Method for initializing short time and groundfault protection heat index accumulators in a trip unit

A method of initializing short time and groundfault protection heat index accumulators to compensate for current flow during the time interval between the application of phase current and application of current measurement and protection algorithms in an electronic trip unit is presented. The electronic trip unit includes a microcontroller and associated ROM having trip unit application code, e.g., main functionality firmware, including initializing parameters, and boot code, stored therein and EEPROM having operational parameter code, e.g., encoded enumerations for parameters such as 50/60 Hz., UL/ANSI/IEC, etc., stored therein. The time internal comprises first and second time components. The first time component is the time between the instant phase currents are applied and the instant the microcontroller is initialized and the second time component is the time between the instant the microcontroller is initialized and the instant the microcontroller begins execution of short time and groundfault algorithms (i.e., protection algorithms). The first time component is approximated by applying an initial current measurement to index a calibrator. The second time component is measured by a counter controlled by the initialization firmware. The first and second time components are applied to the initial current measurement (or a derivative thereof) to determine an initial value (or a component thereof), for short time and/or groundfault heat index accumulators.

BACKGROUND OF THE INVENTION 
The present invention relates generally to trip units. More specifically, 
the present invention relates to a method of heat index accumulator 
initialization for short time and groundfault protection algorithms in an 
electronic trip unit. 
Electronic trip units are well known. Electronic trip units typically 
comprise voltage and current sensors which provide analog signals 
indicative between the power line signals. The analog signals are 
converted by an A/D (analog/digital) converter to digital signals which 
are processed by a microcontroller. The trip unit further includes RAM 
(random access memory), ROM (read only memory) and EEPROM (electronic 
erasable programmable read only memory) all of which interface with the 
microcontroller. The ROM includes trip unit application code, e.g., main 
functionality firmware, including initializing parameters, and boot code. 
The EEPROM includes operational parameters for the application code. 
These trip units are required to meet certain standards, e.g., UL/ANSI/IEC, 
which specify a short time delay from the instant power is applied to when 
a trip unit must be ready to trip. In this context, "short time" delay 
refers to a specific protection characteristic, i.e. short time vs. long 
time, instantaneous, or ground fault protection. Although the delay is 
literally "short", the context of the statement is in reference to the 
short time protection characteristic curve. The standards also define trip 
time curves specifying under what conditions a trip must occur. 
Short time and groundfault protection heat index accumulators are 
initialized to compensate for current flow during the time interval 
between the application of phase current and application of current 
measurement and protection algorithms. This time interval is assumed to be 
a predetermined amount of time. This assumed time delay is applied to the 
initial current measurement (or a derivative thereof) to determine an 
initial value (or a component thereof, for short time and/or groundfault 
heat index accumulators, as is well known in the art. 
SUMMARY OF THE INVENTION 
The above-discussed and other drawbacks and deficiencies of the prior art 
are overcome or alleviated by the method of initializing short time and 
groundfault protection heat index accumulators to compensate for current 
flow during the time interval between the application of phase current and 
application of current measurement and protection algorithms in an 
electronic trip unit of the present invention. The electronic trip unit 
comprising voltage and current sensors which provide analog signals 
indicative the power signals. The analog signals are converted by an A/D 
(analog/digital) converter to digital signals which are processed by a 
microcontroller. The trip unit further includes RAM (random access 
memory), ROM (read only memory) and EEPROM (electronic erasable 
programmable read only memory) all of which communicate with the 
microcontroller. The ROM includes trip unit application code, e.g., main 
functionality firmware, including initializing parameters, and boot code. 
The EEPROM includes operational parameter code, e.g., encoded enumerations 
for parameters such as 50/60 Hz., UL/ANSI/IEC, etc. 
In general, trip units are required to meet certain standards, e.g., 
UL/ANSI/IEC, which specify a short time delay from the instant power is 
applied to when a trip unit must be ready to trip. These standards also 
define trip time curves specifying under what conditions a trip must 
occur. The trip curves are based on temperature which is proportional to 
current as a function of time. 
An object of the present invention is to initialize short time and 
groundfault protection heat index accumulators to compensate for current 
flow during the time interval between the application of phase current and 
application of current measurement and protection algorithms. In 
accordance with the present invention, this time internal is divided into 
a first time component and a second time component. The first time 
component is the time between the instant phase currents are applied and 
the instant the microcontroller is initialized and the second time 
component is the time between the instant the microcontroller is 
initialized and the instant the microcontroller begins execution of short 
time and groundfault algorithms (i.e., protection algorithms). 
The first time component is approximated by applying an initial current 
measurement to index a calibrator to approximate the first time component 
as a function of the phase current, CT ratio, and 50 Hz. vs. 60 Hz. 
application. The second time component is measured by a counter controlled 
by the initialization firmware. The first and second time components are 
applied to the initial current measurement (or a derivative thereof) to 
determine an initial value (or a component thereof), for short time and/or 
groundfault heat index accumulators, in the same manner the predetermined 
time delay is employed in the prior art. 
The above-discussed and other features and advantages of the present 
invention will be appreciated and understood by those skilled in the art 
from the following detailed description and drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a general schematic of a trip unit is generally shown 
at 10. Trip unit 10 comprises a voltage sensor 12 which provides analog 
signals indicative of voltage measurements on a signal line 14 and a 
current sensor 16 which provides analog signals indicative of a current 
measurements on a signal line 18. The analog signals on line 14 and 18 are 
presented to an A/D (analog/digital) converter 20, which converts these 
analog signals to digital signals. The digital signals are presented over 
a bus 22 to a microcontroller (microcomputer) 24. Trip unit 10 further 
includes RAM (random access memory) 26, ROM (read only memory) 28 and 
EEPROM (electronic erasable programmable read only memory) 30 all of which 
communicate with the microcontroller 24 over a control bus 32. EEPROM 30 
is non-volatile so that system information and programming will not be 
lost during a power interruption or outage. Data, typically the status of 
the circuit breaker, is displayed by a display 34 in response to display 
signals received from microcontroller 24 over control bus 32. An output 
control 36, in response to control signals received from microcontroller 
24 over control bus 32, controls a trip module 38 via a line 40. 
Calibration, testing, programming and other features are accomplished 
through a communications I/O port 43, which communicates with 
microcontroller 24 over control bus 32. A power supply 42 which is powered 
by the service electricity, provides appropriate power over a line 44 to 
the components of trip unit 10. ROM 28 includes trip unit application 
code, e.g., main functionality firmware, including initializing 
parameters, and boot code. EEPROM 30 includes operational parameter code, 
e.g., encoded enumerations for parameters such as 50/60 Hz., UL/ANSI/IEC, 
ect. It will be appreciated that these parameters or options are stored in 
the trip unit at the factory and are selected to meet customers' 
requirements. 
Trip units are required to meet certain standards, e.g., UL/ANSI/IEC, which 
specify a short time delay (designated "c" in FIG. 2) from the instant 
power is applied to when a trip unit must be ready to trip. This short 
time delay differs for each of the standards. These standards also define 
trip curves specifying under what conditions a trip must occur. The trip 
curves are based on temperature which is proportional to current as a 
function of time. 
Referring to FIG. 2, in general, an object of the present invention is to 
initialize short time and groundfault protection heat index accumulators 
to compensate for current flow during the time interval between the 
application of phase current and application of current measurement and 
protection algorithms. In accordance with the present invention, this time 
interval is divided into a first time component "a" and a second time 
component "b". The first time component is the time between the instant 
phase currents are applied and the instant microcontroller 24 is 
initialized, i.e., comes out of reset and begins executing instructions, 
and the second time component is the time between the instant 
microcontroller 24 is initialized and the instant microcontroller 24 
begins execution of short time and groundfault algorithms (i.e., 
protection algorithms). 
The first time component is primarily dependent upon hardware design 
architecture and components. The first time component is approximated by 
applying an initial current measurement to index a calibrator, i.e. an 
equation or a digitized form of an equation such as a calibration table. 
The index calibrator provides an approximation of the first time component 
as a function of the phase current, CT ratio and line frequency (i.e., 50 
Hz. vs 60 Hz.) The initial current measurement is requested by the 
initialization firmware. Once initialization functions necessary to 
facilitate sampling (measurements), and consequently the application of 
protection algorithms, has been performed, the initialization firmware 
enables interrupts to initiate data sampling (measurement) and protection 
algorithm scheduling. An initial relative current value is determined from 
a peak sample. A rating plug ratio is applied to determine an absolute 
initial current value. A calibration table offset is determined by 
comparing the absolute current value to various reference values selected 
as a function of application, i.e., line frequency. An appropriate 
calibration table base address is selected as a function of line frequency 
(e.g., 60 Hz. or 50 Hz.) and current sensor rating (e.g., 100 mA, 200 mA, 
400 mA, etc.). The offset is added to the base address to develop an index 
into one of the tables which provide delay factors, in units of 
half-cycles, as a function of current. Although only two tables are 
described herein for defining delay as a function of current, i.e., one 
for 50 Hz. applications and another for 60 Hz. applications, the present 
invention is not so limited. An array of tables is preferred over the 
mathematical manipulation (which would be necessary in developing a 
normalized table indexing factor) to reduce a real-time processing 
overhead associated with this function. The delay factor is stored for 
application during initial execution of short time and groundfault 
protection algorithms in developing initial values for the short time and 
groundfault heat index accumulators, respectively. When the short time and 
groundfault protection algorithms are executed for the first time, the 
short time heat index and groundfault heat index accumulators are 
initiated. 
The second time component is primarily dependent upon software design 
architecture and coding methodologies. The second time component is 
measured by a counter (which is internal to microcontroller 30 in the 
present example) controlled by the initialization firmware. 
The first and second time components are applied to the initial current 
measurement (or a derivative thereof) to determine an initial value (or a 
component thereof), for short time and/or groundfault heat index 
accumulators. A derivative of the initial current measurement is applied 
to a composite of the time components to define initial values for each of 
the accumulators (i.e., short time and groundfault heat index 
accumulators). By way of example, derivatives of the initial current 
measurement include, (1) applying a peak current sample from a first 
half-cycle of the phase current to develop an RMS equivalent of the phase 
current and (2) applying a vector sum of multiple phase currents sampled 
during a first half-cycle to develop an RMS equivalent of the phase 
currents. The derivatives from the initial current samples for the short 
time and groundfault heat index accumulators are applied to form initial 
values for their respective heat index accumulators, in the same manner 
the predetermined time delay is employed in the prior art (such being well 
known). 
While preferred embodiments have been shown and described, various 
modifications and substitutions may be made thereto without departing from 
the spirit and scope of the invention. Accordingly, it is to be understood 
that the present invention has been described by way of illustrations and 
not limitation.