sames

sames

SA2007M

7/10

http://www.sames.co.za

TYPICAL APPLICATION

The analog (metering) interface described in this section is

designed for measuring 230V/60A with precision better than

Class 1.

The most important external components for the SA2007M

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

SA2007M.

CT Termination Resistor

The voltage drop across the CT termination resistor at rated

current should be at least 20mV. The CTs have low phase shift

and a ratio of 1:2500. The CT is terminated with a 3.6W resistor

giving a voltage drop of 86.4mV across the termination resistor

at rated conditions (I

for the meter).

max

The termination resistor on the second current channel is

adjustable by means of P2 to facilitate channel equalization

calibration. The termination resistor is chosen so that a 10W

trimpot in parallel will provide a sufficient channel range.

Current Sensor Input Resistors

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

into the current sense inputs of the SA2007M. The resistor

values are selected for an input current of 16µA on the current

inputs of the SA2007M at rated conditions. For a 60A meter at

2500:1 CT the resistor values are calculated as follows:

R10 = R11 = ( I / 16µA ) x R / 2

LSH

= 60A / 2500 / 16µA x 3.6W / 2

= 2.7kW

I = Line current

L

RSH = CT Termination resistor

2500 = CT ratio

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

R13.

Voltage Input IVP

The voltage input of the SA2007M (IVP) is driven with a current

of 14µA at nominal mains voltage. The 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

RB = R7 || (R5 + P1)

A 5k trimpot will be used in the voltage channel for meter

calibration. The center position on the pot is used in the

calibration P1 = 2.5kW.

Combining the two equations gives:

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

Values for resistors R5 = 22kW, P1 = 2.5kW and R7 = 1M W is

chosen.

Substituting the values result in:

RB = 23.9kW

RA = RB x ( 230V / 14V – 1 )

RA = 369kW.

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

120kW, 120kW and 130kW.

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 x 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. Device calibration is done on the

voltage input of the device.

PRELIMINARY