8-channel oscilloscope aids testing of high-power drives

The company, part of the Knorr-Bremse Group, develops and manufactures modular inverter systems for power ratings from 5kW to 7MW. The smaller converter systems of PCS are mainly used in rail vehicles for on-board power supplies, whereas the larger systems are used in test benches and to compensate for the voltage and frequency variations encountered in power supply distribution systems and wind turbine generators.

In test-bench applications, the inverters are capable of controlling electric motors used for driving or braking when testing engines and other components of the powertrains of motor vehicles. For example, it is possible to use a PCS-powered drive to replicate torque surges, which typically occur with every ignition in a combustion engine. Therefore it is a useful setup for tests without requiring the original engine. Another field of application is partial discharge measurement or the determination of various physical and electrical parameters of very large transformers.

These test benches are usually constructed by specialised system integrators for whom PCS provides the appropriate inverter adapted to the specific application.

PCS inverters are also used in wind turbines to minimise the problems caused by short voltage dips on the mains and load shedding when the wind turbine shuts down. This is a problem on large wind farms because the network is suddenly left without several megawatts of generation capacity. These problems can be avoided by using a modern power converter in parallel, and PCS inverters are now being used to upgrade earlier wind turbine systems accordingly. A typical PCS system uses an AC mains voltage input and a DC link voltage (up to 1000V), which in turn is converted to an AC voltage output. The advantage of this approach is that the frequency and amplitude of the output voltage correspond exactly to the requirements of the particular application. This allows optimum motor control with high dynamics. The principle can also be reversed so that, for example, the fluctuating output voltage of a generator can feed directly into the grid.

The PCS inverters are modular and can be simply scaled in terms of size and performance to the particular application. Each module incorporates several IGBTs connected in parallel to achieve the required high currents. Modules can be combined in compact configurations: something that is particularly important for wind turbines, as there is generally little space available and the inverters may have to be installed in the nacelle.

Measurements and tests

Before a new inverter can go into production, numerous measurements and tests are required. In the development department at PCS, a Yokogawa DLM4000 mixed signal oscilloscope is used to carry out these tests. This instrument was selected because of its eight channels, each with a sampling rate of 1.25GS/s. The eighth input channel can also be used as an 8-bit digital input, optionally expandable with an additional 16 bits.

Christian Neisse, Head of Development & Construction Industry at PCS says: "The Yokogawa DLM4000 is used to measure currents of more than 1000 amperes and voltages up to just over 1000 volts. For these measurements, we use Rogowski coils and six high-voltage probes. The Rogowski coils are used for evaluating the current waveform and the time profiles and for locating EMC problems. These can be simply inserted round a cable, so there is no need to disconnect the circuit, which with the thick lines used would be pretty expensive."

Mirko Grimberg, developer at PCS adds: "Because these are 3-phase systems, we usually have to measure three voltages and three currents simultaneously, as well as monitoring the ignition signals of the IGBTs. We therefore needed an oscilloscope with eight analogue channels and some digital inputs. What was also important in this context was the high bandwidth to allow us to look at and record the fast transient signals."

A further benefit was the sophisticated triggering options on the DLM4000, which allowed the PCS team to record and produce a statistical record of intermittent anomalies. Mirko Grimberg says: "We are also currently investigating the harmonics. To this end, we measure the fundamental frequency of 50Hz and the harmonics up to 1MHz. An FFT analysis is performed for pre-compliance testing, and the data is then processed with Matlab software for further calculations. With a four-channel oscilloscope these measurements would be much more time-consuming and complex".

For further information about the DLM4000 8-channel mixed-signal oscilloscope visit tmi.yokogawa.com.