WO2020173544A1

WO2020173544A1 - Mmc statcom with director valves

Info

Publication number: WO2020173544A1
Application number: PCT/EP2019/054646
Authority: WO
Inventors: Aravind MOHANAVEERAMANI; Jan Svensson; Alireza NAMI
Original assignee: Abb Power Grids Switzerland Ag
Priority date: 2019-02-26
Filing date: 2019-02-26
Publication date: 2020-09-03

Abstract

The present disclosure relates to a three-phase Static Synchronous Compensator (STATCOM) arrangement (1) arranged to be connected to a three-phase AC grid. The STATCOM arrangement comprises a Modular Multilevel Converter (MMC) leg (2) and a director valve arrangement (10). The MMC leg comprises a top MMC arm (3a) and a bottom MMC arm (3b) in series connection. Each MMC arm comprises a plurality of series connected converter cells (6). Each cell comprises an energy storage. The top MMC arm is connected between a top point (p) and a middle point (r) of the MMC leg, and the bottom MMC arm is connected between the middle point and a bottom point (q) of said MMC leg. The director valve arrangement is arranged to connect each of the three phases (a, b, c) of the grid to each of the top, middle and bottom points of the MMC leg.

Description

MMC STATCOM WITH DIRECTOR VALVES

TECHNICAL FIELD

The present disclosure relates to a three-phase Static Synchronous

Compensator (STATCOM) connected to an alternating current (AC) power grid.

BACKGROUND

Reduced cost of renewables and accelerated climate change have led to a significant increase in the penetration of renewable energy sources in the power grids. EU countries have already agreed on a new renewable energy target of at least 27% of final energy consumption in the EU by 2030, with few countries as high as 50%. This poses a severe threat to the grid stability. Regulatory bodies like ENTSO-E thus recommend stricter grid codes to be implemented by 2021. One of the notable features in upcoming grid codes is the improved fault handing requirement.

Presently, the requirement on Fault Ride Through (FRT) is to support/boost the positive sequence grid voltage during faults. As an additional feature, the STATCOM could also help damp the negative sequence voltage. This is not a mandatory requirement today. With the new grid codes, it is mandatory for the STATCOM to boost the positive phase-sequence voltage and damp the negative phase-sequence voltage with controller gains set by the grid operator (kp_P and k_pn vary between 2 and 6). The current reference of the STATCOM is to follow the values from the equations shown below:

This requirement may be difficult for the conventional delta (D) connected chain-link STATCOM solution. The arm energy balancing may become a challenge for faults where the ratio of positive and negative phase-sequence voltage approaches unity. SUMMARY

Today, the conventional delta (D) connected chain-link (also called cascaded or Modular Multilevel Converter (MMC), i.e. with series connected converter cells) converters, as illustrated in figure l, are used as STATCOMs in high- power, and/or high-voltage (HV) applications. A drawback with the delta- connected STATCOM may be when operating with unbalanced grid voltages and currents. A differential mode power caused by the unbalanced voltage and currents may lead to charging of some of the delta-connected converter arms while discharging the others. It is to be noted that the net energy absorbed by the converter is typically still zero. However, the energy gets exchanged between the converter arms. To be able to re-balance the capacitor voltages in the converter arms, a calculated zero-sequence current as shown in the expression below is circulated inside the delta branches:

(3)

differential mode power of the converter arms that needs to be corrected. From the above expression, it is evident that the delta-connected chain-link STATCOM would require very high current rating to sustain arm energy balancing as the ratio of negative to positive phase-sequence voltage approaches unity. During severe grid faults, the ratio of negative to positive phase-sequence voltage would reach unity.

Presently, the current reference of the delta-connected STATCOM is adjusted during sever faults so that both the numerator and denominator of the expression becomes zero i.e., the STATCOM behaves as a capacitor for both the positive and negative phase-sequence voltages. Since the new grid codes mandate a STATCOM to behave as an inductor for the negative phase- sequence voltage and a capacitor for the positive phase-sequence voltage, the conventional delta-connected STATCOM fails to operate successfully for some faults. Hence, there is a strong need to investigate new topologies that can handle the new requirements, e.g. regarding FRT, without inadequate harmonic performance of the chain-link converters. According to an aspect of the present invention, there is provided a three- phase STATCOM arrangement arranged to be connected to a three-phase AC grid. The STATCOM arrangement comprises an MMC leg and a director valve arrangement. The MMC leg comprises a top MMC arm and a bottom MMC arm in series connection. Each MMC arm comprises a plurality of series connected converter cells. Each cell comprises an energy storage. The top MMC arm is connected between a top point and a middle point of the MMC leg and the bottom MMC arm is connected between the middle point and a bottom point of said MMC leg. The director valve arrangement is arranged to connect each of the three phases of the grid to each of the top, middle and bottom points of the MMC leg.

According to another aspect of the present invention, there is provided a method of operating an embodiment of a STATCOM arrangement of the present disclosure as a STATCOM connected to the three-phase AC grid. The method comprises operating the director valve arrangement such that at any time over a fundamental phase cycle, the phase having the highest nominal voltage has a conducting connection to the top point, the phase having the lowest nominal voltage has a conducting connection to the bottom point, and the remaining phase of the three phases has a conducting connection to the middle point.

By means of the director valve arrangement, the MMC-director valve combination converter which is herein called a Modular Multi-level DC Neutral Point Clamped (MMDC-NPC) converter can operate as a STATCOM for the three-phase AC grid in accordance with the new grid codes, e.g.

regarding FRT events. The director valve arrangement is called Neutral Point Clamped (NPC) since it may have a topology similar to a three-phase NPC inverter, but connected to the MMC leg instead of a conventional DC source. The MMC is called DC since it outputs typically mono-polar voltage (i.e., voltage with only one polarity but conducts current in both the directions) and takes the place of a DC source vis-a-vis the NPC director valve

arrangement. However, it should be noted that also any other suitable topology for the MMC leg and/or the director valve arrangement may be used for some embodiments of the present invention.

It is to be noted that any feature of any of the aspects may be applied to any other aspect, wherever appropriate. Likewise, any advantage of any of the aspects may apply to any of the other aspects. Other objectives, features and advantages of the enclosed embodiments will be apparent from the following detailed disclosure, from the attached dependent claims as well as from the drawings.

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. The use of “first”,“second” etc. for different features/components of the present disclosure are only intended to distinguish the features/components from other similar features/components and not to impart any order or hierarchy to the features/components.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be described, by way of example, with reference to the accompanying drawings, in which:

Fig l is a schematic illustration of a conventional delta connected MMC STATCOM, according to prior art.

Fig 2 is a schematic circuit diagram of a Modular Multilevel DC Neutral Point Clamped (MMDC-NPC) STATCOM, in accordance with embodiments of the present invention.

Fig 3 is a schematic circuit diagram of a half-bridge converter cell of an MMC arm, in accordance with embodiments of the present invention. Fig 4 is a schematic graph illustrating how different phases of the grid are connected to different points in the MMDC-NPC STATCOM during operation, in accordance with embodiments of the present invention.

Fig 5 is a schematic functional block diagram of a control arrangement for a STATCOM arrangement, in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments are shown However, other embodiments in many different forms are possible within the scope of the present disclosure. Rather, the following embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers refer to like elements throughout the description. Figure 2 illustrates a STATCOM arrangement 1. A Modular Multi-level DC Neutral Point Clamped (MMDC-NPC) converter topology is proposed for the STATCOM arrangement. Thus, the STATCOM arrangement comprises an MMC leg 2 connected to an AC grid with three-phases a, b and c via a director valve arrangement 10. The director valve arrangement 10 is arranged to connect each of the three phases a, b and c of the grid to each of the top point p, the middle point r and the bottom point q of the MMC leg 2. This may be achieved, as in accordance with the figure, with an NPC converter topology.

Herein, the terms“top” and“bottom”, as well as variations thereof, are used to denote different elements of the STATCOM arrangement 1. It should be noted that this is only to distinguish them from other like elements and should not be interpreted as indicating positional relationships. Generally, the terms have been chosen in relation to typical voltage relationships but this may not always be so.

Further, herein it is stated that different parts of the STATCOM arrangement 1 are“connected” to each other. This implies that an electrical current may flow via such a connection during regular operation of the STATCOM arrangement. However, such flow of current may be subject to one or more semiconductor devices/switches being turned ON (conducting) or OFF (non conducting). When it is explicitly stated herein that an electrical connection exist which presently allows a current to flow, the term“conducting connection” is sometimes used.

The MMC leg 2 comprises a top MMC arm 3a and a bottom MMC arm 3b connected in series with each other. Each MMC arm 3 comprises a plurality of series connected (also called chain-linked or cascaded) converter cells 6 (see also figure 3). The top MMC arm 3a is connected between the top point p and a middle point r of the MMC leg 2, and the bottom MMC arm 3b is connected between the middle point r and the bottom point q of the MMC leg 2. Each of the converter cells comprises an energy storage, e.g. a capacitor, and a plurality of semiconductor devices. In some embodiments, it may be preferable that, in each of the converter cells, the semiconductor devices form a half-bridge (HB) topology, since the MMC arms may not need to be bi-polar why HB cells are typically more cost effective. However, full- bridge (FB) or other topologies are not excluded and may be convenient in some embodiments of the invention. Each MMC arm 3 may thus in some other embodiments comprise only full-bridge cells or a mix of half- and full- bridge cells. In some embodiments, mostly half-bridge cells may be used in each MMC arm but with at least one, e.g. a few, full-bridge cells to achieve some benefits at certain conditions.

The director valve arrangement 10 is designed such that it can be operated such that at any time during operation, e.g. over a fundamental phase cycle, the phase a, b or c having the highest nominal voltage uA, uB or uC has a conducting connection to the top point p, while the phase a, b or c having the lowest nominal voltage uA, uB or uC has a conducting connection to the bottom point q. The remaining phase a, b or c, i.e. the phase having a nominal voltage uA, uB or uC which at that time lies between the respective voltages of the other two phases, has a conducting connection to the middle point r. Since each of the phases a, b and c can be provided with a conducting connection to each one of the points p, r and q by means of the director valve arrangement to, which one of the phases that is at any time connected to which one of the points depends on which of the one-directional

semiconductor switches 7 in the director valve arrangement are switched to conducting or non-conducting at that time. It should be noted that during regular operation, each phase is only conductively connected to one of the points p, r or q, between the grid and any of the MMC arms 3.

To this end, the director valve arrangement 10 may comprise a plurality of director valves T. Herein, the director valves T responsible for directing the current of the first phase a are denoted Ta, the director valves T responsible for directing the current of the first phase b are denoted Tb, and the director valves T responsible for directing the current of the first phase c are denoted Tc. When any respective phase is intended, these director valves may herein be denoted Tx. The respective director valves Tx of each phase may then be denoted Txi, Tx2, Txg, Tx4 etc. as in figure 2.

Each of the director valves T may comprises any of a single, or a plurality of series connected, one-directional voltage blocking semiconductor switch 7, e.g. Insulated-Gate Bipolar Transistor (IGBT) or Integrated Gate- Commutated Thyristor (IGCT) or Bi-mode Insulated Gate Transistor (BiGT); a mono- or bipolar chain-link converter, or a mix of mono- and bipolar chain-link converters, e.g. MMC; or a single, or a plurality of series connected, thyristor or Bi-directional Controlled Thyristor (BCT). Typically, with the possible exception of the case of a bipolar MMC, each director valve may also comprise an antiparallel reverse-blocking semiconductor device 8, e.g. comprising or consisting of a diode.

In some embodiments, and as shown in figure 2, the director valve

arrangement 10 may comprise three director valve legs 4a, 4b and 4c, each connected to a respective phase a, b and c and comprising a plurality of series connected director valves (T). In some embodiments, and as shown in figure 2, each director valve leg 4 has four, e.g. only four, series connected director valves T, herein called: a topmost valve Txi connecting the director valve leg to the top point p, a top middle valve Tx2, a bottom middle valve Txg and a bottommost valve Tx4 connecting the director valve leg to the bottom point q. Then, the phase a, b or c which is connected to the valve leg 4 may be connected to the valve leg between the top middle valve and the bottom middle valve. In these embodiments, each valve leg 4 thus consists of four director valves T connected in series with each other, the topmost and top middle valves in a top valve arm 5a to one side of the phase connection point X, Y or Z, and the bottommost and bottom middle valves in a bottom valve arm 5b to the other side of the phase connection point X, Y or Z.

In some embodiments, and as shown in figure 2, each director valve leg 4 is connected to the middle point r, via the MMC connection point x, y or z in the figure, e.g. both between the topmost valve Txi and the top middle valve Tx2 and between the bottom middle valve Txg and the bottommost valve Tx4.

Further, in some embodiments, and as shown in figure 2, the middle point r is connected to the director valve leg 4, e.g. in both places as per the previous paragraph, via a reverse-blocking semiconductor device 9, e.g. comprising a diode, e.g. a respective reverse-blocking semiconductor 9a and 9b for said different places.

Thus, an NPC topology of the director valve arrangement 10 may be obtained, by which each of the phases a, b and c can be conductively specifically connected to each of the top, middle and bottom points p, r and q. Figure 3 illustrates a converter cell 6, herein in HB topology but (as mentioned above) FB topology may also be used with the present invention for one, some or all of the cells of each arm 3. The cell comprises an energy storage 31, e.g. comprising a capacitor, supercapacitor and/or a battery, or a plurality thereof. The cell also comprises a plurality of semiconductor valves S, herein a first semiconductor valve Si and a second semiconductor valve S2 connected in series across the energy storage to form the HB topology. Each semiconductor device S typically comprises a one-directional semiconductor switch 32 and an anti-parallel reverse-blocking semiconductor device 33. The semiconductor switch 32 may comprise or consist of an IGBT or an IGCT, or a plurality thereof. The reverse-blocking semiconductor device 33 may comprise or consist of a diode or a plurality thereof.

The hybrid MMDC-NPC topology typically has half-bridge chain-link cells 6 as wave shapers and director switches T to connect the MMC arms 3 to the AC phases a, b and c, respectively, to achieve energy balancing in the chain- link cell capacitors 31.

The STATCOM arrangement 1 also comprises a control arrangement 20 (see figure 2) for controlling the valves T and S of the MMC leg 2 and the director valve arrangement 10, respectively.

Figure 4 shows that two wave shaping MMC arms 3 are sufficient to generate the three phase voltages, where one MMC arm generates e.g. the line voltage U_ba (i.e. the voltage between phases a and b) and the other generates the line voltage t/_ac (i.e. the voltage between phases a and c), at times when phase a has a nominal voltage which lies between the respective nominal voltages of phases b and c. The dotted vertical lines in the graph illustrates the points in time when switching in the director valve arrangement 10 is needed since the order in voltage magnitude between the three phases changes. Thus, in the time region in which the points p, r and q are schematically shown in the figure, phase b is conductively connected to the top point p, phase c is conductively connected to the bottom point q, and phase a is conductively connected to the middle point r of the MMC leg 2.

When supplying reactive power to the grid, the phase currents h and -h flow through the MMC arms 3a and 3b, respectively. It is to be noted that the phase angle between I/_ha and h is 120° and U_ac and -h is 6o°. Hence, the apparent power in the MMC arms 3a and 3b are, | Uba\*\h\*(-o.5 + jo.866) and I U_ac\ * \I_c\ *(o.5 + jo.866), respectively. Hence, the net active power exchanged between the grid and the MMC arms 3 is zero. However, there is an active power exchange from one MMC arm to the other, leading to charging of one arm and discharging of the other arm. To balance the MMC arm energies, director valve arrangement 10 is used, which may be switched every 6o° to retain the MMC arm energy balancing.

The AC phase-terminals of the STATCOM arrangement 1 are connected to the top (i.e., point‘p’), middle (i.e., point‘r’) and bottom (i.e., point‘q’), respectively, of the MMC leg 2 by switching ON director switches 7 of T_xi-T_x2, T_x2-T_x3 and T_x3-T_x4, respectively. The director switches 7 are switched in such a manner that the phase with the highest, lowest and middle voltages, respectively, is connected to the top, bottom and middle point of the MMC arms 3, respectively. The points‘p’ and‘q’ may be switched between the phases every 120° and point‘r’ may be switched between the phases every 6o° (in accordance with figure 4). With such a switching of the director valves, the voltage potential of point‘r’ is always more than the point‘q’ and the voltage potential of point‘p’ is always more than the point‘r’. Hence, the MMC arms 3 only generate positive voltage i.e., half-bridge cells 6 are sufficient. It is to be noted that the director switches 7 may be switched when the voltage across the switch is zero i.e., Zero Voltage Switching (ZVS) and may be switched at fundamental frequency. Hence, no or low switching losses are expected on the director switches.

An example of the control arrangement 20 of the MMDC-NPC topology is shown in figure 5. A grid voltage controller 21 generates the positive and negative phase-sequence current references I_ref which are sent to the current controller 22. The current controller 22 generates the voltage reference U_abc,ref for the STATCOM arrangement 1 i.e., the voltage that the STATCOM arrangement should generate. This voltage now needs to be translated to the voltage reference Ut_op and Ubot for the top and bottom MMC arms 3a and 3b, respectively, by the MMC reference generator 25. Moreover, switching commands need to be generated for the switches 7 of the director valve arrangement 10. From the voltage reference U_abc,ref generated by the current controller 22, the phase voltage with the highest, the lowest and the middle potential is identified by the max-min identifier 23. This information is then also used to calculate the voltage references U_top and U_bot for the top and bottom MMC arms 3a and 3b i.e., the top arm voltage reference U_top is the difference in voltage of the phases with highest and middle voltage while the bottom arm voltage reference U_bot is the difference in voltage of the phases with middle and lowest voltage. The switching commands for the director switches 7 are chosen by the director switch pulse generator 24 so as to conductively connect the phase with the highest, lowest and middle voltages to the top, bottom and middle points p, q and r of the MMC leg 2,

respectively. The switching commands for the MMC arms 3 are generated from the voltage references U_top and U_bot by the modulator comprising the insert/bypass command generator 26 and the sorting algorithm 27.

It may be important to establish the arm-energy balancing of the STATCOM arrangement 1 under unbalanced operating conditions (unbalance in both voltage and currents). As a first step, it will be proved that the net energy exchanged by the MMC arms 3 with the grid per fundamental cycle is zero. The second step will show that the energy exchanged between the MMC arms 3 is zero.

Let the converter voltage contain positive and negative phase-sequence components and the currents injected by the STATCOM 1 into the grid also contains positive and negative phase-sequence components. The net DC power exchanged between the STATCOM and the grid is given by

P_DC = u_c . i_c = u+ . Ϊ+ + u+ . f- + IJ- . f- + IJ- . Ϊ+ (4)

Since the application is a STATCOM, the common mode power P_DC,c_omm =

P_c ⁺⁺ + P_c ^-_ = 0 . Hence it is necessary to prove P_c ^+_ + P_c ^-+ = 0. Consider the power term due to the positive phase-sequence voltage and negative phase- sequence current u · lc the sum of phase powers can be computed as

Hence, from the above expression, it can be shown that the net power exchanged by the MMC arm 3 with the grid is zero for STATCOM operation even during unbalanced operation.

For any time instant in the line-to-line voltage, the top MMC arm 3a generates the voltage during the positive half-cycle and the bottom MMC arm 3b generates the exact same voltage during the negative half-cycle. Since the generated voltage and current waveforms exhibit half-wave symmetry, the net active power absorbed by the top and bottom arms 3a and 3b are individually zero. In fact, the sum of the cell capacitor voltage ripple waveform on the top and the bottom MMC arms are the same and are 180⁰ out of phase. Hence, the proposed topology has a natural arm-energy balancing even for unbalanced operation. This happens when the director switches 7 are switched to keep the phase with the highest voltage connected to the top point‘p’ of the MMC leg 2 and the lowest voltage to the bottom point‘q’ of the MMC leg 2 and the remaining phase to the middle. Thus, the topology eliminates the need for complex circulating current controllers as in delta-connected chain-link to sustain the arm-energy balancing. Also, the topology is capable to handling the FRT requirements set by the new grid codes which cannot be handled by the existing delta-connected chain-link solution.

The present disclosure 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 present disclosure, as defined by the appended claims.

Claims

1. A three-phase Static Synchronous Compensator, STATCOM, arrangement (l) arranged to be connected to a three-phase AC grid, the STATCOM arrangement comprising: a Modular Multilevel Converter, MMC, leg (2); and a director valve arrangement (10); wherein the MMC leg (2) comprises a top MMC arm (3a) and a bottom MMC arm (3b) in series connection, each MMC arm (3) comprising a plurality of series connected converter cells (6), each cell comprising an energy storage (31), wherein the top MMC arm is connected between a top point (p) and a middle point (r) of the MMC leg, and wherein the bottom MMC arm is connected between the middle point (r) and a bottom point (q) of said MMC leg; and wherein the director valve arrangement (10) is arranged to connect each of the three phases (a, b, c) of the grid to each of the top, middle and bottom points (p, r, q) of the MMC leg (2).

2. The arrangement of claim 1, wherein each of the converter cells in each of the top and bottom MMC arms (3) has a half-bridge topology.

3. The arrangement of claim 1, wherein at least one of the converter cells in each of the top and bottom MMC arms (3) has a full-bridge topology.

4. The arrangement of any preceding claim, wherein the director valve arrangement (10) has a three-phase Neutral Point Clamped, NPC, topology.

5. The arrangement of any preceding claim, wherein the director valve arrangement (10) comprises three director valve legs (4a, 4b, 4c), each connected to a respective phase (a, b c) and comprising a plurality of series connected director valves (T).

6. The arrangement of claim 5, wherein each director valve leg (4) has four, preferably only four, series connected director valves (T): a topmost valve (Txi) connecting the director valve leg to the top point (p), a top middle valve (Tx2), a bottom middle valve (Txg) and a bottommost valve (Tx4) connecting the director valve leg to the bottom point (q), wherein the phase (a/b/c) is connected to the valve leg between the top middle valve and the bottom middle valve.

7. The arrangement of claim 6, wherein each director valve leg (4) is connected to the middle point (r), both between the topmost valve (Txi) and the top middle valve (Tx2) and between the bottom middle valve (Txg) and the bottommost valve (Tx4).

8. The arrangement of claim 7, wherein the middle point (r) is connected to the director valve leg (4) via a reverse-blocking semiconductor device (9), e.g. a diode.

9. The arrangement of any preceding claim, wherein each of the director valves (T) comprises either of: a single, or a plurality of series connected, one-directional semiconductor switch (7); a mono- or bipolar chain-link converter; or a thyristor.

10. The arrangement of any preceding claim, wherein each of the director valves (T) comprises an antiparallel reverse-blocking semiconductor device (8), e.g. a diode.

11. The arrangement of any preceding claim, wherein each of the director valves (T) comprises a Bi-directional Controlled Thyristor (BCT).

12. A method of operating a STATCOM arrangement (1) of any preceding claim as a STATCOM connected to the three-phase AC grid, the method comprising to operate the director valve arrangement (10) such that at any time over a fundamental phase cycle, the phase (a/b/c) having the highest nominal voltage (uA/uB/uC) has a conducting connection to the top point (p), the phase having the lowest nominal voltage has a conducting connection to the bottom point (q), and the remaining phase has a conducting connection to the middle point (r).