VSC based HVDC system

Submitted By
C. Madhu
K.Deepika
K.Mounika
M.Harish
R.Ephraim
DEPARTMENTOFELECTRICALANDELECTRONICS ENGINEERING

Chapters
1. 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 INDIA
Back-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.

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 water
Use of this system is mainly due to cost considerations
A metallic return may be used where earth resistivity is too
high
This 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 Links
There 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 Links
In 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 Links
This 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 Systems
Converters
 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

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.

Introduction
Conventional 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 converter
The 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

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

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
p
n
n
n
n
n
Q
Q
P
P
V
V
V
V
J
1
2
2
1
1
2
2
2




δ
δ
(1)
where the Jacobian matrix is divided into submatrices as






=
2221
1211
JJ
JJ
J (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
n
n
n
n
n
V
P
V
V
P
V
V
P
V
V
P
V
J
1
1
2
2
1
2
1
2
2
2
12

















∂
∂
∂
∂
∂
∂
∂
∂
=
++
n
nn
n
pp
QQ
QQ
J
δδ
δδ
1
2
1
2
2
2
21



















∂
∂
∂
∂
∂
∂
∂
∂
=
+
+
+
+
+
+
p
p
p
p
p
p
n
n
n
n
n
n
V
Q
V
V
Q
V
V
Q
V
V
Q
V
J
1
1
1
2
1
2
1
2
1
2
2
2
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)
.

and δ(0)
to calculate a total np number of
injected reactive power Qcalc
(0)
and equal number of reactive power
mismatch ∆Q(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
0
01
1
V
V
VV (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).

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
1
1
1
1
1
1
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([)( 01010
12
01
12
RvRRRRvRRRRvRaRRswRRaRoR BGVVmVGGmP ϕθθϕθθ +−++−−+=
)]cos()sin([)( 01010
12
01
12
RvRRRRvRRRRvRaRReqRRaRoR BGVVmVBBmQ ϕθθϕθθ +−−+−−+−=
Likewise, another set of equations may be developed for the inverter
)]sin()cos([ 0101
12
1 IIvIIIIvIIoIvIaIvIIvI BGVVmVGP ϕθθϕθθ −−+−−−=
)]cos()sin([ 0101
12
1 IIvIIIIvIIoIvIaIvIIvI BGVVmVBQ ϕθθϕθθ −−−−−−−=
)]sin()cos([)( 0101
12
01
1
0
2
IvIIIIvIIIoIvIaIIswIIaI BGVVmVGGmP ϕθθϕθθ +−++−−+=
)]cos()sin([)( 0101
12
01
1
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 0.97680
178.19−∠ 0
813.0∠

SUMMARY OF POWER LOSSES INCURRED BY
THE VARIOUS MODELS
Model
Active 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.

VSC based HVDC system

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