GB2149128A - 3-phase solid state energy meter - Google Patents

Abstract

Three-phase solid state power measurement apparatus determines the amplitude and sign of a series of single samples of the fundamental frequency component of n- 1 line currents in an n- conductor supply source, which when multiplied by the associated voltage samples effect a measurement proportional to a conductor power component. The total load power is derived by a single controlling and computational device whose function is to control sampling times (relative to zero-crossings of the phases of the supply) according to the requirements of VA, KW and KWR quantities, to algebraically add component powers and therefrom to derive a sequence of pulses indicative of the total load energy consumptions. <IMAGE>

Description

SPECIFICATION 3-phase solid state energy meter This invention relates to the field of power and energy measurement in 3-phase alternating current electricity supplies, using all solid state electronic circuitry.

The measurement of electricity supply power flow and totalised energy is conventionally executed by the rotating induction-wheel meter. This is an electromagnetic device whose rotation is proportional to energy and whose speed of rotation is proportional to circuit power.

The induction wheel meter is the conventional means for measuring, for example, domestic single phase supplies. A variation of the single phase induction wheel meter is also commonly used for the measurement of industrial three phase supplies. Such a three phase meter typically consists of three sets of voltage and current coils and their associated fields acting upon a single aluminium rotating wheel. The power in each set of coils is proportional to the energy flow in each phase with respect to neutral and the total rotation is proportional to the sum of energies flowing in all three phases.

An induction wheel meter can also be configured in such a manner that where there is no neutral, ie, a three phase 3 wire supply, the total energy flowing to the load can also be assessed. Alternatively two single phase meters can be utilised connected according to the well known "2-watt meter" method of measurement for three phase, 3 wire supplies.

The object of this invention is to effect power measurement on three phase 3 wire and/or three phase, 4 wire systems using solid state circuitry and dispensing with the need for electromechanical power sensing devices.

Traditional solid state means of measuring power in single phase systems utilises an analogue mutiplier sensitive to instantaneous current and voltage. The output is utilised to create a train of pulses whose rate is proportional to instantaneous power. The count of pulses is then proportional to energy consumed.

My patent, British Application No. 8319888 disclosed a novel means of effecting power measurement in single phase circuits, whereby only a single sample of the current waveform per main half-cycle effected a precise measure of power. This invention is totally accurate for linear loads. When 50 Hz filtering is applied to the current waveform before sampling the method is theoretically and practically precise also for non-linear current waveforms such as phaseswitched and electronic equipment loads. Using this principle our application No. 8319888 shows how a single sample of the magnitude of the current waveform fundamental leads to an apparatus which fully simulates the power measurement effected by the induction wheel meter.

When the aforegoing principles are extended to measurement on general three phase systems problems arise due to phase relationships of the various supply voltages and the associated line currents. The object of this invention is to effect measurement on three phase supplies in such a manner that the principles of our single-sample one-phase solid state metering methods can be incorporated in novel apparatus for measuring three phase energy.

Furthermore our invention describes means whereby the requirement for three separate multipliers or power sensors can be dispensed with and only a single microelectronic device can be utilised to accommodate power in the sum of all phases for either three phase 3 wire, or three phase 4 wire systems.

The basic function of our invention is to measure power in a three phase circuit. Such supplies may or may not include a neutral conductor and one object of our invention is to be able to measure power by the same or similar apparatus whether there is a neutral or not.

Fig. 1 illustrates a three wire supply and Fig. 2 illustrates a four wire supply. Fig. 1 shows the conventional 2-wattmeter measurements and Fig. 2 a conventional "3-wattmeter" method when four wires are present. In general it can be seen that by considering any of n wires as a 'chassis' the total power is measured by making n-1 power flow measurements on the n-1 remaining conductors each paired with 'chassis'.

As a consequence it is possible to monitor the power in a variety of ways on either 3 or 4 wire systems. Fig. 3 for example shows how power in a 4 wire system can be measured analogously to the 3 wire system of Fig. 1.

However, if measuring connections are made to a load such as in Fig. 1 or 3 it is possible that the phase of a line current with respect to an associated voltage can be greater than + 90 and this condition can not occur in a single phase power measurement unless the load is generating power. Consequently, a single phase current sample will always be positive and implementation of the apparatus need only concern itself with current magnitude. The first novel feature of our three phase measurement apparatus is that when sampling current we have to ensure that the amplitude of current is sensed, that is both its size and direction.By sensing direction we are then assured of sensing the negative power contribution that can occur in one current-voltage pair power measurement, even though the net power in all conductors must be positive for a power-absorbing load.

Fig. 4 illustrates the phases of currents and interphase voltages for a three phase, 3 wire system. Shown in Fig. 4 is the range of phases possible for 1A the A conductor line current. For a balanced load the phase can vary + 90 on 1A Since 1A is at 30 to VAB for a resistive load, it is clear that a negative power contribution can result. Fig. 5 shows another equivalent, connection of measuring apparatus to sense total power.

Referring to Fig. 5 it is clear that two current transformers are required and three voltage probes. By summing algebraically the powers as derived products VAC 1Aand V BCIB total power can be measured.

Fig. 6 shows an analogous connection for a 4-wire system and Fig. 7 a more conventional power measuring connection. Although the connection of Fig. 6 is possible it suffers from the disadvantage that the neutral current is often small and consequently currents measured are not conveniently of similar size and one voltage, VNC is smaller than the other two VAC and VBC.

Consequently the preferred connection is that of Fig. 7.

Since the one apparatus is designed to be capable of measuring either 3 or 4-wire systems it follows that the voltage sensing must be capable of accepting phase to phase voltages as on Fig. 5 as well as phase to neutral as in Fig. 7. It can be seen that the chassis connection is made by a voltage probe VR to one conductor. Pairs consisting of a current transformer and a voltage probe or connection labelled as P1, P2 and P3 are made to the conductors.

Power is monitored independently on each pair so that any pair can be associated with any conductor. In the 3 phase 3 wire system one pair is just not connected and consequently senses no power. When power to drive the equipment is taken from the same supply that is being measured it is only necessary to ensure that the voltage connections from the monitored circuit are also routed approximatey to provide power to the power supply of the meter.

Having described the essential features of our invention with respect to connection of voltage and load current sensors it is now instructive to describe the principles of the power measurement. This is effected by taking a single sample of the sensed 50 Hz filtered current waveform passing through the current transformer at the time of the voltage maximum on the associated voltage probe. Multiplication of the current sample by the probe voltage is then proportional to the energy consumed in that < cycle period of 10 milliseconds on that phase or line current with respect to 'chassis' or common.

Fig. 8 illustrates in broad principle how a single processing device can effect control of measurement functions to allow computation of both total true power KWH and total reactive power KWHR.

Voltage timing references indicative of zero crossing times T1, T2, T3, detected on the three voltage probes are indicative to the processor of when to examine current samples sensed in transformers CT1, CT2, CT3. Automatic selection of the appropriate range of voltage and current voltage magnification is selected so as to ensure good accuracy over a typically 1000:1 current range and up to 5:1 voltage range. An important feature is the fundamental filter on the current waveform ensuring accuracy on non-linear loads and a vital element of the system is that the polarity of the signal at the ADC output should be sensed by the processor. It is here that a negative sample (with respect to the mean level) indicates negative power. The sum of all powers derived within the processor must produce a positive result.Clearly all current transformers are connected round the load conductors in the same sense.

Voltage sensing is made most simply on the mean voltages derived from full or half wave rectificed derivatives of the three voltages sensed by the three voltage probes. Since these voltages are sensibly constant they can be sampled at a time convenient to the processor.

In the same manner that a processor samples a mean voltage related to phase voltage it can also sample a voltage related to peak filtered current. By so doing the processor can compute total VA instead of Vl or in addition to VI. There is the same freedom in sampling peak current since it too is made sensibly constant for the duration of a few cycles of the mains voltage and varies only slowly as the power demand changes.

Fig. 9 shows a timing diagram of the samples that must be taken, by example on a 3 phase-3 wire system. It can be seen that the processor has 60 of phase (ie 3.33 ms) to effect sampling and Vl multiplication before the next sample is due. This is quite adequate time using a fast processor operating at around 11 MHz clock time. Samples of voltage and peak cycle current can be taken at any time convenient to the processor, at intervals of one, or if preferred several, half cycles. If reactive power is also measured the samples, shown dotted, are required.

The samples required for a full 3 phase, 4 wire system are shown in Fig. 10.

A specific embodiment of the invention will now be described by way bf example with reference to the accompanying drawings in which Figure 1 shows power flow on a 3 phase, 3 wire supply Figure 2 shows one representation of power flow on a 3 phase, 4 wire supply Figure 3 shows an alternative representation of power flow on a 3 phase, 4 wire supply Figure 4 is a phasor diagram for a 3 phase, 3 wire supply Figure 5 illustrates placement of voltage sensing and current transformer sensors for a 3 phase, 3 wire system Figure 6 illustrates placement of voltage sensing and current transformer sensors for a 3 phase, 4 wire system analogous to Fig. 4 Figure 7 illustrates conventional placement of voltage sensing and current transformer sensors for a 3 phase, 4 wire system Figure 8 is a block diagram illustrating how a single processor can be configured to produce pulses indicative of 3 phase energy consumption for KWh, and KWhR.

Figures 9 s 10 are timing diagrams indicative of the sampling actions required for 3 wire and 4 wire measurements respectively.

Figure 11 is a detailed diagram of specific embodiment of the invention Figures 12, 13, 8 14 are flow charts indicative of functions to be performed by the digital program controlling the specific embodiment Figs. 1 to 10 have already been referred to whilst explaining the essential technical features of our invention and illustrate the reasoning behind the specific example of an embodiment shown in Fig. 11.

Fig. 11 shows a complete embodiment of a solid state 3 phase, 3 wire/3 phase, 4 wire power and energy meter. Facilities provided by this detailed embodiment include the following: Displays of True power KW Apparent power KVA Reactive power KWR Maximum demand in previous maximum demand period (KVA or KW).

A time reference input for establishing maximum demand periods.

A time reference reset input for external syncronisation of maximum demand periods.

Pulses indicative of True energy KWh Reactive energy KWhR Apparent energy KVAh A data transmission facility for relaying maximum power demand.

Handshake facilties for resetting output energy pulses optionally employed by an external controller sensitive to the energy pulses.

In this specific embodiment voltages V1, V2, V3 are provided as outputs from the voltage isoiating transformers, 1. The voltage outputs follow the voltages between the selected common (in this diagram it is Neutral) and the phase voltages.

The body of the circuitry can be at earth potential since the monitored voltages are isolated.

Current transformers 2 produce currents which are typically 1/1000 of the current circulating in the three line conductors.

The current transformers can be of any standard type. These output currents are intrinsically isolated from the supply and can feed directly to load resistors, 3, typically of value 3 ohms for a maximum load current of 100 Amp or 0.3 ohms for a maximum load current of 1000 Amps.

The power supply, 4, can be fed from the load supply or a separate source as is most convenient.

An alternative measuring connection, 5, is shown when the apparatus is used to measure a 3 phase, 3 wire supply. One pair of voltage-current measuring sensors is just not connected.

Voltages V1, V2, V3 are utilised directly to detect the times of their zero crossings in the zero crossing detect circuits 6, and zero crossing logic 7. Outputs from the zero crossing logic indicate the time of occurence of the zero crossing, which voltage it has occured on, and whether it is on the positive-going or negative-going portion of the cycle.

A dc level indicative of the amplitude of the three voltages V1, V2, V3 is derived by process of rectification and detection of the mean level of the rectified signal in the voltage level detect circuits 8. Such a process gives a voltage which is reliably proportional to mains RMS voltage even in the presence of noise spikes and as long as the mains is near to sinusoidal.

The three currents 11, 12 and 13 are sensed in the load resistors 3 and fed into 50 Hz low pass filters, 9. The output of these filters is such that there is a 40 dB rejection at 1 50 Hz so that the filter output is near to a sinewave for any load current waveform. The magnitude and phase of the output filter sinewaves when multiplied by the associated voltage amplitude is directly proportional to the power in each circuit.

The filtered outputs are peak detected in the current amplitude sensing circuitry, 10. The amplitude of these current voltages when multiplied by the associated mains voltages is directly proportional to the voltamperes in the measured load.

In order to derive total 3 phase power the processor, 11, in conjunction with an analogue to digital converter, 12, must measure all three voltages V1, V2, V3 and three current voltages.

For true power the current voltage X is proportional to the current (I cos (p) at the filter output at the time of the peak of the associated voltage V and + is the phase difference between V and 1.

When sensing KVA, apparent power, the peak detected currents Al, A2, A3 can be sampled at any time since the time constant of these circuits is several cycles of mains voltage and remain sensibly constant in one cycle of mains.

The ADC utilised in this embodiment is an 8 bit analogue to digital converter with 8 separate selectable analogue inputs. 3 each of these are utilised as current inputs from the current amplifiers, 13, and voltage amplifiers, 14.

Selection of voltage to be measured is effected by control of the quad bilateral switch, 15, a CMOS 4066 integrated circuit.

Selection of current phase to be measured is effected by control of the parallel quad bilateral switches, 1 6. Selection of I or A is effected by control of the two way switch, 1 7. Selection of I effects measurement of a varying voltage and software in the processor decides the timing for a sample I sin f (reactive) or a sample I cos (p true power component. Selection of the slowly varying A allows a sample to be taken any time convenient to the processor.

At all times it is the processor and its program which is controlling the analogue switches according to the dictates of the zero crossing inputs to effect the right sample measurements at the correct time and in the correct sequence.

In order to preserve good accuracy the current inputs to the processor are multiplied by 1 by 10 and by 100. This means that a range of currents varying over a 1000 to 1 range can be accurately sensed. In order to cope with the requirement for phase to neutral and phase to phase voltage inputs typically 240 and 41 5 volts respectively three ranges of voltage sensing inputs are provided to the processor. By having gains of X1, X2, X4 voltages between 11 0V and 550V can be measured and maintain an accuracy of 1%. For both current and voltage inputs the processor decides by examining the output of each amplifier which is the appropriate amplifier in range for that measurement.

The processor provides pulse outputs indicative of set increments of energy. The energy increment is set to 10 Wh, 100 Wh, 1 KWh or 10 KWh by the input 18. Pulse duration (or handshake option) is set by inputs 19 at 1 sec at 60 seconds 1/10 second 1/100 second or handshake.

Pulse outputs are provided, 20, for true energy, apparent energy and reactive energy driving through pulse interfacing circuitry 21. The pulse interfacing circuitry provides opto isolated 10 mA signals and clean relay output circuits for interfacing the pulses to other equipment. The handshake input is also opto isolated and accepts up to 50 mA input current. In order to effect all the necessary samples and multiplications within the mains cycle time, a 8749 processor is utilised running at 11 MHz controlled by the crystal 22.

A peripheral clock chip, 23, provides a + hour or other period time reference. This can be synchronised to an external interval reference by the maximum demand input, 24. The processor then keeps a record of the maximum power drawn by the load in each half hour (or other) period.

The maximum demand output is in the form of a serial digital signal, 25, indicative of the maximum demand, repeated at intervals and available for reception by an external controller.

The maximum demand quantity can be selected to be either KVA or KWatts according to the select input 26.

In this embodiment a display 27 is provided which allows the display of output information from the processor. The display indicates the load true power, apparent power and reactive power as well as the maximum demand of true apparent or reactive power in the current half hour maximum demand period.

Figs. 12, 1 3 and 14 are flow diagrams of the software routine followed by the processor to effect all of the functions so far described in this detailed embodiment. Fig. 12 is the executive which oversees the complete energy monitoring function and reacts to the position of presetting switches.

Fig. 13 shows the functions required in monitoring the various powers and energies flowing, totalising these over 2 or 3 voltage current pairs and setting pulse output signals.

Fig. 14 shows the display and pulse output functions.

A complete flowchart and its equivalent listing is not provided since from the foregoing description of my invention the software functions that need to be implemented in producing a three phase measurement utilising a single processor are quite clear.

The complete embodiment consists of appropriate software as outlined above operating in conjunction with a single processing device to control appropriate single sampling on each half cycle of two or three voltage current sensing pairs. The energy in each half cycle (or several half cycles) of each voltage-current pair is algebraicly added in the processor's data registers and output pulses are produced each time a preset energy increment is consumed corresponding to VA, VI, and Vl Reactive. Auxilliary displays of all powers and maximum period demands are maintained and a data output is available of a selected maximum demand quantity.

Claims (14)

1. Apparatus for the measurement of 3-phase alternating electricity supply consumption whose measurement circuitry consists solely of electronic components, such circuitry being responsive to the times of voltage zero crossings of the supply voltages in such a manner as to effect a single sample of the fundamental component of each of a same number of associated load currents in each half cycle of said voltages at times bearing a fixed relation to said zero crossings, said samples being sensed in both magnitude and sign being indicative when multiplied by the associated voltage magnitude of the magnitude and direction of flow of power in each conductor.

2. Apparatus according to claim one whereby total power flow to a load is derived from the algebraic summation of a number of invidual powers each such power being derived from the algebraic product of a single sample of current in one half cycle of an associated voltage and the amplitude of the said associated voltage.

3. Apparatus according to claims one and two whereby total power flow is derived from measurements of current flow on a number of supply conductors each paired with one common conductor and associated measurements of voltage magnitude between the same conductors and the same common conductor, such that the total number of power components contributing to the algebraic summation of total power is equal to one less than the total number of conductors constituting the source of electricity supply to a load.

4. Apparatus according to claims one to three where the power flow in each half cycle of measured current and voltage pair or the algebraic sum of each power flow in the duration of a complete mains cycle is convered by multiplication by an appropriate constant to a quantity indicative of energy in each half cycle of one measured voltage or the total energy in a complete cycle of one measured voltage.

5. Apparatus according to claims one to four whereby total energy flow in a complete cycle is derived from the algebraic summation of constituent power components such energy being accumulated in successive complete mains cycles such that means are provided whereby a succession of pulses is provided each pulse being indicative of the consumption by the load of a present value of energy.

6. Apparatus according to claims one to six whereby the algebraic summation of power and accumulation of energy and the production of pulses indicative of energy increments as well as the control of the sampling of constituent voltage current pairs is effected by a single controlling and computation device.

7. Apparatus according to claims one to six whereby current and voltage samples are taken by one or several analogue to digital converters, in combination with a number of analogue switches whose outputs are provided to said analogue to digital converters and whose several inputs are voltages proportional to measured load currents and measured supply voltages.

8. Apparatus according to claims one to seven whereby a succession of outputs from one or several analogue to digital converters is indicative of the amplitudes of supply voltages and currents such analogue to digital converter outputs being utilized by a single or several computation and controlling devices to compute total supply power flow and accumulated load energy consumption.

9. Apparatus according to claims one to eight whereby sample measurements of the filtered fundamental magnitude component of current are made at the peak of associated voltages therefrom being derived by the apparatus real load power and accumulated energy.

10. Apparatus according to claims one to eight whereby sample measurements of the filtered fundamental component of current magnitude are made at the zero crossing time of associated voltages therefrom being derived by the apparatus reactive load power and reactive or oscillatory energy.

11. Apparatus according to claims one to eight whereby sample measurements of current amplitudes are made at any time during the period of a mains cycle or half-cycle such current amplitudes being the peak value of the filtered fundamental component of load current with respect to its mean level and therefrom being derived the apparent power, KVA, or apparent energy KVAH.

1 2. Apparatus according to claims one to eleven whereby are provided three current sensing inputs and four voltage probe inputs sufficient to measure load power in 3 phase 4 wire supplies or any lesser number of phases and/or wires.

1 3. Apparatus according to claims one to twelve where means are provided to display or to communicate in a data transmitting means the real, apparent and reactive powers and the respective energies consumed within the duration of a selectable or preset period.

14. Apparatus according to claims one to thirteen where a time reference, synchronisable to external synchronising means, effects sensitivity to consecutive time periods wherein controlling and computing means records the maximum real, reactive or apparent total load power and means whereby said maximum power.or;powers is displayed or communicated via data transmitting means.

1 5. Apparatus according to claims one to fourteen whereby pulses indicative of various total load energy consumptions may be settable to be indicative of various incremental values of energy such pulses being of selectable duration.

1 6. Apparatus according to any of the preceeding claims one to fifteen wherein phase shift occurring within a fundamental mains frequency filter is accommodated by a compensating shift of sampling times effected by a controlling device.

1 7. Apparatus according to any of the preceding claims one to sixteen as illustrated by the accompanying drawings one to fourteen which effects measurement of multiphase power and comprising means effective to produce a series of output pulses indicative of various energies consumed by a load.