Tutorial discussion on long duration variations assessment
The long-duration variation category refers to the steady-state rms voltage level of the power system and is divided into three subcategories; interruption, overvoltage and undervoltage. This category is assessed in intervals equal to the time needed for equipment to reach the steady-state temperature. The heating time constant varies with the size and nature of the equipment. Equipment can been divided into three classes:
highly sensitive electronic systems with a heating time constant of less than 600 ms (usually related to components on the DC side of power supplies),
varistors and power electronics with a 2-min heating time constant, and
electrical apparatus with a heating time constant exceeding 24 min (motors, cables, switches, transformers etc).
According to the rule of thumb, equipment reaches its steady-state temperature after five times its heating time constant. Therefore, the long duration variations are rms voltage levels over 3 s (5×600 ms), 10 min (5×2 min) and 2 h (5×24 min) intervals.
ANSI indicates that the rms AC voltage level excludes voltage transients and temporary overvoltages caused by system conditions such as faults and load rejection. In light of this, the related level specified in standards should be used as the basis for comparison for assessing the 10 min interval rms value. These rms valuea can be calculated with the instantaneous rms voltage level measured in small 12-cycle windows.
The assessment should be referred to the declared voltage level Vd. The declared voltage is the voltage level specified in the utility's contract with the end-user and, unless modified by the utility in agreement with the latter, equal to the nominal voltage specified in ANSI C84.1-1989, CSA/CAN3-C235-83 or any equivalent standards available in each country.
The assessment of the long-duration variation category can be given as the percentage deviation from the declared voltages. The instantaneous steady-state voltage ISV assessed on short windows such as 12-cycle width can be used to assess the deviation ISVd from the declared voltage Vd :
When negative, the ISVd value should be classified in the undervoltage IRU subcategory, otherwise in the overvoltage IRO subcategory. To conserve the accuracy of the cumulative function, a 0% level is accumulated in the overvoltage subcategory when the negative ISVd is accumulated in the undervoltage category, and vice versa.
For benchmarking application class, the assessment is performed every consecutive 10-min interval accumulating one IRU and IRO value during each interval. For example, if each deviation ISVd is assessed over 12-cycle windows (» 200 ms) and this assessment is performed 3 times per second, 1800 IRO and IRU are available for assessing the 10-min values using the following equations:
References
Roger Bergeron, "Voltage Unbalance on Dstribution Systems - Phase I", Canadian Electrical Association, Project No. 231 D 488, Montréal, Québec, January 1989.
Westinghouse Electric Corporation. "Applied Protective Relaying", Silent Sentinels publication, Newark, New Jersey.
Cumming, P.G. "Protection of Induction Motors Against Unbalance Voltage Operation", IEEE-PCI, September 1983, IEEE-PIT, June 1984 and IEEE-IAS October 1984.
Roger Bergeron, "A Measurement Protocol for Power Quality Coordination", CIRED 1991, paper 2.17, April 1991.
IEC 27-1 (1992).- Letter Symbols To Be Used in Electrical Technology. Part 1: General.
IEC 146. Parts 1 to 6 .- Semiconductors Convertors.
- IEC 375 (1972).- Conventions Concerning Electric and Magnetic Circuits.
- 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.
