WO2005085880A2

WO2005085880A2 - Electrical power meter

Info

Publication number: WO2005085880A2
Application number: PCT/EP2004/014920
Authority: WO
Inventors: Andrew Stevens; David Mackie
Original assignee: Actaris Uk Limited
Priority date: 2003-12-31
Filing date: 2004-12-17
Publication date: 2005-09-15
Also published as: WO2005085880A3; GB0330239D0; GB2409732A

Abstract

The present invention relates to the field of shunt based electrical power meter. Said meter (10) comprises Live In and Live Out terminals LI and LO adapted to be series connected in the live wire of the electrical power distribution circuit, a neutral terminal N adapted to be connected to the neutral wire of the electrical power distribution circuit, a voltage sensor in order to generate a signal responsive to a voltage provided to said load, a current sensor in order to generate a signal responsive to a current provided to said load, said current sensor being a shunt resistor R8h connected between said Live In LI and Live Out LO terminals, a voltage test terminal VTT adapted to be connected to one terminal of the voltage source of a test rack. Said meter (10) further comprises a resistor Rtt connected permanently between said voltage test terminal VTT and said Live In terminal LI.

Description

ELECTRICAL POWER METER

The present invention relates to the field of shunt based electrical power meter. It is well known to provide electrical utility meter allowing the measurement of electrical power. Such a meter measures signals proportional to current and voltage in the power line being metered. The current and voltage signals can then be readily multiplied. The product of multiplication provides the electrical power and the consumption of electrical energy by integration of said electrical power. Such a power meter comprises both voltage and current sensing elements. One of the most common current sensor technologies is the low resistance current shunt. The Low resistance current shunt offers good accuracy at low cost and the current measurement is very simple, the voltage drop across said shunt resistor being proportional to the current. In conventional manner, such meters are tested on a test rack of multiple units provided with a source of current I and a source of voltage V. Normally, the test rack uses a multi-tap transformer to provide an isolated voltage supply to each meter in the rack, allowing multiple units to be calibrated in parallel. Occasionally the end customer will test samples of the meters prior to installation in the field and there is a possibility that this testing will be performed in a rack with a single voltage supply. Such an old test rack has only one voltage feed to all meters because it assumes isolation between voltage and current circuits (case of electromechanical meters having separate voltage and current coils). However, such a solution using one single voltage supply raises some difficult problems with shunt-based meters as illustrated in figure 1. Figure 1 shows schematically a test rack 1 using a single source of current I and a single source of voltage V. Three meters M1 , M2 and M3 are tested. Each of the meters comprises a shunt resistor Rsh connected between the Live In LI and Live Out LO terminals of the live phase conductor. The voltage source V is connected between the LI terminal and the neutral terminal N of each meter M1 to M3. The current source I is connected between the LI terminal of the first meter M1 and the LO terminal of the third meter M3. In this situation, the voltage source will cause a short across the shunt element in the meters under test. More precisely, as shown by the bold arrows F, all the meters except the last meter M3 will have their shunt shorted by the use of said single voltage source V. One object of the present invention is to provide a meter for metering the electrical power on a power load, said meter comprising a shunt resistor for current sensing and being able to be tested on a test rack without individual voltage supplies to each meter by avoiding that the shunt be shorted out by the common voltage feed to all meters. It is already known from US 5, 264, 786 to provide an electrical meter comprising three terminals (Live In, Live Out and Neutral terminal) with a supplementary Voltage terminal for test purpose in order to enable the ' simultaneous test of several identical meters on a test rack with a single voltage supply. In the disclosed solution, this voltage terminal is either put at the same potential than the Live In terminal, or put at a different potential, thanks to a removable conductor. However, the disclosed solution is complex since there is a need to remove manually all the removable connector when test of several meters is needed. In addition, the meter is also provided with a voltage divider between the Live In terminal modification at the level of the measuring circuit and the voltage terminal, and a differential amplifier in the voltage sensor. The present invention proposes a very simple solution, where few modification are needed compare to the meters shown in figure 1. More precisely, the present invention provides a meter for metering the electrical power on a power load, said meter comprising: - Live In and Live Out terminals adapted to be series connected in the live wire of the electrical power distribution circuit, - a neutral terminal adapted to be connected to the neutral wire of the electrical power distribution circuit, - a voltage sensor in order to generate a signal responsive to a voltage provided to said load, - a current sensor in order to generate a signal responsive to a current provided to said load, said current sensor being a shunt resistor connected between said Live In and Live Out terminals, - a voltage test terminal adapted to be connected to one terminal of the voltage source of a test rack, said meter being characterized in that said Live In teminal is permanently connected to said voltage test terminal via a test resistor. Thus, according to the invention, the physical connection for the voltage test is made via a Voltage Test Terminal VTT on the meter. In order to prevent the voltage connection from shorting the shunt, there is a proposal to add a series resistor in the VTT. This resistor would appear in parallel with the shunt at the Live In terminal. Other characteristics and advantages of the invention will appear reading the following description of an embodiment of the invention, given by way of example and with reference to the accompanying drawings, in which: - Figure 1 shows schematically a test rack with meters according to the prior art, - Figure 2 shows schematically a meter according to the invention, - Figure 3 shows schematically a test rack with a plurality of meters as shown in figure 2, - Figures 4 (a) and (b) show schematically a meter as shown in figure 2 in two different configurations. Figure 1 has already been described by reference to the prior art. Figure 2 shows a single-phase meter 10 according to the invention.

In using this configuration, said meter 10 is operably connected to power lines and to a load of a power consuming facility, not shown, such as a residence or industrial establishment. One has to note that only the features necessary to the understanding of the invention are shown in figure 1 , said meter further comprising, in a conventional manner, several other features for metering the electrical power on the power load and not shown in figure 1. Said meter 10 comprises: - Live In and Live Out terminals LI and LO adapted to be series connected in the live wire of the electrical power distribution circuit, - a neutral terminal N adapted to be connected to the neutral wire of the electrical power distribution circuit, - two series resistors RCAL and Rπ connected between the LI terminal and the N terminal, - a shunt resistor R_sh connected between LI and LO terminals, - a vo Itage test terminal VTT, - a test resistor t connected between the LI terminal and the VTT terminal. The shunt resistor R_srι is operable to generate a current measurement signal representative of the current waveform on the power lines. The main voltage applied to the load is measured by a potential divider between consisting of the resistor R_n in series with the variable resistor RCAL- The variable resistor, RCAL, and the resistor R_π further represent the calibration network which is used to calibrate the meter during manufacture. As can be seen from the circuit, the additional test resistor, R_tt, appears in parallel with Rsh for the current path (Live In LI to Live Out LO) but in series with the calibration network for the voltage path (VTT to Neutral). The advantage of the test resistor Rt will appear with reference to the figure 3 that shows a test rack 11 with N tested meters 10ι to 10N as represented in figure 2. Said N meters 10ι to 1 ON are tested simultaneously. Said rack 11 comprises a single source of current I and a single source of voltage V. The voltage source V is connected between the VTT terminal and the neutral terminal N of each meter 10ι to 1 ON- The current source I is connected between the LI terminal of the first meter 10ι and the LO terminal of the third meter 1 ON- The fact of having a VTT test terminal with a resistor Rtt connected between it and the LI terminal prevents the voltage source connection from shorting the shunt resistors R_S . Of course, such a configuration is advantageously feasible if the value of R_H is chosen to be significantly higher than the value of the shunt resistor Rsh. There are two distinct cases which must be considered when calculating the value of R , namely the effect on the shunt current and on the calibration voltage. These effects can be considered separately as described below. We are going to consider in the first instance the effect of R_tt on the shunt current k. The current, k, into the first meter sees the VTT resistor,

in parallel with the shunt resistor Rj . The calibration network resistance is also in parallel but is sufficiently large to be considered open circuit for this calculation (approximately R_shx10⁹):

If R_tt is chosen such that: R_tt = K.R_sh (2) then

^{Ittl =} κ and

~> --^~-~- O) 1+ — K The current into the second meter is \ so, from equation (3), the shunt current in this meter is given by:

It can be shown that for the penultimate meter, (N-1 ), the shunt current is given by:

In the final meter, the parallel currents from the preceding meters are recombined through R_tt so that: N ~ ^S (6) Thus for the final meter in the rack, there is no error in the shunt current through the meter but for any meter, n, where n=1 to (N-1 ), the shunt current error is given by: Error = -I = -

Expressing the error as a percentage of the source current, fe:

• Error xlOO (8)

In addition to the value of the test resistor, equation (8) shows that the shunt current error in an individual meter will depend on where the meter is in the chain of meters in the rack. The effect of Rt on the calibration voltage will now be considered below with reference to figures 4 (a) and (b). Figures 4 (a) and (b) show schematically a meter as shown in figure 2 in two different configurations: a first configuration with the meter in use (or in calibration) shown in figure 4 (a) and a second configuration with the meter in a test rack shown in figure 4 (b). Since the test voltage is applied to each meter in parallel, the number of meters being tested together is immaterial. Therefore the calculation of the error is based on an individual meter. Considering the equivalent circuit in figure 4 (a), the calibration voltage, V_CAL, is given by: R, V. 'CAL ^■XV;, (9) ^RN ^+RCAL Similarly, the equivalent voltage in the test rack configuration is given . by: V. ^^■CA ^"*^" ^TΓ TEST ^■XV,. (10) ^RN ^{+ R}CAL ^{+ R}TΓ The voltage error between the two configurations is given by: V ^ve =V ^V _™TE.ST -V ^VCAL

_N.K_TΓ v. :XY, (11) (RN+RCAL+RTΓXRN+RCA Expressing the error as a percentage of the calibration voltage: V. Error -xlOO N CAL

^■ Error 100

(12)

Thus the error in the calibration voltage is dependent on the F _\L setting, as well as the value of \ . Consequently the voltage error will be expressed as a percentage range set by the minimum and maximum available values of the calibration network. In order to illustrate the impact of Rt in the test situation, consider a typical shunt meter where the shunt resistance, F^_h, is 200μΩ. Assuming that a test rack is capable of testing up to 60 meters simultaneously, the error due to R_tt can be calculated using the equations (8) and (12) above. Some examples of error for selected Rt values are shown in the table 1 below.

Table 1 Clearly the error in the calibration voltage is too large for R_t with K=10⁶, so a preferred solution would be to use K belonging to the range [10⁴; 10⁵]. Naturally, the present invention is not limited to the example and embodiment described and shown, and the invention can be the subject to numerous variants that are available to the person skilled in the art.

Claims

CLAIM

1. Meter (10) for metering the electrical power on a power load, said meter (10) comprising: - Live In and Live Out terminals (LI, LO) adapted to be series connected in the live wire of the electrical power distribution circuit, - a neutral terminal (N) adapted to be connected to the neutral wire of the electrical power distribution circuit, - a voltage sensor (R_CAL, R_Π) in order to generate a signal responsive to a voltage provided to said load, - a current sensor (R_Sh) in order to generate a signal responsive to a current provided to said load, said current sensor being a shunt resistor connected between said Live In (LI) and Live Out (LO) terminals, - a voltage test terminal (VTT) adapted to be connected to one terminal of the voltage source (V) of a test rack, said meter being characterized in that said Live In teminal (LI) is permanently connected to said voltage test terminal VTT) via a test resistor (Rtt).