OVERCURRENT PROTECTION DEVICES

Abstract: An overcurrent protection device incorporates a microprocessor (42) the ROM of which stores data representing a multiplicity of points on a plurality of different overcurrent margin/trip delay characteristic curves. A characteristic selector device (48) is provided to enable a user to select which of the stored characteristic curves is employed. The microprocessor is programmed to accept data relating to the current in a circuit to be protected, calculate the required trip delay and actuate a trip relay (46) if the delay expires.

Description: , ~ Technical field, of invention This invention relates to overcurrent protection devices such as circuit breakers. Background Art Conventional thermal circuit breakers are designed so that their thermal properties cause a given relationship between the overcurrent level and the time taken for the breaker to trip. Thus at a current level only marginally in excess of the rated current a relatively long time - may be several seconds - is required whereas when the current becomes greatly in excess of the rated current much faster tripping occurs - perhaps in a few milliseconds. The shape of the . current - time curve is of consider¬ able importance particularly when a multi-tier protection system is being designed, to ensure that the circuit breaker connected nearest the particular load trips first so that loads in which no overcurrent condition exists are not dis¬ connected unnecessarily. Clearly where the current-time curves of the devices in different tiers of the system have different shapes it becomes difficult for the designer to ensure that proper tripping occurs at all overcurrent levels. It lias previously been proposed to utilize a relay with an electronic current detection circuit which deter¬ mines the current time curve, but such circuits are not generally readily adjustable to meet all the requirements. Accordingly it is an object of the present invention to provide an overcurrent protection device -in which the current-tripping time curve can be wideJLy varied to suit differing requirements. OMPI Summary of Invention Broadly, an overcurrent protection device in acco dance with the invention comprises current sensing means for producing an electrical signal related to the current in a conductor, an electronic circuit connected to receiv said electrical signal, and output means connected to be controlled by said electronic circuit so as to interrupt the current in said conductor when such current has excee ed a set value for a time dependent on the margin by whic the current exceeds the set value, characterised in that said electronic circuit includes a read-only memory pro¬ grammed with data determining the current - time characte istic. More particularly an overcurrent protection device accordance with the invention comprises current sensing means for producing an electrical signal related to the current in a conductor, an electronic circuit connected t receive said electrical signal, and output means connecte to be controlled by said electronic circuit so as to inte rupt the current in said conductor when the current in sa conductor exceeds a set value for a time dependent on the margin by which the current exceeds the set value, charac '-- • terised by current - time characteristic selection means settable to determine a plurality of points on the curren time characteristic curve, said electronic circuit operat ing to interpolate between said points. The electronic circuit preferably includes a micro processor device which carries out the interpolation func tion referred to above. The microprocessor memory may be pre-programmed wi the co-ordinates of said points on a multiplicity of different characteristic curves, the selector means actin to select which of the pre-programmed curves is to be use Alternatively, the selector means may comprise an array o "£fU switches which can be separately and independently set to designate the co-ordinates of the points. In either of these cases the end-user of the device can himself select the co-ordinates. As a further alternative the selector means may com¬ prise a permanent wired connection array which determines the co-ordinates of the points , the end user then having no control, but having an assured current-time characteris¬ tic built into his device. Brief Description of Drawings In the accompanying drawings, Figure 1 is a diagram of an example of an overcurrent protection device in accordance with the invention, Figures 2, 3 a *π*d k are diagrams showing three altern¬ ative forms of a characteristic curve selector forming part of the device of Figure 1, Figure 5 is "fc-ke flow chart of a main programme used in a microprocessor in the device of Figure 1, Figures 6 and 7 are flow charts of two sub-routines used in tne programme, Figure 8 is a more detailed flow chart of a part of --.. . the programme illustrated by Figure 5 , &Ω<3- Figure 9 is the flow chart of a modified version of the part of the programme illustrated by Figure 8. Best Mode of Carrying out Invention Referring firstly to Figure 1 the ' device shown inclu¬ des a set of current transformers having secondary windings 10, 11, 12 and 13 magnetically linked to the phase conductors l-'÷ , 15 and 16 the neutral conductor 17 -of a three phase supply to a circuit to be protected. One end of the neutral winding 13 is grounded and this winding has a load resistor l8 connected across it. Bach of the three phase windings 10, 11, 12 has one of its ends earthed via an associated one of three load resistors 19, 20 and 21 and its other end connected to the other end of the neutral winding 13. It will be appreciated that this arrangement is such tha the voltage across the resistors 1 , 20, 21 will be prop tional to the current in the three phases and, provided there is no earth leakage, ' the voltage across the resist 18 will be zero. Increasing earth leakage causes an inc asing voltage across the resistor 18. The three resistors 19, 20 and 21 are connected t a circuit for producing a signal corresponding in magnit to the amplitude of the current in that phase which is carrying the heaviest current. This circuit includes th comparators 22, 23 and 24 which have their non-inverting inputs connected to the "live" ends of the resistors 19, 21 and their inverting inputs connected together and to output of the circuit. Three diodes 25, 26, 27 have thei anodes connected to the.outputs of respective ones of the comparators 22, 23 and 24. The cathodes of the diodes 2 2β, 27 and connected together and to one side of a capac 28, the other side of which is grounded. A resistor 29 connected across the capacitor 28„ A diode 30. has its anode connected to the cathode diodes 25■ » 2β, 27 and its cathode connected to one side '-- . a capacitor 31 » resistor 3 being connected across the diode 30. T e other side of the capacitor 31 is grounde The voltage appearing at any time is proportional the amplitude of the maximum phase current in the conduc 14, 15, l6 the resistor 29 and capacitor 28 and the resi 32 with capacitor 31 acting to filter out high frequency changes in the current signals. The diode 30 enables ra changes in the current to be followed.* The voltage on resistor 18 is applied to a simila circuit comprising a comparator 33j a diode 3k, a capaci 35 a resistor & , a further diode 37 further resistor and a further capacitor 39. __ - The signals on capacitors 31 and 39 are applied to the non-inverting inputs of two further comparators 40 and 41 providing two inputs to a micro-processor circuit 42. An 8-bit digital output from the circuit 42 is supplied to a digital-to-analog converter 43 which supplies an analog output which is applied to the inverting inputs of the com¬ parators 40, 4l. Part of the microprocessor circuit soft ¬ ware is designed to vary the eight-bit output referred to bring the analog output of the converter 4 as close as possible to the signals on capacitors 31 and 39 at different stages of the programme, under the control of the outputs of the comparators so as to convert these signals into digital form within the circuit 42. The microprocessor circuit 42 is used to compare the "current" data it receives with threshold level data stored in its memory and -to output to a digital timer 44, a digital signal indicating the length of time for which the current can be allowed to persist. This digital signal, which is used to load a counter incorporated in the timer 44 is calculated by the circuit 42 as a function of the margin by which the current exceeds the threshold as will be herein¬ after explained. The timer 44 provides an output signal when it has counted out the time interval corresponding to the digital signal with which it is loaded and this output signal is applied to an INTERRUPT terminal IRQ of the circuit 42. ϊ-hen such an output signal is received by the circuit 42 whilst an overcurrent condition still exists the circuit 42 produces a trip signal which is applied via a buffer amplifier 5 to a trip relay 4 which opens contact 47 controlling the conductors 14, 15 and lβ. S The relationship between the overcurrent margin and the time delay between this margin being detected and trip¬ ping occurring is controlled as mentioned above by the cir¬ cuit 3 . To this end the read only memory of the circuit 32 is programmed with data defining the co-ordinates in a current-time plane of 3 points on each of sixteen different ( - O PI - (, - current time characteristic curves. The 32 points are equally spaced on the digital "current" axis, but prefer ly, as shown, the digital-to-analog converter 43, is pro ded with a feedback circuit to give it a non-linear conv sion characteristic, such that a digital step on the cur axis represents a relatively small change in actual curr at low current, but a relatively large change in current high current. The selection of the one of the sixteen character tic curves s.tored is to be utilized in the calculation o the time delay is determined by a characteristic selecto 48 which provides a four-bit input to circuit 42. ' One f of selector 48 is shown in Figure 2 and includes four independent two-way switches 48a, 48b, 48c_ and 48<d, each which is connected to select a "1" or a "0" for each bit Resistors and capacitors are associated with the switche as shown in ensure that changing over of the switches do not create spurious pulse signals at the circuit 42 inpu An alternative selector 148 is shown in Figure 3 » which a socket l48a is arranged to receive any one of si teen different plug-in connector boards l4Sb to provide appropriate 4-bit word. The board l48b may be provided shown with a pictorial representation of the characteris defined by the board. Finally, as shown in Figure 4, the selector may b four pole - lβ-way switch connected to provide a directl coded binary output. In any event the selector 48, x 148 or 248 provide ':s 4-bit code determining the address of the 32 ppooiinnttss ssttoor_ in the ROM. Turning now to Figure the microprocessor softwa is illustrated by a flow sheet. A reset entry input to programme occurs when the trip relay is reset and as shown this reset entry causes the IRQ vector to be set to point to the trip routine, i.e. ensures that if an output is received from timer 44 during an overcurrent (or earth fault) condition this interrupt signal initiates the TRIP sub¬ routine shown in Figure 6. After initialising all the support chips (e.g. the timer 44 and d. to a converter 42) the IRQ facility is reset and disabled, so that any output from the timer 44 until an IRQ enable condition has been established will have no effect. At this stage of the programme, the signal on capaci¬ tor 31 is converted into a digital signal as already men¬ tioned and stored in a register in circuit 42. Next the signal on capacitor 39 is converted and stored in another register and in the next programme interval a comparison between these stored signals and the respective stored threshold levels is carried out. If neither comparison results in an overcurrent or earth fault condition being found to be present the loop back to the reset and disable IRQ step is initiated and this loop persists until an over- current or earth fault condition is found to exist. In this event an enable IRQ flag is set so that an input rom timer 44 will now initiate the TRIP sub-routine . On the next few programme s * teps the trip time delay .is calculated (see Figure 8 and subsequent description) and an "adjusted" trip time is then calculated by carrying out the calculation, "adjusted" trip time = old trip time left x new trip old trip time time. The trip time initially set in the timer is the maximum its counter can hold and counting was starred when the enable IRQ flag was set. Thus this "adjusted" trip time calcula¬ tion involves inputting the current count in the timer, dividing by the trip time last calculated (on the first cycle the initial maximum time) and multiplying by the new¬ ly calculated trip time appropriate for the last input O PI current signals. The new trip time is now stored (to be used in the above "adjusted" trip time calculation as the old trip ti on the next programme cycle) and the current level data i also stored. The "adjusted" trip time is loaded into the timer. Next, the current signals form capacitors 31 and 39 are input again and a determination is then made as to whether the current signals have changed from the stored levels by more than x o (e.g.2.5 / £) . f no such change has occurred a loop is created to repeat the inputting of the current signals and if the currents are constant this loo persists until the timer 44 produces its output and cause the TRIP sub-routine to be entered. If on the other hand a change of more the % has occurred in the current levels, these are again compared with the thresholds and, if the fault condition still exi a loop is created which takes the programme back to the enable IRQ stage so that the new trip time step, the "adj ed" trip time determination etc are repeated. Again, whe the timer 44 finally produces its output the TΗIP routine entered, the main programme being discontinued. If the fault condition is found not to exist after current change .of more than _x.°/o the IRQ vector is set to point to the RECOVERY routine (Figure 7) so that when tne timer 44 times out this RECOVERY routine will be entered instead of the TRIP routine, Next the timer 44 is loaded with its maximum count and the current signals are then input yet again. If tne current signals are still below threshold, a loop is established in which the current signals are repeated input and compared with the threshol until the maximum time (8 minutes) has expired when the timer 44 output will cause the RECOVERY routine to be ent ed. If, during this 8 minutes the fault condition recurs the old time is loaded with the maximum delay and calcula¬ tion is carried out and the result (8 minutes - timer con¬ tents + a fixed compensation delay) is loaded into the timer. The IRQ vector is set to point to the TRIP routine and the programme returns to the ENABLE IRQ point. The TRIP routine consists simply of outputting a TRIP signal to the buffer 45 and latching this programme step. The RECOVERY routine consists of setting the IRQ vector to point to the TRIP routine and then returning immediately to the reset and disable IRQ point in the pro¬ gramme (the timer then stopping with its contents at maximum). Figure 8 is a flow sheet showing the "calculate new trip time" programme steps in more detail. As shown these steps include inputting the curve data from the selector, multiplying by a set value and adding a constant to arrive at. t.hβ ROM addresses of the appropriate 3 point co-ordinates in the ROM. A register, is then loaded with the actual curr¬ ent signal and a divide by 8 calculation is made and the result of this and the remainder are both stored. The result is used to select the point in the ROM to represent the stored "current" co-ord nate below the actual current and the resulting time data T, is loaded into a register. Then the time data T„ from the next ROM location is loaded into another register. The new trip time T' is then calculated from T - Φ + [ (T„ - T τ ) x REMAINDER i8 It will be appreciated that the programme described ensures that interpolation between ' the stored fixed points on the characteristic curves is carried, out in the time calculation stage. The "adjusted" trip time calculation ensures that when the current is changing the new trip time is effectively shortened in proportion to the amount of the already expired portion of the previous trip ime. This corresponds approximately to the thermal behaviour of a conventional thermal circuit breaker in which the trip time - OMPI - |0 - " " for a given overcurrent condition will depend on the immediately pre-existing current condition, i.e. if the current rises to 1.5 x threshold tripping will occur mor quickly if the current was previously 1.25 x threshold t it would if the current was previously less than the threshold. The circuits and all the components used in the a described embodiment can be common to a wide range of de only the current transformers being changed to determine the actual thresholds. In a modified form of the invention the technique storing in the ROM of the co-ordinates of fixed points o the characteristic curves is not employed. Instead the selector means is a more complicated switch array, or ev a keyboard, which is or are used to input to the circuit the co-ordinates of a number of points (e.g. 6 points) o the desired curve, each point being individually set ind endently of the others. In this case, since fewer point on the curve are specified it becomes desirable to plot curves on log-log co-ordinates rather than linear co-ord ates and the converter 42 would then have a linear chara eristic (i.e. the feedback network mentioned would be omitted). To enable log-values to be determined the ROM would include a log look-up table "and an anti-log look-u table may also be included, or alternatively a programme could be employed to obtain anti-log values from the log look-up table. Figure 9 shows the modified "calculated new trip programme steps used in this modified form of the invent As shown these include inputting all the time curve co¬ ordinate data to a register, inputting the input current and storing the log of this. Finding the co-ordinates o the specified points above and below this log value and ing these to calculate the slope of the characteristic _ - - between these points, calculating * in terms of the actual current, one co-ordinate and the slope and then outputting the an i-log of If . OMPI

Claims: - IX - •* " CLAIMS 1. An overcurrent protection device comprising curren sensing means for producing an electrical signal related to the current in a conductor, an electronic circuit conn ted to receive said electrical signal, and output means connected to be controlled by said electronic circuit so to interrupt the current in said conductor when such curr has exceeded a set value for a time dependent on the marg by which the current exceeds the set value, characterised in that said electronic circuit includes a read-only memo programmed with data determining the current - time characteristic. 2. An overcurrent* protection device comprising curren sensing means for producing an electrical signal related the current in a conductor, an electronic circuit connect to receive said electrical signal, and output means conne ed to be controlled by said electronic circuit so as to i terrupt the current in said conductor when the current in said conductor exceeds a set value for a time dependent o the margin by which the current exceeds the set value, characterised by current - time characteristic selection means settable to determine a plurality of points on the current time characteristic curve, said electronic circui operating to interpolate between said points. 3. A device as claimed in claim 2 in which said elec¬ tronic circuit includes a microprocessor device which carries out the interpolation function. 4. A device as claimed in claim 3 n which the micro¬ processor has a memory preprogrammed with the co-ordinates of said points on a multiplicity of different characteristic curves, said selector means acting to select which one of the pre-programmed curves is to be used. 5. A device as claimed in claim 3 in which said selec¬ tor means comprises an array of switches which can be separately and independently set to designate the co-ordin¬ ates of the points. 6. A device as claimed in claim ' 3 in which said selec¬ tor means comprises a permanent wired connection array which determines the co-ordinates of the points.

Inventors: BURGESS T, SMART I
Applicants: BURGESS T, ELLISON GEORGE LTD, SMART I, GEORGE ELLISON LTD
Application year: 1981
Main classification: H02H3
IPC codes: H02H3