General Overview of HVDC Transmission System · Comparison With HVAC ITEM HVAC HVDC 1. Power Transmission Capability Low High (e.g. 3000 MW Bipole) 2. Distance Limited by Stability

General Overview of HVDC Transmission System

WHY HVDC ?

Asynchronous connection (enables to connect two different electrical networks having different frequency & voltage)

Power flow control (enables the stability of electrical network)

Added benefit to the transmission like stability, power quality etc.

WHY HVDC?

Environmental advantages (lesser right of way requirement)

Lower line losses compared to AC line (no corona & charging current)

Economical (only two conductor for transmission & lesser tower height)

Comparison With HVAC

ITEM HVAC HVDC 1. Power Transmission Capability Low High (e.g. 3000 MW

Bipole) 2. Distance Limited by Stability

considerations. Switching Stations required.

No Limitation. Cheaper alternative for Long Distances

3. System Connection Synchronous Asynchronous

4. Right of Way requirements High Low 5. Power Control No Yes 6. Features – Frequency Control,

Reactive Power Control, Damping of Oscillations etc.

Not Available Available

Comparison With HVAC

ITEM HVAC HVDC 7. Tapping of Power Connection Simple Costly, Multi-terminal

Scheme required 8. Economical Alternative for Low to Medium distance,

Medium Power Range. Long Distance Bulk Power Transmission

9. System SCL (for consideration in developed AC systems due to high fault currents)

Contributes to System SCL

Does not Contribute to System SCL

10.

Pollution Effects Relatively Lesser More Pronounced Higher insulator creepage distance is required

SO WHY HVDC RATHER THAN HVAC ?

Long distances make HVDC cheaper

Improved link stability

Fault isolation

Asynchronous link

Control of load flow (DC voltage can be exactly controlled)

96 m 46 m

400 kV AC Lines 500 kV DC Line

Comparison of right of way

Cost comparison of ac and dc transmission

Cost of DC terminal

Cost of AC terminal

CostBreak even distance

Distance in km

Cost of AC Line

Cost of DC Line

500 – 700 km

Types of HVDC Transmission system

Mono Polar System:

One pole, one conductor for transmission and current return path is through earth.

Mainly used for submarine cable transmission.


Bipolar System:

Two poles, two conductors in transmission line, one positive with respect to earth & other negative

The mid point of Bi-poles in each terminal is earthed via an electrode line and earth electrode.

In normal condition power flows through lines & negligible current through earth electrode. (in order of less than 10 Amps.)


Homo Polar System:

Two poles at same polarity & current return path is through ground.

This system was used earlier for combination of cable & over head transmission.


Back to Back HVDC Coupling: Usually bipolar without earth return. Converter & inverters are located at the same place. No HVDC Transmission line. Provides Asynchronous tie between two electrical

network Improves system stability Power transfer can be in either direction.


Multi Terminal HVDC System:

Three or more terminal connected in parallel, some feed power and some receive power from HVDC Bus.

Provides Inter connection between the three or more AC network.

System stability of AC network can be improved.

HOW HVDC WORKS

POWER FLOW EQUATIONS

FOR AC TRANSMISSION:

POWER(P) = V1 V2

XSinδ

POWER FLOW EQUATIONS

FOR DC TRANSMISSION:

POWER(P) = Vdr (Vdr-Vdi)

R

HVDC Transmission Normal Power Direction

Rectifier Inverter

Note! Only a small voltage difference

Id

Page 28

RectifierInverter

Note that the current flow is in the same direction.Only the polarity is reversed.(changing a).

Id

HVDC Transmission Reverse Power Direction

Note! Only a small voltage difference

Page 29

Basic Diagram of HVDC System

DCTRANSMISSION

LINE

Pd = Vd Id

FILTER

Vd

AC SYSTEM A TERMINAL A

Ld Id

FILTER

TERMINAL B

Ld

AC SYSTEM B

6-Pulse Convertor Bridge

3

6

CiLs

4

E1 Ls

Ls

Bi

iA

1

2

I

V'd

5

Vd

IddL

d

12-Pulse Convertor Bridge

Y

Filter

12-Pulse Valve Group With AC Filters

BIPOLE HVDC

MODES OF OPERATION

BASIC HVDC Single Line Diagram

DC OH Line

Converter Transformer

DC Filter:DT 12/24DT 12/36


ThyristorValves

400 kV AC Bus

AC Filters

Smoothing Reactor




ThyristorValves

400 kV AC Bus

AC Filters

Smoothing Reactor

Modes of Operation

DC OH Line


ThyristorValves

400 kV AC Bus

AC Filters,Reactors

Smoothing Reactor


ThyristorValves

400 kV AC Bus

AC Filters

Smoothing Reactor

Bipolar

Current

Current

Modes of Operation

DC OH Line


ThyristorValves

400 kV AC Bus

AC Filters,Reactors

Smoothing Reactor


ThyristorValves

400 kV AC Bus

AC Filters

Smoothing Reactor

Monopolar Ground Return

Current

Modes of Operation

DC OH Line


ThyristorValves

400 kV AC Bus

AC Filters,Reactors

Smoothing Reactor


ThyristorValves

400 kV AC Bus

AC Filters

Smoothing Reactor

Monopolar Metallic Return

Current

HVDC Thyristor Valves

Modern Thyristor Valves

Valve Modular Design

Suspended Installation

Water Cooled

Valve Hall Equipments & Thyristors at Vindhyachal

THYRISTOR VALVE:

AIR INSULATED

WATER COOLED

DESIGNED FOR INDOOR USE

OCTUPLE UNIT (Each Physical Structure Contain 08 Valve Functions)

Each single valve has 04 Thyristor modules connected in series

Each Thyristor Module has 06 thyristors with Voltage Divider & Control circuits

Each Thyristor Module in a Valve is series connected to a Reactor

Total no of Thyristors in one valve hall : 576

Each valve has 1 Thyristor redundant out of 24

Valve Hall Equipments & Thyristors at Vindhyachal

VALVE HALL EQUIPMENTS & THYRISTOR

CONFIGURATION OF ONE THYRISTOR LEVEL

Thyristor

TCU

C1 C2

Heat sinks

HVDC Vindhyachal Thyristor Rating

CURRENT RATING: RATED DIRECT CURRENT: 3600 A MAX. DIRECT CURRENT AT RATED POWER: 3700 A MAX. DIRECT CURRENT AT OVERLOAD: (i) Id MAX FOR 2HR IN 12 HRS. : 4150 A (ii) Id MAX FOR 5 SEC. IN 5 MIN. : 4650 A

VOLTAGE RATING: NON REPETITIVE REVERSE VOLTAGE: 5350 V NON REPETITIVE FORWARD VOLTAGE: 4850 V

Valve Reactor

Thyristor

Thyristors

The Thyristor is a solid state semiconductor switching device with four layers of alternating P and N type material.

It has three terminals; Anode, Cathode & Gate

The Thyristor continue to conduct as long as they are forward biased ( that is as long as the voltage across the device has not reversed).

N

P

P

N

Anode

Cathode

Gate

J1

J2

J3

Thyristor vs Diode

Like the Diode, Thyristor is also unidirectional device that blocks current flow from cathode to anode.

Unlike the Diode, a Thyristor also blocks current flow from anode to cathode until it is triggered by a proper gate signal between gate & cathode terminals.

Static I-V Characteristics of a Thyristor

V bo +Va

+Ia

-Va

-Ia

Vbo = Forward break over voltage

Forward blocking mode

Forward conduction mode

Reverse blocking mode

Latching current

Holding current

Vbr = Reverse breakdown voltage

Vbr

Forward leakage current

Reverse leakage current

Effect of Gate Current on Forward Breakover Voltage

The effect of gate current on the forward break over voltage of thyristor can be understood by the figure alongside.

For Ig = 0, forward break over voltage is Vbo

For Ig1>Ig, forward break over voltage is V1<Vbo

For Ig2>Ig1, forward break over voltage is V2<V1<Vbo

For Ig3>Ig2>Ig1, forward break over voltage is V3<V2<V1<Vbo

+Ia+Ia

+Va+Va

Ig=0Ig=0

Ig1Ig1 Ig2Ig2Ig3Ig3

Ig3>Ig2>Ig1Ig3>Ig2>Ig1

V1V1 V2V2V3V3

V3<V2<V1V3<V2<V1

VboVbo

Cooling System

Basic Control Concepts

DC Line

α γ

Current Reference Voltage Reference

InverterRectifier

Powercontrol

Iorder Currentcontrol

amplifier

Converterunitfiring

control Id

Iresponse

Voltagemeasuring

system

Porder

Pmod

Ud response

To inverter

HVDC control system


1.0

1.0

Ud (p.u.)

Id (p.u.)

Rect Id Cont

Oper. Point

Inverter Characteristics

Rectifier Characteristics

Converter Characteristics Converter Characteristics


Converter Characteristics Converter Characteristics

1.0

1.0

Ud (p.u.)

Id (p.u.)

Rect Id Cont

Oper. Point

Inv Id Cont

Inverter Characteristics

Rectifier Characteristics

YY

Y

iy

i

iy i+

%

5 7 11 13 17 19 23 25

11 13 23 25

510

510

iy

in

iy + i in

iy

i

iy i+

AC Side Harmonics

Transformer function in HVDC system

•Supply of AC voltages into two separate circuits feeding the rectifier bridges with a phase shift of 30 electrical degrees for reduction of low order harmonics esp. 5th & 7th harmonics. •As a galvanic barrier between AC and DC systems to prevent DC potential entering into the AC system •Reactive Impedance in the AC supply to reduce short circuit currents and to control the rate of rise in valve current during commutation.

Converter Transformers

Type of Connections No. of design X No. of units

Spares required

3 Phase Star-Delta & Star-Star

2 X 2 2

Single phase 2 winding 2 X 6 2

Singe phase 3 winding 2 x 3 1

Extended delta-connection

2 X 2 1

3 Phase 3 winding 2 X 2 1

DC High Speed Switches Transfer between configurations

Transfer between configurations by means of MRTB and MRS(GRTS)

DC MEASURING DEVICESDC MEASURING DEVICES

Measurement on DC side for control, monitoring and Protection AC CTs cannot be used on DC side – saturation DC current measuring devices –

DC shunt – low value resistor mV drop from the shunt will be taken for determining the current To solve insulation problems – electrical signals are converted to optical at the

shunt and at control system converted to electrical Supply for the conversion process is obtained from the control panels in the form

of optical power DC voltage divider

Capacitive & resistor divider circuit Drop across the resistor scaled for determining the voltage Optical conversion process is same as the current measuring device

Smoothing Reactor -

Connected in series in each converter with each pole Decreases harmonic voltages and currents in the DC line Smoothen the ripple in the DC current and prevents the

current from becoming discontinuous at light loads Limits crest current (di/dt) in the rectifier due to a short

circuit on DC line Limits current in the bypass valve firing due to the

discharge of the shunt capacitances of the dc line

To reduce the magnitude of the harmonic currents circulating in the HVDC transmission line to avoid unacceptable interferences.

DC filters are needed for HVDC transmission with Bipole Link and overhead line.

In the case of back-to-back and cable transmission systems, there are no requirements for dc filters.

DC Filters

Thank You

Documents

General Overview of HVDC Transmission System · Comparison With HVAC ITEM HVAC HVDC 1. Power Transmission Capability Low High (e.g. 3000 MW Bipole) 2. Distance Limited by Stability