Power Metering - Application Notes - Atmel Corporation

Power Meter Front End Design:

The Delta Connection

Atmel’s AT73C500 + AT73C501-based

meter chipset measures power and

energy in three-phase systems or, alter-

natively, the chipset can be set to

operate in a mode where it measures

three separate, single-phase connec-

tions. In normal modes of operation, the

chipset measures Y-connected three-

phase, four-wire environments, but there

is no dedicated operating mode for

Delta-connected three-phase, three-wire

systems. This application note describes

how to implement the chipset in three-

phase, three-wire environments.

Three-Phase Basics

To understand the measurement

method, it is important to outline some of

the following basic concepts.

Terminology

Voltages (and currents) in a three-phase

system are referred to as either phase

voltages (currents) or main voltages

(currents), depending on the point of

reference.

•A Phase Voltage(1) (UP1, UP2 and UP3)

is always referenced to the neutral

wire. In systems where there is no

physical neutral wire, the phase

voltages are referenced to a virtual

neutral.

Note: 1. Also called neutral voltage. This

should not be confused with the

ground neutral that is at zero

potential.

• The Main Voltage (U1, U2 and U3) is

defined as the voltage between two of

the phase conductors.

•A Phase Current (IP1, IP2 and IP3) is

the current flowing through the load

impedance. Depending on the type of

connection, this current is not

necessarily directly measurable from

the meter connection points, but must

be derived from the vector sum of

other currents.

• The Main Current (I1, I2 and I3) is

defined as the current that flows

through the phase conductor.

Depending on the type of connection,

this current is not necessarily the

same that flows through the load.

The Star Connection

A typical three-phase, four-wire service

with a grounded neutral is illustrated in

Figure 1. The system shown is com-

monly referred to as a YN-configuration,

with the subscript denoting there is a

neutral wire present. The wiring method

is sometimes also referred to as Star

connection.

In Figure 1, the circuit on the left illus-

trates the power generator and the

circuit on the right the load. In this type of

connection, the energy meter would

interface to measure the phase volt-

ages (UP1, UP2 and UP3) and phase

currents (IP1, IP2 and IP3).

In a balanced (all loads are equal) Star

connection, the neutral line carries no

current and the main current is equal to

the phase current. The main voltage is

related to the phase voltage as follows:

U3U ×=

Rev. 1680A–07/00

Power Meter

Front End

Design:

The Delta

Connection

Application

Note

Meter Front End Design

Figure 1. Three-phase, Four-wire, YN-connection

The Delta Connection

The number of wires may be reduced from four to three, if

the load is wired in Delta-configuration. It is noted that the

generator does not necessarily have to be wired in Delta-

configuration, it can also be Star-connected. An example of

both the generator and the load-connected in Delta-config-

uration is illustrated in Figure 2.

The connection in Figure 2 contains no neutral wire and the

inherent voltage marking therefore refers to main voltages

(measured between phase conductors). When recalculat-

ing main voltages to phase voltages, the voltages are

referenced to a virtual neutral wire.

It is noted, that in a balanced Delta connection, the relation-

ship between phase current and main current is:

Figure 2. Three-phase, Three-wire, Delta Connection

I3I ×=

VIRTUAL GROUND POTENTIAL

Meter Front End Design

Front End Connections

The schematic diagram on page 7 shows a method for con-

necting the A/D converter to three-wire service. The

example Delta connection is rated for a 230/400V system,

but may easily be adapted to any other rating.

The Voltage Front End

The two transformers (Tr1 and Tr2) together with the resis-

tor network R1 through R6 convert the main voltages to

phase voltages. The conversion factor (the number of pri-

mary windings per number of secondary windings) of the

transformers is 1:1.

Resistors R1, R2 and R3 must be scaled to match the volt-

age of the system in order for the ADC inputs not to exceed

the maximum peak rating of 1 volt. ADC voltage maximum

should be reached at maximum main voltage of the system

front end. It may be assumed that the main voltage is equal

for all three phases and, therefore, that the maximum input

voltage to the converter, ûIN, can be calculated from any

one of the following:

We assume a system where the phase voltage is 230V

and, consequently, the main voltage 400V. The analog

front end should allow for a 15% voltage overhead, as

specified in IEC standards (see Table 1).

To achieve the ratings in Table 1, we start by defining resis-

tors R4 = R5 = R6 = 1 kohm, and get:

Using a standard valued resistor of 390 kohm, the maxi-

mum voltage for this configuration will be approximately

275V (i.e., about 390V peak amplitude). The voltage maxi-

mum should later be fine-tuned to 270V during factory

calibration.

The above calculations may be repeated to adapt the

design for any voltage system, but the maximum voltage of

the meter front end must be considered when reading data

or pulses from the meter. It should be noted that the default

bit resolution of data registers and the meter constant of

pulse outputs are based on a 270V maximum voltage.

It should also be noted that the DSP now measures phase

voltages of a Delta-connected load. For example, the meter

software of the ATEK5003 (The Evaluation Kit for the

AT73C500 chipset) displays RMS voltage values with

respect to the phase voltage and not the main voltage..

R6R3

R5R2

R4R1

ˆPFSPFSPFSIN ××

=××

380kohmR41

R3R2R1

PFS ≈×











−

===

Table 1. Voltage Ratings at Various Circuit Nodes

Path Node Symbol Nominal Maximum Unit

Voltage

Phase1

Phase2

Phase3

400 460 VRMS

Tr1 / 3 - G N D

Tr1 / 4 - G N D

Tr2 / 4 - G N D

UP1

UP2

UP3

230 270 VRMS

VI1

VI2

VI3

0.602 0.707 VRMS

Current

Tr 3 / P r i m a r y

Tr 4 / P r i m a r y

Tr 5 / P r i m a r y

-80A

RMS

CI1

CI2

CI3

-0.707V

RMS

Meter Front End Design

The Current Front End

Current transformers Tr3, Tr4 and Tr5 are used for sensing

the phase current, which goes through the primary winding.

The transformers should have a conversion factor, M, such

that, at maximum rated phase current, the secondary cur-

rent, which goes through the shunt resistors (R7, R8 and

R9) does not produce more than 1-volt peak amplitude at

the ADC input. The higher the conversion factor of the

transformer, the lower the value of the shunt resistor and

vice versa.

It should be noted that for high resistive values, the signal

distortion in the current transformer is low, but the thermal

noise in the resistor is high. On the other hand, for low

resistive values, the thermal noise is lower, but the power

dissipation of the resistor is higher. In addition, the current

transformer may distort the signal if the load on the second-

ary winding is too high.

As a compromise, the example wiring uses current trans-

formers with a conversion factor of 2500 and shunt

resistors of 22 ohms. The full-scale current rating is 80A, at

which the RMS voltage at the ADC input is:

The peak amplitude at the ADC input is then:

It should be noted that the current measured by the

chipset is the main current. In a Start Connection, this is of

no importance, since the phase and main currents

are equal, but in the Delta-configuration this must be

accounted for.

Measurement Method

The DSP uses the same calculation methods, regardless of

type of connection, since there is no special mode setting

for connecting the device to a Delta load. The differences in

measurement method, as compared to the default Star

connection are due to the wiring of the front end and the

conditioning of signal amplitudes.

Using the front end connections illustrated in the schematic

on page 7, the DSP will measure phase voltages and main

currents for each phase. The measurement results, avail-

able via the data registers, do not necessarily give

straightforward indications on the condition of the phase

loads in a Delta connection. For example, the RMS values

of phase voltages and main currents cannot be used to

derive the power consumption of one phase load, since the

Delta-wired load is connected between two phase nodes,

as shown in Figure 3.

The DSP will give measurement results as if the load was

connected in Star. Measurement results must therefore be

related to the equivalent Star connection as shown in

Figure 4. Please note that the load impedances of the

equivalent connection are not the same as those for the

actual Delta connection.

For example, the active power readings for phase one indi-

cates how much power has been consumed in the

equivalent load of Z1 and not how much power has been

consumed in any of the true loads of the Delta connection.

Figure 3. The Delta-connected Load

0.704V

2500

80A

RU 3

7IN =×Ω=×=×=×=

0.9956VU2u

ˆININ =×=

VIRTUAL GROUND POTENTIAL

Z12

Meter Front End Design

Figure 4. Equivalent, Unbalanced Star Connection without Neutral Wire

Summary

In Delta-configuration, data registers should be interpreted as shown in Table 2.

VIRTUAL GROUND POTENTIAL

IP1

IP2

IP3

Table 2. Delta-configuration

Variable Register Measure Note

Active Power Power consumed in equivalent phase load

Reactive Power Power consumed in equivalent phase load

Apparent Power Power consumed in equivalent phase load

PF1

PF2

PF3

Power Factor Between main current and phase voltage

WPE 12 Active Power Exported

Total power in all loads

WPI 13 Active Power Imported

WQI 14 Reactive Power, Ind. Load

Total powerin all loads

WQC 15 Reactive Power, Cap. Load

TIME 16 Time Since Reset Not affected

FREQ 17 Line Frequency Not affected

RMS Voltage Phase voltage

RMS Current Main current

Meter Front End Design

Typically, it is of no importance if the individual phase pow-

ers reported are relative to a Star or Delta connection, as

long as the overall power readings are correct. Regardless

if the chipset is wired to a default Star load or the Delta

load, as illustrated in the schematic on page 7, the overall

power readings are always obtained by summing the indi-

vidual phase registers.

When wired to a Delta load, it must be noted that individual

phase power information is correct only in the special case

when both the generator and the load are balanced. If the

load or supply is unbalanced (even if the generator is bal-

anced, impedances in the transmission lines may result in

unbalanced voltages), then only the sum of the individual

phase power readings will be valid for the Delta connection.

Individual phase powers cannot be evaluated for an unbal-

anced Delta load.

Meter Front End Design

Figure 5. Front End Connection Schematic

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