Comparative Feasibility Studies of High Voltage Circuit Breaker Operating in the LFAC System With Different Frequencies

With the development of the flexible low frequency transmission technology following the improvement of offshore wind power transmission, it is of great significance to investigate the operation adaptability of the main equipment especially under the low frequency conditions. A comparative study was performed in the present work to discuss the dynamic operation performance of a 252 kV/50 kA puffer-type circuit breaker influenced by the system operational frequency. Since the decreased operational frequency would result in a longer arc duration, a pre-designed driving mechanism with characteristics of high speed control was proposed and compared with a conventional unit. Moreover, as one of the reasonable indicators to evaluate the interruption performance of the circuit breaker, the critical rate of rise of recovery voltage (RRRV) that the circuit breaker can withstand after the arc extinguishment under different low frequencies were computationally obtained by an established arc model. The effects of the frequency change on the arc dynamic characteristics during the interruption of circuit breaker were comprehensively discussed as well and it could reasonably provide technical optimisation for the development of low-frequency-based circuit breaker for further engineering applications by analysing its interruption feasibility.

reasonable distance approximately within 100 km [1]. Due 28 The associate editor coordinating the review of this manuscript and approving it for publication was Guido Lombardi .
to the limited transmission distance, HVDC technology is 29 developed more feasible for the long distance transmission. 30 However, it is noticed that the application of offshore wind 31 farms should be equipped with expensive converter stations, 32 resulting in high cost of construction and maintenance. The 33 space charge accumulation caused by HVDC transmission 34 is a non-negligible problem as well [2], [3], [4]. With an 35 in-depth consideration of above limitations, the flexible low 36 frequency AC (LFAC) system was proposed with reducing 37 the expense and increasing system reliability for the grid 38 interconnection, which not only achieves the technique exten-39 sion of HVAC transmission but also reduces the cost for using 40 the converter stations during DC transmission. 41 The selected frequency for constructing the LFAC system 42 should be satisfied with the demand for offshore wind power 43 transmission and this was firstly suggested by Xifan Wang in 44 VOLUME 10,2022 This work is licensed under a Creative Commons Attribution 4.0 License. For more information, see https://creativecommons.org/licenses/by/4.0/ circuit breaker. Considering the long-time interval before the 101 current passing its zero point in LFAC system, the extended 102 arcing time of the circuit breaker, the high arc energy and the 103 difficulty for interruption, the corresponding study could not 104 only reasonably propose modified solutions for interrupting 105 low frequency-based short circuit currents, but also essential 106 for product optimum design. It also could provide references 107 for subsequent research of phase selection techniques. 108 With considering the reasonable frequency range to sat-109 isfy the demand of grid connection with far offshore wind 110 farms, the frequency variation researched in the present work 111 ranges from 12.5 Hz (50/4) to 25 Hz (50/2) and a kind of 112 252 kV/50 kA puffer type circuit breaker is chosen as the 113 model circuit breaker. The research strategy of present work 114 is organised in following order, presenting an overview of the 115 conventional AC circuit breaker with discussing the involved 116 factors that influence its operation performance in the first 117 part of section 2. A magnetohydrodynamics (MHD)-based 118 arc model and its validity is then described in the second part 119 of section 2. The model settings are verified with experimen-120 tal results obtained by performing an interruption test duty 121 of the short line fault type (SLF) L90 for the model circuit 122 breaker, which is shown in the third part. To study the effect 123 of the driving mechanism on the operational performance of 124 the model circuit breaker, two different travel characteristics 125 of the moving components driven by the driving mechanism 126 are compared in section 3. Furthermore, the influences of 127 frequency variation on the arc dynamic behaviour and inter-128 ruption capability of the model circuit breaker are further 129 discussed in section 3. Conclusions are finally drawn in 130 section 4. As a key protection apparatus, the circuit breaker is operated 134 to isolate a faulty part of the grid system timely and maintain 135 the system stability. The essential role of the circuit breaker 136 is to interrupt the fault current passing through it successfully 137 and it involves complicated physical-chemical processes. The 138 interruption capability of the circuit breaker is predominantly 139 determined by the establishment of the dynamic environment 140 of the flow field, especially before the current passes its final 141 zero point. It is mainly affected by numbers of factors such as 142 current level, arc duration, travel characteristic of the moving 143 contacts and the design of nozzle geometry. To complete the 144 interruption duty successfully, the circuit breaker should be 145 designed with ensuring two representative aspects. First of 146 all, the gas between the two arcing contacts would change 147 from a conducting medium at high current to a poor conductor 148 after the current passes the final current zero. Secondly, suffi-149 cient insulation of the gas needs to be restored at a short time 150 scale (∼100 µs) after current passes its final zero. A well-151 designed circuit breaker should also ensure that the arc can 152 always be successfully quenched with the moving contact 153 being at all its possible positions inside the arcing chamber 154 and the core question is how to achieve the interruption 155 of a fault current with optimised gas dynamics and contact travel characteristic. It involves the in-depth understandings 157 of the physical arcing process that directly determines the arc 158 behaviour during the interruption.

159
With comprehensive understandings of the arc physics, 160 the interruption capability of the circuit breaker can be 161 evaluated using computer-aided design (CAD) tool appro-162 priately. With the development of advanced computing tech-163 nologies, there are numerous investigations related to the 164 physical-chemical properties of the arc plasma [12], [13] 165 and the accumulated understandings of such underlying pro-166 cesses also enable the arc modeling being a commonly used 167 approach to study the arc dynamic behaviour during the cir- characteristics and it could also provide the basis for the 173 product optimum design [14], [15], [16].

175
The arc is formed by the breakdown of the gas medium 176 in the contact gap (SF 6 gas in present work) and the flow 177 inside the arcing chamber is assumed to be the turbulent 178 [13], [17]. The arc behaviour could be described by modified can be written as: where φ is the solved-for variable and ρ is the density of the 193 gas mixture, V the velocity vector which includes the radial

194
(v) and the axial (w) velocity components. Source term S φ and 195 diffusion coefficient φ are shown in Table 1, of which each 196 notation has its conventional meaning [13]. Besides that, the 197 thermophysical properties of the SF 6 -PTFE mixture used in 198 present work are given in [18].

199
Ohmic heating is a physical process that converts electric 200 energy generated by the flowed current to thermal energy 201 and the gas medium will be heated up to a high temperature.

202
To determine ohmic heating (σ E 2 ), the electrical conductiv-203 ity σ is calculated as the function of temperature and pressure 204 and the electric field is obtained by solving the following 205 current continuity equation: The boundary conditions for solving the current continuity 208 equation are defined following the long-range nature of the 209 electric field. The computation domain should be sufficiently 210 large and the uncertainties caused by the settings of boundary 211 conditions would not affect the solution in domain of interest. 212 With the consideration of the turbulence model [19], it has 213 been observed that the Prandtl mixing length model has more 214 computational advantages in comparison with the k-ε model. 215 The turbulence parameter is sensitive to the geometry of the 216 nozzle [20] and the turbulence viscosity is calculated by: where the length scale L s is assumed to be proportional to the 219 local thermal radius of the arc column r δ , and c is the turbulence parameter which should be calibrated 222 in accordance with at least one set of experiment results. To scientifically describe the interactions between hot arc 224 column and solid nozzle, a semi-empirical radiation model is 225 applied by assuming that radiation only travels in the radial 226 direction of the axis-symmetric arc column. It means that the 227 cell on nozzle surface which is radially shadowed by a solid 228 object receives no radiation from the arc [21]. Moreover, this 229 model defines an arc core region which starts from axis to the 230 isotherm of 83%T max and this is actually a defined radiation 231 emission region and the emitted radiation energy from the 232 arc column will reach the nozzle surface and cause ablation. 233 The total amount of the radiation energy can be calculated by 234 the net emission coefficient, which is expressed as a function 235 of temperature, pressure and cylindrical arc column radius. 236 For the puffer type circuit breaker, the cylindrical column 237 radius is defined as the radial distance from the axis to the 238 4,000 K isotherm. Except for the emission part, the remained 239 amount of radiation would be absorbed within the arc region 240 from 83%T max to the 4,000 K isotherm [22], [23], [24]. 241 In this model, the radiative flux normal to the defined surface 242 VOLUME 10, 2022 element of nozzle can be obtained by:

223
where ξ is a coefficient calculated by ablation surface incline, 245 q is the net radiative power loss per unit volume, R 0.83Tmax is 246 the radius of the arc core and α is the proportion of radiation 247 re-absorption. Mass loss rate per unit length is calculated by: where h v is the energy required to convert one kilogram of 250 PTFE (nozzle material) to vapour of 3,400 K (11.9 MJ/kg)

251
[25] and the mass density of PTFE is 2.2 g/cm 3 . process after the current interruption, regarding as thermal 282 recovery process.

283
After current zero, a low post arc current still flows in the 284 residual plasma between the arcing contacts under the action 285 of the imposed system transient recovery voltage (TRV). The 286 TRV appears across the circuit breaker terminals after current 287 interruption and it is one of the decisive factors to assess the 288 interruption capability of circuit breaker. It highly correlates 289 to the occurred fault types and characteristics of the power 290 grid system. This TRV is linearly increased immediately at a 291 given rate of rise (dV/dt). The post arc current will be solved 292 using Ohm's Law with the given dv/dt to assess the thermal 293 recovery of insulating gas medium, which indirectly implies 294 the interruption capability of the circuit breaker. A successful 295 interruption is achieved if the post arc current decreases to an 296 insignificant low value in a few microseconds.

297
The critical RRRV which is the threshold value of dv/dt is 298 determined by calculating the post arc current with different 299 values of dv/dt. This critical value is an important indicator to 300 describe the thermal interruption capability of circuit breaker. 301 A value of dv/dt higher than this critical RRRV would lead to 302 unsuccessful thermal recovery. In present work, the thermal 303 interruption capability of the circuit breaker is evaluated by 304 interrupting the SLF L90 since it is more difficult and harsher. 305 The initial dv/dt is much higher, resulting in a more serious 306 condition for the circuit breaker interruption.

308
To study the impacts of the grid system frequency variation 309 on the arc dynamic behaviour, the accuracy and reliability of 310 the calculation results should be confirmed by proven CFD 311 arc model with a satisfactory solution convergence. Actually, 312 the model has been applied to predict the pressurisation in 313 an auto-expansion circuit breakers [18] and the arc voltage in 314 the puffer circuit breaker [26]. The results match reasonably 315 well with the measurement results. In addition, during current 316 zero and post arc phases, the turbulence cooling becomes the 317 predominant mechanism to remove remained thermal energy 318 in the residual arc. In present work, the Prandtl mixing length 319 model was used, of which the calibration of the turbulence 320 parameter c is performed with two experiment results.

321
The model circuit breaker has passed the L90 test in 50 Hz 322 with an arcing time of 10 ms by sustaining the TRV imposed 323 from power grid system with a RRRV of 9.18 kV/us while 324 the test was failed when arc duration reduces to 8.8 ms. The 325 second test is performed as a typical thermal recovery failure 326 case to determine the shortest arc duration the circuit breaker 327 could interrupt. Both of these two tests were controlled by a 328 conventional driving mechanism. The average velocity of the 329 moved arcing contact is 3.5 m/s before the separation of the 330 two arcing contacts and it grows to 9.6 m/s during the arcing 331 process with the short arc duration.

332
The selection of the turbulence parameter c depends on the 333 calibration of the turbulence model. A normally used range 334 of c from 0.32 to 0.35 is then chosen to calculate the post 335 arc current. It is found that the results from prediction for 336   it is highly associated with the fluid flow dynamics inside 361 the arcing chamber. The circuit breaker should be designed 362 with sufficient capacity to interrupt different kinds of faults 363 within a permitted arc duration. During the interruption of 364 the circuit breaker, the moving contact has to accelerate to a 365 minimum speed over the ''over travel'' distance. Such arcing 366 time and contact acceleration are both realised by an effective 367 control of the driving mechanism and the moving contact 368 would stop at various axial positions with different arcing 369 time. With the considerations of the LFAC system, there 370 are two proposed scenarios for designing the driving mecha-371 nism characteristics, i.e. conventional driving mechanism and 372 high speed driving mechanism. When uses the conventional 373 driving mechanism in power frequency, the designed travel 374 characteristic of the moving contact is sufficient to complete 375 the establishment of the flow field for arc extinguishment. 376 However, the physical current zero point is delayed due to the 377 frequency decrease. It implies that the original travel distance 378 of the moving contact becomes shorter and it needs a further 379 movement to quench the arc at a new current zero point. The 380 mechanical property of the conventional driving mechanism 381 needs to be optimised with a much longer travel distance 382 of the contact. The second approach is proposed based on 383 the technology of high speed interruption. The high speed 384 driving mechanism could reduce the breaking time of the 385 circuit breaker by compressing the ''over travel'' time with 386 VOLUME 10, 2022  of 22000 J. Table 2 provides a comparison of the characterised 406 parameters for the two driving mechanisms.

407
From the comparison, the arcing time for the model circuit 408 breaker interrupting the short arc duration case was shortened 409 to 7 ms while that for the conventional driving mechanism is 410 10 ms. The total action time before arcing contacts separation 411 also largely decreases from 28 ms to 8 ms while the averaged 412 velocity of the moving contact is increased from 9.6 m/s to 413 14 m/s during the flow field establishment process and such 414 faster acceleration leads to the shorten of the arc duration. 415 Figure 5 presents the accumulated electrical energy during 416 the whole arcing process. The total energy input for Com.1 is 417 79.33 kJ while only 39.81 kJ is accumulated for Com.2.

418
Although the accumulated electric energy input for Com.1 419 is almost two times larger than that of Com.2, the maximum 420 pressure inside the upstream puffer cylinder is almost equal, 421 respectively 1.60 MPa and 1.61 MPa for Com.1 and Com.2, 422 as shown in Figure 6. The fluctuation of the pressure curve is 423 primarily resulted from the propagated pressure waves from 424 arcing space to the puffer cylinder. The pressure wave cannot 425 propagate immediately from one position to another and the 426 compressible characteristics of the gas flow inside the arcing 427 chamber also leads to the disturbance during the propagation 428 and reflection of the pressure waves. It is also found from Figure 6 that the pressure within the   to the start position of main nozzle flat throat at this time 462 instant. The proportion of the PTFE vapour in SF 6 -PTFE gas 463 mixture flows through cross section C is less than 8% of the 464 total mass flow while the thermal energy brought by PTFE 465 vapour occupies 60.1% of the total thermal energy. Since the 466 mass and energy flow through cross section C for Com.2 467 shows a similar pattern, it would not be discussed in this part 468 (Figure 7 right figure).

469
From Figure 8, it is found that the PTFE vapour does not 470 reach the interior part of the puffer cylinder, which total mass 471 flow and thermal energy are both dominantly controlled by 472 SF 6 gas. Therefore, it could be concluded that PTFE vapour 473 backflow has a negligible effect on the pressurisation process 474 in the puffer cylinder. In addition, the mass and energy both 475 flow out from the puffer cylinder (value above zero) during 476 the whole arcing time, which indicates that the effect of gas 477 VOLUME 10, 2022  puffer cylinder speedily rushes out with a low temperature 503 and a high mass flow rate. This mass flow rate largely impacts 504 the gas flow velocity and energy transfer between hot arc 505 column and surrounding cold gas. In addition, this high mass 506 flow rate causes an additional mass flowing into contact 507 space, leading to the last temporary pressure peak within the 508 contact space, as shown in Figure 6. The stronger mass flow 509 rate is designated by a lower pressure difference between the 510 puffer cylinder and middle of main nozzle flat throat. Such 511 relationship is due to the fact that the pressure within the 512 heating channel starts to recover shortly before final current 513 zero.  Based on previous studies, it could be known that high speed 555 interruption could relatively reduce the breaking time and the 556 moving contact travel to the specified position at particular 557 time to establish the required flow field environment with the 558 pre-designed fast motion characteristic. It still could ensure 559 sufficient arc quenching capability although the arc duration 560 is shortened.

561
Combined with preliminary discoveries of the feasibility to 562 use the high speed interruption technology for the operation 563 VOLUME 10, 2022 is appeared with a variation of interrupting current waveform, 572 as shown in Figure 12. It is also found that the overall current 573 becomes lower with reduced frequency for the low frequency 574 condition. The variation of the accumulated electrical energy 575 input as a result of frequency reduction is firstly studied, 576 as shown in Figure 13.

577
For the two cases with frequency of 20 Hz and 25 Hz, the 578 current increases to its maximum at the very beginning of the 579 arc burning. Although the current is slightly lower during this 580 stage, the overall accumulative impact is still stronger so that 581 the total electrical energy input presents a growth with raised 582 frequency.

583
The pressure distribution at point A also displays a similar 584 pattern within the whole arcing time. The maximum pressure 585 in the puffer cylinder slightly grows with increased frequency, 586 as illustrated in Figure 14, respectively from 1.51 MPa, 587 1.52 MPa, 1.53 MPa, 1.56 MPa to 1.63 MPa. Actually, an 588 initial filling pressure of the working gas inside the puffer 589 cylinder is 0.7 MPa and it roughly increases to 0.88 MPa 590 during the period of moving contact acceleration within the 591 over-travel distance. The minor growth is resulted from gas 592 compression due to the movement of the piston. After the 593 arc initiation, the accelerated velocity of the piston becomes 594 larger so that the pressure starts to increase considerably 595 with a combined effect between gas compression and hot gas 596 backflow.

597
As presented in Figure 15, the overall mass and thermal 598 energy flowing out through cross section D inside the puffer 599 cylinder are both reduced due to the increased frequency. 600 As explained previously, the flow through cross section D 601 is the summation of compressed gas with positive flowing 602 direction and hot gas backflow with negative direction into 603 the puffer cylinder. Because of the frequency increase, the 604 total electric energy input to the arc column also increases, 605 which results in an improvement of the flow reversal effect so 606 that the total mass and thermal energy flow from arcing space 607 backwards the upstream puffer cylinder becomes more. This 608 is why the difference between outflow and inflow through 609 cross section D respectively for the total mass and thermal 610 energy becomes increasingly smaller. It implies that the gas 611  The transportations of mass and thermal energy both reach the 618 first peak roughly at 11 ms, which is because the current just 619 increases to its maximum of 56.1 kA under 50 Hz while the 620 current for low frequency conditions has already decreased 621 below 40 kA. It indicates that the energy injected into the 622 arc column around this time instant becomes the largest for 623 the 50 Hz case and the gas flow transportation also becomes 624 stronger so that the total amount of mass and thermal energy 625 flowing out through cross section D drops rapidly during 626 11 ms to 13 ms. After that, the current decreases to a lower 627 value below 20 kA for all the cases. A lower value decreased 628 by frequency indicates a less effect of the gas backflow so that 629 the mass and energy flowing out become more.

630
Effect of frequency change on temperature distribution is 631 discussed in Figure 16. The highest temperature is increased 632 from 11,300 K to 12,300 K at different frequencies. It could 633 be observed from the figure that the flat throat of main nozzle 634 has already been completely cleared by the moving contact at 635 final current zero. In the vicinity of moving contact, there is a 636 stagnation area where the thermal energy produced by ohmic 637 heating could not be sufficiently removed by the slower gas 638 flow so that the temperature keeps high. It is due to a hitting 639 of the arc flow on the contact surface. In addition, another 640 stagnation area is existed between auxiliary nozzle and main 641 nozzle since the gas flowing out from the heating channel 642 splits into two directions: hollow contact and downstream 643 exit. The imposed system TRV is later shared with these two 644 sections which are divided by the flow stagnation point.

645
As discussed above, the pressure within arcing space drops 646 during current zero phase since the electrical power supply is 647 insufficient due to the current decrease. Then, the gas in the 648 puffer cylinder rushes out towards arcing space with a high 649 mass flow rate to speedily cool down the arc in comparison 650 with that at high current phase. As a dominant mechanism for 651 arc cooling before current zero, the turbulence is influenced 652 by mass flow rate and corresponding velocity of the gas flow. 653 With the turbulence cooling, the size of arc column is largely 654 compressed by cold gas flow and the temperature is reduced 655 by interacting with the surrounding cold gas and the arc will 656 finally extinguish due to the stop of thermal ionization.

657
With the increased frequency, the maximum temperature 658 of the arc is slightly increased when current passes its zero. 659 It implies that the effect of cold gas blowing becomes rela-660 tively weak due to the insufficient flow field establishment. 661 The arc column diameter is thus marginally bigger with the 662 increased frequency, which undoubtedly increases the diffi-663 culty for arc quenching.

664
On that basis, the impact of frequency difference on the 665 arc voltage is evaluated as well, as shown in Figure 17. The 666 arc voltage is sensitive to the arc column size (diameter and 667 length), electrical conductivity of the arc medium and the gas 668 pressure distribution in the contact space. Actually during the 669 high current phase, the current influences the energy dumped 670 into the contact space and the pressure. From Figure 14, it is 671 noticed that the pressure slightly increases with the increased 672 frequency. Due to the variation of interrupting current, the 673 time interval for high current phase which stops at the current 674 below 15 kA becomes marginally longer with the increased 675   As shown in Figure 18, the temperature of arc is almost 693 stable during the high current phase so that the arc resistance 694 is also roughly stable and the variation of the arc voltage is 695 insignificant. Before final current zero, the arc temperature 696 significantly reduces and results in a sharp growth of the arc 697 resistance, which consequently increases the arc voltage to a 698 much higher value. The higher peak voltage at final current 699 zero point indicates that the ionization degree of the particles 700 within the arcing medium becomes insufficient so that the arc 701 conductance is reduced faster correspondingly, which means 702 that the recovery process of the gas medium becomes faster 703 accordingly and enhanced the thermal interruption capability 704 of the circuit breaker.  breaker when controlled by high speed driving mechanism is 749 slightly higher although the interrupting condition is harsher.

750
(2) In low-frequency-based environment, during the whole 751 arc quenching process, the peak voltage at final current zero, 752 as a critical indicator to evaluate the ionization degree of the 753 gas medium, is decreased due to frequency increase and the 754 critical RRRV also reduces gradually. Such decrease implies 755 a deterioration of thermal interruption capability of the model 756 circuit breaker.

757
(3) Since the arcing time will be prolonged with frequency 758 drop, the model circuit breaker is recommended to operate 759 with high speed interruption technology to expand its scope 760 of the applications within a wide frequency range. Detailed 761 knowledge gained from the research would be useful for 762 the optimization of the circuit breaker in terms of identi-763 fying the effects of the frequency change on arc behaviour 764 during the interruption especially for reliable interruption 765 with different arc durations in LFAC system.