Characterization Power Converters for Low and Medium Voltage

In our Multi-Megawatt Lab, we have unique equipment for testing power electronic devices and systems up to the multi-megawatt range. With an exclusive connection to the 110 kV high-voltage grid and our own medium-voltage grid (available exclusively for the lab), we have ideal conditions for developing and improving measurement procedures and researching new characterization methods for power electronics.

Our R&D-Services in the field of »Characterization Power Converters for Low and Medium Voltage« include:

Impedance Spectroscopy

Prinzip der Impedanzspektroskopie
© Fraunhofer ISE
Priniciple of impedence spectroscopy. In addition to the 50 Hz components, the device under test, pictured as a Thévenin equivalent, is excited by a high-frequency voltage.
Beispielhafter Verlauf der Thévenin-Impedanz
© Fraunhofer ISE
Example of the course of the Thévenin impedance of a PV inverter in the frequency range up to 10 kHz.

The differential impedance spectroscopy method can be used to determine the frequency-dependent impedance as well as the internal harmonic voltage sources of an inverter. For this purpose, the test object (converter) is excited with a small-signal voltage with variable frequency during operation. The resulting current is measured and transformed into the frequency domain. For each excited frequency, a Thévenin equivalent (voltage source with internal impedance) can be calculated. The impedance may show dependencies on the operating point, the switching frequency, and the control parameters. Using the resulting impedance curve of an inverter determined in this way, it is possible now to determine the inverter behavior at an individual grid connection point, thus identifying possible resonances and unacceptably high harmonic currents. The impedance also allows the damping characteristics of high-frequency signals to be estimated. The impedance curve is also increasingly being requested by grid operators for the grid connection of power plants. (The internal harmonic voltage sources are independent of the grid connection point. They can be used as a measure of harmonic emissions.

We offer impedance spectroscopy for power converters with a capacity of up to 1 MVA and rated voltages of up to 1100 V. For this purpose, we have a highly dynamic AC amplifier with a frequency range of up to 20 kHz.

Measurement of Harmonic Emissions

Oberschwingungsemission und Grenzwerte eines Wechselrichters
© Fraunhofer ISE
Harmonic emission of an inverter and limits values at a specific connection point.
Hochdynamische Stromsensoren
© Fraunhofer ISE
Highly dynamic current sensors for harmonic emission measurements of inverters up to the megawatt range.

In current grid-connection guidelines, the limits for the emission of harmonic currents are defined. In our Multi-Megawatt Lab, high-precision measurement devices are available for measurement of the harmonic emission of power converters up to a power of several megawatts.

In addition, we investigate the propagation of harmonics in the grid, across the grid levels. In the laboratory, we have highly dynamic measurement equipment for low, medium, and high voltage levels and can perform time-synchronized measurements across grid levels of the interaction of multiple generators can also be studied in the laboratory or in field measurements. A measurement system with up to 96 measurement channels is available for this analysis, which also allows synchronized measurements at multiple locations in the field.

In addition to the analysis of harmonic emissions on the power quality, we optimize the hardware and software of power converters to achieve low harmonic emissions as well as to minimize resonances. Innovative control methods are developed to reduce the harmonic distortion of the mains voltage. Active filters or grid-forming control methods are used to provide harmonic currents for non-linear loads in the network and thus improve the voltage quality.

Fault-Ride-Through (FRT)

Stationäre FRT-Anlage für Prüflinge bis 10 MVA
© Fraunhofer ISE
Stationary FRT test facility for devices under test up to 10 MVA.

The behavior of conventional and renewable generators as well as storage systems during short-term grid disturbances, or Fault Ride Through (FRT), is an important issue for grid-integration.

A distinction is made between different types of faults. On the one hand, there are undervoltage faults, or Under Voltage Ride Through (UVRT) events, which typically occur due to short circuit occurrences in the power grid. On the other hand, voltage rises, or Over Voltage Ride Through (OVRT) events, can occur, for example when large loads or generators are tripping. For this reason, the various international feed-in guidelines stipulate that grid-connected converter systems behave in a grid-stabilizing manner during such voltage faults and provide voltage-supporting reactive current.

Besides this, an imbalance of active power in the grid leads to grid frequency changes. In these cases, generators and storage facilities must adapt the active power fed to the grid in order to stabilize the grid frequency. The plants, which can adjust their output power during so-called Limited Frequency Sensitive Mode (LFSM) events, reduce their output power at over-frequency or increase it at under-frequency.

In order to be able to investigate and verify such behaviors, the Multi-Megawatt Lab has test facilities for test objects with powers of up to 10 MVA (FRT) or 1 MVA (LFSM).

Efficiency Measurement

Large climatic chamber designed for multi-megawatt class central inverters.

The following facilities are available for measuring performance and efficiency under various climatic conditions: 

1.       Unidirectional DC sources for the measurement of solar inverters up 1 MW, especially for                inverters with multiple MPP trackers

2.       Bidirectional DC source for the measurement of battery converters up to 1 MW

3.       Bidirectional AC source up to 1 MW for increased measurement accuracy of efficiencies                  compared to direct operation on transformer

4.       6 MW DC source will also be available in the near future


Using our own software tools, we achieve a high degree of automation in efficiency measurements. The customer benefits through faster measurement times and thus lower costs. The TestLab Power Electronics is accredited for measuring the efficiency standards DIN EN 50530 and DIN EN 61683.

Closely related to efficiency measurements are the investigations on the derating behavior of an inverter. Our climatic chamber allows such tests to be carried out between temperatures from -30°C to + 80°C at almost any humidity. The particularly large design of the climate chamber also enables the measurement of large central inverters of the multi-megawatt class.

Troubleshooting in PV Power Plants

Mobile Prüfgeräte bei Messungen im Feld.
© Fraunhofer ISE
Mobile test equipment during measurements in the field.

Due to their complexity and particularly challenging operating and environmental conditions, inverters have been among the most frequently failing components in PV power plants and as a result can cause additional costs due to maintenance and downtime. Occasionally, problems also occur in large plants that cannot be explained and solved by a single component, but only by the interaction of many components. This concerns e.g. subsynchronous oscillations or system resonances.

We have many years of experience in solving problems in PV systems and offer a multi-stage procedure for this purpose. A quick data analysis may already be able to narrow down sources of errors. For a detailed examination in the field, we have mobile and high-resolution measuring equipment for low voltage up to 1500 V and for currents up to 5000 A at our disposal. If required, a mobile fault ride-through test container (FRT) can also be applied for power ratings up to 4.5 MVA. If a control-engineering solution is required, we can model all relevant components of the large-scale plant and associated grid connection points and develop suitable algorithms. For detailed investigations of individual inverters by our experienced staff, our multi-megawatt laboratory provides the appropriate test environment. In addition, we also offer functional tests of DC arc detectors.

Quick Analysis for PV Power Plants

The reasons for yield losses and faults in PV parks can be complex, e.g.,

  • PV module and inverter degradation
  • faulty planning or execution
  • faulty devices (e.g., inverters, grid protection, ICT)
  • grid interactions

Fraunhofer ISE detects the causes of these problems und provides suitable solutions. As a start, we offer a lean preliminary analysis that focuses on typical problems, without obligation.

For a preliminary analysis we require the following data:

  • plant information (location, tilt, azimuth, …)
  • measurement data from monitoring (P_AC, U_DC, I_DC, …)

Our digital decision path allows us to check the data for typical problems without much effort. In this analysis typical problems with modules, inverters, system design, etc. are considered. Based on the results of this preliminary analysis, we offer fast advice on how to solve the problem quickly, or, if necessary, we suggest further analysis of the problems by our inhouse experts.

We have developed a lean problem-solving process that allows us to narrow down the technical problem at an early stage and offer you professional advice. Our knowledge covers the entire PV system, including the cell and module technology, power plant design, inverter technology, and grid behavior.

Fill out web form and get troubleshooting analysis

© / Adam Smigielski
Yield losses? Inverter failures? System damage? We have transferred our experience from our research and from industry collaboration into an analysis tool that allows us to automatically identify typical problems in a data set.

High-resolution Measurements of Medium Voltage Systems Up To 20 MVA

© Fraunhofer ISE
Medium voltage connection terminal at the Multi-Megawatt Lab.

Our 20 kV laboratory network is fed from the 110 kV network via our own 40 MVA transformer, which allows us to test and measure systems, converters, and components without affecting third parties. Our focus is on customer-specific tests and investigations of system components up to 36 kV. In our large-scale medium-voltage test hall with direct connection to our own 20 kV grid, we can measure any kind of medium-voltage components. Converters, transformers, etc. up to the size of a 40-foot container can be tested here. Our strength here is customer-specific measurements.

Our equipment includes:

  • Direct MV connection to 20 kV / 20 MVA
  • 3-36 kV / 2.5 MVA laboratory transformer
  • 3-12 kV / 150 kVA three-phase variable transformer
  • 15 kV (16.7 Hz) / 200 kVA railroad transformer
  • 1 MVA highly dynamic grid simulator
  • 7-40 kV / 600 kW DC source
  • 1 MW / 20 kV AC resistive load
  • High-precision power analyzer up to 36 kV / 5 kA
  • Galvanically isolated small signal measurements on high potential
  • LVRT and HVRT tests for converters with up to 10 MVA rated power

You are welcome to contact us, if you wish to perform a special measurement and you lack the equipment or the correct connection power.

More Information on this Research Topic

Accredited Laboratories

TestLab Power Electronics

The TestLab Power Electronics is accredited according to DIN EN ISO/IEC 17025 to perform studies on inverters and other components up to the megawatt range.

Research Project


Stable Power System Behavior with Converter-based Generation.

Research Project


Stable regenerative power supply optimized for parallel operation of inverters.

Data and facts

Energy Charts

Interactive graphics on electricity production and exchange electricity prices.

Research Project


Over Voltage Ride Through – Temporary Overvoltages and Derived Rules for Efficient and Safe Grid Operation