With the growing importance of underground power systems and the need for greater reliability of the power supply, cable monitoring and accurate fault location detection has become an increasingly important issue. The presence of inherent random fluctuations in power system signals can be used to extract valuable information about the condition of system equipment. One such component is the power cable, which is the primary focus of this research.

This thesis investigates a unique methodology that allows online monitoring of an underground power cable. The methodology analyzes conventional power signals in the frequency domain to monitor the condition of a power cable.

First, the proposed approach is analyzed theoretically with the help of mathematical computations. Frequency domain analysis techniques are then used to compute the power spectral density (PSD) of the system signals. The importance of inherent noise in the system, a key requirement of this methodology, is also explained. The behavior of resonant frequencies, which are unique to every system, are then analyzed under different system conditions with the help of mathematical expressions.

Another important aspect of this methodology is its ability to accurately estimate cable fault location. The process is online and hence does not require the system to be disconnected from the grid. A single line to ground fault case is considered and the trend followed by the resonant frequencies for different fault positions is observed.

The approach is initially explained using theoretical calculations followed by simulations in MATLAB/Simulink. The validity of this technique is proved by comparing the results obtained from theory and simulation to actual measurement data.

Electric substation physical plans are developed with consideration given to lightning protection. To develop these plans utility design engineers use various methods. This thesis focuses on developing a computer program for two methods/models for substation shielding against direct lightning strokes. The first method is being used currently in the industry to protect the substation structures. The second model is a new and more physics based approach towards lightning phenomenon. Both the methods consider only direct lightning strikes that can hit the substation equipment. Hence, the travelling waves, indirect strokes or over-voltage arriving at the substation equipment are not considered. The Electro-Geometric method (EGM) based Rolling Sphere Method (RSM) is used to develop first part of the program. The aim of the program is to design the protection system for the substation equipment quickly and error free. The protection system uses lightning masts and/or shield wires to protect the station equipment. These are grounded solidly with low impedance to earth. The MATLAB based program gives a two dimensional visual representation of the zone of protection and therefore helps utility engineers to position shielding system. As this program is converted further into an executable file, it can be used on any computer to produce the results without need of any other software. The second part of the thesis focuses on developing the MATLAB code for protection of substation equipment using the Rizk model which is not used as of now for shielding system design in industry. Using more physics based model, simulation of downward lightning leader and connecting upward leader is shown.

Finally both the methods are compared. This includes consideration of a 220 kV substation layout arrangement. The equipment are protected using shielding masts and the comparison is made in terms of number of the protective equipment needed. It is found that the classical rolling sphere model gives more conservative results than the physics based model. Hence the results shows that it is possible to use present methods and still protect the equipment sufficiently.

Underground transmission cables in power systems are less likely to experience electrical faults, however, resulting outage times are much greater in the event that a failure does occur. Unlike overhead lines, underground cables are not self-healing from flashover events. The faulted section must be located and repaired before the line can be put back into service. Since this will often require excavation of the underground duct bank, the procedure to repair the faulted section is both costly and time consuming. These added complications are the prime motivators for developing accurate and reliable ratings for underground cable circuits.

This work will review the methods by which power ratings, or ampacity, for underground cables are determined and then evaluate those ratings by making comparison with measured data taken from an underground 69 kV cable, which is part of the Salt River Project (SRP) power subtransmission system. The process of acquiring, installing, and commissioning the temperature monitoring system is covered in detail as well. The collected data are also used to evaluate typical assumptions made when determining underground cable ratings such as cable hot-spot location and ambient temperatures.

Analysis results show that the commonly made assumption that the deepest portion of an underground power cable installation will be the hot-spot location does not always hold true. It is shown that distributed cable temperature measurements can be used to locate the proper line segment to be used for cable ampacity calculations.

ABSTRACT

This dissertation introduces a real-time topology monitoring scheme for power systems intended to provide enhanced situational awareness during major system disturbances. The topology monitoring scheme requires accurate real-time topology information to be effective. This scheme is supported by advances in transmission line outage detection based on data-mining phasor measurement unit (PMU) measurements.

A network flow analysis scheme is proposed to track changes in user defined minimal cut sets within the system. This work introduces a new algorithm used to update a previous network flow solution after the loss of a single system branch. The proposed new algorithm provides a significantly decreased solution time that is desired in a real- time environment. This method of topology monitoring can provide system operators with visual indications of potential problems in the system caused by changes in topology.

This work also presents a method of determining all singleton cut sets within a given network topology called the one line remaining (OLR) algorithm. During operation, if a singleton cut set exists, then the system cannot withstand the loss of any one line and still remain connected. The OLR algorithm activates after the loss of a transmission line and determines if any singleton cut sets were created. These cut sets are found using properties of power transfer distribution factors and minimal cut sets.

The topology analysis algorithms proposed in this work are supported by line outage detection using PMU measurements aimed at providing accurate real-time topology information. This process uses a decision tree (DT) based data-mining approach to characterize a lost tie line in simulation. The trained DT is then used to analyze PMU measurements to detect line outages. The trained decision tree was applied to real PMU measurements to detect the loss of a 500 kV line and had no misclassifications.

The work presented has the objective of enhancing situational awareness during significant system disturbances in real time. This dissertation presents all parts of the proposed topology monitoring scheme and justifies and validates the methodology using a real system event.

The electromagnetic fields near power lines that may produce adverse effects on humans are of increasing interest in a variety of situations, thus making it worthwhile to develop general-purpose software that estimates both the electric and magnetic fields accurately. This study deals with the simulations of the electric and magnetic fields near high-voltage power lines for the triangular, horizontal and vertical conductor arrangements under both balanced and unbalanced conditions.

For all three conductor arrangements, the shapes of the electric field distribution curves are different with the vertical arrangement best for minimizing right of way consideration, while the shapes of the magnetic field distributions curves are similar. Except for the horizontal arrangement, the maximum electric field magnitudes with shield conductors are larger than those without shield conductors. Among the three different arrangements, the maximum field value of the vertical arrangement is most vulnerable to the unbalanced conditions.

For both the electric and magnetic fields, increasing the heights of phase conductors gradually results in diminishing return in terms of the field reduction. In this work, both the maximum electric field magnitudes and the maximum magnetic field magnitudes produced by 500 kV power lines at 1 m height from the ground are all within the permissible exposure levels for the general public. At last, the dynamic trajectories of both fields with time are simulated and interpreted, with each field represented by a vector rotating in a plane describing an ellipse, where the vector values can be compared to high-speed vector measurements.

The demand for cleaner energy technology is increasing very rapidly. Hence it is

important to increase the eciency and reliability of this emerging clean energy technologies.

This thesis focuses on modeling and reliability of solar micro inverters. In

order to make photovoltaics (PV) cost competitive with traditional energy sources,

the economies of scale have been guiding inverter design in two directions: large,

centralized, utility-scale (500 kW) inverters vs. small, modular, module level (300

W) power electronics (MLPE). MLPE, such as microinverters and DC power optimizers,

oer advantages in safety, system operations and maintenance, energy yield,

and component lifetime due to their smaller size, lower power handling requirements,

and module-level power point tracking and monitoring capability [1]. However, they

suer from two main disadvantages: rst, depending on array topology (especially

the proximity to the PV module), they can be subjected to more extreme environments

(i.e. temperature cycling) during the day, resulting in a negative impact to

reliability; second, since solar installations can have tens of thousands to millions of

modules (and as many MLPE units), it may be dicult or impossible to track and

repair units as they go out of service. Therefore identifying the weak links in this

system is of critical importance to develop more reliable micro inverters.

While an overwhelming majority of time and research has focused on PV module

eciency and reliability, these issues have been largely ignored for the balance

of system components. As a relatively nascent industry, the PV power electronics

industry does not have the extensive, standardized reliability design and testing procedures

that exist in the module industry or other more mature power electronics

industries (e.g. automotive). To do so, the critical components which are at risk and

their impact on the system performance has to be studied. This thesis identies and

addresses some of the issues related to reliability of solar micro inverters.

This thesis presents detailed discussions on various components of solar micro inverter

and their design. A micro inverter with very similar electrical specications in

comparison with commercial micro inverter is modeled in detail and veried. Components

in various stages of micro inverter are listed and their typical failure mechanisms

are reviewed. A detailed FMEA is conducted for a typical micro inverter to identify

the weak links of the system. Based on the S, O and D metrics, risk priority number

(RPN) is calculated to list the critical at-risk components. Degradation of DC bus

capacitor is identied as one the failure mechanism and the degradation model is built

to study its eect on the system performance. The system is tested for surge immunity

using standard ring and combinational surge waveforms as per IEEE 62.41 and

IEC 61000-4-5 standards. All the simulation presented in this thesis is performed

using PLECS simulation software.

Shunt capacitors are often added in transmission networks at suitable locations to improve the voltage profile. In this thesis, the transmission system in Arizona is considered as a test bed. Many shunt capacitors already exist in the Arizona transmission system and more are planned to be added. Addition of these shunt capacitors may create resonance conditions in response to harmonic voltages and currents. Such resonance, if it occurs, may create problematic issues in the system. It is main objective of this thesis to identify potential problematic effects that could occur after placing new shunt capacitors at selected buses in the Arizona network. Part of the objective is to create a systematic plan for avoidance of resonance issues.

For this study, a method of capacitance scan is proposed. The bus admittance matrix is used as a model of the networked transmission system. The calculations on the admittance matrix were done using Matlab. The test bed is the actual transmission system in Arizona; however, for proprietary reasons, bus names are masked in the thesis copy in-tended for the public domain. The admittance matrix was obtained from data using the PowerWorld Simulator after equivalencing the 2016 summer peak load (planning case). The full Western Electricity Coordinating Council (WECC) system data were used. The equivalencing procedure retains only the Arizona portion of the WECC.

The capacitor scan results for single capacitor placement and multiple capacitor placement cases are presented. Problematic cases are identified in the form of ‘forbidden response. The harmonic voltage impact of known sources of harmonics, mainly large scale HVDC sources, is also presented.

Specific key results for the study indicated include:

• The forbidden zones obtained as per the IEEE 519 standard indicates the bus 10 to be the most problematic bus.

• The forbidden zones also indicate that switching values for the switched shunt capacitor (if used) at bus 3 should be should be considered carefully to avoid resonance condition from existing.

• The highest sensitivity of 0.0033 per unit for HVDC sources of harmonics was observed at bus 7 when all the HVDC sources were active at the same time.

This thesis provides a cost to benefit analysis of the proposed next generation of distribution systems- the Future Renewable Electric Energy Distribution Management (FREEDM) system. With the increasing penetration of renewable energy sources onto the grid, it becomes necessary to have an infrastructure that allows for easy integration of these resources coupled with features like enhanced reliability of the system and fast pro-tection from faults. The Solid State Transformer (SST) and the Fault Isolation Device (FID) make for the core of the FREEDM system and have huge investment costs.

Some key features of the FREEDM system include improved power flow control, compact design and unity power factor operation. Customers may observe a reduction in the electricity bill by a certain fraction for using renewable sources of generation. There is also a possibility of huge subsidies given to encourage use of renewable energy. This thesis is an attempt to quantify the benefits offered by the FREEDM system in monetary terms and to calculate the time in years required to gain a return on investments made. The elevated cost of FIDs needs to be justified by the advantages they offer. The result of different rates of interest and how they influence the payback period is also studied. The payback periods calculated are observed for viability. A comparison is made between the active power losses on a certain distribution feeder that makes use of distribution level magnetic transformers versus one that makes use of SSTs. The reduction in the annual active power losses in the case of the feeder using SSTs is translated onto annual savings in terms of cost when compared to the conventional case with magnetic transformers. Since the FREEDM system encourages operation at unity power factor, the need for installing capacitor banks for improving the power factor is eliminated and this re-flects in savings in terms of cost. The FREEDM system offers enhanced reliability when compared to a conventional system. The payback periods observed support the concept of introducing the FREEDM system.

This research primarily deals with the design and validation of the protection system for a large scale meshed distribution system. The large scale system simulation (LSSS) is a system level PSCAD model which is used to validate component models for different time-scale platforms, to provide a virtual testing platform for the Future Renewable Electric Energy Delivery and Management (FREEDM) system. It is also used to validate the cases of power system protection, renewable energy integration and storage, and load profiles. The protection of the FREEDM system against any abnormal condition is one of the important tasks. The addition of distributed generation and power electronic based solid state transformer adds to the complexity of the protection. The FREEDM loop system has a fault current limiter and in addition, the Solid State Transformer (SST) limits the fault current at 2.0 per unit. Former students at ASU have developed the protection scheme using fiber-optic cable. However, during the NSF-FREEDM site visit, the National Science Foundation (NSF) team regarded the system incompatible for the long distances. Hence, a new protection scheme with a wireless scheme is presented in this thesis. The use of wireless communication is extended to protect the large scale meshed distributed generation from any fault. The trip signal generated by the pilot protection system is used to trigger the FID (fault isolation device) which is an electronic circuit breaker operation (switched off/opening the FIDs). The trip signal must be received and accepted by the SST, and it must block the SST operation immediately. A comprehensive protection system for the large scale meshed distribution system has been developed in PSCAD with the ability to quickly detect the faults. The validation of the protection system is performed by building a hardware model using commercial relays at the ASU power laboratory.

The utilization of power cables is increasing with the development of renewable energy and the maintenance replacement of old overhead power lines. Therefore, effective monitoring and accurate fault location for power cables are very important for the sake of a stable power supply.

The recent technologies for power cable diagnosis and temperature monitoring system are described including their intrinsic limitations for cable health assessment. Power cable fault location methods are reviewed with two main categories: off-line and on-line data based methods.

As a diagnostic and fault location approach, a new passive methodology is introduced. This methodology is based on analyzing the resonant frequencies of the transfer function between the input and output of the power cable system. The equivalent pi model is applied to the resonant frequency calculation for the selected underground power cable transmission system.

The characteristics of the resonant frequencies are studied by analytical derivations and PSCAD simulations. It is found that the variation of load magnitudes and change of positive power factors (i.e., inductive loads) do not affect resonant frequencies significantly, but there is considerable movement of resonant frequencies under change of negative power factors (i.e., capacitive loads).

Power cable fault conditions introduce new resonant frequencies in accordance with fault positions. Similar behaviors of the resonant frequencies are shown in a transformer (TR) connected power cable system with frequency shifts caused by the TR impedance.

The resonant frequencies can be extracted by frequency analysis of power signals and the inherent noise in these signals plays a key role to measure the resonant frequencies. Window functions provide an effective tool for improving resonant frequency discernment. The frequency analysis is implemented on noise laden PSCAD simulation signals and it reveals identical resonant frequency characteristics with theoretical studies.

Finally, the noise levels of real voltage and current signals, which are acquired from an operating power plant, are estimated and the resonant frequencies are extracted by applying window functions, and these results prove that the resonant frequency can be used as an assessment for the internal changes in power cable parameters such as defects and faults.