A transformer turns ratio test provides a quick verification of the most fundamental operational characteristic of a transformer – its ability to transform voltage as anticipated. In doing so, the test provides invaluable reassurance to the operator. Open- and short-circuit conditions in transformer main and tap windings may cause the transformer turns ratio to change and therefore this test is at once providing useful diagnostic information.

Engineers have noted, however, that traditional "hand-crank" TTR instruments sometimes give values for turns ratio that are different from those given by modern automated TTR instruments. Indeed, many insist that the traditional instruments provide results that are more dependable. But is this actually true? And why do the instruments produce different results?

Mexico already has over 3 GW of installed wind power capacity and plans are in place to increase this to more than 9.5 GW by the end of 2018. This ambitious target can only be achieved if the installation and commissioning of a new plant proceeds smoothly and the plant proves reliable in operation.

Most engineers amass a collection of invaluable reference materials throughout their careers. Most have also dealt with the angst resulting when a valued reference piece goes M.I.A., which fuels subsequent reluctance to loosen a tight grip on these libraries. Since in most cases, personal financial investment has grown these libraries, which typically include university textbooks and the like, often when these valued employees move on, these treasure troves follow.

This highlights an interesting aspect to succession planning; that is, the advantages of establishing and maintaining a protected corporate library, accessible to all. A corporate library serves important roles, including: (1) spreading valued knowledge much further within the organization and (2) aiding with succession planning as an additional way to pass knowledge.

Performing high voltage insulation diagnostic tests (power factor, tan delta) on large capacitive specimens like generators, motors, capacitive voltage transformers and long lengths of cable always presents a challenge. Typically, these tests are performed with the aid of a resonating inductor to neutralise the load presented to the test set by the capacitance of the specimen. Unfortunately, these inductors are expensive, heavy and complicated to operate, which means that an alternative approach is desirable.

Embedded or distributed generation schemes,  where a local generator is connected directly to the distribution network, are becoming widespread. The operators of these schemes face an important challenge: how to ensure that the system behaves safely and predictably if the local generator becomes isolated from the network – a condition known as islanding.

There are numerous potential hazards associated with islanding, not the least of which is that engineers working to restore the network connection may not realise the system is still powered. Another hazard is that the generator may continue to supply local loads but, without support from the network, this may result in it being heavily overloaded.

The solution usually adopted to address these hazards is to immediately shut down the islanded generator, but this can only be done if a fast and dependable way of detecting islanding is available. Many approaches are possible, but one that is widely used is rate of change of frequency (ROCOF) protection, which has established a reputation for responding faster and more reliably than alternative protection techniques.

In This Issue

• Protective System Maintenance Program
• Transmission Line Loadability
• Generator Loadability
• Relay Performance during Stable Power Swings
• Coordination for Performance during Fault

Traditionally, testing protection systems in power distribution networks has been a time-consuming process involving many separate steps. Now however, an alternative approach is available which allows all key aspects of the protection system to be tested simultaneously, leading to big time savings. Stefan Larsson of Megger explains.

New Three Phase SEVERKER 900, the highly regarded SVERKER is now for the first time in a three phase standalone unit.

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