Power analysis

Power is the rate of doing work. It is equivalent to an amount of energy consumed per unit of time. The power of an electrical system is the multiplication of the voltage with the current, integrated over and then divided through the periodic time. The periodic time (equals the frequency) must be known in order to calculate the power of an electrical system

If you would like to learn more about solutions for every application, please download our POWER BROCHURE

Also check the Power Analysis application page for more details.

In physics power is the rate of doing work. It is equivalent to an amount of energy consumed per unit of time. The unit of power is Joule per second [J/s], also known as Watt [W]. The integral of power over time defines the energy (performed work).

Power calculation

The power of an electrical system is calculated by multiplying the voltage with the current.

But is it really that simple? What about measuring a 7-phase system or measuring frequency inverters? Power calculation can be very easy for example when measuring DC systems, but it can also be challenging when measuring inverters with multiple phases.

In this reference manual the correct way to measure the electrical power of different systems will be explained as well as for different applications such as grid, motor and inverters among others. First of all, the theoretical part will be covered, including a theoretical background on power calculations. Secondly, the calculation of easy power parameters and explanations of how to configure the power module and how to visualize the power parameters in the measurement screen of the DEWESoft software. Lastly, there is a practical part showing step-by-step how to measure DC power, single phase, 2-phase, 3-phase in star, delta, Aron and V-connection AC power, as well as what is to consider when measuring inverters and an example of how to calculate the power of a 6-phase motor.


Electrical power is the rate at which electric energy is transferred by an electric circuit. The SI unit of power is watt [W].

The following formula describes the calculation of the electric power for AC or DC systems.

  • P is power in Watt [W]
  • u is voltage in Volt [V]
  • i is current in Ampere [A]
  • T is periodic time in seconds [s]

So, the power is not just voltage multiplied with current as was initially stated, it’s the integration over the periodic time of this term, divided through the periodic time. This means that is a requirement to know the period of time (frequency) to calculate the power of an electrical system.

Measuring DC power is not difficult as the voltage and current are constant and there is no frequency as seen in the image below. The time interval for the integration just defines the average interval of the power calculation.

To measure AC systems the periodic time needs to be known. It cannot be done by taking the normal grid frequency (e.g. 50 Hz) as a constant. This is due to the fact that the grid frequency is ever changing depending on the balance of the energy supply and the electric load at that exact moment. With variable drives, where the frequency continuously changes with a large bandwidth (ranging from 1 Hz up to 2000 Hz) makes it even more difficult to calculate the power.

Therefore, the period time must be determined. This is especially difficult when measuring inverters where the voltage is not a sinusoidal waveform anymore but packets of pulses.

Frequency determination

This is where the DEWESoft power analysis tool distinguishes itself from the conventional power analyzers. Conventional power analyzers use a zero-point detection to determine the periodic time. This means that they evaluate when the voltage or current crosses that x-axis and use that value to calculate the periodic time. This works well most of the time, but especially when measuring strongly distorted signals this method can lead to measurement errors.

DEWESoft was not happy with “works well most of the time”. Therefore, a special FFT (Fast Fourier Transform) algorithm (Software PPL) to determine the periodic time (frequency) was developed. The algorithm determines the periodic time of the signal via a special FFT algorithm using a sampling window of multiple periods (typically 10 periods, definable in the power module). The calculated frequency is highly accurate (mHz) and works for every application (motor, inverter, grid, …).

How cheap watt-meter calculates the power of an AC system

A cheap watt-meter calculates the power of an AC system out of the peak values from the voltage and the current, according to this formula:

In order to calculate the power, they simply multiply the RMS values of the voltage and current. This way of measuring the power works well when the waveform for both current and voltage is an ideal sinusoid (as it is produced in the generators at power plants). Nowadays the waveform of both voltage and current are never ideal due to non-linear loads and also non-linear generation units. This way of calculating the power is outdated, especially when measuring inverters, the calculation will mostly be completely wrong.

How a conventional power analyser calculates the power of an AC system

Conventional power analyzers calculate the RMS values of the voltage and current out of each sample point. The RMS values are calculated out of the square root of all squared sample points of the curve divided by the number of samples.

How Dewesoft calculates the power of an AC system

As mentioned earlier conventional power analyzers calculate the power in the time domain. Dewesoft on the other hand calculates power in the frequency domain. With the predetermined period time, an FFT analysis for voltage and current is done for a definable number of periods (typically 10 with electrical applications) and a definable sampling rate. The FFT yields an amplitude for the voltage, current and the cos phi for each harmonic. One major benefit of this FFT transformation is that the behavior of amplifiers, current or voltage transducers in amplitude and phase for the whole frequency range (using the Sensor XML) can now be corrected. This way of power analysis has the highest possible accuracy. Another benefit is that harmonic analysis and other power quality analysis can be done completely synchronized to the fundamental frequency.

With the FFT corrected values, the RMS voltages and currents are calculated out of the RMS values of each harmonic.

The power values for each harmonic and the total values are calculated with the following formulas:

Benefits of the Dewesoft power calculation against other approaches:

  • Raw data storing together with power analysis
  • Additional sensor calibration for amplifier and sensors for amplitude and phase for the full frequency spectrum
  • Easy power quality analysis (harmonics, inter harmonics, higher frequencies)
  • Resampling
  • Period values for power, voltage, current and symmetrical components

Active power - P

Active power is measured in Watt (W) and refers to the energy transfer from an electric generator to a load. The active power is the power which can be used by electric loads (useful power).

Reactive power - Q

The reactive power is measured in Volt-Ampere-reactive (VAr). The reactive power doesn’t consist of energy but is necessary for most types of magnetic equipment (motors, transformers) to work. Reactive power is provided by generators, synchronous condensers or electrostatic equipment such as capacitors and directly influences the electric system voltage and also the capacity of the power transmission line.

Apparent power - S

Apparent power is measured in Volt-Amperes (VA) and is the voltage on an AC system multiplied by the total current that flows in it. It is the vector sum of the active and the reactive power.

Power factor - PF

The ratio between active power and apparent power in a circuit is called the power factor. For two systems transmitting the same amount of active power, the system with the lower power factor will have higher circulating currents due to energy that returns to the source from the energy storage in the load. These higher currents produce higher losses and reduce the overall transmission efficiency. A lower power factor circuit will have a higher apparent power and higher losses for the same amount of active power.

Cos-phi

The cos phi is the angle difference between a phase voltage relative to the current. 

The difference between cos phi and the power factor is that the cos phi is calculated for each individual harmonic starting at the fundamental frequency compared to the power factor which includes the whole spectrum (all harmonics).

Power triangle

The power triangle illustrates the relation between active, reactive and apparent power.

In the diagram, P is the active power, Q is the reactive power (in this case positive), S is the complex power and the length of S is the apparent power. Reactive power does not do any work, so it is represented as the imaginary axis of the vector diagram. Active power does do work, so it is the real axis.

Power triangle - analogy to beer

To easier understand the power triangle an analogy with beer is given. The reactive power is like the foam (head) on top of the beer. The active power is the actual liquid gold in the glass. The foam in the glass does take up space and thereby reduces the amount of space that the actual beer has in the glass. The foam is a natural part of the process of pouring beer, but one does try and keep it as low as possible to increase the capacity for the Beer. By comparison the reactive power will always be in the transmission lines and reduce the capacity in which real power can be transported, but it is kept as low as possible. Only the real power can be used.

Calculation of power values (P, Q, S) for each harmonic

To easier understand how the power of each harmonic component is calculated, refer to the image below illustrating the calculation of several harmonic active power values. The same calculation is done for both apparent and reactive power, just using different equations.

The NEW power triangle

Nowadays the typical power triangle that was illustrated above doesn’t comply with the times anymore because other parameters like the distortion or harmonic reactive power have to be considered in the new power triangle as illustrated below. This new triangle shape is mainly due to more and more non-linear loads (inverter, electronic ballast unit, etc...) and also new power generation units (the wind, PV, etc...). The new power triangle, therefore, has an additional dimension added to it:

Calculation of power values (P, Q, S) of the NEW power triangle

Harmonic reactive power - QH

The sum of the harmonic reactive power. This reactive power occurs through the phase shift between voltages and currents of the same frequency.

Name in DEWESoft (e.g. for L1): QH_L1, reactive power of all harmonics where u and i have the same harmonic order.

Distortion reactive power - DH

The combination of voltages and currents of different frequencies which produce the distortion power.

Name in DEWESoft (e.g. for L1): DH_L1, distortion power of all harmonic components’ reactive powers where u and i have different harmonic orders.

Distortion - D

Distortion power considers everything except the first harmonic.

Name in DEWESoft (e.g. for L1): D_L1, distortion power of all harmonic components’ reactive powers (u and i have the same order but not equal 1 or have different order).

These are the power quality (PQ) parameters. All other PQ parameters which are calculated in Dewesoft (Harmonics, THD, Rapid voltage changes, Symmetrical components …) you will find in the POWER QUALITY PRO training.

The following table shows the calculation of the power values for different connections.

The power module is one of the most complex mathematic modules in Dewesoft X. It allows measurements of different frequency power grids in different configurations and even variable frequency sources. This section will demonstrate how to use it.

After the configuration of the voltage (please refer to the voltage pro training) and current (please refer to the current pro training) inputs it is now possible to add a new power module by clicking on the “+” button in the channel set-up. Then click on the “power analysis” button by doing this the power icon will be available.

The "Power" icon is now available on the main dashboard: 


The following screenshot gives you an overview of the power module.




In the power module, there are several system configurations available. The most common ones are 1-phase and 3-phase star and 3-phase delta connections. 2-phase is used with special motors and also in some applications of the grid. The Aron and V configuration are basically star or delta configuration but measuring only 2 currents instead of 3 (see 3-phase measurement). Special configurations like 6-, 7-, 9- or 12 phase motor measurement can be done with multiple single-phase systems and adding up the power values in the Math library.

Line frequency

Firstly, the line frequency must be set. In the public grids the frequencies are 50 Hz for Europe and 60 Hz in the USA (please make sure that the correct frequency is set for the region where the measurement will take place). There are also other line frequencies available (16,7 Hz, 25 Hz, 400 Hz, 800 Hz) for special applications. For inverter measurements “variable frequency” must to be selected. The “variable frequency” setting searches automatically for the fundamental frequency in the signal via an FFT algorithm (highest peak).

This algorithm calculates the frequency with very high accuracy (mHz) but is also very CPU intensive (CPU power). For optimal performance, it’s recommended to set a range (start and end frequency) where the fundamental frequency could be via the “Exact frequency settings”. For example, when measuring an inverter driven motor and it is known the fundamental frequency is no higher than 200 Hz it is recommended to add an end frequency a little higher than the known frequency for example at 250 Hz.

Output units

When measuring high power, it could be useful to change the output unit to a higher unit. Available are Watt, Kilowatt and Megawatt.

Frequency source

A special functionality in the power module is the selection of the frequency source. As a source, the voltage, the current, external signal or an arbitrary channel can be selected.

Arbitrary channel uses filters for better and more stable frequency determination. For all different types of frequency sources, it is possible to select the channel which should be used for the frequency calculation. If there are more than one power module, it is possible to synchronize the modules with arbitrary channels. (E.g. by selecting one phase out of another power module)

As seen in the following image of an inverter measurement, the voltage (green) isn’t a sinusoidal waveform any more. It has become a packet of pulses. In this case, if voltage is selected as the frequency source there is a chance that the determination might be faulty. On the other hand, the current waveform (orange) has a nice stable sinusoidal form and should then be selected as the frequency source as this will yield much better results. The oscilloscope function in the DEWESoft software is extremely helpful in analyzing the signals for frequency source determination.

Number of cycles

In this option, the number of cycles for the power calculation can be set as shown in the image below on the left-hand side. As standard, this value is 10 periods for 50 Hz measurements and 12 periods for 60 Hz applications (required in 61000-4-30). In the drop-down list only 10 and 12 cycles are selectable, but any arbitrary number up to 999 can be entered into the field, the lowest number of periods is 5. For all other applications, if there is a need for faster values the “period values” functionality can be used, ensure that this function is enabled by clicking in the small box, it is activated when there is a tick in the box as seen in the image on the right-hand side.

Nominal voltage

The entry of the nominal voltage is important if you want to calculate the Flicker values. For other measurements the voltage should be set to at least an approximate voltage of the measurement. If this value is set very high (e.g. measuring inverter with 20 V output and the nominal voltage is selected to 400V) the frequency determination can fail.

Example:

  • 230 V - line to earth voltage for star configuration
  • 400 V - line to line voltage for delta configuration

In the drop-down list 120 V and 230 V is available, but it is possible to enter any arbitrary voltage that is required for the measurement in the nominal voltage field.

Calculation sample rate

The calculation sampling rate in the power module is like a sample rate divider for the power calculations. At high sampling rate (>100 kHz) this is often necessary due to performance problems (CPU power at the limit). So just select the calculation rate you need for your measurement. If you store all data in the full sampling rate (always fast) you also can calculate the power in the full calculation rate via the post-processing functionality.

Typical calculation rate:

  • grid measurement - 10 to 20 kHz
  • wind, renewable, etc. - 50 kHz
  • inverter measurement - 100 kHz or more

Channel list

After all the configurations have been done, the channel list will show which parameters will be calculated by the DEWESoft X software:

The power module calculates many parameters. Most of the time not all these parameters are needed for the current application. In this case it is possible to deselect some of the unneeded parameters. This will reduce the data file sizes (The size of the reduction relies strongly on the number of deselected parameters but also on the parameter calculation intensity as these also differ).

Vector scope

The vector scope functionality in the power module depicts a fast overview of all voltage and current channels that are connected to the power module and if they are connected correctly to the measurement device.

The bottom right of the vector scope depicts what the voltages and currents should look like. If a voltage or current has been connected incorrectly the hardware connections don’t have to be changed, this can be corrected in the Dewesoft X software using the wiring schematics. In addition to the correct positioning of the phases, it is also possible to verify that the current transducers have been connected correctly. Should this not be the case, it can also simply be changed within the software, in the analog set-up using the scaling option just click on the “±” to change the polarity.

Multiple power modules

In the Dewesoft X software multiple power modules can be created. This means that power can be measured at multiple points completely synchronous. In the math library the power modules can be further refined, for example the efficiency can be calculated automatically (see efficiency calculation). This is also very helpful when measuring multi-phase motors (6-12 phases). Just click on the + next to the already active power modules to add another one.

Harmonics, THD, Flicker, Flicker emission, Rapid voltage changes, Background harmonics

These features will be described in the Power quality course in the PRO training.

Period values

Period values are needed to perform a detailed analysis of electrical equipment (e.g. analyzing behavior at faults or switching processes) and for fault recording (as a trigger argument). The period values are calculated for voltages, currents, active, reactive, apparent power, power factor as well as other parameters.

The period values can be calculated with a definable overlap (up to 99%) and for a definable number of periods (up to 4). Using an overlap of 99% at a 50 Hz measurement you can calculate the power values for every 0,2 ms. That’s a unique feature of DEWESoft.

Overlap:           25%, 50%, 75%, 90%, 95%, 99%

Periods:            1/2, 1, 2, 4

Period values are not corrected in amplitude and phase as is the case for the other power calculations in the power module.

Considering the period values for the symmetrical components (for more details please see the Power Quality Pro training) there are more than 50 parameters available.

After configuring the power module, the measurement screen(s) can be configured in measurement mode. As discussed earlier different visualization modes can be inserted by switching to design mode. The visualizations that are available are located at the top of the window. On the right side of the window is the channel list and on the left side is the properties of the different visualizations.

The most interesting visualizations for power analysis are:

  • Digital meters (to see the different instantaneous values calculated out of the power module)
  • Recorder (to see the chart of power values)
  • Scope (to see the waveform of the voltages and currents)
  • Vector-scope (to see the relation of the voltages and currents)
  • Harmonic FFT (to see the harmonics of voltage, current, power and reactive power synchronized to the fundamental frequency of the signal)

Channel list

In the channel list you now find all the parameters which are automatically calculated out of the power module. They are categorized to easily find the desired values:

The Vector Scope and Harmonic FFT visualizations are described in more detail because they are specially implemented for the power module calculations.

Vector scope

“Vector scope” means that not only the absolute values of current and voltage are important but also the phase relation between current and voltage. This is essential as only current that is in phase with the voltage can be used for producing useful work.

 Therefore, in the vector scope both the absolute values and the angle known as the angle Φ (phi) is measured. From the angle Φ the value of cos Φ can also be calculated (which in principle is just the cosine of the angle Φ, this is important because it is the direct ratio of work that is done compared to the consumed current). In the vector scope the voltage vector (hollow point arrow) and the current vector (full point arrow) for each harmonic can be shown.

Dewesoft X has a special feature where the vector scopes orientation can be changed. This means that the scope can be shown in a clockwise and in an anti-clockwise manner, and it is also possible in some special applications to change the vector scope orientation to the right. This can be changed in  “Settings” → “Extension” → ”Power grid analysis”. Default settings is to show the vector scope for the first harmonic. Via the input field “shown harmonic” in the visualization properties each individual harmonic can be selected.

Furthermore, the scaling for voltage and current can be defined either manually or automatically. The option “Show measured values” adds digital values of basic power parameters for each phase on the right and left a side of the vector scope. The option “Tick count” defines the number of circles to be shown.

  • upper, CW - zero is top, the positive phase angle is right
  • right, CW - zero is right, the positive phase angle is right
  • upper, CCW - zero is top, the positive phase angle is left
  • right, CCW - zero is right, the positive phase angle is left

Harmonic FFT

In the power module the harmonics can be calculated for apparent, active and reactive power for each individual harmonic. But is it necessary to calculate the harmonics?

In theory, voltage and current have a perfect 50 (or 60) Hz sine wave. This is the case if there are just linear loads (Ohmic) connected to the grid (e.g. incandescent light bulbs). But as there are more and more non-linear loads connected (ballast unit, inverter, etc.), some generation units are not linear anymore either e.g. Wind, PV, etc., therefore the waveform of voltage is not an ideal sinusoidal wave anymore. The images on the below show the voltage and current of a conventional light bulb (left) and a LED (right). It can clearly be seen that the light bulb waveform for current (blue) is sinusoidal but the current of the LED (red) is a pulse waveform with a high crest factor.

The following Harmonic FFTs depicts the big difference of the two different loads (current harmonics):

What is the effect of harmonics. Using an AC electromotor, the first harmonic (line frequency) drives the motor. The rest of the harmonics are either producing vibrations or noise. In truth there are bad and then there are worse harmonics. The 2nd, 5th, 8th... harmonics are really bad ones since they are braking the motor. The 3rd, 6th, 9th... harmonics are either driving or braking, while 4th, 7th, 10th... harmonic are driving the motor, but they are still producing high noise and vibrations.

In the Harmonic FFT the following can be visualized, harmonic - voltage, current, active and reactive power as well as the line voltage each fully synchronized to the fundamental frequency.

In the visualization properties, there are additional options available:

  • Show Data Panel – The power values of each harmonic can be displayed in the top right corner of the FFT when “Show Data Panel” is selected. The individual harmonic values can be shown by clicking on the order number.
  • Y-Axis – Here the harmonics can be displayed on a linear or logarithmic scale, as well as if they are shown in an absolute or percentile relative to the fundamental frequency.
  • Draw full FFT - The option “Draw full FFT” shows all frequencies, not only multiples of the fundamental frequency. As seen in the Line Voltage FFT in the Image above.

More details about harmonics and full FFT will be given in the Power Quality Pro training.

The practical part will kick off with a simple DC power measurement

Hardware configuration

For a simple DC measurement please connect the voltage and the current to the Sirius as it is shown in the following image.

Analog setup

In the next step, the analog set-up needs to be done for both the voltage and the current (Please refer to the pro training on voltage and current as a reference). The image below shows what a typical DC set-up would look like.


Power module set-up

In the power module, the wiring must be set to DC measurement, this is illustrated in the image below in the red square. Then configuration for the selected application must to be done.

On the wiring schematic page, it is possible to select two different calculation modes, this is illustrated in the blue square in the image. Firstly, it is possible to select or type the required calculation rate, or the measurement can be synchronized to another channel. This synchronized channel can be from another power module e.g. a 3-phase power module. Additionally, an energy calculation can be added, as seen in the yellow square in the image. 



Math

This only needs to be done in the DEWESoft X2 software, as there is a dedicated calculation in DEWESoft X3. 

In Dewesoft X, a simple math formula needs to be created, by multiplying the DC voltage with the DC current the DC power is calculated as seen below.

Never calculate the DC power using a 1-phase power module. This won’t work and will yield incorrect results.

Measurement screen

If you switch to the Measurement screen you can visualize the voltage, the current and the power. In this example, the battery power of an electric vehicle is shown. As you can see the voltage (purple) is relatively constant while the current is just present if power (acceleration) is required.

Hardware configuration

For a single-phase AC measurement please connect the voltage and the current to the Sirius as it is shown in the following picture.

Analog setup

As next step, the analog setup for the voltage and current input must be done (please refer to the pro training on voltage and current for more information).

Use the XML correction factor for the used current transducer to ensure that the highest accuracy is yielded in the power calculation. (Please refer to the current  pro training for more information on how to do this)

Power setup

In the power module, the wiring must be set to single phase and the configurations for the specific application has to be done.

For example, measuring a load connected to the public grid the line frequency will be set to 50 Hz (60 Hz in North America, parts of South America, Japan, etc.), the output unit should be Watt, the frequency source is voltage, number of cycles are 10 and the nominal voltage (line to earth) is 230 Volts.

Measurement screen

After switching to measurement mode, the screen design for the measurement can be set up to the requirements of the user.



2-phase measurements are really seldom, but some motors (e.g. step motor) for example are operated with two phases (one phase has a phase shift of 90°).

Hardware configuration

For a 2-phase AC measurement please connect the voltage and the current to the Sirius as it is shown in the following picture.

Analog setup

As next step, the analog setup for the voltage and current input has to be done (see pro training voltage and current).

Use the XML correction factor for the used current transducer to ensure that the highest accuracy is yielded in the power calculation. (Please refer to the pro training for more information on how to do this). This will ensure the highest accuracy for the measurement.

Power setup

In the power module, the wiring must be set to 2-phase and the configurations for the specific application need to be done.

Measurement screen

After switching to measurement mode, the screen design for the measurement can be set up to the requirements of the user. In this case, the scope and vector scope of a 2-phase step motor is shown as well as the single-phase voltage and current of the grid.

The star connection is mainly used for measuring 3-phase systems, especially if a neutral line from the grid or the star point of a motor is available. The three phase voltages are connected to the Sirius HV modules on the high voltage side. The low voltage side of the three inputs is on the potential of the neutral line or motor start point. If both are not available an artificial star-point can be created by short-circuiting the low sides of the amplifiers.

Hardware configuration

The following image shows the connection for a 3-phase star measurement including three zero flux transducers for current measurement. In this measurement zero flux transducers are used therefore, a Sirius  MCTS slice need to be connected as well. This is because the zero flux transducers need more power that the Sirius  4xHv 4xLV can deliver. The Sirius  MCTS is designed to deliver power up to 20 W per channel.




 

Analog setup

The next step is setting up the analog channels for the voltage and current inputs.

Please see the pro training on voltage and current for more information


Use the XML correction factor for the used current transducers to ensure that the highest accuracy is yielded in the power calculation. (Please refer to the current pro training for more information on how to do this)

Power setup

In the power module, the wiring has to be set to 3-phase star and the configurations for a certain application have to be done.

Measurement screen

After switching to measurement mode, the screen design for the measurement can be set up to the requirements of the user. In this example the load of a household is shown.

On the top left of the screen there are digital meters that display the voltage and current values in RMS, in the middle is the current power that is being consumed from the grid and on the top right-hand side current power values of the three phases are shown. The left scope in the middle of the screen shows the waveform of the voltage, and the right one the waveform of the current (which is quite distorted). At the bottom of the screen the load profile is shown in a recorder.



The delta connection is used in the absence of a neutral line or star point on a motor. The three phase voltages are connected to the Sirius HV modules on the high voltage side to the live terminals (Red). The neutral terminals of the HV amplifier must be connected to the next live terminal (neutral terminal L1 to live terminal L2, neutral terminal L2 to live terminal L3 and then the neutral terminal L3 to the live terminal L1).

Hardware configuration

The following image shows the connection for a 3-phase delta measurement including three zero flux transducers for current measurement. In this measurement zero flux transducers are used, therefore, a Sirius MCTS slice need to be connected as well. This is because the zero flux transducers need more power than the Sirius 4xHV 4xLV can deliver. The Sirius MCTS is designed to deliver power up to 20W per channel.

Analog setup

The next step is setting up the analog channels for the voltage and current inputs (please see the pro training on voltage and current for more information).

Use the XML correction factor for the used current transducer to ensure that the highest accuracy is yielded in the power calculation. (Please refer to the current pro training for more information on how to do this)

Power setup

In the power module, the wiring must be set to 3 phase-delta and the configuration for the specific application must be done.

Measurement screen

After switching to measurement mode, the screen design for the measurement can be set-up to the requirements of the user. In this example the measurement of a PV inverter in delta configuration is shown. The vector scope on the top right shows that the power is being fed into the grid, therefore the current vectors have a phase shift of 180° to the voltage vectors compared to where they would be if the system was consuming power. Further illustrated are the wave forms of both voltage and current on the scopes as perfect sinusoidal wave forms.

As a comparison in the next image a 1-phase PV inverter is depicted, with unfavorable wave-forms for both voltage and current. The voltage has a rectangular wave form, electrical devices like this one puts large amounts of stress on the grid.This is mainly due to harmonics that are present in the signals, they cause distortion in voltage and current wave-forms causing them to take on different wave-forms compared to the ideal sinusoidal wave form that they should have. 

Please refer to the Power Quality pro training for more information on Harmonics and the influence that they have.



A special feature in the Dewesoft X power module is the star-delta calculation.

This feature allows for the calculation of all values of a delta connection out of a star connection (waveform, RMS values) and vice versa. This means that no matter the hardware connection to the system, both connection types can be measured. 

For example, to see the analog voltage signal of a delta connection when a star connection is being used, just select the option “Calculate waveforms” highlighted in BLUE in the figure. The next option highlighted in RED “Calculate line voltages” allows RMS voltages and harmonics to be displayed.

These features are available in the power module in the set-up screen on the bottom right, next to the wiring schematic. 

The following table shows the calculations that are used in Dewesoft X to do the star-delta and the delta-star conversions.

This feature is included as standard in the power module and therefore completely free of charge (with other power analyses this feature needs to be purchased separately)


With some applications, only two currents and / or voltages are measured instead of three in a three-phase arrangement. The main reason for this type of measurement is to save costs.

This is done for measurements where it is completely sure that the load is absolutely synchronous, the third current can then be calculated from the two currents that were measured. This is often done with grid measurements (expensive current transducers, symmetrical load).

These measurement options can ONLY be used when there is NO neutral line.
Never use these measurement options with an inverter measurement. The measurement will be wrong.


Aron-connection

Hardware configuration - The most common way of measuring active power with symmetrical and asymmetrical loads without an N connection is the two-power meter circuit or Aron circuit. It has the advantage over the three-power meter circuit that it saves one measuring device and that cos phi and reactive power determination is also possible with a symmetrical load. The Aron connection is a star connection where only two currents are measured.Power setup - in the power module, the wiring must be set to 3-phase Aron and the configurations for the specific application must be done.

V-connection

Hardware configuration – the V-connection is a delta connection where only two currents are measured, but fundamentally works with the same principle that the Aron connection.

Power setup - in the power module, the wiring must be set to 3-phase V and the configurations for the specific application must be done.

3-Phase 2-meters connection

Hardware configuration - the 3-phase 2-meters Connection is a delta connection where just two voltages and two currents are measured.

Power setup - in the power module, the wiring must be set to 3-phase 2-meters and the configurations for the specific application must be done.

When measuring inverters or after the inverters care must be taken as there are a few things that have to be considered to ensure that the correct results are yielded with the highest possible accuracy.


High-speed amplifiers

Always use high-speed (HS-HV and HS-LV) amplifiers for measuring inverters. As the voltage is modulated (amplitude or phase) with switching frequencies up to several hundred kilo hertz therefore, it is absolutely necessary to use high-speed amplifiers to get correct results.

Current sensors

AAs there are a lot of higher frequency components and some DC components in measurements of inverters or after inverters. Ensure to always use current sensors with a high bandwidth and are capable of measuring both DC and AC currents. 

The recommended current transducers to use would be DEWESoft zero-flux transducers, flux-gate current clamps or hall-compensated AC current clamps.

Frequency source

It is important to set the current as frequency source for this measurement, because the voltage out of an inverter hasn't got a sinusoidal waveform any more. The inverter has modulated the signal by pulse width or amplitude, so it’s a packet of pulses. In the following image an inverter modulated voltage (orange) is depicted. The current (green) has a sinusoidal waveform and should therefore, be used as the frequency source.

Measurement configuration

The choice of either using a star or a delta connection for measuring after inverters is open and can be freely selected by the user. Please note that an Aron or a V-connection should never be used for this type of measurement. For basic power analyses, a delta connection is a commendable choice, especially if there is no star-point available. But please use caution as the measurement is made between voltages this connection is not always suitable for detailed inverter analyses (analyses of switching pulses).

When measuring with a star point connection a star point adapter should always be used. The artificial star point via the modules doesn’t conform due to differences in impedance's. The active power analysis will not be affected by this, but the analog signals that are displayed will be false. The apparent power and the power factor that this type of measurement will yield will also be false. Using a star connection with a star point adapter will yield very accurate power values as well as the true analog signal which can then be used for a detailed power analysis.

The best way of measuring an inverter is using the star connection with a star point adapter. If this is not possible then opt for the delta configuration.

Motor cable shielded or not shielded

A frequently asked question when measuring an inverter: Is there is a difference when measuring with or without a shielded motor cable?

The answer is, yes, there could be a difference. Due to the high switching frequencies of the inverter there could be leakage currents via the shield of the cable. This leakage current can affect the results in the following ways:

  • Phase shift - it’s possible that a phase shift occurs. Comparative measurements, where a motor cable was measured parallel with and without shielded motor cables, has shown that the phase shift can be more than 15°.
  • Bandwidth damped - with shielded motor cables it is very likely that the signal is damped, especially at higher frequency parts.
  • Higher DC current - with the measurement of the shielded motor cables there is a possibility of a higher DC current occurring.

Therefore, this capacitive leakage can have low-pass characteristics (phase shift, no higher frequencies) and can affect the measurement results

All the most common configuration setups are available in the DEWESoft power module. But especially in E-Mobility there are applications and configurations for 6-, 7-, 9- or even 12 –phase motors needed. There can be various reasons for this such as lowering the voltage level or reducing the stress on the components.

With conventional power analyzers there is no way to do a comprehensive analysis of such motors. But with the modular hardware systems of DEWESoft in combination with its powerful software these types of applications do not present any type of challenge.

For example, a 6-phase motor can be measured by using 6 single-phase power modules and calculating the total power in the power module.

This application should demonstrate how powerful the DEWESoft Power analyzer is. Should there be a special application or any uncertainty on how to measure or set up such a special application please feel free to contact:

 Rupert Schwarz[email protected], +43 676 700 48 47.

In the Dewesoft X software 

the energy can automatically be calculated using the power module. It’s possible to calculate the total energy consumption or it can be split up into positive (energy consumption) and negative energy (energy delivery). This is helpful when measuring electric vehicles with recuperation (energy recovery) or when measuring the load profile of households/industry which have a power generation unit e.g. photovoltaic.

The energy calculation is a simple integration of all power values. To get the positive energy, the positive power values are integrated, for the negative energy the negative power values are integrated. A selection can be made between positive, negative or both.

P is power in Watt [W] and E in energy in Watt hours [Wh].

In the following image the settings for the Energy calculation in the power module is shown.

In addition to the basic power calculations it is also possible to start/reset the energy calculation at trigger events. Otherwise, the energy will be calculated as soon as you switch into measure mode. It is also possible to reset values at start of acquisition or with a channel.

The efficiency of electrical devices can easily be calculated using the Dewesoft X Math library, using the Formula editor. It can be calculated for power or for energy values, during the measurement as well.

Where η is efficiency in [%]

There are applications where the energy is measured at multiple points for instance in electric vehicles that use recuperation of energy. The following Sankey diagram is a representation of the energy flow that would likely be measured with a recuperation electric vehicle.

This course will end with a few tips that can be used if the computing power of the measurement computer seems to be inefficient!

Should the situation arise where the computer that is being used for the measurements have inefficient computing power to calculate all the parameters that are available, there are a few tips that could help with that. 

  • Acquisition rate - the acquisition rate should be set to suit the respective application. Often lowering the acquisition rate doesn't affect the measurement results. With inverter measurements, the sampling rate should be 10 - 20 times that of the switching frequency.
  • Calculation rate in power module - lowering the calculation rate often doesn't influence the power calculation. The benefit of this compared to lowering the acquisition rate is that the analog signal is still available. If the data is stored with the "always fast" option, the power module can be calculated after the measurement using the full sampling rate.
  • Post Processing - the power module is already present (Offline mode) or recording raw data, power modules are subsequently added and calculated (Analyze mode).
  • Downsizing - deselection of options and channels (analog, math, power) that are not needed for the measurement. In the power module a lot of options are very performance intensive, especially the following ones: Harmonic Smoothing Filter, Flicker and Period Values, and in Math especially Filters, Statistics, FFT, Formulas with cosine and sine calculations need a lot of CPU power.


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