A Short, but Representative, ‘Perspective Piece’ on Industry Uptake of ‘Near 50 Hz’ PD Testing Internationally
When assessing what might at first appear to be a “new technology” in the power industry, it is reasonable of any client to seek a degree of evidence of its established provenance offshore and in the application to which it is intended to be applied in the ‘local’ context.
When that ‘new technology’ is in fact well established offshore but new to the local market, one must consider the question as to what would constitute a suitable level of information to inform, address, and allay in reasonable fashion any uncertainties or questions as to the suitability of the technology for the intended application and also the quality of its said provenance. Clearly, no technology will ever hold 100% of the market or technical opinion but it would be a fair observation that evidence of widespread uptake over a longer term, coupled with a selection of technical papers that all concur on the technical quality and advantages of the technology over alternative or conventionally-deployed solutions, should be deemed to be a tipping point to allow that technology’s confident and optimistic deployment locally.
The purpose of this letter to convey to you and your colleagues the background, deployment level, international uptake, and technical evidence enough to allow your company to embrace with confidence the ‘near 50Hz’ partial discharge cable diagnostic testing technology applied to both new and in-service condition assessment of your 11-33 kV cable population.
…AVO NZ proudly delivers this long-sought capability
HV Cable Assets constitute in many EDB’s and private networks one of the largest asset groups by replacement value. In some of the larger EDB’s this may represent as much as $1.5 to $2 billion. Alarmingly, and at odds with almost all other asset classes managed by such networks, almost nothing is known about the asset condition of each individual cable. Effective HV cable asset management is seriously fraught by this situation. With the parallel scenario of above-ground infrastructure management challenges now painfully troubling the Industry, the risks posed by not knowing the condition of HV cable assets is now a very pressing problem, given the age profile of such assets and the uncertain dynamic of asset aging modelling.
We all know the single most effective way to prevent power failures and shortened equipment lifespan is by keeping moisture to a minimum, along with heat and oxygen. It's fair to say that most equipment failure and subsequent losses (time, resources, etc) could be mitigated, through routine insulation testing.
When it comes to improving testing standards, the process is often long and expensive. Because of this, many industry players focus on improving test and measure equipment, which means we have very advanced tools that can often be let down by outdated regulations.
It also means when we do see updates to international standards, it's a very good thing and often welcome. One example is the adoption of DFR (Dielectric Frequency Testing) over traditional PF (Power Factor) and DF (Dissipation Factor), over 25 years after it was first introduced.
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.