Submitted ByC. MadhuK.DeepikaK.MounikaM.HarishR.Ephraim

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

Chapters1. HVDC Transmission system

2. Voltage source converters

3. VSC-HVDC Transmission

4. Newton - Raphson method

5. Modelling of VSC

6. Test cases and results

7. Future scope

HVDC stands for High Voltage Direct Current. HVDC transmission is an

efficient technology designed to deliver large amount of electricity over long

distances with negligible losses.

The world’s first commercial HVDC link situated between the Swedish

mainland and the island Gotland was delivered by ABB in the year of 1954

with the capacity of 20MW, 100kv

INRTODUCTION

The longest HVDC link in the world is currently the Xiangjiaba–Shanghai.

It was built on owned by State Grid Corporation of China(SGCC)

Total length - 2071km

Power ratting - 6400MW

DC voltage - 800KV

HVDC IN INDIABack-to-Back

HVDC LINK CONNECTING REGION

CAPACITY (MW)

Vindyachal North – West 2 x 250

Chandrapur West – South 2 x 500

Vizag – I East – South 500

Sasaram East – North 500

Vizag – II East – South 500

NEEDS OF HVDC

As the load demand increases as the time progresses , there should be two

possibilities:

Either to increase the generation

To minimize the losses

The losses are occurred at various levels are Generating level, transmission

level and distribution level

So the losses at transmission level can be greatly reduced by HVDC

transmission

WHY TO PREFER HVDC THAN HVAC?

Long distance transmission

5 times more energy transmits than AC(same lines)

Less losses (no inductance, capacitance).

Cost of transmission is low.

Maintenance & operation cost is low.

Initial cost is high but overall cost is low than ac.

Cont…

HVDC System Configurations and Components

HVDC links can be broadly classified into:

Monopolar links Bipolar links Homopolar links Multi terminal links Back-to-back links Point-to-point links

Monopolar Links

It uses one conductor The return path is provided by ground or waterUse of this system is mainly due to cost considerationsA metallic return may be used where earth resistivity is too

highThis configuration type is the first step towards a bipolar link

Bipolar Links It uses two conductors, one positive and the other negative Each terminal has two converters of equal rated voltage, connected in

series on the DC side The junctions between the converters is grounded Currents in the two poles are equal and there is no ground current If one pole is isolated due to fault, the other pole can operate with

ground and carry half the rated load (or more using overload capabilities of its converter line)

Homopolar Links

It has two or more conductors all having the same polarity, usually negative

Since the corona effect in DC transmission lines is less for negative polarity, homopolar link is usually operated with negative polarity

The return path for such a system is through ground

Multi terminal LinksThere are more than two sets of converters like in the bipolar case.Thus, converters one and three can operate as rectifiers while converter

two operates as an inverter. Operating in the opposite order, converter two can operate as a rectifier

and converters one and three as inverters

Back-to-Back LinksIn this case the two converter stations are located at the same site and no

transmission line or cable is required between the converter bridges. The connection may be monopolar or bipolar. The dc-link voltage is regulated by controlling the power flow to the ac

grid.This system having fast control of the power flow.

Point-to-Point LinksThis configuration is called as the point to point configuration, when the

converters are located in different regions and need to be connected

with a transmission line to transmit power form one converter side to

another.

In that case one converter acts as a rectifier, which provides the power

flow and another one acts an inverter which receives that power.

Components of HVDC Transmission Systems

1. Converters

2. Smoothing reactors

3. Harmonic filters

4. Reactive power supplies

5. Electrodes

6. DC lines

7. AC circuit breakers

Components of HVDC Transmission SystemsConverters They perform AC/DC and DC/AC conversion They consist of valve bridges and transformers Valve bridge consists of high voltage valves connected in a 6-pulse or 12-pulse

arrangement The transformers are ungrounded such that the DC system will be able to establish its

own reference to ground

Smoothing reactors They are high reactors with inductance as high as 1 H in series with each pole They serve the following:

They decrease harmonics in voltages and currents in DC lines They prevent commutation failures in inverters Prevent current from being discontinuous for light loads

Harmonic filters Converters generate harmonics in voltages and currents. These harmonics may cause

overheating of capacitors and nearby generators and interference with telecommunication systems

Harmonic filters are used to mitigate these harmonics 18

Contd….

Reactive power supplies Under steady state condition conditions, the reactive power consumed by the

converter is about 50% of the active power transferred Under transient conditions it could be much higher Reactive power is, therefore, provided near the converters For a strong AC power system, this reactive power is provided by a shunt

capacitor

Electrodes Electrodes are conductors that provide connection to the earth for neutral. They

have large surface to minimize current densities and surface voltage gradients

DC lines They may be overhead lines or cables DC lines are very similar to AC lines

AC circuit breakers They used to clear faults in the transformer and for taking the DC link out of

service They are not used for clearing DC faults DC faults are cleared by converter control more rapidly 19

ADVANTAGES OF HVDC

Disadvantages

Power loss in conversion, switching and control

Expensive inverters with limited overload capacity

High voltage DC circuit breakers are difficult to build.

Provision of special protection to switching devices & filtering

elements.

IntroductionConventional thyristor device has only the turn-on control; its turn-

off depends on the current coming to zero as per circuit and system

conditions.

With some other types of semiconductor device such as the

insulated-gate bipolar transistor(IGBT), both turn-on and turn-off

can be controlled, they can be used to make self-commutated

converters.

In such converters, the polarity of DC voltage is usually fixed and

the DC voltage, being smoothed by a large capacitance, can be

considered constant. For this reason, an HVDC converter using

IGBTs is usually referred to as a voltage sourced converter.

Types of Converters

Line commutated converters Use switching devices such as thyristor.

Classical HVDC system

VSC based HVDC system

Self commutated converters Use fast switching devices such as IGBT’s, GTO’s.

There are two basic categories of selfcommutating converters:

1. Current-sourced converters in which direct current always has one

polarity, and the power reversal takes place through reversal of de

voltage polarity.

2. Voltage-sourced converters in which the de voltage always has one

polarity, and the power reversal takes place through reversal of de

current polarity.

Why Self commutated Converters preferred than Line commutated Converters

A major drawback of HVDC systems using line-commutated

converters is that the converters inherently consume reactive power.

The AC current flowing into the converter from the AC system lags

behind the AC voltage so that, irrespective of the direction of active

power flow, the converter always absorbs reactive power, behaving

in the same way as a shunt reactor. The reactive power absorbed is at

least 0.5 MVAr/MW under ideal conditions.

It suffers from occasional commutation failures in the inverter mode

of operation.

Self commutating voltage source converterThe direct current in a voltage-sourced converter flows in either

direction, the converter valves have to be bidirectional, and also,

since the de voltage does not reverse, the turn-off devices need not

have reverse voltage capability; such tum-off devices are known as

asymmetric turn-off devices. Thus, a voltage-sourced converter valve

is made up of an asymmetric tum-off device such as a GTO with a

parallel diode connected in reverse.

voltage-source converters maintain a

constant polarity of DC voltage and power

reversal is achieved instead by reversing the

direction of current.

Basic Voltage source converter

Contd..Function of capacitor:

On the de side, voltage is unipolar and is supported by a capacitor. This capacitor is large enough to at least handle a sustained charge/discharge current that accompanies the switching sequence of the converter valves and shifts in phase angle of the switching valves without significant change in the de voltage.

Function of inductor:

Reducing the fault current, this coupling reactance stabilises the AC current, helps to reduce the harmonic current content and enables the control of active and reactive power from the VSC.

Types of voltage source converters

1. Two level converter

2. Three level converter

3. Modular Multi level converter

Three-phase, two-level voltage-source converter for HVDC

Three-phase, three-level, diode-clamped voltage-source converter for HVDC

Three-phase Modular Multi-Level Converter (MMC) for HVDC.

Why we are looking towards VSC HVDC instead of Conventional HVDC

Conventional HVDC uses line commutated converters, these

converters requires large amount of reactive power for rectification

and inversion. Commutation failures in inverter mode of operation.

These converters require a relatively strong synchronous voltage

source in order to commutate.

Contd…

These problems can be eliminated in self-commutated conversion by

the use of more advanced switching devices with turn-on and turn-

off capability.

The present self-commutating HVDC technology favours the use of

IGBT-based VSC, combined with high-frequency sub-cycle

switching carried out by PWM

Classical HVDC system

VSC Based HVDC system

This high controllability allows for a wide range of applications. From a

system point of view VSC-HVDC acts as a synchronous machine without

mass that can control active and reactive power almost instantaneously.

And as the generated output voltage can be virtually at any angle and

amplitude with respect to the bus voltage, it is possible to control the active

and reactive power flow independently.

Components of VSC-HVDC System and its operation 1. Physical Structure

2. Converters

3. Transformers

4. Phase Reactors

5. AC Filters

6. Dc Capacitors

7. Dc Cables

8. IGBT Valves

9. AC Grid

Contd…

1. Physical Structure: The main function of the VSC-HVDC is to

transmit constant DC power from the rectifier to the inverter. As

shown in Figure.1, it consists of dc-link capacitors Cdc, two

converters, passive high-pass filters, phase reactors, transformers

and dc cable.

2. Converters: The converters are VSCs employing IGBT power

semiconductors, one operating as a rectifier and the other as an

inverter. The two converters are connected either back-to-back or

through a dc cable, depending on the application.

3. Transformers Normally, the converters are connected to the ac system

via transformers. The most important function of the transformers is to

transform the voltage of the ac system to a value suitable to the

converter. It can use simple connection (two-winding instead of three to

eight-winding transformers used for other schemes). The leakage

inductance of the transformers is usually in the range 0.1-0.2p.u

4. Phase Reactors: The phase reactors are used for controlling both the

active and the reactive power flow by regulating currents through them.

The reactors also function as ac filters to reduce the high frequency

harmonic contents of the ac currents which are caused by the switching

operation of the VSCs. The reactors are usually about 0.15p.u.

Impedance.

5.AC Filters: High-pass filter branches are installed to take care of these

high order harmonics. With VSC converters there is no need to

compensate any reactive power consumed by the converter itself and the

current harmonics on the ac side are related directly to the PWM

frequency. The amount of low-order harmonics in the current is small.

Therefore the amount of filters in this type of converters is reduced

dramatically compared with natural commutated converters.

6.Dc Capacitors: On the dc side there are two capacitor stacks of the

same size. The size of these capacitors depends on the required dc

voltage. The objective for the dc capacitor is primarily to provide a low

inductive path for the turned-off current and energy storage to be able to

control the power flow. The capacitor also reduces the voltage ripple on

the dc side.

7. Dc Cables: The cable used in VSC-HVDC applications is a new

developed type, where the insulation is made of an extruded polymer

that is particularly resistant to dc voltage. Polymeric cables are the

preferred choice for HVDC, mainly because of their mechanical

strength, flexibility, and low weight.

8. IGBT Valve: The insulated gate bipolar transistor (IGBT) valves used

in VSC converters. These devices having low forward voltage drop and

high switching frequency. A complete IGBT position consists of an

IGBT, an anti parallel diode, a gate unit, a voltage divider, and a water-

cooled heat sink. The gate-driving electronics control the gate voltage

and current at turn-on and turn-off, to achieve optimal turn-on and turn-

off processes of the IGBT.

9. AC Grid: Usually a grid model can be developed by using the

Thevenins equivalent circuit. However, for simplicity, the grid was

modeled as an ideal symmetrical three-phase voltage source.

Comparison of classical HVDC and VSC-HVDC

Advantages of VSC-HVDC

Independent control of active and reactive power without extra

compensating equipment.

Mitigation of power quality disturbances.

Reduced risk of commutation failures.

Communication not needed.

Multiterminal DC grid.

APPLICATIONS

Long-distance bulk power transmission

Underground and submarine cable transmission

Interconnection of asynchronous networks

Newton – Raphson Method:• The Newton-Raphson method is a powerful method of solving

non-linear algebraic equations.

• It works faster and is sure to converge in most cases as

compared to the Gauss – Siedel method.

• It is indeed the practical method of load flow solution of large

power networks.

• Its only drawback is the large requirement of computer memory

which has been overcome through a compact storage scheme.

• Convergence can be considerably speeded up by performing the

first iteration through the GS method and using the values so

obtained for starting the NR iterations.

Load flow by Newton- Raphson method Let us assume that an n-bus power system contains a total number of

np P-Q buses while the number of P-V (generator) buses be ng such

that n = np + ng + 1. Bus-1 is assumed to be the slack bus. We shall

further use the mismatch equations of ∆Pi and ∆Qi respectively.

The approach to Newton-Raphson load flow is similar to that of

solving a system of nonlinear equations using the Newton-Raphson

method: at each iteration we have to form a Jacobian matrix.

∆

∆∆

∆

=

∆

∆∆

∆

+

+

+p

p

pn

n

n

n

n

Q

Q

P

P

V

V

V

VJ

1

2

2

1

1

2

2

2

δ

δ

(1)

where the Jacobian matrix is divided into submatrices as

=

2221

1211

JJ

JJJ (2)

It can be seen that the size of the Jacobian matrix is (n + np − 1) × (n +

np − 1). For example for the 5-bus problem of Fig According to our

thesis, this matrix will be of the size (7 × 7). The dimensions of the

submatrices are as follows:J11: (n − 1) × (n − 1), J12: (n − 1) × np, J21: np × (n − 1) and J22: np × np

The submatrices are

∂∂

∂∂

∂∂

∂∂

=

n

nn

n

PP

PP

J

δδ

δδ

2

2

2

2

11

∂∂

∂∂

∂∂

∂∂

=

++

++

p

p

p

p

n

nn

n

n

n

V

PV

V

PV

V

PV

V

PV

J

1

12

2

1

21

2

22

12

∂∂

∂∂

∂∂

∂∂

=++

n

nn

n

ppQQ

QQ

J

δδ

δδ

1

2

1

2

2

2

21

∂

∂∂

∂

∂∂

∂∂

=

+

++

+

++

p

p

p

p

p

p

n

n

n

n

n

n

V

QV

V

QV

V

QV

V

QV

J

1

1

12

1

2

1

21

2

22

22

(2.1) (2.2)

(2.3)(2.4)

Newton – Raphson Load Flow Algorithm

The Newton-Raphson procedure is as follows:

Step-1: Choose the initial values of the voltage magnitudes |V|(0) of all np

load buses and n − 1 angles δ(0) of the voltages of all the buses except

the slack bus.

Step-2: Use the estimated |V|(0) and δ(0) to calculate a total n − 1 number

of injected real power Pcalc(0) and equal number of real power mismatch

∆P(0).

Step-3: Use the estimated |V|(0) and δ(0) to calculate a total np number of

injected reactive power Qcalc(0) and equal number of reactive power

mismatch ∆Q(0).

Step-3: Use the estimated |V|(0) and δ(0) to formulate the Jacobian matrix J(0).

Step-4: Solve (3.10) for ∆δ(0) and ∆|V|(0)÷|V|(0).

Step-5: Obtain the updates from( ) ( ) ( )001 δδδ ∆+=

( ) ( )( )

( )

∆+= 0

001

1V

VVV (4)

(3)

Step-6: Check if all the mismatches are below a small number.

Terminate the process if yes. Otherwise go back to step-1 to start the

next iteration with the updates given by (3) and (4).

Chapter 5

The complex power model for the rectifier can be obtained from the

nodal admittance matrix as shown in below equation

=

*

*

0

0

o

vR

o

vR

o

vR

I

I

V

V

S

S

o

vR

V

V

0

0

Χ

++Φ−Φ−

Φ+Φ−

o

vR

eqaswa

a

V

V

BjYmGYjm

YjmY

)()sin(cos

)sin(cos

11

11

11

1

2

=

Following equations are the nodal active and reactive power expressions for the rectifier are arrived at

)]sin()cos([ 01010'2

1 RRvRRRRvRRRvRavRRvR BGVVmVGP ϕθθϕθθ −−+−−−=

)]cos()sin([ 01010'2

1 RRvRRRRvRRRvRavRRvR BGVVmVBQ ϕθθϕθθ −−−−−−−=

)]sin()cos([)( 0101012

0112

RvRRRRvRRRRvRaRRswRRaRoR BGVVmVGGmP ϕθθϕθθ +−++−−+=

)]cos()sin([)( 0101012

0112

RvRRRRvRRRRvRaRReqRRaRoR BGVVmVBBmQ ϕθθϕθθ +−−+−−+−=

Likewise, another set of equations may be developed for the inverter

)]sin()cos([ 010112

1 IIvIIIIvIIoIvIaIvIIvI BGVVmVGP ϕθθϕθθ −−+−−−=

)]cos()sin([ 010112

1 IIvIIIIvIIoIvIaIvIIvI BGVVmVBQ ϕθθϕθθ −−−−−−−=

)]sin()cos([)( 010112

011

0

2

IvIIIIvIIIoIvIaIIswIIaI BGVVmVGGmP ϕθθϕθθ +−++−−+=

)]cos()sin([)( 010112

011

0

2

IvIIIIvIIIoIvIaIIeqIIaI BGVVmVBBmQ ϕθθϕθθ +−−+−−+−=

Since both converters are connected their DC side to a common DC bus 0, it should be noted that buses OR and OI are the same bus in this back-to-back VSC-HVDC application.

Fig: Back-to-back VSC-HVDC linking two equivalent AC sub-systems. The following parameters are used: (i) Transmission Line 1 and 2: RTL = 0.05p.u., and XTL = 0.10p.u., BTL = 0.06p.u.,; (iii) VSC 1 and VSC 2 initial shunt conductance for switching loss calculation Gsw = 0.01p.u.,; (iv) LTC 1 and 2 series reactance: 0.06 p.u.; (v) active and reactive power load at bus 2: 1 p.u. and 0.5 p.u.; (vi) active and reactive power load at bus 5: 1.5 p.u. and 0.5 p.u.

Nodes 1 2 3 0 4 5 6

V(p.u) 1.02 1.002 1 1.414 1 0.99 1.02

(deg)Ө 0 -16.04 -19.664 0.000 1.154 -2.53 0.000

VSC 1 2 LTC 1 2

ma 0.838 0.831 Tap 1.1105 0.97680178.19−∠ 0813.0∠

SUMMARY OF POWER LOSSES INCURRED BY THE VARIOUS MODELS

ModelActive Power Losses(MW)

Reactive Power losses (MVAR)

AC1 AC2 VSC-HVDC AC1 AC2 VSC-HVDC

PV buses 2641 1.29 N/A 72.95 5.13 N/A

Sources 26.58 1.28 0.57 73.27 5.19 5.74

New model 26.89 1.31 1.97 7445 5.38 5.91

A new model suitable for VSC-HVDC links using Newton-Raphson

power flows solutions has been developed. In this model properties

of PWM , ohmic losses and conduction losses of VSCs are included.

The phase angle of the complex tap changer represents the phase

shift that would persist in a PWM inverter. More significantly, this

would be the phase angle required by the voltage source converter to

enable either reactive power generation or absorption purely by

electronic processing of the voltage and current waveforms within

the operation of voltage source converter.

Comparisons were also made against a model where the VSC-HVDC

link is represented as two PV-type nodes at its connecting nodes with

the two AC sub-systems.

A new VSC HVDC model has been presented and power flow has

been analysed with Newton’s Raphson method. It should be noted

that although no multi-terminal VSC-HVDC test cases are addressed

in thesis, the formulation here presented is also suitable for solving

such systems.

The natural idea of connecting dc links together to form the VSC-

HVDC system will quite possibly lead to the emergence of dc grids,

which may profoundly affect the future of the electric power grid.

The new model developed may be extended to power flow analysis

for multiple grids connected through VSC-HVDC links.