SA2007H
ssaammeess
TYPICAL APPLICATION
Voltage Divider
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.
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
Current Input IIN1, IIP1, IIN2, IIP2
Values for resistors R4 = 10W, R5 - 22kW and R7 - 1MW is
chosen.
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.
Substituting the values result in:
RB = 21.526kW
RA = RB x (230V / 14V -1)
RA = 332.12kW.
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
Standard resistor values for R1, R2 and R3 are chosen to be
100kW, 100kWand 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:
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:
C = 1 / (2 xp x Mains frequency x R5 x tan (Phase shift angle))
C = 1 / ( 2 xp x 50 x 1MW tan (0.18 degrees ))
C = 1.013µF
R10 = R11 = ( I/ 16µA ) x R / 2
SH
= 60A / 2500 / 16µA x 3.6W / 2
=2.7kW
Reference Voltage Bias resistor
I =Line current
L
R6 defines all on chip and reference currents. With R6 = 24kW
optimum conditions are set. Calibration should be done in the
micro controller software.
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.
7/10
http://www.sames.co.za
PRELIMINARY
SAMES [ SAMES ]
