BHEL HVDC Training Report Summary

Summer Training Report on HVDC Transmission System at BHEL, Noida
Training Report- HVDC Transmission System Page 1
Summer Training
Subject: HVDC Transmission System and its Applications
Trainer: Mr. M I Khan,
Senior Specialist-HVDC System,
BHEL, Noida
Trainee: Mr. Mohammed Azadar Naqvi,
Student- B. Tech-F (Electrical),
Faculty of Engineering & Technology,
Jamia Millia Islamia, New Delhi

What is HVDC?
A high voltage direct current electric power transmission
system uses direct current for the bulk power transmission in
contrast with more common alternating current.
The first commercially used HVDC link in the world was
built in 1954 between the mainland of Sweden and island of
Gotland. The first HVDC link to be commissioned in India was
Rihand-Dadri in 1991.
Why (HVDC) required?
AC technology has proved very effective in the field of
generation, transmission and distribution of electrical energy.
Nevertheless, there are tasks which cannot be performed
economically or with technical perfection by this method.
Hence a need of alternating technique: HVDC transmission
Inductive and capacitive elements of overhead lines and
cables put limits to the transmission capacity and the
transmission distance of AC transmission links. In case of HVDC
transmission Inductive and capacitive parameters do not limit
the transmission capacity or the maximum length of a DC
overhead line or cable. The conductor cross section is fully
utilized because there is no skin effect.
This limitation is of particular significance for cables.
Depending on the required transmission capacity, the system
frequency and the loss evaluation, the achievable transmission

distance for an AC cable will be in the range of 40 to 80 km. For
a long cable connection, e.g. beyond 40 km, HVDC will in most
cases offer the only technical solution because of the high
charging current of an AC cable. This is of particular interest for
transmission across open sea or into large cities where a DC
cable may provide the only possible solution.
Direct connection between two AC systems with different
frequencies is not possible. However a DC link allows power
transmission between AC networks with different frequencies
or networks, which cannot be synchronized, for other reasons.
Direct connection between two AC systems with the same
frequency or a new connection within a meshed grid may be
impossible because of system instability, too high short-circuit
levels or undesirable power flow scenarios. However Input of
additional power without increasing the short circuit ratio of
the network concerned is possible with HVDC system.
The land coverage and the associated right-of-way cost for an
HVDC overhead transmission line is not as high as that of an AC
line. This reduces the visual impact and saves land
compensation for new projects.
In AC mesh network power is generally controlled by the
load. However in HVDC transmission digital control system
provides accurate and fast control of the active power flow
(10% to 100%). This feature of HVDC system makes it unique
for commercial operation of power transmission.

DC transmission can be controlled by the firing angle, as the
result, it is independent to events in linked ac system,
eliminating the inherent power fluctuation which may
endanger the stability.
Fast modulation of DC transmission power can be used to
damp power oscillations in an AC grid and thus improve the
system stability.
Economic Consideration- HVDC system is more economical
compared to alternative AC system for long distance. Break
even distance is in the range of 500 to 800 km.
For a given transmission task, feasibility studies are carried out
before the final decision on implementation of an HVAC or
HVDC system can be taken.
Environmental Consideration- HVDC transmission system is
basically environment friendly because improved energy
transmission possibilities contribute to a more efficient
utilization of existing power plants. There are, however, some
environmental issues (like Audible noise, Electromagnetic
Compatibility, Visual Impact and Use of ground or sea return
path in mono polar operation) which must be considered for
the converter stations.

HVDC Function, Principle and Configuration
HVDC technology is use to transmit the bulk electrical power
over a large distance. A simplified single line diagram for HVDC
system can be drawn as:
The dc output voltage magnitude is controlled by varying the
firing angle of thyristor valves in the converter. At rectifier the
firing angle is varied from 50
to 900
while at inverter it is varied
from 900
to 1600
. Simplified block diagram for HVDC system is
as:
This can be further simplified in to equivalent circuit as:

And hence power can be reverse simply by changing the
magnitude of voltage.
And hence the basic principle of HVDC system and its simplified
DC circuit diagram can be summarized as:

There are various configuration of HVDC system:
Back to Back Converter
The expression Back-to-back indicates that the rectifier and
inverter are located in the same station.
Back-to-back converters are mainly used for power
transmission between adjacent AC grids which cann’t be
synchronized. They can also be used within a meshed grid in
order to achieve a defined power flow.
Example: 158kV DC, 500MW Bheramara Back-to-Back project
between India (400kV) and Bangladesh (230kV).
Monopolar Long-Distance Transmissions
For very long distances and in particular for very long sea cable
transmissions, a return path with ground/sea electrodes will be
the most feasible solution.

In many cases, existing infrastructure or environmental
constraints prevent the use of electrodes. In such cases, a
metallic return path is used in spite of increased cost and
losses.
Example: 300kV DC, 300 MW Thailand–Malaysia monopolar
metallic return, Long distance transmission, 110km substation
at Khlong Ngae (230kV AC) and Gurun (275kV AC).
Bipolar Long-Distance Transmissions
A bipolar is a combination of two poles in such a way that a
common low voltage return path, if available, will only carry a
small unbalance current during normal operation. The
advantages of a bipolar solution over a solution with two
monopoles are reduced cost due to one common or no return
path and lower losses. The main disadvantage is that
unavailability of the return path with adjacent components will
affect both poles.
Following are achievable configuration with bipolar HVDC:

Bipolar System, Bipolar with Ground Return Path:
Example: ± 500 kV DC, 2500MW Ballia- Bhiwadi bipolar, Long-
distance transmission, 800 km (400kV AC).
Bipolar System, Bipolar with Dedicated Metallic Return Path
for Monopolar Operation:
Example: ± 250 kV DC, 2×200 MW COMETA, Spain (400kV AC)–
Mallorca (230kV AC) bipolar with metallic return conductor,
Submarine cable transmission, 250 km.
Bipolar System, Bipolar without Dedicated Return Path for
Monopolar Operation:
Monopolar operation is possible by means of bypass switches
during a converter pole outage, but not during an HVDC
conductor outage.

Example: ± 450 kV DC, 1000 MW BritNed, bipolar with fast
bypass switches without metallic or ground return, Submarine
cable transmission, 260 km (400kV AC).
Other achievable configurations with bipolar HVDC are as:
Bipolar System, Monopolar, Ground Return with two DC Lines

MAIN COMPONENTS OF AN HVDC SYSTEM
Thyristor valves are most important converting station
equipment and other main equipment’s are converter
transformer, DC reactor, harmonic filtering equipment, control
equipment and reactive power compensation equipment.
AC Switchyard (1)
The purpose of AC switchyard is to connect the terminal to the
AC system and Links Important HVDC components together. It
consist of AC bus bar for incoming and outgoing feeder,
Switchgear equipments (like Circuit breaker, Isolator,
Grounding Switch, Surge arrestors) and instrument
transformers (CT and VT for measuring & protection).

AC Filters, Capacitor Banks (2)
The purpose of AC filters, capacitor bank and shunt reactor is to
supply the reactive power and filter harmonic currents. As a
thumb rule HVDC system with thyristor valves requires reactive
power around 50% of active power. AC filter are RLC circuits
connected between phase and earth. They offer low impedance
to harmonic frequencies. Thus, AC harmonic current are passed
to earth. An n-pulse converter generates harmonics
predominantly of the order of nx±1 on AC side where x is an
integer and n is usually 6 or 12.
Two 400kV, 3-phase AC filters used in Ballia-Bhiwadi HVDC
project are shown below:

Converter Transformer (3)
Converter transformer requires to obtain the AC Voltage
needed for the required DC voltage and to obtain 12-pulse
operation (star and delta connection). Special attention must
be paid to the DC pre-magnetization of the core due to small
asymmetries during operation and stray DC currents from the
AC voltage network. The effects of DC pre-magnetization must
be compensated by appropriate design and manufacturing
efforts (e.g. additional core cooling ducts, avoidance of flux
pinching in the core sheet). A single phase two winding
converter transformer is shown below:
For six-pulse converter, a conventional 3-phase or three single
phase transformers is used. For a 12 pulse converter bridge, the
following converter transformer may be used:
a) 2 X 3-phase 2-winding transformers
b) 3 X 1-phase 3-winding transformers
c) 6 X 1-phase 2- winding transformers

Thyristor Valves (4)
Thyristor valves convert AC to DC and vice-versa at rectifier and
inverter station respectively. It also connects 6-pulse bridges in
series for required DC voltage. To achieve the high voltage and
current various thyristors are connected in series and parallel.
Thyristor is line commutated device and it requires a triggering
signal for turn-on at the Gate. This signal can be electrical or
optical.
A 3-phase thyristor valve used in Ballia-Bhiwadi HVDC project is
shown below:

Smoothing Reactors and DC Filters (5)
Smoothing reactors are require for smoothing the DC current
ripples, to avoid resonance with DC line and to reduce the risk
of commutation failures by limiting the rate of rise of the DC
line current at transient disturbances in the AC or DC systems.
While DC filter limit the interference caused by DC side
harmonics. An n-pulse converter generates harmonics
predominantly of the order of nx on DC side where x is an
integer and n is usually 6 or 12.
Smoothing Reactor and DC filter used in Ballia-Bhiwadi HVDC
project are shown below:
HVDC converter may produce electrical noise in the carrier
frequency band from 20 kHz to 490 kHz. they also generate
radio interference noise in megahertz range of frequency. High
frequency filter is mainly aimed to reduce the interference to
the power line carrier communication. Such filter is connected
between the converter transformer and the station AC bus.
Two three-phase AC filter for 400kv 500kv DC filter

DC Switchyard (6)
DC switchyard require in HVDC system to achieve DC side
transmission configuration. It consists of DC breaking switches,
Isolator and grounding switches, surge arrestors and DC current
and voltage measuring devices. DC measurement is done by
optical devices or zero flux measurement technique.
Depending on type of configuration of HVDC system the
component of DC switchyard varies. Back-to-Back HVDC
configuration does not require a DC Yard. Similarly DC yard in
monopolar HVDC configuration is quite simpler compared to
bipolar HVDC system.

Advantages of HVDC
Various advantages of HVDC system have been mention earlier
in chapter why HVDC. Few advantages are summarized here:
More power can be transmitted per conductor per circuit.
Use of Ground Return Possible
Smaller Tower Size
Higher Capacity available for cables
No skin effect
Less corona and radio interference
No Stability Problem
Asynchronous interconnection possible
Lower short circuit fault levels
Power transmission is easily controllable
Economically transmission of bulk power over a long distance
Inherent problems associated with HVDC
Expensive convertors
Reactive power requirement
Generation of harmonics
Difficulty of circuit breaking
Difficulty of voltage transformation
Difficulty of high power generation
Absence of overload capacity

List of HVDC project in India
Following table summarize the various HVDC project
commissioned in India:
S.
No.
Project
Name
Connecting
Region
Commissioned
On
Power
Rating
(MW)
AC
Voltage
(KV)
DC
Voltage
(KV)
Mode of
Operation
No. of
Poles/
Blocks
Length
of Line
(km)
1 Rihand-Dadri ER-WR
December-
1991
1500 400 500 Bipole 2 816
2 Talcher-Kolar ER-SR June-2003 2000 400 500 Bipole 2 1369
3
Ballia-
Bhiwadi
ER-NR
Pole1: March-
2010
Pole2: March-
2011
2500 400 500 Bipole 2 780
4
Chandrapur-
Padge
CR-WR 1999 1500 400 500 Bipole 2 752
5
Mundra-
Mohindergarh
ER-NR 2012 2500 400 500 Bipole 2 986
6
Bishwanath-
Agra
NER-ER 2015 6000 400 800
Multi-
Terminal
2 1728
7 Vidyanchal WR-NR April-1989 2×250 400 70
Back-to-
Back
2
8 Chandrapur WR-SR
December-
1997
2×500 400 205
Back-to-
Back
2
9 Sasaram ER-SR
September-
2002
1×500 400 205
Back-to-
Back
2
10 Gazuwaka ER-SR March-2005 2×500 400
Block1:
205kV
Block2:
177kV
Back-to-
Back
2

References
"Special features & indigenization efforts NHVDC -II" by M I Khan, A K
Tripathy
"Total Indigenous efforts NHVDC-II" by M I Khan, A K Tripathy
HVDC – High Voltage Direct Current Transmission Unrivaled practical
experience by Siemens AG
High Voltage Direct Current Transmission – Proven Technology for Power
Exchange by Siemens AG
An Overview to HVDC links in India by Prof. Kusum Tharani, Aahuti Gupta
and Apoorva Gupta at International Journal of Electrical, Electronics
"Commissioning Experiences of NHVDC -II" by M I Khan, A K Tripathy

BHEL HVDC Training Report Summary

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