Modern Hvdc

Modern HVDC State of the Art

Driving Forces for Modernization      

Valves Converter Transformers Filters AC-DC Measurements Digital Signal Controllers Compact Substation Design

1. Thyristor Valves 

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

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Rating 8-9 kV on silicon wafers of 150 mm diameters. Less number of units for series connection. Light Triggered Thyristors (LTT) are used eliminating high components on firing circuits. Valves are air-insulated & housed outdoor.

2. Self Commutated Valves 

Voltage Source Converters are now widely used with increased power rating.



They produce almost sinusoidal output

Comparison Of Semi-conductor Devices Thyri stor

GTO

IGBT

SI

MCT

MOSFET

Max.V

9000

6000

1700

2500

3000

1000

Max. I

4000

6000

800

800

400

100

Gating

pulse

current

voltage current

voltage

current

V. Drop

1.2

2.5

3

4

1.2

resistive

Sw. Freq(kHz)

1

5

20

20

20

100

Target V

10000 10000

3500

5000

5000

2000

Target I

8000

2000

2000

2000

200

8000

IGBT : Review 

Insulated Gate Bipolar Transistors Combine high impedance and speed of the MOSFETS with the high conductivity of Bipolar Transistors



IGBT then handle High Power and High Speed.

POWER DEVICE HISTORY GTO:GATE TURN OFF THYRISTOR BPT:BIPOLAR POWER TRANSISTOR

THYRISTOR

GTO IGBT

BPT IGBT:INSULATED GATE BIPOLAR TRANSISTOR

IGBT: Review Basic Configuration FET and Bipolar

Review: IGBT 

IGBTs of higher power and speed are being developed by optimizing the physical structure and designs (design rules, parallel packing)

Review: IGBT The IGBTs of the future will continue to give high power and speed, at higher and higher voltages (>3000 V), smaller packages and greater temperature capacity (>400 C) 

Technical Trend of IGBT Gate Structure & Image

Generation

1'st Gen.

(VCES=600V)

VCE(sat)

tf

(1'st：1.0)

1.00

3.0V TYP.

0.5μ s TYP.

0.60

3.0V TYP.

0.2μ s　TYP.

0.25

2.1X TYP.

0.15μ s　TYP.

0.06

1.6V TYP.

0.15μ s　TYP.

Cell size

EMITTE GATE R

2'nd Gen.

N+ P+ NN+ P+

3'rd Gen.

COLLECTOR

Trench Photo EMITTER

4'th Gen.

GATE

N P+

(Trench)

+

N N+ P+

(PhaseⅡ ) COLLECTOR

12

WAVEFORM EXAMPLES THREE PHASE PWM CONVERTER CIRCUIT P

L C

AC Input Voltage N

AC Input Current

Sign Wave form ・ pf=1.0 図３ .18 　 PWM 制御三相ブリッジコンバータ

WAVEFORM EXAMPLES 12 PULSES RECTIFIER CIRCUIT AND INPUT CURRENT WAVEFORM Ic Id

図３ . ７（５）　１２相サイリスタ整流回路と交流入力電流波形

WAVEFORM EXAMPLES 6 PULSES RECTIFIER CIRCUIT AND INPUT CURRENT WAVEFORM

Ic

INPUT CURRENT

HARMONIC CONTENTS %

18 16 14 12

6-PULS THYRISTOR RECTIFIER 12-PULS THYRISTOR RECTIFIER PWM IGBT CONVERTER

10 8 6 4 2 0

5f

7f 11f 13f 17f 19f

3. Tunable AC Filters 



Conventional filters can be detuned due frequency deviation, capacitance & inductance changes. Tunable filters can be achieved by: 1. Mechanical or electronic inductor taps. 2. Orthogonal Magnetization of Iron core.

3. Tunable AC Filters Orthogonal Magnetization of Iron core.  Core Magnetization does not require moving parts.  It can be controlled by direct current

3. Tunable AC Filters

4. AC-DC Measurements  

  

Optical Current Transducers are used. A/D converter is powered by laser (light) source via fiber optic link. Current is transmitted in digital form. Fiber links up to 300 m are used. Better reliability and lower costs.

4. AC-DC Measurements

5. Digital Signal Controller 

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 

DSP algorithms have grown exponentially The Million Instructions per second (MIPS) exceed 3000. Converter firing control circuits are fully digital Size of cubicles decreased- lower costs. Faults diagnosis features Online monitoring

6. Substation Design 

Modern substation are compact and occupy less area.



Example 2000 MW in 1980’s is 300x300 m



Today 2000 MW is 100x200 m.

7. Deep Electrode 

Electrode needs to be located where resistivity is low and may require a large area.



Deep hole Electrode can be placed with low resistivity.

7. Deep Electrode

7. Deep Electrode 

Advantages: 1. Electrodes are closer to converter. 2. Use of shorter lines . Low power loss. 3. Reduced Interference 4. Less risk of lightning strikes. 5. Sites are easy to find. 6. Possibility of homopolar operation, if needed.

7. Deep Electrode 



Advantages: 1. Electrodes are closer to converter. 2. Use of shorter lines . Low power loss. 3. Reduced Interference 4. Less risk of lightning strikes. 5. Sites are easy to find. 6. Possibility of homopolar operation, if needed.

Figure 15.15.

HVDC fault performance 

DC overhead line faults 1.The fault is detected by the DC line fault protection. 2.This protection orders the rectifier into inverter mode and this discharges the line effectively. 3. After some 80 - 100 ms the line is charged again by the rectifier. 4. If the fault was intermittent, due to e.g. a lightning strike, then normally the line can support the voltage and the power transmission continues. 5. Full power is then resorted in about 200 ms after the fault.

HVDC fault performance 

DC overhead line faults 1. The DC line fault clearing does not involve any mechanical action. 2. It is faster than for an AC line. 3. It should also be pointed out that DC line faults on a bipolar line affect only one pole (if fallen line towers is disregarded). 4. The bipolar DC line is equivalent to a double circuit AC line!

HVDC fault performance  

AC network faults When a temporary fault occurs in the AC system connected to the rectifier, the HVDC transmission may suffer a power loss. Even in the case of close single-phase faults, the link may transmit up to 30 % of the prefault power. As soon as the fault is cleared, power is restored to the pre-fault value.

HVDC fault performance 

AC network faults



When a fault occurs in the AC system connected to the inverter, a commutation failure can occur interrupting power flow. A commutation failure is an unwanted, but natural process in a classic HVDC inverter that the system can handle. If the AC-fault is temporary the power is restored as soon as the fault is cleared.

HVDC fault performance 

AC network faults A distant fault with little effect on the converter station voltage (< 10 percent) does not normally lead to a commutation failure. A CCC (Capacitor Commutated Converter) HVDC converter can tolerate about twice this voltage drop before there is a risk of commutation failure.

DC Breakers A: Surge arrestor C: Commutating Capacitor R: Discharge Resistor CS: Main Switch IS : Isolating Switch Gc: Spark Gap

DC Breakers CS operates .1 .1 2. Arc voltage causes A Gc to spark over. 3. : Commutating Capacitor charges 4. A limits capacitor voltage & takes over full current 5. Current through breaker reduces to zero. 6. Breaker CS then completes its opening.