Doctor of Philosophy
Electrical and Computer Engineering
Sidhu, Tarlochan Sing.
Electrical power system components are designed three-phase balanced and symmetric with the internal connection of wye or delta. The common point of the wye-connected equipment, which is called neutral, is impedance grounded for many reasons such as fault ride through by controlling transient overvoltages, and limiting the ground overcurrents. Depending on the application, different neutral impedance grounding methods exist that employ resistors or reactors with/without neutral grounding transformers. These apparatuses are known as Neutral Grounding Devices (NGD). The most well-known sort of NGDsarethe Neutral Grounding Resistor (NGR) and Neutral Grounding Reactor (NGL) which are the main focus of this research work.
As said, NGDs provide many benefits; however, they fail due to many reasons such as corrosion, lightning, and extended service life. Upon this failure, the advantages of impedance grounding are replaced by disadvantages of the ungrounded or solidly grounded traditional systems. Consequences of such a failure are the false sense of security, ungrounded system, transient overvoltages, overcurrents, line-to-ground voltage test non-safety, and so on. In order to prevent these issues, the intactness and integrity of the neutral-to-ground circuit shall be ensured. However, this cannot be done easily since the neutral-to-ground circuit is dead or de-energized during the steady-state condition. However, there has to be a continuous and online monitor, which without it there is no guarantee or indication that these apparatuses have failed. That is why the Canadian Electric Code (CEC) mandates monitoring of the neutral-to-ground circuit in industrial and commercial networks.
Accordingly, this research work first reviews the existing monitoring methods to understand the fundamentals, and performance of these techniques. The performed literature survey results in a conceptual classification of the existing methods into three categories called passive, active, and passive-active. This part of the carried-out research highlights the advantages and disadvantages of the methods on one hand, and the evolution trend of the methods on the other. It also reveals that all of the existing methods suffer from one shared issue which is the hard-to-achieve continuous monitoring. In fact, they cannot provide continuous or uninterrupted operation in all system conditions, i.e., normal, faulted, and de-energized. It is this major shortcoming of the literature which motivates towards making a difference. Therefore, the mission is to resolve this issue relying on the existing measurement instruments and protection installations. As the results, three new or enhanced methods are achieved.
The first technique is a cost-effective combination of two existing techniques resulted in a better performance. The performance of this proposed method is comprehensively studied using software analysis, and a fabricated prototype of the invented mechanism for full-range neutral voltage measurement. The resulted method provides reliable monitoring during both faulted and unfaulted conditions of the power system which is the most prominent advantage of the proposed technique since none of the existing methods, with the same measurements, provide the such a performance.
The second proposed technique is an economical solution that employs the third harmonic of neutral and residual voltages for monitoring the NGR installed at the neutral of the unit-connected generators. The proposed technique is comprehensively studied including further hardware validations using an available industrial generator protective relay. The required measurement instruments and protection infrastructures are readily available which means that the proposed method could be implemented with no additional cost. In fact, the proposed method could be easily incorporated into the core of the existing digital protective relays.
Lastly, the third technique employs an existing sub-harmonic injection based generator stator ground protection for monitoring the neutral-to-ground circuit of the same generator, which is equipped with either the neutral grounding resistor or neutral grounding reactor. This alternative is also a money-saving solution since it only demands a current sensor to measure the injected current. It is also easily retrofitted to installed digital protective relays. The other advantage of this proposed method is its functionality in de-energized condition of the power system besides its reliable performance in both faulted and unfaulted operation conditions. It is this one last accomplishment that brings the mission to completion.
Jafari, Rahim, "Continuous Monitoring of Neutral Grounding Resistors and Reactors" (2018). Electronic Thesis and Dissertation Repository. 5877.