How to measure current and voltage

To measure current and voltage signals in a power system a data acquisition system has to be designed for this specific purpose. Data acquisition is when the measured signal is gathered from the source and then digitized for storage and prepared for analysis and presentation. This is done for testing, measuring and automation applications. A complete data acquisition system consists of roughly five parts: transducers/sensors, signals, signal conditioning, hardware and software.

The data acquisition starts with measurements of the physical phenomenon that is going to be analysed. In this case where alternating voltage and current harmonics are the physical phenomenons, the signals are analog signals where the frequency is of importance. This means that the sampling rate is of importance for the results of the measurements. The levels of the measured signals are also of importance, as the magnitudes of the signals are used for calculation of THD and IHD. For harmonic components that are measured, they are added the total signal, meaning they are not measured as pure sinusoidal waveforms, symmetrical around the x-axis. This makes the shape important as well, as the harmonic components are seen with a voltage offset.

For most measurements a transducer is necessary for conversion of the physical phenomenon into a measurable signal. For current and voltage the measured signals are already readable for the data acquisition system. Transducers are commonly used for adjustments of the signal

44

magnitudes to fit within the input range of the measurement system. Another important attribute with the use of transducers for current measurements is safety. When large currents are measured, direct measurements can be dangerous to humans and measurement equipment. By adjusting the current level by using a current transducer this safety risk can be limited. Further, for systems with a settled mounting, for examples cables that are hard to dismount and connect directly to the measurement system, transducers/sensors which are clipped on around cables makes the installation of the measurement system very simple and easy [20]. For current sensors there are two types of output signals. Either a current output or a voltage output can be chosen for a current transducer, depending on which of the two the

measurement system uses as input. The output signal is proportional to the measured signal on the transducers primary side. The relationship between the primary input signal and the

secondary output is often given as 0.1V per ampere for voltage output transducers, and 500:5 CT for current output transformers.

Figure 4.19 - Current transducer with voltage output [21]

For current transducers with a voltage output signal the sensors typically contain a precise burden resistor internally which allows the output to be a voltage signal. This makes current measurements very easy and flexible as the transducer can be connected to most measurement equipment as long as the voltage is within the equipment’s voltage rating. Regarding safety, the voltage output signal is a low energy signal, and can safely be used by the user.

Connecting and disconnecting the sensors won’t damage them. However, due to the low energy, the signal is sensitive to interference from the outside, and connections via long cables are not recommended.

45

For the current output current transducer, known as a current transformer, the current output is a high energy signal that can cause damage to equipment and is hazardous for the user. The standard output is typically of 5A or 1A and cannot be connected directly to most equipment as it would cause damage. If the sensors are connected to an open circuit, the voltage

occurring can be very high, permanently damaging the sensors. For this reason fuses are not used on the secondary side of a current transformer.

When choosing a transducer for a measurement system, one can choose between split core and open core CTs. For existing installations the split core CT is a good choice as it is easy to install in any system. The split core CT is more expensive than the solid core, but if rewiring of an existing system is difficult this is often the best choice. The solid core CT is cheaper than the split core, and also gives better readings. For measurements that require the best measurements this is the best solution, and also for testing of temporary lab set-ups the solid core is preferable due to cost and performance.

After measurements are made the acquired data must be analysed, virtualized in a good way and stored if necessary. This is done with computer software programs. When choosing a data acquisition software there are some things one need to consider. The time it takes to learn the software, integration time of the software, performance and opportunities that comes with the software, available help like customer support, online communities/forums and available courses, and the stability, reliability and reputation of the software.

46

The data acquisition software can be ready-to-run software, where the measurement device can be plugged in and then a run button is pressed to get measurement results. This is very specific task solution good for specific tasks at that exact moment. Programmable software on the other hand allows more flexibility in the usage. It can be programmed to fit a larger range of measurement devices and it can be used for several specific tasks. The trade-off with programmable software is the time spent learning the programming and designing the programs for the different measurements.

The programming language must then be taken into account when programming software is chosen. The ANSI C/C++ for examples can be difficult and time consuming to learn. Easier programming like the graphical programming makes it easier and quicker to learn the

software as it is more intuitive and built in a way that is more in the direction of an engineer’s way of thinking.

To avoid much extra work the software must be able to communicate directly with the data acquisition hardware. This is to avoid work related to import and export of acquired data between the software and hardware. Post processing, analysis and storage are made less complicated by choosing software and hardware capable of communicating directly with each other.

The software of choice needs a good way of visualization and presentation of the results acquired. A good and easy presentation helps speeding up the analysis process and the understanding of the results from the measurements don’t need any deeper analysis to get the information needed [21].

47

5 International measuring standards

For the measurements conducted in this project the following standards are applicable:  IEC 61000-4-7:2009

 IEC 61000-4-30:2015

The IEC 61000-4-7 standard is a general guide on harmonics and interharmonics

measurements and instrumentation. It is applicable for instrumentation intended for measuring spectral components in the frequency range up to 9 kHz and defines the measurement

instrumentation intended for testing of equipment in accordance with emission limits given in certain standards (IEC 61000-3-2) as well as the measurement of harmonic currents and voltages in actual power systems.

The IEC 61000-4-30 standard is made for power quality measurement methods. It defines methods for measurement and interpretation of results for power quality parameters in AC power systems with a frequency of 50 Hz and 60 Hz. The parameters measured in this project; magnitude of supply voltage, voltage harmonics and current measurements are all covered by this standard.

5.1 IEC 61000-4-7:2009