GB2061641A - Method and apparatus for controlling maximum electricity demand - Google Patents

Abstract

The invention relates to the control of maximum electricity demand to an installation. The invention provides a maximum demand controller including means for measuring the current, 23, and means for measuring the voltage, 22, supplied to an installation, means for multiplying the measured current and voltage to determine the power consumed by the installation, 25, and a microcomputer, 30, for periodically comparing the power consumption with a preset target value, and for controlling the electricity supply to individual loads within the installation to ensure that the average power consumption of the installation during any demand interval does not exceed the preset target value. <IMAGE>

Description

SPECIFICATION Method and apparatus for controlling maximum'electricity demand The present invention relates to the control of the maximum electricity demand of an electricity consumer.

According to certain tariff systems, electricity consumers are charged under two headings.

One is a charge for the total numer of units (kilowatt-hours) used during the billing period. The other is a charge based on the maximum demand. The demand is measured in kilowatts and is obtained by calculating the average power consumption during consecutive time periods known as 'demand intervals'. The 'maximum demand' is the highest demand recorded for any of these demand intervals during the demand period.

The demand intervals, which may be of 1 5 minutes duration, may occur between certain hours, for example between 08.00 and 21.00 hours, Monday to Friday inclusive. The actual charges to the consumer which are based on his maximum demand are somewhat complex, but invariably however the higher the maximum demand, the greater these charges.

A consumer with a high maximum demand and a poor load factor indicates a high degree of load fluctuation. If it were possible to smooth out these load fluctuations, an improvement in load factor and hence a reduction in maximum demand could be achieved, thus reducing electricity costs to the consumer. In addition, the smoothing of consumers' load profiles contributes to a reduction in system peak and improves the utility load factor.

Since many consumers have non-critical loads hereinafter called "optional loads" such as water heating, air-conditioning, refrigeration, and the like, improved load factor can be achieved by interrupting the supply to these loads when demand is high.

An apparatus for switching in and out optional loads to control the supply to a plurality of loads to ensure, inter alia, that it does not exceed a predetermined value is known as a "maximum demand controller". A maximum demand controller monitors the demand and, within the demand interval, interrupts and reconnects the supply to the optional loads. A consumer provided with a demand controller may set a power consumption value on the demand controller which he does not wish to exceed. Such a value is hereinafter called the "preset target value".

The present invention provides a maximum demand controller including means for measuring the current and means for measuring the voltage supplied to an installation, means for multiplying the measured current and voltage to determine the power consumed by the installation, and a microcomputer for periodically comparing the power consumption with a preset target value, and for controlling the electricity supply to individual loads within the installation to ensure that the average power consumption of the installation during any demand interval does not exceed the preset target value.

In a preferred embodiment of the invention, the demand controller includes signal multiplexer ing means for routing the measured current and voltage signals sequentially from each phase of a multiphase supply to the multiplier means, a voltage to frequency converter for receiving the output from the multiplier means, and a counter for receiving the output from the voltage to frequency converter and for converting the output into a form acceptable to the microcomputer.

The demand controller may also include conditioning means intermediate the voltage measuring means and multiplexing means and intermediate the current measuring means and multiplexing means for providing signals acceptable to the multiplexing means.

Advantageously, the demand controller incorporates means for biassing the voltage to frequency converter so that the latter has an output in the absence of an input signal from the multiplier and can therefore be calibrated by the microcomputer.

The demand controller also has timing means governed by the frequency of the supply voltage for enabling the microcomputer to control the sequential operation of the multiplexing means, and for disabling the counter between measurement of each phase. The timing means also enables the microcomputer to maintain a real time clock in software, and enables the maximum demand controlled to provide a real time based record of the microcomputer control operation.

In the preferred embodiment of the invention, the demand controller includes means for recognizing signals from the supply at the commencement and conclusion of peak hour periods and demand intervals, and means which provide operator access to the programme, enabling the target value to be set and enabling the real time software clock to be corrected.

A maximum demand controller according to the invention, hereinafter referred to as the Demand Manager, will now be described more particularly with reference to the accompanying drawings in which: Figure 1 is a schematic view of the Demand Manager, showing the input and output; Figure 2 is a block diagram of the Demand Manager; Figure 3(a) is a circuit diagram of the interface for conditioning the measured voltage signals; Figure 3(b) is a circuit diagram of the interface for conditioning the measured current signals; Figure 4 shows the circuit for converting the conditioned voltage and current signals to a train of pulses to be passed to a counter; and Figure 5 shows the zero-crossing detector timing circuit.

The Demand Manager and the method will initially be described in broad outline.

A maximum demand figure (0-9999kW) which the consumer does not wish to exceed is entered on the Demand Manager by means of thumbwheel switches 1 0.

To enable the Demand Manager to calculate the average power consumption during the demand intervals, it is fed with the three phase voltages and neutral at the supply point and also three current transformer currents corresponding to the three line currents at the supply point.

To ensure that the Demand Manager synchronises with the Utility's measuring periods, the start of each demand interval is signalled by the actuation of a voltage free contact of a relay which may be provided by the Utility and operated from its metering equipment. On receipt of this signal, the Demand Manager commences measuring the energy consumption. The energy consumed divided by the time over which the measurement is made is the average power consumption for that time period. Thus at the end of the demand interval the above calculation would yield a value corresponding to the value recorded on the Utility's Maximum Demand Meter.

The Demand Manager does this calculation each minute (the switching interval) and compares the result with the target value set on the thumbwheel switches 1 0. If at the end of any minute in the demand interval the average power consumption exceeds the target value, loads are switched out according to a programme stored in the microcomputer section of the Demand Manager. Likewise, if it is less than the target value, loads are switched back on (assuming they have already been switched out). The programme will be described in more detail later in the specification.

To provide for load switching, the Demand Manager has eight output ports 1 5 to which the neutral terminal of three-phase (or single phase) contactors are connected. Loads which the consumer decides can be shed when necessary are connected via these contactors. The Demand Manager switches these contactors on or off simply by providing or removing neutral at the output ports.

For consumer information, eight light emitting diodes 16 indicate the state of each port (on/off).

There are also two pairs 20,21 of seven-segment displays, one pair 20 showing the number of minutes elapsed during the demand interval and the other pair 2t showing the average power consumption for the number of minutes elapsed, expressed as a percentage (%) of the target value.

The operation of the Demand Manager in achieving the various features already outlined will now be described in detail. The configuration of the system will be described initially.

(i) The system is microprocessor based.

(ii) For power measurement purposes the instrument is fed with the three phase voltages and three current transformers currents. Ta transform these quantities into values which can be processed by the subsequent electronic equipment a series of active buffers is used. These comprise the voltage conditioning 22 and current measurement 23 blocks of Fig. 2.

(iii) To calculate power the current and voltage signals must be multiplied. However rather than provide a multiplier for each phase, a multiplexer 24 is used which takes each phase in sequence i.e. the signals corresponding to phase one voltage and current are fed to the multiplier 25, then phase two signals, then phase three signals and then back to phase one signals again and so on. This is explained in greater detail later in the specification.

(iv) The output of the multiplier 25 is effectively a signal corresponding to power which is converted to a series of pulses by the voltage to frequency converter 26. These series of pulses are then counted by counter 27 and the value of the count is stored in the microcomputer 30 every lOOm sec. The sum of these counts is effectively the energy consumption (allowing for the appropriate correction factor which must be made for the multiplexing arrangement).

Dividing by the time over which these counts are summed yields the average power consumption for that period.

(v) The target value entered on the thumbwheel switches 10 is stored in random access memory. The micrncomputer 30, by virtue of the programme stored in read only memory, compares the value obtained for the average power consumption with the preset target value and takes switching action as determined by the programme.

(iv) The utility signals the start of "peak hours" i.e. the higher tariff rate, at the start of each demand interval. The Demand Manager recognises these signals and the programme acts accordingly. The remainder of the timing functions are referenced to the zero crossing of the mains. A zero crossing detector 40 is used to produce 100 msec pulses which drive the maskable interrupt line 41 of the microcomputer. This is shown in the timing circuit of Fig. 5. A 24 hour clock is maintained in software derived from the T00 msec pulls and this clock is used in conjunction with the printer option which is described later in the specification. The 24 hour clock may be adjusted by means of the thumbwheel switches 10 to correct for any inaccuracy due to drift in the frequency of the supply.

(vii) Each minute during the demand interval the microcomputer 30 updates the information on the pairs 20,21 of seven segment displays and the eight light emitting diodes 16 i.e. the number of minutes elapsed in the interval, the power consumption as a % of the target value, and the status. of each output port 1 5.

The hardware involved in the various stages of the system will now be described.

Voltage 22 and Current 23 interface circuits are used to transform the voltage and current inputs to suitable values, as has been mentioned earlier. Each phase of the voltage circuit (voltage conditioning circuit) 22 shown in Fig. 3 (a) consists of an operational amplifier 50 with a gain of 40:1. Neutral is also fed through an operational amplifier 51 and then to the interface circuits of each phase as shown, so that measurement is with respect to neutral. Thus the phase voltages (220V. RMS) are reduced to 5 Volts RMS signals approximately.

The current interface circuit 23 is shown in Fig. 3 (b). The input is a 0-5A current. Taking the normal maximum current that will appear in the secondary of the current transformers (not shown) as 5A RMS this develops a 0.5V RMS signal across the 0.1 ohm resistor 60. The gain of operational amplifier 61 is 10 which yields an output signal of 5V RMS.

Multiplexer 24, multiplier 25 and voltage to frequency converter 26 will now be described with reference to Fig. 4. The six outputs from interface circuits 22,23 are fed to the multiplexer 24. The multiplexer 24 addresses the signals associated with each phase in turn and feeds them to the multiplier 25. Each phase is switched through to the multiplier 25 for a period of 100 msec before changing to the next phase. Since a settling time is required in the multiplexer 24 between each change of address there is a 100 msec pause between changes. The microcomputer 30 supplies the 100 msec pulses which cause multiplexer 24 to address each phase in the sequence described above.

Multiplier 25 which receives the signals from multiplexer 24 and multiplies them has a gain of 10:1. In the case where a 5V RMS phase voltage signal and a 5V RMS current signal are supplied to multiplier 25 an output signal of 2.5V RMS is obtained.

The output of multiplier 25 is fed to voltage to frequency converter 26 which operates in the following manner: an input of + 10V gives an output pulse rate of 100KHz; an input of OV gives an output pulse rate of 50KHz; an input of - 10V gives an output of 0 pulses.

The output of voltage to frequency converter 26 is fed to sixteen bit counter 27 which is read and cleared by microcomputer 30 at 100 m sec. intervals.

To correct for inaccuracies in voltage to frequency converter 26 the following technique is used. After the signals associated with the three phases and their respective pause periods are fed to voltage to frequency converter 26, ground is then fed to voltage to frequency converter 26 for 100 m sec. This should yield an output pulse rate of 50 KHz. If the count result indicates other than 50 KHz a correction factor is developed in software which will adjust the results for the phase signals accordingly. This correction factor is determined on each cycle of multiplexer 24.

In summary, the sequence of signals through the multiplexer circuit is as shown below.

Phase 1 Pause Phase 2 Pause Phase 3 Pause Ground Pause 100 msec The zero crossing detector timing circuit will now be described with reference to Fig. 5. The input to the base of TR1 is an 1 1V 50 Hz signal tapped off a winding of the transformer (not shown) which supplies the current to the current interface circuit 23. The output of transistor TR 1 is fed to a Schmitt Trigger circuit which converts the signal to a 50 Hz square wave. This in turn is fed to a divide by 10 counter which produces the 100 msec. signal mentioned above.

This is used to drive the interrupt line 41 of the microcomputer.

In more detail, each edge of this square wave generates an interrupt. A leading edge (low to high state) tells microcomputer 30 that sixteen bit counter 27 did not record in the previous 100 msec. (This is the pause period for multiplexer 24). A trailing edge (high to low) indicates that a pulse count did occur for the previous 100 msec.

From the circuit of Fig. 5 it can also be seen that when the 100 m sec signal is high the AND gate 70 is enabled and the output from voltage to frequency converter 26 is sent to sixteen bit counter. When the signal is low (i.e. during the pause period) the gate is disabled and any pulses from voltage to frequency converter 26 are prevented from getting to counter 27.

The switching circuit for the output ports will now be described. Eight peripheral interface drivers are enabled, by microcomputer 30, via AND gates which drive light emitting diodes 16.

The enabling of a peripheral interface driver energises a change-over relay whose voltage free contacts are connected to an output port. A link on each relay permits its contacts to be set in the normally-open or normally-closed position. The changeover of a relay's contacts connect or disconnect a load from the respective output port.

The details of the microcomputer 30 are as follows: (i) Motorola MC6802 Microprocessor with 1 28 bytes of RAM on board.

(ii) Motorola MC6821 Peripheral Interface Adaptor (PIA) used for receiving data from the input switches and sending date to the switching circuits and displays.

(iii) 2K of Erasable Read Only Memory (Motorola MC7442) in which the operating programme is stored.

The remaining hardware, which includes power supplies, reset circuitry, latches and seven segment displays are of conventional type and will not be described in detail.

The software routines employed in the Demand Manager to implement the features described, will now be described.

The main features to be implemented are as follows: (f) Transferring information from counter 27 to microcomputer 30.

(2) Transferring information from microcomputer 30 to triac 83 drive circuitry.

(3) Operating a 24 hour clock for the printer option.

(4) Implementing the Demand Manager's control algorithm.

(5) Other special features.

Items (1), (2), (3) are conventional software routines and will not be described further.

Referring first to feature (4) above, the Demand Manager control algorithm attempts to keep the average power consumption for the demand interval below the value entered on thumbwheel switches 10. It does this by comparing the measured average power consumption at each minute of the demand interval with the value on thumbwheel switches 10 and switching loads in or out in response to this comparison. The loads are switched according to a simple sequential priority scheme, the eighth output port being always the first to be switched out, and the first output port being always the last. The number of loads to be switched is determined according to the routine outlined in Table 1.

No. of Minutes elapsed 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 P120%T 0 0 -3 -3 -3 -3 -3 -3 -3 -3 -3 -3 -4 -5 R 100%TP120%TO 0 -1 -1 -3 +1 -1 -1 -1 -1 -1 -2 + -4 E 80%TP100%T 0 0 +1 +1 +1 +1 +1 +1 +1 +1 +1 0 +2 +2 S P80%T 0 0 +2 +2 +2 +2 +2 +2 +2 +2 +2 +1 + +3 E T Table 1. The number of loads switched at 1 minute intervals during the demand interval for various values of P.

In the table, P is the measured Power Consumption, T is the target value, + indicates a load is switched on, and - indicates a load is switched off.

It can be seen that no load switching takes place during the first 3 minutes. This guarantees a minimum service to all loads. At the end of each demand interval all loads are switched back on.

To avoid hunting, a load is never switched back if any switching out has taken place in the previous minute.

The various special features included under feature (5) above will now be described.

(i) At the end of each demand interval the metering relay provided by the Utility falls off for 9 seconds thus closing a contact connected to an input port of the PIA for 9 seconds. This resets the displays to zero, all loads are reconnected and microcomputer 30 commences the next demand interval. If however the relay does not reoperate after 9 seconds microcomputer 30 will know it has come to the end of the Maximum Demand measuring period for that day and will not exercise the cqntrol regime. The displays are kept at zero. This situation continues until receipt of the next synchronisation pulse the following morning i.e. the next operation of the relay at which point it commences a control period.

(ii) Immediately on power-up of the Demand Manager, if the maximum demand relay is operated, it commences controlling according to its prqgramme. Initially it may be out of step with the Utility demand interval but will synchronise properly on receipt of the next synchronising pulse.

(iii) When a number of loads are to be switched at any stage in the interval they are switched singly with a one second interval (approximately) between them. This is to avoid severe voltage dips on the system; for example eight loads being switched back at the end of the demand interval could cause an unacceptably high voltage dip.

(iv) As previously mentioned, the power consumption is displayed on pairs 21 of seven segment displays as a percentage of the target value. Since only two digits are available readings greater than 100% cause the display to flash the two least significant digits.

(v) If at any stage the consumer wishes to change the target value; thumbwheel 10 setting is simply changed to the desired figure and pressing a switch 11 transfers the information to microcomputer 30.

(vi) The current transformer sizes may vary from installation to installation. A switching arrangement is available which allows the user to tell microcomputer 30 which ratio is being used. This avoids having to use a scaling factor on the results obtained from the Demand Manager. The ratios that can be selected are 100/5, 300/5, 600/5, 1500/5 and 2000/5.

A printer option (not shown) is available for use with the Demand Manager. It prints out time and demand in kW at the end of each demand interval. The additional hardware involved consists of a plug-in circuit board and a printer. The circuit board has a PIA for transferring information to the printer. The information to be printed is received from microcomputer 30 via the system data bus. The printer does not print outside control times. However, a switch can be operated which puts the Demand Manager in Monitor mode. In this condition the Demand Manager does not require a relay synchronising pulse but generates its own reset every fifteen minutes. Thus it can operate on a twenty four hour basis printing out time and demand every quarter of an hour.

A further feature of the Demand Manager is that thumbwheel switches 10 used for entering the target value are also used to enter the time of day. To ensure that switches 10 are set back to the target value the time display flashes until the target value is again entered. This is important should mains failure occur since on return of power the Demand Manager will read whatever value is set on thumbwheel switches 10 as its target value.

A still further feature of the Demand Manager is that a switch can be operated which will cause the Demand Manager to print out time and demand every minute. Thus the Demand Manager not only controls the maximum demand but also provides information on how the demand is constituted and how it varies with time.

Claims (11)

1. A maximum demand controller including means for measuring the current and means for measuring the voltage supplied to an installation, means for multiplying the measured current and voltage to determine the power consumed by the installation, and a microcomputer for periodically comparing the power consumption with a preset target value, and for controlling the electricity supply to individual loads within the installation to ensure that the average power consumption of the installation during any demand interval does not exceed the preset target value.

2. A maximum demand controller according to Claim 1, including signal multiplexing means for routing the measured current and voltage signals sequentially from each phase of a multiphase supply to the multiplier means.

3. A maximum demand controller according to Claim 1 or 2 including a voltage to frequency converter for receiving the output from the multiplier means, and a counter for receiving the output from the voltage to frequency converter and converting the output into a form acceptable to the microcomputer.

4. A maximum demand controller according to Claim 2, or Claim 3 as dependant on Claim 2, including conditioning means intermediate the voltage measuring means and multiplexing means and intermediate the current measuring means and multiplexing means for providing signals acceptable to the multiplexing means.

5. A maximum demand controller according to Claim 3, including means for biassing the voltage to frequency converter so that the latter has an output in the absence of an input signal from the multiplier and can therefore be calibrated by the microcomputer.

6. A maximum demand controller according to Claim 2 or any of Claims 3, 4 or 5 as dependant on Claim 2 including timing means governed by the frequency of the supply voltage for enabling the microcomputer to control the sequential operation of the multiplexing means, and for disabling the counter between measurement of each phase.

7. A maximum demand controller according to Claim 6 in which the timing means also enables the microcomputer to maintain a real time clock in software, which enables the maximum demand controller to provide a real-time based record of the microcomputer control operation.

8. A maximum demand controller according to any preceding claim including means for recognising signals from the supply at the commencement and conclusion of peak hour periods and demand intervals.

9. A maximum demand controller according to any preceding claim including means which provide operator access to the programme, enabling the target value to be set and enabling the real-time software clock to be corrected.

10. A maximum demand controller accoding to any preceding claim wherein, at periodic intervals within a demand interval, the microcomputer operates to connect or disconnect optional loads.

11. A maximum demand controller according to Claim 10, which operates according to a stored programme, generally so that the greater the difference between the measured average power consumed during one of said periodic intervals and the preset target value, the greater the number of optional loads switched at the end of said periodic interval.

1 2. A maximum demand controller substantially as hereinbefore described with reference to the accompanying drawings.