Note: Descriptions are shown in the official language in which they were submitted.
~'7
1 479128
CIRCUIT INTERRUPTER t~7ITH SOLID
STATE DIGITAL TRIP UNIT
CROSS-REFE~ENCE TO ~ELAT~D CANAD ~ L~L~3l9
me present invention is related to material
disclosed in the *ollowing Canadian applications g all of
which are assigned to the same assignee of the pres~nt
application.
Canadian Serial No~ 374,755, "Circuit Int~3rrupter
With Front Panel Numeric Display" ~iled ~pril 6, 1981
by J. C~ Engel, R. T. Elms, and G. F~ Sal~tta;
Canadian Serial No. 374,764, "Cirouit Int~rrupter
With So~id State Digi-tal Trip Unit And Positlve Power-Up Fea-
ture" filed April 6, 1981 by R. T. Elms, G. F. Saletta, and
B. ~. Mercier;
Canadian Serial No. 374,776, "Circuit Interrupter
With Digital Trip Uhit And Optically Coupled Data Input/
: 15 Output System" *iled April 6, 1981 by J. C. Engel, J. A.
Wafer, J. T. ~ilson, and R. T. Elms;
Canadian Serial No. 374,716, 'ICircuit Interrupter
~7ith Energy Management ~unctions" ~iled April 69 1981
by J. T. Wilson, J. A. ~lafer, and J. C. Engel;
Canadian Serial No. 374,735, "Circuit Interrupter
With Digital Trip Unit And Style Designator Circuit" filed
April 6, 1981 by J. J. Matsko, E. W. Lange, J. C. Engel,
and B. J. Mercier;
Canadian Serial No. 374,742, "Circuit Interrupter
With Overtemperature Trip Device" ~iled April 6, 1~1 by
J. J. Matsko, and J~ A. Wa~er;
Canadian Serial No. 374,754, "Circuit Inter~upter
With
~`
S3~7
2 47J128
Digital Trip Unit And Means To Enter Trip Settings" filed
April 6, 1981 by R. T. Elms, J1 C. En~el, B~ J. Mercier3
G. F. Saletta, and J. T. ~ilson;
Canadian Serial No. 374~792) "Circu~t Interrupter
With Digital Trip Unit And Power Supply" ~iled April 6~ 1981
by J~ C. EngelJ J~ A~ Wa~er, R. T. Elms, and G. F. Saletta;
Canadian Serial No~ 374,696~, "Circuit Interrupter
Wlth Multiple Displar And Parameter E~ltry Mean~l' filed
April 6, 1981 by J. J. Matsko, J. A. Wafer, J. C. Engel, and
B. J. Mercier;
Canadian Serial No. 374,771~ "Circuit Interrupter
~rith Remote Indicator And Power Supply" ~iled April 69 1981
by J. C. Engel~ J. A. Wafer, B. J. Mercier, and J. J. Matsko;
Canadian Serial No. 374,724, "Circuit Interrupter
15 With Digital Trip Unlt ~nd Automatic Re~et" ~iled April 6,
19~1 by B. J. Mercier and J. C. Engel; and
Canadian Serial No. 374,748, "Circuit Interrupter
With Digital Trip Unit And Potentiometers For Parameter Entr~
flled ~pril 6, 1981 by J. C. Engel, B. J. Mercler, a~d R~ T.
Elms.
~C~ 0~ 'IU~ ~v~n~
The invention relates to circuit interrupters
having means for electronically analy2ing the electrical
conditions on the circuit being protected, and means for
automatically openlng to interrupt the current ~low when-
ever electrical conditions exceed predetermined lim.its.
DescriE~tion of the Prior Art:
Circui-t breakers are widely used in industrial
and commercial applications for protecting electrical con-
ductors and apparatus connected thereto from damage due to
excessive current flow. Circuit breakers were initially
designed to interrupt when the current flowing through
them exceeded a certain level. Gradually, however, more
elaborate time-current interrupting characteristics were
required such that a circuit breaker would rapidly open
~;.
;~`3 47,128
upon very high overload conditions but would delay inter-
~ruption upon detection of lower overload currents, the
`'~delay time being roughly inversely proportional to the
degree of overload. Additionally, circuit breakers were
called upon to interrupt upon the detection of ground
;fault currents. As the complexity of electrical distri-
~bution systems increased, the control portions of circuit
'.;breakers on a system were interconnected to provide selec-
tivity and coordination of interruption sequences. This
.~10 allowed the system designer to specify the order in which
the various circuit breakers would interrupt under speci-
fied fault conditions.
During the late 1960's, solid-state electronic
control circuits were developed for use in high power, low
voltage circuit breakers. l`hese control circuits perform-
ed functions such as instantaneous and delayed tripping
.which were traditionally achieved by magnetic and thermal
means. The improved accuracy and flexibility of the solid
state electronic controls resulted in their wide-spread
acceptance, even though the electronic control circuits
were often more expensive than their mechanical counter-
parts.
The earliest electronic control circuit designs
utilîzed discrete components such as transistors, resist-
:25 ors, and capacitors. More recent designs have included
~-integrated circuits which have provided improved product
performance at reduced cost.
, "As the cost of energy continues its rapid rise,
.there is increasing interest in more efIectively control-
ling the usage of electrical energy through the clesign of
more sophisticated electrical distribution systems.
Therefore, there is required a circuit breaker providing a
more complex analysis of electrical conditions on the
circuit being protected and even greater capability for
coordination with other breakers. As always, it is ex-
.tremely desirable to provide this capability at the same
or lower cost.
, .
.
4 ~7,128
SUMMARY OF THE INVENTION
Circuit interrupter apparatus is provided foruse on an electrical power distribution system. The
apparatus includes separable contacts, releasable means
operable when actuated to automatically open the contacts,
means operable upon energization to actuate the releasable
means, means ~or sensing current flow through the con-
tacts and ~or providing operating power to the apparatus,
means for converting analog signals to digital values,
a digital arithmetic and logic processor, and memory
array means for storing a plurality o~ values corres-
ponding to the desired time current tripping character-
istics of the apparatus.
The processor periodically genera~es signals to
cause the analog-to-digital conversion means to supply a
digital representation of the value oE curren~ ~low
through the contacts. The processor then compares the
current :Elow value representation to the digital represen-
tations o~ the t.ime-current tripping characteristic stored
in the memoryl and generates a signal to energize the
actuating means and separate the contacts when current
flow therethrough exceeds the time-current tripping char-
acteristic of the apparatus.
47,12~; 48,335; 48,336
`BRIEF DESCRIPTION OF THE D~AWINGS
. . .
Figure 1 is a perspective view of a circuit
breaker embodying the principles of the present invention;
Fig. 2 is a functional block diagram of the
circuit breaker shown in Fig. l;
Fig. 3 is an electrical schematic diagram of a
distribution system employing the circuit breaker of Figs.
1 and 2;
Fig, 4 is a graph of a typical time-current
` 10 tripping characteristic, plotted on a log-log scale;
Fig. 5 is a block diagram of the trip unit shown
in Figs. 1 and 2;
Figs. 6A and 6B are partial schematic diagrams
of the trip unit circuitry of Fig. 5;
Fig. 7 is a general flow chart o the program
stored in the memory of the microcomputer;
Fig. 8 is a flow chart of the analog-to-digital
routine stored in the memory of a microcomputer which is a
component of the trip unit;
Fig. 9 is a flow chart of the short delay trip
and instantaneous trip functions of the program of Fig. 7;
Fig, 10 is a flow chart of the long delay trip
function of the program of Fig. 7;
Fig. 11 is a flow chart of the ground trip func-
tion of the program of Fig. 7;
Fig. 12 is a flow ~hart of the self-checking
routine of the program of Fig. 7; and
Fig. 13 is a flow chart of the routine to read
external programmable read-only-memory of the program of
Fig. 8.
DESCRIPTION OF THE PREFERRED EMBODIMENT
General Physical and Electrical D~scription:
Reference may now be had to the drawings, in
which like reference characters refer to corresponding
components. A perspective view and a functional block
diagram of a molded case circuit breaker 10 employing the
principles of the present invention are provided in Figs.
1 and 2, respectively. Although the circuit interrupter
6 47,128; 48,335; 48,336
10 is a three-pole circuit breaker for use on a three-
phase electrical circuit, the invention is, of course, not
so limited and could be used on a single-phase circuit or
another type of multiphase circuit.
A power source such as a transformer or switch-
board bus is connected to input terminals 12 and an elec-
trical load is connected to output terminals 14. Internal
conductors 16 connected to the terminals 12 and 14 are
also connected to interrupting contacts 18 which serve to
selectively open and close an electrical circuit through
the circuit breaker. The contacts 18 are operated by a
mechanism 20 which responds to manually or automatically-
initiated commands to open or close the contacts 18.
Current transformers 24 surround each of the
internal phase conductors 16 to sense the level of current
flow through the conductors 16. The output from the
- current transformers 24 is supplied to a trip unit 26,
along with the output from a current transformer 28 which
senses the level of ground fault current flowing in the
circuit. The trip unit 26 constantly monitors the level
of phase and ground fault currents ~lowing in the circuit
to which the breaker lO is connected and initiates a
command signal to a trip coil 22 which actuates the mech-
anism 20 to open the contacts 18 whenever electrical
conditions on the circuit being protected exceed predeter-
mined limits stored in the trip unit 26. During normal
conditions, the mechanism 20 can be commanded to open and
close the contacts 18 through manually-initiated commands
applied through the manual controls 32.
Referring to Fig. 1, it can be seen that the
circuit breaker 10 includes a molded insulating housing
34. The terminals 12 and 14 are on the rear of the hous-
ing 34 and are thus not shown in Fig. 1. A handle 36 is
mounted on the right-hand side of the housing 34 to allow
an operator to manually charge a spring (not shown) in the
mechanism 20. The manual controls 32 are positioned in
the center of the housing 34. Windows 38 and 40 indicate
the state of charge of the spring and the position of the
. ~ '
' :
7 47,12~; 48,335; 48,336
contacts 18, respectively. A push-button 42 allows an
operator to cause an internal electric motor to mechanic-
ally charge the spring in the same manner as ~he manual
charging operation which can be performed by the handle
36. A pushbutton 44 allows an operator to cause the
spring to operate the mechanism 20 to close the contacts
18. Similarly, a pushbutton 46 allows an operator to
cause the spring and mechanism 20 to open the contacts 18.
The panel of the trip unit 26 is positioned on
the left side of the housing 34 as can be seen in Fig. 1.
This panel includes a numeric display device 80 to permit
an operator to observe the/electrical parameters on the
circuit being protected, a plurality of light-emitting
diode (LED) indicators 84, 86, and 88, a rating plug 78 to
determine the maximum continuous current of the breaker,
and a plug-in programmable read-only memory (PROM) chip 82
to define the time-current trip characteristic of the
breaker.
Use of a_Circuit Breaker in an
Electr~icaI P wer ~istrlbution System
Before explaining the operation of the trip
unit, it will be helpful to describe in greater detail the
function of a circuit breaker in an electrical power
distribution circuit. Fig. 3 shows a typical electrical
distribution system. A plurality of electrical loads 48
are supplied through circuit breakers 50, 52 and 54 from
either of two sources of electrical energy 56 and 58. The
sources 56 and 58 could be transformers connected to
separate high voltage electrical feeder lines, diesel-
3o powered generators, or a combination of the two. Powerfrom the first source 56 is supplied through the first
main circuit breaker 50 to a plurality of branch circuit
breakers 60, 62, 64, and 66. Similarly, power from the
second source 58 may be supplied through the second main
circuit breaker 52 to a second plurality of branch circuit
breakers 68, 70, 72, and 74. Alternatively, power from
either source 56 or 58 may be supplied through the tie
circuit breaker 54 to the branch circuit breakers on the
8 47,128; 48,335; 48,336
opposite side~ ~enerally, the main and tie circuit break-
ers 50, 52 and 54 are coordinated so that no branch cir~
cuit is simultaneously supplied by both sources. The
capacity of the main and tie circuit breakers 50, 52 and
54 is usually greater than that of any branch circuit
breaker.
If a fault (abnormally large current flow)
should occur at, for example, the point 76, it is desira-
ble that this condition be detected by the branch circuit
breaker 62 and that this breaker rapidly trip, or open, to
isolate the fault from any source of electrical power.
The fault at the point 76 may be a large over-current
condition caused, for example, by a short circuit between
; two of the phase conductors of the circuit, or an overload
only slightly above the rating of the breaker such as
might be caused by an overloaded motor. Alternatively, it
may be a ground fault caused by a breakdown of insulation
; on one of the conductors, allowing a relatively small
amount of current flow to an object at ground potential.
In any case, the fault would also be detected by the main
or tie breakers S0, 52 or 54 through which the load fed by
branch breaker 62 is supplied at the time of the fault.
However, it is desirable that only the branch circuit
breaker 62 operate to isolate the fault from the source of
electrical power. The reason for this is that if the main
or tie circuit breaker should trip, electrical power would
be lost to a greater portion of the entire system than
merely the load attached to the branch circuit on which
the fault occurred. It is therefore desirable that the
main and tie circuit breakers 50, 52 and 54 should have a
longer delay period following detection of a fault before
they initiate a tripping operation. The coordination of
delay times among the main, tie, and branch circuit break-
ers for various types of faults and the need for inter-
locking between breakers are major reasons for the need to
provide sophisticated control in a trip unit.
Time-Current Tripping Characteristics:
In order to achieve the coordination between
. , `
-
9 47,128; 48,335; 48,336
circuit breakers as described a~ove, the time vs. currenttripping characteristics of each circuit breaker must be
specified. Circuit breakers have traditionally exhibited
characteristics similar to that shown in Fig. 4, where
both axes are plotted on a logarithmic scale. ~hen cur-
rent below the ~aximum continuous current rating of the
breaker i5 flowing, the breaker will, of course, remain
closed. As current increases, however, it is desirable
that at some point, for example the point 300 of Fig. 4,
the breaker should trip if this overload current persists
for an extended period of time. Should a current flow
equal to the maximum con~inuous current rating as speci-
fied by point 300 persist, it can be seen from Fig. 4 that
the breaker will trip in approximately 60 seconds.
At slightly higher values of current, t;he time
required for the breaker to trip will be shorter. For
example at 1.6 times maximum continuous current as speci-
~ied by point 302, the breaker will trip in about 20
seconcls. 'rhe portion o:E the curve between the points 300
and 304 is known as the long delay, or thermal, character-
istic of the breaker, since this characteristic was pro-
vided by a bimetal element in traditional breakers. It is
desirable that both the current level at which the long
delay portion begins and the trip time required for any
point on that portion be adjustable. These parameters are
known as long delay pick-up and long delay time, respec-
tively, the variation of which is indicated by the arrows
306 and 308.
At very high overcurrent levels, for example 12
times the maximum continuous current and above, it is
desirable that the circuit breaker trip as rapidly as
possible. This point 312 on the curve is known as the
"instantaneous" or magnetic, trip level, since traditional
breakers employed an electromagnet in series with the
contacts to provide the most rapid response. The instan-
taneous pick-up level is usually adjustable, as indicated
by the arrow 314.
To aid in coordinating breakers within a distri-
47,128; ~8,335; 48,336
bution system, modern circuit breakers have added a shortdelay trip characteristic 316 between the long delay and
instantaneolls portions. The present invention allows
adjustment of both the short delay pick-up level and the
short delay trip time as indicated by the arrows 318 and
320.
Under certain conditions it is desirable that
the trip time over the short delay portion vary inversely
with the square of the current. This is known as an I t
characteristic and is indicated in Fig. 4 by the broken
line 310.
Trip Unit Functions and Modes:
The functions and modes of the trip unit 26
employing the principles of the present invention will now
be described. A rating plug 78 is inserted into the front
panel of the trip unit 26 to specify the maximum contin-
uous current to be allowed in the circuit being protectecl
by the circuit breaker. This may be less than the actual
capacity of the circuit breaker, which is known as the
frame size. For example, the ~rame size for the circuit
breaker may be 1,600 amperes; however, when the breaker is
initially installed the conductors of the circuit being
protected may be sized so as to continuously supply only
l,200 amperes of electrical current. Therefore, a rating
plug can be inserted in the trip unit to ensure that the
maximum continuous current allowed by the circuit breaker
will be only 1,200 amperes even though the circuit breaker
itself is capable of safely carrying l,600 amperes contin-
uously.
Throughout the remainder of the description of
the invention, current levels may be described as multi-
ples of the maximum continuous current as specified by the
rating plug. This convention will be expressed as, for
example, 3 per unit or 3 p.u. to indicate a current level
of three times the maximum continuous current.
As can be seen in Fig. l, the trip unit panel
contains a numeric display indicator 80 and a number of
LED indicators 84, 86, and 88. The electronic circuitry
-
~ -
11 47,128; 48,335; ~8,336
internal to the trip unit causes the numeric display
indicator 80 to sequentially display the present value of
electrical conditions on the circuit being protected and
the various limit settings defining the time-current trip
curve of the breaker as currently set. The LED's 84, 86
and 88 indicate whether a ground fault, long delay over-
current, or "instantaneous" overcurre~t was the cause of a
trip operation.
To the right and below the numeric display indi-
cator 80 and rating plug 78 is a plug-in programmable
read-only memory (PROM~ module 82, such as a type 3601
manufactured by the Intel Corporation, in which are stored
the various limit values and settings which specify the
time-current tripping characteristic of this particular
circuit breaker. The method of loading the settings into
this module and the manner in which the module is used by
the trip unit circuitry will be described in a later
section.
~c~e~
2Q The trip unit circuitry includes a digi~al
arithmetic logic and control processor 154 such as the
type 8048 microcomputer manufactured by the Intel Corp-
oration, and is presented by block diagram in Fig. 5.
This section will describe each block of Fig. 5 and pre-
sent a description of the operation of the trip unit.
The microcomputer 154 contains an arithmetic
logic unit (ALU) 153, 64 ~ 8-bit bytes of read-write
random acc~ss memory (RAM) 155, lK x 8-bit bytes of read-
only memory (ROM) 157, an 8-line data bus 172, and two
8-line input-output ports Port 1 and Port 2. Other types
of digital arithmetic logic and control processors could
be used, such as those requiring outboard memory circuits
rather than having the on-chip ~AM and ROM circuits of the
8048. However, for a detailed description of the micro-
computer, reference should be made to the MCS-48 Micro-
computer User's Manual published by the Intel Corporation.
Circuit Description:
Referring to the system block diagram of Fig. 5
12 47,128; 4~,335; 48,336
and the detailed schematic diagrams of Figs. 6A and 6B,
the display section 79 is first described. It consists
of four data la~ches IC5, IC6, IC7 and IC8 and the four-
digit liquid crystal numeric display 80. The data latches
may be the type MC 14543. Display data is multiplexed on
the data bus 172 of the microcomputer; the four least
significant bits represent data and the ~our most signi-
ficant bits its position on the display. The liquid
crystal display 80 derives its back plane clock from the
interval timer 92. This interval timer also fulfills the
function of resetting the microcomputer if it does not
receive its clock signals from the microcomputer 154.
Under normal operation, the micropro~essor outputs a pulse
on every execution of the main program loop.
It can be seen on the diagram of Fig. 5, that
the PROM 82 receives its address from the data bus 17~ and
outputs its contents via Port 1. Since the displays sec-
tion 79 and the address lines of the PROM 82 are both
connected to the data bus 172, the address information for
the PROM would tend to cause a garbled display. However,
the address information appears on the bus or only a
small fraction of a second, to be immediately ~ollowed by
valid display information. The LCD display therefore does
not have time to respond to the PROM address in~ormation
and the operator observes only the valid display informa-
tion.
The output subsystem 94 consists of 112 of a
type A775 comparator IC2, and of quad NOR gate IC10 and
quad NAND gate ICll. Through comparator IC2 the micro-
computer 154 via Port 2 sets an interlock output signalafter a ground fault pickup. Through the NAND gate of
ICll the mi~rocomputer sets the corresponding LED indicat-
or 84, 86 or 88 after a trip.
The NOR gates IC10 provide the high-level output
signal to trip a single SCR 98 under ground, short delay,
long delay, or instantaneous trip. It also forces this
trip signal to follow the RESET signal during power-up
thus eliminating false tripping during the 10 ms period of
l3 ~7,128; 48,335; 48,336
microcomputer instability after power is first applied.
The input subsystem 100 consists of two peak
detecting circuits including capacitors 90 and 91, a type
ZN425J D/A converter IC4, the other half of comparator
~C2~ and the analog switches of IC3. The capacitors 90
and 91 store the peak value of phase and ground current,
respectively, for each cycle of the AC line. The peak
values are then read every cycle by the microcomputer.
The capacitors 90 and 91 are reset (discharged) later in
each cycle by the microcomputer through a transistor 96
and IC11 activated by Port 2.
The analog-to-digital conversion of the signal
from the input subsystem 100 is accomplished by an itera-
tion technique employing the D/A converter IC4 and compar-
ator IC2. A digital value is supplied to the ~D/A con-
; verter IC4 by the microcomputer 154. This value is con-
verted to an analog value and supplied to IC2~ IC2 then
compares this value to the value supplied from capacitors
or 91 through the analog switch IC3 and indicates
whether or not the value supplied by IC4 is larger. The
result of this comparison is supplied via the Tl test
input to the microcomputer 154, which then generates a new
value to IC4. This process continues until the value
generated by the microco~puter 154 is very close to that
supplied by the analog switch IC3, and the result is re-
tained in the accumulator of microcomputer 154. The
technique is shown in greater detail in the flow chart of
. Fig. 8.
The function of transistors 102 and 104 and
their associated components is to direct the phase (or
ground) currents from the CT's 24 and 28 to the rating
plug resistor 105 during non-tripping operation. However,
when a trip condition is sensed and the trip SCR 98 is
turned on, transistors 102 and 104 are turned off, thereby
directing essentially all of the phase (or ground) current
signal into the shunt trip coil for a positive tripping
action.
Power for the trip unit circuitry is supplied by
, .
1~ 47 ~ 12~; 48 ~ 335; 4~3 ~ 336
rechargeable battery with charging power produced by the
current transformers 24. Alternately, power could be
derived directly from the current transformers 24 or
independently via connections to the conductors 16.
Description of Operation
The operation of the invention is described in
detail in this section. In the first part, a general flow
chart of the program and the allocation of memory are
`~ presented. Major subroutines called from the main loop
will then be ~t~i-~e~in the second part.
Data Memory Allocation:
The alloca~ion of the internal RAM 155 of the
microcomp~ter 154 is shown in Table I.
, .
47,128; 48~335; 48,336
TABLE I
DATA MEMORY MAP (RAM)
63 Long Delay Pick-up (LDP)
62 Long Delay Time (LDT)
61 Short Delay Pick up (SDP)
Short Time (SDT)
59 Instantaneous Trip Setting (ITS)
58 Ground Fault Piclc-up (GFP)
57 Ground Fault Time (GFT)
56
54 Sum 6 = Tally of GFT
53 Sum 4 = Tally of SDT
52 Sum 45 = Self-checking Su~ 4
51 Sum 65 = Self-checking Sum 45
49
48
~ 47
; 20 46 Sum 3 = Lower Tally of LDT
4S Sum 2 = Middle Tally of LDT
44 Sum 1 = Upper Tally of LDT
43
42
41 Trip Flag
Cycle Counter
39 Present value of inst. current
38 Present value of GND current
37
36 Trip value
; 35
34 Display inde~
33 Low byte of addr. of next display
32 High byte of addr. of next display
As can be seen, the top eight locations are used
to load the limit value sett~ngs, sucù as Long Delay
~ .
.
16 47,128; 4~,335; 48,336
Pick-Up and Long Delay Time. The values in these loca-
tions are refreshed every 4 seconds, after a reading of
the external PROM 82. The tallies for ground fault, short
delay, and long delay timing functions are also kept in
RAM. The address of the next information to be displayed,
the present value of ground and instantaneous current, and
i the trip value are stored in locations shown. The addrcs-
` sing of those values is done indirectly through Register
(R~) or Register 1 (Rl) which contains the particular
address.
The lower 32 words of data memory are used for
` standard "housekeeping" functions of the microcomputer, as
explained in the previously referenced Intel Userls
~; Manual.
Main Loop
~; Refer to the flow chart of the main loop shown
. in Fig. 7. After the system is powered-up or the reset
'~ button on the front panel is pushed, the program counter
o the microcomputer 154 is loaded automatically with
hex. An instruction at this location brings the microcom-
puter to three initialization routines: clcar RAM, load
display with ~.0, and perform discriminatary trip func-
tion. In the latter function, the present value of the
. phase current is compared with 9.0 p.u., i.e. nine times
rated current. Thus, if the breaker is experiencing a
high overload when the trip unit is firs~ powered-up, the
program is able to trip the breaker within 0.5 ms. These
~: initialization routines are executed only during power-up
, or reset.
At this point the program counter is decremented
to FF hex or 255 decimal. This count signals the micro-
computer 154 to read the external PROM 82. If the PROM 82
' is unreadable (contents = ~H or FFH) or the checksum is
invalid, minimum limit value settings (from ROM 157 in-
~35 ternal to the microcomputer) are loaded in corresponding
.~RAM locations. Otherwise, the last sixteen memory loca-
tions of the PROM 82 are read. The use of a 2K PROM will
thus allow the user to reprogram a new set of limit values
17 47,128; 48,335; 48,336
into the PROM 16 times, before a new PROM must be employ-
ed. (16 x 16 values x 8 bits per value = 2048). After
reading values from PROM, the program jumps to entry
location BEGIN. From then on, this will be the starting
point of the main loop.
The internal ROM 157 of the microcomputer 154
includes a look-up table containing the addresses of the
subroutines which prepare the formats to enable the var-
ious parameter values to be displayed. Through an index
R34 (initialized at ~ and updated by each display routine)
the address of the next display routine is read and stored
in R33 and R32 of RAM 155.
Next, the four main functions of the program are
entered; the instantaneous trip function, the short delay
trip function, the long delay trip function and the ground
trip function. Those functions will be presented in
detail in the next section.
A self-checking subroutine is next executed. In
this subroutine, the analog-to-digital converter, short
delay pick-up, and ground test functions are checked. If
a failure is detected, a failure flag is set and an error
code stored in RAM 155.
The capacitors 90 and 91 for storing peak phase
and ground current are then discharged and a time delay
executed equal to 16.667 ms less the time expended in
executing the main loop instructions.
; A flag is next checked to determine if a trip-
ping operation has occurred. If so, the value of phase or
ground current which caused the trip is now displayed.
Since the trip unit is powered externally, a tripping
operation will not inhibit execution of the microcomputer
software.
After the first cycle, the main counter is at
254D. This number signals the microcomputer 154 to select
another parameter to be displayed by the indicator 80.
Realizing that this count is circular, it can be seen that
the selection is done immediately after reading the PROM
82 and 255 x 16.667 ms ~4.27 sec) thereafter.
18 47,1~8; 48,335; 48,336
The parameter display is a three-digit number in
per unit format, the parameter being displayed is identi-
fied by a numeric code which appears concurrently with the
parameter value in the left-most digit of the numeric
display 80, as follows:
1. Present Phase Current
2. Long Delay Pick Up
3. Long Delay Time
4. Short Delay Pick Up
5. Short Delay Time
6. Ground Fault Pick Up
7. Ground Fault Time
8. Instantaneous Trip Level
9. Present Ground Current
When the counter reaches 125 (2.1 sec) and if an
error was found in the self-checking routine, an error
code will be displayed in the indicator 80 instead o~ a
parameter value: 1 for A/D conversion failure or instan-
taneous trip function failure, 2 for short delay function
failure, 3 for ground trip function failure, and 4 to
indicate that minimum settings are being used. This will
cause the indicator 80 to change from parameter value to
error code every two seconds, indicating to the user that
an error was found.
Detailed Description of Operation:
This section will describe, in detail, the func-
tion blocks shown in the general flow diagram~ Reference
should be made to the flow diagrams presented for each
block.
Considering the instantaneous trip function and
short delay trip function first, refer to the flow chart
of Fig. 9. Upon entering those two routines, the micro-
computer 154 switches the D/A converter IC4 analog output
to the phase peak detecting circuitry through resistors
108, 110 and 112, having values of 6.8K, 220X, and 220K,
respectively. This produces a scale factor of 1 p.u.
(with a digital representation of 160). The A/D conver-
sion (Fig. 8~ subroutine is now called which lasts 0.26 ms
.
19 47,12~; 48,335; 48,336
(104 instructions x 2.5f~cs average execution time).
The A/D conversion subroutine operates by clear-
ing the accumulator, then setting the most significant bit
thereof as a test value. This value is sent to the D/A
converter which produces a corresponding analog value.
This analog value is compared to the phase current value
provided by the peak detecting capacitor 90. If the trial
analog value is smaller than the phase current, then the
trial value consisting of one bit is added to the digital
successive approximation of the phase current value which
is retained in register R3. The test bit in the accumula-
tor is then shifted one place to the right, a correspond-
ing analog test value generated, a comparison made, and
the bit is retained or not in register R3 according to the
results of the comparison. In a similar manner all eight
bits of the accumulator are tested and at the completion
of the eighth bit, the retained value in R3 is transferred
to the accumulator.
The digital value of present phase current (PPC)
is then stored in RAM 155 in order to be displayed and
used in the Short Delay routine. If PPC is greater than
the instantaneous trip setting ~ITS), a tripping operation
is executed, which includes the function of saving the
current value which caused the trip (to be displayed on
indicator 80) and lighting the proper LED 84, 86 or 88 to
indicate cause-of-trip. Otherwise, the short delay trip
function is entered.
In the Short Delay routine, a tally is incre-
mented every cycle if the PPC is larger than the short
delay pickup. The tally is then compared to a value
corresponding to the short delay time setting (SDT). If
the tally is greater than the SDT value, a trip operation
is called for. Otherwise, the Long Delay Test routine is
entered. If the PPC is smaller than the short delay
pickup the short delay tally is reset to zero. At this
point the Long Delay Test (LDTST~, as shown in Fig. 10, is
entered.
Upon entry, the LDTST function switches (through
~0 47,128; 48,335; 48,336
IC3) to the phase peak detecting circuitry. However, this
is done through resistors 114 and 116 having values of
3.3K and 220K, respectively (see Fig. 6). Thus, the
threshold level in the A/D conversion process is doubled.
Keeping in mind that 1 p.u. was encoded as 16D in the in-
stantaneous trip and short delay functions, it can be seen
that now 1 p.u. is encoded as 32D (a reso].ution of 3.12%).
For long delay timing a quantity proportional to
(i)2 must be calculated. This value is added to an accum-
ulating register and then compared to the Long Delay Time(LDT) setting whenever the Long Delay Pick-up (LDPU)
setting is exceeded. The accumulating register then
represents "(i)~t". The use of an example will illustrate
the procedure used:
21 47,128, 48,335; 48,336
Suppose LDP~ = 1 PU = 32 D
LDT = 2 sec
I (PPC) = 6 PU = 32 D x 6 = 192 D
i2 = (192)2 = 36,864
2 Instead of storing i2, however, the quantity
i ~4 is retained since less memory space is required, and
sufficient resolution still maintained. Thus:
i2/4 = 36,~64/4 = 9216.
If i2/4 is accumulated into a tally of 24 bits
every 1/60 of a second, in two seconds the tally will be:
9216 x 60 x 2 = 1,105,920 D
which brings the upper eight bits of ~he tally to the
value:
~ - = 17 D
Thus, an LDT setting for 2 seconds, encoded as 17 D or
11 H, is reached in exactly 2 seconds as desired. There-
fore, LDT setting = # of seconds x 17/2. It must be
realized that with lower PPC the trip unit will take
longer time to reach that count, and with larger PPC the
trip unit will reach that count faster (time will be
inversely related to (i)2).
~ Referring to the flow chart of Fi~. 10~ it can
; be seen that when the PPC is less than LDPU the tally is
decremented with a fixed value of A4 H = 164 D. This
number represents the (LDP min)2/4 or (.8 x 32 D)2/4 =
164 D.
The ground fault test function is now performed.
In prior art trip units, on non-ground faults in which the
phase current is between three and ten times the breaker
frame rating, the ground fault pick-up is desensitized so
that the fictitious ground fault current (an artifact of
the current transformers) will not cause an improper trip.
In the present trip unit as can be seen in the 10w chart
22 47,128; 48,335; 48,336
of Fig. 11, further corrective action is provided. The
ground fault pick-up is desensitized, as in the prior art,
when PPC is greater than or equal to 7.0 PU; however, for
PPC between 1.0 and 7.0 PU, the fictitious ground current
is accounted for by subtracting from the ground current
sensed, the PPC divided by 4. This method could, of
course, be accomplished by other ~eans, such as analog
circuitry.
If the present ground current is greater than
the ground current pick-up setting, the ground interlock
output is set, to signal other breakers that this breaker
is monitoring of a ground fault. Next a tally similar to
the short delay tally is incremented. If this tally is
now greater than the ground fault tally trip value, a trip
operation is performed. Otherwise, the program enters the
self-checking routine.
If the present ground current is less than the
ground current pick-up setting, but grea~er than 1/2 the
setting, ~he ground interlock outpu~ is se~. In addition,
for all values of ground current less than the setting,
the tally is decremented (not reset as in Short Delay~ and
the self-checking routine is entered.
Refer to the self-checking routine in Figure 12.
This routine, performed every cycle, resets the peak-
detecting capacitors 90 and 91 and checks the runningtally of ground fault and short delay functions, alerting
the user to a malfunction of the main loop. This is done
by setting flags which are checked every 2.1 seconds in
the main loop, and storing an error code. If the flag is
set, the main loop causes an error code number to appear
on the numeric display 80. Thus, instead of a four-second
display of parameter values, there would be alternate 2.1
second displays of error codes and parameter values.
As stated previously, the READ routine shown in
Fig. 13 allows the user to reprogram the external PROM
chip with new set point limit values up to 16 times
via a PROM programmer. It also loads minimum settings
for the breaker, if the PROM was not correctly programmed
or the PROM is missing.
23 47,128; 48,335; 48,336
As an example, the settings may be encoded in
PROM 82 as fo~lows:
E~LE
(~ 32) LDPU of .8 PU = .8 x 3~ = 26 D = IA H
(x 8.5~ LDT of 2 sec = 2 x 8.5 - 17 D = 1} H
(x 16) SDPU of l.s PU = l.S x 16 = ~4 D = 18 H
(x 1) SDT of 20 cycles = 20 x 1 = 20 D = 14 H
(x 64) GFP of .2 PU = .2 x 64 = 12.8 D = OD H
(x 1) GFT of 20 cycles = 20 x 1 = 20 D = 14 H
(x 16) ITC of 8.0 P~ = 8 x 16 = 128 D = 80 H
In this format the settings are ready to be used
by the program. However, in order to be displayed ~every
4 seconds) they must be each converted to recognizable
decimal characters.
Thus, every display routine calls a routine to
convert the integer and fraction portions of the display
value from hex format to B-~.' The BCD values are then
converted to 7 segment format by the latch decoders.
