The Characteristic Analysis for Sub-module in Modular Multilevel

Converters

Chengyong Zhao2014.09.23

1 Introduction of the development of the VSC-HVDC

2 Typical SM topologies of the MMC

3 A novel improved SM topology with DC fault ride-through capability

4 Summary

OUTLINE

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Traditional LCC-HVDCAs a traditional DC transmission mode ,LCC-HVDC has been widely applied.

the voltage rating of LCC-HVDC Projects in China will be up to + 1100kV in near future, and there will be about 32 LCC-HVDC projects in 2020.

Main problems of the LCC-HVDC:

Thyristors as commutation devices, with high possibility of commutation failures

Operating performances largely depends on AC system, lack of independence

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Voltage source converter based HVDC

(VSC-HVDC)

The core of the VSC-HVDC transmission is the converter. It uses full controlled devices such as IGBT, GTO and so on.

So it has great advantages over the LCC-HVDC. It can realize the independent control of the active and reactive power, supply a passive grid, improve the stability of the AC system, realize the fast power reversal, etc…

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Some projects in China

Zhoushan 5-terminal MMC-HVDC project: ±200kV, 400MW.(Commissioned)

Nanao 3-terminal MMC-HVDC project: ±160kV, 200MW.(Commissioned)

Nanhui MMC-HVDC project:+30kV,18MW. (Commissioned)

Tianguang project Reconstructing the traditional LCC project (under plan)

Luxi back-to-back projectConstructing for Asynchronous interconnectio n between Yunnan and Guangxi

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The 2 or 3 level VSC topology have been used widely. Compared with the traditional 2 or 3 level VSC, the modular multilevel converter (MMC), which was firstly proposed in 2001, has great advantages, including good harmonic performance, low loss, no need for series-connected IGBTs and so on. At present, the MMC or some similar topology are being adopted in the most of VSC-HVDC projects under planning and construction. It has become the development trend of VSC-HVDC.

Two level VSCMMC

The topology of the voltage source converters

The typical structure of MMC

Each phase unit of the converter has two arms, the upper arm and the lower arm.

Each arm is constituted by n number of SMs.

A multi-level output voltage of the AC side can be generated by distributing the SM number of the upper arm and the lower arm.

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Main problems of the development of VSC-HVDC

The limited voltage and power ratings

Limited by the development of the power electronic devices

Traditional VSC topologies are not adaptable to over-head lineTraditional half-bridge MMC can’t ride-through serious DC fault

High cost and immature technology of high-voltage DC breakers

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1 Introduction of the development of the VSC-HVDC

2 Typical SM topologies of the MMC

3 A novel improved SM topology with DC fault ride-through capability

4 Summary

OUTLINE

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3 typical SM topologies

HBSM FBSM CDSM

There are three representative structures of sub-modules which are shown in the figure.

Half-Bridge Sub-Module Full-Bridge Sub-Module Clamp Double Sub-Module

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T1 T2 iSM USM state

normal operation

1 0 - uCinsert

0 1 - 0 bypass

Start or DC fault

0 0 >0 uC

0 0 <0 0

(1) HBSM

The advantage of this topology is less power electronic devices, most of the projects under construction are using this topology.

Shortcomings:

The topology can’t ride-through serious DC faults.When a bipolar DC short circuit fault happens , AC system will infeed short circuit current continuously to short-circuit point through D2 .

The SM can’t output minus level, so it has low flexibility. 11

(2) Clamp-Double Sub-Module (CDSM)

CDSM is composed of 5 groups of IGBT and the diode switch group (T1&D1,T2&D2,..., T5&D5), two diodes (D6 and D7) and two independent capacitance.

T1 T2 T3 T4 T5 iSM USM state

Normal operation

1 0 0 1 1 - 2uC inserted0 1 1 0 1 - 0 bypassed1 0 1 0 1 - uC inserted0 1 0 1 1 - uC inserted

Start-up or during DC fault

0 0 0 0 0 >0 2uC blocked

0 0 0 0 0 <0 -uC blocked 12

the work state of the CDSM

(b) Blocked state with negative current

(a) Blocked state with positive current

Compared with the HBSM, the additional devices of the CDSM make it have the ability to ride-through serious DC faults, because the DC fault current can be cut off by the clamping of the capacitor.

Less additional costsGood DC fault ride-through capabilityLow flexible 13

T1 T2 T3 T4 iSM USMstate

Normal operation

1 0 0 1 - uC Inserted0 1 1 0 - -uC Inserted1 1 0 0 - 0 Bypassed0 0 1 1 - 0 Bypassed

Start-up or during DC

fault

0 0 0 0 >0 uC Blocked

0 0 0 0 <0 -uC Blocked

(3) FBSM

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The work state of FBSM

The AC infeed current will flow through the capacitors, so the fault current can be cut off by the clamping of the capacitors.

Besides, since the output voltage of the FBSM can be positive and negative as well, the flexibility of the system can be improved greatly. Eg: the power reversal in a hybrid HVDC system(1 terminal VSC and 1 terminal LCC) 15

Simulation results for 3 SM topologies of MMC blocked after DC pole to pole fault

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DC short-circuit current (KA)

CDMMC and FBMMC can ride through severe DC faults, so they’re more suitable for multi-terminal VSC-HVDC applications and overhead line applications.FBMMC has the ability to output negative level, so it can increase the DC voltage utilization rate and has better flexibility.HBMMC has the lowest costs and losses, with sacrificing some technical performance.

Conclusion of the 3 typical topologies

characteristics HBMMC FBMMC CDMMCcut off DC fault -- instant fast

sub-module/arm 2N 2N NIGBT/arm 4N 8N 5Ndiode/arm 4N 8N 7N

capacitor/arm 2N 2N 2Nmaximum level 2N+1 4N+1 2N+1

The basic characteristics of different topology comparison

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1 Introduction of the development of the VSC-HVDC

2 Typical SM topologies of the MMC

3 A novel improved SM topology with DC fault ride-through capability

4 Summary

OUTLINE

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Novel Double-Clamped Sub-Module (NDCSM)

The left figure shows the topology of Novel Double-Clamped Sub-Module (NDCSM).(a) and (b) shows the off-state and on-state of NDCSM separately.

According to this figure, the operating principle is shown by the current loop. We can conclude that if the NDCSM is on-state, the output voltage will be 2Uc.

(a) Arm current path of NDCSM at off-state(b) Arm current path of NDCSM at on-state

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Operating Status T1 T2 T3 T4 T5 USM State

1 1 0 1 0 1 0 off

2 0 1 0 1 1 2UC on

3 0 0 0 0 0 —— block

If T1 T3 T5 is triggered on, NDCSM operates at off-state; If T2 T4 and T5 is triggered on, NDCSM operates at on-state; If all IGBTs are triggered off, NDCSM is blocked.

0

-+

- +

T2

T3 T4D3 D4

T5 D5 D6

T1 D1 D2

CU

CU -+

- +

T2

T3 T4D3 D4

T5 D5 D6

T1 D1 D2

CU

CU

C2U

(a) (b)

The operating status of NDCSM.

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The advantage of NDCSM is fewer devices, it can cut off the short-current (including the AC fed).When NDCSM is blocked, figure (a) shows the current loop if Iarm>0, figure (b) shows the current loop if Iarm<0 . The short-current will be cut off by backward voltage which come from the quick charging of capacitor in NDCSM.

The NDCSM only has six Diodes and the investment will be lower. 21

When it is blocked, capacitor in NDCSM will be charged by short-current and it will cause a backward voltage. The short-current will be cut off by this backward voltage which is shown in the figure at right side.

The figure at right side uses phase A and B as an example. A formula is shown as follows.

ab cu cd 0 0+ + + +ap bndl

di diU U U L L U

dt dt

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A 21 level NDCMMC-HVDC system was built in PSCAD/EMTDC. The line voltage of both AC systems is 230kV and frequency is 50Hz. The capacity of both transformers is 450MW and ratio is 230kV/225kV. The positive sequence leakage reactance is 0.15 p.u.. The control strategy of NDCMMC1 is DC voltage control and AC voltage control. The control strategy of NDCMMC2 is power control and AC voltage control.

Udcref=±200kVSN=420MWL0=0.061H, C0=5000μF, UC=20kV, N=10

Simulation analysis

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At 2.1s there occurs a pole to pole fault at DC sideThe system blocks at 2.108s and deblocks at 2.3s. The fault continued until 2.3s.

Fig.1 AC RMS voltage at NDCMMC1 side Fig.2 DC voltage at NDCMMC1 side

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1 Introduction of the development of the VSC-HVDC

2 Typical SM topologies of the MMC

3 A novel improved SM topology with DC fault ride-through capability

4 Summary

OUTLINE

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MMC-HVDC, as a very potential candidate of voltage source converter, has become the development trend of VSC-HVDC

There’re 2 main challenges for the development of VSC-HVDC: enlarging the voltage and power ratings, and riding-through serious DC faults.

3 typical SM topologies are introduced and their advantages and disadvantages are compared

To improve the CDSM, a new topology ,with DC fault ride-through capability and less power electronic devices, is proposed and the simulation has validated the fault ride-through capability of all the topologies.

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Conclusion

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Thanks for your attention!

Chengyong Zhao

Ph.D and Professor, NCEPU

Email:chengyongzhao2@163.com

Phone:86-10-61773760