sames

sames

SA2007H

7/10

http://www.sames.co.za

TYPICAL APPLICATION

The analog (metering) interface shown in figure 8, is designed

for measuring 230V/60A with precision better than Class 1.

The most important external components for the SA2007H

integrated circuit are the current sense resistors, the voltage

sense resistors and the bias setting resistor. The resistors

used in the metering section should be of the same type so

temperature effects are minimized.

Current Input IIN1, IIP1, IIN2, IIP2

Two current transformers are used to measure the current in

the live and neutral phases. The output of the current

transformer is terminated with a low impedance resistor. The

voltage drop across the termination resistor is converted to a

current that is fed to the differential current inputs of the

SA2007H.

CT Termination Resistor

The voltage drop across the CT termination resistor at rated

current should be at least 20mV. The CT’s have low phase shift

and a ratio of 1:2500. The CT’s are terminated with a 3.6W

resistor giving a voltage drop of 86.4mV across each

termination resistor at rated conditions (I

for the meter).

max

Current Sensor Input Resistors

The resistors R10, R11 and R12, R13 define the current level

into the current sense inputs of the SA2007H. The resistor

values are selected for an input current of 16µA at rated

conditions. For a 60A meter and a CT Ratio of 2500:1 the

resistor values are calculated as follows:

R10 = R11 = ( I / 16µA ) x R / 2

SH

= 60A / 2500 / 16µA x 3.6W / 2

= 2.7kW

I = Line current

L

R = CT Termination resistor

SH

2500 = CT ratio

The two current channels are identical so R10 = R11 = R12 =

R13.

Voltage Input IVP

The voltage input of the SA2007H (IVP) is driven with a current

of 14µA at nominal mains voltage. This voltage input saturates

at approximately 17µA. At a nominal voltage current of 14µA

allows for 20% overdriving. The mains voltage is divided with a

voltage divider to 14V that is fed to the voltage input pins via a

1MW resistor.

Voltage Divider

The voltage divider is calculated for a voltage drop of 14V.

Equations for the voltage divider are:

RA = R1 + R2 + R3

RA = R7 || (R5 + P1)

Combining the two equations gives:

(RA + RB) / 230V = RB / 14V

Values for resistors R4 = 10W, R5 - 22kW and R7 - 1MW is

chosen.

Substituting the values result in:

RB = 21.526kW

RA = RB x (230V / 14V -1)

RA = 332.12kW.

Standard resistor values for R1, R2 and R3 are chosen to be

100kW, 100kW and 120kW.

The capacitor C1 is used to compensate for phase shift

between the voltage sense inputs and the current sense inputs

of the device, in cases where CTs with phase errors are used.

The phase shift caused by the CT may be corrected by

inserting a capacitor in the voltage divider circuit. To

compensate for a phase shift of 0.18 degrees the capacitor

value is calculated as follows:

C = 1 / (2 x p x Mains frequency x R5 x tan (Phase shift angle))

C = 1 / ( 2 x p x 50 x 1MW tan (0.18 degrees ))

C = 1.013µF

Reference Voltage Bias resistor

R6 defines all on chip and reference currents. With R6 = 24kW

optimum conditions are set. Calibration should be done in the

micro controller software.

PRELIMINARY