Pertti Pakonen

Dielectric properties and partial discharge endurance of polypropylene-silica nanocomposite

This paper presents the results of the dielectric properties and partial discharge endurance measurements conducted on polypropylene (PP)-silica nanocomposite. The material compounds were analyzed with micro-Raman spectroscopy, X-ray tomography and transmission electron microscopy (TEM). ac and dc breakdown strength of the materials was measured. Dielectric response, capacitance and loss factor of the film samples were measured as a function of temperature and frequency. Partial discharge (PD) endurance of the reference PP and PP Silica nanocomposite was studied as a function of ac voltage. Material surfaces were analyzed after PD stress with optical microscopy. All dielectric measurements were done for oriented thin films with a thickness of 11-23 μm. The results were analyzed statistically to determine the effects of the additive on the properties of PP. The paper discusses the potential of PP Silica nanocomposite with a view to high voltage applications, especially power capacitors.

Characteristics of partial discharges caused by trees in contact with covered conductor lines

Characteristics of partial discharges (PD) caused by trees in contact with covered conductor (CC) lines have been studied. A tree in contact with a CC distorts the electric field around the conductor and causes partial discharges. According to tests conducted in a high voltage laboratory the rise time of the PD current pulses caused by the trees is relatively long, in the order of 60 ns...0.8 mus. The fault impedance and PD magnitude caused by a tree in contact with a CC and the propagation attenuation of PD pulses was studied in different seasonal conditions in September, November and March on a full scale 110 kV CC trial line. In case of an intact CC the fault impedance is several MOmega. After breakdown of the conductor covering the fault impedance collapses in summer to tens and in winter to hundreds of kOmega. The average PD magnitude measured in March was approximately one fifth of that in September. On a 9.3 km line portion the observed propagation attenuation of the PD signal was 0.37...3.7 dB. The variation in the attenuation is attributed to the variation in the rise times of the PD pulses and the modal composition of the PD signal. The results indicate that PD measurements can be used to detect trees fallen on CC lines.

Interference of touch dimmer lamps due to PLC and other high frequency signals

The interference of touch dimmer lamps due to PLC signals has found to be a common interference problem in Finland after the installation of smart meters that use PLC. One reason for this is the lack of equipment immunity standards for voltage frequencies below 150 kHz. This paper will study controlled discontinuous high frequency signal burst effect to two touch dimmer lamps in the laboratory environment. Both lamps were disturbed due to these signals. Based on the results it is possible that interference problems can occur even if the distance between lamp and the PLC device is close to 100 m. Paper will also show the measurement results on how one plug-in type filter improve interference situation. Results show that the amplitude of the signal burst will decrease at lamp side but filter also affects signal levels at mains side.

Experiences on narrowband powerline communication of automated meter reading systems in Finland

According to Finnish legislation, at least 80 % of the energy meters have to be remotely readable and provide hourly based energy data by the end of 2013. In April 2011 Tampere University of Technology (TUT) in co-operation with Finnish Energy Industry conducted a questionnaire to Finnish distribution network operators (DNOs) to map the installation status of the remotely readable meters, the communication technologies used in the meters and the interference problems experienced so far, related to, especially, PLC (power line communication) systems. The questionnaire dealt with both the interference caused by customer apparatus to PLC and vice versa. On-site measurements to study common customer apparatus disturbing the PLC were also conducted in 2012 in the networks of a few DNOs. Most common disturbance sources were frequency converters and switch mode power supplies. The measurements indicate that in addition to the lack of emission standards below 150 kHz the aging of customer equipment may be a relevant cause of interference problems.

Novel inductive sensor solutions for on-line partial discharge and power quality monitoring

Networks of medium voltage have spread over a long distance through underground cables. The aim to prolong the life span of existing network assets increases the need for condition monitoring to prevent unplanned and long lasting interruptions. Continuous on-line partial discharge (PD) measurement is an excellent way to determine the overall health of the medium voltage (MV) components and to detect developing faults in underground cables. A sensor is an essential part of the PD monitoring system which measures the high frequency PD signals. PD sensors can be used to measure power frequency current and harmonic currents on the MV side to estimate the thermal loading of transformer or incoming cable provided that the sensors have a suitable frequency response for measuring low frequency signals as well. A novel inductive sensor is described in this paper which allows both PD measurements as well as power quality (PQ) measurements at frequency range below 2.5 kHz. Authors did experiments on different ferrite cores in order to design the best possible sensor which can be used for both PD and PQ measurement. Characteristics of the sensors, including sensitivity, saturation current, and frequency bandwidth, as well as relative errors are provided and analyzed under laboratory conditions. At the end, developed sensors are compared with a commercial HFCT sensor, the Rogowski coil and power quality current sensors to show the capability of the sensors to be used for PD and PQ measurements.

Grid-connected PV power plant induced power quality problems — Experimental evidence

This paper presents new findings on phenomena contributing to flicker and voltage variations caused by grid-connected photovoltaic (PV) inverters. The voltage variations caused by two different 6 kW single-phase grid-connected PV inverters were studied during climatic variations by varying their grid-coupling impedance. Two different methods for characterizing the PV-plant induced voltage variations were studied: the short-term flicker index (Pst) and the 10 minute very-short voltage variation value (VSV). The results clearly indicate that PV inverter power fluctuations induced by cloud shading and enhancement have a significant effect on the VSV value, but not on Pst. PV inverters have a clear effect on the Pst as well, but the main contributors are related to the inverter design rather than the power fluctuations caused by clouds. The main contributor in the elevated Pst values could be traced back to the poor design of the maximum power point tracking (MPPT) of the inverters. The MPPT caused subharmonic current variations at a frequency of approximately 8 Hz which is close to the most sensitive frequency of human eye. Another factor causing rapid voltage variations in low irradiance conditions was the current transients related to the inverter start-up and shut-down. Harmonic current distortion is also a potential PV inverter related power quality (PQ) issue. This study indicates that although the current total harmonic distortion (THD) may be very large at low power levels the total demand distortion (TDD) of the PV inverters is almost constant regardless of the output power and the harmonic current had only a very limited effect on the voltage quality even at the weakest network having a short-circuit current of Isc=250 A. Thus, voltage variations caused by the PV inverters were the main PQ issue in the studied networks. The investigations also clearly show that a part of the power quality problems found in the PV plants are caused by the poor design of the PV inverters.