Schneider Electric HRB1684301-01 user manual VT and CT considerations, Example calculations

Chapter 15—Verifying accuracy PowerLogic™ PM5500 series user manual

VT and CT considerations

The test points are always taken at the secondary side, regardless of whether VTs or CTs are used. Ptot is derived from the values of the voltage and current inputs at the secondary side, and takes into account the VT and CT ratios.

If VTs and CTs are used, you must include their primary and secondary ratings in the equation. For example, in a balanced 3-phase Wye system with VTs and CTs:

Ptot = 3 × VLN ×	---------------------------------VT primary	× I ×	----------------------------------CT primary	× PF ×	------------------1 kW
	VT secondary		CT secondary		1000 W

Example calculations

A balanced 3-phase Wye system uses 480:120 volt VTs and 100:5 amp CTs. The signals at the secondary side are 119 volts line-to-neutral and 4.99 amps, with a power factor of 0.85. The desired pulse output frequency is 20 Hz (20 pulses per second).

1. Calculate the typical total output power (Ptot):

Ptot = 3 × 119 × 480--------	× 4.99 × 100--------	× 0.85 ×	------------------1 kW	= 121.14 kW
120	5		1000 W

2. Calculate the pulse constant (K):

K =	3600------------------------------------------------------------× (pulse frequency)	=	3600-----------------------------------------------------------------------------------------second/hour × 20 pulses/second
	Ptot		121.14 kW

K = 594.4 pulses/kWh

3.At full load (200% of nominal current = 10 A) and power factor (PF = 1), calculate the maximum total output power (Pmax):

Pmax = 3 × 119 × 480--------	× 10 × 100--------	× 1 ×	------------------1 kW	= 285.6 kW
120	5		1000 W

4. Calculate the maximum output pulse frequency at Pmax:

Maximum pulse frequency =	K-----------------------× Pmax	=	594.4-------------------------------------------------------------------------pulses/kWh × 285.6 kW
	3600		3600 seconds/hour
Maximum pulse frequency = 47.2		pulses/second = 47.2 Hz

5.Check the maximum pulse frequency against the limits for the LED and digital outputs:

· ≤

47.2 Hz LED maximum pulse frequency (2.5 kHz)

· >

47.2 Hz digital output maximum pulse frequency (25 Hz)

NOTE: The maximum pulse frequency is within the limits for LED energy pulsing. However, the maximum pulse frequency is greater than the limits for digital output energy pulsing. Pulse output frequencies greater than 25 Hz will saturate the digital output and cause it to stop pulsing. Therefore in this example, you can only use the LED for energy pulsing.

Adjustments to allow energy pulsing at the digital outputs

If you want to use the digital output, you must reduce the output pulse frequency so it is within the limits.

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