Modelling and Dynamic Operation of the Zhoushan DC Grid: Worlds ...

International High Voltage Direct Current 2015 Conference October 18-22, 2015 Seoul, Korea

Modelling and Dynamic Operation of the Zhoushan DC Grid: Worlds First Five-Terminal VSC-HVDC Project Yousef Pipelzadeh, Balarko Chaudhuri, Tim C Green

Yanan Wu, Hui Pang, Junzheng Cao

Imperial College London, Control and Power Group, Electrical and Electronics Engineering {yp508, b.chaudhuri, t.green}@imperial.ac.uk

Smart Grid Research Institute, State Grid Corporation China, Beijing, China {wuyanan, panghui, caojunzheng}@sgri.sgcc.com.cn

Abstract— This paper highlights the world’s first operational MTDC grid, namely the 5-terminal Zhoushan DC grid. The scheme went under operation in 2014. The topology and operation of the Zhoushan DC grid are demonstrated with recorded measurements obtained from the converter station, after being subjected to system disturbances. A generic modeling framework for the Zhoushan DC grid is developed in PSCAD/EMTDC. One particular concern is how the Zhoushan DC grid would react to DC side faults and the resulting power imbalance. Despite the completion of the Zhoushan MTDC grid, technological barriers such as the unavailability of fast protection systems, DC circuit breakers and highly efficient VSCs with DCside fault-clearing capabilities have all been bottlenecks at the time of commissioning, but are now under extensive research and development. The challenges and importance of DC grid protection are highlighted through case studies performed on the DC grid model in PSCAD/EMTDC.

Wind power generated is fed into China's regional power grid, safeguarding future energy supply as power generation methods transition from coal towards renewables solutions. This innovative project, tackles the challenge of building advanced smart grids to keep up with China's growing electricity demand and provides confidence to the global industry for development of the so-called "HVDC super grids" in the future.

Keywords—DC grid; Modular Multi-level Converters, DC breakers; PSCAD/EMTDC; Multi-terminal DC; Voltage Source Converter

Many papers in the context of modelling the dynamic operation of DC grids have been proposed with an attempt to resolve the main technical issues, which need to be overcome to realize a DC grid in practice. For example, several methods for efficiently simulating point-to-point DC grids with MMC topologies have been proposed [1], [2]. However, they do not consider potential interaction with neighboring AC networks. The work of [3] provides an in-depth stability assessment of AC-MTDC grid system; however the converters are represented by averaged two-level schemes and neglect certain slower loop dynamics that exist in MMC schemes.

I.

INTRODUCTION

This paper presents the modelling and dynamic operation of the world's first five-terminal DC grid in China, which will help integrate clean renewable energy into the power grid. In recent years, the efficiency, size and ratings of voltage-sourced converters (VSCs) have improved significantly, making them potential candidates for use in HVDC applications. The emerging modular multilevel converter (MMC) topologies are a major step forward in VSC converter technology for high voltage DC transmission. The Zhoushan MTDC project uses VSC transmission based on modular multi-level converter (MMC) technology. This project establishes a critical interconnection between Mainland China and five isolated islands raising electric power supply and enhances the grid reliability by total 1000 MW capacity. The islands have relatively weak connection to the mainland power grid due to the geographical restriction. As a self-commutated device, the VSC converters can operate into the weak systems, or even passive AC systems with no sources of generation. This makes it ideal for connecting island loads or “blackstarting” AC systems.

Currently, there are three MMC-HVDC projects operational in China, namely the Nanhui MMC-HVDC demonstration project in Shanghai, Nanao three-terminal MMC-HVDC project in Shantou and the five-terminal MMC-HVDC in Zhoushan. Among them, Nanhui and Nanao projects were put into operation in 2011 and 2013, respectively. The Zhoushan DC grid was completed in 2014, becomes the world’s first fiveterminal MMC-MTDC project.

Another critical aspect is the protection of the MTDC grid in the event of a fault in the DC cable network. There are effectively two options for clearing DC grid faults without causing a large loss-of-infeed. One way is to use DC breakers (when commercially viable) to isolate only the faulty cable while continuing to operate the rest of the DC grid as usual [4]. The other way is to use converter topologies which are capable of interrupting the DC fault current [5]. This paper is organized as follows. In Section II, the planning principles and design considerations for the Zhoushan DC grid system are described. Section III, highlights the modelling and experimental results of the Zhoushan DC grid. Section IV illustrates the dynamic operation of the DC grid with simulations performed in PSCAD/EMTDC and Section V concludes this paper.

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II. DESIGN CONSIDERATIONS FOR THE ZHOUSHAN DC

GRID A single-line diagram of the AC power grid in the five northern Zhoushan islands before the construction of the Zhoushan DC grid is shown in Fig 1. The highest voltage level is 220 kV AC. The islands are mainly connected to China’s mainland (Ningbo Grid) through AC double circuits rated at 220 kV and 110 kV. Yangshan power grid is connected Daishan through a 110 kV AC cable [6]. To Shanghai Luchao Station 2×30 MW/±50 kV Yangshan

Sijiao

The final phase is to construct the five-terminal DC grid system. The conceptual thought process is that the DC breakers will gradually be included to the existing fiveterminal DC transmission system to form the so-called “Zhoushan DC grid”. The five converters rated at ±200 kV in the Zhoushan DC grid are located at the Zhoushan mainland (Dinghai), Daishan island, Qushan island, Yangshan island and Sijiao island. They are connected via modular multi-level voltage source converter (MMC-VSC) HVDC links to form a 5-terminal DC grid, as shown in Fig. 3. Cables connect the converters with total length of 140 km. Detailed technical parameters for the DC grid can be found in the appendix. To Shanghai Luchao Station 2×30MW/±50kV

Qusha n

er it w u to irc e - e-c l b u

o

n

d

o

er it w u to irc e - e-c l b u

o

n

d

o

Yangshan

Sijiao

Daishan 32.3km

Zhoushan 39km Qushan 46km

110 kV 220 kV ±50 kV LCC-HVDC

Ningbo Grid

17km

Daishan

The design and construction of the Zhoushan DC grid is divided into three phases – the first phase is the construction of the 5-terminal DC transmission system located across the five major islands of northern Zhoushan, as shown in Fig. 2, which will provide exchange of energy supply between the islands and allow for the connection of wind farms.

Sijiao Yangshan

Qushan Daishan

er w it to rcu ne ci O o tw

Fig 1: Grid connection of northern Zhoushan islands

Zhoushan

110 kV

Ningbo Grid

220 kV ±200 kV VSC-HVDC ±50 kV LCC-HVDC Windfarm

Fig. 3. Zhoushan MTDC grid (power flow directions also shown)

Following faults on the DC side, isolating only the faulty component (a converter or a cable) of the MTDC grid is a challenge. Besides protection and dc breaker development issues, there are primary control problems, such as autonomous sharing of power imbalance among the converters following a converter or cable outage. If a converter station needs to be isolated from the remaining (operational) system, the insolation switch on the DC pole line should be opened. The system should be discharged before the switch is opened and the converters should be blocked with AC breakers opened.

Zhoushan

Fig. 2: Geographical location of the 5 converter stations The second stage is to design and construct the DC breakers. Extensive analysis will be carried out to understand the optimal configuration for the DC breakers and their strategic placements, in order to meet the system design requirements, which include fault clearance, system recovery, etc.

After the DC transmission system is put into operation, the Zhoushan converter station will serve as the transmitting station while the remaining four converter stations serving as receiving station. If Dinghai (Zhoushan) station is under operational maintenance or an outage condition has occurred; then Daishan station will serve as the sending end with the remaining three converter stations as the receiving end. The five-terminal DC transmission system works in parallel with the AC lines. The Zhoushan and Daishan converter stations can simultaneously inject power to the DC system side, and serve to maintain voltage stability and power balance of the whole DC transmission system [7].

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III. MODELLING AND PRACTICAL OPERATION OF THE ZHOUSHAN DC GRID

This section highlights the modelling aspects and practical results from field tests, which demonstrate the Zhoushan converter in operational mode.

arm can be realized by a horizontal array of such units and, if necessary, by assembling them in a vertical arrangement to meet the specific project requirements. Other arrangements are also possible. I+ dc

a) Multi-terminal DC Grid The MTDC grid as shown in Fig 4 was modelled in PSCAD/EMTDC. The MTDC grid is flexible and allows for: • Monopole or bipolar MTDC grid converter stations, • Grounding considerations with the metallic return network, • DC cable model (pi section or distributed), • Type and location of fault on the dc side, • Cable outage or converter outage leading to unbalanced (on DC side) operation.

C

+

+

+

Vc

Vb

C

L

Vc

I c+

Larm

Larm

Larm

Larm

Larm

I b-

C

Vdc

I c-

C

Vc

C Vc

Vc

Sijiao C

75km

Vc

Larm

32.3km 39km

17km

45km

arm

V

C

C

Vc

Dinghai (Zhoushan)

C

C Vc

I b+

I a+

I a-

Yangshan

Vc

Vc

Va

Ia Ib Ic

C

C

Vc

Vc

C Vc

Vc

C

+ Varm

DC cable AC line

C

Vc

Vc

Vc

-

-

Va

Vb

Vc

C Vc

-

C

C Vc

Vc

I dc-

Sub module (SM)

Daishan

Qushan

Fig 4: Single-line diagram of the Zhoushan MTDC model in PSCAD/EMTDC

b)

Modular Multi-level Converters

The converter valves used in the Zhoushan DC grid use semibridge modular multilevel converter valves. The MMC valves shown in Fig 5 depicts the Zhoushan scheme. The MMC deployed in the Zhoushan project consists of six converter arms. Each of them comprises a high number of power modules (PM) and one converter reactor connected in series. The power modules contain an IGBT half bridge as a switching element and the DC capacitor unit for energy storage. The rated operating voltage of each sub module (SM) is set to 1.6 kV. Therefore, each converter arm should be cascaded with a minimum of 250 SMs. The electronics for the control of the power semiconductors, the monitoring of the capacitor voltage, and the communication with the higherlevel controllers are not presented in this paper. c) Converter valve equipment Thanks to its modular construction, the MMC is very scalable, i.e. conveniently adaptable to any required power and voltage ratings. The required number of power modules per converter

Arm

Phase Module (PM)

Fig 5: Modular Multi-Level Converter (MMC) Topology

Fig 6 depicts a view of an MMC design. In principle, both a standing and a suspended construction can be readily achieved. However, a standing construction was chosen, since in that case the converter design imposes less special requirements to the converter building. The single-row valve tower of one bridge arm comprises 288 sub-modules, and is arranged in three layers. Each layer comprises three valve modules, and each valve module comprises eight submodules.

Fig 6: Typical converter arrangement configuration

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Practical Operation of the Zhoushan DC Grid 16

Active power

20

12

15

8

10

4

5

Reactive power

0

0 0

235

470

505

740

965

Time (ms)

Active power 0

4

Reactive power

-5

0 -4

-15

-8

-20 0

235

470

505

740

965

-12

Time (ms) Fig. 10: Practical test results of reactive power step response [8]

Uas Ubs Ucs Uav Ubv Ucv Ias Ibs Ics Iav Ibv Icv 0

Fig. 8: Zhoushan converter valve hall [8]

The Zhoushan MTDC system was commissioned in July 2014. The practical operation of the system under step changes in reference active and reactive power, along with a single phase to ground fault are shown in Figs. 10 and 11, 12 respectively.

Reactive Power (Mvar)

Active Power (MW)

Fig. 9: Practical test results of active power step response [8]

-10

Fig.7: Zhoushan converter station [8]

Reactive Power (Mvar)

The Zhoushan five-terminal DC grid system was officially approved in December 2012 to build five converter stations – Zhoushan, Daishan, Qushan, Sijiao and Yangshan. In December 2013, construction of the main body of the Zhoushan converter station was first completed, consisting of the AC switching station, linking area, valve hall, DC yard, complex building and a comprehensive fire pump room. The total floor area is 6764 m2, as shown in Fig.7. The converter station covers an area of 5481 m2, the complex building an area of 1082 m2, and the comprehensive fire pump room has covers an area of 201 m2, as shown in Fig. 8. [8]

Active Power (MW)

d)

200

400

600

800

1000

1200

Time (ms) Fig. 11: Practical recordings of single phase to ground fault [8] {Uas, Ubs and Ucs: AC system voltage; Ias, Ibs and Ics: AC system current; Uav, Ubv, and Ucv: converter side voltage; Ias, Ibs and Ics: converter side current}

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IV. DYNAMIC OPERATION OF THE DC GRID IN PSCAD/EMTDC

The Zhoushan multi-terminal DC grid as described above was modeled in PSCAD/EMTDC. The technical parameters of the DC grid are listed in the Appendix. There are essentially two ways of clearing DC grid faults without causing a large loss-of-infeed. One option is to use DC breakers to isolate only the faulty cable while continuing to operate the rest of the DC grid as usual. Another option is to use converter topologies which are capable of interrupting the DC fault current. DC breaker configurations: the following section demonstrates the operation of the DC grid when subjected to a DC fault.

Fig.13: DC line currents at each converter station

Case study I: Without DC breakers In the first case study, the dynamic operation of the multiterminal system is examined assuming no DC breakers are installed. The power reference values for S2, S3, S4, S5 are all set to - 0.5pu (see Table I. A large disturbance, in the form of dcside faults is considered. In this example, a DC bipolar short fault is applied at 1.0 s (at the midpoint connecting converter stations #2 and #3, as shown in Fig.12) with all the converters blocked after around 2-4 ms. The AC breakers in each of the stations are opened about 50 ms after the valve blocking. #5yangshan

Fig.14: DC line voltages at each converter station

#4sijiao

#1-dinghai #2daishan

#3qushan

Fig.15: DC line Power at each converter station

Fig.12: Five-terminal Zhoushan scheme without DC breakers

DC Fault outage simulation The dynamic response of the system following the DC fault mentioned above is shown in Figs. 14-17. When the DC fault occurs, the AC breakers on all converter stations are opened, which results in the DC grid to shut down. The transients shown in the DC grid are shown in Figs. 14-16. The blocking signal for each converter is shown in Fig. 16 (value of ‘1’ refers to unblocking and ‘0’ is for blocking).

Fig. 16: Converter blocking signal (‘1’ unblocking, ‘0’ blocking)

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Case study II: Inclusion of DC breakers In this case study, the DC breakers are installed at across all the DC lines, except at Sijiao and Qushan converter stations. This configuration reduces platform size and costs, but also satisfies the protection requirements of the five converter stations. However, two converter stations (Sijao and Qushan) cannot be switched in STATCOM mode of operation after a fault is cleared.

DCCB (open)

#5yangshan

#4sijiao

DCCB (closed)

Fig. 19: DC line voltages at each converter station

#1-dinghai (Zhoushan) #2daishan

#3qushan

Fig. 17: Five-terminal Zhoushan scheme with DC breakers (except at converters #4 and #5). Next, a DC cable fault was considered at the midpoint of the bipolar cable connecting converter stations 2 and 3. The fault was created at 1 s and was cleared by opening the faulted cable within 5.0 ms. As shown in Fig.18-22 the system recovery happens around 50 ms later. The ac breaker in Qushan station is opened around 50 ms after the valve blocking ( Fig.18-22).

Fig. 20: DC line Power at each converter station

Fig. 21: Converter blocking signal (‘1’ unblocking, ‘0’ blocking)

Fig.18: DC line currents at each converter station

A third configuration with DC breakers installed at both ends of the DC lines is shown in Fig. 22, providing the highest level of protection. After a fault is cleared, a converter station may quickly switch toward the STATCOM mode. However, this configuration scheme requires a large number of breakers.

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Yangshan

Sijiao

DC breakers closed

REFERENCES

DC breakers opened

[1] U. Gnanarathna, A. Gole, and R. Jayasinghe, “Efficient modeling of modular multilevel hvdc converters (mmc) on electromagnetic transient simulation programs,” Power Delivery, IEEE Transactions on, vol. 26,no. 1, pp. 316–324, 2011

Zhoushan

[2] M. Saeedifard and R. Iravani, “Dynamic performance of a modular multilevel back-to-back hvdc system,” Power Delivery, IEEE Transactions on, vol. 25, no. 4, pp. 2903–2912, 2010 [3] N.Chaudhuri, R.Majumder, B.Chaudhuri,and J.Pan, “Stability analysis of vsc mtdc grids connected to multi-machine ac systems,” Power Delivery, IEEE Transactions on, vol. 26, no. 4, pp. 2774–2784, 2011.

Daishan

Qushan

Fig. 22: Five-terminal Zhoushan scheme with DC breakers

V. CONCLUSION This paper introduced the world’s first operational MTDC grid, namely the Zhoushan 5-terminal DC grid which went under operation in 2014. Practical measurements taken from field tests demonstrated the successful operation of the Zhoushan DC grid. However, despite the Zhoushan project commissioned, the unavailability or technological readiness of fast protection systems, DC circuit breakers for DC grids have been bottlenecks. A generic Zhoushan DC grid model was developed in PSCAD/EMTDC and simulation studies illustrate the importance of DC grid protection. The results show that when the system is subjected to DC-side faults, and considering without DC protection (as is the case today), the entire DC system will have to be shut down. However, with DC breakers in place, it is shown to be possible to isolate only the faulty cable while continuing to operate the rest of the DC grid. The experience gained from designing, developing, and constructing the Zhoushan DC grid will serve as a great motivation and confidence for further developments of DC grids worldwide.

[4] N. Chaudhuri, , B.Chaudhuri, R. Majumder, and A. Yazdani. Multiterminal Direct-current Grids: Modeling, Analysis, and Control. John Wiley & Sons, 2014. [5] M. Merlin., T. C. Green, , P. D. Mitcheson, D. R. Trainer, R. Critchley, W. Crookes & F. Hassan (2014). The alternate arm converter: A new hybrid multilevel converter with dc-fault blocking capability. Power Delivery, IEEE Transactions on, 29(1), 310-317. [6] State Power Economic Research Institute, “Preliminary Design Report for Zhoushan Multi-Terminal VSC-HVDC Demonstrate Project,” State Power Economic Research Institute, 2013. [7] G. F. Tang, X. Luo, and X. G. Wei, “Multi-terminal HVDC and DC-grid technology,” Proceedings of the CSEE, vol. 33, no. 10, 8–17, Apr. 2013. [8] G.F. Tang, Z. He, H. Pang, X. Huang, X. P. Zhang, Basic topology and key devices of the five-terminal DC grid. Power and Energy Systems, CSEE Journal of, 1(2), 22-35

ACKNOWLEDGEMENT

The authors gratefully acknowledge the financial support received by the Engineering and Physical Sciences Research Council (EPSRC) and the National Natural Science Foundation of China (NSFC) under the project Enhanced Renewable Integration through Flexible Transmission Options (ERIFT) programme under grants EP/K006312/1 (UK) and 51261130471 (China).

93

APPENDIX The load flow results calculated by PSS/E program are shown in Fig. 23. The basic system parameters for load flow calculations of the grid and some further data for building an electromagnetic transient model are given in the tables below. Ba-A0 615 220.0 GE

2*51

100km

DC/AC Line

40km

2*103

2*154

Yangshan 199.7

220kV

100

32.3km

Sijiao 198.9

Ba-A1 212.2

200

20km

VSC

300

Ba-A6 213.9 30km

Cb-A1 100

±200kV

Ba-A5 212.8

AC Grid Equivalent (GE)

100

Cb-A5 100

WF

2*51 75km

Ba-A3 219.3 406

100

39km

Cb-A3

AC Load

30

1 17

Zhoushan 200.0

231

Daishan 199.8

2*52 Ba-A2 300 211.8 20km 400 Cb-A2

Ba-A4 220.0 Qushan 200.2

99

100 Cb-A4 100

17km

45km

Fig. 23: Load flow results for the Zhoushan DC grid (in PSS/E) Assume that the active power received or transmitted by the converter stations is as described in Table I when Zhoushan DC power grid operates in steady state. The negative “-” sign refers to active power received, whilst the positive sign “+” means that active power is being transmitted. ACTIVE POWER OF CONVERTER STATIONS Zhoushan

Daishan

Qushan

Yangshan

Shengsi

+400 MW

-200 MW

-50 MW

-100 MW

-50 MW

TECHNICAL PARAMETERS OF ZHOUSHAN MULTI-TERMINAL DC TRANSMISSION SYSTEM AC System Voltage AC Grid Equivalent Short Circuit Ratio AC Grid Equivalent X/R Ratio AC Load Power Factor Nominal DC Voltage Configuration High-level control Rated DC Voltage Rated Capacity of Converter Station Total cable length

220kV 25kA 20 0.95 ±200kV Symmetrical monopole Droop control ±200 kV Zhoushan: 400 MW Daishan: 300 MW Qushan, Sijiao,Yangshan: 100 MW 141.5 km

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PARAMETERS OF DC CABLE DC Line

Distance (km)

Ampacity* (A)

Resistance* (ohm/km)

Zhoushan-Daishan Zhoushan-Yangshan Daishan-Yangshan Daishan-Qushan Yangshan-Sijiao

45 75 39 17 32.3

1092 1092 722 282 282

0.0176 0.0176 0.0366 0.193 0.193

* Actual DC cable data was not available at the time of this study. The data is extracted from ABB, “It’s time to connect-technical description of HVDC Light technology”, PP.33-34.

PARAMETERS OF AC OVER HEAD LINE Resistance Rac (Ω/km) 0.053

AC Line 220kV OHL

Reactance Xac (Ω/km) 0.305

Susceptance Bac/2 (S/km) 1.81*10-6

Rated current（kA） 1.03

PARAMETERS OF PRIMARY DEVICES OF DIFFERENT CONVERTER STATIONS Equipment Converter Bridge arm reactor Bypass transformer

Zhoushan 400 MW/450 MVA Dry air-core, 90 mH 3-phase 3-winding 450/450/150 MVA 230/205/10.5 kV

Parameters Qushan 100 MW/120 MVA Dry air-core, 350

Daishan 300 MW/350 MVA Dry air-core, 120 mH

mH 3-phase 3-winding 350/350/120 MVA 230/204/10.5 kV

Yangshan 100 MW/120 MVA Dry air-core, 350 mH

3-phase 3-winding 120/120/40 MVA 115/208/10.5 kV

Sijiao 100 MW/120 MVA Dry air-core, 350 mH



Transformer shortcircuit impedance

15/50/35 %

15/50/35 %

15/50/35 %

15/50/35 %

Transformer connection

Yn, d, d11 450/450/150 MVA 230/205/10.5 kV

Yn, d, d11 350/350/120 MVA 230/205/10.5 kV

Yn, d, d11 120/120/40 MVA 230/205/10.5 kV

Yn, d, d11 120/120/40 MVA 230/205/10.5 kV

Yn, d, d11 120/120/40 MVA 230/205/10.5kV

250 A

250 A 50Hz

250 A 50Hz

DC cable

1,000 A

AC frequency DC reactor Starting resistance AC breaker AC outlet switch DC outlet switch Number of sub-modules per arm Sub-module capacitance

Grounding

750 A

50Hz

50Hz

50Hz

15/50/35 %

Dry air-core, 20 mH

Dry air-core, 20 mH

Dry air-core, 20 mH

Dry air-core, 20 mH

Dry air-core, 20 mH

6 kΩ/20kW

9 kΩ/20 kW

26 kΩ/20 kW

26 kΩ/20 kW

26 kΩ/20 kW

4 kA/50 kA 3150 A/50 kA(3s)/125 kA (Peak) DC 200 kV/1,600 A/30 kA (3s)





250

250

250

250

250

12 mF

9 mF

3 mF

3 mF

3 mF

star point reactors grounding on the ac side of the converters

star point reactors grounding on the ac side of the converters

High resistance (2k Ω ) grounding at transformer neutral point (Yn)



Notes：The number of the MMC levels can vary, depending on the application and simulation speed requirement. For PSCAD/EMTDC version 4.5 or above, we recommend you to develop the 251-level MMC model and use the sub-module capacitance provided above.

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