key technologies of protection and fault isolation of dc distribution network · 2014-10-31 · key...

Key Technologies of Protection and Fault Isolation

of DC Distribution Network

Tuesday, October 28, 2014

Bin LiBin LiTianjin University, ChinaTianjin University, [email protected]@tju.edu.cn

4

Backgrounds1

2

3

DC Distribution Network

Protection and Fault Isolation

Conclusions

2

Outline

DC system development

•1886, the 1st

generator•1879, electric light•1882, DC power supply system

AC system development

•1830s, transformer•ext 2•1891, 15.2kV AC transmission line•1895, simple AC power system (USA)Text

DC system

AC system

early DC system AC systemvoltage conversion hard easypower transmission hard easy

motor commutation brushed motors brushless motors

Operation condition serially connected, poor reliability

network controlled with high reliability

1. Backgrounds

DC distribution system is developing while the AC network system is matured.

Development of DC technology

mercury-arc converter

Thyristor converter

self-turn-off device

Thyristor is invented in 1956.

The first thyristor is developed in1957 and commercialised in 1958.

1st DC transmission system using thyristor converter is established in 1972.

The concept of self-turn-off device is proposed in McGill,Canada,1990.

ABB：HVDC Light；Siemens：new HVDC；China：flexible HVDC；Alstom：HVDC MaxSine.

It is suitable for DG combination and island power supply.

Mercury-arc converter is invented in 1928

1st transmission project u s i n g m e r c u r y - a r c converter is operating in 1954

high failue rate,poor reliability and high price.

1st experimental project operated in 1997.

DC distribution networks are suitable for： Non-synchronously integrated with AC system; Power supply to users; DG integration, new energy sources interconnection

Content DC ACreactive power Unnecessary Necessary

reliability high Comparative lowsynchronization none Existmax transmission

distance unlimited Limited

connecting to DGs suitable Has difficultiesChange rate of fault

current large small

difficulty in voltage conversion hard easy

difficulty in interrupting current hard easy

Comparision of AC system and DC system

Features and Benefits of DC grid

4

Backgrounds1

2

3



Conclusions

7

Outline

PROBLEM: MISMATCHED AC&DC POWER REQUIREMENTS

ENERGY SOURCES– MIXED AC&DC

ELECTRIC DEVICES– TYPICALLY DC

AC/DC SiteGeneration

PV

DC Wind Power

AC Grid

DC Fuel Cells

Electronic LightingData&Telecom Centers

PC

Phone

EV

DCAC

Battery

ACDC

Lost opportunity to reduce energy consumption

2. DC Distribution Network

SOLUTION: DC POWER DISTRIBUTION SYSEM

High Efficiency:less power conversion stages

Simple Control:only consider the dc bus voltage

Improved Reliability:bi‐directional dc‐ac converters

DC VERSUS AC Opened during Islanding Mode

UtilityGrid

ACDC

DCDC

DCDC

Energy Storage

DC Bus

DCAC

…

AC Microgrid

Bi-directionalPower converter Pdc

AC Bus

PV Wind Turbine

DCDC

TV,PC

DCDC

EV

Protection technologies of the dc systemOne of the most interesting topics:


Protection and fault isolation play the key roles to e n s u r e reliability and stability of t h e DC p o w e r distribution system.

DC Converter TechnologiesLine-Commutated Current-Source Converter

Voltage-Source Converter

Large transmission capacity and low investment

Power thyristor: control conduction

AC system voltage is necessary for commutation

Requirement of reactive power

Large space for filters and capacitors

IGBT: turning-on and turning-off are controllable

Self-commutation, supply power to passive network without commutation failure

Control active and reactive power

Small space, high investment

Topologies of DC Converterstwo-level converter

simple construction

PWM modulation

high switch losses

harmonic of PWM

modular multilevel converter（MMC）

low switching loss

low harmonic distortion

Topologies of DC Converters

modularity, scalability

Development Progress of VSC-HVDC

Company Name Time

ABB HVDC Light 1997

STATE GRID HVDC Flexible 2006

Siemens HVDC Plus 2007

Alstom HVDC MaxSine 2010

Voltage Source Converter-HVDC”（VSC-HVDC）.


Closed loopRadial Parallel Connection Mixed Connection

It is difficult to break DC current because the fault current has no natural zero-crossing point.To avoid DC system collapse, DC circuit breaker should be able to interrupt fault as fast as possible. At least the action time should reach a few hundreds microseconds level or a few milliseconds.

Mechanical Solid State Hybrid

Breaking Speed Breaking Capacity Topology Control Loss

Mechanical Slow（Tens of milliseconds） Large Simple Easy Low

Solid StateFast（Hundreds

of microseconds）

Small Moderate Moderate High

Hybrid Medium（A few milliseconds） Medium Complicated Difficult Medium

Key Technologies of Fault IsolationCircuit Breaker of DC system

The current in DC power grid rises rapidly with high amplitude, so the time of

protection action should be in several milliseconds or even microseconds.

The configuration of protection depends on the power grid scale, converter

topology, the configuration of circuit breaker and many other factors, so

protection strategies are complex and diverse.

Key Technologies of Fault IsolationProtection of DC system

0.00 0.02 0.04 0.06 0.08 0.100.0

0.2

0.4

0.6

0.8

1.0

续流

电流

/kA

时间/s

电容放电电流

放电

Fault current in DC power system

4

Backgrounds1

2

3



Conclusions

17

Outline

1

23 4

5

67

8

1. VSC valve2. valve reactor3. Transformer4. starting resistance

3. Protection and Fault Isolation Main Electric Connection of DC System

AC system protection

Transformer protection

Converter protection DC system

protection

5. Arrester6. grounding resistance7. DC cable8. lightning arrester

3. Protection and Fault Isolation Fault Types and Positions

External AC system ①②

common short-circuit and grounding fault

Internal AC

system

③④ AC bus fault

⑤⑥ bridge arm reactance grounding fault

Valve

⑦⑧ valve grounding fault

⑨ valve flashover fault

⑩ valve phase flashover fault

DC system

⑾⒀ Singlepolar grounding fault

⑿⒁ Bipolar short-circuit fault

Time(s)1t

2t

1Di 3Di 5Di1 3 5 AD , ,i （）

ai bi ciAabci （）

Ali（）

Vdcu（）

DC capacitor discharging

3t

Diodes naturallyCommutation conducting

Diodes conductsynchronously

Fault Analysis of VSC-DC System

Rs Lsav

bvcvdcU

C

sabci

dci li /2R /2L

/2R /2L

sausbuscu

ci1D 3D 5D

4D 6D 2D

DC side faults (VSC)

singlepolar-to-ground

Bipolar fault

I. DC capacitor discharging stage;

II. Freewheel Diodes naturally

commutation conducting stage;

III. Freewheel Diodes conducting

synchronously stage.

Time(s)1t

2t

1Di 3Di 5Di1 3 5 AD , ,i （）


Ali（）

Vdcu（）

DC capacitor discharging

3t

Diodes naturallyCommutation conducting


Fault Analysis of VSC-DC System

DC side faults(VSC)


DC capacitordischarging

Diodes naturallycommutationconducting

Converter：IGBTblocked

AC side：current not rising

DC side：DC voltage dropping

AC side：Feeding small current to the diodes

DC side：DC voltage drop

AC side：Three-phase fault

Diodes：Overcurrent shocking

DC side：Inductance discharging

DC capacitordischarging

Diodes naturallycommutationconducting

singlepolar-to-ground Bipolar fault

Bipolar short-circuit fault（most serious）

Time S

li

La _ upi

abci ai bi ci

Unit(A)

Fault Analysis of MMC-DC System

sau

sbu

scu

Rs sL LR

1T

LR

Rs sL LRLR

Rs sL LRLR

2T

1D

2DC

1T

2T

1D

2DC

1T

2T

1D

2DC

1T

2T

1D

2DC

1T

2T

1D

2DC

1T

2T

1D

2DC

1T

2T

1D

2DC

1T

2T

1D

2DC

1T

2T

1D

2DC

1T

2T

1D

2DC

1T

2T

1D

2DC

1T

2T

1D

2DC

2lR / 2lL /

2lL /2lR /

I. SM not blocked;

II. Initial stage after blocking SM

III. Uncontrolled rectification stage

Bipolar short-circuit fault（most serious）

Time S

li

La _ upi

abci ai bi ci

Unit(A)

Fault Analysis of MMC-DC System

SM notblocked

Initial stage after blocking SM

uncontrolled rectificationstage

SM capacitor and AC source feed fault current. Converter arm current and DC current rise fast.

Arm reactance freewheeling, AC source feeding three-phase fault current.

Due to the equivalent DC load is very small, so the currents of the AC side, converter and the DC side are still very large

Open AC side circuit breaker

Block IGBT Circuit breakers of AC side trip

Isolate fault linewith DC switch

Reclose AC side circuit breakerRestore AC side power supplyReduce area of blackout

Converter blockClear up the DC faults by the improvement of the converter topology and the cooperation of the control strategy.

• Long switching time

• High investment• Large power loss

DC circuit breaker • Action time• Switching capacity

11SW 12SW

31SW

33SW

22SW

23SW

1Line

3Line 2Line

11SW 12SW

31SW

33SW

22SW

23SW

1Line

3Line 2Line

11SW 12SW

31SW

33SW

22SW

23SW

1Line

3Line 2Line

1T

2T

1D

2DC

3T

4T

3D

4D

1T

2T

1D

2DC

3T

4T

3D

4D

Fault Isolation Strategies

• Low reliability of power supply

Vdcu（）

Ali（）


1Di 3Di 5Di1 3 5 AD , ,i （）

Time(s)

AC side

Converter

SFCL

SFCL

Normal state

Superconducting state

Ic

Hc

Tc

Normal state

Normal state

Superconducting limiting

DC faultSFCL quenchResistance rises to increase damping effect

Isolate the fault by DC-CB

AC side ：Three-phase fault is avoided Converter：Overcurrent of the diodes

is eliminated DC side：Overcurrent is limited DC-CB：Action time is extended

Switching capacity is reduced

Fault Isolation Technologies

Decreased significantly

No zero-crossing point

Converter limiting

T

D

lim iterRCC

1D 1D 2D2D

1T 1T2T 2T

T

D

lim iterRCC

1D 1D 2D2D

1T 1T2T 2T

T

D

lim iterRCC

1D 1D 2D2D

1T 1T2T 2T

Time S

li A

abc Ai ai bi ci

_La upi A

Fault currents of AC side, DC side and converter are limited

Investment and power loss are low The requirements for the action

time and switching capacity of the DC-CB are decreased

Fault Isolation Technologies Decreased significantly

Over-current Protection Simplicity. If i>imax, protections will operate All branch currents are increased when a

fault occurs. Action velocity, Selectivity, Sensitivity.

Reliability

Differential Protection intrinsic selectivity Problems in communication with

long lines: complexity and delay Action velocity

Distance Protection

Protection Technologies

The cable impedance is so small that the measurement errors will result in protection failure. So the traditional distance protection is not available

4

Backgrounds1

2

3



Conclusions

32

Outline

4. Conclusions

DC distribution networks is suitable for integration of

DGs, improving efficiency of energy uThe current in DC

power grid rises rapidly with high amplitude, so the time of

protection action should be in several milliseconds or even

microseconds.

The configuration of protection depends on the power

grid scale, converter topology, the configuration of circuit

breaker and many other factors, so protection strategies

are complex and diverse.

tilization, etc.

DC Distribution Platform in Tianjin University

±200VDC

DCDC

ACDC

10 kVA

BUS1

5 kW

380/220V

PV BATTERYSTORAGEDEVICES

DC LOADDC LOAD

QF8 QF9LINE1 QF1-2QF1-1

LINE6 QF6-2QF6-1

ACDC5 kW

WIND POWER

±200VDCBUS2

5 kW

5 kW 5 kW 5 kW 5 kW

QF2-2QF2-1 LINE2

QF5-2QF5-1 LINE5

±200VDCBUS3

ACDC

ACDC

10 kVA220/380V

DCDC

DCDC

QF3-2QF3-1 LINE3

±200VDCBUS4

QF4-2QF4-1 LINE4

DCDC

DC LOAD

DCDC

±200VDCBUS6

±200VDCBUS5

DC LOAD

Low-Voltage Bipolar-Type ±200VDC Two Bi-directional DC/AC Converters Multi-terminal DC-links --- 6 DC Buses Meshed Network

Characteristics:

key technologies of protection and fault isolation of dc distribution network · 2014-10-31 · key...

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