IEEE Std C37.26 pdf free download

IEEE Std C37.26 pdf free download.Power- Factor Measurement for Low-Voltage (1000 V AC or lower) Inductive Test Circuits.
6. Phase relationship method
6.1 General
The methods of determining the test circuit power factor dependent upon asymmetrical values of the test current (i.e., the ratio method; see Clause 4) or the decay of the dc component of the test current (i.e., the dc decrement method; sec Clause 5) generally are not suitable for the measurement of the power factor of circuits above 30% power factor, where the dc component is severely reduced. This method can also be used for tests with lower power factor if the test current is symmetrical or nearly symmetrical. The phase relationship method, using the time relationship of the current and the voltage waves, is the recommended method on test circuits having power factors above 30%.
6.2 Procedure for determining the power factor
This method involves the closely controlled closing of the test circuit and determines the power factor of the test circuit under essentially symmetrical current conditions.
a) Determine the “zero point” of the “true” axis of the test current at the end of the first major half-cycle by constructing the current waveform envelope and a line midway between the two envelope lines.
b) Determine the open-circuit voltage zero that occurs just prior to the current zero of interest by extension in time of 360 degrees from the second voltage zero before the initiation of the test current.
c) The power factor of the test circuit is represented by the time in degrees (or in ms) between the extended open-circuit voltage zero and the current zero at the time of measurement, which is the end of the first major half-cycle of test current.
d) This procedure is repeated on each phase of a three-phase test circuit. The current zero of each phase shall be related to the phase-to-neutral voltage for that phase. Greater accuracy results if the power factor of each phase is determined with the test circuit closed to produce a symmetrical current for the phase tinder investigation. The test circuit power factor is considered to be the average of the individual phase power factors.
4. Ratio method
4.1 General
Devices such as current-limiting fuses, fused circuit breakers, and similar fast-clearing devices may have total interrupting times of 0.5 cycles or less. The ratio method permits measurement to be made within the operating time of these devices. The ratio method is generally not suitable for use on test circuits with a power factor above 30%.
Because this method requires closing the test circuit to produce the maximum current asymmetry, the resulting high electrical loading and mechanical forces on the bus supports and circuit components may jeopardize the test plant equipment. In such cases, the test laboratory may request to determine the power factor of the test circuit using a reduced current level (such as 50% of the rated test current), with the agreement of the manufacturer and the witnessing test inspector.
4.2 Procedures for determining the power factor
The power factor of the test circuit is determined at an instant of time one-half cycle (based on the fundamental frequency timing wave) after the initiation of the test current flow by determining the symmetrical and asymmetrical (total) currents at this point of time (see Figure 1, Figure 2, and Table 2). Both the rms symmetrical current and the total rms asymmetrical current are to be measured, and the ratio MA (for three-phase test circuits) or M1 (for single-phase test circuits) is to be calculated as follows:
a) Construct the envelope of the test current wave as shown in Figure 1.
b) Construct a vertical line at the one-half cycle time after the initiation of the test current.
C) Measure the deflection of the trace from the zero line to the maximum point of the envelope at the one-half cycle line (this is the value shown as A’).
d) Measure the deflection of the trace from the zero line to the minimum point of the envelope at the one-half cycle line (this is the value shown as B’).IEEE Std C37.26 pdf download.