NZ227975A - Load shedding at consumer premises - Google Patents

Description

22 7 975 Priority Datc(s): .. 3, a. < ^ Complete Specification Filed: Class: .'./.'.V.--. . .

Publica:ion Date: $jj( P.O. Journal, Mo: ' . o ORIGINAL NEW ZEALAND PATENTS ACT 1953 COMPLETE SPECIFICATION E * ^ *\\ -f»\\ "ELECTRICAL LOAD SHEDDING CIRCUIT //* i/y \ 1 4 FEB 1989^ \f.

WE, STANDARD TELEPHONES AND CABLES PTY. LIMITED, A Company of the State of New South Wales, of 252-280 Botany Road, Alexandria, New South Wales, 2015, Australia, hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: 1 227975 This invention relates to emergency load shedding means for AC electricity power supply systems supplying power to a plurality of consumers. In such systems when power demand exceeds supply the generators providing the power to the system become overloaded. This overload may occur rapidly and it is necessary to shed non-essential loads quickly in order to avoid a collapse of the entire system.

In a known method to reduce demand on an electricity supply system in the event of power demand exceeding supply, a command signal is transmitted from the power utility's control centre to a remotely operated switch at the consumers premises, turning off non-essential loads such as large capacity hot water heaters, swimming pool filters etc.

A major problem with such a method, however, is the amount of time taken to transmit the command signal to the consumers premises which may take up to several minutes and require a relatively complex communication link. Under emergency conditions, such as the loss of a generator, the power begins to drop immediately, possibly leading to a collapse of the system before commands can be issued to turn off the non-essential loads. One known system devised to solve this problem utilizes the drop in supply frequency, hereinafter referred to as an under-frequency, which occurs 2 227975 when a generator becomes overloaded. By detecting a drop In the supply frequency at a substation the load of a whole suburb is shed. This causes great inconvenience to the consumers affected.

Another known system, described in the specification of Australian Patent No. 5^2,286 provides a device at the consumers premises which, if a phase-lock loop fails to maintain synchronization between a clock signal generator and the incoming waveform of the AC supply, produces a phase error signal which is used to control switching means to shed at least part of the load at the consumer's premises.

A disadvantage of the last mentioned known system is that the lock range of the phase lock loop is determined by analogue components whose tolerances do not permit the accurate setting of the threshold frequency in a reproducable way. Moreover, the threshold frequency must be set during manufacture and cannot be easily altered.

It is desirable to provide a method and device for controlling the load of an AC electricity supply system in which the threshold frequencies can be set precisely using standard components.

It is also desirable to provide a method and device for controlling the load of an AC electricity supply system 3 227975 whose threshold frequencies are programmable during manufacture, on site or remotely.

A further desirable feature of such a system is to provide a method and device for controlling the load of an AC electricity supply system which may employ a simple telephone line as a link to the utility command centre.

This specification describes a method of controlling the load of an alternating-current electricity supply system supplying power to a plurality of consumers, said method comprising the steps of, for one or more of the plurality of consumers, measuring the frequency of the alternating-current supply, comparing the measured frequency to at least one threshold value stored in storage means which corresponds to a predetermined frequency below the normal frequency of the supplied voltage, and upon the measured frequency falling below said threshold value causing, after a predetermined delay, at least part of the consumers lead to be disconnected from the said supply system, said load being re-connected to the said supply system when the measured frequency has risen above said threshold value.

This method is implemented in an apparatus for use by a consumer using power for a load from an alternating-current electricity supply system supplying power to a plurality of 227975 o consumers, comprising frequency measuring means arranged to measure the frequency of the alternating-current supply and provide a frequency output signal indicative of the measured frequency, comparison means to which said frequency output signal Is applied and which compares said frequency output signal to at least one threshold value, said threshold value corresponding to a first predetermined frequency which is below the normal frequency of said alternating current supply, wherein when said frequency output signal indicates that the measured frequency has fallen below said one threshold value, a switch control signal is provided by said comparison means after a first predetermined disconnect delay provided by disconnect delay means, said switch control signal being extended to a load switch means which is rendered non-conducting thereby to disconnect at least part of said consumer's load from the said supply means, said load switch means being rendered conducting upon said measured frequency rising above said threshold value, thereby reconnecting said at least part of said consumers load to the said supply system.

The present invention ensures, that if an under-frequency condition occurs, electrical loads are shed before load-shedding commands could be issued over conventional 227975 load-control systems. Load-shedding is achieved with the present invention on a priority basis, by dropping first non-essential loads, and then more important loads for successively more severe failures, reducing network load before indiscriminate (suburb-wide) blackouts occur.

In order that the invention can be readily understood, an embodiment thereof will now be described in relation to the drawings.

Embodiments of the invention are shown in the drawings in which: Pig. 1 is a block diagram illustrating basic elements employed in a first embodiment; Fig. 2 illustrates an embodiment employing a microprocessor controllable over the telephone line; Fig. 3 illustrates a further refinement of the embodiment of Fig. 2.

In these figures, common reference numbers have been used to refer to similar elements and the invention will be described with reference to all drawings. A difference between figures two and three is that in Fig. 3 the zero-crossing detector 13 is shown as a separate item whereas in Fig. 2 it is incorporated in interval timer 1. 227975 Referring to the drawings, a load 6 is connected to an AC mains 5 via a load switch 4. Across mains 5 is coupled a zero-crossing detector 13 whose output is coupled to an input of a counter means 3 associated with a crystal oscillator 2. Outputs of counter means 3 are respectively connected to inputs of comparator means 11 and 12 each of which compares the outputs from the counter with respective stored threshold or limit values. The outputs of comparator means 11 and 12 are connected to inputs of processor means (CPU) 8.

A memory associated with processor 8 includes four delay registers and two memory registers respectively associated with two of the delay registers.

An output of processor 8 is coupled to load switch 4. A phone line 9 is connected to an input of processor means 8 via an interface means 10 for connection to the power utility 's System Control and Data Acquisition (SCADA). Alternatively, the AC mains or a radio link could be utilized for providing the connection to SCADA.

It will be understood, that although the preferred embodiment Includes a connection to the power utility's SCADA, the load control arrangement of the present invention can be programmed to operate independently of commands for SCADA in 7 ■< m 227975 j'—'S w c which case interface 10 and phone line 9 would not be needed.

In operation the frequency of the power supply on mains 5 is monitored by zero crossing detector 13, which produces a pulse every time the power supply waveform passes through zero, producing nominally 100 or 120 pulses per second for 40Hz or 60Hz respectively. Preferably, the zero-crossing detector is filtered in hardware (not shown) to reduce the incidence of spurious outputs due to noise impulses on mains 5 or voltage harmonics, which can both produce spurious zero-crossings.

A counter 3 is continually incremented by crystal oscillator 2 at a rate which will preferably count at least 1000 counts per AC cycle, giving a resolution of better than 0.01 ms. This provides a fine control over the order of shedding different types of load. Counter 3 is reset by a pulse from zero-crossing detector 13 twice for every AC cycle. The value of the output of counter 3 is continually compared in comparator means 11 and 12 to threshold values programmed into corresponding limit registers associated therewith. A threshold value corresponding to a severe under-frequency is contained in the first limit register associated with comparator 11 and a threshold value corre- 8 227975 sponding to a less severe under-frequency is contained in the second limit register associated with comparator 12.

When the threshold value is exceeded in second limit register due to a less severe under-frequency, an interrupt signal is provided at the output of the comparator means 12. This signal causes the processor means to prepare to operate load switch 4 and shed load 6.

To ensure that load 6 is not shed prematurely due, for example, to an occasional noise impulse or single cycle power loss, the processor means includes filter means in the form of software. Two delay registers are provided in a memory associated with the processor means. In the case of an interruption of a less severe under-frequency condition, a "1" is added to the first of the delay registers. This delay register is decreased at regular intervals, for example, by 1 every 40 ms. The processor means waits until the count exceeds the preset maximum value stored in a corresponding first memory register associated with the first delay register. Typically, this maximum value is 1750 which causes a delay of about 25 seconds at 50 Hz. This method imposes no interrupt load on the processor means software until an under-frequency condition occurs. After 25 sec- \ - 'J cN\ 'J TA ■\' 18D£Ci990'' e i 227975 onds, the processor means operates load switch H and sheds load 6.

Similarly, when the threshold value programmed in the second limit register is exceeded, due to a severe under-5 frequency, an interrupt signal Is provided at the output of comparator means 11. This signal causes the processor means to prepare to operate the load switch 4 and shed load 6. The processor means waits until the count In the second delay register exceeds the preset maximum value stored in the 10 associated second memory register. Typically, the maximum value is 10 resulting in a delay of about 150 ms at 50 Hz. After 150 ms the processor means operates the load switch 4 and sheds load 6.

After the AC frequency returns to above the threshold 15 determined by the second delay register the processor 8 switches load switch 4 and restores power to load 6. Restoration, however, only occurs after a delay. Two further registers provide a maximum and minimum delay time limit and processor 8 selects a delay within those limits. Typically 20 the minimum delay is 5 minutes and the maximum delay Is 10 minutes. This allows the highest priority loads to be switched to the supply quickly, and the lowest priority loads remain shed for a longer period. This delay may be O o \10 * y * u ^ ?8DEC|990^ 227975 G selected randomly to prevent all loads being restored simultaneously. The delay also allows sufficient time for the SCADA system to transmit overriding commands over telephone line 9 if required. The load may be restored or restoration inhibited by these commands.

The power utility's SCADA system coupled to phone line 9 is programmed with the characteristics of all nonessential loads controlled by the load shedding arrangement of the present invention. The under-frequency limit values are programmed into each of the limit register and delay registers for the type of load controlled by these registers which allows gradual load shedding as the power is lost. The registers may be programmed remotely via phone line 9, or via the supply line 5 or manually during manufacture, or later on site.

Processor means 8 may form part of an energy management terminal such as, for example, the one described in New Zealand Patent Application No. 218,262.

Although two limit registers are shown in the preferred embodiment, further limit registers may be Included to provide further frequency/delay profiles for a single load, or provide respective under-frequency thresholds for a plurality of loads.

V v; ■«> --j; v-^r-^v.•••»»> -,\.v»W.' '• -• ',«• >t: ■ 227975 Ssw^ In an AC electricity power supply system whose normal frequency is 50 Hz, the threshold values programmed in the limit registers may be : 1. Less severe under-frequency condition : 49.0 Hz for 5 25 seconds 2. Severe under-frequency condition : 48.5 Hz for 150 ms.

The less severe under-frequency condition might occur, say, during a cold snap when many consumers switch on elec-10 trical heating apparatus. The load Increases gradually and the frequency of the power supply correspondingly falls. If it reaches 49.0 Hz and remains there for at least 25 seconds load shedding commences.

The severe under-frequency condition might occur in the 15 event of a generator failing. Under this condition the load rapidly exceeds supply and there is a corresponding rapid fall in the power supply's frequency to 48.5 Hz. After 150 ms the load shedding commences.

While the present Invention has been described with re-20 gard to many particulars, It is to be understood that equivalents may be readily substituted without departing from the scope of the invention.

Claims (27)

What we claim is:

1. A method of controlling the load of an alternating- current electricity supply system supplying power to a plurality of consumers, said method comprising the steps of, for each of the plurality of consumers, measuring the frequency of the alternating-current supply, comparing the measured frequency to at least one threshold value stored in storage means which corresponds to a predetermined frequency below the normal frequency of the supplied voltage, and upon the measured frequency falling below said threshold value causing, after a predetermined delay, at least part of the consumers load to be disconnected from the said supply system, said load being re-connected to the said supply system when the measured frequency has risen above said threshold value.

2. A method as claimed in claim 1, including the step of delaying for a predetermined time the re-connexlon of the at least part of the consumers load to the said supply system.

3- A method as claimed In claim 2, wherein the re- connexion of at least part of the consumers load is selectively determined by command signals transmitted from a remote control centre via a communications link. 227975

4. An apparatus for use by a consumer using power for a load from an alternating-current electricity supply system supplying power to a plurality of consumers, comprising frequency measuring means arranged to measure the frequency of the alternating-current supply and provide a frequency output signal indicative of the measured frequency, a first comparison means to which said frequency output signal is applied and which compares said frequency output signal to at least one threshold value, said threshold value corresponding to a first predetermined frequency which is below the normal frequency of said alternating current supply, wherein when said frequency output signal indicates that the measured frequency has fallen below said one threshold value, a switch control signal is provided by said comparison means after a first predetermined disconnect delay provided by disconnect delay means, said switch control signal being extended to a load switch means which is rendered nonconducting thereby to disconnect at least part of said consumer's load from the said supply means, said load switch means being rendered conducting upon said measured frequency rising above said threshold value, thereby re-connecting at least part of said consumers load to the said supply system. 227975

5- An apparatus as claimed in claim 4, wherein said frequency output signal is compared to a further threshold value which corresponds to a second predetermined frequency below the first predetermined frequency, and wherein when said frequency output signal indicates that the measured frequency has fallen below said further threshold value a switch control signal is provided by a second comparison means after a second predetermined disconnect delay provided by the disconnect delay means, said second predetermined delay being shorter than said first delay, said switch control signal being applied to said load switch means which is rendered non-conducting thereby to disconnect at least part of said consumer's load from the said supply system.

6. An apparatus as claimed in claim 4 or claim 5 including re-connect delay means arranged to delay said re-connexion of at least part of said consumer's load for a predetermined period.

7. An apparatus as claimed in any one of the preceding claims, wherein each threshold value is stored In a respective limit register means associated with each said comparison means.

8. An apparatus as claimed In claim 7, wherein said limit register means are programmable'. 15 2279

9- An apparatus as claimed in any one of claims 4 to 8, wherein said disconnect delay means comprises first and second delay register means each associated with memory means to provide first and second predetermined disconnect delays.

10. An apparatus as claimed in claim 9, wherein said first and second disconnect delay register means are programmable .

11.An apparatus as claimed in any one of claims 6 to 10 wherein said re-connect delay means comprise third and fourth delay register means to provide first and second predetermined re-connect delays.

12. An apparatus as claimed in claim 11, wherein said third and fourth delay register means are programmable.

13. An apparatus as claimed in claim 11 or 12, wherein said third and fourth delay register means provide respectively minimum and maximum re-connexion delay limits, for providing a selectable delay within said limits.

14. An apparatus as claimed in any one of claims 9 to 13, wherein said disconnect delay means includes additional delay register means and associated memory means.

15. An apparatus as claimed In any one of claims 11 to 14, wherein said re-connect delay means includes additional delay register means. • •{! 16 ,/ *"is;1^29 JAN 1991*$ ■i. n 227975

16. An apparatus as claimed In claim 14 or 15 wherein the additional delay register means are programmable.

17- An apparatus as claimed In any one of claims 4 to 15, including processor means to whose input means is coupled said switch control signal and processed therein, an output of said processor means being coupled to said load switch means for controlling the load switch means with a control signal produced in said processor means.

18. An apparatus as claimed in claim 17, wherein a communications line is coupled to a further input of said processor means, whereby command signals from a remote control means coupled to the communications line are processed for overriding or delaying the re-connexion of at least part of said consumer's load.

19. An apparatus as claimed in claim 17 or 18, wherein said limit register means and/or said delay register means are remotely programmable via said communications line.

20. An apparatus as claimed in any one of claims 17 to 19> wherein said processor means is incorporated In an energy management terminal means. 17 227975

21. An apparatus as claimed in claim 20, wherein said remote control means is a system control and data acquisition (SCADA) means.

22. An apparatus as claimed in any one of claims 18 to 21 wherein said communications line Is a telephone line.

23. An apparatus as claimed in any one of claims 18 to 21, wherein said communications line is provided by the AC mains.

24. An apparatus as claimed in any one of claims 17 to 23, wherein said processor means incorporates filter means adapted to filter noise Impulses and single cycle losses.

25- An apparatus as claimed In any one of claims 4 to 24, wherein said frequency measuring means comprises an arrangement which counts pulses from an oscillator means that occur between successive zero-crossings of the alternating current's waveform, the count being applied to said comparison means and compared to said threshold values.

26. An apparatus as claimed in claim 25, wherein said oscillator means comprises a crystal controlled oscillator. 227975

27. An apparatus for use by a consumer using power for a load from an alternating current electricity supply system, substantially as herein described with reference to the accompanying drawings. STANDARD TELEPHONES AND CABLES PTY. LIMITED // i l P.M. Conrick Authorized Agent P5/1/1703 •'t r .V