FIGURE 2 - available via license: Creative Commons Attribution 3.0 Unported
Content may be subject to copyright.
Overhead section of the bipolar HVDC transmission line. (a) tower structure (not drawn to scale). (b) circuit model for electromagnetic transient simulation.
Source publication
Growing urbanization coupled with increased power demands have led to an increasing use of mixed power transmission lines with sections of overhead lines (OHL) and the underground cables. Due to differences in surge impedance of cables and OHL, voltage surges experience reflections and refractions at their boundaries which make the transient behavi...
Contexts in source publication
Context 1
... accurately represent the overvoltage across insulators, the vertical section of tower is divided into two sub-sections connected at cross arms as shown in Fig. ...
Context 2
... parameter line with the same surge impedance for upper and lower sections [31], [32]. The wave propagation on the tower is retarded by cross arms and braces. Therefore, the velocity of the traveling wave on the tower is assumed to be 85% of the speed of light [33]. The approximate surge impedance of the waisted tower structure, as shown in Fig. 2, is calculated using (1) [34]. ...
Context 3
... strings are composed of 29 disks of cap and pin suspension ceramic insulators with a 5 m air gap between arc horns as shown in Fig. 2. The capacitance of one insulator disk is approx. 80 pF ...
Context 4
... the PSCAD simulation, the insulator breakdown is represented by a switch connected in parallel with an equivalent insulation capacitance cins, as shown in Fig. 2. The inductive nature of the arc path is taken into consideration by inserting an inductance Larc in series with the switch, according to the recommendation of ...
Context 5
... choice of the cable and overhead conductor is made to ensure of a power rating of 660 MW per pole of the HVDC line. Sections of the bipolar overhead line, including the OHGWs, are modelled according to geometry shown in Fig. 2. Midspan conductor sag is 10 m for all conductors. OHPCs are modelled based on Parrot ACSR 54/19, whereas ½ high strength steel is used to model the ...
Similar publications
Operation data show lightning faults account for >70% for the main ultra-high voltage (UHV) DC transmission channels, very different from the design view. In order to accurately master the lightning characteristics and the lightning protection performance of the line so as to propose solutions to weak points, this study firstly obtains and analyses...
Citations
... However, this advantage requires the extension of HVDC lines over wide geographical space making them vulnerable to different lightning strikes and increasing pollution levels arising due to global climatic change. The destructive effect of transient overvoltages initiated by lightning strikes upon HVDC networks has been studied extensively in literature [28][29][30][31][32][33][34][35][36][37]. Also, the effect of pollution layer upon insulators was studied also in literature [38][39][40][41][42][43]. ...
The increasing effect of climatic change has mandated the investigation of its negative impact upon electrical power systems. One of the most vulnerable parts of the power system to climatic conditions is the transmission line. Among transmission line technologies, high voltage direct current (HVDC) transmission systems are gaining attention in last decades for their various applications in long distance power transmission and asynchronized connections. In this paper, HVDC transmission system was simulated using ATP/EMTP software package. The insulators of the system were tested for different lightning types under clean and polluted conditions. To evaluate the amount of reduction in insulation performance, two factors were identified as risk of back flashover factor (RBF) and the degradation factor. The computation of RBF was dependent on the parameters of the strike leading to back flashover and the probability of its occurrence. The degradation factor was defined as the percentage of increase of RBF due to polluted conditions. The degradation factor was computed for negative, positive and indirect strikes to be 92%, 29.2% and 33.3% respectively. The results indicate that the climatic pollutive impact could lead to severe degradation that could double the RBF as in negative strikes.
... Currently, significant lightning research has been conducted on "Line Commutated Converter" (LCC) DC transmission lines at HVDC level including "Mixed" transmission Links [8]. Contrary to MVDC projects which are 10-100km in length, HVDC systems have long-length transmission lines thus reflection and refractions from the converter substation are not taken into consideration for lightning studies. ...
... Since there are no proper guidelines for riser section modelling. Asif et al. [8] has modelled it like an OHL (i.e., similar speed of wave propagation, geometry, and surge impedance). However, practically riser section should have a geometry transitioning from tower section to cable. ...
... The most vulnerable sections of MVDC system from lightning overvoltage's are the line transition areas i.e., the cable section adjacent to the overhead transmission segment and the substation region [8]. Thus, lightning impact on them has been studied based on IMSF (10kA) on positive power conductor and 110kA BF lightning on OHGW. ...
Medium voltage direct current (MVDC) transmission system are growing due to their assistive quality in conventional grid and compatibility with renewable power network. MVDC distribution links with “Mixed” overhead (OH) & underground (UG) sections could be devised based on urban planning. UG Cables or substations are indirectly exposed to lightning strikes due to adjacent tower sections. In case of MVDC converter or cable, present researchers do not specify lightning voltage impulse level for related system voltage. Therefore, preluding electromagnetic (EM) transient investigation are required to determine the maximum lightning overvoltages for system peripherals i.e. cable & Modular Multilevel Converter (MMC) substation. This research focuses on analyzing lightning performance of OH transmission towers-cable junction & tower-substation link in case of a shielding failure (SF) and back flashover (BF) for a ±35kV short-length mixed MMC-MVDC transmission scheme. This article provides broad-band modeling method for MMC substation for lightning investigation. In addition, based on a detailed time-domain parametric evaluation in
PSCAD/EMTDC
program, lightning impulse voltage across the transmission line’s pole insulator and embedded cable section are estimated along with numerical validation relying on travelling wave theory. Effect of project parameters such as tower grounding resistance, riser section surge impedance (which connects cable & OH line) and cable length on lightning overvoltage impacting the cable and connected tower section has been demonstrated.
... Theoretical and experimental results have described the flashover characteristics of simple electrode systems such as rod-rod and rod-plane electrodes [18][19]. Researchers have also focused on the impulse flashover characteristics of towers without cross arms [20][21]. Nonetheless, the actual air gap of transmission towers cannot be well described by ideal models or previous structures, which made it necessary to conduct overvoltage flashover tests using a full-scale tower with composite cross arms. ...
Overhead transmission lines (OTLs) have always been the major means of power delivery. With the significant increase of transmission voltage and transmission capacity, the dimensions of transmission towers are increasing accordingly, resulting in extensive occupation of land resources. Towers with composite cross arms are a promising solution to this problem, considering the remarkable performance of composite line insulators. In this research, a full-scale alternating current (AC) 500 kV model of a transmission tower with composite cross arms is manufactured and applied under a lightning overvoltage of different polarities. The developing process of streamer-leader discharge is recorded with a high-speed camera, and the major path of the flashover is identified. The flashover voltages are measured and corrected to standard conditions while considering the air humidity and air density, and clearly confirm the polarity effect. The tower’s lightning-withstand level is calculated based on the tower structure and the flashover characteristics. Based on the results obtained from full-scale experiments, the feasibility of composite cross arms is confirmed, and a structural optimization is proposed.
... Early studies on the growth of over voltages in mixed transmission lines that consider the cable insulation level are accounted for [12,13]. Some studies [14][15][16] have provided parametric analysis and formulations to explain the traveling wave behavior. Nonetheless, bipolar lightning is a multi-flash phenomenon, and transient overvoltages due to subsequent lightning strokes and the response of traveling waves in a mixed HVDC link have been overgeneralized in the literature. ...
Bipolar lightning strokes are associated with multiple polarity electrical discharge with no current intervals in between, making their behavior quite peculiar. This work presents a fast front analysis of a mixed high voltage direct current (HVDC) transmission link, evaluating the factors that influence the line transients due to shielding failures and back flashovers (BFOs), considering both overvoltage and repeated polarity reversal at the cable sending terminal. The research process includes a detailed modeling of a bipolar lightning stroke, frequency-dependent HVDC overhead, and underground transmission line sections. Noticeable findings include the occurrence of only a positive polarity insulator BFO for the adjacent and subsequent tower, despite the dual polarity of the lightning stroke with relatively small values for the lightning parameters. The influence of traveling waves on the insulator flashover performance of the line with varying parameters (such as the riser section length, the tower grounding impedance, and the location of the lightning stroke) is recorded and explained.
... Many researchers have studied the impact of lightning strikes on mixed transmission lines since the 1990s and several resulting articles have attempted to address various aspects of lightning transients in mixed HVDC as well as HVAC lines [6][7][8][9][10][11][12][13][14][15][16]. Many of these studies focused on specific design aspects such as the shunt reactor on cable terminal, presence of surge arrester/sheath voltage limiter, OHL geometries and tower footing impedance. ...
... Others focused on the modelling details such as corona, frequency dependent sheath grounding impedance as well as tower footing impedance. In our previous study [15], we presented a thorough theoretical analysis supported by fast front transient simulation regarding the transient behavior of mixed HVDC transmission lines. The insulators of OHL most vulnerable to flashover were identified and the reason for their high vulnerability was explained. ...
... It has been found that the insulation coordination measures do not alter the transient behavior of the mixed transmission line studied in our previous work [15]. The second tower closest to the cable boundary remains most vulnerable to flashover despite the installation of a surge arrester at cable terminals and lowering the sheath grounding impedance. ...
Many geographical constraints and aesthetic concerns necessitate the partial use of cable sections in the High Voltage DC (HVDC) transmission line, resulting in a mixed transmission line. The overhead sections of mixed lines are exposed to lightning strikes. The lightning strikes can not only result in flashover of overhead line (OHL) insulators but can enter the cable and permanently damage its insulation if adequate insulation coordination measures are not taken. In this work, we have analyzed the factors that affect the level of overvoltage inside the cable by simulating a fast front model in PSCAD. It has been determined that surge arresters must be provided at cable terminals when the length of cable sections is less than 16 km to limit the core-ground overvoltage within the lightning impulse protective level (LIPL). The level of sheath-ground overvoltage is independent of the length of cable; however, it can be limited within LIPL by lowering the sheath grounding impedance to 1.2 Ω. Insulation coordination measures do not impact the likelihood of OHL insulators’ flashover. The flashover performance of OHL can be improved by lowering the footing impedance of the second tower closest to the cable terminals, which is otherwise most likely to flashover.
... It has been successfully applied in three phase AC cables to suppress circulating currents in the sheaths [11,[21][22][23]. In DC cables, sheath grounding at terminals is applied to suppress the sheath voltage in [7,[24][25][26]. However, the steady state sheath voltage and losses have not been discussed by any of these papers. ...
... However, the steady state sheath voltage and losses have not been discussed by any of these papers. The need for investigating steady state sheath voltage and losses in DC cable considering various sheath grounding schemes has been emphasized in [1,26,27]. ...
The current and voltage in High Voltage DC (HVDC) line is not pure DC but contain superimposed ripple components. The current ripple in core of HVDC cable magnetically induces a voltage in the sheath, whereas the voltage ripple causes the flow of charging current from core to sheath. The knowledge of sheath voltage is necessary to ensure compliance with the specification of utility companies. In this work, we have reported that the models available in commercial Electromagnetic Transient (EMT) simulation software erroneously introduce a DC bias in steady-state sheath voltage and sheath current. We have also demonstrated that by removing the DC bias accurate steady-state evaluation of sheath voltage and sheath current is possible. Additionally, we have analyzed the sheath voltage and currents in HVDC cable considering different cable lengths and sheath grounding schemes. It has been found that grounding the sheath at the terminal of HVDC cable can limit the sheath voltage to acceptable levels without causing substantial joule loss in the sheath.
... Moreover, significant power imbalance occurs between the reduced active power exported by ON-HVDCCs and the wind power in-feed by OF-HVDCCs which will be stored as an electrical field in the respective capacitors of VSCs and HVDC lines. Accordingly, transient direct over-voltage might appear due to the limited stored EPRP of VSCs that might cause equipment failure/aging in turn [2]. ...
This paper proposes a communication-free control strategy at the offshore wind farm (OWF) level to enhance onshore fault ride-through (FRT) grid code compliance of the voltage source converter (VSC)-based multi-terminal high voltage direct current (MT-HVDC) grid. In this proposal, the emerging virtual synchronous generator (VSG) concept is employed to equip the Type 4 wind turbine generator (WTG)s with inherent grid forming ability. Accordingly, it is proposed to switch the offshore HVDC converters control mode from grid forming to grid feeding during onshore FRT period to realize direct wind power in-feed reduction as a function of the severity of MT-HVDC grid’s overvoltage. The related dynamics are mainly characterized by the high-speed current control loop, so improved OWF response is achieved during onshore FRT period as conventional voltage/frequency modulation strategies are not employed. New analysis/amendments are also proposed to study and improve the transient active power reduction sharing between the WTGs in first few power cycles under wind wake effect. Finally, with the objective of a smooth transfer of HVDC converters and WTGs in several proposed operation states, a set of state machines are proposed considering whole WTG’s dynamics. Comprehensive time-domain simulations are performed with averaged electromagnetic transient models to demonstrate the improved onshore FRT behavior in terms of minimizing the electrical stress at both MT-HVDC grid and OWF levels.
In direct current (DC) transmission lines, power cables are susceptible to the invasion of switching overvoltage or lightning overvoltage. This causes the accumulation of space charge in the insulation due to the higher transient composite electric stress. Space charge is an important factor in dielectric breakdown, especially under high electric stress. However, until now, there are few reports on space charge characteristics under the composite stresses of DC voltage and impulse voltage. In this paper, a modified pulsed electroacoustic space charge measurement system used for DC voltage superimposed by impulse voltage is developed by reasonably choosing resistor and capacitor elements in the measuring system. Then, space charge characteristics in cross-linked polyethylene (XLPE) are measured under DC voltage, impulse voltage and DC voltage superimposed by impulse voltage. The test results show that hetero-charges dominated in the XLPE bulk under a positive DC voltage, whereas homo-charge injection occurred from the semiconductor (SC) electrode under a negative DC voltage. However, a continuous impulse voltage with high amplitude can cause a large degree of homo-charge injection from both electrodes. Moreover, greater homo-charge injection appeared under the negative impulse voltage than that under the positive impulse voltage. Under a DC voltage superimposed by an impulse voltage, the superposition of voltages with the same polarities facilitated homo-charges injection from both electrodes to a greater extent than did the superposition of voltages with opposite polarities. Furthermore, regardless of the polarity of the applied DC voltage, greater space charge accumulation appeared in XLPE when a negative impulse voltage is superimposed.
