While switched mode power technology represents an increasing proportion of power conversion devices, large scale power transmission continues to rely almost exclusively upon Power transformers and Distribution transformers.
They operate on the principle of electromagnetic induction and while the specific demands of these two transformer types differ for reasons we need not discuss here, they both present a challenge for measurement instrumentation with which they need to be tested.
Efficiency and Loss measurement
Given the size of power distribution transformers, the importance of minimising losses becomes particularly important. Only a small percentage loss represents a large absolute power value, and this represents a greater problem than just the loss of efficiency in power distribution. An additional challenge is to maximise the reliability and associated lifespan of power transformers in a network that is expected to provide power without interruption. So, given the fact that the reliability of electrical components is directly related to operating temperature, it follows that transformer designs must pay close attention any sources of heat through losses.
Two particular measurement challenges are phase accuracy, where almost perfect inductance means that a small phase error will result in large power error, and harmonic distortion, where non-linear magnetic materials result in non-sinusoidal waveforms that may fail to meet distortion criteria and will increase core losses.
Power Factor and Cos Phi
In power electronics, Power Factor is defined as the ratio between ‘real’ or ‘useful’ power measured in Watts and ‘apparent’ power, VA, the product of RMS Volts and RMS current. In a perfect sinusoidal AC system with a resistive load, the Watts would equal the VA, so the Power Factor would be ‘1’. However, in a real system, phase shift, harmonic distortion or both, will reduce the power factor toward ‘0’.
The term Cos Phi, an abbreviation for cosine of the phase angle between voltage and current, is often used interchangeably with Power Factor, since it results in the same number in an AC power system with no harmonic distortion.
In reality however, it is unlikely that a power transformer will have equal Power Factor and Cos Phi, because power transformers usually use a laminated steel core with an associated non-linear B-H curve, that results in a non-sinusoidal current. It is for this reason that measurement protocol in large power transformer manufacturing companies continue to apply conversion factors and special terms like ‘K’ factor, that account for the limitation of conventional measurement instruments with respect to harmonically distorted waveforms.
SFRA (Sweep Frequency Response Analysis)
While the general principle of power transformers is relatively easy to understand, the design and mechanical structure is complex.
Core material, winding assembly and positioning, interconnection quality, insulation properties and thickness, cooling oil purity and many more elements of a transformer assembly will influence its functionality when first built, then when shipped to its destination and over time, when the environment, maintenance and material ageing will change the transformer behaviour.
Sweep Frequency Response Analysis is a measurement technique that characterises the behaviour of a specific transformer resulting from its particular and unique mechanical characteristics, to produce what is commonly called a ‘fingerprint plot’, named as such because it is unique to the transformer to which it applies. These plots are used by the manufacturer to validate correct assembly of the transformer, commissioning engineers to confirm that damage has not occurred during transit and service engineers, for ongoing predictive maintenance.
Power Line Communications Analysis
Communication between substations in a power network use analogue or digital power line communication. Selective level meter instruments support test and maintenance of analogue communication systems with a range of measurements defined in section 10 of this application tab.