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Accredited high frequency calibration – the only way to truly validate real-word power measurements

Feature

By Kelvin Hagebeuk, Marketing Manager – Test and Measurement, Yokogawa

The use of electricity was very different around the beginning of the last century than today. Carbon incandescent lamps had just begun to replace gas lighting, and the first main-line locomotive powered by single-phase alternating current went into operation in Austria in 1904. Coal consumption had significantly increased and accounted for half the global energy. In the face of this rapid increase in the supply of electricity, understandably no thought was given to its sustainable use and possible impact on the environment.

The first standard for electricity metering, the ANSI C12 code, was developed in 1910. At that time, electrical controlling systems were essentially linear, and little attention was given to the need to accurately measure consumption at frequencies other than the supply frequency, which generally ranged from 25 to 60Hz (with some up to 140Hz).

Power measuring solutions

Nowadays, devices such as switch-mode power supplies, electronic lighting ballasts, soft starters in motor controls and frequency converters in traction applications consume power at much higher frequencies, and even though supply frequencies worldwide are now standardised at 50 and 60Hz, calibration of high-frequency power has lagged behind the development of power meters to address these applications. Few national laboratories can provide traceability up to 100kHz – the frequency at which instruments have to be calibrated to provide accurate results in these application sectors.

With standards such as IEC62301 Ed2.0 and EN50564:2011 covering standby power consumption, and the SPEC guidelines, there are many measuring solutions being offered to measure power at higher frequencies. However, how can it be shown that a power measuring instrument truly does meet the manufacturers’ specifications? It is no longer sufficient to ignore real-world conditions by only calibrating an instrument using pure sine waves at 50/60Hz.

Before this question can be answered, the specifications of the measuring instrument itself need to be examined. Irrespective of the specified accuracy, users need to be assured that the readings today will be the same as the readings tomorrow or in a year’s time, so the stability of the instrument is just as important as accuracy. Any suggestion by a manufacturer that their instrument will regularly require adjustment to maintain its specification should be considered with caution.

It is also necessary to consider what may be missing in the specifications. It is not easy for an engineer to use the specifications to estimate the actual possible error for their specific measurement, particularly when the power accuracy is not specified directly but written as a function of the applied input currents and voltages and their measuring ranges. It will be impossible to estimate if the effects due to the common mode, phase shift, power factor and distorted waveforms are also not explicitly specified.

With regards to the meaning of “calibration”, it can be defined as the comparison of an instrument’s performance with a standard of known accuracy.

Correct measurements

No measurement is ever correct. There is always an unknown, finite, non-zero difference between a measured value and the corresponding “true” value. In other words, a user can never be 100% sure that an instrument is operating within its specified tolerance limits.

Fortunately, there are steps that can be taken to minimise the possibility of a measurement falling outside specified tolerance or uncertainty bands. Regular traceable calibration is a method for gaining quantifiable confidence in a measurement system by comparing the instrument’s performance to a standard of known accuracy. A calibration report can therefore prove that a measuring instrument meets its accuracy specifications or potentially exceed them.

However, all laboratory standards and even national standards have uncertainties of measurement; hence it is difficult to be absolutely confident that an instrument is operating within its stated tolerance limits. Of concern then is not only if the calibration is carried out using standards that are traceable to national and/or international standards, but also if the methods used to perform the calibration are defined and controlled. This is a key difference between a “traceable” calibration and one which is performed in an ISO 17025 accredited laboratory.

Generally, an ISO 9001 quality system requires a traceable calibration, which doesn’t define how the measurements are conducted. On the other hand, an ISO 17025 accredited laboratory has also demonstrated that they are technically competent and able to produce precise and accurate calibration measurements. Thus, an ISO 17025 certificate shows the measurement uncertainty for each measurement; there is never a guarantee that the measurements on a ‘traceable’ ISO9001 certificate are correct.

Calibration accreditation

It is no accident that the power transformer industry requires extremely high levels of accuracy and accreditation for its calibrations. Potentially large sums of money are at stake in penalties if a transformer doesn’t meet its specifications for power losses. The difference in percentage terms between the specified and actual power loss could be small, but when the amount of power being transformed is large, the financial value of the additional ‘lost’ energy could become huge over time.

Once the decision has been made to use an accredited calibration laboratory, their capabilities or ‘scope of calibration’ needs to be considered. This scope is in the public domain, and often can be viewed on-line via the national accrediting body’s website. The scope will include details of not only the types of measurements included in the accreditation but also their ranges, and the Calibration and Measurement Capability (CMC) uncertainties.

The accurate measurement and hence calibration of AC power is much less straightforward than for voltage and current. Although national standards laboratories invariably are able to offer accredited power calibrations at 50/60Hz, relatively few have developed this capability for much higher frequencies.

As applications for precise power measurement can range from home and office appliances, using small currents, to industrial applications such as inverters, electric motors and renewable energy, employing much higher voltages and currents, this capability will also need to be covered in the scope of the laboratory.

Yokogawa is the first manufacturer of power measuring instruments with its own accredited laboratory for calibrating power up to 100kHz.

Long history of solutions

As a company which has been around and involved in electrical measurement since 1915, Yokogawa’s history is almost as long as our use of electricity. Utilising its own ISO 17025 laboratory it can readily demonstrate that its own flagship products are the world’s most accurate and stable power analysers. The recently introduced new flagship model, the WT5000, introduced the concept of modularity to this class of instrument. This means that even if a user needs to change or increase the number of channels they require for their measurements, it may not be necessary to buy a whole new instrument. Additionally, it ensures that the year-on-year accredited calibration history of the instrument will continue to demonstrate the validity of users’ measurements on their own products, due to the inherent accuracy and stability of the WT5000. Hence, users will also be able to show proof to their customers/consumers why their specifications can be trusted.

The path to obtain accreditation for the calibration laboratory took 3½ years for Yokogawa and a significant investment in time and money. With a focus on providing the means to prove the validity of real-world measurements, one of Yokogawa’s primary objectives is to confirm the results of all our efforts to be more sustainable with its energy consumption. Small changes in the power measurement efficiency of systems and devices requires not only high accuracy and precision, to be able to measure very small changes in power consumption, but also independent corroboration of the measuring instrument. Accredited calibration is the only method which provides this corroboration.

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