Wye service power meter

A device for monitoring power in a wye power service having three phases includes a sampling element arranged and configured to sample a current value and a voltage value from each of the three phases. A processor, coupled to said sampling element, calculates power either (i) by using the current value and the voltage value for the at least one phase when the voltage value for the at least one phase is available, or (ii) by using the current value for the at least one phase and the voltage values for the other two phases if the voltage value for the at least one phase is unavailable.

BACKGROUND AND SUMMARY 
This invention relates to a meter for monitoring power usage in a multiple 
phase electrical service. 
In order to deliver electrical energy to a factory or other high power 
user, a four wire wye service, having one wire for each of three phases of 
power plus one wire for neutral, is often employed. A representation of a 
four wire wye service is illustrated in FIG. 1, in which three phases, A, 
B, C, and neutral N are arranged in a wye ("Y") configuration. Symbols 
V.sub.A, and V.sub.B, and V.sub.C represent voltage vectors for potentials 
between N and terminals A, B, C, respectively. Symbols I.sub.A, I.sub.B, 
and I.sub.C represent current vectors for the three phases A, B, C, 
respectively. 
Power for each of the phases A, B, and C is determined by calculating the 
scalar product (or "dot" product) of the voltage vector and the current 
vector for the phase under consideration, as represented in Equations (i), 
(ii), and (iii): 
EQU P.sub.A =V.sub.A .multidot.I.sub.A (i) 
EQU P.sub.B =V.sub.B .multidot.I.sub.B (ii) 
EQU P.sub.C =V.sub.C .multidot.I.sub.C (iii) 
The total power for the service is calculated by summing the power for each 
of the three phases A, B, and C, as represented in Equations (iv) and (v): 
##EQU1## 
In an ideal power distribution system, because the voltage and current 
vectors are in phase with each other, all power would be real (or 
"active") power, measured in units of Watts. In practical applications, 
however, reactive loads cause the voltage and current vectors to include a 
component that 90.degree. out of phase. This out-of-phase component 
produces imaginary (or "reactive") power consumption. The imaginary 
component is measured in units of volts-amps reactive, or VARs. The 
imaginary component of power is calculated by delaying the voltage vector 
from the current vector by -90.degree., as represented in Equations (vi), 
(vii), and (viii): 
EQU VAR.sub.A =V.sub.A(-90.degree.) .multidot.I.sub.A (vi) 
EQU VAR.sub.B =V.sub.B(-90.degree.) .multidot.I.sub.B (vii) 
EQU VAR.sub.C =V.sub.C(-90.degree.) .multidot.I.sub.C (viii) 
In order to accurately monitor power usage, the power provider must be able 
to determine the total amount of power--both real and imaginary--that is 
consumed by a customer. In many applications (e.g., so-called 9S and 16S 
metering), the power monitoring calculations are performed by providing 
both voltage and current for each of the three phases in the service to 
the meter, as well as a parameter representative of the neutral, N. This 
allows Equation (v) (known as the "unrestricted" metering equation) to be 
used to calculate total real power consumption. Similarly, total imaginary 
power consumption may be calculated by summing the individual imaginary 
power components for each of the three phases represented in equations 
(vi), (vii), and (viii). 
In certain types of power metering (e.g., 6S metering), however, only two 
of the three line-to-neutral voltages are brought to the meter from the 
four wire wye service. Accordingly, the unrestricted metering equation 
cannot be used by a 6S meter in calculating power for a four wire wye 
service because one of voltages V.sub.A, V.sub.B, and V.sub.C is not 
provided to the meter. Instead, the 6S meter uses a derivative of the 
unrestricted metering equation, which is estimated by placing a 
phase-balance restriction upon the service, which simplifies the equation. 
FIG. 2 shows that V.sub.A, V.sub.B, and V.sub.C maintain approximately the 
same magnitude and phase relationship. The phase diagram of FIG. 2 uses a 
plurality of voltage vectors V.sub.A, V.sub.B, and V.sub.C, which have 
substantially the same magnitude. Each consecutive vector is separated by 
approximately 120.degree.. 
The above assumption allows power for a particular phase to be calculated 
by using the current vector for the phase under consideration and the 
voltage vectors for the other two phases. For example, assuming the 
voltages for phases A and C are available, and the voltage for phase B is 
unavailable, the power for phase B is calculated as follows. First, 
because each of the three phases is assumed to have the same magnitude and 
phase relationship, voltage vector B is represented by the following 
equation: 
##EQU2## 
Similarly, in order to calculate the reactive power, voltage vector B is 
delayed by -90.degree., as represented by the following equation: 
EQU V.sub.B(-90.degree.) =-(V.sub.A(-90.degree.) +V.sub.C(-90.degree.))(xi) 
Next, the power (Watts) for phase B is calculated by Equation (xii), below, 
which is derived by substituting Equation (x) into Equation (ii). 
Similarly, the reactive power (VARs) is calculated by Equation (xiii), 
below, which is derived by substituting Equation (xi) into Equation (vii), 
as follows: 
EQU P.sub.B =-(V.sub.A +V.sub.C).multidot.I.sub.B (xii) 
EQU VAR.sub.B =-(V.sub.A(-90.degree.) 
+V.sub.C(-90.degree.)).multidot.I.sub.B(xiii) 
Because voltages and currents for the remaining two phases--A and C--are 
available, the real and reactive power components for phases A and C can 
be calculated using Equations (i), (iii), (vi) and (viii), respectively. 
The total power for a 6S meter hence may be calculated using the following 
equation: 
EQU P.sub.Total =V.sub.A .multidot.(I.sub.A -I.sub.B)+V.sub.C 
.multidot.(I.sub.C -I.sub.B) (xiv) 
Equation (xiv) represents the equation that is implemented on an 
electromechanical 6S meter. The current I.sub.B is subtracted from I.sub.A 
to form a single current which is then multiplied by V.sub.A to form one 
element of the metering. The current I.sub.B also is subtracted from 
I.sub.C to form a single current which is then multiplied by V.sub.C to 
form the other element of the metering. The sum of the power registered on 
these two elements represents the total energy registration of the 6S 
meter. 
The inventors of the present invention recognized that these two materially 
different ways of metering power could not be handled by a single unit, 
and developed a way to obviate this drawback. 
The control circuitry for a solid state power meter according to the 
present invention allows the same basic circuit board to be used in a 
number of different meters having varying metering requirements; the 6S, 
9S, and 16S described above. The circuit board has the capabilities to 
accept all three currents from a three phase, four wire wye service and to 
accept either all three or only two of the voltages from the wye service 
depending on the meter wiring scheme. Power is calculated using either (i) 
an unrestricted metering equation which uses the voltage and current for 
each phase; or (ii) a phase-balance restricted metering equation in which 
power for the phase for which the voltage is unavailable is calculated 
using the voltages of the other two phases and the current for the phase 
under consideration. Hence, the same control circuitry can be used in 
different types of meters. Because the same circuit board may be used in 
6S and 9S meters, among others, a marked increase in manufacturing 
efficiency, and corresponding decreases in time and expense, are realized. 
Other advantages and features will become apparent from the following 
description and claims.

DETAILED DESCRIPTION 
In many industrial applications, a polyphase solid state meter, as opposed 
to a mechanical or electromechanical meter, is used to determine real 
power (Watts) and imaginary power (VARs) associated with a particular 
power line. FIG. 3 shows a front perspective view of a typical solid state 
meter 34 which includes a digital display 36 for displaying power usage; a 
protective face plate 38; and a cover 40. 
The main components of a solid state meter are illustrated in block diagram 
form in FIG. 4. The meter system includes two A/D converters 26 and 28 for 
respectively sampling voltage values and current values from the wye 
service. The sampled current and voltage values are provided to central 
processing unit (CPU) 22 which controls various meter functions and 
calculates energy usage. A single chip computer (e.g., Motorola 68HC11 
series) having on-board read only memory (ROM), random access memory 
(RAM), various timers and input/output ports may be used as CPU 22. The 
on-board ROM contains a firmware program that is executed by CPU 22 to 
control meter functions. A storage memory 24 is provided to store sampled 
data values, while logic element 30 includes circuitry to perform, under 
control of CPU 22, metering functions, energy demand calculations, meter 
calibration, etc. Display 32 can be a digital display like display 36 of 
FIG. 3. 
Although four wire wye service meters such as 9S and 16S use currents and 
voltages from all three phases in registering power consumption, 6S 
metering, in which all three currents but only two phase voltages are 
available, remains in use in many service installations. Accordingly, the 
polyphase solid state meter of the present invention uses analog/digital 
(A/D) sampling for calculation of voltages, currents, and energy 
registration, and provides the capability of registering power using 
conventional 6S metering techniques. 
Specifically, the solid state meter according to the present invention that 
is employed in a 6S metering environment meters energy as accurately as an 
electromechanical 6S meter in both polyphase and series configurations and 
does so using only two of the three phases' line-to-neutral voltages. 
At the same time, the control circuitry for the present solid state meter 
retains the ability to employ the unrestricted metering equation for 
registering power when placed in a metering environment such as the 9S 
environment. In this way, although the 6S and 9s are uniquely different 
meters, they may use essentially the same circuitry. All that differs is 
the connection of the meter to the service. Accordingly, a uniform 
configuration of control circuitry can be used among different types of 
meters, thereby streamlining the development process and reducing 
attendant manufacturing time and costs. 
One embodiment of the control circuitry for a polyphase solid state meter 
has the capability to register power both using the unrestricted metering 
equation, as well as the restricted phase-balance equation (i.e., the 6S 
metering equation). In addition, the control circuitry has the capability 
to receive currents from all three phases of a four wire wye service, and 
to receive voltages either from two or three phases, depending on how many 
are available. Consequently, the control circuitry may be used either in a 
6S meter or in other metering systems that make use of the unrestricted 
metering equation (e.g., 9S or 16S metering). 
The preferred meter of the present invention is a fully solid state, 
polyphase meter that employs the above-described features. This meter has 
a basic configuration which handles 6S, 9S and 16S metering environments. 
In the 9S and 16S mode, the meter control circuitry accepts voltages from 
all three phases and calculates power using the unrestricted metering 
equation. 
In the 6S metering mode, in contrast, only two of the three line-to-neutral 
voltages are brought to the meter from the four wire wye service. In this 
mode, the meter control circuitry accepts the two available voltages and 
calculates power using the phase-balanced restricted metering equations 
(xiii) and (xiv). 
As shown in FIG. 5, the present meter includes front-end module 42, 
register module 48, and liquid crystal display (LCD) 54. Front-end module 
42 (including Front End CPU 44, A/D converters 26 and 28, storage memory 
45, and general logic 46) controls metering functions such as voltage and 
current measurements, power calculations, power calibrations, etc., by 
means of a firmware program stored in on-board ROM and executed by Front 
End CPU 44. Register module 48, on the other hand, has its own firmware 
program to control non-metering system functions such as display and 
instrumentation, input/output, and the like. 
Operation of a 6S meter according to the present invention is described in 
more detail with reference to the flowchart in FIGS. 6a and 6b. Upon 
start-up of the meter in step 601, Front End CPU 44 reads, in step 603, a 
meter identifying variable from the firmware embedded in the on-board ROM 
to identify the type of metering environment (e.g., 6S, 9S, 16S) in which 
the control circuitry is to be used. The sample counter, n, is initialized 
to 0 in step 605. 
In step 607, Front End CPU 44 causes current A/D converter 28 to sample a 
current value and voltage A/D converter 26 to sample a voltage value from 
phase A of the wye service. These current and voltage samples are 
represented as I.sub.A (n) and V.sub.A (n), respectively, where n is the 
sample counter. 
Next, in step 609, the Watt product (V.times.I) for phase A at sample n is 
accumulated for later use in calculating calibrated and aggregate Watts 
and VARs, as discussed in greater detail below. Similarly, at step 611, 
the VAR product (V.sub.-90. .times.I) for phase A at sample n is 
accumulated for later use. The 90.degree. phase shift of the phase A 
voltage is achieved by using sample number (n-2)--i.e., VAR 
product=V.sub.A (n-2).times.I.sub.A (n). 
At step 613, Front End CPU 44 examines the meter identifying variable to 
determine if the meter is operating in a 6S metering environment. When 
Front End CPU 44 determines at step 613 that the meter identifying 
variable in the firmware is set to correspond to a meter other than a 6S 
meter, step 637 is executed in which Front End CPU 44 causes current A/D 
converter 28 and voltage A/D converter 26 to sample, respectively, a 
current value and a voltage value for phase B of the wye service. These 
current and voltage samples are represented as I.sub.B (n) and V.sub.B 
(n), respectively, where n is the sample counter. 
At steps 639 and 641, the Watt product and the VAR product for phase B are 
calculated in essentially the same manner as for phase A, discussed above 
with respect to steps 609 and 611. 
On the other hand, if, at step 613, the meter identifying variable 
indicates that the meter is operating in a 6S metering environment, Front 
End CPU 44 causes current A/D converter 28 to sample a current value for 
phase B, I.sub.B (n), and voltage A/D converter 26 to sample voltage 
values from that used for phases A and C, V.sub.A (n) and V.sub.C (n), 
respectively, in step 615. 
Next, in steps 617 and 619 the Watt and VAR products for phase B are 
accumulated. However, because the phase B voltage in 6S metering is 
unavailable for measurement, the Watt and VAR products for phase must be 
calculated in a manner different from phases A and C. Specifically, the 
Watt product for phase B in a 6S meter environment is determined according 
to Equation (xv): 
EQU Watt Product=(-V.sub.A (n).times.I.sub.B (n))+(-V.sub.C (n).times.I.sub.B 
(n)) (xv) 
The VAR product for phase B in a 6S meter environment is determined 
according to Equation (xvi): 
EQU VAR Product=(-V.sub.A (n-2).times.I.sub.B (n))+(-V.sub.C 
(n-2).times.I.sub.B (n)) (xvi) 
In step 621 Front End CPU 44 causes current A/D converter 28 to sample a 
current value, I.sub.C (n), and voltage A/D converter 26 to sample a 
voltage value, V.sub.C (n), from phase C of the wye service. In steps 623 
and 625, the Watt and VAR products for phase C at sample n are accumulated 
in essentially the same manner as for phase A, discussed above with 
respect to steps 609 and 611. As with phases A and B, the phase C Watt and 
VAR products will be used subsequently in calculating calibrated and 
aggregate Watts and VARs. 
After the sample counter has been incremented in step 627, Front End CPU 44 
determines, in step 629, if a predetermined number of samples, k, has been 
measured. Multiple samples are used because power usage varies 
significantly from instant to instant in an actual installation. Because 
the sampled values of current and voltage represent discrete values 
measured at isolated points in time, using only a single set of sampled 
values would result in incomplete and potentially erroneous power 
registration. Accordingly, in order to calculate actual power consumption 
more accurately, the meter registers power by summing over the 
predetermined number of samples, k (e.g., 481). The quantum of power that 
is measured over k samples is referred to an "energy increment." 
In the case that the number of measured samples n is less than the 
predetermined number of samples k, the process returns to step 607 to 
sample the next iteration of current and voltage values for phase A. Steps 
607 through 629 are repeated k times--i.e., until k sets of Watt products 
and VAR products are accumulated for each phase. 
After an appropriate number of products have been accumulated, power 
calculations are performed in step 631 to determine the calibrated Watts 
and VARs for each of the three phases of the four wire wye service. Power 
for phases A and C is calculated using the following equations in which 
Equation (xvii) represents the active power component and Equation (xviii) 
represents the reactive power component: 
EQU P.sub.f =Q/K {n=1 to k}.SIGMA.V.sub.f (n).times.I.sub.f (n)!(xvii) 
EQU VAR.sub.f =Q/K {n=1 to k}.SIGMA.V.sub.f (n-2).times.I.sub.f (n)!(xviii) 
In Equations (xvii) and (xviii), f is the phase under consideration (A or 
C); V.sub.f (n) is individual voltage sample number n for the phase under 
consideration; V.sub.f (n-2) is individual voltage sample number n, 
delayed by -90.degree., for the phase under consideration; I.sub.f (n) is 
individual current sample number n for the phase under consideration; k is 
the predetermined number of samples over which power is measured; and Q is 
a measured, meter-specific constant to account for gain, calibration, and 
other such parameters that vary from meter to meter. 
In the above calculations, Front End CPU 44 can use the unrestricted 
metering equation to calculate active and reactive power for phases A and 
C because voltage and current values are available to the 6S meter for 
each of those phases. However, because an actual voltage for phase B is 
not provided to the voltage A/D converter to be sampled, the following 
phase-balance restricted equations are used by Front End CPU 44 to 
calculate active and reactive power, respectively, for phase B: 
EQU P.sub.B Q/K {n=1 to k}.SIGMA.(-V.sub.A (n).times.I.sub.B (n))+(-V.sub.C 
(n).times.I.sub.B (n))! (xix) 
EQU VAR.sub.B Q/K {n=1 to k}.SIGMA.(-V.sub.A (n-2).times.I.sub.B 
(n))+(-V.sub.C (n-2).times.I.sub.B (n))! (xx) 
In Equations (xix) and (xx), V.sub.A (n) and V.sub.C (n) are individual 
voltage samples number n for phases A and C, respectively; V.sub.A (n-2) 
and V.sub.C (n-2) are individual voltage samples number n, effectively 
delayed by -90.degree., for phases A and C, respectively; and I.sub.B (n) 
is individual current sample number n for phase B. Q and k have the same 
meaning as in Equations (xvii) and (xviii). 
After Front End CPU 44 calculates the power for each individual phase, the 
total, aggregate power is calculated in step 633 by summing the individual 
phase components for Watts and VARs, respectively. 
Next, in step 635, Front End CPU 44 passes the calculated energy increment 
to Register CPU 50, which, under control of display logic 52 and its own 
ROM-embedded firmware, displays the power registration information on LCD 
54. Register CPU 50 also stores the energy increment information in 
storage memory 51 for future reference. 
Thereafter, Front End CPU 44 returns to step 605 to begin sampling for the 
next energy increment. 
The same control circuitry and operation as shown in FIGS. 5 and 6a-b, and 
as described above with respect to the 6S meter, can be used in other 
types of meters, for example, in 9S and 16S meters, which utilize the 
unrestricted metering equation rather than the restricted phase-balance 
equations employed in the 6S meter. All that needs to be done is to change 
the value of the meter identifying variable in the firmware embedded in 
the on-board ROM in Front End CPU 44 to identify the type of metering 
configuration in which the control circuitry is to be used. FIGS. 6a and 
6b comprise an example of a selection mechanism. 
As indicated in FIGS. 6a-b, operation of the meter control circuitry in a 
16S meter is the same as its use in a 9S meter. However, certain burden 
resistors on the circuit card have to be replaced for use in the 16S meter 
to account for a difference in current ratings between the meters--namely, 
a 16S meter is rated at 200 amperes whereas the 9S (as well as the 6S) is 
rated only at 20 amperes. 
Although the above-described embodiment uses firmware embedded in on-board 
ROM, those skilled in the art will recognize that various modifications 
could be made without departing from the spirit of the invention as 
defined by the appended claims. For example, a separate ROM (i.e., off 
board the CPU) or other type of non-volatile memory element (e.g., 
magnetic media, battery backed-up RAM, etc.) could be used to store the 
firmware control program. Alternatively, if a volatile RAM is used in 
place of the ROM, the firmware appropriate to the particular metering 
configuration could be downloaded to the RAM from a host computer each 
time the meter was turned on. 
Further details of the control circuitry used in a solid state power meter 
are described in co-pending U.S. patent application Ser. No. 08/037,938, 
which is incorporated herein by reference. 
Other embodiments are within the scope of the following claims.