Design of digital synchronism checking relay for interconnected power grids
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Date
2012-10-09
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UMT Lahore
Abstract
In this project we have designed a circuit to connect to parts of an interconnected system energized by two different sources. By interconnecting separate utilities with the high voltage transmission system, it is possible to pool both generation and demand, not only providing a number of economic and other benefits, including a more efficient bulk transfer of power from generation to demand centers. The interconnected transmission system, by linking together all participants across the transmission system, makes it possible to select the cheapest generation available. Transmission circuits tend to be far more reliable than individual generating units, and enhanced security of supply is achieved because the transmission system is better able to exploit the diversity between individual generation sources and demand. An interconnected transmission system enables surplus generation capacity in one area to be used to cover shortfalls elsewhere on the system, resulting in lower requirements for additional installed generation capacity, to provide sufficient generation security for the whole system. Without transmission interconnection, each separate system would need to carry its own frequency response to meet demand variations, but with interconnection the net response requirement only needs to match the highest of the individual system requirements to cover for the largest potential loss of power (generation) rather than the sum of them all.
The main idea is to calculate voltage, frequency, and phase angle of both sides using digital sampling and compare these values. If they lie in the tolerable range, then permission to close command is issued for the respective circuit breaker.
A high-performance PIC18F452 RISC CPU is used for processing the inputs. Sinusoidal signals from both ends of the system are converted to digital form, and after half-wave rectification these samples are used for calculation of voltage. To measure frequency, the input sine wave is converted to a square wave and then positive edge-triggered external interrupts are used to count the number of cycles in one second.
Measuring phase angle was quite an interesting task and required a lot of effort. The time difference between the starting of two waves is used to compute the phase angle.
Monitoring the system using a synchro-check relay before synchronization of supplies for the confidence of the utility can save a lot of trouble caused by out-of-step trippings.