Manufacturers recommend that users should not exceed 1% unbalance in the voltage supply to motors designed before 1978 (NEMA MG1, 1978). However, several laboratory tests have demonstrated that these motors could tolerate more than 2% unbalance [20] . Yet despite such evidence, manufacturers have been unwilling to change their specification, claiming that the weakness of the assessment technique available for field tests did not reflect the severity level of the laboratory tests.

In actual fact, most users still assess unbalance with three voltmeters to measure each phase-to-phase rms voltage and then calculate the ratio between the highest difference and the mean measured value. However, since the voltage and the power system impedance fluctuate over time, manufacturers find it unfair to compare the single instantaneous assessment value with the 2% applied during the laboratory tests over a period of several hours, which is long enough for motor temperature to reach a steady-state level. In fact, according to the rule of thumb used by most laboratories, the steady-state temperature is reached after a test duration equal to five times the heating time constant, i.e. 2 h or over.

The voltage unbalance measurement method should therefore use a severity scale based on the laboratory susceptibility test. This monitoring method, which was developed before 1930 as a means of providing protection against voltage unbalance, is based on the use of negative-sequence current time-delay relays.

Westinghouse and General Electric (GE), on the other hand, were recommending the use of an inverse time protection relay, based on the square of current unbalance multiplied by the disturbance duration, which was to be set at 40 (p.u.)² -s for Westinghouse motors [ref. 21] and 120 (p.u.)² -s for GE motors [ref. 22] . Cummings's method [ ref. 22] developed in 1980 was therefore used [ ref. 23] to calculate the positive- and negative-sequence components of voltage unbalance matching the recommended current for coordination and protection, and to use these components to derive the equivalent voltage unbalance coordination limit ranging from 1.48 to 4.32 (p.u.)² -s (14 800 to 43 200 %² -s), i.e. 2% voltage unbalance during 2 h or 3 h for Westinghouse and GE motors respectively.

Until 1978, energy savings were not an important factor to be taken into consideration compared to today’s criteria and computer-based assistance for designing power apparatus was unavailable. Consequently, older motor designs allowed much higher disturbance levels than can be tolerated now (from 1 to 2 p.u.). Motor manufacturers are therefore calling for a power supply with a lower voltage unbalance level.

However, due to the general introduction of high-speed trains and other single-phase loads, voltage unbalance on the power distribution system is actually increasing, with the result that power distributors want equipment to be able to tolerate higher voltage unbalance.

For these reasons, voltage-unbalance assessment requires more and more sophisticated techniques so as to apply a severity scale that would match the laboratory susceptibility tests.

Furthermore, an appropriate protection coordination technique is needed to reduce the safety margin that both manufacturers and electric-power distributors have added to the standards, with the result that the user is paying for the overdesign of motors and power systems.

Voltage unbalance assessment technique

This chapter describes an optimised technique that takes account of the requirements of the laboratory susceptibility tests and the appropriate protection coordination technique suggested above. This tutorial contains the basic knowledge needed for development.

The basic requirements for assessing voltage unbalance are the measurement and observation periods, the calculation method, and the evaluation technique.

Voltage-unbalance assessment calls for sampling each phase to ground or phase-to-phase voltage at a rate of at least 32 samples/cycle. A lower sampling rate can be used if an analog filter is used to remove the harmonic content from the signal. Three high-precision voltage transformers (<0.6%) are normally used to measure the voltage of MV or HV power systems. The voltage transformers used to assess voltage unbalance should be identical and loaded with identical impedances. Transformers feeding a 2-½ element metering system or an open-delta phase-to-neutral configuration should be avoided. The open delta voltage transformer connected phase-to-phase can be used if the load is identical on each transformer and if no load is connected on the open side of the open delta connection.

The instantaneous values of u_u are generally of no interest. In fact, equipment sensitive to such short intervals could have problems whenever a fault occurs on the power system or an asynchronous three-phase switch operates. However, the instantaneous values recorded can be used to calculate the level for longer assessment intervals.

CIGRE-CIRED Working Group 02 (CC02) recommends that the rms value of u_u recorded in every consecutive 3 s interval be assessed, using the following equation:

[27]

The parameter m represents the number of u_us recorded during every 3 s interval. Every 3 s interval during the full monitoring period serves to assess the highest level with a not to be exceeded specific certainty.

The equation [27] describes the measurement in a specific 3 s interval which appears to be very rigorous and unique. However, researchers have found several types of the 3 s interval which can be continuous, sliding, instrument sliding or re-synchronised intervals. The voltage unbalance survey result can also depend upon the type of interval used. The user should consult the tutorial on 3-s interval rms assessment to understand the differences between the interval types and to know which instrument to use.

CC02 also recommends the assessment of a short duration 10 min interval, which is calculated using equation [28].

[28]

The u_u10min value gives an indication of the overheating that takes place in the power electronics of the three-phase DC converter, which is reported to be sensitive to the negative-sequence component, since one of the three phases conducts for a longer period of time than the others. However, as no utility has reported such a case so far, there is no foundation upon which to define the compatibility level.

To compare measurements with planning levels, CCO2 has recommended sampling voltage unbalance during one week and to process the samples to find the maximum rms value calculated with samples of every 10 min interval. The 95% daily cumulative frequency of the unbalance voltages assessed in every 3 s interval should be used for comparison. CC02 also recommends comparing the maximum value of assessments over 3 s intervals with 1.5 to twice the planning level.

For assessing the contribution of a specific load, the voltage unbalance measurement period may correspond to the normal operating cycle of the load.

The user should bear in mind that measurements are performed to verify the differences between the electromagnetic environment and the susceptibility tests performed under ideal conditions in the laboratory. When the survey is aimed at determining the severity factor related to electric motors and power apparatus, the u_u2h value assessed on the basis of a 2 h interval should be used [ref.: 27]. The u_u2h value is assessed at every 10 min interval with the 12 previous u_u10min values assessed. This value is assessed using the following equation:

[29]

The maximum value that is calculated for each interval is then used to plot the following coordination graph (Fig 12). An adequate margin should be maintained between the susceptibility curve and the measurement curve [ref.: 27].

Disturbance level measurements adjusted in terms of equipment heating

The assessment technique presented in the previous section supplies the voltage unbalance factors needed to compare the planning level with an assessment of the actual disturbance levels occurring on the power system.

However, the 3 s, 10 min and 2 h intervals used to assess the power quality factors such as voltage unbalance give only approximations of disturbance duration severity levels, since disturbances preceding the measured interval are not considered.

For example, tests on motors by the Worcester Polytechnic Institute (USA) have demonstrated that calculated harmonic levels based on standard intervals can give lower severity assessment than the actual severity value when short interval burst disturbances on allow energy to accumulate in the motor. This accumulated energy and its consequences are ignored in the measurement method proposed by CC02.

The maximum values of disturbances measured in standard intervals are, therefore, not always sufficiently exact to calculate the loss-of-life related to high-level disturbances lasting only a short time and derating factors caused by burst disturbances.

In the case of thermal stress due to fluctuating disturbances in particular, the thermal inertia of the equipment significantly alters the calculation results. The thermal time constant is therefore an important aspect that can be considered using the procedure given in the Canadian Electrical Association report 220 D 711 on "Power Quality Measurement Protocol, CEA Guide to Performing Power Quality Surveys" (see reference given in the tutorial on 3-s interval rms value assessment)..

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References

20. Roger Bergeron, "Voltage Unbalance on Dstribution Systems - Phase I", Canadian Electrical Association, Project No. 231 D 488, Montréal, Québec, January 1989.

21. Westinghouse Electric Corporation. "Applied Protective Relaying", Silent Sentinels publication, Newark, New Jersey.

22. Cumming, P.G. "Protection of Induction Motors Against Unbalance Voltage Operation", IEEE-PCI, September 1983, IEEE-PIT, June 1984 and IEEE-IAS October 1984.

23. Roger Bergeron, "A Measurement Protocol for Power Quality Coordination", CIRED 1991, paper 2.17, April 1991.

24. IEC 27-1 (1992).- Letter Symbols To Be Used in Electrical Technology. Part 1: General.

25. IEC 146. Parts 1 to 6 .- Semiconductors Convertors.

26.     IEC 375 (1972).- Conventions Concerning Electric and Magnetic Circuits.

27. Roger Bergeron, "Power Quality Measurement Protocol, CEA Guide to Performing Power Quality Surveys," CEA report 220 D 711, Canadian Electrical Association 1 Westmount Square, Suite 1600 Montréal, Québec, Canada H3Z 2P9, 1996, 216 pp.