Switching operations in power systems can produce significant overvoltages under specific circumstances. With the increasing application of underground cables in transmission systems, the statistical distribution of energization overvoltages is expected to change substantially due to the different electrical characteristics of cables and OHLs. Therefore, it is crucial to perform an insulation coordination study by analysis of the statistical distribution of energization overvoltages. This paper presents a statistical switching analysis on a hybrid OHL-Cable circuit to investigate how such hybrid circuits can affect the distribution of overvoltages. The literature has addressed the distribution of energization overvoltages only for OHLs or cables, but such an study is not available for hybrid systems consisting of OHLs and cables combined. The study is carried out for different cable lengths in the case study to identify how an increasing cable share in the circuit influences the overvoltages distribution due to no-load energization. Moreover, the impact of symmetrical and asymmetrical circuit structures is also addressed. The study is carried out on a distributed frequency-dependent parameter model of the Dutch 380 kV grid in PSCAD/EMTDC. ... Read more

Zero-missing is a phenomenon in shunt compensated cable systems in which the current through the line breaker does not cross the zero point for several cycles. This paper deals with a thorough investigation on countermeasures of the zero-missing phenomenon in transmission systems and determines the requirements, benefits, and risks of applying each method. The effectiveness of countermeasures is studied on a simulated cable project with different cable lengths in an actual grid model of the Dutch 380 kV transmission system. Results are analyzed based on three criteria related to the IEC standards and the Dutch grid code. In addition, the switching sequence of circuit-breakers is specified to maximize the effectiveness of the countermeasures. A statistical switching analysis is performed for the insulation coordination study since the application of some countermeasures increases the probability of high transient switching overvoltages. Moreover, the closing variation threshold of circuit-breakers is calculated as a function of the circuit impedance and the shunt compensation degree. ... Read more

Extra high voltage (EHV) power transmission systems have been traditionally constructed by using overhead lines (OHL) to transfer power over long distances. During the last decade, the opposition against the construction of new OHLs has significantly increased due to societal and environmental concerns, which has caused major obstacles for the grid development. Under this circumstance, system operators have been urged to find solutions and alternatives for the future grid developments. A promising solution of this challenge is to underground the transmission grids (fully or partially) by means of EHV AC underground cables. In this regard, the future transmission grids will be composed of OHLs in non-sensitive areas and underground cables in sensitive areas like populated neighbourhoods and environmentally sensitive locations, which implies on a large-scale utilization of long cables in future EHV grids. These grids are known as hybrid OHL-Cable grids. Although this is very encouraging from the societal and environmental points of view, new challenges arise mainly from the technical perspective and high capital cost. Regarding the technical perspective, the large-scale application of long cables in transmission grids is not yet a well-practiced technique for system operators. In fact, cables have been widely used in low and medium voltage distribution grids, but not in EHV transmission grids. The electrical, thermal, and mechanical characteristics of cables and OHLs are significantly different. These differences can cause various technical problems in the grid, which in return may increase the chance of damage to system components and reduce the reliability of the power supply. Therefore, a decision for the large-scale utilization of EHV cables will be very risky without gaining the complete knowledge and insight of the expected hazards and their countermeasures. This was the main driving force for system operators and manufacturers to carry out research and investigation on the technical performance of EHV grids with long cables. So far, lots of researches have been performed to investigate the design and operation of long EHV cables in transmission grids. These studies have answered many questions and unknowns, but there are still several important scientific gaps that have to be tackled. As a result, the Dutch transmission system operator, TenneT, began an extensive ten-year cable research program together with the Technical Universities of Delft and Eindhoven to investigate the technical possibilities of utilizing long EHV underground cables in the future transmission projects. This thesis, as the last part of the Dutch cable research program, provides robust and comprehensive answers to the most crucial scientific gaps and addresses the required techniques for the reliable operation of cable projects. These techniques can be used in practice by system operators since they are based on realistic assumptions and reliable simulations on an accurate model of an actual power transmission system. This thesis focuses on crucial phenomena related to the steady-state operation, harmonic behaviour, and transient operation of hybrid OHL-Cable systems. A hypothetical future project in the Dutch 380 kV grid with 80 km transmission length was selected as the case study, for which all phenomena were studied according to the most recent standards and grid code. The main scientific contribution of this thesis is the rigorous and comprehensive analysis of a hybrid OHL-Cable system to identify the impact of long cables, system parameters and topology on the system operation. The thesis proposes a methodology for optimal compensation of the cable reactive power in order to enhance the system performance. Moreover, the significance of energization overvoltages is investigated by a robust statistical analysis, which is the first of its kind for hybrid OHL-Cable grids. Last but not the least, two new countermeasures for the zero-missing phenomenon have been developed and several other countermeasures have been also investigated. The main conclusion of the thesis is that the large-scale application of underground cables in transmission systems is technically possible under the condition that all technical phenomena and issues are properly addressed in the planning and designing phases of each project. A case-by-case study for cable projects is a “must” as each project has its own electrical and geographical characteristics. System parameters and topology are different in different areas and consequently the severity of phenomena and challenges will be different. Several countermeasures are available for each technical issue, where the most optimal one should be selected by conducting an in-depth technical analysis. The decision to choose the right countermeasure is highly dependent on the project specifications. Finally, for each cable project, it is always recommended to perform a step-by-step study similar to the presented approach in this thesis, in which all the relevant phenomena from the steady-state operation to the electromagnetic transient behaviour are investigated. The study should follow the guidelines, grid code, manufacturer requirements, and standards in order to guarantee that all requirements for a reliable system operation are met accordingly. ... Read more

Energization overvoltages are among the severest overvoltages stressing insulations of EHV power system components. Since these overvoltages have a statistical nature, the insulation level should be determined with the use of a statistical approach by which the distribution of overvoltages is calculated. Literature has properly studied the distribution of energization overvoltages in purely OHL or cable systems, but such a study is not available for hybrid systems consisting of both OHLs and cables. It is expected that the overvoltage distributions change substantially when both OHLs and cables are used in a transmission line. This paper tackles this issue by analyzing the overvoltage distributions due to the energization of a 380 kV hybrid OHL-Cable circuit, in which the cable length is variable. The study includes various sensitivity analyses to find out the impact of system parameters and topology on overvoltages. By the statistical analysis, it has been discovered that energization overvoltages of a hybrid OHL-Cable circuit are higher than those of a fully-cable circuit and very likely lower than those of a fully-OHL circuit with the same transmission lengths. ... Read more

Integration of long EHV XLPE underground cables in traditionally OHL-based transmission systems can result in abnormal transients caused by large cable shunt capacitance. Transients associated with switching of long cables may result in severe temporary or transient overvoltages. This paper investigates energization transients related to the installation of long 380 kV cables in transmission systems. Voltages and currents at multiple locations were analyzed in the time-domain after energization of different cable lengths in a 80 km mixed OHL-cable connection in the future Dutch 380 kV grid. Moreover, zero-missing phenomenon, as one of the likely events during energization transients of shunt compensated cables, and its countermeasures are discussed. The study comprises time-domain simulations in PSCAD/EMTDC environment,
where an accurate distributed frequency-dependent parameter model of the complete Dutch 380 kV grid is developed. ... Read more

This paper deals with the shunt compensation design of long 380kV-50Hz XLPE cables by simulating a double-circuit partially cabled connection with the transmission length of 80km in the Dutch transmission system. The proposed procedure for shunt compensation sizing is fully elaborated in this paper. Four sizing criteria are used to find the minimum required size of compensation. All simulations are performed for different cable lengths. Moreover, different compensation arrangements including line-end and distributed arrangements are compared in terms of minimum required compensation size. Finally, the influence of mixed-line configuration, i.e. the number and the location of cable sections, on the minimum required compensation size is investigated by simulating five mixed-line configurations. All simulations are performed for two load-flow scenarios representing two extreme situations in the future planning of the Dutch transmission grid.
... Read more

The worldwide demand for electricity, which is steadily increasing, leads to a continuous need for developing and extending the electrical transmission networks. However, the installation of new overhead lines (OHL) faces many challenges due to societal and environmental reasons. One solution that gains widespread public support is the installation of EHV AC XLPE underground cables (UGC). Although this development is quite encouraging from a societal perspective, new challenges might arise, mainly from a technical perspective. This is due to different electrical characteristics of underground cables compared to OHL as well as long repair time in the case of failure in cable systems. These aspects highlight the necessity of investigating power system technical performance issues related to application of EHV cables to prevent any unwanted condition in partially cabled grids. This paper, among various system operation issues, investigates optimum shunt compensation sizing, reliability analysis, and condition monitoring of EHV cable systems. Reactive power compensation by means of shunt reactors should be allocated for long cables to consume their reactive power surplus. It is crucial to have sufficient size of shunt compensation because both undercompensation and overcompensation can lead to undesirable system operation like overvoltage and zero-missing phenomenon. In this work, the shunt compensation sizing of a 80 km double-circuit connection in the Dutch transmission system consisting of series connected OHL and cable sections is studied. The sizing is performed for two load-flow scenarios and based on four sizing criteria and different cable lengths in the case study. Moreover, the most decisive sizing criterion in each case is determined too. The next studied issue is the reliability analysis of partially cabled EHV grids. The additional components of UGC (joints and terminations) together with a significantly larger repair time (compared to OHL), reduce the reliability of the whole system. In this work, a reliability assessment approach is developed in order to examine how the installation of EHV UGC in transmission networks impacts the overall reliability level. A contingency analysis regarding failures of 380 kV OHL and UGC is performed through state enumeration. Reliability indicators are calculated by performing a dc load-flow calculation and applying remedial actions (if necessary) to relieve overloads. Varying cable length is installed in three connections in the Dutch network, the probability of load curtailment is calculated and the main factors that influence the probability of overload with EHV UGC are explored. The third studied aspect is the condition monitoring of cable systems. An advanced real time Condition Monitoring System (CMS) was installed in the Dutch 380 kV grid and its goal is to monitor the status of the cable connection and its impact on the 380 kV electricity transmission network. In this paper, a first comparison between measurement and simulation results, in both time and frequency domain, is presented. In this way a first validation of the simulation models is performed and the reasons of possible deviations are discussed. ... Read more

Application of long EHV XLPE underground cables has divers effects on the transient behavior of transmission grids, mainly due to their large capacitive shunt impedance. Low-order resonance frequencies are one of the features of long cables that deserve special attention. This paper investigates the
resonance behavior of partial cable-based EHV networks by simulating a 380 kV double-circuit mixed OHL-cable connection in the future Dutch transmission system. The Sweep Frequency Response Analysis (SFRA) is performed for the distributed frequency-dependent parameter model of the whole Dutch 380
kV grid with and without cables in the case study project. The analysis is carried out for different cabling scenarios (i.e. different cable lengths) and for several sensitivity analyses including the influence of shunt compensation size, shunt
compensation location, and mixed-line configuration on the resonance behavior of the grid. The results are compared in terms of order of first harmonic resonance frequency and number of resonance frequencies. The EMT studies are carried out in the PSCAD environment. ... Read more