Study of the Inertia Support Potential From HVDC Cables in Power Systems With High Renewable Energy Source Penetration

Stimulated from the de-carbonization targets which was set up by governments worldwide, more and more conventional generation has been decommissioned. Lack of power system inertia support has gradually emerged as a major challenge for system operators around the world. It is especially the case for weak power systems with higher renewable energy source penetration. Existing research in providing inertial support relies on the installation of super capacitors onto the existing high voltage direct current (HVDC) systems. This technique imposes additional cost and reliability concerns. This article proposed a novel strategy which unlocked the potential from HVDC cables to provide inertia support. A comprehensive feasibility study has been performed on a prototype system in which the dynamic frequency response is analyzed. The results showed that for a feasible control strategy, the released energy from HVDC cables is sufficient to replace the vast majority of synchronous machines within a typical power system. This demonstrated the feasibility of utilizing existing HVDC cables for inertia support which significantly reduces the economic costs and improves the power system transient stability.

the Renewable Portfolio Standard (RPS), which has been 29 enacted by 29 states [2]. For instance, California increased 30 its RPS target to 60% by 2030 and mandated a 100% clean 31 target by 2015 [3]. The EU planned to significantly reduce 32 its dependency on energy imports, as well as to invest on dis-33 ruptive innovations for decarbonization [4]. All these ambi-34 tious targets require large scale commissioning of renewable 35 energy sources such as wind or photovoltaic (PV). 36 Replacing synchronous generators with wind turbines or 37 PVs reduces the inertia of power systems. Traditional grids 38 rely on synchronous generators to provide bulk system inertia 39 and participate in frequency regulations. When fault occurs, 40 the kinetic energy stored within the synchronous generators 41 (known as inertia) is released to compensate the imbalance 42 between generation and demand. This prevents the frequency 43 to fall (or rise) rapidly beyond its statutory limit. With the 44 increased level of renewable energy source (RES) penetra-45 tion, synchronous generators are gradually decommissioned, 46 characteristics of the frequency transient change are ana-89 lyzed. It is concluded that for a feasible control strategy, 90 the released energy from HVDC cables is enough to replace 91 the clear majority of synchronous machines within a typical 92 power system. 93 Operational experience shown the main constraint for inte-94 grating more RES is lack of system inertia. Techniques pro-95 posed in this article solve this critical issue with low cost and 96 high reliability. This article reports disruptive innovation for 97 decarbonizing power system and facilitates net zero targets 98 set by the twenty-sixth session of the Conference of the Par-99 ties (COP 26) to the UNFCCC (United Nations Framework 100 Convention on Climate Change) MMC. It has the potential 101 to produce significant social and economic benefits. The fea-102 sibility of utilizing existing HVDC cables for inertia support 103 which significantly reduces the economic costs and improves 104 the system reliabilities. It will increase the stability of power 105 systems with higher renewable energy source penetration. 106 It is believed that the findings of this study are close related 107 to the scope of the journal and will gain great credits from a 108 large number of audiences. 110 Inertia plays a key role in handling frequency disturbances 111 within power systems. The power grid is evolving to inte-112 grate more and more RES, mainly wind and solar, which do 113 not consist mechanical inertia. In the context of large-scale 114 connection of RES, the low inertia features weakened the 115 robustness of the power system and its ability to recover 116 frequency from disturbances [6]. Intensive research work has 117 been focused on improving the system inertia levels with var-118 ious proposals. According to their mechanisms, the existing 119 inertia support methods can be divided into two catalogues: 120 rotating machines and power electronics devices. As a conventional generator, the traditional thermal power 123 unit can supply electricity to the grid as well as provide iner-124 tia support. To achieve the same, large-scale energy storage 125 equipment is widely utilized [7], with mechanical energy stor-126 age capacities of up to 100 MW from compressed air energy 127 storage (CAES) devices [8], [9] and pumped-hydro (PHS) 128 plants [10]. Like thermal units, the energy stored in the CAES 129 and PHS can be used to drive synchronous generators for 130 providing inertia support. In addition to these, synchronous 131 compensator (SC) [11]- [13], as a rotating equipment, can 132 store kinetic energy when spinning. Although a SC does not 133 generate active power, it is able to release rotational kinetic 134 energy to support system inertia in the event of sudden loss 135 of generation. The existing methods to support power system 136 inertia are summarized in Table 1.  • the control algorithm can be designed to damp the fre-182 quency disturbances when power system is subject to 183 contingencies.

184
Although the existing methods provide inertia support in 185 some extent, they require significant investment on hardware 186 and equipment which is not cost effective. Other key dis-187 advantage is the needs of large amount of power electronic 188 devices and super capacitors whose reliability is not adequate. 189 In contrast of this, an innovative technique is brought into 190 consideration to utilize the energy stored within the HVDC 191 cables for inertia support. This method does not introduce 192 additional equipment which resolve both the economic and 193 reliability issues. It is also able to boost its inertia support 194 capacity in the future with large-scale HVDC cables being 195 laid.  The first type of approach proposes an integrated inertia 208 control method for different MMC-HVDC systems: a coor-209 dinated control that integrates the capacitive energy of the 210 MMC-HVDC sub-module and the overspeed reserve capac-211 ity of the wind turbine when the frequency drops signifi-212 cantly. The second type of method is the focus of this paper. 213 Reference [27] uses a full or half bridge converter based 214 MMC to extract energy from the HVDC cable to provide 215 additional active power to the AC system. The literature 216 argues that the increased costs are disproportionate to the 217 benefits reaped when the dc voltage drop is kept small 218 (around 10%).

219
To keep the active power constant, the cable current will 220 increase accordingly when operating at reduced voltage. 221  Instantaneous power for inertia support is computed as: Characteristic parameters model of a coaxial cable.
In (1)  The inertia, usually expressed as J with a unit 'kg · m 2 ', 281 is a standard physics term defined in various international 282 standards. In engineering, the inertia of motor is usually 283 expressed as 'GD 2 '. The relationship between these two is: 284 For synchronous generators, since pole pairs p can vary, 286 the angular velocity ω (rad · s −1 ) refers to electrical angu-287 lar velocity is different from the rotor's mechanical angular 288 velocity (rad · s −1 ). The relationship among mechanical 289 angular velocity , electrical angular velocity ω, and stator 290 current frequency f (Hz, s −1 ) is shown as: System frequency f is either 60 Hz (for US and part of 295 Japan) or 50 Hz (for the rest of the world). The number 296 of polar pair p varies with different types of synchronous 297 generators. For example, salient pole machines are widely 298 used for hydraulic turbines while cylindrical rotor is domi-299 nating steam turbines. To achieve the same electrical angular 300 speed, a quad-pole hydro machine only needs n N = 750 r/min 301 while a cylindrical steam rotor needs a rotor speed of 302 n N = 3000 r/min.

303
When the rotor rotates at the rated mechanical angular 304 speed 0 (synchronous speed), its rotational kinetic energy 305 (unit: J) can be calculated as: At the same time: Take the QF-15-2 steam generator as an example, when the 312 system frequency drops from 50 Hz to 49.8 Hz, the energy 313 released by the its rotor is calculated as following: Comparing (1) and (11) The kinetic energy stored in the rotor of a synchronous gen-327 erator is expressed as Inertial time constant is: Energy stored within a HVDC cable is: The virtual inertia of the cable can be expressed as: It can be rewritten as (17) in the similar form as the 338 rotational inertia of a synchronous motor.
where k Cable represents ratio between the rate of change of W c 345 and the rate of change of the angular velocity of the generator. 346 As shown in (19), k Cable is only related to the step-down speed 347 of the voltage, which is dependent to the control circuit but 348 independent to the cable's intrinsic parameters such as its type 349 or length etc.
Therefore, the equivalent inertia time constant of the 352 HVDC cable is: As shown in (20), k Cable is the key parameter to measure 356 the HVDC cable's inertia support capability. It links the speed 357 of voltage variation with the rate of change of the system 358 frequency (RoCoF), and is the key parameter to optimize 359 when designing the control circuits for HVDC cables' inertial 360 support. The derivation presented in Section 3.1 has provided the the-364 oretical demonstration on providing inertia support for power 365 system through rapid reduction on the terminal voltage of a 366 HVDC cable. HVDC cables are widely utilized in modern 367 power networks, especially for the transmission of offshore 368 wind power. Therefore, through appropriate control circuit to 369 reduce the operating voltage of cables when a frequency dip 370 in the system occurs, the energy stored in cables can be fully 371 utilized for integrated inertia control.

372
To make full use of the energy stored in DC cables, the 373 cable operating voltage is required to decrease rapidly when 374 the frequency change reaches a certain threshold, thus releas-375 ing energy rapidly for inertia support.

376
The design of step-down control circuit can be inspired 377 from the reduced voltage operation of HVDC transmission 378 projects. By considering the existing control methods in 379 HVDC transmission projects, the simplest and most feasible 380 method is to reduce the operating voltage by increasing the 381 trigger angle α of the rectifier. Since the DC voltages on the 382 rectifier and inverter side of the converter station is cosine 383 related to its corresponding trigger angle, the control system 384 can increase the α angle to reduce the DC voltage. This 385 method allows for fast and easy voltage reduction operation. 386  As explained before, the terminal voltage of the HVDC 469 cable is reduced rapidly to release the energy for inertia 470 support. Keeping the rest of the simulation same, but vary 471 the voltage depletion rate, the transient stability is analyzed 472 below.

473
As shown in Figure 6, a 3 MW load is added at t = 20 s, 474 resulting in an instantaneous 2% increase in static load. With 475 the same magnitude of disturbances, the transient stability 476 worsened with more wind integrated. Comparing the results 477 with simulation without the inertia support, it is observed 478 that increasing the equivalent inertia of the cable achieves a 479 significant reduction of the frequency fluctuation and thereby 480 its system stability is improved.    from the HVDC cable results in a slower frequency change 497 and improved stability than the system without support. The 498 maximum RoCoF after the disturbance is inversely propor-499 tional to the system inertia level. The lower the grid inertia, 500 the higher the RoCoF, and the more embedded generations 501 to be tripped by the relay protections, which then results 502 even deeper frequency drop etc. This positive feedback loop 503 endangers the system security and could end up with a fre-504 quency collapse. The proposed method using HVDC cable to 505 provide inertia support reduces the risk of frequency collapse 506 when subject to high percentage of wind generations.

507
The reduction in the DC operating voltage of the cable 508 leads to a reduction in the power transmitted by the DC 509 system, an increase in system line losses and a reduction in the 510 operating efficiency. To make better use of the energy stored 511 in the HVDC cable, future work should focus on further 512 study into this transient process, which can start with different 513 step-down methods and investigate and compare the effects 514 of different step-down waveforms on the system transient 515 process when the step-down speed requirements are met. Up to now, 28 countries around the world have committed 521 to decarbonize their energy system, among which the United 522 Kingdom is pioneering the race for reducing carbon emission. 523 VOLUME 10, 2022 are gradually de-commissioned and replaced by renewable 565 energy sources. From 2010 to 2020, the proportion of fossil 566 fuels has been halved from 80% to 40%. The proportion of 567 coal-fired power has been reduced to 2% and the proportion 568 of clean energy such as wind, solar, hydro, and bioenergy 569 is approaching the proportion of fossil energy sources such 570 as coal oil and gas. This marks the end of the UK's power 571 sector's reliance on coal power and its progression into the 572 non-fossil energy era. New energy source such as solar power and wind power is 576 developing rapidly. However, when frequency drops caused 577 by power shortages, RoCoF protection trips, resulting in the 578 removal of the turbine and further increases in power short-579 ages. The inertia support method based on energy storage rep-580 resented by super capacitors has received extensive attention. 581 It was proposed to configure a super capacitor for each 582 wind turbine to provide extended inertia for the double-fed 583 induction generator (DFIG) [30]. A 1.5MW DFIG is accom-584 panied with a 1150 V×110 F super capacitor energy storage 585 device. According to the literature [31], the cost of lithium 586 ion supercapacitors is approximately $250-$1,000/kWh. For 587 the super capacitor configuration in this example, where C = 588 110F and V dc = 1150V, the stored energy is 1 is optimized to equally split the charging and discharging 591 energy storage capability.

592
The energy stored within a set of lithium ion supercapaci-593 tors is approximately 20 kWh calculated by V dcmax = 1300V 594 and V dcmin = 977V. The average cost of the capacitors is 595 approximately $12,500. The rated voltage of a single lithium 596 ion capacitor unit is ranged 2.2-3.8 V and its capacitance is 597 ranged 1400-2500 F [32]. A single supercapacitor unit with a 598 rated voltage of 3 V and a capacitance of 2500 F is selected 599 for inertia support. To achieve the same nominal voltage 600 (1150 V) of the DFIG, 384 units need to be connected in 601 series. The capacitance of 384 series connected capacitors is 602 6.5 F, to increase this to desired level (110 F), 17 series capac-603 itor branches connected in parallel to build the capacitor bank 604 (384 * 17). The maximum discharge current limits the power 605 extraction and absorption of the capacitor bank to 73.4 kW. 606 The practical Rate of Change of Frequency (RoCoF) limit 607 in future GB power system is estimated to be 0.5 Hz/s [33]. 608 Therefore, 4 GW of power needs to be provided within 609 1 second to meet the requirement of system frequency bal-610 ance. If the missing inertia of 100 GVA · s in net zero sys-611 tem is supplied only by supercapacitor banks as described 612 above, around 27,500 such banks are needed. This makes the 613 one-time investment approximately $350 million, excluding 614 the cost of supplement devices, operation and maintenance 615 throughout their whole life.  The main idea of this method is to fully utilize the energy 645 released from the HVDC cables during the voltage deple-646 tion process. No additional energy storage device is required 647 which means zero construction investment, low mainte-648 nance cost and low operational cost. The reduced installation 649 of power electronics devices and supercapacitors can also 650 reduce its complexity as well as improve the reliability.

652
With the increased percentage of renewable energy sources, 653 system inertia continues to fall due to the decommissioning 654 of synchronous generators. To maintain the system security 655 during the operational time scale, the system inertia must be 656 kept above its threshold. As discussed in the previous section, 657 two methods can provide inertia support:  Table 2.

662
From commercial point of view, technologies using super-663 capacitor banks to provide inertia support incur excessive 664 investment, which involve construction cost, operation and 665 maintenance cost, auxiliary cost etc. O&M cost is calculated 666 as a percentage of the initial construction cost, which is 667 assumed to be 10% in this paper. For reliability analysis, 668 assume the failure rate of individual supercapacitor is identi-669 cal as: λ SC . The failure rate of a multi-element supercapacitor 670 bank is calculated from (22)