dm00028436

AN3398
Application note
Fast digital calibration procedure for
STPMC1 based energy meters
Introduction
The STPMC1 device functions as an energy calculator and is an ASSP designed for
effective energy measurement in power line systems utilizing Rogowski coil, current
transformer, and Shunt or Hall current sensors. Used in combination with one or more
STPMSx ICs, it implements all the functions needed in a 1, 2, or 3-phase energy meter.
Due to its internal structure and features, STPMC1 allows a more effective and innovative
calibration procedure, which is explained in this document.
Advantages of this procedure are:
■
reduced calibration time
■
no need for re-calibration (calibration parameter can be written in a permanent way).
For further information about the device please refer to the STPMC1 datasheet.
This application note integrates the AN2299 application note for the STPMC1 metering chip.
Sections 1, 2, and 3.1 of the AN2299 application note can be considered valid also for the
STPMC1 device, sharing the same architecture as STPM01 and STPM10 devices, whilst
the calibration calculations shown in section 3.2 of AN2299 and in this document are slightly
different and are reported below.
November 2011
Doc ID 018812 Rev 1
1/11
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Contents
AN3398
Contents
1
Calibration flow chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1
2
2/11
Calibration procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.1
Working point setting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1.2
Algorithm choice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.3
Offline parameter calculations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.1.4
Online procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
1.1.5
Coherency check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Revision history . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Doc ID 018812 Rev 1
AN3398
Calibration flow chart
1
Calibration flow chart
The calibration procedure can be summarized in the following steps, which are examined in
Section 1.1.
Figure 1.
Calibration flow chart
Start
S
ttaarrtt
Start
S
Working
W
oorrkkiinngg point
ppoint
Working
W
pooiinntt
1. Working point setting
R1
R
11 or
oor
SV
VV
R1
R
orr S
S
SV
2. Algorithm choice
Offline
Off
O
fffflline
Offline
O
liinnee
Decrease LED frequency
Online
O
nnlliinnee
Online
O
3. Offline parameters calculations
K?
K
??
K?
K
4. Online procedure (for each phase)
5. Coherency check (for each phase)
End
E
nndd
End
E
AM09391v1
1.1
Calibration procedure
1.1.1
Working point setting
According to the information contained in AN2299, the STPMC1 device can also be
calibrated in a single point for each phase.
Therefore, voltage and current nominal values of the selected phase must be defined before
running the calibration procedure, for example:
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Calibration flow chart
Table 1.
AN3398
Working point setting
Parameter
Value
Vn
230 V
In
5A
Description
Phase to neutral RMS voltage
Phase RMS current
The other parameters, which follow, and the constants of the STPMC1 metering device (and
relative tolerances) are also known:
Table 2.
STPMC1 internal parameters
Parameter
KV
KI
Value
Description
0.875
Voltage calibrator ideal value if PM = 0(1)
0.9375
Voltage calibrator ideal value if PM = 1
0.875
Current calibrator ideal value if PM = 0
0.9375
Current calibrator ideal value if PM = 1
len_i
2^16
Current register length
len_u
2^12
Voltage register length
Kint_comp
1.004
Gain of decimation filter
π
3.14159
FM
4 * 10^6
If oscillator frequency is 4.000 or 8.000 MHz
2^22
If oscillator frequency is 4.194 or 8.388 MHz
4915200
If oscillator frequency is 4.915 or 9.830 MHz
DUD
2^17
Internal parameter
Vref
1.23
Internal voltage reference
4
Amplification of voltage ADC for STPMS1
2
Amplification of voltage ADC for STPMS2
Au
8
Amplification of current ADC for STPMS1
32
Ai
2
Amplification of current ADC for STPMS2
16
0.815
Gain of integrator @ line frequency = 50 Hz
0.679
Gain of integrator @ line frequency = 60 Hz
0.6135
Gain of differentiator @ line frequency = 50 Hz
0.7359
Gain of differentiator @ line frequency = 60 Hz
Kint
Kdif
1. PM is CFG 21, which sets the meter precision (Class 1 or Class 0.1).
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AN3398
Calibration flow chart
Only analog parameters are objects of calibration because they introduce a certain error.
Voltage ADC amplification Av is constant, while Ai is chosen according to the used sensors.
The calibration procedure has, as a final result, correction parameters called KVR, KVS, KVT
and KIR, KIS, KIT and KIN (if used) which, applied to STPMC1 voltage and current
measurements, compensate small tolerances of analog components that affect energy
calculations.
As KVR, KVS, KVT and KIR, KIS, KIT and KIN calibration parameters are the decimal
representation of the corresponding configuration bytes CVR, CVS, CVT and CIR, CIS, CIT,
CIN, the values of those bits are obtained at the end of calibration.
In the following procedure CVR, CVS, and CVT are indicated as Cv; CIR, CIS, CIT, and CIN
are indicated as Ci; KVR, KVS, and KVT are indicated as Kv, and KIR, KIS, KIT, and KIN are
indicated as Ki.
Through hardwired formulas, Kv and Ki tune measured values varying from 0.75 to 1 in 256
steps, according to the value of Cv and Ci (from 0 to 255).
If PM=1, two bits are appended to each Cv and Ci (see the STPMC1 datasheet for details),
and the corresponding tunings vary from 0.875 to 1 in 1024 steps.
To initially obtain the greatest correction dynamic, calibrators are set in the middle of the
range, therefore obtaining a calibration range of ± 12.5% (± 6.25% when PM is set) per
voltage or current channel:
Table 3.
Calibrator value according to PM
PM
Calibrator value
0
Kv = Ki = 0.875
Ci = Cv = 128
1
Kv = Ki = 0.9375
Ci = Cv = 512
In this way it is possible to tune Kv and Ki having a precise measurement: for example, with
PM=0, Cv=0 generates a correction factor of -12.5% (Kv=0.75) and Cv=255 determines a
correction factor of +12.5% (Kv=1), and so on.
According to the information above, the following formulas, which relate Kv,i and Cv,i to each
other are obtained:
Equation 1
Kv,i = (Cv,i/128) * 0.125 + 0.75
Equation 2
Cv,i = 1024 * Kv,i - 768
or when PM = 1
Equation 3
Kv,i = (Cv,i/512) * 0.0625 + 0.875
Equation 4
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Calibration flow chart
AN3398
Cv,i = 8192 * Kv,i - 7168
Indicating, with AvI and AvV, the average values read from the device and with XI and XV the
ideal values of RMS current and voltage readings, the following can be reported:
PM = 0:
Equation 5
XV = (Kv * AvV) / 0.875
Equation 6
XI = (Ki * AvI) / 0.875
PM = 1:
Equation 7
XV = (Kv * AvV) / 0.9375
Equation 8
XI = (Ki * AvI) / 0.9375
1.1.2
Algorithm choice
It is possible to use two different algorithms to calculate the parameters to be used during
calibration:
1.
R1 and R2 constant in order to carry out the sensor sensitivity KS
2.
Current sensor sensitivity and R2 constant in order to carry out R1.
The methods are the same and the choice is left to the designer.
According to the chosen algorithm, the next calibration step produces the value of sensor KS
or resistor R1 to be mounted on the measurement board to achieve calibration.
Algorithm formulas are reported below for both current transformer/Shunt and Rogowski coil
current sensors.
1.1.3
Offline parameter calculations
First of all, it is necessary to determine the target power sensitivity (from the LED pin) to be
achieved with the calibration process, for example:
C = 128000 pulses/kWh
The calibration procedure outputs Cv and Ci values that allow the above power sensitivity of
the meter.
This sensitivity is used to calculate target frequency at the LED pin for nominal voltage and
current values:
Equation 9
XF = f * 64
with:
Equation 10
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AN3398
Calibration flow chart
f = (C/64 * In * Vn) / 3600000
Current transformer or Shunt - Constant R1
In this algorithm voltage divider sensitivity is fixed, therefore, resistor values R1 and R2 are
the known values of the voltage divider resistor.
From the values above and for both the given amplification factor Ai and the initial calibration
data, the following target values can be calculated:
Voltage divider output:
Equation 11
VDIV = Vn * R2 / (R1 + R2)
Target RMS reading for given Vn:
Equation 12
XV = (VDIV / VREF) * 2 * Kdif * Av * Kv * Kint_comp* Kint * len_u
Target RMS reading for given In:
Equation 13
XI = f * len_u * len_i * DUD / (FM * XV)
From which current sensor sensitivity KS is obtained:
Equation 14
KS = XI * VREF * 1000 / (In * Ai * Ki * Kint_comp * Kint * Kdif * len_i) [mV/A]
Current transformer or Shunt - Constant KS
In this case the type of current sensor and its nominal value of sensitivity must be known
and is equal to KS.
From the values above and for both the given amplification factor Ai and the initial calibration
data, the following target values can be calculated:
Target RMS reading for given In:
Equation 15
XI = In * KS * Ai * Ki * Kint * Kint_comp * Kdif * len_i / (VREF * 1000)
Target RMS reading for given Vn:
Equation 16
XV = f * len_u * len_i * DUD / (FM * XI)
Voltage divider output:
Equation 17
VDIV = XV * VREF / (2 * Kdif * Av * Kv * Kint_comp * Kint * len_u)
From which R1 resistor value is obtained:
Equation 18
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Calibration flow chart
AN3398
R1 = R2 * (Vn - VDIV) / VDIV [Ohm]
Rogowski coil - Constant R1
As before, the voltage divider sensitivity is fixed, therefore, resistor values R1 and R2 are the
known values of the voltage divider resistor.
From the values above and for both the given amplification factor Ai and the initial calibration
data, the following target values can be calculated:
Voltage divider output:
Equation 19
VDIV = Vn * R2 / (R1 + R2)
Target RMS reading for given Vn:
Equation 20
XV = (VDIV / VREF) * Av * Kv * Kint_comp * len_u
Target RMS reading for given In:
Equation 21
XI = f * len_u * len_i * DUD / (FM * XV)
From which current sensor sensitivity KS is obtained:
Equation 22
KS = XI * VREF * 1000 / (In * Ai * Ki * Kint_comp * Kint * len_i) [mV/A]
Rogowski coil - Constant KS
The sensor’s nominal value of sensitivity must again be known, for example, it is KS.
From the values above and for both the given amplification factor Ai and the initial calibration
data, the following target values can be calculated:
Target RMS reading for given In:
Equation 23
XI = In * KS * Ai * Ki * Kint * Kint_comp * len_i / (VREF * 1000)
Target RMS reading for given Vn:
Equation 24
XV = f * len_u * len_i * DUD / (FM * XI)
Voltage divider output:
Equation 25
VDIV = XV * VREF / (Av * Kv * Kint_comp * len_u)
From which R1 resistor value is obtained:
Equation 26
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Calibration flow chart
R1 = R2 * (Vn - VDIV) / VDIV [Ohm]
1.1.4
Online procedure
According to the used current sensor and the chosen algorithm, a component (resistor or
current sensor) of the value calculated through the formulas above must be mounted on the
board.
To start the online calibration procedure, the following must be verified:
●
EM is connected to the calibration system and is properly configured according to the
chosen application
●
EM calibrator parameters are preset to initial data
●
Target values of line signals are stable.
A 3-phase voltage signal must be provided to all phases and current signal only to the phase
under calibration.
When the system is connected and powered on, a certain number of readings of the RMS
values must be performed.
Due to the fact that 0.4% of ripple is present in the measured RMS values, more than ten
readings of these values should be gathered each cycle (20 ms at 50 Hz) and the average
values of RMS current and voltage readings AvI and AvV should be computed.
Consequently, having the average values AvI and AvV, a pair of final 8-bit (or 10-bit if PM =
1) calibration data can be calculated as shown below:
Equation 27
Ci,v = 896 * XI,V / AvI, V - 768; (PM = 0)
Equation 28
Ci,v = 8192 * XI,V / AvI, V - 7168; (PM = 1)
where XV and XI are those calculated in one of the four previous cases.
1.1.5
Coherency check
We can assume that the EM works correctly and that built-in voltage and current sensors
allow the target power sensitivity constant to be achieved, because the correction
parameters Ki and Kv can tune measured values within the calibration range of ±12.5% or
±6.25% if PM =1 per voltage or current channel.
If, after the calibration, calculated values for Cv or Ci are out of their 8 or 10-bit range, it may
mean that the application cannot reach the target value of power sensitivity. In this case,
steps 3 and 4 must be repeated choosing a smaller power sensitivity value. If the values of
Cv or Ci are out of range even for small values of PM, it may mean that the energy meter
board is not good enough to perform such measurements, possibly because the tolerance
of the components is too big, or no care has been taken in the layout phase, so the
application must be re-designed.
Otherwise, if the final calibrator data is written into STPMC1, the average RMS readings are
very close to target values XI and XV and the frequency of the LED output are very close to
the target value f.
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Revision history
AN3398
2
Revision history
Table 4.
Document revision history
Date
Revision
16-Nov-2011
1
10/11
Changes
First release
Doc ID 018812 Rev 1
AN3398
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