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5 Basis of measurement and calculations
Reactive and Apparent Power
Active powers are calculated directly by multiplication of voltage and current. Reactive powers
are calculated using frequency corrected quarter phase time delay method. Apparent power is
calculated as the square root of sum of squares of active and reactive powers. For 4 wire
products, overall powers are the sum of the per phase powers. For 3 phase 3 wire products, the
"two wattmeter" method is used for overall powers.
Energy resolution
Cumulative energy counts are reported using the standard IEEE floating point format. Reported
energy values in excess of 16MWh may show a small non cumulative error due to the
limitations of the number format. Internally the count is maintained with greater precision.
The reporting error is less than 1 part per million and will be automatically corrected when the
count increases.
Power Factor
The magnitude of Per Phase Power Factor is derived from the per phase active power and per
phase apparent power. The power factor value sign is set to negative for an inductive load and
positive for a capacitive load.
The magnitude of the System Power Factor is derived from the sum of the per phase active
power and per phase apparent power. The system power factor value sign is set to negative for
an inductive load and positive for a capacitive load. The load type, capacitive or inductive, is
determined from the signs of the sums of the relevant active powers and reactive powers. If
both signs are the same, then the load is inductive, if the signs are different then the load is
capacitive.
The magnitude of the phase angle is the ArcCos of the power factor. It's sign is taken as the
opposite of the var's sign.
Maximum Demand
The maximum power consumption of an installation is an important measurement as power
utilities often levy related charges. Many utilities use a thermal maximum demand indicator
(MDI) to measure this peak power consumption. An MDI averages the power consumed over a
number of minutes, such that short surges do not give an artificially high reading.
Integra uses a sliding window algorithm to simulate the characteristics of a thermal MDI
instrument, with the demand period being updated every minute.
The demand period can be reset, which allows synchronisation to other equipment. When it is
reset, the values in the Demand and Maximum Demand registers are set to zero.
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Integra 1540, 1000, 0640, 0440, 0340, 0240 Issue 1 04/03