hvdc, facts and sub-stations

2. Unit - II
HVDC, FACTS and SUBSTATIONS
CO-02
Explain HVDC transmission system and its components,
understand the objectives of FACTS and distribution
automation.

3. • What is HVDC ?
• High voltage direct current (HVDC) power
systems use D.C. for transmission of bulk power
over long distances.
• For long-distance power transmission, HVDC
lines are less expensive, and losses are less as
compared to AC transmission. It interconnects
the networks that have different frequencies
and characteristics.

4. HVDC (High Voltage Direct Current)

5. Block diagram of HVDC (High Voltage Direct Current)
Transmission lines

6. • When using direct current to provide an
asynchronous link between two ac systems, it is
necessary to have two convertor stations one at
each end, connected by a dc transmission line. The
main equipment in a convertor station is
transformers and thyristor valves.
• Chokes and filters are provided at each end to
ensure smooth direct current and suppress
harmonics. At the sending end the thyristor valves
act as rectifiers to convert ac into dc which is
transmitted over the line. At the receiving end the
thyristor valves act as inverters to convert dc into
ac which is utilized at the receiving end.

7. • Single line diagram of a HVDC transmission system is
shown in Fig., where A and B are the two converter
stations. Converter station A is supplied from the
generating station G. In converter station at the
sending end the voltage is stepped up to appropriate
value by step-up transformer and then converted
into direct current by the thyristor valves.
• Thus at the start of transmission line, we have high
voltage direct current. This rectified current flows
along the transmission line to the receiving-end
converting station B, where it is converted into 3-
phase ac current by the thyristor valves and then
stepped down by the step- down transformer to low
voltage for further distribution.

10. Main components of HVDC transmission system.
• Converter Unit
• Converter Transformers
• Filters
– AC filter
– DC filter
– High-frequency filter
• Shunt capacitors or Reactive Compensation
• Smoothing Reactor
• Transmission medium or lines or cables
• DC and AC switchgear

11. • Converter Unit
• HVDC transmission requires a converter at each end of the
line. The sending end converter acts as a rectifier which
converts AC power to DC power and the receiving end
converter acts as an inverter which converts DC power to
AC power.
• This unit usually consists of two three phase converter
which are connected in series to form a 12 pulse converter.
The converter consists of 12 thyristor valves and these
valves can be packaged as single valve or double valve or
quadrivalve arrangements.
• Due to the evaluation of power electronic devices, the
thyristor valves have been replaced by high power handling
devices such as gate turn-off thyristors (GTOs), IGBTs and
light triggered thyristors.

12. • The valves are cooled by air, water or oil and
these are designed based on modular concept
where each module consists of a series
connected thyristor levels.
• Firing signals for the valves are generated in the
converter controller and are transmitted to each
thyristor in the valve through a fiber optic light
guide system. The light signals further converted
into electrical signals using gate drive amplifiers
with pulse transformers.
• The valves are protected using snubber circuits,
gapless surge arrestors, and protective firing
circuits.

13. • Converter Transformers
• The transformers used before the rectification of AC in HVDC
system are called as converter transformers. The different
configurations of the converter transformer include three
phase- two winding, single phase- three winding and single
phase- two winding transformers.
• The valve side windings of transformers are connected in star
and delta with ungrounded neutral and the AC supply side
windings are connected in parallel with grounded neutral.
• The design of the control transformer is somewhat different
from the one used in AC systems . These are designed to
withstand DC voltage stresses and increased eddy current
losses due to harmonic currents.
• The content of harmonics in a converter transformer is much
higher compared to conventional transformer which causes
additional leakage flux and it results to the formation of local
hotspots in windings. To avoid these hotspots, suitable
magnetic shunts and effective cooling arrangements are
required.

14. • Filters
• Due to the repetitive firing of thyristors,
harmonics are generated in the HVDC system.
These harmonics are transmitted to the AC
network and led to the overheating of the
equipment and also interference with the
communication system.
• In order to reduce the harmonics, filters and
filtering techniques are used. Types of filters
include

15. • AC filters
• These are made with passive components and they
provide low impedance and shunt paths for AC
harmonic currents. Tuned as well as damped filter
arrangements are generally used in HVDC system.
• DC filters
• Similar to AC filters, these are also used for filtering
the harmonics. Filters used at DC end, usually smaller
and less expensive than filters used in AC side. The
modern DC filters are of active type in which passive
part is reduced to a minimum.
• Specially designed DC filters are used in HVDC
transmission lines in order to reduce the disturbances
caused in telecommunication systems due to
harmonics.

16. • High frequency filters
• These are provided to suppress the high
frequency currents and are connected
between converter transformer and the
station AC bus. Sometimes these are
connected between DC filter and DC line and
also on the neutral side.

17. • Shunt capacitors or Reactive Compensation
• Due to the delay in the firing angle of the converter
station, reactive volt-amperes are generated in the
process of conversion. Since the DC system does not
require or generate any reactive power, this must be
suitably compensated by using shunt capacitors
connecting at both ends of the system.
• Smoothening reactor
• It is a large series reactor, which is used on DC side to
smooth the DC current as well as for protection
purpose. It regulates the DC current to a fixed value by
opposing sudden change of the input current from the
converter. It can be connected on the line side, neutral
side or at an intermediate location.

18. • Transmission medium or lines or cables
• Overhead lines act as a most frequent transmission
medium for bulk power transmission over land. Two
conductors with different polarity are used in HVDC
systems to transfer the power from sending end to
receiving end.
• The size of the conductors required in DC
transmission is small for the same power handling
capacity to that of AC transmission. Due to the
absence of frequency, there is no skin effect in the
conductors.
• High voltage DC cables are used in case of
submarine transmission. Most of such cables are of
an oil filled type. Its insulation consists of paper
tapes impregnated with high viscosity oil.

19. • DC and AC switchgear
• The switchgear equipment provides the
protection to the entire HVDC system from
various electrical faults and also gives the
metering indication. The switchgear
equipments include isolator switches,
lightening arrestors, DC breakers, AC breakers,
etc.

20. • The power dispatched from the generating station PS less the power received at
the receiving end PR i.e., (PS – PR) represents the power losses due to conversion
and transmission. The convertor at the sending end acts as a rectifier and is
suitable for power frequency (i.e. frequency of generator) on its ac side while
the converter at the receiving end acts as an inverter and its frequency is
determined by the frequency of the load system. This frequency is independent
of the sending-end frequency provided the two ends A and B are not
additionally connected by the 3-phase lines.
• The dc output voltage magnitude is controlled by varying the firing angle of the
thyristor valves in the converter. In rectifier the firing angle is between 0° and
90° while in inverter it is between 90° and 180°. As the dc output voltage is a
function of cosine of the firing angle hence the converter voltage becomes
negative when firing angle α exceeds 90°. This makes the converter to operate
as an inverter. The two converters at sending end and receiving end are identical
and whether they have to work as rectifier or inverter is determined by the
direction of power flow.
• In practical HVDC converter stations three-phase bridge converters are
employed at both ends (sending as well as receiving ends). Reversible operation
of converters as well as bidirectional power flow in HVDC link is possible simply
by the control of firing angle.
• https://circuitglobe.com/hvdc-transmission-system.html
• https://www.electronicshub.org/high-voltage-dc-transmission-
system/#What_is_HVDC

21. Advantages of HVDC Transmission System
1. The basic D.C transmission line requires only 2
Conductor. (+ ve & - Ve) and if ground is used as a
return path, then only one conductor is sufficient.
2. If ground is used as return path, then only 2
conductors are sufficient for double circuit.
3. As number of conductor required are less, so load
on tower is less. This make Tower design simple
and lighter.
4. Tower required less ground area as its base is less
than AC tower. ( Right Of Way ) So land use
benefits are more.
5. No intermediate substation is required like HVAC
transmission line.
6. Due to above advantages, Cost of transmission
line per KM is less.

22. 7. Skin effect is absent.
8. No proximity effect.
9. Less radio interference.
10. No Ferranti effect.
11. String efficiency 100%
12. Low corona loss.
13. Copper losses are less, transmission efficiency is more.(As dc
resistance is less than AC resistance by 1.6 times)
14. As Copper loss are less So transmission efficiency is more
15. As effect of L & C is absent and value of DC resistance of
conductor is less, so voltage drop in transmission line is less.
16. Voltage regulation is better than HVAC transmission line.
17. Voltage control easy for long distance HVDC transmission line.
18. Power flow control is easy for long distance transmission.
19. There is no limit for transmission of power.
20. Asynchronous tie possible.
21. HVDC line has more stability than HVAC.

23. 22.If power is to be transmitted through cable than
there is no limit on the length of cable as
charging current is absent.
23.There is no need of reactive power
compensation.
24.Two transmission lines of different frequencies
can be inter connected to grid system through
HVDC link OR Asynchronous tie is possible
through HVDC link

24. Disadvantages HVDC Transmission System
1. It is difficult to step up and step down DC voltage
like AC voltage.
2. Special cooling arrangements are necessary for
converter, so it increases cost of substation.
3. Cost of DC substation is more than AC substation,
due to additional equipment required like rectifier,
inverter etc.
4. Maintenance cost of DC substation is more due to
additional equipment.
5. Space required for DC substation is more due to
additional equipment.

25. 6) Losses in DC substation are more due to additional
equipment.
7) Over load capacity Converter is very less.
8) Reliable DC circuit breakers are not available like AC
circuit breakers.
9) Cost of DC circuit breaker is more than AC circuit
breaker.
10) If ground is used as the return path, then it leads
• Corrosion of underground metallic structure of
buildings, pipes, etc.
• Causes disturbance in underground communication
cable.
11) HVDC is economical only for bulk amount of power is
to be transmitted (1000MW and above) and for long
distances (800KM and above) Transmission line.

26. Applications of HVDC transmission system
1) HVDC is economical to transmit bulk amount of power
1000 MW & above. Over a long distance 800 Km &
above.
2) Interconnection of two transmission lines having
different frequencies is possible through HVDC link.
3) HVDC is preferred for underground cable when power
transmission through underground cable is greater
than 40-50 KM than only HVDC uniquely suited.
4) HVDC is preferred for underground cable transmission
as incoming line in Megacities/City centre in- feed.
5) HVDC is preferred for underground cable transmission
for crossing long lake, ocean etc.

27. 6) HVDC is preferred for underground cable transmission
where atmospheric conditions are too bad for overhead
transmission line, e.g. High wind pressure, rainfall, icefall
etc.
7) HVDC is preferred for underground cable for long
distance underwater power links.
8) HVDC is preferred for underground cable for powering
island from onshore.
9) HVDC is preferred for underground cable for taking
power from offshore wind farm.
10) HVDC is preferred for underground cable for powering
oil and gas offshore floating platform.
11) Integration of generation( conventional/non-
conventional)
12) Increasing existing grid utilization.
13) Interconnection of different grids or networks

28. TypesofHVDCD.Clinks or Transmissionsystem
1. Monopolar HVDC transmission line (System).
2. Bipolar HVDC transmission line (System).
3. Homopolar HVDC transmission line (System).
4. Back to Back HVDC coupling System.
5. Multi-terminal HVDC System.

29. 1. Monopolar DC link.

30. • In this DC system, sending end and receiving
end converters are connected by a single
conductor (or line) with positive or negative
polarity. Mostly negative polarity is preferred
on overhead lines due to lesser radio
interference.
• It uses ground or sea water as a return path.
Sometimes a metallic return is also used. It is
to be noted that earth offers less resistance to
DC as compared with AC.

31. 2. Bipolar DC link.

32. • This is the most commonly used configuration of HVDC
system. It uses two conductors; one is a positive
conductor or pole and the other negative conductor of
the same magnitude (typically of ± 650V).
• Each terminal has two sets of converters of identical
ratings connected in series on DC side. The neutral
points (junction between the converters) are grounded
at one or both ends and hence the poles operate
independently.
• Normally, both poles are operated at same current and
hence there is no ground current flowing under these
conditions.
• In the event of a fault in one conductor, the other
conductor with ground return can supply half the rated
load and thus increase the reliability of the system. The
bipolar link has two independent circuits and it can be
operated as a monopolar link in an emergency situation.

33. 3. Homo-polar DC link

34. • This link has two or more conductors with the same
polarity, usually of negative and they are operating with
ground return. If fault takes place in one conductor, the
converter equipment can be connected to healthy pole and
it can supply more than 50% of the rated power by
overloading at the expense of increased line loss.
• This is not possible in case of bipolar link where graded
insulation is used for negative and positive poles. This
system is preferred when continuous ground currents are
inevitable.
• The advantage of the system is that less corona loss and
radio interference due to the negative polarity on the lines.
However the large earth return current is the major
disadvantage.

35. Comparison of HVDC and HVAC Transmission System
HVDC Transmission System HVAC Transmission System
Low losses.
Losses are high due to skin effect and
corona.
Better Voltage regulation and
Control ability.
Voltage regulation and Control ability
is low.
Transmit more power over longer
distance.
Transmit less power compared to
HVDC system.
Less insulation is needed. More insulation is required.
Reliability is high. Low Reliability.
Asynchronous interconnection is
possible.
Asynchronous interconnection is not
possible.
Reduced line cost due to fewer
conductors.
Line cost is high.
Towers are cheaper, simple and
narrow.
Towers are bigger compared to HVDC.

36. What is FACTS- (Flexible AC Transmission systems) ?
A Flexible AC transmission System
refers to the system consisting of power
electronic devices along with power system
devices to enhance the controllability and
stability of the transmission system and
increase the power transfer capabilities.

37. • Objectives of FACTS
1. To increase the power transfer capability of
transmission systems.
Power flow in a given line should be able to be
increased up to the thermal limit by forcing
the necessary current through the series line
impedance & same time stability of the
system is maintained with real time control of
power flow.

38. 2. To keep power flow over designated routes.
This objective implies that by being able to
control the current in a line (Example : Changing
the effective line impedance), the power flow
can be restricted to selected transmission
corridors.

39. • Benefits of FACTS
1. Improved power transmission capability
2. Improved system stability and availability
3. Improved power quality
4. Minimized environmental impact
5. Minimized transmission losses

40. • Basic types of FACTS controllers
1. Series connected controllers
2. Shunt connected controllers
3. Combined series-series controllers
4. Combined shunt-series controllers

41. 1.Series connected controllers
The series controller could be a variable
impedance or a variable source both are power
electronics based and all series controllers injects
voltage in series with the line.

42. 2. Shunt connected controllers
The shunt controllers may be variable
impedance connected to the line voltage causes
a variable current flow hence represents
injection of current into the line.

43. 3. Combined series-series controllers
The combination could be separate series
controllers or unified series-series controller-
Interline Power Flow Controller

44. 4. Combined shunt-series controllers
The combination could be separated series and
shunt controllers or a unified power flow
controller.

45. • Meaning of Sub-station.
The assembly of apparatus used to change
some characteristic (e.g. voltage, a.c. to d.c.,
frequency, p.f. etc.) of electric supply is called a
Sub-station.
• Meaning of Receiving Station.
At Receiving Station, the level of voltage
reduced by step-down transformers up to
132kV, 66 or 33 kV, and Electric power is
transmit by three phase three wire overhead
system to different sub stations.

46. • Functions Sub-station & Receiving Station.
• Maintain adequate line capacity to secure
power supply
• Transmit data for control, protection and
network monitoring
• Control voltage
• Limit power surges
• Determine energy transfer

47. Classification of substations
• According to service requirement.
1. Transformer sub-stations.
2. Switching sub-stations.
3. Power factor correction sub-stations.
4. Frequency changer sub-stations.
5. Converting sub-stations.
6. Industrial sub-stations.
• According to constructional features.
1. Indoor sub-station
2. Outdoor sub-station
3. Underground sub-station
4. Pole-mounted sub-station

48. • Comparison between outdoor and indoor sub-station
Outdoor Sub-station Indoor Sub-station
Voltages level beyond 66 kV, Voltages level up to 66 kV.
More space required Less space required
Less time required for
erection
More time required for
erection
Easy future extension Difficult future extension
Easier fault location because
of equipment being in full
view.
Difficult fault location
because of equipment not
being in full view.
Low capital cost High capital cost
Difficult operation Easier operation

49. Symbols for Equipment in Sub-Stations

52. Draw single line diagram of 220KV/66 KV MUSS-(Master Unit Sub-station)

56. Main components of substation
1. Bus-bars.
2. Insulators.
3. Isolating switches.
4. Circuit breaker.
5. Power Transformers.
6. Instrument transformers.
(i) Current transformer (C.T.)
(ii) Potential transformer (P.T.)
7. Metering and Indicating Instruments.
8. Miscellaneous equipment.
(i) Fuses
(ii) carrier-current equipment
(iii) sub-station auxiliary supplies

57. • Functions of Main components of substation
• Bus-bars : When a number of lines operating at the same
voltage have to be directly connected electrically, bus-bars are
used as the common electrical component. Bus-bars are copper
or aluminium bars and operate at constant voltage.
• Insulators : The insulators serve two purposes. They support the
conductors (or bus-bars) and confine the current to the
conductors. The most commonly used material for the
manufacture of insulators is porcelain.
• Isolating switches : In sub-stations, it is often desired to
disconnect a part of the system for general maintenance and
repairs. This is accomplished by an isolating switch or isolator. An
isolator is essentially a knife switch and is designed to open a
circuit under no load.

58. • Circuit breaker : A circuit breaker is an equipment
which can open or close a circuit under normal as
well as fault conditions.
• Power Transformers : A power transformer is used
in a sub-station to step-up or step-down the
voltage. Except at the power station, all the
subsequent sub-stations use step-down
transformers to gradually reduce the voltage of
electric supply and finally deliver it at utilization
voltage.
• Instrument transformers : The function of these
instrument transformers is to transfer voltages or
currents in the power lines to values which are
convenient for the operation of measuring
instruments and relays. There are two types of
instrument transformers viz.,(i) Current transformer
(C.T.) (ii) Potential transformer (P.T.)

59. • Metering and Indicating Instruments : There
are several metering and indicating
instruments (e.g. ammeters, voltmeters,
energy meters etc.) installed in a sub-station to
maintain watch over the circuit quantities. The
instrument transformers are invariably used
with them for satisfactory operation.

60. • Types of Bus bar arrangement
1. Single bus-bar system.
2. Single bus-bar system with sectionalisation.
3. Double bus double breaker.
4. Ring or Mesh main bus bar
5. Duplicate bus-bar system or Main and transfer bus system.
6. One and half circuit breaker system.
7. Breaker and half system .

61. • Single bus-bar system.

62. • it consists of a single bus-bar and all the incoming and outgoing
lines are connected to it. The chief advantages of this type of
arrangement are low initial cost, less maintenance and simple
operation. However, the principal disadvantage of single bus-bar
system is that if repair is to be done on the bus-bar or a fault
occurs on the bus, there is a complete interruption of the supply.
This arrangement is not used for voltages exceeding 33kV. The
indoor 11kV sub-stations often use single bus-bar arrangement.
• Fig. shows single bus-bar arrangement in a sub-station. There are
two 11 kV incoming lines connected to the bus-bar through circuit
breakers and isolators. The two 400V outgoing lines are
connected to the bus bars through transformers (11kV/400 V) and
circuit breakers

63. • Single bus arrangement with sectionalisation

64. • In this arrangement, the single bus-bar is divided
into sections and load is equally distributed on
all the sections. Any two sections of the bus-bar
are connected by a circuit breaker and isolators.
• Two principal advantages are claimed for this
arrangement. Firstly, if a fault occurs on any
section of the bus, that section can be isolated
without affecting the supply from other sections.
Secondly, repairs and maintenance of any
section of the bus-bar can be carried out by de-
energising that section only, eliminating the
possibility of complete shut down. This
arrangement is used for voltages up to 33 kV.

65. • Fig. shows bus-bar with sectionalisation where
the bus has been divided into two sections.
There are two 33 kV incoming lines connected
to sections I and II as shown through circuit
breaker and isolators. Each 11 kV outgoing line
is connected to one section through
transformer (33/11 kV) and circuit breaker. It is
easy to see that each bus-section behaves as a
separate bus-bar.

66. • Double bus double breaker bus bar arrangements.

67. • The double bus–double breaker arrangement
involves two breakers and two buses for each
circuit (With two breakers and two buses per
circuit, a single bus failure can be isolated without
interrupting any circuits or loads. Furthermore, a
circuit failure of one circuit will not interrupt other
circuits or buses.
• Therefore, reliability of this arrangement is
extremely high. Maintenance of switching devices
in this arrangement is very easy, since switching
devices can be taken out-of-service as needed and
circuits can continue to operate with partial line
relay protection and some line switching devices in-
service, i.e., one of the two circuit breaker.

68. Advantages of double bus double breaker system:
1. During fault conditions, the load can be transferred to
one bus so there will not be an interruption in power
supply.
2. Here we are not using a bus coupler so there will not
be much delay in power supply while closing circuit
breaker to transfer load from one bus to another bus.
3. High flexibility.
Disadvantages of double bus double breaker system:
1. The number of circuit breakers used is high so cost is
very high.
2. Maintenance cost will also be high. So this type of
arrangement is used very rarely.

69. Ring Main Bus Bar Arrangements.

70. • In ring main bus bar arrangement , all breakers are arranged
in a ring with circuits connected between two breakers. From
a reliability standpoint, this arrangement affords increased
reliability to the circuits, since with properly operating relay
protection, a fault on one bus section will only interrupt the
circuit on that bus section and a fault on a circuit will not
affect any other device .
• Protective relaying for a ring bus will involve more
complicated design and, potentially, more relays to protect a
single circuit.
• From a maintenance point of view, the ring bus provides good
flexibility. A breaker can be maintained without transferring
or dropping load, since one of the two breakers can remain
in-service and provide line protection while the other is being
maintained.
• Cost of the ring bus arrangement can be more expensive than
a single bus, main bus and transfer, and the double bus–single
breaker schemes since two breakers are required for each
circuit, even though one is shared.

71. • Advantages of mesh system.
1. Provides protection against the fault.
2. For substations having a large number of
circuits, this arrangement is suitable.
• Disadvantages of mesh system.
1. It doesn’t provide switching facility.
2. Not suitable for all type of substations.

72. • Importance of interconnecting substations in
large power systems.
1. Enhanced reliability is assured for the most critical
loads in the system
2. Backup power remains available in the event of
incoming line cut off.
3. Greater power availability, which also provides
coverage during maintenance.
4. Costs are lower for the breakers and other system
components.

73. • Functions of Load Dispatch Stations.
1. Data processing and system studies.
2. Load forecasting for a working day & holiday and analysis of
system behaviour.
3. Checking station wise outage position every morning at 6 a.m.
4. To assess the generation availability.
5. quality maintenance of supply to the consumer by way of
improved voltage regulation.
6. Load Scheduling And Economic Operation.
7. Integration With Regional Load Dispatch Centers.
8. Co-ordination with neighbor grids.
9. Maintenance Of Grid Discipline.
10. System security, SCADA management and communication.
11. Consumer Interaction And Public Relation.

74. MODEL QUESTIONS BANK
• Cognitive Level: UNDERSTAND, APPLICATION
1. Briefly explain the principle of HVDC system operation with sketch.
2. List the advantages and limitations of HVDC transmission.
3. Compare HVAC and HVDC transmission.
4. Briefly explain types of DC links with diagrams.
5. Explain Monopolar DC link with diagram.
6. Explain Bipolar DC link with diagram.
7. Explain Homopolar DC link with diagram.
8. Briefly explain Flexible AC Transmission systems (FACTS).
9. State objectives of FACTS.
10. Name the different types of FACTS controllers with functions.
11. Explain the functions of Substation.
12. Classify the substations.
13. Compare outdoor and indoor substations.