Dr. Chengxiong Mao Professor School of Electrical and ... · PDF filesynchronous generator and applications of high power electronic technology to ... Fast responding and ... Proposed

Received his B.S., M.S. and Ph.D. degrees in Department of Electrical Engineering, from HUST, in 1984, 1987 and 1991, respectively.

Visiting scholar in University of Calgary, Canada, from Jan. 1989 to Jan. 1990, and in Queen’s University of Belfast from Dec. 1994 to Dec. 1995 respectively. He was doing researches in Technical University of Berlin from Apr. 1996 to Apr. 1997 under the support of Humboldt Foundation.

Research fields: power system operation and control, the excitation control of synchronous generator and applications of high power electronic technology to power systems.

Dr. Chengxiong Mao，ProfessorSchool of Electrical and Electronic EngineeringHuazhong University of Science and Technology (HUST)P. R. China

1

Multi-level DC Distribution System and Key Technologies

Chengxiong Mao

[email protected] of Electrical and Electronic Engineering

Huazhong University of Science and TechnologyOct. 2014

Panel 4: DC Distribution Technologies PN4-05

Outline Background

Multi-level DC Distribution System Grid Configuration Voltage Class Series Operation & Control

Key Technologies DC Transformer DC Circuit Breaker DC Transducer

EPT-based AC/DC Hybrid Distribution System EPT Concept Application Scenarios Demo System & Simulations

Future Development

3


Outline

Background Multi-level DC Distribution System

Grid Configuration Voltage Class Series Operation & Control



Future Development4


Background

5


Wind FarmPV

Energy StorageFuel Cell

Electric VehicleMicroturbineDC LoadsCable Transmission

Conventional AC Distribution

System

Big Challenge

DC Distribution System

Hybrid AC/DC Distribution System

。。。

Heavy Load Supply

DC Power System AC Power System DC Power System ???

Low fault rate in circuitCapability of monopole operation, which transmits part of the

power without interruptionFast responding and quick recovery Inherent self-protection of power electronics preventing internal

faults affecting external system

1

6

Lower transmission losses (no reactive power, lower resistance)More efficient for DC loadsStronger power supply relatively

2

Multi-level DC Distribution System and Key TechnologiesBackground

Advantages of DC Distribution System

7

Buried cables are superior to overhead lines in view of reliability and urban landscape.

Buried cables are superior to overhead lines in view of reliability and urban landscape.

In AC distribution, frequency and reactive power control are necessary in addition to voltage control.In AC distribution, frequency and reactive power control are necessary in addition to voltage control.

3

AC transmission via buried cables is not practical because of severe charging current.

AC transmission via buried cables is not practical because of severe charging current.

DC distribution could avoid these problems.



4

5

8

By instant regulation of power electronic equipments

Output of distributed sources ranges from DC to aperiodic AC and their dynamic performances vary. So it's convenient to convert outputs to DC uniformly for interconnection and control



9


DC Distribution SystemAlso faced

Many challenges

Outline Background




Future Development10


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Multi-level DC Distribution System——Grid Configuration

The Critical Equipments:DC TransformerDC Circuit BreakerDC CableDC Transducer…

High PerformanceHigh ReliabilityLow Cost

Topology of multi-level DC distribution system (a simple case)

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Low voltage

Electric vehicle

AC loads

DC/ACHigh voltage

Low voltage

DC loads

AC loads

Medium voltage

Medium voltage

DC/ACDC-EPT

DC-EPT

DC-EPT DC-EPT

Small DG、 Energy storage

Large renewable sources

Rail Transit

Multi-level DC Distribution System——Grid Configuration

Satisfy power supply requirements of different levels

Allow integration of electrified railway, Electric Vehicle charging station, urban metro system, large renewable source and DG at user side

Decouple different stages of DC network

Decline power requirements of DC circuit breakers, since DC transformer(DC-EPT) can restrict rising of fault current due to its fast-responding features

Features：

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——Grid ConfigurationMulti-level DC Distribution System

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High load density in the future

Load Density of Major City in China in 2020

City Type Load Density(MW/km2)Developed

10～40

Developing 5～10

Undeveloped 3～5

Constraints：——Voltage Class Series

Multi-level DC Distribution System

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The development level of related technology ( e.g. VSC-HVDC )

Profile of typical VSC-HVDC projectsProject Nation Time Rating/MW Voltage/kV Length/km

Gotland Sweden 1999 50 ±80 70

Cross Sound Cable USA 2002 330 ±150 40

Estlink Estonia-Finland 2006 350 ±150 105

Trans Bay Cable USA 2010 400 ±200 88

Shanghai China 2011 20 ±30 8

Nanao China 2013 200 ±160 32

Zhoushan China 2014 1000 ±200 134

Dalian China CIP 1000 ±320 60

DolWin1 Germany CIP 800 ±320 165

INELFE France-Spain CIP 2×1000 ±320 64

Multi-level DC Distribution System——Voltage Class Series

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Optimization of grid structure

To optimize grid structure, DC distribution system should support DC loads without superfluous converters, so proper integration voltage class should be set for certain items.

To optimize grid structure, DC distribution system should support DC loads without superfluous converters, so proper integration voltage class should be set for certain items.


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Other equipment engaged in aperiodic, non-power frequency or non-three-phase operations suggests to be supported by DC. Other equipment engaged in aperiodic, non-power frequency or non-three-phase operations suggests to be supported by DC.


Optimization of grid structure (cont’d)

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Transition from traditional AC distribution system

In this case, the insulation requirement of DC voltage should be

no higher than the original AC voltage .

In this case, the insulation requirement of DC voltage should be

no higher than the original AC voltage .


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Proposal of DC voltage class series：

Proposal of DC Voltage ClassVoltage Capacity Cross section Supply radius（km）

(kV) (MW) （mm2） Al Cu ±320 1024 1200 408 685 ±150 225 600 204 343 ±30 18 300 51 86 ±10 7 200 6.8 11.4

±0.75 0.04 90 3.06 5.14 0.4 0.01 60 2.04 3.43


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±320 kV ,±150 kV are for the dense load demand in the future, they have stronger supply ability than 500 kV, 220 kV (in AC) respectively, and they are standard voltage-levels in domestic VSC-HVDC projects.

±30 kV meets the IEC standard that voltage ratio may be greater than 5 between 50~150 kV, and it allows integration of electrified railway andlarge renewable energy sources.

±10 kV has equal power supply ability with 20 kV(AC) while maintaining 10 kV(AC) power lines.

±750 V supports urban metro and small DGs while 400 V and 48 V support most household and enterprise appliance.

Each DC voltage has corresponding voltage in AC system, thus making it easier for interconnection and transition.

Explanations：


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Steady state operation & control Transient state operation & control Stability analysis Economical operation & control Protection Reliability evaluation …

——Operation & ControlMulti-level DC Distribution System

Many AC distribution system operation & control methods can also be referenced by DC distribution system.

Outline Background






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Requirements for DC transformer：

Safe Efficient Smart

——DC TransformerKey Technologies

DC DC

？？AC AC

very easy

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Existing schemes for DC transformer：

Conventional boost converters Cannot achieve high gain and high power Unidirectional, poor controllability

Switched capacitor converters Need too many modules to achieve high gain, which implies

significant loss and complexity

Resonant converters Increased switching losses Poor power quality Difficulties with power direction reversal

Most of these schemes are not suitable for DC distribution system because of limitation in power scale, efficiency and controllability.


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EPT(Electronic Power Transformer)：DC/DC

General structure of DC-EPT

Middle/high frequencytransformer


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Features :

Internal MF/HF ac transformers to achieve potential isolation, bidirectional power flow and flexible phases and gain

Using cascade H-bridge (LV) or Modules Multilevel Converters (MMC) to achieve high power and reduce volume and weight of transformers and capacitors

Lowest switching losses are achieved with a step modulation to obtain high efficiency

The primary and secondary converters are blocked when dc circuit fault occurs, thus the main circuit breaker can be rated lower


——DC Transformer——Middle/High Frequency Transformer

Key Technologies

At present, high voltage, high power, low loss & low cost middle/high frequency transformer is still very difficult.

10kV/42kVA/1kHz middle frequency transformers

Magnetic core materials:Silicon Steel, Amorphous, Ferrite, Nanocrystalline, …

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No current zero crossing, so DC breakers need to tolerate ultra overvoltage and current caused by generating zero crossing

fault penetration is much faster and deeper because of low impedance, hence it is necessary to clear the fault within a few milliseconds

Features of DC short circuit fault:

Key Technologies

DC circuit breaker based on LC oscillation system

——DC Circuit Breaker

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Existing types of DC breakers：

Key Technologies——DC Circuit Breaker

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——DC Circuit BreakerKey Technologies

ABB Hybrid HVDC Breaker

The hybrid HVDC breaker is designed to achieve a current breaking capability of 9.0 kA in an HVDC grid with rated voltage of 320 kV and rated HVDC transmission current of 2 kA.

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Proposed DC Circuit Breaker for DC distribution System

i

swidcI

2L1L1C 2C

RQB1u 2udu

Topology of the DC circuit breaker

Waveform of superimposition current

Produce current of which size and waveform can be changed flexibly to superimpose on the DC fault current

Generate artificial zero crossing & reduce positive amplitude of the superimposed current

Decrease effectively the electric-arc and damage to the switching contact

——DC Circuit BreakerKey Technologies

——DC Transducer

Key Technologies

How to measure DC voltage & DC current quickly and precisely and in low cost is very important for multi-level DC distribution system.

Hall effect current/voltage sensors

Outline Background






EPT-based Hybrid Distribution System

Electronic Power Transformer (EPT) A static device that transfers electrical energy from

one circuit to another through medium- or high-frequency (MF or HF) electromagnetic induction-based transformation and power electronic conversion technology.

Also named Solid State Transformer (SST) or Power Electronic Transformer (PET).

Conceptual diagram of EPT34

——EPT concept

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In fact, EPT is now a general concept

EPT can satisfy the different requirements of different power sources and loads.

Hybrid-EPT

Interface for ac & dc

DC-EPT

DC grid application

AC-EPT

Without dc-link

AC grid application

With dc-link

AC AC

DC DC

EPT-based Hybrid Distribution System——EPT concept

Interface for dc micro-source

Interface for ac micro-source

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EPT-based Hybrid Distribution System——Application Scenarios

Interface for AC/DC distribution system

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Power generation by remaining heat, pressure & gas, etc.

EPT-based industry distribution system

Several systems can be interconnected to form a cluster of systems, and the loads in each system could be supplied from others.

In the large-scale industry enterprises, the conventional captive power plants may be integrated to enhance the supply reliability.

Single system Multi system

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Main Parameters of the Demo System

——Demo System & SimulationsEPT-based Hybrid Distribution System

0

3000

6000

9000

12000

0 0.1 0.2 0.3

time (s)

Out

put p

ower

of P

V (W

)

Case I: Standalone state

-200

-100

0

100

200

0 0.1 0.2 0.3

time (s)

Vol

tage

of A

C b

us (V

)

0

500

1000

1500

2000

2500

3000

3500

0 0.1 0.2 0.3

time (s)

DC

load

pow

er (W

)

0

1500

3000

4500

6000

7500

0 0.1 0.2 0.3

time (s)

AC

load

pow

er (W

)

Output voltage control operating modeEPT with PV & ESS

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time (s) time (s)

time (s) time (s)

Demo system

The PV and battery together guarantee the reliability of the power supply for critical loads

The power of PV varies from 10.1 kW to 6.8 kW at 0.2 s, while the DC bus is loaded 3 kW and the AC bus is loaded 7 kW.O

utpu

t pow

er o

f PV

(W)

Volta

ge o

f AC

bus

(V)

DC

load

pow

er (W

)

AC

load

pow

er (W

)EPT can maintain the AC bus voltage. The entire system can satisfy the load demand regardless the variations from PV.


Case II: Grid-tied state

0

3000

6000

9000

12000

0 0.2 0.4 0.6 0.8 1.0

time (s)

Out

put p

ower

of P

V (W

)

0

1000

2000

3000

4000

0 0.2 0.4 0.6 0.8 1.0

time (s)D

C lo

ad p

ower

(W)

0

2500

5000

7500

10000

0 0.2 0.4 0.6 0.8 1.0

time (s)

Pow

er fl

owin

g th

roug

h EP

T (W

)

-3000

0

3000

6000

0 0.2 0.4 0.6 0.8 1.0

time (s)

Pow

er p

rovi

ded

by u

tility

grid

(W)

Real power tracking operation mode

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time (s) time (s)

time (s) time (s)

Both the PV and utility grid power the loads reliably and greenly

The DC loads are set at 3 kW and the AC loads are set at 7 kW.

The maximum output power of PV varies from 10.1 kW to 6.8 kW at 0.1 s, and back to 10.1 kW at 0.4 s.

And half of AC loads are switched off at 0.6 s.

Power provided by utility grid (W)Power flowing through EPT (W)

Output power of PV (W) DC load power (W)Demo system

The system can always obtain the maximum output power from the PV, and the excess power is transferred to the utility grid.


0

50

100

150

0 0.25 0.50 0.75 1.00 1.25 1.50

time (s)

RM

S vo

ltage

of A

C b

us (V

)R

MS

volta

ge o

f AC

bus

(V)

Case III: EV Powering state

-6000

-5000

-4000

-3000

-2000

-1000

0

0 0.25 0.50 0.75 1.00 1.25 1.50

Out

put p

ower

of E

V (W

)

0

500

1000

1500

0 0.25 0.50 0.75 1.00 1.25 1.50

time (s)

DC

load

pow

er (W

)

0

1000

2000

3000

4000

0 0.25 0.50 0.75 1.00 1.25 1.50

AC

load

pow

er (W

)

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time (s)

time (s)

time (s)

time (s)

EV can enhance the power supply reliability of the critical loads in an emergency

EV can be integrated to serve as a temporary power supply.

The critical loads are set at about 4.8 kW, including DC loads 1.3 kW and AC loads 3.5 kW.

At 0.3 s, the AC loads are reduced to half.

Output voltage control operating modeEPT with EV

Out

put p

ower

of E

V (W

)

AC

load

pow

er (W

)

DC

load

pow

er (W

)

Demo system

EV can power loads through the EPT in a critical condition, and the power supply reliability would be enhanced.


Brief introduction:

10kV/400V/500kVA AC-EPT Industrial Prototype

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By far, one 10 kV/400V/500kVA industrial prototype of AC-EPT was been developed. This valuable experience may attribute to development of DC-EPT.


C

B

AInput stage output stageisolation stage

MF

The prototype will be installed in Wuhan Iron & Steel (Group) Corp. 10kV distribution system to increase

the power supply quality and reliability in 2014

Industrial Distribution System

10kV/500kVA EPT Industrial Prototype

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Steady state input voltage & currents

Steady state output voltages & currents

10 kV Laboratory Experimental Results

45


Startup Loaded

46

10 kV Laboratory Experimental Results


Outline Background






Future Development

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Future DevelopmentMulti-level DC Distribution System

Power Electronicshigh voltage & high power

low cost & low losshigh reliability

Advanced Control Strategyhigh performance

Advanced Materialless volume & low loss & low cost

DC Loadshigh efficiency

Advanced DC Transducerhigh accuracy & low cost

Background






Summary

Thanks！

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Documents

Dr. Chengxiong Mao Professor School of Electrical and ... · PDF filesynchronous generator and applications of high power electronic technology to ... Fast responding and ... Proposed