EN
 
English
French
German
Italian
Russian

Current measurement

The electric current is a physical quantity caused by voltage and means a flow of electrons between different electric potential. An electric current is the flow of the electric charge.

Now let's return to the water analogy of electricity to explain what current is.

Electric current is a physical quantity caused by voltage and means a flow of electrons between different electric potentials (usually from the positive to the negative poles in the case of direct current, this differs with alternating current). This means in the water analogy the current is the actual water flow rate flowing from the upstream in the downstream direction. Concretely this means that the current is the flow of an electrical charge between two poles.

As mentioned above there are two types of current, these are direct current (DC) and alternating current (AC). The simpler of the two is DC where the electrons only flow in one direction and the flow is constant, on the other hand there is AC where the electrons change direction and amplitude with the frequency (depending on the grid frequency, in Europe that is 50 Hz meaning that the electrons change direction and amplitude 50 times a second). AC is difficult to explain with the water analogy as water in most cases does not change direction and only flows in one direction.

The cause of the direct current is "direct" or constant voltage, for example, a battery. But for generating an alternating current we need a source of alternating voltage, which is, for example, an AC generator in the power plants. The normal wave form of an alternating current is a sine wave, where the positive half cycle corresponds to the positive flow of the current and the negative half cycle corresponds to the reversed flow of the current.

Now let's take a look at how current measurements are done. The simplest way to do this is using an ammeter. In order to do the measurement, the circuit must be opened, and the ammeter connected in series with the circuit. To affect the flow of current as little as possible, ammeters must have a very low impedance.

Since there are many current transducers available for DAQ instruments, current can be measured in a variety of ways. Current measurement is usually divided in two major groups. One is "direct“ this is when the conductor must be disconnected and a sensor is connected in series with the circuit. The second type of sensors allows a measurement of current flowing through a conductor without opening the circuit, which means we can measure the current with a galvanic isolation of the sensor from the conductor.

The “direct” measurement method of currents works without any additional logic circuits. The most common method used in this kind of measurement is using a shunt resistor, which is then connected in series with the measured electrical circuit.

What is a shunt resistor?

A shunt resistor is a resistor with a very low resistance that is accurately predetermined by the manufacturer. A shunt resistor works on the principle that it is connected in series with the electrical circuit and as the current flows through it and the voltage drop on the resistor is measured. The voltage is directly proportional to the flowing current according to Ohm's Law, because we know the exact resistance of the shunt. Choosing a shunt with high accuracy is essential because it will actually define the precision of the measurement itself.

With this method, AC (alternating) or DC (direct) current can be measured but there are a few things that should be considered. Firstly, the declared current of the shunt should not be exceeded as this may burn (destroy) the resistor. Secondly the shunt will heat up and eventually over heat if the maximum declared current flows through it for extended periods of time. The shunts resistance changes with increasing temperature, and if the shunt overheats the resistance can change permanently. Due to this a shunt is usually only used up to about 60 % of its declared current level.

Further there is the common mode voltage, which we discussed earlier. This might cause some complications in the early stages of the current measurement. An example of this would be: When measuring the current that is flowing through a normal incandescent light bulb using a shunt resistor, the difference in voltage at the amplifier will be very small. Although, the measured “voltage points” are still higher than the ground, they can go as high as the grid voltage. If the grid voltage is connected to a 10 V range amplifier the measurement instrument will be destroyed and the only thing left to “measure” will be the sparks coming out of it. To ensure that this doesn’t not happen it is recommended to use an isolated measurement instrument.

To simplify measurements with DEWESoft instruments we can choose between two different DSI adapters with an integrated shunt resistor. For example, inside the DSI 20mA adaptor, there is a 50 Ohm 0.01%, 0.25W shunt. Below is some information about these shunt adapters in the table.

Dewesoft adapterRangeShunt resistor valueResistor tolerance
DSI 5A5A10 mΩ +/- 0.01%
DSI 20mA4mA-20mA50 Ω
+/- 0.01%

Measurements with these two adapters are simple, as there are no additional calculations to be made. The adapters have built in TEDS that recognize the sensors automatically in the DEWESoft software. This saves valuable time in sensor configuration, but the conductor must still be split and connected to the sensor in order to do the measurement.

Interrupting the conductor in order to attach the adapter for the current we wish to measure is sometimes not possible. Flowing current can also be measured with current sensors. This is possible because the flowing current causes a magnetic field around the conductors and current sensors measure the intensity of the magnetic field around the conductor in many different ways and they are also galvanically isolated.

A quick overview of the sensors and how they measure current via the magnetic field. These kinds of sensors are isolated from the conductors which mean easier, faster and safer measurements. This type of measurement is safer for both the user and the measurement instrument because the galvanic isolation eliminates the possibility of a high common mode voltage, which is present when measuring high voltage currents with shunt resistors. 

We must bear in mind that these kinds of sensors have a phase shift to the output voltage compared to the measured current. The extent of the phase shift depends on the type of current sensor and on the measured frequency. With high accuracy current sensors, the phase shift is nearly zero; with cheaper sensors the phase shift can be more than 10° at the fundamental frequency and even more at higher frequencies. Phase shift itself can be problematic but if we have this in mind when setting up the measurement configuration this shouldn't cause any problems at all. Furthermore, DEWESoft offers an additional sensor calibration in the software (Sensor Editor) which improves the accuracy and phase shift even more.

In the next sub-chapters the following current sensors will be described in more detail:

  • Rogowski coil
  • Iron-core clamp
  • Hall compensated AC/DC clamp
  • Zero flux transducers
  • Current transducers in public grids


Overview

The following table shows the main differences between the different types of current transducers and the applications for which they are used.


NOTE: Please always use the analogue setup DC coupling and as an input type bipolar for all types of current measurements. When AC-coupling is selected a high pass filter will be activated which can lead to deviation in the phase measurement (e.g. at 50 Hz). The AC coupling option is only used in special applications.

ADVICE: Should the situation arise where a smaller current needs to be measured with a current transducer with an over-proportional range, simply lead the conductor through the transducer several times. For example, if you lead a 20 A conductor through the transducer 5 times the measurement will yield 100 A. Please do not forget to consider this scaling in the setup of the corresponding analog input channel!

Rogowski coil

A Rogowski coil is a simple measurement device which allows an AC current measurement without splitting the conductor. It consists of a helical coil of wire with the lead from one end returning through the center of the coil to the other end so that both terminals are at the same end of the coil. This coil must be wrapped around the conductor where the current measurement will take place. This allows for a measurement to be done without cutting, disconnecting or stripping the wire. The alternating current in the conductor will cause a voltage induction in the coil.

Measurement with the Rogowski coil has several advantages. Rogowski coils are available for measuring very small currents (some 100mA) up to very high currents (>100 kA). The coil itself is flexible, thin, light and robust. Since there are no magnetic materials, the Rogowski coils cannot saturate and, therefore, has a high overload withstand capability. They are very linear and immune to DC currents which allow for measuring small AC currents with the presence of a large DC component. The bandwidth of the Rogowski coils depends on the type and price and can go up to several Mhz.

There are also some disadvantages. Because the principle of measurement with the Rogowski coil is the measurement of the induced voltage caused by the current flowing inside of the coil, which is proportional to the derivation of the current, an integrator circuit must be used on the output side to make the output voltage proportional to the current flowing through the conductor. Therefore, an external power supply is necessary. It’s not possible to measure DC currents (exception: special types of Rogowski coils are able to measure DC currents). The biggest disadvantage of the Rogowski coil is the phase shift. The phase shift also depends heavily on the positioning of the coil (vertical and horizontal). This positioning error of the coil cannot be compensated using the DEWESoft sensor editor. But the phase and amplitude error due to frequency behavior can be compensated using the sensor editor.



To measure an AC current simply use a DEWESoft current sensor which works with the use of a Rogowski coil. These sensors are integrated similarly to the DSI shunt adapters, with built-in TEDS chips with all the configuration data stored. TEDS only available on certain models.

Iron-core current clamps

Current clamps allow the measurement of current flows with galvanic isolation. Clamps have two jaws which can be opened, and the clamp can simply be clamped around the conductor. The measurement with a clamp is based on the Hall's effect or current transformer technology, which means that the magnetic field of the flowing current is used to cause a voltage output on the current clamps.

The iron-core clamp works on the principle of a transformer. Depending on the number of windings on the primary side compared to the secondary side (turns ratio), a certain current will be induced on the secondary side. Like any transformer, this only works for measuring AC current.

.



The advantages are that the current clamps are cheap, they don’t need an external power supply and they are available for small to very high current measurement ranges. The disadvantages are that they are heavy, inflexible and it is not possible to measure DC currents. Furthermore, the bandwidth is limited (maximal 20 kHz).

Hall-compensated AC/DC current clamps

The Hall Effect is conveniently used to measure both the AC and DC current with a wide amplitude and frequency range (up to 100 kHz) with high sensitivity. For this reason, it is recommended to use hall effect-based clamps to measure DC currents.



The advantages of hall-compensated AC/DC current clamps are the high accuracy (0,5 %), a high bandwidth (100 kHz), the measurement of AC and DC currents and the circuit doesn't need to be opened.

DEWESoft offers a variety of clamps that function using the Hall effect for measuring current, these are listed in the table below.

The voltage output of these kind of clamps are also directly proportional to the current. Current clamps also produce a phase shift which can be up to ~10°, but really good clamps can reduce the phase shift to under 1°. The phase shift of current sensors changes with frequency, this is important to remember when doing any power related measurements.

Zero-flux transducers

Current transducers allow the measurement of current flows with galvanic isolation. They reduce the high voltage currents to a much lower value. The conductor with the measured current must be guided through the loop of the sensor because current transducers function on the principle of a transformer, which means they have a current output signal and this low current signal can then be measured with the DAQ.

Zero-flux current transducers are not simple transformers, they also have sophisticated constructions and integrated electronics. They have two windings which are operated in saturation to measure the DC current, one winding for the AC current and an additional winding for compensation. This kind of current measurement is very precise because of the zero-flux compensation.

This is a very important point because the magnetic core of the transformer stays magnetized with the residual magnetic flux, which destroys the accuracy of the measurement. In these transducers, the parasitic flux is perfectly compensated. Therefore, zero-flux current transducers are used for measuring currents with high precision, but they are not suitable for simple and fast measurement like iron-core clamps or Rogowski coils.

Zero-flux transducers are used to measure currents with the highest accuracy for both AC and DC and have high bandwidth capabilities (up to 1 MHz). They are very linear and have low phase and offset errors.


  1. Connecting a zero-flux transducer to a Sirius system

This chapter explains how to connect zero-flux transducers to a Sirius system. The connection of the zero-flux transducers is illustrated by means of        IT 400-S transducers. 

Required components for the set up 

Step1:
Firstly connect the zero-flux transducer IT 400-S with the D9m-D9f-5M-MCTS cable to the SIRIUSi-PWR-MCTS slice at the Sensor 1 input. 

The D9m-D9f-5M-MCTS cable is a simple extension cable and can be used for all zero-flux transducers (60A up to 1000A). 

Step 2
Take the DSI-MCTS-400-03M cable and use Output 1 of the SIRIUSi-PWR-MCTS to connect it to the first LV input of the Sirius PWR amplifier. 

Note: The DSI-MCTS-XXX cable can only be used for certain zero-flux transducers. The cables have a build-in shunts which only correspond to certain transducers. Please refer to the following table for information on

Repeat Step 1 and 2 for all zero-flux transducers that need to be connect to the system.
A three-phase star system configuration will resemble the image below: 

You will find how to connect voltage and current transducers to the system for different wiring configurations (DC, 1-phase, 2-phase, 3-phase delta-star-aron-V, etc.) in the Dewesoft PRO training course “POWER ANALYSIS”
https://training.dewesoft.com/online/course/power-analysis 

Software Configuration

In the DSI-MCTS-XXX cable there is a TEDS chip integrated, where data about scaling, calibration etc. of the zero-flux transducer is stored. If you connect this shunt cable to the Low-Voltage input of the Sirius amplifier all these configurations are done automatically. Therefore the MSI adapters and TEDS sensors have to be activated. Please check at “Settings” – “Settings” if this option is enabled, see screenshot below. 

After connecting the sensor (e.g. MCTS 400) you will see in the column Ampl.name the current transducer (e.g. DSI-MCTS-400) and the type of measurement will be changed to “Current”. 

Finally you just have to set a suitable measurement range and set a low-pass filter if necessary. 

Current transducers in public grids

Current transducers are used to monitor the current flow in the public grid and protect the equipment from overload. A current transducer is easily explained as a transformer which is operated in short-circuit on the secondary site (or with only a small load). On the output (secondary) side of the current transducer, we get a low current signal which is directly proportional to the current on the primary side. In public grid operation, the secondary current is standardized with a level of 1A or 5A.

There are different measurement classes of current transducers which describe the accuracy and the phase shift of the transducers. The classes range from 0,1 to 5. Class 0,1 means that the accuracy of the measured amplitude is 0,1% and the phase shift is ± 5 minutes. At class 5 the accuracy is 5% and the phase shift ± 120 minutes.

The description of the current transducers also defines the overload factor, the rated power (load) and the application of the transducer (protection, measurement). The load (input resistance of the measurement device) is important because it influences the overload capability of a current transducer. If the load is higher than the rated load, the transducer will go into saturation prematurely and, therefore, will lose the overload capability.

Please also consider the bandwidth of current transducers when measuring Power Quality parameters like Harmonics.

Attention: Never operate current transducers in open-loop mode on the secondary side. This creates high voltages which can destroy the transducers and can be hazardous for people.

Now let's take a look at the classic 40 W light bulb. The first thing to notice is that the load on the grid is linear to the voltage . The measured power is exactly 40 W, but the vector scope looks strange. In fact, since the light bulb is a purely ohmic load, the voltage and current should be perfectly aligned, but as we can see, they are not. What is the reason for this? Do you remember the previous chapters where we have seen the difference between the current clamps and the shunt resistor? Since we are using the current clamps, we have amplitude and phase errors. As a result, the current clamp is the main source of the calculation error in this case.

In Dewesoft X we have a chance to compensate these errors. Let's take a look at it!

As it was explained before every current sensor has a frequency dependent behavior regarding the amplitude and phase. In Dewesoft X, it is possible to correct this behavior in the Sensor editor and make the sensor even more accurate as the manufacturer of the sensor specifies it. This is unique in the market.

Let's choose the settings "Sensor editor" menu item to get a list of all possible sensors. Now let's add one sensor and enter the Sensor type and Serial number. Enter the Physical (input) unit, which is A (amperes) in our case and the Electrical (Output) unit, which is V (volts).

Next let's enter the SCALING factor. Since the sensor is linear with the amplitude, we only need to enter the scaling factor, which is 1 in our case (1A=1V). Do not worry about the polarity of the sensor, it can be reversed in the channel setup.

Now we come to the most important part - the definition of the transfer curve. In the table under the TRANSFER CURVE column, we select Yes to signify that a transfer curve will be defined. Now we need to enter the points of the curve. We need to enter the a[dB] - amplitude deviation in dB and the phi[deg] - phase angle in degrees. The next question is: Where do we to get this transfer curve? There are a lot of transfer curves for the most common sensors that have already been measured, so it's worth checking if it already exists. A second option is to copy it from the calibration sheet of the sensor if the calibration sheet includes a transfer curve. The third option is to measure it with the FRF option, but this requires some equipment. When we get this transfer curve, we just need to enter it in the table. We see that at 50 Hz, the angle is around 10 deg, which explains the phase shift we saw in the measurement.

Save the sensors with the Save file button and close the sensor editor with Exit. Now let's go back to the analog setup and choose the sensor for the current channel. Open the Sensors tab and select the serial number of the sensor previously entered in the Sensor field of the editor. Nothing much happens, but note that we can't enter the normal scaling or sensitivity any more. To reverse the polarity of the sensor you have to choose the Scaling by function and select Sensitivity. With clicking the ± button, you can reverse the polarity.

That's it. For the next setup we don't have to define a sensor anymore, instead we can just select it from the sensors list.

Now let's see what the effect of sensor correction on our measurement is. The results are much better. The phase angle is virtually eliminated and the power is calculated correctly.

Now we will make some current measurements with the Dewesoft X software and a measurement device.

We will measure the current which is consumed by a classic 40W light bulb and 11W energy saving light bulb. For this measurement, we will use two approaches, the first will be the direct voltage measurement on a shunt resistor and the other will be a measurement with current clamps.

Before the measurement we must do some calculations that will help us to choose the SIRIUS amplifier and range of the amplifier and current clamps. If we turn on both light bulbs the declared power will be 51W and the RMS value of the grid voltage is 230V, so let's take those numbers into our calculations.

After the rough calculations, we get the results that our RMS value of the current is approximately 0.22A. We know that the max value of the sine wave signal is √2 times RMS, but since the energy saving light bulb doesn't use the current in sine waveform we should have some reserve in our measurement ranges due to the higher crest factor of the energy saving bulb. This means that we will choose the 10A range on the current clamps and use the MSI SHUNT 5A adapter. The shunt resistance is 0.01Ω which means 1A current will cause 10mV drop on shunt. This information is needed when we are setting the measurement channel on which we measure a voltage drop on the shunt. Since Dewesoft MSI adapters are already equipped with this information, the software is able to configure the setup in the correct way. This is one thing less for taking care of when we are using Dewesoft MSI adapters.

Now we can start with our measurement. We will use two different SIRIUS amplifiers, LV and ACC. Let's see how the connection for our measurement looks like. Current clamps are directly connected to the ACC module and the MSI SHUNT 5A adapter is connected directly to the LV module like on the photo below.

As you can see in the photo we must split the wire for the shunt installation. This can be dangerous because of the grid voltage and we should be careful when doing this at home. Now let's see how the configuration of channel 1 with the shunt is done. First we rename the channel to Shunt current so we will later know the output of which sensor's output we are looking at when the measurement is in progress. Physical quantity should be set to Current and Unit is set to Amperes(A) by default. Once these settings are done we should "calibrate" our sensor. We will choose calibration by two points in this case because we already know that 1V equals 10A. We just simply type this two values in the prepared place. If we set all the parameters correctly and the classic light bulb is turned on, we can already see the sine form of our current in Scope mode on the left bottom side of the setup window.

For channel 8, where we have connected the Current clamps, the settings will be a little different because we are using HV module for this measurement. Since the current clamps are set on 10A range they provide 1mv/1mA on the output (scaling factor is 1). That means we can't get more than 10V on output and range of our amplifier should be set to 50V to achieve greater resolution of the measurement. We should also set the Physical quantity to "Current" and measured unit to Amperes.

In the next snapshot, you can see the combined waveform of the energy saving bulb and the light bulb. The waveform changed mostly due to the non-sine waveform and the high crest factor of the energy saving light bulb.

Now when we switch to Measure mode, we can see the phase shift of the current clamps compared to the shunt resistor. At first sight, there is no big phase shift(around 10°) on the picture below, but with applications like the power measurement phase shift is very important for correct results. The phase shift is around 10° in the picture below, and can influence the measurement results for detailed power analysis significant (especially reactive and apparent power). The phase-shift of this current can be compensated using the sensor editor.

Calculate the AC RMS value

To see the RMS value of the current signal, add a Basic statistic math function.


Select the input channel (current signal) and the RMS as the output channel. We can display one value per measurement or we can display new values for each defined block of data.

 

Another option is also to display the RMS value of the signal directly in the recorder. Select the RMS as the Display type (in the left corner of the screen).

This website uses cookies to ensure you get the best experience on our website. Learn more