71M6513 Neutral Current Measurement

Abstract

Measuring neutral currents enhances tamper-detection techniques. To measure neutral currents, a fourth current input channel is used, along with the existing three voltage and three current inputs. This application note discusses how the Teridian 71M6513, a device for 3-phase electronic power-metering applications, can implement the neutral-current measurement, with only minor hardware and firmware modifications.

Introduction

The ability to measure Neutral Currents is a key feature to enable the user to enhance tamper detection techniques. The current demo board firmware has the capability to measure Neutral Currents using the Compute Engine (CE) Firmware. All that is required is a minor modification to the existing demo board and firmware, which will be discussed below.

To measure Neutral Currents, a 4th current input channel needs to be created to be used along with the existing three voltage and three current inputs (VA, VB, VC, IA, IB, IC). The Neutral Current measurement can be implemented using the existing 71M6513 demo board with minor hardware and firmware modifications as given below:

Hardware Modifications:

  1. The Neutral Current input can be connected to the 71M6513 Auxiliary input V3 (pin 86)
  2. Analog Auxiliary input V3 is measured with respect to VBIAS (pin 81)
  3. The input circuit for Neutral Current measurement is to be added as shown in the figure below:

Firmware Procedure for Measuring Neutral Current

  1. The Neutral Current measurement is performed by acquiring the auxiliary input data from the V3 pin (pin 86).
  2. The analog data measured is referenced to the VBIAS pin (pin 81).
  3. Analog input V3 data can be read using the MUX_ALT mode bit in the I/O memory.
  4. The TERIDIAN Demo Code revision 3.04 accesses the MUX_ALT mode once a second.
  5. To acquire Neutral Current with a higher number of samples the existing Demo Board firmware has been modified with an MPU register named ALT_MUX_RATE.
  6. The modified firmware stores the ALT_MUX_RATE at MPU RAM (XRAM) location 0x0F, and the value is configurable using the )f=xx command via the serial interface.
  7. The default value for the ALT_MUX_RATE is 2520. The multiplexer is driven once in 2520 samples to acquire the auxiliary data from the Temperature and V3 inputs. The default sampling rate for the auxiliary data is 1Hz. The resulting sampling rate is 2520/ALT_MUX_RATE.
  8. If ALT_MUX_RATE is set to 10 then the auxiliary data is sampled at every 10th sample and thereby the auxiliary data sample rate is 252 Hz.
  9. The Minimum value for ALT_MUX_RATE is 10. That is 252Hz is the maximum recommended sample rate.

The Data collected for several ALT_MUX_RATE settings is given below:

S. No Applied
Current
Neutral Current
ALT_MUX_RATE = 10
(252Hz)
Neutral Current
ALT_MUX_RATE = 25
(100Hz)
Neutral Current
ALT_MUX_RATE = 50
(51Hz)
1 200 200.66 201.150 202.335
2 150 150.63 150.778 150.824
3 100 99.820 99.765 100.221
4 50 49.859 49.760 49.333
5 25 24.889 24.861 24.951
6 10 10.042 10.134 10.193
7 5 4.988 5.132 5.155
8 2.5 2.486 2.459 2.535
9 1 1.036 1.067 1.071
10 0.5 0.564 0.581 0.592
11 0.25 0.324 0.328 0.334