Aspects of ultra-high voltage half-wavelength power transmission technology

1 Introduction

Global Energy Interconnection (GEI) is a globally interconnected strong and smart grid with UHV grid as the backbone, which will serve as a platform for extensive development, deployment and utilization of clean energy globally [1]. GEI allows operators to balance consumption and generation across the globe, and eases the major strains on today’s power grids： soaring energy demand,intermittent renewable energy, and the ever-increasing need to secure grids [2]. Half-wavelength transmission (HWLT)line is a kind of three-phase AC power transmission that the line length between its sending and receiving ends is about half of the wavelength (i.e. 3000km for 50 Hz). The charging power and reactive power loss reach equilibrium and no voltage-supported devices are required along HWLT line. Therefore, HWLT technology has positive effect on the construction of GEI.

HWLT technology was first proposed by former Soviet scholars in 1940’s [3]. Experts from the United States,India, Italy and other countries also conducted researches on natural or tuned HWLT in the following decades [4-7].In recent years, in-depth theoretical analysis and simulation researches of HWLT have begun. The previous studies only concern about the characteristic analysis, but recently,especially in three years, the research directions pay more attention to theoretical analysis, technology research and engineering test. Moreover, feasibility studies have been carried out in Brazil and other countries [8-10].

This paper comprehensively analyzes research achievements of HWLT technology. The operating characteristics of HWLT line are summarized and some aspects are presented such as simulation method, steady voltage control, relay protection, manual tuning, secondary arc suppression, overvoltage suppression, economic evaluation and engineering technology.

2 Application scenarios of HWLT

Due to the inverse distribution of energy resources and loads in China, large-scale power transmission is still an important direction for grid development. In the future, eastern and western synchronous girds will be interconnected by several ultra-high voltage (UHV) DC and AC lines. A new grid pattern will be formed with its feature of clear sending-end and receiving-end with DC and AC systems developed coordinately. For the development and distribution of clean energy in China, huge amounts of clean energy that’s generated in the west will get consumed in the east by large-scale and long-distance transmission line. Hence, the clean energy will be utilized efficiently and the air pollution and heavy smog will be alleviated.

In the global perspective, solar resources near the Equator and wind resources near the North Pole are abundant, while load centers are mainly distributed in mid-latitudes. By transmitting the renewable energy to load centers with long distance transmission technology, a safe and reliable clean energy supply will be provided worldwide.

From the viewpoint of energy development, it is necessary to develop large-scale and long-distance transmission technology in China and around the world. Innovative research on the key technologies of UHVAC power transmission is of important academic and practical value. The HWLT technology is one of the promising large-scale and long-distance transmission technologies with its length about 3000km.Wind power in Inner Mongolia and Xinjiang Uygur Autonomous region, solar power in Xinjiang and Tibet and hydro power in the southwest mountainous area can be delivered to North, East and Central China load centers by UHV HWLT lines.

As shown in Fig. 1 [1], double or multiple HWLT lines are series connected to form a full-wavelength or even longer transmission line worldwide. Ultra-long power transmission will be achieved. Intercontinental or global energy interconnection becomes possible.

Fig. 1 Worldwide application scenarios of HWLT

3 Operation characteristics of HWLT line

3.1 Fundamental theory of HWLT line

The length of HWLT line is approximately 3000km for 50Hz system. The lump parameter circuit model is not suitable for the analysis of HWLT line characteristics. Hence,the distributed parameter model for transmission line is adopted instead. z0=r0+jx0 and y0=g0+jb0 are the impedance and admittance per unit length respectively,and are the sending-end voltage and current respectively,and are the receiving-end voltage and current respectively,and are the voltage and current at x point respectively,andrepresent the voltage and current at point, dx is infinitesimal of the line.

The voltage and current relationship on both sides of the transmission line can be represented by the hyperbolic function of the sinusoidal steady-state solution：

Fig. 2 Distribution parameter circuit of long

Where is the line propagation coefficient; is the line characteristic resistance.

The charging reactive power generated in l-length line is represented by integration of susceptance per unit length：

The reactive power loss consumed in l-length line is represented by integration of reactance per unit length：

Based on equation (2), the charging reactive power of long transmission line is：

Similarly, the reactive power loss of long transmission line is

According to the equations (4) and (5), it can be found that the charging power and reactive power loss of the long circuit are related to the length of the line and the transmission power of the line. When the transmitted power factor is 1, Fig. 3 describes the excess reactive power of the line (charging reactive power subtract reactive power loss) affected by the length of the line and the active power delivered. According to Fig. 3, when the length of ideal transmission line is exactly half wavelength, the line charging power is equal to the line reactive power loss and the terminal voltage always maintains the same amplitude[11]. Therefore, no additional reactive power compensation is needed to balance excess reactive power.

3.2 Steady state voltage and reactive power

The voltage profiles along HWLT line are significantly affected by transmitted active power and power factor [11-15].It is concluded that the HWLT maximum line voltage increases with the increase of transmitted active power or the decrease of power factor. Based on the superposition of incident wave and reflected wave, the voltage profile of HWLT line is reasonably explained in [12]. Based on the formula in [15], the maximum steady-state voltage along the HWLT line and its location can be quantitatively calculated. It is shown that the power factor has significant influence on voltage profiles along the line. Lowing HWLT line transmitted reactive power is an effective method to improve power factor and avoid power frequency overvoltage.

Fig. 3 Distribution of reactive power along the line

3.3 Power transmission characteristics

From the deduced power transmission equation, it is known that when the length of transmission line is exactly half wavelength, the electrical distance is extremely short [16].Theoretically, the transmission power limit tends to infinity, but actually it is constrained by factors such as the maximum line voltage, insulation level and transient stability.It is shown in [17] that the power limit of HWLT line can reach 5300MW with reasonable start-up mode and voltage level. In order to better control the line power flow, meet the stability margin and ensure characteristics of low frequency,the actual length of HWLT line is generally little longer,and the electrical angle is maintained around 190 °.

3.4 Transient voltage

The HWLT line has unique power frequency overvoltage and switching overvoltage characteristics. In 1969, Prabhakara F S et al. found that the grounding shortcircuit fault can cause partial power frequency overvoltage.The fault location has significant effect on the amplitude of the caused overvoltage [5]. The power frequency overvoltage caused by load shedding or ground fault is simulated and analyzed in [18-19]. The overvoltage caused by load shedding is relatively lower, and its amplitude increases with the increase of load power [18]. The overvoltage caused by ground fault is more severe, as the fault has mischaracterized the HWLT system. The most serious fault area is between 75% and 90% of the HWLT line, and the area of maximum overvoltage appears at about 1/4 wavelength from the short-circuit point [19]. Based on the electromagnetic transient simulation under various operating conditions in literature [20-21], it is found that the overvoltage profiles of the closing operation of breakers are in the shape of ‘saddle’ with high at both ends and low in middle. The maximum overvoltage appeared near the end of unloaded line.

In [14, 22], the principle of series resonance is used to explain the cause of the most serious faults. The location and the maximum overvoltage amplitude are related to many factors such as the fault type, system strength and line structure.

4 Key technologies of UHV HWLT

4.1 Simulation method

Choosing a reasonable simulation method has a huge impact on the credibility of the analysis results. Distributed parameter model is adopted in [11] for the analysis of HWLT line steady state characteristic, and the accuracy of the model is verified. Electromagnetic transient simulation method is adopted for the analysis of HWLT line transient characteristic. Due to the calculation burden, it is difficult to apply to the analysis of large-scale power system.Electromechanical transient simulation method is adopted instead, but large error may appear. Dynamic phasor model is introduced in [23] to improve the electromechanical transient simulation method, and the simulation accuracy is increased.

4.2 Steady state voltage control

As mentioned above, the steady state voltage along the HWLT line is significantly affected by the power factor.A multi-objective reactive power optimization model is proposed to reactive power/voltage control in [24]. Reactive compensation equipment and generating units nearby are considered as the optimization strategy to control the voltage alone HWLT line. A voltage and reactive power control scheme is presented in [15] for hybrid power systems consisting of HWLT line and DC transmission line.

4.3 Protection technique

The fault features of HWLT line have significant differences from conventional transmission line both in space and time. On the space scale, the voltage along the line presents nonlinear and non-monotonous fluctuation characteristics. On the time scale, there are significant time differences on both sides of the line [25]. Hence,targeted principle and scheme of relay protection should be presented to meet the HWLT requirements. It is shown in [26] that the current differential protection based on lump parameter circuit model cannot meet the performance requirements. The effect of frequency dependent parameters on differential protection is analyzed in [27], and the use of power frequency components can avoid the influence of frequency dependent parameters. The current differential protection based on Bergeron model is applied in [28]. The expression for differential current is further deduced in [29],and the principle of choosing current differential point is proposed. A method to calculate the optimal differential point is presented in [30] based on time difference. In [31],it is found that the adjugate impedance characteristics have significant differences between external and internal faults.Accordingly, an adjugate impedance protection principle is proposed. Based on analysis of symmetrical components during the short circuit, an impedance-based algorithm for locating faults in long lines with lengths greater than 1/4 wavelength is proposed in literature [32]. A novel faulted phase selection algorithm is proposed in [33] based on the variation in apparent impedance. For fault location problem, traveling wave-based approaches are robust to the atypical operation features of half-wavelength power transmission lines [34]. A new mitigation method to single-phase auto-reclosing switching in half-wavelength transmission lines is presented in [35].

4.4 Tuning technique

In actual projects, the transmission length is hard to exactly match with half-wavelength. The length can be compensated by manual tuning. Three types of tuning mode, π-type, T-type and capacitance-type, are first presented in [6]. The tuning effect of π-type and T-type under various conditions is shown in [36], and the feasibility of manual tuning is verified. The capacitance tuning system is analyzed in literature [37]. When the number of tuning capacitances reaches eight, the tuning effect begins to stabilize. It is concluded in [38] that the π-type and T-type tuning networks are suitable to make centralized compensation on the line side, while the capacitance tuning network is suitable to make decentralized compensation. When asymmetrical faults occur, a preliminary study on the compensation of zerosequence network is presented in [39].

4.5 Secondary arc and suppression

The secondary arc extinction characteristics are one of the key technical problems [7]. In [40], it is found that the arc root motion of secondary arc has obvious polarity effect. Meanwhile, it is proposed that appropriate compensation can accelerate the extinction of secondary arc. In [41], considering multi-field coupling such as the electromagnetic force, the thermal buoyancy, the wind load force and the air resistance, a dynamic model of secondary arc is established. The research results provide reference to the actual operating conditions for the suppression technology of the UHV HWLT line secondary arc.

4.6 Overvoltage suppression and insulation coordination

The overvoltage problem is a key factor restricting the application of HWLT technology. Reasonable overvoltage suppression measures and insulation coordination are the premises of system reliable operation. Surge arresters are used in [20] to suppress the power frequency overvoltage and switching overvoltage. Due to the fact that the overvoltage amplitude of UHV HWLT line caused by internal fault is much larger than the conventional UHV transmission line, it is necessary to set multiple surge arresters along the line and require more energy absorption capacity. For line insulation coordination, the insulation level of power equipment in substations and the distances of air gaps, the insulation configuration for transmission line and the related distances of air gaps are determined in[42]. The results show that the insulation configuration of UHV HWLT equipment and lines can basically follow the configuration of conventional UHV project, and only part of the tuning parameters need to be adjusted.

4.7 Economic analysis

Based on the minimum annual cost method, it is concluded that the UHV HWLT is more economical than± 800kV and ± 1000kV DC transmission [43]. Samorodov G et al. pointed out that the investment for the singlecircuit HWLT with reserve phase could be considered 30% lower than that for HVDC [44]. However, taking the scenario of hydropower delivering in the Amazon Basin as an example, the Brazilian team presented that the 1000kV HWLT scheme did not reflect the economic advantages compared with ± 800kV HVDC and many technical problems need to be solved [8]. To sum up, the economy of UHV HWLT technology remains to be further studied, and the corresponding conclusion needs to be drawn combining actual transmission scenarios.

4.8 Engineering design

In recent years, the research on HWLT engineering practice and its impact have been gradually increasing.On the basis of the UHV power grid planning in China,six feasible experimental schemes of HWLT line are proposed in [45]. Power extraction system laid out along HWLT line is studied in [46]. The topology of series or parallel FACTS is designed in [47] to achieve the power flow control and tapping. It is shown in [48] that with 5000MW power transmission, the HWLT system can maintain power angle stability. The concept of stereoscopic power grid is given in [49]. It is concluded that point-togrid HWLT with double drop points can shorten the electric distance between landing points, and enhance the stability and synchronous power support of synchronous grid. The cross power characteristics of point-to-grid HWLT with double drop points are further analyzed in [50]. Result shows that the small voltage difference between nodes can cause high cross power, and higher demand of system meticulous control is needed. The influence of actual transposition representation and adequacy of AC link have been analyzed in [51].High frequency is generated in [52] to make transmission distance equal to half-wavelength. Hence, the length limitation of large capacity, long distance transmission line is overcome by frequency conversion technology.

5 Conclusions

UHV HWLT is another option for long-distance transmission beyond UHV DC transmission. It can be used for transmission projects of 3000km, 6000km or even longer distances and gives an important technical support in setting up global energy interconnection.

The steady-state and transient characteristics of UHV HWLT line have significant differences from conventional UHV transmission line. As the significant factors in causing power frequency overvoltage, the line overloading operation, low power factor operation and line internal fault need to be focused on. Due to the high transient overvoltage level, overvoltage suppression equipment such as large-capacity surge arrester needs to be developed. The adaptability of manual tuning scheme to different operating conditions and its transient characteristics after large disturbance need to be further studied.

This paper discusses the advantages and hot topics in half-wavelength power transmission technology in the past decade. Current research results are summarized from simulation method, steady-state voltage control, relay protection, tuning network, secondary arc and overvoltage suppression, economic evaluation and engineering design. There is still a very long way to go to realize halfwavelength power transmission technology into practical engineering. Up to now, research is mainly focused on the point-to-point transmission mode. If it is connected with the existing synchronous grid, point-to-grid or even gridto-grid transmission mode can be formed. Interaction with synchronous grid and interplay with HVDC transmission system also needed to be explored.

Acknowledgements

This work was supported by State Grid Corporation of China (SGSDDKOOKJJS1600149) and National Key Technology Research and Development Program of China(No. 2015BAA07B01).