SA2007M Datasheet, PDF (7/10 Pages) Sames – Single Phase Power/Energy Metering IC with Tamper Detection

SA2007M
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_max for the meter).
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_L/ 16µA ) x R_SH / 2
= 60A / 2500 / 16µA x 3.6W / 2
= 2.7kW
I_L= Line current
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
sames
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.
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PRELIMINARY