Quantifying Middle Frequency Transient Currents in Power Consuming Devices

Sammanfattning: The complexity of electrical systems present in vehicles is continuously rising and the number of interconnected electrical devices is growing during the strive for optimization and reaching higher efficiency levels. Actuation of said devices have the possibility of injecting voltage and current harmonics into the system, thus negatively affecting neighbouring devices. A measurement method for detecting the transient currents originating from such devices was therefore desirable. Focus was placed on current transients with frequency components within the range 1 kHz to 1 MHz. The report presents theory regarding current sensing techniques, where sensors based on the Rogowski coil were chosen for further implementation. Signal analysis theory included choice of sampling frequency and the FFT approach. Different data acquisition hardware were considered for usage with the current sensors, and evaluated both in field and a laboratory environment. Compiled application notes for the end-user of the measurement method are presented. Laboratory tests showed that Rogowski sensors were capable of measuring the transient currents with the desired spectral resolution. Caution should however be taken when measuring and interpreting low frequency components due to the droop effect. Among the evaluated Rogowski sensors, the sensor type 30LFB was found to be best fitted for measuring current transients originating from electrical loads in a vehicle. Different power cable set-ups can act on the system with low-pass characteristic. Field tests showed that the energy demand of an electrical unit is not solely supplied by the battery but also neighbouring devices. Most prominently were a 40 kHz component that could be seen propagating from the electronic clutch actuator to other loads. However, the majority of the currents’ frequency components with considerable amplitudes were located outside the delimited spectrum. It was concluded that sampling hardware have the possibility of injecting the quantified waveform with frequency components that are related to the sampling frequency.