked power grids, the combined inertias of many large rotating generators operating synchronously provide an extremely stable system operating frequency. Power system deviations from nominal frequency rarely occur on large networks in developed countries. When significant deviations do occur, they usually result from slow clearance of severe faults, or cascading system failures that result in severe mismatches between available generation and connected loads. In less developed countries, there can be long term mismatches between available generation and connected loads that result in steady state under-frequency operation.

In small, isolated power systems, frequency deviations are more likely to occur when changes in connected loads are an appreciable fraction of the available generator kVA. Examples of this situation occur when isolated industrial loads are served from on-site generation and when commercial loads are served from on-site back-up generators. Remote communities served by local generators without a connection to a larger networked power grid may also experience frequency deviations due to switching on and off of "large" loads.

The results of frequency deviations on connected loads vary by type of load. Per IEEE Standard 446-1995, the majority of end-use equipment can tolerate frequency changes by as much as +/-0.5 Hz, which is slightly less than a one percent deviation on a 60 Hz system. Frequency deviations can affect the performance of electronic timers, where a timing interval is determined by detecting the zero-crossings of the ac supply voltage. Frequency deviations can also affect measurements of voltages where (frequency dependent) reactive components are involved.

Typically, power frequency variations on modern, interconnected power systems are rare. When they do occur on large systems, however, there exists the possibility for severe consequences such as sustained interruption of power. In such a scenario, frequency measurements are of interest in characterizing the dynamics of the power system during stressful conditions.

2. Measurement Techniques.

A variety of means are available for the measurement of synchronous power frequency. These include, and are not necessarily limited to, the following.

• Measured time interval between zero crossings of an ac input signal.
• Measured output frequency of a phase-locked-loop operating on an ac input signal.
• Use of a frequency-to-voltage converter, with an input derived from either the ac input signal or from a phase-locked-loop.
• FFT analysis of digitally sampled ac input waveform.

Rapid progress in electronic measurement technology of ac waveforms has opened the way for a variety of measurement application techniques. This standards document does not provide a specific recommended approach to ac frequency measurements, rather it outlines a test protocol for the "certification" of a meter’s ability to correctly measure ac input frequency under a variety of dynamic and steady-state conditions.

3. Instrumentation Test Protocol for Frequency Measurements

The objective of the frequency measurement test protocol is to determine the ability of a test instrument to properly report the frequency of a standard input signal. The input signal must be precisely defined, and have specific attributes that embrace the range of possible operating conditions in the field. The ability of the instrument in test to properly record [or indicate] the standard input signal will be measured in terms of acceptable levels of measurement uncertainty. Measurement uncertainty is defined as a function of the class of instrument under test. These measurement uncertainties are as defined below.

Standard Input Signal. The instrument under test shall have the following set of signal waveforms applied to its input channels. The test input signal will exercise the instrument in measuring off-nominal operating frequencies and rate-of change of frequency, both positive and negative.

A method to verify the performance specifications should be provided. These procedures should specify common, readily available test equipment (arbitrary [random??] waveform generator). If a specification is beyond the user's normal capacity to test, IEEE 1159 should describe an alternative method which can be implemented in the instrument or supplied as a separate unit.

8. Bibliography

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