A maximum-power monitoring device and method whereby a direct-current voltage is derived which is proportional to the instantaneous power and is converted into a train of pulses in a voltage-frequency converter proportional to the power and these pulses are summed during an interval between two successive cyclic pulses. When the total exceeds a threshold value, a flip-flop is triggered. The state of the flip-flop controls a shift register which is periodically stepped with each cyclic pulse and "read". The reading is used to control a logic circuit, when it exceeds a predetermined value, for cutting off in accordance with predetermined (programmed) priorities, the loads or load sections of the monitored system. As a result, the mean peak demand is held at a predetermined level.

FIELD OF THE INVENTION 
The present invention relates to a maximum-power monitor and monitoring 
method and, more particularly, to a system for maintaining a predetermined 
mean electrical power demand over a predetermined period. 
BACKGROUND OF THE INVENTION 
The cost of supplying electrical energy includes fixed costs which depend 
upon the size of the installation and variable operating costs which are 
proportional to the output thereof. In order to distribute these costs 
most effectively to the consumers, electric power companies operate in 
accordance with a maximum tariff. Thus, apart from the actual consumption 
charge, there is applied a fee, generally known as the demand charge, 
which is determined by the highest power actually supplied over a 
predetermined period. 
The measurement of electric power demand is carried on by a maximum or peak 
indicator or a demand meter. 
For calculating the demand charge, it is desirable that brief or 
intermittent power peaks not be considered since they do not generally 
affect the fixed costs of generating electric power. As a result, it is 
desirable to use the value of the mean power over a relatively long period 
of 15 to 30 minutes for determining the power maximum. Conventional 
maximum indicators have a circular scale provided with a drag indicator. 
At the end of the desired reading time, the position of the drag indicator 
or pointer against the scale indicates the highest power maximum which has 
been detected. 
Naturally, each consumer of power would like to hold its power maximum 
(demand) as low as possible. To this end, monitoring devices have been 
provided so that, when the maximum indicator reaches a predetermined power 
level, various loads or groups of loads are cut off from the supply 
network. Such automatic load-shedding reduces the drain and hence the 
power consumption of the monitored system and thereby prevents an increase 
in the demand charge. 
The disadvantage of conventional monitoring devices is that they generally 
must be coupled with the maximum-power indicator and hence must be 
connected by control lines with the loads or groups of loads to be cut 
off. In a large system, e.g. a large industrial plant, farm or the like, 
these control lines may run distances of the order of kilometers. Another 
important disadvantage is to be found in the nature of the monitoring 
process. Conventional devices only become effective when a predetermined 
power level is attained to operate the load-shedding switches. This power 
level thus constitutes a switching threshold. 
However, when the rate of increase of the power consumption is sharp, the 
actual triggering of the load-shedding devices may be too late to prevent 
an increase in the demand charge. On the other hand, the need to respond 
quickly once the threshold is reached may result in the cut off of a 
greater number of loads than is desirable so as to interrupt operation of 
the factory or otherwise interfere with the normal operating procedures of 
the system. Frequently, therefore, the use of the prior art system is a 
compromise between premature and undesirable cut off loads and delay in 
effecting the cut off of loads so that the demand charge will increase. 
OBJECTS OF THE INVENTION 
It is the principal object of the present invention to provide a device, 
unit or system, which obviates the disadvantages of earlier arrangements 
for the purposes described and which is not prone to excessive delay in 
responding to an excess power demand but also does not tend to cut off an 
excessive number of loads or load groups. 
Still another object of the invention is to provide an improved method of 
maintaining a mean power demand over a predetermined period. 
SUMMARY OF THE INVENTION 
These objects and others which will become apparent hereinafter are 
attained, in accordance with the present invention, in an apparatus for 
monitoring the power drawn by a system having a plurality of loads and 
which comprises a device for generating a direct current voltage which is 
proportional to the power. 
According to the invention, a voltage-frequency converter is connected to 
the power detector which has the aforementioned DC voltage output to 
convert this signal into a train of pulses of a frequency which is 
proportional to the detected power. The pulses of this signal are summed 
during the intervals between two cyclic pulses and, when the sum reaches a 
level above a threshold, a flip-flop is switched. The state of the 
flip-flop is detected in the form of an input to a shift register whose 
contents are evaluated at every cyclic pulse, the evaluated value, e.g. a 
count, being used to operate a logic circuit when it exceeds a 
predetermined value. 
The logic circuit carries out a load-shedding or cut off of individual 
loads or groups of loads in accordance with programmed priorities and, of 
course, reconnects the cut off of loads or groups of loads to the main 
when the measured power is reduced sufficiently. 
More specifically, the mean maximum power demand of an electrical 
installation, e.g. a factory, locality or farm, is monitored so as to 
monitor the power demand for which charges are applied substantially 
constant. In this monitoring method and apparatus, the current can pass 
through a current transformer connected in series with the load and 
provided with an internal load resistance which develops a maximum voltage 
which is proportional to the current flow. The current range can be broken 
down into a plurality of partial ranges. The output of this current 
detector is applied to an amplifier stage of variable gain so that, at the 
output for a nominal current flow, the signal is a voltage of, for 
example, 1 V.sub.eff. 
The mains voltage is transformed with phase correction from, for example, 
220 V.sub.eff to a suitable lower level. 
The current voltage signals are applied to a low-drift digital multiplier 
with appropriate phase-angle correction to generate an output which is 
passed through a low pass filter to eliminate the sin.sup.2 factor and 
provide a direct current voltage which is proportional in power. An 
amplifier can be provided at the output of the low-pass filter. 
When the multiplier input has a nominal value of 1 V.sub.eff for the 
current and 2 V.sub.eff for the voltage with a phase angle cos .phi. of 
which, the output is a direct voltage of 0.5 volt. 
This direct-current voltage proportional to power is applied to a very 
stable linear voltage frequency converter at the output of which a pulse 
train is obtained with a frequency proportional to power. 
This power-proportioned frequency is applied to a frequency divider which, 
for each 408 input pulses, reverses the state of an IK flip-flop. The 
state of the flip-flop is detected with cyclic pulses controlled by the 
mains frequency (50 or 60 Hz) at an interval of 14.0625 seconds and is 
simultaneously reset. The state of the flip-flop is read into a 64-bit 
shift register. The cyclic pulses read the 64-bit register, the contents 
thereof are evaluated, e.g. in a counter for use in a process controller 
so that the shift regulator acts in a ring-counting system through each 64 
steps. Each such cycle of the shift register brings about an internal 
organization of the information which is identical to the organization of 
the information which is identical to the organization before rotation. 
The number of positive bits is counted and represents the mean value of 
the power in the previous 15 minutes although the information is obtained 
every 14.0625 seconds. 
When the positive bit count exceeds a predetermined setpoint value, the 
counter provides an output signal which is applied to the priority logic. 
In response to the logic circuit, a relay is controlled to cut off the 
load with the lowest priority. If the mean power during the next cycle is 
still in excess of the setpoint value, a further relay is controlled to 
cut off another load or group of loads with the next higher priority. 
When the mean power is smaller than the setpoint value during the next 
cycle, after four cycles the relay with the highest priority is again 
operated to connect its load group to the mains. Four cycles have been 
chosen as the delay so that small load variations do not create a hunting 
situation in the control system. After a further four cycles, depending 
upon the count and its relationship to the setpoint value, a further load 
group can be connected.

SPECIFIC DESCRIPTION 
In the drawing, a three-phase alternating-current source, such as an 
electric power generating plant, is represented at 10 an is connected to a 
power-distribution network 11 which runs to a large number of electrical 
installations which can be defined as groups of loads serviced by a common 
meter. Only one such installation has been shown in the drawing and the 
meter has not been illustrated but can be of a conventional type as 
described, for example, in MARKS' MECHANICAL ENGINEERS' HANDBOOK, 
McGraw-Hill Book Co., New York, 1958, chapter 15, pages 34ff. 
Within the installation, main lines 13 run to a multiplicity of loads or 
load groups represented at I, II, III, . . . To facilitate the load 
shedding or cutting off of these loads in accordance with a desired 
priority, each of the loads I, II, III is connected in series with a set 
of contacts C.sub.1, C.sub.2, C.sub.3, operated by respective relays 
R.sub.1, R.sub.2, and R.sub.3. 
According to the invention, a power detector 12 of the type described above 
provides low voltage signals representing current and voltage values as 
shown at 16, which values are corrected for phase angle, to a digital 
multiplier 14, the output of which is applied to a low-pass filter 15. 
As previously described, therefore, the low-pass filter, e.g. via an 
amplifier (not shown), delivers at 17 a direct current voltage which is 
proportional to the instantaneously detected power. 
This signal is applied to a voltage/frequency converter 18 which transforms 
the input voltage into a train of pulses of a frequency proportional to 
the detected power and applied at 19 to a frequency divider. The output of 
the frequency divider 20 is applied to a summer 23 preferably by serving 
as an input at 21 to an AND gate 22 which is triggered by a frequency 
controlled timer 28. The latter is, in turn, triggered by or synchronized 
with the mains frequency delivered at 29 thereto. The timer output to the 
gate 22 corresponds to the interval between two working cycles as 
described above. 
The output of or summer of adder 23 is applied to a flip-flop 24 which is 
read each operating cycle pulse from the timer 28, the state of the 
flip-flop 25 upon reading being applied to the 64-bit shift register 25 
which can be the ring-connected type. The contents of the shift register 
are shifted through one stage with each operating cycle by the operating 
cycle pulse delivered from the timer 28. At each such pulse, moreover, at 
an interval of 14.0625 seconds, the contents of the shift register 25 are 
evaluated by a threshold counter 26 which can have a setpoint value and, 
when the count exceeds the setpoint value, can operate the 
priority-selection logic circuit 27 to first cut off one load group I, II, 
or III then another etc. in accordance with preprogrammed priorities. To 
this end the logic circuit is connected to the relays R.sub.1 through 
R.sub.3 and can be connected to other relays for other load groups as 
required.