RM4301ASEA datasheet(2/19 Pages) SAMES

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PRELIMINARY

RM4301ASEA

Current Value

Load

Power Factor

Class 1 Error Limits

0.05lb

£ I < 0.1lb

Balanced three phase

± 1.5%

0.1lb

£ I £ IMAX

Balanced three phase

± 1.0%

0.1lb

£ I < 0.2lb

Balanced three phase

0.5 inductive (lag)

± 1.5%

0.1lb

£ I < 0.2lb

Balanced three phase

0.8 capacitive (lead)

± 1.5%

0.2lb

£ I £ IMAX

Balanced three phase

0.5 inductive (lag)

± 1.0%

0.2lb

£ I £ IMAX

Balanced three phase

0.8 capacitive (lead)

± 1.0%

0.1lb

£ I £ IMAX

Single phase

± 2.0%

0.2lb

£ I £ IMAX

Single phase

0.5 inductive (lag)

± 2.0%

Table 1: IEC62053-21 Accuracy Specifications

CIRCUIT DESIGN PRINCIPLES

CURRENT SENSING NETWORKS

The primary function of the current sensing networks is to

sense the load currents and convert them to the input

current signals required by the SA4301A. The current

sensing network for one phase is shown in Figure 2. The

sensing networks for all phases are identical.

The amplitude of the input current into the SA4301A at

maximum current (IMAX) should be set as close as possible

to 16μARMS. The current input of the device saturates at

25μA peak current, so the 16μARMS input current (22.62μA

peak) allows for an over-current up to 110% IMAX before

saturation occurs. The SA4301A can be used with most

available current transformers.

The burden resistor of the current transformer should be

selected such that the voltage across the resistor at

maximum current (IMAX) is in the order of 100mVRMS. It is

best that this voltage does not exceed 200mVRMS. The

reference level should be connected in the centre of the

burden resistor to create purely differential current inputs.

This will result in the best linearity for the meter. Resistors

RX14 and RX15 form the burden resistor. The best value of

each burden resistor is determined by using

MAX

100mV

RX15

RX14

(1)

to calculate the theoretical value of the burden resistor and

then rounding this up to the nearest available resistor value.

NCT is the turns ratio of the current transformer.

The internal current feedback present on the differential

current inputs IIN and IIP of the SA4301A creates a virtual

short circuit between the two current input pins. This means

that the resistor value required to generate the correct input

current can be calculated using:

MAX

RX19

RX18

RX17

RX16

(2)

where RB is the actual value of the burden resistor used.

A secondary function of the current sense networks is to

attenuate all high frequency components that could disrupt

the accuracy of the SA4301A. These high frequency

components may occur due to high frequency surges (fast

transient burst), may be induced through strong electric

fields or may simply be noise on the power lines. Certain

high frequency components, typically those close to integer

multiples of the sampling frequency of the analog to digital

converters will be mapped close to 50Hz once sampled (a

process known as aliasing) and will distort the accuracy of

the converters. This can be prevented by adequately

attenuating all high frequency signal components. The

typical oscillator frequency is 3.58MHz and the analog to

digital converters of the SA4301A operate at one half of this

frequency, so the filters should be designed to give sufficient

attenuation at 1.79MHz. This can readily be achieved by

placing a single order RC low pass filter on each current

input as shown in Figure 2. The capacitors cannot be placed

directly on the input pins IIN and IIP because no differential

voltage signal exists between these pins due to the virtual

short circuit created by the input network of the SA4301A.

The input resistance is therefore split into two equal

resistors (RX16/RX17 and RX18/RX19) and the capacitor is

placed between these resistors. Now a differential voltage

can appear across the capacitors and hence filter high

frequencies. The lowest -3dB cut-off frequency (and hence

best filtering ability) for a given capacitor value is achieved

when all four input resistors are equal (RX16 = RX17 =

RX18 = RX19 = RC). The current input networks must be

balanced so both capacitors must also be equal (CX2 = CX3

= CC). In this case the equivalent resistance associated with

each capacitor is ½RC and the -3dB cut-off frequency is:

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