Tutorial on measured intervals

     

This annex was extracted from CEA Report 220D711 [27] with the permission of the Canadian Electrical Association. This report deals with measurement techniques associated with the equipment susceptibility.

The research conducted by the Worcester Polytechnic Institute (US) describes thermal response testing procedures based on hypothetical fluctuations in the amplitude of the disturbance in the motor supply. However, the fluctuation pattern used for an academic analysis of motor overheating may never occur in the field. Actually, the pattern of the amplitude of real disturbances on the system is totally unpredictable, with the result that manufacturers find it difficult to match their thermal test results to all the possible curves of each fluctuation pattern. For this reason, they tend to use a simple pattern for the thermal response test, which consists of applying a disturbance level to the supply voltage for a period equivalent to five times the time constant of the equipment under test (Figure IV.1).

The lack of precision in these tests due to neglecting the shapes of the system disturbances is offset by the use of digital processing for the three consecutive intervals of measured disturbances. This involves adjusting the amplitude of the disturbances measured on a severity scale comparable to the disturbance level used in the simple testing procedure described above. This is done by considering the energy accumulated in the equipment connected to the system during the events that preceded the interval analyzed. The intervals measured following the resynchronized-interval method (see  tutorial on 3-s interval assessment) have then to be re-evaluated for shorter intervals which, when they are added to a virtual interval related to the previous disturbances, give a sum equal to the standard intervals of 3 s, 10 min and 2 h (Figure IV.2).

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Therefore, the observation periods should be selected with a view to finding a virtual interval representing the heat accumulated during the previous interval (Fig. IV.2), of the motor exposed to voltage unbalances. The purpose of the procedure described below is to assess the time Tj (Fig. IV.2) and a virtual interval Tacc in such a way that the total is equal to the standard interval used in the tutorial on 3-s interval assessment.

Steps IV.1 to IV.5 describe the procedure used to assess the voltage unbalance in terms of the energy accumulated in electric motors. Motors are characterized by a heating time constant exceeding 24 min (1440 s).

The assessment of each interval contains a u(u-2h-rms) value that was found using the resynchronized-interval method (see annex V) and these values are used to assess the u(u-2h-e) level which takes account of the energy accumulated during the periods preceding the interval measured. So as to evaluate u(u-2h-e), let us define the terms illustrated in Figure IV.3, namely:

Figure IV.3: Energy accumulated during earlier events

uu-2h-e: severity level that takes account of the energy accumulated during previous disturbances

uu-e-pr: u(u-2h-e) level of the preceding interval

T1: interval (in seconds) between the end of the interval containing uu-e-pr and the beginning of the interval used to determine the value of uu-2h-e

ugap: rms disturbance level incorporated into the interval T1

u(u-2h-rms): The level of the interval to be reassessed, which was calculated by the resynchronized-interval method

The following procedure describes steps to assess the energy-related level of the voltage unbalance that overheats motors characterized by a heating time constant t of 24 min. The basic calculating interval D t increases in steps of 10 min.

IV.1 Number from 1 to 12 the uu-10min-rms values used in the resynchronized-interval method to calculate the value of u(u-2h-rms) and set k = 1. If the interval used to determine the value of u(u-2h-rms) is less than 2 h or if no uu-10min-rms value exceeds by 5% the u(u-2h-rms) level, set all the uu-2h-e(k) = uu-2h-rms and proceed to step IV.5.

IV.2 Calculate the ulevel(k,j) for j = k to 12 using the following equation:

[IV.1

if ulevel(k,j) is less than uu-2h-rms, set uu-2h-e(k,j) = uu-2h-rms

IV.3 Calculate the interval Tlevel(k,j) for each ulevel(k,j)
i.e. Tlevel(j) = 600 (j-k+1) and calculate the interval Tacc(k,j) equivalent to the accumulated energy using the following equation:

                                                    [IV.2]

 

IV.4 Find the value of ulevel (k,j) which gives the sum Tacc(k,j) + Tlevel(k,j) nearest to 7200 s (2 h) and record this as being the value uu-2h-e(k), increment the value of k and return to IV.2 until k = 12.

IV.5 Record the uu-2h-e(k) value found in step IV.4 (or in IV.1) which gives the sum Tacc(k,j) + Tlevel(k,j) nearest to 2 h. This record becomes the uu-2h-e of the interval considered. Record the uu-2h-e as the new weekly uu-2h-e-max when it exceeds the previous uu-2h-e-max.

 

       

       

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References

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

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

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

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

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

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

  1.     IEC 375 (1972).- Conventions Concerning Electric and Magnetic Circuits.
  1. 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.