Patent GB-545150-A (GB)
Title: Apparatus for determining and/or controlling the maximum demand of an electric power-load
IPC: H02P29/00
Filed: 1940-08-09
Abstract: 545,150. Automatic control systems for electricity consumption; current measuring. WATFORD ELECTRIC & MANUFACTURING CO., Ltd., COATES, H., and VUILLE, B. A. Aug. 9, 1940, No. 12807. [Class 38 (iv)] [Also in Group XXXVI] The maximum demand of an electric power consumer's load is automatically reduced when at any instant in a maximum demand period the energy consumed exceeds that which it is permissible to consume, or when at any point the energy remaining available is less than that which a constant or standard load would consume in the remainder of the period, or when the instantaneous load at a given point in the period is greater than a constant load equivalent to the energy then available. At other times the demand is permitted to fluctuate freely The invention is described with reference to a steelwork's load comprising an electric furnace, automatically controlled to a normal constant load (which is adjustable by manually adjusted contact elements and follow up contacts operated by a reversible motor LRM varying diverting resistances associated with load current relays) and an annealing furnace which, after automatic reduction of load on the furnace to the controlled value, is automatically reduced to a minimum value sufficient for effective operation by a reversible motor AFC. A polarized relay G2 effects load reduction by means of a relay CL and permits load increases through a relay CR. The relay G2 is operated by unbalance in a Wheatstone bridge of resistances RL, RT and RM, RX operative until a predetermined time towards the end of a demand period which is determined by the setting of cam actuated switches TCIRb/c, whereupon it is subject to a bridge of resistances RL, RT1 and RI, RX1. Of these RL is progressively reduced until the predetermined time at a rate varying with the load demand, RT is reduced at a constant rate (RL and RT maximum being proportional to the energy consumption permitted in a demand period). The ratio RM, RX is proportioned to a permitted instantaneous demand or standard load, and RI is continuously adjusted by load responsive means to be proportional to the actual instantaneous demand. The sensitivity of the relay G2 is increased in steps in cycles of 30 seconds by shunting out series resistances. As shown the instantaneous demand of the plant is measured by a Wheatstone bridge comprising temperature responsive resistances R7, R8 in a passage through which fluid rises at a rate depending on the current in resistors 21 fed by series transformers 22 on the three phase power mains 20. To change the relation between the response of the bridge and the power current from a square law to a linear law the flow of liquid is accelerated by further heating resistances 24 and final correction is obtained by a network R 3 , R 5 , R 6  in the bridge of which the bridge-balancing reance R. is adjusted by a motor BM and a galvanometer relay G 1 . Alternatively to resistances 24 and R 3 , R 5  the motor BM may operate through arms designed to give a linear law. The sensitivity of the galvanometer G1 is also increased in stages in cycles of 30 seconds. The bridge-balancing adjustments also maintain the resistance R1 at a value corresponding to the instantaneous load demand and set switch contacts 43 in corresponding position with reference to a zero 40 and fixed switch contacts 45. A synchronous motor MM drives a shaft 39 at the rate of one revolution in 30 secs. whereby the contacts 45, 43 are closed in succession to determine the successive periods of operation of a clutch 52 through which an impulsing switch LIG is operated to cause stepping of a shaft 98L whereby the resistance RL measures the energy remaining available at each point in the maximum demand period. An impulsing switch TIG causes stepping up of a shaft 98T at a constant rate so that resistance RT corresponds with the energy which would remain available if the load remained constant at an average maximum demand value. A motor RM is energized to reset the shafts 98L,, 98T at the end of each demand period. A system of electrically interlocked relays is described whereby the apparatus is set in motion by and maintained in synchronism with, impulses established by the supply authority's maximum demand meter. The switch LIG may be electromagnetically actuated by a coil energized periodically by a magnet separated by a piece of glass from a cam of magnetic material driven by a kV motor and the current measuring resistance R1 may be controlled by solenoids 87, Fig. 7, supplied from the series transformers 22 and maintained balanced against a spring 89 which, with the resistance RI, is adjusted by a motor M controlled by contacts 91. The complete and Provisional Specification describe other modifications. The galvanometer contacts may be additionally controlled as in Specification 501,917. Specifications 418,782 and 540,087 also are referred to.