ELECTRICITY METERING FOR THREE-WIRE NETWORK SERVICE SYSTEM DIAGRAMS

Two-Stator Meter
Three-wire network service is obtained from two of the phase wires and the neutral of a three-phase, four-wire wye system, as shown in Figure 7-2. It is, in reality, two two-wire, single-phase circuits with a common return circuit and it has voltages that have a phase difference of 120 electrical degrees between them.

The voltage is commonly 120 volts line-to-neutral/208 volts line-to-line.

The normal method of metering a network service is with a two-element (two-stator) meter connected as shown in Figure 7-2. With this connection, which follows Blondel’s Theorem, each stator sees the voltage of one phase of the load. The phasors representing the load phase currents, IAN and IBN, are shown in the diagram lagging their respective phase voltages.

Figure 7-2 Two-Stator Meter on Three-Wire Network Service.

The meter current conductors carry the line currents, IAN and IBN, and inspection of the circuit shows that these currents are identical to the load phase currents. Hence, the meter correctly measures the total load power. Any loads connected line-to-line, between A and B in Figure 7-2, will also be metered properly. With this type of meter there are no metering errors with imbalanced load voltages or varying
load currents and power factors.


As such, the watt metering formula for any instance in time is:

Watts  (VAN X IA) + (VBN X IB)

Accumulating the watts over time allows the metering of watthours.


Single-element (single-stator) meters for measuring network loads have been developed and may be used with reasonable accuracy under particular load conditions.  The conventional three-wire, single-element (single-stator), single-phase meter cannot be used for network metering.

It will, of course, measure the 208-volt load correctly; but the two 120-volt loads are metered at 104-volts rather than at 120 volts and at a phase angle which is 30 degrees different from the actual. Therefore, for 120-volt balanced loads, meter registration will be close to 75 percent of the true value; but with imbalanced loads, the resulting meter error varies, rendering such metering useless.

Single-Stator Meters for Three-Wire Network Service
Single-stator meters have been developed for use on three-wire network services. These meters do not conform to Blondel’s Theorem and are subject to metering errors under certain conditions noted in the following paragraphs.

The schematic connections for the two types of meters now in use are shown in Figure 7-3. Each meter has one voltage coil and two current coils. In one case, Figure 7-3a, the meter is designed to use line-to-line voltage (208 V) on the voltage coil and the other, Figure 7-3b, uses one line-to-neutral voltage (120 V) on its voltage coil.

The currents in the meter current coils are shifted in phase to provide correct metering. Obviously, any imbalance in line-to-neutral voltages will cause metering errors and, where imbalanced voltages exist, a two-stator meter should be used for accurate results.

In electromechanical meters, the current phase shifting is accomplished by impedance networks of resistors and inductors along with the current coils to split the total line currents and shift the phase position of the meter-current-coil current the desired amount. The number of turns and the impedance of the current coil may also be varied in design to obtain a usable meter.

The current-impedance networks are shown in Figure 7-3. Electronic meters can accomplish the phase shifting using a variety of techniques. Phase sequence of voltages applied to these meters is extremely important since such meters can usually only be designed to provide the correct phase shift of meter-coil current for only one phase sequence.

If they are installed on the wrong phase sequence their energy registration is useless. All meters of this type have a built-in phase-sequence indicator.

Figure 7-3 Single-Stator, Three-Wire Network Meters.