Technical Field

This invention relates to emergency load shedding means for AC elec¬ tricity power supply systems supplying power to a plurality of consumers.

In such systems when power demand exceeds supply the generators pro¬ viding 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. Background Act

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 ca¬ pacity 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 when a generator becomes overloaded. By detecting a drop in the supply fre¬ quency 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. 512,286 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 er-

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ror 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 therefore an object of the present invention to provide a method and device or controlling the load of an AC electricity supply system in which the threshold frequencies can be set precisely using standard compo¬ nents.

It is a further object of the present invention to provide a method and device or controlling the load of an AC electricity supply system whose threshold frequencies are programmable during manufacture, on site or remotely.

A still further object of the present invention 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. Summary of the Invention

According to a first aspect of the invention there is provided 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 fre¬ quency 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.

According to a further aspect of the invention there is provided 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 consum¬ ers, comprising frequency measuring means arranged to measure the frequency of the alternating-current supply and provide a frequency output signal in¬ dicative of the measured frequency, comparison means to which said fre¬ quency 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 consum¬ er'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 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 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.

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Best Mode of Carylng Out The Invention

In order that the invention can be readily understood, a ^: embodiment ^• thereof will now be described in relation to the figure of the drawing. ^•

Referring to the drawing, a load 1 is connected to an AC mains 2 via a load switch 3. Across mains 2 is coupled a zero-crossing detector 4 whose output is coupled to an input of a counter means 5 associated with a crys¬ tal oscillator 6. Outputs of counter means 5 are respectively connected to inputs of comparator means 7 and 8 each of which is associated with respec¬ tive limit register means 9 and 10. Respective outputs of comparator means 7 and 8 are connected to inputs a and b of processor means (CPU) 11.

A memory 12 associated with processor 11 includes four delay registers 13, 14, 15 and 16 and two memory registers 17 and 18 respectively associ¬ ated with delay registers 13 and 14.

An output of processor 11 is coupled to load switch 3. A phone line

19 is connected to an input c of processor means 11 via an interface means

20 for connection to the power utility's System Control and Data Aquisition (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 com¬ mands for SCADA in which case interface 20 and phone line 19 would not be needed.

In operation the frequency of the power supply on mains 2 is monitored by zero crossing detector 4, 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

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spurious outputs due to noise impulses on mains 2 or voltage harmonics, which can both produce spurious zero-crossings.

A counter 5 is continually incremented by crystal oscillator 6 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 5 is reset by a pulse from zero-crossing detector 4 twice for every AC cycle. The value of the output of counter 5 is continually compared in comparator means 7 and 8 to threshold values programmed into two limit registers 9 and 10 respectively associated therewith. A threshold value corresponding to a severe under- frequency is contained in limit register 10 and a threshold value corre¬ sponding to a less severe under-frequency is contained in limit register 9.

When the threshold value is exceeded in limit register 9 due to a less severe under-frequency, an interrupt signal is provided at the output of the comparator means 7. This signal is extended to input a of the processor means 11 which prepares to operate load switch 3 and shed load 1.

To ensure that load 1 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 13 and 14 are provided in a memory 12 associated with the processor means. In the case of an interruption of a less severe under-frequency condition, a "1" is added to delay register 13. The 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 memory register 15 as¬ sociated with delay register 13. 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 seconds, the processor means operates load switch 3 and sheds load 1.

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Similarly, when the threshold value programmed in limit register 10 As.. exceeded, due to a severe under-frequency, an interrupt signal is provided at the output of comparator means 8. This signal is extended to input b of: the processor means which prepares to operate the load switch 3 and shed load 1. The processor means waits until the count in the delay register 14 exceeds the preset maximum value stored in memory register 18 associated with the delay register 14. Typically, the maximum value is 10 resulting in a delay of about 150 ms at 50 Hz. After 150 ms the processor means op¬ erates the load switch 3 and sheds load 1.

After the AC frequency returns to above the threshold determined by register 14 the processor 11 switches load switch 3 and restores power to load 1. Restoration, however, only occurs after a delay. Delay registers 15 and 16 provide a maximum and minimum delay time limit and processor 11 selects a delay within those limits. Typically the minimum delay is 5 min¬ utes 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 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 17 if required. The load may be restored or restoration in¬ hibited by these commands.

The power utility's SCADA system coupled to phone line 17 is pro¬ grammed with the characteristics of all non-essential loads controlled by the load shedding arrangement of the present invention. The under- requency limit values are programmed into each limit register 9 and 10 and delay registers 13 and 14 for the type of load controlled by these regis¬ ters which allows gradual load shedding as the power is lost. The regis¬ ters may be programmed remotely via phone line 17, or via the supply line 2 or manually during manufacture, or later on site.

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Processor means 11 may form part of an energy management terminal such as, for example, the one described in Australian Patent Application No. 65,908/86.

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 thresh¬ olds for a plurality of loads.

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 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 electrical heating apparatus. The load increases gradually and the frequency of the power supply correspond¬ ingly falls. If it reaches 49.0 Hz and remains there for at least 25 sec¬ onds load shedding commences.

The severe under-frequency condition might occur in the 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.

VJhile the present invention has been described with regard to many particulars, it is to be understood that equivalents may be readily substi¬ tuted without departing from the scope of the invention.

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