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MeggerBook Cable & Report Edition

In the UK, we have two common power systems supplying locomotives. A 25 kV overhead line system and a 750 V third rail system. Both systems use the track as a return path for current flow (see figures 1 and 2).

The Narrow Band Dielectric Frequen-cy Response (NBDFR) test method consists of a series of power factor measurements ranging from 1 to 1000 hertz. This aggregate of the CHL (pri-mary to secondary) measurements constitutes the dielectric response of the test specimen. The subsequent evaluation consists of a geometric analysis of a plot where the mea-sured power factors and correspond-ing frequencies are graphed. As an insulation system ages, the response migrates towards the high frequency end of the plot. When the frequency corresponding to the lowest magni-tude within the response (the trough) is used as a reference point, the move-ment of the response may be quanti-fied; enabling the transformer may be classified in terms of condition.

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The original design of electrical test instrumentation was often focused on taking a measurement and little else. A lot of the operation and interpretation was left to the operator. This was fine for people who liked the challenge or had experience. But it could be tough on apprentices, new hires, and those who were simply handed the instrument with no training.Constant improvement in instrumentation has addressed these problems by adding to the capabilities of the instrument. Some are for convenience, some safety and some the advancement in technology over the years that helps standardize testing and provide more accurate results. A prime example is that of insulation testers. Let’s examine some of the key improvements in features and functions, and what they mean to the operator.

When considering earth (ground) testing, people will often ask about what type of test leads should be used. This includes questions about the type of wire used, the resistance of the leads and the length of the leads, as well as concerns over safety. Fortunately, most of the answers to these questions are addressed by the design of the earth tester itself.

One of the simplest and most common electrical tests performed by electricians and test technicians today is the continuity test. This test option is featured on most multi-meters and insulation testers. The continuity test is simple to perform – connect two leads across opposite ends of the item under test (IUT), press the test button, and the instrument quickly provides a resistance reading. A low reading is considered good and a high reading bad. In fact, this test is so simple that it is often used for applications that actually require a more demanding performance from the test instrument. In this note we’ll look at some of these applications and the resulting requirements that highlight the differences between a low resistance ohmmeter (DLRO) and a digital multi-meter (DMM) – and the importance of using a DLRO to find issues that a DMM cannot.

Winding resistance measurements are conducted for detecting various faults in transformers – including shorted turns, loose connections, broken strands, and malfunctioning tap changer mechanisms. Winding resistance test sets such as the MTO106 are also capable of testing electric rotating machinery – motors and generators. Winding resistance measurements detect problems in a motor that other tests may not find. These problems include partial or fully shorted coils, poor crimps/connections, imbalance between phases (improper turns on a phase), and incorrect coil (phasing) connections. 

Power swing which is principally caused by an oscillation in active and reactive power of transmission line, consequent to an enormous disruption in power system, which if not blocked, could cause wrong operation to the distance relay which may lead to tripping the healthy part of the transmission line. In the absence of power swing function it may result in severe damage to the machines or cascading tripping in the grid resulting in blackouts. To prevent such scenarios, intelligent electronic devices (IEDs) have power swing block (PSB) detection and trip logics incorporated with distance schemes. 

Transformers are critical assets of power system network and their adequate protection is a matter of vital importance. Modern protection relays are usually defined as intelligent electronic device (IED) s, and they provide huge flexibility and enhanced protection features to assure correct operation of the equipment. Transformer protection IEDs are equipped with key features such as differential and restricted earth fault (REF) protection along with several other functionalities as per the guidelines of IEEE C37.91.