Q; What is the first trip test how is it used to evaluate my circuit breaker?
A: The first trip test uses small clamp-on current transformers that connect to the coil circuit and the load, or protection transformers on the circuit breaker while the breaker is still in service.The breaker is then tripped and the coil current is measured along with the voltage drop. The current extinction times of the three phases are also measured. The coil current trace and other parameters can be compared to previous measurements to see if the breaker is operating normally. This test ensures that no operation is left unmeasured and actually gives a picture of “real life” conditions and how the circuit breaker operates after remaining closed for months or even years.
Q: Since first trip evaluates “real life” conditions, do I still need to perform traditional offline timing on my circuit breaker?
A: Because first trip is relatively easy and quick, some people have tried to replace traditional timing tests with first trip testing. However, it is important to keep in mind that first trip testing complements but does not replace offline time and travel analysis. With first trip you are comparing previous measurements and trending, but time and travel analysis allows you to not only compare and trend results, but to actually verify that the circuit breaker is operating within manufacturer and IEEE/IEC specifications.
Q: You mention time and travel analysis, but connecting the transducer is often difficult. With this in mind, are travel measurements really necessary? Why can’t I just perform timing?
A: Timing ensures that the three phases are synchronized and that the contacts are opening at the correct time, but travel measurements provide a lot more information on how the contacts are actually performing. Travel verifies the stroke of the circuit breaker, as well as the velocity of the contacts. The circuit breaker times can be out of specification, but as long as the velocity of the interrupter is correct it will still be able to clear the fault. Additionally, travel will reveal mechanical issues such as overtravel and over damping. In order to simplify transducer connections, Megger provides a variety of transducers and connection adapters that fit multiple circuit breakers.
Klaus Spitzenberg - Megger Support, Germany
Condition analysis of gas-insulated medium voltage circuit breaker systems is now practical and cost effective.
Working with energy distribution utilities Syna and Westnetz, Megger has developed techniques that make it possible to determine the condition of enclosed medium-voltage circuit breakers safely and cost-effectively. These techniques, which also allow first-trip testing to be performed, are based on connecting a Megger TM1800 or TM1700 circuit breaker analyser to the Voltage Detection System (VDS) via an adaptor. This opens up a whole range of new possibilities for system operators, as it means that it is no longer necessary to spend time isolating the circuit breakers for tests.
A definite need for testing
As gas-insulated medium-voltage circuit breaker systems are housed in enclosures, it is often impossible to use established condition analysis methods – the process would simply be too laborious, time consuming and uneconomical. However, there is a definite need to test these systems, as even components that are nominally maintenance-free need testing.Indeed, German law, and the law of most other countries, dictates that all work equipment must be tested regularly, using tests based on an assessment of the hazards that might arise from the equipment. And gas-insulated medium voltage circuit breakers most certainly fall into the category of work equipment. The new testing techniques make it easier for switchgear users to meet their legal obligations, as well as providing them with invaluable information to aid effective maintenance planning.
Isolation no longer needed
With the new techniques, it is no longer necessary to isolate the circuit breakers when evaluating their condition. The test instrument is connected to the capacitive Voltage Detection System via an adapter. It’s also connected to a trigger box and, via current clamps, to the medium-voltage circuit breaker system. A TM1700 circuit breaker analyser or a TM1800 analyser , shown in Figures 1 and 2, may be used. All key parameters can be measured and recorded without the time-consuming isolation and grounding procedures that are needed with other testing methods. In addition, no connections are needed to secondary circuits.
Tony Wills - Application engineer
Sweep frequency response analysis (SFRA) testing on transformers provides invaluable information about the mechanical integrity of the transformer’s components – information that can’t be obtained in any other way without dismantling the transformer or at the very least, by performing a thorough internal inspection. Mechanical movement or deformations within the transformer produce changes in the transformer’s inductance and capacitance distribution. It is these changes that SFRA testing measures.
Some engineers and technicians however, find SFRA test results daunting to analyse, particularly if they have not had much experience with using this technique. Having comparative results, ideally from tests performed when the transformer was new or in known good condition, is a big help, because SFRA test results should not change throughout the life of a transformer. But when changes in the test results are seen, what do they signify? International standards organisations have developed guides and brochures to help answer this question but for those who prefer a more practical hands-on approach, Megger’s FDB101 demonstration tool is a very attractive alternative.
Part 1 of this article, which appeared in the September's 2017 edition of Electrical Tester online and is available by clicking here, looked at the basics of time and travel measurements for circuit breakers. This second and final part presents a practical case study, and also discusses how to proceed when the circuit breaker manufacturer provides little or no information to aid time and travel measurement.
Robert Foster - Application engineer
Case study: Siemens SPS2-38-40-2 Circuit Breaker
Centrix City and Centrix City Compact vans are equipped with the latest generation of non-destructive cable diagnostic systems. These include an SPG 40 test set, which is a multi-functional system for testing, prelocation, pinpointing and burning of cable faults in low and medium voltage networks.
In the City van, this is partnered with an Teleflex T 30-E reflectometer, and all functions are easily managed from fully integrated control unit that provides testing, diagnostics and fault location on one screen. In the City Compact van, the SPG 40 test set is used in conjunction with a detachable Teleflex SX reflectometer to provide testing and fault location functionality. Diagnostic functions are provided via a laptop computer.
The systems in both vehicles have been designed to be easy to work with and they follow the Megger easyGO operating philosophy that enables even inexperienced users to efficiently carry out cable checks, fault location and diagnostics. Comprehensive safety features are also incorporated, with all safety-relevant parameters automatically monitored in line with current codes and standards.
Niclas Wetterstrand - Program manager Megger Sweden
Standby battery installations provide electricity to key elements of power generation, transmission and distribution systems, such as circuit breakers and protective relays, computers, control panels and telecommunication equipment, when other power sources have failed. The batteries can provide power instantly, either directly as DC or via an inverter as AC, thereby ensuring that critical systems continue to operate until emergency generators are ready to take over, or the main electricity supply is restored.
Dennis Neitzel - Director emeritus - AVO training institute
A great deal of attention is devoted to safe working practices relating to electrical construction, maintenance and repair work. Industry electrical publications regularly report on safety issues, including the use of the proper tools and equipment for energised and de-energised work, as well as using the correct personal protective equipment (PPE) for each workplace situation. However, electrical test instruments are given very little, if any, discussion in safety articles. Even the dangers of using the wrong test instrument or using an instrument improperly, which can have catastrophic results, are rarely mentioned.
Some of the most frequently used test instruments include non-contact voltage testers, multimeters, insulation testers and ground-resistance testers. A big issue with using non-contact or proximity devices, for example, is that to prove a circuit is de-energised it is necessary for that circuit to be tested phase-to-phase and phase-to-ground, which cannot be done using this type of tester.
When electrical safety is discussed, the subjects of shock, arc flash, and arc blast predominate in the discussions. The question is often asked: How do I identify when these hazards are present, or likely to be present, when I am using electrical test instruments on electrical circuits and equipment? This article discusses electrical hazards, along with requirements for assessing the workplace to identify electrical hazards, and also discusses personal protective equipment (PPE) associated with using test instruments.
Jill Duplessis - Global technical marketing manager
Winding resistance testing is a very revealing electrical diagnostic test for the routine screening of power transformers to determine their “state of health”. In fact, it’s an essential test when dissolved gas analysis indicates overheating in the oil or of the paper, and no condition assessment is complete without it.
Moreover, if you were to ask experienced substation maintenance engineers for their thoughts about which electrical tests should be done on a routine basis (which is not necessarily the same, of course, as the tests they are actually doing!), the winding resistance test would be likely to take second place only to the transformer turns ratio (TTR) test.
The TTR test typically takes top honours, not necessarily because of its diagnostic capabilities, but because it provides validation and reassurance that the transformer is actually doing what it’s supposed to do - transforming voltage. Nevertheless, the DC winding resistance test would be a very strong contender for the top spot if the list of important transformer tests were ever re-ordered purely on the basis of diagnostic value and screening prowess.