WO2020173549A1

WO2020173549A1 - Statcom arrangement comprising energy storage

Info

Publication number: WO2020173549A1
Application number: PCT/EP2019/054700
Authority: WO
Inventors: Jan Svensson; Aravind MOHANAVEERAMANI; Marta IBANEZ
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 StatCom arrangement (1). The StatCom arrangement comprises a Modular Multilevel Chain-Link Converter (MMC) (2) arranged to be connected to a high-voltage AC power grid (8) and act as a Static Synchronous Compensator(StatCom). The StatCom arrangement also comprises an Energy Storage System(ESS) (3) comprising a Voltage-Source Converter (VSC) (4) and an energy storage (ES) (5), and arranged to connect the ES to the power grid via the VSC. The StatCom arrangement also comprises a coordinated controller (6) configured to control both the MMC and the ESS based on the same reference.

Description

STATCOM ARRANGEMENT COMPRISING ENERGY STORAGE TECHNICAL FIELD

The present disclosure relates to a StatCom arrangement comprising an energy storage. BACKGROUND

The accelerated climate change on earth has triggered an increase of renewable generation installations and in the same time the number of traditional power plants using coal is reduced. Thus, less synchronous generators, which are used in the traditional power plants, will be connected to the power system. The synchronous generators are contributing to the stability of the power system with their inertia and short-circuit capability. Moreover, the continuous price drop for renewables will result in an even increased penetration of renewables in the near future. Since renewables such as solar and wind are using power electronic interfaces to the power grid, the renewables will not automatically contribute to the inertia therein and the short-circuit power of the power system.

To safe guard the operation of the power system, new grid codes are emerging to cope with the future situation with an increased number of renewables in the power system. Today, the delta (D) and wye (Y, also called“star”) connected Modular Multilevel Chain-Link Converters (MMC) are used for reactive power compensation in the power system. A drawback with the MMC converters is occurring when operating with unbalanced grid voltages and currents.

To be able to balance the capacitor voltages in the converter cells in the branches, a circulating zero phase-sequence current is added for the delta connected chain-link converter. The zero phase-sequence current increases when the negative phase-sequence voltage increase and the current of the converter cells increases that results in an overrated converter. A singularity point occurs when the negative phase-sequence voltage is equal to the positive phase-sequence voltage. This singularity point implies that there is no balancing solution and the converter cell rating needs to be infinity in order to balance the converter cell voltages.

For the wye-connected chain-link converter, the zero phase-sequence voltage increases when the negative phase-sequence current increase and the rated voltage of the converter branch increases. A singularity point occurs when the negative phase-sequence current is equal to the positive phase-sequence current.

To conclude, the new requirements on handling unbalanced voltages and currents that are set by the future grid codes result in that the delta- and wye- connected chain-link converters require overrating and singularities might occur at certain operation conditions.

On the other hand, the integration of energy storage (ES) into the StatCom to provide short-term active power support presents some challenges. One of the major issues is the degradation of lithium-ion batteries and

supercapacitors when exposed to high ambient temperatures. For that reason, it is preferable to use an external energy storage solution separated from the converter compartment (valve hall).

WO 2016/150466 discloses a converter arrangement where each converter cell comprises an energy storage connected via a DC-DC converter interface.

SUMMARY

For the expected future grid code scenario, two major issues have been identified:

1) During fault support, the StatCom should be able to boost positive phase- sequence voltage and dampen negative phase-sequence voltage ( _pp and _pn may vary between e.g. 2 and 6)

2) Short-term active-power support should be provided by the future

StatCom to deal with the lack of inertia of the future power system. According to available information regarding future grid codes, the StatCom should be able to provide e.g. 30% of active power for 20 seconds.

According to an aspect of the present invention, there is provided a StatCom arrangement. The StatCom arrangement comprises an MMC arranged to be connected to a high-voltage AC power grid and act as a StatCom. The

StatCom arrangement also comprises an Energy Storage System (ESS) comprising a Voltage-Source Converter (VSC) and an energy storage (ES), and arranged to connect the ES to the power grid via the VSC. The StatCom arrangement also comprises a coordinated controller configured to control both the MMC and the ESS based on the same reference.

According to another aspect of the present invention, there is provided a method performed by a coordinated controller of controlling a StatCom arrangement. The StatCom arrangement comprises an MMC connected to a high-voltage power grid and acting as a StatCom. The StatCom arrangement also comprises an Energy Storage System (ESS) comprising a Voltage-Source Converter (VSC) and an energy storage (ES), wherein the ES is connected to the power grid via the VSC. The StatCom arrangement also comprises the coordinated controller, communicatively connected to both the MMC and the ESS. The method comprises obtaining a reference for the StatCom

arrangement. The method also comprises, based on the received reference, outputting an MMC reference to an MMC controller of the MMC. The method also comprises, based on the received reference, outputting an ESS reference to an ESS controller of the ESS.

By means of the coordinated controller, the ESS may support the MMC during load balancing and fault support to avoid the overrating and possible singularities of some operation conditions, and the harmonics of the VSC may be compensated by the MMC. 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 1 is a schematic circuit diagram of a StatCom arrangement in accordance with embodiments of the present invention.

Fig 2 is a schematic block diagram of a control arrangement for a StatCom arrangement of figure 1, in accordance with embodiments of the present invention.

Fig 3 is a schematic flow chart of a method 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 l illustrates a StatCom arrangement l comprising an MMC 2 and an ESS 3 which are both connected to the Alternating Current (AC) power grid 8, typically a high-voltage (HV) grid. The MMC and the ESS are thus connected in parallel to the grid in that they are not connected to each other prior to the respective grid connections.

The MMC and the ESS of the StatCom arrangement are in the figure schematically connected to the grid via a transformer arrangement 7, e.g. comprising or consisting of a three-way transformer, but the MMC 2 and the ESS 3 may alternatively be connected via respective transformer

arrangements.

The MMC 2 may be any type of power electronic MMC, e.g. having a delta or wye topology, or a combination thereof, or having any other topology suitable for the MMC to act as a StatCom for the grid 8, typically a three-phase topology. The MMC comprises a plurality of converter arms, each comprising a plurality of series connected (also called chain-link or cascaded) converter cells. Each converter cell comprises semiconductor valves forming a full- bridge or half-bridge (preferably full-bridge) topology across an energy storage, e.g. a capacitor arrangement, of the cell.

The VSC 4may be of any suitable topology for acting as a power electronics interface between the ES 5 and the grid 8. The VSC may conveniently be a 2- level or 3-level VSC, e.g. in accordance with a conventional topology thereof. The ES 5 may e.g. comprise a battery arrangement comprising at least one battery, whereby the ESS may be regarded as a Battery Energy Storage System (BESS), and/or comprise a supercapacitor arrangement comprising at least one supercapacitor which may be preferred in some embodiments. The ES is preferably external to the MMC 2, typically situated outside of the valve hall of the MMC, in order to avoid degradation due to the heat produced by the MMC valves.

In accordance with the present invention, both of the MMC 2 and the ESS 3 of the StatCom arrangement 1 are controlled in a coordinated manner by means of a coordinated controller 6 of the StatCom arrangement 1. The coordinated controller is configured to control both the MMC 2 and the VSC 4 based on the same reference for the StatCom arrangement 1. The

coordinated controller 6 thus obtains a reference for the output of the StatCom arrangement 1 to the grid 8, and based on this reference calculates respective references to both the MMC 2 and the VSC 4 such that the MMC and VSC in combination act to operate in accordance with the obtained reference for the output of the StatCom arrangement.

Figure 2 illustrates a control arrangement 20 for controlling the StatCom arrangement 1. Typically, the reference obtained by the coordinated controller 6 is a current reference I_ref, and the outputted references to the MMC 2 and VSC 4, respectively, are also current references I_ref, MMC and

Iref, ESS.

A grid controller 11 may output the current reference I_ref to the coordinated controller 6, based on input of e.g. a measured grid voltage U_grid of the grid 8, a measured grid current I_grid of the grid 8 and/or a measured converter current I_COnv of the StatCom arrangement 1.

Based on the thus obtained current reference I_ref, the coordinated controller 6 may then calculate the MMC current reference I_ref, MMC, and the ESS current reference I_ref, ESS, respectively. Optionally, the coordinated controller may also receive measurements 14 and 15 of the respective outputs to the grid 8 of the MMC 2 and the VSC 4, e.g. current, voltage and/or power measurements 14 and 15, and any of the MMC current reference I_ref, MMC and the ESS current reference I_ref, ESS may also be based on any of those measurements 14 and 15. The MMC current reference I_ref, MMC may typically be sent to an MMC controller 12 of the MMC 2, for control of said MMC, and the ESS current reference I_ref, ESS may typically be sent to an ESS controller 13 of the ESS 3 for control of the VSC 4 of said ESS. The MMC controller 12 may also control the MMC 2 based on measurements of the MMC output. Similarly, the ESS controller 13 may also control the VSC 4 based on measurements of the VSC output.

In figure 2, current references are specified. However, the invention is not limited to current references. Rather, the coordinated controller 6 may handle any type of references for controlling the MMC 2 and ESS 3, respectively. In some embodiments, current controllers are used, why the coordinated controller 6 provides current references to the MMC and ESS controllers 12 and 13, respectively, and then each of these controllers 12 and 13 will generate voltage references based on the respective current references Iref, MMC and Iref, ESS from the coordinated controller. However, in other embodiments, the coordinated controller 6 may output voltage reference(s) to either or both of the MMC and ESS controllers 12 and 13. Also, in some embodiments, the reference received by the coordinated controller, from the grid controller, may be a voltage reference.

So, as we have thought the invention, the output of the coordinated control should be current references.

Thus, the present invention, in some embodiments thereof, proposes a coordinated control for a chain-link converter 2 together with a 2-level (2L) or 3-level (3L) VSC 4 to meet future grid code scenarios (power balancer and fault support) and to provide a short-term active-power support.

The proposed StatCom arrangement 1 comprises:

- A chain-link converter 2 (delta or wye connected), herein called an MMC; - A Energy Storage System (ESS), e.g. a BESS, 3 that comprises a 2L/3L VSC 4, which is connected/interfaced to an external energy storage 5.

- A coordinated controller 6 that provides the references, e.g. current or voltage references, for the chain-link converter 2 and the ESS 3 to meet the future grid codes with an optimal rating.

Some of the key features of the StatCom arrangement 1 are:

- The chain-link converter 2 provides reactive power compensation during balanced operation.

- The ESS 3 provides active power for inertia support.

- The ESS 3 supports the chain-link converter 2 during load balancing and fault support to avoid the overrating and the singularities of the chain-link converter during some operation conditions.

- The harmonics of the VSC 4 can be compensated by the chain-link converter 2.

An analysis of the proposed arrangement 1 during fault support in a StatCom station composed of a wye chain-link converter 2 and a BESS (2L-VSC with energy storage) is given below. Same analysis could be applied to a delta chain-link converter as well.

The current references during fault support for future grid codes are:

with k_pp, k_{pn \} 2 ® 6

Assuming \ u_ref \ is l pu,

= f_u = 0 , and \ U_ySC\ + \u_vsc \ is l pu, the current references can be written as: Note that a current limiter is included in the grid controller to limit the maximum phase current to 1 pu. It results in a limitation of |¾_;C| and of \l_vsc to 0.58 pu when k_pp = k_pn.

For chain-link converters (MMC), to keep the capacitors balanced during unbalanced operation, a zero phase-sequence component (voltage for floating neutral wye configuration and current for delta configuration) is added to the converter references. The zero phase-sequence voltage for the wye chain-link converter is given by:

It can be seen from the expression above that the required zero phase- sequence voltage can go to infinity when the denominator of the expression becomes zero i.e., magnitudes of the phase-sequence currents are the same. According to the current reference expressions for the fault support, the magnitudes of the phase-sequence current would be the same when n k-pp = ^ kpn_*

It is herein proposed to decrease the negative phase-sequence current that is provided by the chain-link converter 2 by injecting the remaining negative phase-sequence current by the ESS 3. In this way, the negative phase- sequence current component injected to the grid 8 meets the requirements, and the singularity and overrating of the chain-link converter 2 are avoided.

For worst case scenario, when k_pp is equal to k_pn, phase voltage of the chain- link 2 becomes infinity when all the negative sequence current reference is provided by the chain-link converter, i.e., the magnitudes of the positive and negative phase-sequence current components are the same. With the introduction of the ESS 3 to the system, the negative phase- sequence current reference can be shared between the chain-link converter and the ESS. Thus, the singularity can be avoided and the required phase voltage of the chain-link converter to keep the capacitors balanced can be reduced.

For instance, if the ESS 3 can provide 0.3 pu current, the negative phase- sequence that should be provided by the chain-link converter 2 is reduced to 0.28 pu and the required phase voltage becomes 1.97 pu.

The coordinated controller 6 may decide the optimal current references Iref _MMc and Iref _ESS for the wye-connected chain-link converter 2 and the ESS 3 to minimize the phase voltage of the chain-link converter and the phase current of the ESS.

The optimal value of the negative phase-sequence reference for the chain-link 2 will depend on how the cost function is defined. As an example, a dummy cost function considering the maximum phase voltage and current may be considered. However, the actual cost function could be defined after a detailed cost system evaluation. For the study case, the phase current of the chain-link 2 and the phase voltage of the ESS 2 (e.g. a 2L-VSC 4 thereof) are set to the minimum of 1 pu. j_jWyeChL . wyeChL _T ,2LVSC

Cost = i2 LVSC

uph ¹ ph ^uph ^lph (8)

As a result, the cost is given by the phase voltage of the chain-link 2 and the phase current of the 2L-VSC 4. A minimum of 2.07 pu is obtained in the example, where the negative phase-sequence current rating of the chain-link is 0.12 pu, and, thus, the ESS 3 must provide the difference to 0.58 pu, which is 0.46 pu. This results in a maximum phase voltage of the wye chain-link 2 of 1.6 pu. With the above analysis, it has been shown that the proposed solution allows to meet future grid codes by combining existing products together with an external energy storage 5 together with minimum ratings of the power electronics converters 2 and 4.

Figure 3 schematically illustrates some embodiments of the method performed by the coordinated controller 6, in accordance with the present invention. A reference, e.g. a current reference I_ref, for the StatCom

arrangement is obtained Mi, e.g. received. Then, based on the obtained Mi reference, an MMC reference I_ref, MMC is outputted M2 to an MMC controller 12 of the MMC (2), and an ESS reference I_ref, ESS is outputted to an ESS controller 13 of the ESS 3.

Optionally, measurements 14 and 15 of the MMC 2 and the VSC 4 outputs, respectively, are received M4, before the outputting M2 and M3 of the MMC reference I_ref, MMC and the ESS reference I_ref, ESS, e.g. before, after or

concurrently with the obtaining Ml of the reference wherein in those cases the MMC reference I_ref, MMC and the ESS reference I_ref, ESS may be based also on these received M4 measurements.

In some embodiments of the present invention, the reference I_ref is obtained Mi by being received from a grid controller 11.

In some embodiments of the present invention, the obtained Ml reference I_ref is a current reference.

In some embodiments of the present invention, the outputted M2 MMC reference I_ref, MMC is an MMC current reference.

In some embodiments of the present invention, the outputted M3 ESS reference I_ref, ESS is an ESS current reference.

In some embodiments of the present invention, the coordinated controller 6 is configured to receive the reference I_ref from a grid controller 11 and output an MMC reference I_ref, MMC to an MMC controller 12 of the MMC 2 and an ESS reference I_ref, ESS to an ESS controller 13 of the ESS 3 based on the received reference. In some embodiments, the coordinated controller 6 is also configured to receive measurements 14 and/or 15 of the MMC 2 and/or VSC 4 outputs, respectively, whereby the MMC reference I_ref, MMC and/or the ESS reference I_ref, ESS may be based also on these received measurements.

In some embodiments of the present invention, the VSC 4 is a 2-level or 3-level power electronic converter.

In some embodiments of the present invention, the MMC 2 has a delta or wye topology.

In some embodiments of the present invention, the ES 5 comprises a battery arrangement and/or a supercapacitor arrangement.

In some embodiments of the present invention, the MMC 2 and the ESS 3 are connected to the grid 8 via the same transformer arrangement 7, e.g. a three- way transformer which is preferred in some applications.

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 StatCom arrangement (l) comprising: a Modular Multilevel Chain-Link Converter, MMC, (2) arranged to be connected to a high-voltage AC power grid (8) and act as a Static

Synchronous Compensator, StatCom; an Energy Storage System, ESS, (3) comprising a Voltage-Source Converter, VSC, (4) and an energy storage, ES, (5), and arranged to connect the ES to the power grid (8) via the VSC; and a coordinated controller (6) configured to control both the MMC and the ESS based on the same reference (I_ref).

2. The StatCom arrangement of claim 1, wherein the coordinated controller (6) is configured to receive the reference (I_ref) from a grid controller (11) and output an MMC reference (I_ref, MMC) to an MMC controller (12) of the MMC (2) and an ESS reference (I_ref, ESS) to an ESS controller (13) of the ESS (3) based on the received reference.

3. The StatCom arrangement of claim 2, wherein the coordinated controller (6) is also configured to receive measurements (14, 15) of the MMC (2) and VSC (4) outputs, respectively, and wherein the MMC reference

(Iref, MMC) and the ESS reference (I_ref, ESS) are based also on these received measurements.

4. The StatCom arrangement of any preceding claim, wherein the VSC (4) is a 2-level or 3-level power electronic converter.

5. The StatCom arrangement of any preceding claim, wherein the MMC (2) has a delta or wye topology.

6. The StatCom arrangement of any preceding claim, wherein the ES (5) comprises a battery arrangement and/or a supercapacitor arrangement.

7. The StatCom arrangement of any preceding claim, wherein the MMC (2) and the ESS (3) are connected to the grid (8) via the same transformer arrangement (7), e.g. a three-way transformer.

8. A method performed by a coordinated controller (6) of controlling a StatCom arrangement (1), said StatCom arrangement comprising: a Modular Multilevel Chain-Link Converter, MMC, (2) connected to a high- voltage power grid (8) and acting as a Static Synchronous Compensator, StatCom; an Energy Storage System, ESS, (3) comprising a Voltage-Source Converter, VSC, (4) and an energy storage, ES, (5), wherein the ES is connected to the power grid (8) via the VSC; and the coordinated controller (6), communicatively connected to both the MMC and the ESS; the method comprising: obtaining (Ml) a reference (I_ref) for the StatCom arrangement; based on the obtained (Mi) reference, outputting (M2) an MMC reference (Iref, MMC) to an MMC controller (12) of the MMC (2); and based on the received (Mi) reference, outputting (M3) an ESS reference (Iref, ESS) to an ESS controller (13) of the ESS (3).

9. The method of claim 8, wherein the reference (I_ref) is received (Ml) from a grid controller (11).

10. The method of claim 8 or 9, wherein the obtained (Ml) reference (I_ref) is a current reference.

11. The method of any claim 8-10, wherein the outputted (M2) MMC reference (I_ref, MMC) is an MMC current reference.

12. The method of any claim 8-11, wherein the outputted (M3) ESS reference (I_ref, ESS) is an ESS current reference.

13. The method of any claim 8-12, further comprising: receiving (M4) measurements (14, 15) of the MMC (2) and VSC (4) outputs, respectively, wherein the MMC reference (I_ref, MMC) and the ESS reference (Iref, ESS) are based also on these received (M4) measurements.