Power quality

The different power quality parameters describe the deviation of the voltage from its ideal sinusoidal waveform at a certain frequency. These deviations can lead to disturbances, outages or damages of electrical equipment connected to the grid.

The different power quality parameters describe the deviation of the voltage from its ideal sinusoidal waveform at a certain frequency. These deviations can lead to disturbances, outages or damages of electrical equipment connected to the grid. It is essential to permanently track these parameters: starting during the development phase (of the electrical equipment) until live operation: e.g. continuous monitoring of a couple of points in the electrical grid in order to prevent and correct quality disturbances

The Dewesoft Power Analyser is able to measure all of these parameters according to IEC 61000-4-30 Class A. In comparison to other power quality analysers it’s possible to do more detailed analysis (e.g. raw data storing, behavior at faults, calculation of additional parameters etc.).

The purpose of this chapter is to cover all power quality parameters which can be calculated in Dewesoft X.

To learn more about basic Power analysis and measurement, please visit our POWER ANALYSIS course.

Harmonics are integer multiples of the fundamental frequency (e.g. 50 Hz) and cause a distortion in voltage and current of the original waveform. Harmonic voltages and currents caused by non-sinusoidal loads can affect operation and lifetime of electrical equipment and devices. Harmonic frequencies in motors and generators can increase heating (iron & copper losses), can affect torque (pulsating or reduced torque), can create mechanical oscillations and higher audible noise, causes ageing of shaft, insulation and mechanical parts and reduce the efficiency.

Current harmonics in transformers increase copper and stray flux losses. Voltage harmonics increase iron losses. The losses are directly proportional to the frequency and, therefore, higher frequency harmonic components are more important than lower frequency components. Harmonics can also cause vibrations and higher noise. The effects to other electrical equipment and devices are very similar and are mainly reduced efficiency and lifetime, increased heating, malfunction or even unpredictable behaviour.

Dewesoft allows measuring harmonics for voltage, current and additional active and reactive power up to the 3000th order. All calculations are implemented according to IEC 61000-4-7 and can be selected in the power module according to the next picture:

Harmonics

Up to 500 harmonics can be calculated, in addition there is the option to choose all harmonics or just even or odd ones. If there are current channels used in the power module it is also possible to calculate phase angles, P, Q and the impedance.

Number of sidebands

The basic idea of sidebands is that a certain frequency range is considered as one harmonic.

Example: 1 full sideband (equals +/-5Hz) at a frequency of 50 Hz means that a frequency range from 45-55 Hz is considered to be the first harmonic (it's the same for all other harmonics). If you select 2 sidebands the first harmonic will reach from 40 to 60 Hz.

Number of halfbands

The IEC 61000-4-7 (page 22) requires for the grouping of the harmonics sidebands where only the square root of the quadratic half of them should be added. This is required for the lowest and highest line and is defined as halfbands in Dewesoft.

Example I: 1 sideband and 1 halfband at a frequency of 50 Hz mean that a frequency range from 45-55 Hz and the square root of the quadratic half of the 40 Hz to 60 Hz lines are considered to be the first harmonic.

Example II: 2 sidebands and 1 halfband at a frequency of 50 Hz mean that the lines from 40 Hz to 60 Hz have the full amplitude, while the lines at 35 Hz and 65 Hz are only considered with the square root of the quadratic half.

Interharmonics

The interharmonics cover all lines not covered by the Harmonics (see page 26 of IEC 61000-4-7).

Example: 1 sideband and 1 halfband at a frequency of 50 Hz, the first interharmonic is the area between 0 Hz and 45Hz.

Group FFT lines

The higher frequency parts can be grouped in 200 Hz bands up to 150 kHz.

Attention: Please be aware that according to page 29 of IEC 61000-4-7 these groups start at -95 Hz to +100 Hz around the middle frequency.

Example: For 2.1 kHz, lines from 2005 Hz to 2200 Hz are considered to be one group.

Full FFT

This option calculates the full FFT which can be exported to the database and displayed via 2D graph.

Harmonics smoothing filter

This option enables the low pass filter which is required in IEC 61000-4-7 according to page 23.

Background harmonics

With this option it is possible to subtract an existing and known harmonic pattern (magnitude and phase) from measured values. This is a typical application for the commissioning a powerful power converter in order to know the noise of this converter.

You can use this function for the voltage and/or the current, as you can see in the following picture.

All you need to do is to enter the magnitude und phase angle of the harmonic pattern into the following input mask.

Example

The following screens show a certain harmonic pattern measured and then subtracted in Dewesoft X using the background harmonics function.

Voltage, current, active and reactive power, phase angle, impedance, interharmonics and higher frequencies can be displayes in Dewesoft X whether as numeric display, recorder or 2D graph.

There are two possibilities for displaying harmonics. You can choose between:

  • Harmonic FFT
  • 2D graph

With the Harmonic FFT you can display the harmonics of the voltage, the current, power or reactive power. In the picture below you can see the voltage harmonics of a three-phase system.

With the 2D graph you can display voltages and currents of different phases in one graph. In addition, you have a wide range of display options. In the following figure you can see the harmonics of the phase voltage L1.

On the picture below you can see the display options of the 2D graph. There you can choose the graph type (line or histogram) and linear or logarithmic display.

Persistence means that when the harmonics change during a measurement, the changes are displayed blurred, for example like in the following figure:

Higher frequencies

Example: higher frequencies from 2 Hz up to 20 kHz shown in a 2D graph as histogram (application: HVDC converter station).

Interharmonics

Example: interharmonics shown in 2D-Graph as histogram. Peak at 900 Hz which is the switching frequency of a HVDC converter operated in the public grid.

Dewesoft X calculations for each harmonic/whole waveform:

Dewesoft X calculations for a 3 phase system:

The total harmonic distortion (THD) for voltage and current can be calculated up to the 3000th order. In general it is defined as sum of all harmonics to the fundamental.

The most important producers of harmonics are loads which are controlled by converters (diodes, thyristors, transistors). In the following two pictures you find a typical comparison of lamps and the produced harmonics and a representative illustration of different converter technologies and the according THD value.

Fundamental symmetrical components

Normally the electric power system operates in a balanced three-phase sinusoidal steady-state mode. Disturbances, for example a fault or short circuit, lead to a unbalanced condition.

By using the method of the symmetrical components it is possible to transform any unbalanced threephase system into 3 separated sets of balanced three-phase components, the positive, negative and zero sequence.

The main advantage of the symmetrical components is that it makes life (and calculation) much easier. In case of a fault or short circuit the unbalanced system can be easily transformed into symmetrical components, wherewith the calculations can be done straight forward. In the end the results are transformed back into the „real-life“ phase voltages and currents.

In general a 3-phase system can be displayed and described as following:

A balanced 3-phase system may look like this: same RMS-value for all line voltages and currents, and a 120° phase shift between each of them.

In order to explain the basic idea of the symmetrical components, the first step is to define the operator „a“ as unit vector with an phase angle of 120° or 2pi/3.

So the voltages can be described in different ways now:

Calculation of zero-sequence system

In an symmetrical system the following equation is valid:

In a real system this sum is not equal to zero. A voltage difference occurs:

This voltage difference divided through 3 represents the so called zero-sequence system:

The zero-sequence systems for the three phases (u10, u20, u30) have the same aplitude and phase. Therefore, only value for the zero-sequence system "U_0" will be shown.

The calculation of the zero-sequence current is analogue to this procedure.

HINT: If you multiply the currents for the zero-sequences system with 3 (=I_0 x 3) you will get the current of the neutral line U_N.

Calculation of positive-sequence system

The positive sequence system has the same rotating direction as the original system (right). This means it will have the same rotating direction of an electrical machine connected to the grid.

As the values of the positive-sequence system for all three phases have the same amplitude (now they are symmetrical) and an phase shift of exactly 120°, it's enough to show one value. The value for the positive-sequence system in Dewesoft X is called „U_1“.

Calculation of negative-sequence system

The negative sequence system has the opposite rotating direction as the original system (left). This means it will rotate in opposite direction of an electrical machine connected to the grid.

As the values of the negative-sequence system for all three phases have the same amplitude (now they are symmetrical) and an phase shift of exactly 120°, it's enough to show one value. The value for the negative-sequence system in Dewesoft X is called „U_2“.

Matrix of zero, positive and negative-sequence system

According to the following equations the phase voltages and currents are transformed into the symmetrical components. The result are three balanced 3-phase systems, the positive (U 1, I1), negative (U 2, I2) and zero sequence (U0, I0).

NOTE: The basic values of symmetrical components (U_0, U_1, U_2, I_0, I_1, I_2) are calculated for each harmonic and added up geometrically.

As you can see in the next pictures, a unbalanced system can be composed by using the positive, negative and zero symmetrical components. The following picture shows an screenshot of Dewesoft X showing the real system:

The following picture shows an screenshot of Dewesoft X showing the three systems (positive, negative and zero) of the symmetrical components:

This screen is provided by Kurt Stranner (KNG Netz GmbH).

Out of the parameters of the symmetrical components (positive-, negative- zero- sequence) the original system can easily be rebuild:

The following variables are calculated in Dewesoft X and shows the components of the zero- and negative-sequence system compared to the positive-sequence system (for total and fundamental harmonic).

Extended positive sequence parameters (according to IEC 614000)

The following calculations are based on Annex C of IEC 61400-21.

Based on the measured phase voltages and currents, the fundamental's Fourier coefficients are calculated over one fundamental cycle T as first step.

It is important to mention that the index a stands for the line voltage L1. The coefficients for L2 (ub) and L 3 (uc) as well as the coeffiecients for the currents (ia, ib, ic) are calculated exactly the same. Furthermore f 1 is the frequency of the fundamental. The RMS value of the fundamental line voltage is:

Extended negative sequence parameters (according to IEC 614000)

Extended zero sequence parameters (according to IEC 614000)

Flicker is a term for the fluctuations (repeated variations) of voltage. Flashing light bulbs are indicators for a high flicker exposure. Flicker is especially present at grids with a low short-circuit resistance and is caused by the frequent connection and disconnection (e.g. heat pumps, rolling mills, etc.) of loads which affects the voltage. A high level of flicker is perceived as psychologically irritating and can be harmful to people.

The Dewesoft power analyser allows to measure all flicker parameters according to IEC 61000-4-15. The flicker emission calculation is implemented according to IEC 61400-21 and allows the evaluation of flicker emission in to the grid caused by wind power plants or other generation units.

The flickermeter architecture is described by the block diagram of the next figure. It is divided into two parts, simulation of the response of the lamp-eye-brain chain und the on-line statistical analysis of the flicker signal leading to the known parameters.

Block 1

This block contains a voltage adapting circuit that scales the input mains frequency voltage to an internal reference level. This method permits flicker measurements to be made, independently of the actual input carrier voltage level and may be expressed as a per cent ratio.

Block 2

The purpose of the second block is to recover the voltage fluctuation by squaring the input voltage scaled to the reference leve, thus simulating the behaviour of a lamp.

Block 3

This block is composed of a cascade of two filters, which can precede or follow the selective filter circuit. The first low-pass filter eliminates the double mains frequency ripple components of the demodulator ouptut.

The high pass filter can be used to eliminate any DC voltage component. The second filter is a weighting filter block that simulates the frequency response of the human visual system to sinusoidal voltage fluctuations of a coiled filament gas-filled lamp (60W/230V and/or 60W/120V).

Block 4

Is composed of a squaring multiplier and a first order low-pass filter. The human flicker perception, by the eye and brain combination, to voltage fluctuations applied to the reference lamp, is simulated by the combined non-linear response of blocks 2,3 and 4.

Block 5

The last block of the whole chain performs an on-line analysis of the flicker level, thus allowing direct calculation of significant evaluation parameters.

In the next figure you can see an example of a rectangular voltage flicker.

With Dewesoft X you can calculate the Pst and Plt value according to the IEC norm with a calculation time of 10 minutes respectively 120 minutes, though it is also possible to adapt the calculation time to your needs, set a calculation overlap and filter.

Flicker emission

The flicker emission (also called current flicker) calculates the proportion of the flicker, which is added to the grid by a producer or a consumer. In addition the internal voltage drop is calculated by the grid impedance of the current flow.

The voltage drop is added to an idealized voltage source vectorial (U = Usim + R*I + L * di/dt). Using the flicker algorithm and the new voltage the current flicker values are calculated.

Enable "Flicker" and "Current Flicker" and input the grid parameters. You can type in the short circuit power or the impedance of the grid. The phase is the impedance angle of the grid. You can also type in a number of different phase angles (e.g. 30;50;70;85).

NOTE: In the power module the nominal voltage must be set. This value is also the value for the idealized voltage source!

Now let's take a look at a flicker measurement with the settings from above. The following parameters are calculated with Dewesoft X:

The rapid voltage changes are parameters which are added as a supplement to the flicker standard. Rapid voltage changes describe all voltage changes which are changing the voltage for more than 3% at a certain time interval. These voltage changes can afterwards be analysed with different parameters (depth of voltage change, duration, steady state deviation, etc.).

The rapid voltage changes (RVC) are a special calculation in Dewesoft X which allows to calculate the maximal voltage drop (d max), the stationary deviation after the voltage drop (dc) and the time where the voltage drop is below 3,3% of U n. All values are calculated according to the IEC 61000-4-15. Analysis can be done for example for IEC 61000-3-3 and IEC 61000-3-11. The following picture shows the calculated parameters (IEC 61000-4-15 page 35).

Measurements with Dewesoft X

Let's take a look at the rapid voltage changes with Dewesoft X:

Steady state duration is defined in seconds.

Hysteresis is the condition for the stationary deviation (du_dc), see IEC 61000-4-15 page 8.

EXAMPLE: If you define a hysteresis of a 0,2% and a steady state duration of 1s, the stationary condition is reached if the voltage doesn't leave +- 0,2% for 1 second.

NOTE: The rapid voltage changes values (du_max, du_duration, du_dc) are calculated out of the defined settings for period values (number of periods and overlap). Take care of right settings for analysis according to related standards (½ period values for RVC determination according to IEC61000-4-15)

Dewesoft has created a special FFT algorithm (software PLL) to determine the periodic time (frequency). The algorithm determines the periodic time of the signal via a special FFT algorithm at a sampling window of multiple periods (typically 10 periods, definable in power module). The calculated frequency is highly accurate (mHz) and works for every application (motor, inverter, grid, …).

How Dewesoft X calculates the power of an AC system

While other power analysers calculate the power in the time domain, in Dewesoft X it is calculated in the frequency domain. With the before determined 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. Out of this FFT analysis, we get an amplitude for the voltage, current and cos phi for each harmonic. One major benefit of this FFT transformation is that we can now correct the behaviour of amplifiers, current or voltage transducers in amplitude and phase for the whole frequency range (using the Sensor XML). 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:

Power grid

  • fault and transient recording
  • power quality analysis (IEEE 1159, EN50160)

Transformer

  • efficiency analysis (IEC 60076-1)
  • no-load and short circuit testing
  • vibration, noise

Wind, solar and CHP

  • power performance (IEC 61400-12)
  • power quality (IEC 61400-21 / FGW-TR3)
  • active and reactive power (FGW-TR3)
  • behaviour at faults (FGW-TR3)

Nuclear power plant

  • turbine and generator
  • testing rod drop
  • castor testing

Turbine and generator

  • modal analysis
  • order tracking
  • balancing
  • rotational vibration
  • efficiency measurement

Smart grid and energy management

  • power system testing
  • load profile
  • demand side management

Aircraft

  • power system testing
  • fault and transient recording
  • hybrid testing
  • harmonics analysis

Marine

  • power system testing
  • fault and transient recording
  • hybrid testing

Railway

  • power system testing (AC and DC rails)
  • power quality analysis
  • fault and transient recording
  • short-circuit analysis
  • pantograph and current shoe testing

E-mobility

  • electric two wheeler
  • electric vehicle
  • hybrid vehicle (series and parallel)
  • hydrogen vehicle

Equipment testing

  • fans and pumps testing
  • circuit breaker testing
  • filter analysis
  • harmonics analysis according to IEC 61000-3-2/-12
  • voltage changes according to IEC 61000-3-3/-11
  • CE conformity of electrical devices (harmonics, flicker) and a lot more

To learn more about POWER APPLICATION please download our POWER BROCHURE

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