An encoder is a sensor that converts rotational or linear displacement into digital signals. As shown in Figure 1, a typical optical encoder consists of: (1) light source (LED), (2) code disk, (3) stationary mask, (4) light detector (photodetector), and (5) signal conditioning circuit. The disk sits between the light source and the photodetector. As […]
The relay operation is a function of the input quantities, such as current, voltage, impedance, and/or phase angle. The relay can be made to respond to either a single quantity or a combination of two or all input quantities. When the relay is operated by a single quantity, its response is strictly a function of
Instrument transformers are essential parts of many electrical metering and relaying systems. The quality of instrument transformers will affect directly the overall accuracy and performance of these systems. Instrument transformer performance is critical in protective relaying since the relays can only be as good as the instrument transformers. They serve two basic functions: To change
The main tests that are conducted on instrument transformers are as follows: Polarity Test of Instrument Transformers Instrument transformers are marked to indicate the instantaneous direction of primary and secondary currents. Usually, one primary and one secondary terminal are marked with a cross (×) or a dot (•) or a square (■) to indicate the
A perfect cable can be electrically modeled by a single capacitor. The longer the cable, the larger the capacitance of this capacitor. As the insulation of the cable degrades, losses in this capacitor increase which can be measured by tan delta testing of the cable. The measurements can be compared with previous test measurements and
Cable testing is conducted to chart the gradual deterioration over the years, to do acceptance testing after installation, for verification of splices and joints, and for special repair testing. It is always appropriate to conduct the cable insulation resistance measurement test first, and if the data obtained looks good, then proceed with the DC overpotential
The insulation resistance test of transformer complements other transformer tests. The solid winding insulation tests are not conclusive in themselves but provide valuable information on winding conditions, such as moisture content, and carbonization. These tests are considered non-destructive even though at times they may cause a winding failure. It should be pointed out that a
The hot collar test on bushings may be performed on compound-type, porcelain dry-type bushing, oil-filled bushings, and cable potheads. The collar is energized by the test voltage (thus the term hot collar), while the center conductor is grounded. It is well-established fact that the compound and dry type bushing fail from leaks that develop in
Power transformers are critical and costly assets in the electric power system. They are one of the most important electric apparatuses for providing reliable energy flow. They also constitute one of the largest investments in a utility system or in an industrial complex. For this reason, transformer condition assessment and management are a high priority.
An oil-filled transformer insulation system consists of insulating oil and cellulose (paper) materials. Under normal use, transformer insulation deteriorates and generates certain combustible and noncombustible gases. This effect becomes more pronounced when the transformer insulation is exposed to higher temperatures. When cellulose insulation (i.e., winding insulation) is overheated to temperatures as low as 140°C, carbon