Theory Of Operation - GE 735 Instruction Manual

1 INTRODUCTION 1.1 OVERVIEW

1.1.3 THEORY OF OPERATION

1
A block diagram of the 735/737 hardware is shown on the following page. A 16-bit single chip microcomputer handles data
acquisition, input/output and control. Program memory, data RAM, 10 bit A/D and UART are internal to the microcomputer.
Phase and ground current are monitored via external CTs which are connected to internal interposing CTs for isolation and
signal conditioning. Low pass filters, level shifters and gain amplifiers transform the input signal to a level suitable for con-
version by the 10 bit A/D. A/D values are converted, using software, to the true RMS value of the input sinewave. Separate
× 1 and × 10 gain amplifiers are continuously sampled by the A/D convertor with program logic dynamically choosing the
appropriate range.
Eight rotary switches and 2 banks of DIP switches are periodically read and decoded to determine settings. Using the
appropriate curve settings, the microcomputer computes instantaneous and time overcurrent values closing the trip relay
when a trip value is reached. This relay will remain latched until all phase and ground currents have dropped to zero. True
RMS current is calculated and bar graph segments are driven under program control to indicate the value. All output relays
are driven in response to computed conditions. These drivers are opto-isolated and a separate relay supply is used to pre-
vent noise coupling for external sources to the microcomputer.
To prevent possible lockup of the relay in case of abnormal transient conditions, a separate hardware timer is continuously
reset by the microcomputer under normal conditions. In the event of the program hanging up, this external watchdog will
time out and issue a system reset.
An internal UART buffered by an isolated RS485 driver controls the serial communications. Baud rate is selectable through
an internal timer. Like all other inputs/outputs transient protection is applied to ensure reliable operation under real condi-
tions.
A flyback switching power supply generates multiple isolated supply voltages of +12 I/O, +5 digital, +12 analog and +5
RS485. Two different versions are available to cover the range 20 to 60 V DC or 90 to 300 V DC. Front end rectification and
filtering enable these supplies to also be used with 50/60Hz control power sources.
Structured firmware design running under a real time operating kernel ensures robust program operation under different
conditions. It also contributes to bug free code maintenance.
GE Power Management 735/737 Feeder Protection Relay 1-
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