Friday, January 27, 2012

Power-quality testing

A Yokogawa DL850 ScopeCorder is playing a key part in tests on synchronised switching devices in power systems being carried out by Harmonizer Power Quality AB: a Swedish company that provides power-quality services for industrial and utility customers.

Among the services provided by Harmonizer are measurements of active and reactive power, power factor, current and voltage variations, harmonics and transients. Typical applications include analysing the best location for reactive power compensation in a network, calculating regulation cycles for optimum harmonic- and power-flow control, sizing capacitor banks for shunt or series compensation, calculating and designing harmonic filters for the suppression of harmonic distortion, and sizing surge arresters for transient overvoltage limitation.

The application for which the Yokogawa DL850 ScopeCorder is used relates to the synchronised switching of high-voltage filter banks: in particular, programming and checking the functioning of the synchronising device and the circuit breaker. Unsynchronised switching of capacitor and filter banks often results in high-voltage and high-current transients that can have adverse effects on power systems performance and reliability. In order to eliminate these high switching transients and the resultant power-quality problems, the closing and opening operations of the circuit breaker have to be synchronised, which requires accurate monitoring of the relevant electrical parameters.

In the ideal scenario, the circuit breaker should be closed when the voltage across the load is close to zero and opened in such a way as to avoid short arcing times which can lead to re-ignitions or re-strikes. To check that these requirements are met, it is necessary to carry out measurements on the circuit-breaker closing and opening times while recording the network voltage and filter current during opening and closing operations.

These criteria impose stringent requirements on the measuring instrument involved. A typical test might require measurements on five voltage channels and three current channels in an environment where voltages can be several hundred volts. More significantly, the operating times that need to be measured mean that the instrument has to have a resolution as low as 1 µs.

Previously, the Harmonizer engineers had used test instruments such as conventional network analysers and digital storage oscilloscopes (DSOs), but found that these were not adequate for this particular application because of their limitations in terms of time resolution, accuracy and numbers of channels.

"What we needed was an instrument combining multi-channel capability with high measurement accuracy, high sample rate and time resolution along with good display quality and the ability to zoom in on the fine detail of measured waveforms", comments Natan Gothelf, Managing Director of Harmonizer: "After evaluating the available products on the market, we decided that the Yokogawa DL850 ScopeCorder was the one that best met our requirements."

A further benefit of the ScopeCorder is that it has isolated inputs that enable it to be used for high-voltage measurements using standard probes with no additional signal conditioning hardware. In addition, the instrument's powerful display and zoom capabilities make it very easy for the user to examine very short events in recordings taken over a longer period of time.

Harmonizer's tests are typically carried out in the field on a number of different sites, so that the instrument's portability is a further benefit. After field testing, measurement data can be downloaded via the ScopeCorder's various communications interfaces - including a USB connection to a PC - and analysed using evaluation software also supplied by Yokogawa.

The Yokogawa DL850 ScopeCorder is a versatile measuring tool that combines the benefits of a high-speed oscilloscope and a traditional data acquisition recorder in a single, portable instrument. The DL850 incorporates high vertical resolution, channel count, isolation, filtering, and abundant acquisition memory compared to general-purpose oscilloscopes. This makes it ideal for viewing small changes, even across large dynamic ranges, while monitoring more signals simultaneously

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