elcome dear friends of electrical engineering. In our brand-new guest article Roland Bürger (MBS AG) and Jürgen Blum (A Eberle GmbH) explain the most important issues if it comes to PQ measurements in low voltage grids. We hand over:
Changes to the structure of generation and consumption
Over the last few years, the proportion of renewable energy in Germany has grown massively. Wind, biomass, photovoltaic and hydroelectric plants now make up approximately 30% of the country’s energy mix.
Unlike in conventional nuclear or coal-fired power stations, where all synchronous generators are used to produce electricity, here inverters or frequency converters are used. As such, it is not always possible to achieve a clean sine wave.
The distortions are caused by the switching semiconductor elements in the inverter. Harmonics generated in this way are whole multiples of the first harmonic and can extend far into the single-digit kilohertz range. The total harmonic distortion (THD) factor specifies the undesirable distortion ratio of the 50 Hz sinusoidal oscillation and regularly reaches between 10 and 30%.
In addition to the harmonics produced by inverters on the generator side, there have also been changes on the consumer side in recent years. Non-linear consumers such as LED or energy-saving lamps are pushing linear ones, like traditional incandescent bulbs, out of our daily lives almost completely.
Plug-in power supply units for mobile phones and laptops are no longer made from small transformers either, but from semiconductor circuits known as switched-mode power supplies. It would not be possible to create such small, light power supply units any other way. But these benefits are set against one big disadvantage: the current is drawn from the public grid not as a sinusoidal waveform, but in pulses. The figure below illustrates this:
The filter capacitor shown in the diagram not only smooths the required output voltage, it is also recharged in pulses by the rectifier diodes. These steep current peaks generate reactive power on the one hand, and harmonics on the other.
Standards regulate limit values – but not always!
There is already a corresponding set of international norms that limits harmonic currents in end devices with a power consumption > 75 W. Devices under 75 W are not currently covered by standards. In the interests of keeping costs down, manufacturers do not usually implement filter measures or complex power factor correction. The EN 61000-3-2 set of standards does not come into play until the 25 W mark for lamps either; for example, where energy-saving lamps are concerned, THDI values of 30 to 70 % and higher are not uncommon during warm-up and in continuous duty. It should also be noted that, even when they do kick in, the standards only define limit values up to 2 kHz. As a result, manufacturers have hardly taken interference suppression into account at all when developing electronic products for the frequency range > 2 kHz in the past.
In addition, more and more electrical motors with variable-frequency drive technology are being used in the industrial sector. Today already, the percentage of electrical motors sold that have a frequency-controlled drive stands at around 40 %. The majority of these motors utilise pulse width modulation technology, which can generate THDI values in the range from 100 to 120 %. Clean sine waves are almost impossible to identify at these values.
Power electronics have so many benefits that we can categorically state there will be no return to linear consumers such as the traditional incandescent bulb. In fact, we can expect harmonic loads to increase even further in European grids, due to the development of alternative sources of energy and the growth of non-linear consumers. We should also bear in mind that having lots of consumers that are not regulated by standards could cause considerable interference overall.
Filter systems have already had to be installed in office buildings where just computers, telephone systems and energy-efficient bulbs are used, in order to bring problems with harmonics under control.
Effects of harmonics
Grid operators are primarily interested in the economic effects of harmonics. When it comes to harmonic currents, the most important phenomena are as follows :
🌐 Overloading of neutral conductors
🌐 Disturbing noise development (in the frequency range up to 16kHz for the human ear)
🌐 Overheating of transformers
🌐 False tripping of circuit breakers/miniature circuit breakers
🌐 Overstressing of power-factor correction capacitors
🌐 Skin effects
The most important point, however, is that the current harmonics caused by non-linear loads have a lasting effect on the voltage quality. There is a risk that the product quality of the supply voltage will no longer comply with the required EN 50160. Here voltage harmonics are defined in the amplitude and as THDU (max. 8%). If the utility company does not comply with this product standard, the electricity supply contract will ultimately be violated, which in the majority of cases refers to EN 50160. The figure below shows that the voltage quality in a supply system can only be negatively influenced by customers with non-linear loads.
The limit value of the distortion factor (THDU) of 8% defined in EN 50160 is relatively generous in an international comparison. The American counterpart provides significantly lower limit values for critical infrastructure such as hospitals and airports. This is not entirely unjustified. If the distortion level in the supply voltage reaches a value> 10%, the lifetime of the equipment is reduced considerably. The shortening is estimated at the following values:
🌐 32.5% for 1-phase machines
🌐 18% for 3-phase machines
🌐 5% for transformers.
To maintain the lifetime expected from the nominal load, the devices named above must be over-dimensioned. But even then, disturbances cannot be ruled out.
Standard regulation for distribution network operators
The actual standard VDE-AR-N 4100 deals with this matter. Point 126.96.36.199 of this regulation refers to harmonic currents of up to 9 kHz that need to be monitored and covers not only generating plants, but also energy consumers and storage systems. The customer should liaise with the grid operator and take action to reduce harmonic currents – particularly by constructing filter circuits. In future we can assume, therefore, that current measurements up to 9 kHz will be taken continuously across the whole low-voltage network.
Looking at the overall picture of the rise in distributed energy generation plants and non-linear consumers, we can see this is a very sensible move. Grid operators and their customers will need measuring equipment that can accurately record harmonic currents of up to 9 kHz.
Current transformers up to 20 kHz
MBS AG offers the full series of X-type current transformers for measurements up to 20 kHz. These products guarantee high-precision transmission up to 20 kHz on the one hand, and are designed to withstand the thermal demands of running in networks subject to harmonics on the other.
Output signals are 1 or 5 A, just like with the familiar inductive current transformer to IEC 61869-2. Performance data corresponds to standard values too. As a result, these transformers can also be used in conventional 50 Hz applications. An additional rating plate defines the frequency transmission behaviour.
Power Quality Analyzer up to 9 kHz
When choosing the power quality analyzers, the stationary PQI-DE and PQI-DA smart devices from A. Eberle GmbH in Nuremberg, which can perform almost all necessary measurement tasks in electrical networks, have proven their worth.
The devices can be used both as a power quality interface in accordance with network quality standards such as IEC 61000-2-2 / EN 50160 or for checking technical connection guidelines such as DIN VDE AR 4110 and DIN VDE 4120 and much more, be used. Thanks to the available open SCADA standard interfaces such as Modbus RTU / TCP and IEC 61850, the devices can also be used as a high-precision measuring transducer for all physically defined measured variables in three-phase networks in parallel for the seamless recording of measured values over a very long period of time.
In addition to the option of standard evaluations, the devices also have a high-speed fault recorder with a recording rate of 40.96 kHz, as well as a 10ms RMS rms value recorder. This makes a detailed evaluation of network disturbances possible.
The network analyzers record frequencies up to 20 kHz for 4 x voltage and 5 x current and meet the standard evaluation according to IEC61000-4-7 for the frequency range 2 kHz to 9 kHz, which is part of DIN VDE AR 4100.
The voltage is measured directly in the low voltage with the devices. The harmonic levels of current and voltage up to 20 kHz can be mapped in parallel.
Residual current monitoring
The PQI-DE is equipped with a fifth current input for continuous monitoring of residual currents (Residual Current Monitoring - RCM). It is possible to freely program response thresholds for alarm messages or warnings.
Devices from MBS AG are also used here. All differential current sensors DACT and KBU XX D can be used up to 40 kHz without any problems.
Network phenomena such as harmonics, which were previously only detected in laboratory applications, have finally arrived in the supply networks. New problems have arisen. Simple 50 Hz measurements are often no longer sufficient to maintain full control over the networks.
Roland Bürger (MBS AG)
Juergen Blum (A. Eberle GmbH)
1 The THD is the ratio of the harmonic component to the first harmonic
2 Schneider Electric Wiki (accessed 21/08/2020) https://www.electrical-installation.org/enwiki/Effects_of_harmonics_-_Economic_impact