6 Discussion and conclusion
Condition monitoring is becoming an increasingly significant factor to improving the reliability of distribution network. However, the lack of cost-effective monitoring solutions for measuring wide range of disturbances especially at MV side restrains the deployment of condition monitor-ing system extensively in the network. Sensor with high voltage insulation is another essential and equally expensive component which increases the cost of monitoring system even further.
This thesis focused on the development of cost-effective wideband instrumentation for real-time measurements of high frequency phenomena in the distribution network. Another important goal was to study the possibility of developing a multi-mode monitoring concept based on few wide-band sensors and a single measuring unit for a wide range of functionalities i.e. PD and PQ mon-itoring, disturbance recording and fault location, which significantly reduces the cost and com-plexity dealing with installation and maintenance. The idea of combining different monitoring functions into a single unit is driven by previous research done at TUT on on-line PD measure-ment, power quality monitoring and fault location.
Secondary substation is an ideal location for data acquisition since both LV and MV networks can be monitored. Consequently, a novel cost-effective wideband secondary substation monitor-ing solution is presented in this work which includes a multichannel monitormonitor-ing system and a monitoring concept to measure various quantities (shown in Table 3.1) at LV and MV side of secondary substation. The monitoring system includes FPGA-based multichannel data acquisition
& processing unit, filter & amplifier unit, wideband HFCT sensors for MV-side measurement and resistive dividers for LV-side measurement. Multichannel data acquisition interface is developed using an 8 channel, 12 bit, 65 MHz A/D converter so that four channels measure at LV side and four at the MV side. Additionally, the monitoring concept describes what monitoring functions can be incorporated into the monitoring system and how data can be utilized to analyze the elec-trical network during normal operation and different disturbance events.
The author has successfully developed a cost-effective instrumentation for monitoring high fre-quency phenomena and a multi-mode monitoring concept which is based on wideband HFCT sensors and a single measuring unit for monitoring wide range of functionalities i.e. PD and PQ monitoring, disturbance recording and fault location. Most novel part of the thesis is the develop-ment of wideband cost-effective HFCT sensors which made the realization of multi-mode moni-toring concept possible. Wideband HFCT sensors not only monitor PD and PQ but also record disturbances and locate earth faults at MV side of secondary substation, which is a novel approach.
They are fundamental units of the wideband secondary substation monitoring solution which
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nificantly reduces the cost of monitoring at MV side of secondary substation. The proposed mon-itoring solution is not fully integrated into a single unit. However, overall concept is tested and verified through prototype systems in the laboratory and in the field which are summarized below.
An intelligent electronic device (IED) is developed for real-time monitoring of high frequency phenomena in the distribution network. The IED provides a versatile environment to study and analyze the behavior of LV network in real-time. Furthermore, a versatile and complete monitor-ing solution composed of FPGA, multichannel A/D converter, filter & amplifier unit and HFCT sensors was developed for on-line PD monitoring. Laboratory measurements using time-domain and frequency-domain analysis have demonstrated that the monitoring system has successfully captured the PD calibrator pulse as well as real PD data from 20 kV feeder. The monitoring system has eight channels but four were used for PD measurements. The design concept from both de-vices can be extended further for monitoring and real-time processing of the data measured at LV and MV side of secondary substation.
Ferrite cores are used to develop cost-effective wideband HFCT sensors. Split core HFCT sensor M1 with 9 turns winding configuration and 0.3 mm air gap seems to be the best candidate for PD and PQ monitoring. A successful field testing of the wideband HFCT sensors (M1) was carried out at 20 kV feeder phase conductors for PD and PQ monitoring. In addition, RMS-based method for detecting earth fault in MV network was demonstrated using the RTDS. HFCT sensors (M1) have showed promising results in detecting PD and high frequency signals from phase conductors without experiencing saturation. Power quality measurements also showed adequate performance compared with the commercial power quality monitor. The performance of wideband HFCT sen-sors (M1) fulfilled the expectations considering the fact that the same sensor can monitor PD and PQ and locate earth faults.
Secondary substation monitoring plays a vital role in improving the power grid reliability and flexibility. Along with the communication infrastructure, it creates the foundation on which var-ious monitoring, control and network automation applications can be built. Through prototype systems, it is demonstrated that the performance of subsystems of the wideband secondary sub-station monitoring solution were adequate in the laboratory as well as in the field. Thus, it is possible to integrate different subsystems to develop a standalone secondary substation monitor-ing solution. The proposed secondary substation monitormonitor-ing concept would fundamentally im-prove the possibilities of predicting and preventing component failures at secondary substations and connected feeders. Information on the voltage level and quality at secondary substations would be useful in network operation (e.g. to increase the hosting capacity of DG) and network planning. Additionally, fault location would also help automatic fault location, isolation and res-toration process.