The field-testing and calibration of solid-state trip units can be performed by either primary current injection method or secondary current injection method. Photo: YouTube.
The Electronic Trip Unit is a microprocessor controlled, multifunction overcurrent protective device for application with low voltage power circuit breakers. The adjustment flexibility provided by the Electronic Trip Units allows users to easily accommodate load changes and other protective requirements while still assuring optimum coordination.
In addition to the adjustable protection functions, the trip unit is designed to use field interchangeable rating plugs. These rating plugs allow the ampere rating of the circuit breaker to be changed to meet specific applications.
Excellent maintenance is essential for reliability and safety of power circuit breakers. Maintenance programs must be tuned to the specific application, well planned and carried out consistent with both industry experience and manufacturer's recommendations.
Photo: epcc / shutterstock.
All transformers have an inherent sound level that varies with the size and style of the core/coil assembly. Depending upon the transformer operating conditions, sound level measurements at the installation site can be drastically different than those taken at the factory.
While transformer cores are generally recognized as the main source of transformer sound, electromagnetic forces in the windings known as load noise can also be a significant influence in low sound level transformers among other electrical and mechanical factors.
Therefore, the sound producing mechanisms of transformers can be characterized as follows:
Switchgear maintenance is essential for continued reliable operation. Photo: Twins Chip Electrical Industry
Substations and switchgear in an electrical system perform the functions of voltage transformation, system protection, power factor correction metering, and circuit switching. Electrical power aparatus, such as transformers, regulators, air switches, circuit breakers, capacitors, and lightning arresters comprise the components necessary to perform these functions.
This guide provides a general overview for the inspection, testing and maintenance techniques used on switchgear and switchboard assemblies, and their associated components.
Bushings in a high-voltage switchyard. Photo: Wikimedia
The primary function of a bushing is to provide an insulated entrance for an energized conductor into a high-voltage apparatus tank or chamber. A bushing may also serve as a support for other energized parts of the apparatus.
About 90% of all preventable bushing failures are caused by moisture entering through leaky gaskets, cracks, or seals. Moisture causes deterioration of the bushing insulation and can result in explosive failure, causing extensive transformer and other equipment damage, as well as hazards to personnel.
To prevent electrical catastrophes and optimize HV bushing service life, the following preventive maintenance and testing procedures should be performed at regular intervals.
A substation bus scheme is the arrangement of overhead bus bar and associated switching equipment. Photo: ENMAS GB Power Systems
The electrical substation is a junction point where two or more transmission lines terminate. In actuality, most EHV and HV substations can be the point where more than half a dozen of lines terminate. In many large transmission substations, the total numbers of lines terminating exceeds one or two dozen.
A substation bus scheme is the arrangement of overhead bus bar and associated switching equipment (circuit breakers and isolators) in a substation. The operational flexibility and reliability of the substation greatly depends upon the bus scheme.
This guide provides information on the general inspection, operation and maintenance procedures of transfer switches. Photo Credit: Emmerson.
In emergency power systems, transfer switches are used to provide a continuous source of power for lighting and other critical loads (automatically or manually) by transferring from the normal source of power to an emergency source of power in the event that the normal source voltage falls below preset limits.
Its essential that circuit breakers be tested and maintained to ensure proper operation during electrical faults. Photo: Vacuum Interrupter Testing
When a fault occurs on the electrical power system, fast and reliable protection means everything. If a circuit breaker fails to clear the fault at that critical moment, the resulting damage can be disastrous in terms of both personnel injury and equipment damage.
Even though circuit breakers can be very reliable, they tend to gather dirt, moisture, and contaminants while in service. Breakers used in hostile environments can be exposed to various corrosive contaminants, which damage not only the insulation system but also metal components, including the main contacts in air circuit breakers.
For these reasons and more, its essential that circuit breakers be tested and maintained to ensure proper operation during electrical faults. There are three basic electrical tests that should be performed on medium-voltage circuit breakers as part of a preventive maintenance program:
Transformer polarity depends on whether the coils are wound around the core clockwise or counterclockwise, and how the leads are connected. Photo: Youtube
Transformer polarity is important when paralleling transformers for additional capacity or hooking up multiple single-phase transformers to form a three-phase bank.
Polarity marks show the connections where the input and output voltages have the same instantaneous polarity, this is important when hooking up current transformers for relay protection and metering.
Transformer polarity depends on whether the coils are wound around the core clockwise or counterclockwise, and how the leads are connected. Often, polarity marks are shown using symbols, such as a dot or plus-minus marks, on the transformer and nameplate.
The turns ratio test is used as both an acceptance test and a maintenance test, while the polarity test is primarily an acceptance test. Photo: Megger
The transformer turns ratio test is used to determine the number of turns in one winding of a transformer in relation to the number of turns in the other windings of the same phase of the transformer.
The transformer polarity test determines the vectoral relationships of the various transformer windings. The transformer ratio test is used as both an acceptance test and a maintenance test, while the polarity test is primarily an acceptance test.
Testing power factor is a means of measuring the integrity of the insulation. Photo: Doble
Most failures of electrical power equipment are the result of a breakdown in the insulation system. To keep failures at a minimum, periodic maintenance testing of the insulation is recommended to indicate whether it is deteriorating, and how fast.
Power factor tests are used to measure dielectric losses, which relate to the wetness, dryness or deterioration of electrical insulation.