SE1400121A1 - Reduced transformer dependence - Google Patents

Abstract

Det presenteras ett system innefattande en första AC-förbindelse av flerfastyp, en andra AC-förbindelse av flerfastyp, en DC-länk, en första station innefattande ett flertal fasben förbundna mellan terminaler hos DC-länken, varvid mitten av varje fasben är förbundet med en respektive fas hos den första AC-förbindelse av flerfastyp, varvid varje fasben innefattar ett flertal omvandlarceller; och en andra station innefattande ett flertal fasben förbundna mellan terminaler hos DC-länken, varvid mitten av varje fasben är förbundet med en respektive fas hos den andra AC-förbindelse av flerfastyp, varvid varje fasben innefattar ett flertal omvandlarceller.(Fig4)

Description

15 20 25 first station comprising a plurality of phase legs connected between terminals of the DC link, the middle of each phase leg being connected to a respective phase of the ñrst multiphase AC connection, each phase leg comprising a plurality of converter cells; and a second station comprising a plurality of phase legs connected between terminals of the DC link, the middle of each phase leg being connected to a respective phase of the second multiphase AC connection, each phase leg comprising a plurality of converter cells.

A least half of the converter cells of the first station may be full-bridge cells, in which case all of the converter cells of the second station are half-bridge cells.

The second station may further comprise a first multiphase DC/AC converter connected between the phase legs and a positive terminal of the DC link, and a second multiphase DC/AC converter connected between the phase legs and a negative terminal of the DC link, wherein the first multiphase DC/ AC converter and the second multiphase DC/AC converter are both connected to the second AC multiphase AC connection via a respective transformer.

The first station and the second station may be provided on opposite sides of the DC link.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/ an/ the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is now described, by way of example, with reference to the accompanying drawings, in which: 10 15 20 25 Fig 1 is schematic illustration of a point to point HVDC system based on VSCs (Voltage Source Converters); Fig 2 is schematic illustration of trafoless (transformer-less) topology based on CTL topology (MMC with half-bridge cel1s); Fig 3 shows schematic graphs illustrating waveforms of trafoless topology based on MMC with half-bridge cells; Fig 4 is schematic illustration of trafoless topology based on mixed cells MMC topology; Fig 5 shows schematic graphs waveforms of trafoless topology based on MMC with full-bridge cells and half-bridge cells; Fig 6 is schematic illustration of trafoless topology based on mixed-cells MMC topology on both stations; F ig 7 is schematic illustration of hybrid trafoless topology based on MMC topologies with half-bridge cells; and F ig 8 is schematic illustration of hybrid trafoless topology with one side trafoless and other side Quasi- trafoless topology.

DETAILED DESCRIPTION The invention will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description.

VSC stations have a simple converter structure and consist of fewer components compared to Line Commutated Converter (LCC) stations. 10 15 20 25 30 Therefore, they occupy less space and offer more flexibility. Transformers are often the most costly equipment in a converter station and contribute to the substantial volume and weight of the converter station platforms. In addition, transformers produce 30-40% of the total converter station losses of the new generation of HVDC systems. With new semiconductor technologies such as optimized IGCT or Silicon Carbide (SiC), the total transformer loss portion is estimated to be 60-70% of the total station loss. Therefore, it is worth to consider the possibility of the HVDC converter without transformer.

In order to reduce cost and loss, one of the future perspectives is the HVDC systems without transformers. Theoretically it is possible to have DC transmission systems without converter transformers. However, practically, there are still some gaps with trafoless (transformer-less) VSC HVDC.

Removing the transformer has two main impacts on the converter. Firstly, there is no galvanic separation between AC and DC sides. Secondly, there is no AC voltage transformation or adjustment capability.

These two impacts affect the performance of the trafoless converter, compared to conventional converters.

The first impact is common to for all trafoless converters and causes problems such as: grounding issues (no neutral bus, DC system operate with positive and negative pole), blocking zero sequence current at the AC side created by odd harmonics of 3fd multiples, blocking DC current entering the AC side occurs due to asymmetries in circuit or control and fault current limitation.

The second impact is more related to the converter design. The main purpose of the transformer is to adapt the AC voltage to the converter voltage such as: voltage step-up/ step down transformation and/ or voltage level adjustments for two different AC voltages.

Therefore, without the transformer, the DC voltage is more or less established by the AC system voltage. The DC current rating then automatically follows the power rating. Thus, the trafoless converters, unlike converters with 10 15 20 25 transformers or auto- transformers, do not provide the opportunity to optimize the voltage and current rating of the DC side equipment. This particularly true in the case of point-to-point DC links, where the equipment suppliers usually have complete freedom of choosing the DC voltage and current ratings. This problem is intensiñed when two different AC grids are connecting through a trafoless DC link.

However, by properly selecting the DC link voltage and topology, the transformer is not necessary from that point of view and over rating can be minimized.

In order to address the problems associated to the AC voltage mismatching of a trafoless HVDC link, there are two options presented for trafoless topologies herein. One solution is based on elimination of transformer of the point to point HVDC systems. The other solution is called quasi-trafoless or hybrid solution, which contribute to fractional reduction of the total transformers rating.

Trafoless: 100% reduction Two solution using MMC (Modular Multilevel Converters) with half-bridge and full-bridge submodules are presented. In the figures, a sub-module with a single arrow next to it indicates a half-bridge converter and a sub-module with two opposing arrows next to it indicates a f11ll-bridge converter.

MMC with half-bridge cells A trafoless point-to-point HVDC system using MMC with half-bridge submodules on both the first converter station 1a and the second converter station 1b presented in Fig 2. To match the AC voltage on AC side V1 of the first converter station 1a, the first converter station 1a needs to be rated for the V1. As the half-bridges only generate positive voltages, the minimum DC link voltage is m. Therefore, each converter arm of the first converter station 1a is rated for the total DC link voltage at 2V1. To match the AC voltage at the low voltage side V2, the second converter station 1b needs to be rated for the 10 15 20 DC voltage of the DC link and AC voltage at the low voltage side. This results in converter arm rating of V1+V2 in the second converter station 1b. A sampled waveform of the trafoless topologies at each station have been illustrated in Fig 3. In Fig 3, the left hand side relates to the first converter station 1a and the right hand side relates to the second converter station 1b.

In the upper graphs, there is a first wave 10 for the lower arm and a second wave 11 for the upper arm. In the lower graphs, the positive DC voltage DC* and the negative DC voltage DC- are indicated. The second converter station 1b does not need to be rated for the full DC link voltage by a proper PWM technique of converter arm voltages. However, as the power is fixed for both stations, working at reduced modulation index at the second converter station 1b will result in a higher current through the converter arm, which can affect the total silicon area needed for the converter. Converter voltage rating and silicon area of each station have been briefly evaluated in Table 1.

First converter station 1a Second converter station 1b modulatmflifldeï = 1 (SPWM) modulatíonmdex =? (SPWM) 4 [G51 _ BIÖC jag: = lldc v - zv 3"' dCi _ 1 Vdcz = Total Voltage rating N__Total= 12V1 (3 + m) Table 1: Rating evaluation of trafoless using MMC with half-bridges at both stations Highlighted features: The converter of Fig 2 provides a trafoless converter HVDC system which can be matched to two different AC voltages with minimum rating. Moreover, the loss is low due to using half-bridge cells.

MMC with full-bridge and half-bridge cells Looking now to the converter of Fig 4, full-bridge submodules are introduced in MMC arms, offering extra flexibility as the voltage of each submodule can 10 15 20 25 30 be reversed. Operating the full-bridge submodules per arm in reverse polarity results in decoupling AC and DC voltages of the converter. Full-bridge submodules are also advantages in DC fault blocking or current limitation which allows converter operation during the fault without tripping the AC breakers. Maintaining the output (AC-side) voltage higher than the pole (DC- side) voltage nominal value is called “boosting operation”. The difference in voltage is compensated by operating a corresponding amount of full-bridge cells in reverse.

Using “boosting operation' feature in trafoless topologies gives extra freedom to choose the DC link voltage rating.

To match the AC voltage on the AC side V1 of the first converter station 1a, when the DC link voltage can be chosen optionally at 2V2, full-bridges are reversely inserted and operating at boosting operation. Therefore, each converter arm of the first converter station 1a is rated for the total DC link voltage and AC voltage at V,+V2 where at least (V1-V2) of the total arm rating needs to be full-bridge cells. To match the AC voltage at the low voltage side V2, station # needs to be rated for the DC voltage of the DC link and AC voltage at the low voltage side. This results in converter arm rating at 2V2 in the second converter station 1b. This way, the second converter station 1b does not need to boost the voltage and following the chosen DC link voltage thus converter operates at maximum modulation index with half-bridge cells.

Therefore, with some extra switching components paid at full-bridge cells, both converter stations are able to operate at maximum modulation index thus minimizing the current rating and silicon area. Sample waveforms of each converter station is illustrated in Fig 5. In Fig 5, the left hand side relates to the first converter station 1a and the right hand side relates to the second converter station 1b. In the upper graphs, there is a first wave 10 for the lower arm and a second wave 11 for the upper arm. In the lower graphs, the positive DC voltage DC* and the negative DC voltage DC' are indicated.

Converter voltage rating and silicon area of each station in this embodiment have been briefly evaluated in Table 2. 10 15 20 First converter station 1a Second converter station 1b modulation index = 1 (SPWM) Vdcl = 2V2 = Tnzvl modulatíon index = 1 (SPWM) VdCZ = 2V2 = Tnzvl l I Idel '__ ä IdcZ = ä Total Voltage Rating N_Total= 24(1+m)V1 Table 2: Rating evaluation of trafoless using MMC with full-bridge submodules at high voltage and half-bridge submodules at low voltage AC grid It is also possible to use mixed cells in stations #2 to optimize the converter current rating (See Fig 6). In this way, full control can be achieved on the DC link design due to decoupling AC and DC side from both stations. By adding some overrating in the full-bridges, the tap-changer functionality is obtained in order to support more reactive power. Moreover, the arcing on overhead lines in case of faults can be extinguished by suddenly reducing the DC voltage. Consequently, the AC and DC sides are decoupled while the VSC can continue to support the AC network without tripping the AC breaker when DC side fault occurs. It is to be noted that having 50% full-bridge cells in each station can guarantee all the flexibility features with 100% AC/ DC decoupling and fault condition operation. However, it is not the optimum design as the number of full-bridges is proportional to AC side ratios.

The embodiments of Figs 4 and 6 provide a number of positive effects. trafoless converter HVDC system can be matched to two different AC voltages with minimum rating. F ault blocking capability is provided. The fault limiting inductor at the AC side can be reduced. The need for AC side breaker tripping is reduced or even eliminated. There is full freedom to choose DC link voltage (AC and DC is coupled). The number of cells and components is reduced. A higher capability for reactive power control (Boosting operation by small de- rating full-bridges) is provided.

Fractional Trafoless (Less-Transformer) 10 15 20 The embodiment of Fig 7 will now be described. This solution can be called as a hybrid solution or quasi-trafoless as it comprises converters combined with transformers, but only at a fraction of the power of the transformers of the prior art. This results in transferring the fraction of the total power through the transformers and some power directly to the grid, thus reducing the transformer cost while using some of their benefits.

Fig 7 shows a hybrid trafoless solution where one converter station is used as trafoless converter and other side converter station is a transformer based converter. This arrangement could be used if the two AC systems have different voltage levels. As shown, the first converter station 1a is rated for the full AC and DC voltage at 2V1 and AC voltage at low voltage side is adjusted by the transformer ratio. Therefore, the transformer on one side could be used to compensate for the AC system voltage Variations in both AC systems so that is not necessary to over rate the converters or operate with lower modulation index. Some technical issues such as no galvanic separation between the AC and DC systems and the need to block zero sequence harmonics do not apply to this arrangement. Although the Si area of each converter remains at 1pu same as fully trafo-based converters, still 1pu transformer power rating is required. The hybrid HVDC system rating has been summarized in Table 3.

First converter station 1a Second converter station 1b modulation index = 1 (SPWM) modulatíonmdex = 1.15 (SPWM) or Iacl = 'Elda = _ I Vdcl = :Vi IacZ am dc Var-z = 2V1 Total Voltage rating N_Total= 48% Table 3: Rating evaluation of hybrid trafoless using trafo-based on one side and trafoless on other side The embodiment of F ig 7 provides a number of positive effects. The hybrid trafoless converter HVDC system can be matched to tvvo different AC 10 15 20 10 voltages. The fault limiting inductor at the AC side is reduced. Higher capability for reactive power control is provided, (lower de-rating is required). A galvanic separation between the AC and DC systems is provided.

There is no need to block zero sequence harmonics applied to this arrangement. 1pu Si area per converter station.

One station trafoless and one station quasi-trafoless Fig 8 shows another embodiment, where two stations are connected point to point in Quasi-trafoless topology, The first converter station 1a is the CTL topology with only half-bridge cells and the second converter station 1b is quasi-trafoless topology. The idea on the low voltage side is very similar to DC auto-transformer concepts. As shown, some of the power is transferred directly through the phase legs of the converter 22 while the difference between the high and low voltage sides is transferred through smaller AC/ DC converters of 5a, 5b, with voltages 21 and 23 respectively, connected by a galvanic insulation to the AC side. This results in operation of each converter at its maximum modulation index, thus obtaining 1 pu Si area per converter station. The arm of the first converter station 1a is rated for V1 AC and DC at 2V1. The second converter station 1b comprises three converters connected in a stacked fashion. The middle converter connected directly to the AC transfer bulk of the power at higher DC voltage of 2V2 while two smaller top and bottom converters are rated for a lower voltage of V1-V2 and transfer fraction of the power through the transformers. Si area and voltage rating of the proposed quasi-trafoless solution is summarized in Table 4.

First converter station 1a Second converter station 1b modulation index = 1 modulationmdexzl = modulation indexza = 1.15 (SPWhA/p (3PWM) _ 1 I“°1 _ EI” Van = Vana = V1“V2 = ëÜ " "ÛV1 Vain = 2V1 4 fun = 14:23 = fišfac modulatíoníndexzz = 1 (SPWM) Va C22 = ZVz 10 15 11 ull-b 1411:22 = Total Voltage Rating N_Total= 48V1 Table 4: Rating evaluation with MMC as a 3-phase DC/AC converter for upper and lower converters: Upper and lower 3-phase DC/AC low voltage converters in the second converter station 1b (converter 21 and converter 23) can be replaced by a standard 2-level 3-phase converter topologies to further minimize the number of components. The 2-level converter suits this application due to reduced voltage rating of upper and lower converters in the second converter station 1b (converter 21 and converter 23) in Fig 8. Rating evaluation using the standard 2-level converter presented in Table 5.

First converter station 1a Second converter station 1b modufafioflindex = 1 modulationmdexu = moáulationindexfi, = 1.15 (SPWhíI) (3PWM) 1 1"* = Eld” Vana = Vana = V1“V2 = EU _ mwi Van = 2V1 4 Inn = Inez: = :ära modulatíoníndexzz = 1 (SPWM) Vana: = 21/2 4 1111:22 = šfac Total Voltage Rating N_Total= 12V1 (3 + m] Table 5: Rating evaluation with 2-level as a 3-phase DC/AC converter in upper and lower converters.

There are a number of positive effects of the embodiment of Fig 8. The quasi- trafoless converter HVDC system can be matched to two different AC voltages with minimum rating. The new Quasi-trafoless topology avoids extra converter rating through fractional power transformers. This also can benefit from 3PWM modulation. Applying 2-level converter in Quasi-trafoless 12 topology leads to a higher rating and cost reduction. The fault limiting inductor at the AC side is reduced. The number of cells and components is reduced.

The invention has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the invention, as deñned by the appended patent claims.

Claims (4)

10 15 20 25 13 CLAIMS

1. A system comprising: a first multiphase AC connection; a second multiphase AC connection; a DC link; a first station comprising a plurality of phase legs connected between terminals of the DC link, the middle of each phase leg being connected to a respective phase of the first multiphase AC connection, each phase leg comprising a plurality of converter cells; and a second station comprising a plurality of phase legs connected between terminals of the DC link, the middle of each phase leg being connected to a respective phase of the second multiphase AC connection, each phase leg comprising a plurality of converter cells.

2. The system according to claim 1, wherein at least half of the converter cells of the first station are full-bridge cells, and wherein all of the converter cells of the second station are half-bridge cells.

3. The system according to claim 1, wherein the second station further comprises a first multiphase DC/AC converter connected between the phase legs and a positive terminal of the DC link, and a second multiphase DC/AC converter connected between the phase legs and a negative terminal of the DC link, wherein the first multiphase DC/AC converter and the second multiphase DC/AC converter are both connected to the second AC multiphase AC connection via a respective transformer.

4. The system according to any one of the preceding claims, wherein the first station and the second station are provided on opposite sides of the DC link.