dc distribution and power electronics applications …2 d 3 d 4 d 5 d 6 s 1 d 1 d 2 d d 4 d d 6 s 1...

DC distribution and power electronics applications in Smart Grids

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19.4.2012

LVDC20/PJ

DC/AC

AC

LVDC20/1/0.4 kV

DC/AC

0.4 kV 0.4 kV

20/0.4 kV 20/0.4 kV20 kV

0.4 kV 0.4 kV

L

DC

AC

AC

AC

AC

AC

DCDCDC DC

AC

ACAC

AC

P

Backup connection

Suburban district

Indoor DC

AC 20 kV

Urban district

Rural areas Urban areas

LAPPEENRANTAUNIVERSITY OF TECHNOLOGYFINLAND




Detached houses

=

M=

~

=

=

=

M

– Computers– Electronic appliances– Lights

– AC motor drives– Induction heating– Etc.

DC motors

Large resistive loads

=

~

– DC Generators– Power storages

SC

Variable speedAC generators

G

Customer networks/load devices

~

=

DC

…200 Hz

G

GTO-breaker

~

=

Public MV network

=

=

± 750 V

20 kV

=

=

PV

To other customers

LVDC microgrid

Introduction

Reasons to start to study LVDC distribution

- The structure of the electricity distribution is changing- Good power quality and uninterrupted power supply will be more important- The price of energy will rise because of a shortfall in fossil energy sources and

stringent emission regulations -> high efficiency is more important- More distributed generation units and energy reserves will be connected to network- LV and MV networks have to be replaced during next 20 years

Benefits of PWM converters

- Already used in UPS and STATCOM- Bidirectional power flow, control of active and reactive power- Security and communication systems integrated to converters- Connection between different voltage levels, AC/DC voltages

and single/multiphase systems

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19.4.2012

LVDC distribution

Goals

- High power quality to customers- The DC system enables the compensation of voltage sags and short interruptions of the MV

network (DC voltage can decrease 25 %)- Easy to connect distributed generation units and energy reserves -> the islanding operation would

be possible- Security and communication systems integrated to converters- Increased power transmission capacity (lower resistive losses and all power is active power)- Decreased length and complexity of MV network- The reliability of MV network increases since the LVDC network forms its own protection area- Interruption costs of network companies would decrease

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19.4.2012

= =AC DC

1500 V

0

AC

= =

=

=

A

B

DLVAC LVDC

AC

AC

AC

+750 V

-750 V

=C AC

MV

The line converters’ supply voltages

The supply voltage of the 12-pulse half-controlled thyristor-bridge is ULL= 562 V (designed in LUT), 2 % adjustment tolerance is taken into account.

The design method for the supply voltage of the line converters1. The voltage of the MV network can increase10 %2. The adjustment tolerance is 5 %

The line converter produces 750 V DCa) The basic carrier-based sinusoidal pulse width modulation method (PWM) is used, ULL is max.

400 Vb) The modulation index is boosted 15 % by the injection of a third harmonic component to the

sinusoidal modulation references or if the vector modulation is used, ULL is max. 460 V

The line converter produces1500 V DCa) The basic carrier-based sinusoidal pulse width modulation method (PWM) is used, ULL is max.

800 Vb) The modulation index is boosted 15 % by the injection of a third harmonic component to the

sinusoidal modulation references or if the vector modulation is used, ULL is max. 920 V

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19.4.2012

Protection of the LVDC network

- The LVDC distribution network has to fulfil the limits of the standard SFS 6000 about electrical safety.

- The customer’s network is grounded TN system and the LVDC network is ungrounded IT-system -> galvanic isolation is needed

- The load converter cannot be dimensioned according to effective power, but short-circuit current determines its dimensioning.

- Against AC/DC converter switch faults can be protected with protective functions integrated in the converter and with short circuit relaying.

- The short-circuit current can be limited in the converter to protect converters and other components in the network.

- The DC network faults can be covered with combined over current and short circuit protection and with earth fault protection.

- The customer-end network protection can be similar to the solutions used today if galvanic isolation is used. Special equipments are needed only for the DC network.

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19.4.2012

Protection of LVDC network

- The DC network short circuit protection can be made with molded case circuit breakers (A in Fig) which includes circuit breaker and over current relay. The bipolar system poles needs to have its own protection devices.

- The DC breakers (B in Fig) are connected in front of all inverters. - In customer AC network short circuit protection can be used circuit breakers and

fuses (D in Fig) when inverter short circuit capability is as high as used devices require.

- The B type 30mA residual current devices (F in Fig) can be used to increase human safety in double fault situations. Residual current device separates customer network form DC network in case of double fault situations.

- Against DC/AC customer converter switch faults can be protected with functions included in converter and by using surge arresters (G in Fig) and DC circuit breaker to prevent full DC voltage affecting in customer network.

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19.4.2012

Line converters

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19.4.2012

12-pulse half controlled thyristor bridge (operate as a diode bridge in steady-state)

2-level line converter

3-level line converter Vienna

S1 D1

C1

C2

Lgrid Lconv

C

i1i2

+-

+-

iLconv

S1

D1

C1+-

C2+-

iLconv

DD

Y

S1 D1

S2 D2

D5

Lgrid Lconv

C

C1

C2

+-

+-

iLconv

S3

S4

D3

D4

D6

D1

D2

D3

D4

D5

D6

S1Lgrid Lconv

C

C1

C2

+-

+-iLconv

Line converters

- The LVDC network can be supplied by one or two line converters.- The 12-pulse half-controlled thyristor bridge is the simplest rectifier topology which

can be used in the LVDC distribution. Only unidirectional power flow is possible and it produces low-order harmonics to MV network.

- Two-level and three-level line converters enables bidirectional power flow between MV and LVDC networks.

- Two-level and three-level line converters enables better power quality to MV network, these doesn’t produce low-order harmonics, therefore losses in the supply transformers and the MV network are lower.

- The converters can also inject reactive power into the MV network to control the power quality of the MV network. The LVDC distribution network behaves as a resistive load for the MV network if the line converters are used with the unity power factor.

- The drawbacks of PWM line converter are higher costs, more complex structure and control. Furthermore LCL-filter is needed between the line converter and MV network to filter high frequency harmonics caused by modulation.

- The neutral conductor can be connected from the DC link midpoint to the star point of the supply transformer if only one line converter is used and the neutral conductor is grounded.

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19.4.2012

Load converters

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19.4.2012

2-level half bridge 2-level full bridge 2-level 3-phase load converter

3-level half bridge 3-level full bridge 3-level 3-phase load converter

LC-filters should be added between the converter and the load!

Load converters

- Customer AC loads are connected to the DC network by single-phase half or full bridges or three-phase load converters which converge the DC voltage to AC.

- The DC loads can be connected to the network through DC/DC–converters. - The drawback is that the single phase loads produce 2nd harmonic component to the

DC link current.- The half bridges are simple, low-cost and easy to control. Large DC link capacitors

are needed. It is not possible to supply half-wave rectifying loads by half bridges. Half bridges are problematic from the electrical protection point of view.

- The structure of the full bridge is more complex than the structure of the half bridge, but the passive components, which are needed both on the DC and AC side, are smaller. Full-bridges don’t produce harmonic currents at the switching frequency as the half bridges do. It is possible to supply half-wave rectifying loads by full bridges without any problems in the voltage balance of the DC capacitors.

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19.4.2012

NPC converters

- The LVDC distribution is studied before but three-level converters are not used. - Three-level converters are widely used in medium- and high-voltage level because

of smaller voltage stress of the power semiconductor switches, but not in low-voltage level.

- In three-level converter, the load phase terminal can be connected to three voltage potentials, udc/2, 0 and -udc/2

11

n

+-

+-

S1 D1

S2 D2D5

D6 S3 D3

S4 D4

u /2dc

n

+-

+-

S1 D1

S2 D2D5

D6 S3 D3

S4 D4

0

n

+-

+-

S1 D1

S2 D2D5

D6 S3 D3

S4 D4

-u /2dc

n

+-

+-

S1 D1

S2 D2D5

D6 S3 D3

S4 D4

u /2dc

n

+-

+-

S1 D1

S2 D2D5

D6 S3 D3

S4 D4

0

n

+-

+-

S1 D1

S2 D2D5

D6 S3 D3

S4 D4

-u /2dc

NPC converters

The current through IGBTs and diodes are presented

The produced output voltages of two- and three-level single-phase half-bridges are presented

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19.4.2012

2-level 3-level

Output voltage waveforms of two- and three-level three-phase load converters

The produced output voltages of two- and three-level three-phase converters are presented

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19.4.2012

2-level 3-level

NPC converters

Modulation - multicarrier PWM, level shifted PWM methods- space vector modulation (SVM), more state

vectors (two-level converter 8 state vectors, three-level converter 27 state vectors)

DC link capacitors voltage balance- The DC link capacitor voltage balancing has to

be taken into account in the control system when the three-level converters are used, because the output terminal of the converter is possible to connect to the midpoint of the DC intermediate circuit.

- The voltage balance can be destroyed because of fast change in the operating point of the converter or because of unbalanced loading of the capacitors.

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19.4.2012

Vienna rectifier

- The structure of the Vienna rectifier is simpler than the two- or three-level converters’. The produced current distortion and the power factor of the rectifier are possible to control by Vienna rectifier just as with two- or three-level line converters.

- The drawback is that only unidirectional power flow can be used.

- The voltage stress of all the IGBTs and diodes is half of the DC voltage just as with the NPC converter.

- The voltage balance of the DC capacitors has to take into account just as with the NPC converters.

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19.4.2012

D1

D2

D3

D4

D5

D6

S1

D1

D2

D3

D4

D5

D6

S1

D1

D2

D3

D4

D5

D6

S1

D1

D2

D3

D4

D5

D6

S1

D1

D2

D3

D4

D5

D6

S1

Control methods of the LVDC network

- There are multiple different ways to control the operation of the LVDC distribution network.

- In the research work done by Brenna et al., the distributed generation units, battery storages and diesel generation are connected to the LVDC network. The LVDC network is able to bidirectional power flow and can also work in an island mode.

- The purpose of the control system is to allow a simple way to optimize the energy management of the LVDC distribution network.

- The control method is based on the DC link voltage value because it is a common signal for all of the converters. The DC link voltage is directly influenced by the voltage of the MV network, by the demand and the power generated.

- The control is based on four different voltage thresholds. One power system component is predominant with respect to the other devices during each voltage thresholds.

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19.4.2012

Control methods of the LVDC network

- During the normal operating conditions, the line converter controls the DC distribution voltage at Uconv (800 V).

- The voltage sag occurred in the MV network, the DC voltage is controlled to be constant by the line converter

- In the case of AC faults or fault in the converter, the voltage of the MV network decreases to a certain level. The system disconnects from the MV network and the operation of the line converter is stopped. When the DC voltage is fallen below Ubat, the batteries start to supply energy to the LVDC network.

- The DC/DC converter which supplies the energy from the batteries to the LVDC network regulates the DC voltage to the value Ubat (760 V).

- When the energy from the batteries is not adequate, the diesel power system supplies the LVDC network and the converter of the diesel generator regulates the DC voltage to Udiesel (770 V DC).

- The supervisor computer can stop power supplying to some specific loads if there is not enough power production in the LVDC network.

- After the MV network is recovered, the controller of the line converter detects the phase of the MV network by phase-locked loop (PLL).

- If power excess in the DC network, the voltage threshold value UDG is reached and the converters of the distributed generation units limit the generated power on that way that the DC voltage is kept at the value UDG if bidirectional power flow is not possible. Otherwise the supplementary generated power can be transmitted to the MV network.

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19.4.2012

UDG

Uconv

Udiesel

Ubat

UDC

Required AC-filters

- The maximum voltage stress of IGBT in three-level converters is half of those with the comparable two-level converters.

- Harmonic content of the output voltage and current is small -> smaller filter. This is very important because passive components increase the weight, cost and losses of the converters.

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19.4.2012

0 0.5 1 1.5 2 2.5 3

2-level 750 V DC3-level 750 V DCVienna 750 V DC

2-level 1500 V DC3-level 1500 V DCVienna 1500 V DC

2-level 750 V DC

3-level 750 V DC

Vienna 750 V DC

2-level 1500 V DC

3-level 1500 V DC

Vienna 1500 V DC

L _conv [mH] 1.11 0.53 0.53 2.23 1.07 1.07L _grid [mH] 0.14 0.13 0.13 0.28 0.26 0.26C [µF] 10.46 10.46 10.46 5.23 5.23 5.23fres [Hz] 4414 4816 4816 4412 4812 4812m_core_Lconv [kg] 1.53 0.5 0.5 2.74 1.53 1.53m_core_Lgrid [kg] 0.5 0.38 0.38 0.5 0.5 0.5

0 1 2 3 4 5 6

2-level full bridge 750V DC

3-level full bridge 750V DC

2-level 3-phase inv 750V DC


2-level half bridge 1500V DC

3-level half bridge 1500V DC



2-level full bridge

750V DC

3-level full bridge

750V DC



2-level half bridge

1500V DC

3-level half bridge

1500V DC



L [mH] 1.22 0.66 1.32 0.56 5 2.45 1.67 1.32C [µF] 2.98 2.58 4.38 4.38 6.55 5.96 2.8 4.38fres [Hz] 2640 3857 2093 3214 879 1317 2327 2093m_core [kg] 1.64 1.04 1.53 0.83 2.99 2.74 1.64 1.53

Losses of the converters

- The summation losses of the line and load converters are presented. The total power is 20 kW and it is produced by one line converter connected to 1500 V DC or two line converters connected to 750 V DC. The power is consumed by six single-phase load converters (3,3kW/converter) or two thee-phase load converters (3,3kW/phase, 10kW/converter)

- Switching losses of the IGBTs and diodes are marked with blue, IGBTs conduction losses with red and diodes’ conduction losses with green.

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19.4.2012

0

200

400

600

800

1000

1200

1400

2-level 750 V

DC

3-level 750 V

DC

Vienna 750 V

DC

Diode bridge 750 V

DC

2-level 1500 V

DC

3-level 1500 V

DC

Vienna 1500 V

DC

Diode bridge 1500 V

DC

Pow

er lo

ss (W

)

0

200

400

600

800

1000

1200

1400

2-level full

bridge 750 V

DC

3-level full

bridge 750 V

DC

2-level 3-phase 750 V

DC


DC

2-level half

bridge 1500 V

DC

3-level half

bridge 1500 V

DC


DC


DC

Pow

er lo

ss (W

)

Total losses of the converters and filters

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19.4.2012

Total efficiency of the line converters and LCL-filters, load converters and LC-filters at nominal and partial loading conditions

Lconv Lgrid L

0

100

200

300

400

500

600

700

2*2-level 750 V

2*3-level 750 V

2*Vienna 750 V

1*2-level 1500 V

1*3-level 1500 V

1*Vienna 1500 V

Pow

er lo

ss (W

) Ploss_Cu_20kHz

Ploss_Cu_10kHz

Ploss_Cu_50Hz

Ploss_Fe_20kHz

Ploss_Fe_10kHz

Ploss_Fe_50Hz

0

5

10

15

20

25

2*2-level 750 V

2*3-level 750 V

2*Vienna 750 V

1*2-level 1500 V

1*3-level 1500 V

1*Vienna 1500 V

Pow

er lo

ss (W

) Ploss_Cu_20kHz

Ploss_Cu_10kHz

Ploss_Cu_50Hz

Ploss_Fe_20kHz

Ploss_Fe_10kHz

Ploss_Fe_50Hz

0

500

1000

1500

2000

2500

3000

3500

4000

6*2-lev full

bridge 750 V

6*3-lev full

bridge 750 V

2*2-lev 3-phase 750 V

2*3-lev 3-phase 750 V

6*2-lev half

bridge 1500V

6*3-lev half

bridge 1500V

2*2-lev 3-phase 1500V

2*3-lev 3-phase 1500V

Pow

er lo

ss (W

)

Ploss_Cu_20kHz

Ploss_Cu_10kHz

Ploss_Cu_50Hz

Ploss_Fe_20kHz

Ploss_Fe_10kHz

Ploss_Fe_50Hz

0.8

0.82

0.84

0.86

0.88

0.9

0.92

0.94

0.96

0.98

1

25%*Pnom 50% 75% 100%*Pnom

Effic

ienc

y

2-level line converter 750 V DC


Vienna 750 V DC



Vienna 1500 V DC

0.5

0.55

0.6

0.65

0.7

0.75

0.8

0.85

0.9

0.95

1

25% 50% 75% 100%*Pnom

Effic

ienc

y

2-level full bridge 750 V DC

3-level full bridge 750 V DC

2-level 3-phase inverter 750 V DC


2-level half bridge 1500 V DC

3-level half bridge 1500 V DC



Losses of the converters with real load characteristics

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19.4.2012

The most efficient converter topologies in the LVDC distribution with the analyzed load characteristics are the three-level line converters and the three-level, three-phase load converters connected to 750 V DC. The total efficiency of the LVDC network without losses in the DC cable is 80 % at maximum and 33 % at minimum with the analyzed converters. The foremost part of the total losses is caused by high frequency losses of the filter inductor core, especially at low load. The losses would decrease if the iron core was replaced by other materials.

0

1

2

3

4

5

6

7

8

Pow

er lo

ss (M

Wh/

a)

inductor core high frequency losses

converter switching losses

converter conduction losses

inductor low frequency losses and copper losses

0

5

10

15

20

25

30

35

40

Pow

er lo

ss (M

Wh/

a)

Efficiency of the LVDC network

- The power transmission capacity of LV network increases with the use of DC and resistive losses in cables are lower because of higher voltage level.

- The LVDC system is composed of several components -> the total losses are still higher than losses of the traditional distribution system with one transformer.

- The converters should be connected to 750 V DC to minimize the losses. The three-level converters are a more efficient option than the two-level converters.

- A significant part of the total losses are caused by filter inductors -> the mass of the required inductors should be minimized and the material which has lower losses at high frequencies should be used in spite of higher acquisition costs.

- The efficiency would increase for example if the modular converter structure was used (multiple smaller converters and a part of them would be used depending on the loading situation)

- The efficiency would increase if some loads are fed by DC (like heating systems).- The efficiency of the system would be higher if the customer’s network would be

also ungrounded IT-system -> galvanic isolation transformer is not needed. - If galvanic isolation is still used, the efficiency would grow if the 50Hz galvanic

isolation transformer is replaced by isolated DC/DC converter and high-frequency transformer.

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19.4.2012

LVDC field testsTest 1: Orivesi

- Constructed by ABB and LNI Verkko to Orivesi- The customer loads consist of mainly single phase household equipment.- 120kVA ACS800 Island Converter by ABB is used. - The converters are monitored and controlled remotely via wireless GPRS

communication system by SCADA system.

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19.4.2012

Test 1: Orivesi

- According to measurements, the power quality of the customers increased remarkably because of the island converter.

- There had been several short outages in the supply network, but there was no outage in the island network due to the capacitors in the DC-link

- By the island converter, the voltage variation was maintained less than 2 %, the voltages were balanced and short term flicker index was decreased so the requirements of the standard were fulfilled.

- The island converter produced benefits also the supply network: the reactive power is zero because of the control system of the active rectifier. Single phase and distorting load were limited to the island network only so these didn’t load nor distort the supplying distribution line.

- The drawbacks of the island converter were high losses because of oversized converter and audible noise caused by the cabinet’s cooling system.

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19.4.2012

LVDC field testsTest 2: Suomenniemi

- Installed by Lappeenranta University of Technology and Suur-Savon Sähkö in Suomenniemi.

- Because of audible noise problems in the first pilot, there is not any heating or cooling system in the cabinets.

- 12-pulse half-controlled thyristor rectifier is used- Two-level, three-phase customer inverters are used. The LC-filter and 50 Hz

galvanic isolation transformer is connected between the LVDC network and the customer.

- The long DC cable, about 1,7km, enables the possibility to analyze, if there are low or high frequency interferences in the DC cable.

- The communication channel between the rectifier station and the inverters is realized by optical fibre. The pilot system can be controlled though the webportal.

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19.4.2012

MV supply

Rectifier(100 kVA)

Customer 1 Customer 4

Customer 3

DC cable815 m

AC

supp

ly90

m

Distribution cabinet

Customer 3

AC supply10 m

AC

supp

ly5

m

AC supply85 m

Inverter 3 (16 KVA)

DC cable360 m

DC cable360 m

10 kW

7 kW

7 kW

Inverter 2 (16 KVA)

Inverter 1 (16 KVA)

Double tier transformer (Ddy)

(100 kVA)D

C c

able

120

mMV supply

Rectifier(100 kVA)

Customer 1 Customer 4

Customer 3

Customer 4

Customer 3

DC cable815 m

AC

supp

ly90

m

Distribution cabinet

Customer 3

AC supply10 m

AC

supp

ly5

m

AC supply85 m

Inverter 3 (16 KVA)

DC cable360 m

DC cable360 m

10 kW

7 kW

7 kW

Inverter 2 (16 KVA)

Inverter 1 (16 KVA)

Double tier transformer (Ddy)

(100 kVA)D

C c

able

120

m

LVDC field testsTest 2: Suomenniemi

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19.4.2012

dc distribution and power electronics applications …2 d 3 d 4 d 5 d 6 s 1 d 1 d 2 d d 4 d d 6 s 1...

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