Download 11.(a)(i) State the advantages and

IAE-1 Answer Key
1. What are the types of DC link?
1.
2.
3.
4.
Monopolar link
Bipolar link
Homopolar link
Multiterminal link
2. What are the factors to be considered for planning HVDC transmission?
The system planner must consider the factors are,
1. Cost
2. Technical performance
3. Reliability
3. Distinguish between AC & DC transmission.
S.NO
AC Transmission
1.
It requires three conductors
for transmission
There skin effect is present
in AC Transmission
More corona loss
Stability problem occurs.
2.
3.
4.
. 4. Define
DC Transmission
It requires only
2conductors
There is no skin effect in
DC Transmission
Less corona loss
No stability problem.
break even distance- give its range of value for overhead line
The variation of costs of transmission with distance for AC and DC
Transmission. For distances less them break even distance, AC tends to Be
economical them DC. And costlier for longer distances. The break even
Distances can vary from 500 to 800 km in overhead lines
.
5. State at least four HVDC projects in India.
S.NO
System/Project
National HVDC
1.
2.
3.
project-stage-I
NHVDC-stage-II
Rihand-Delhi
Chandrapur-
4.
Year of
Commissioned
Supplier
Power
Rating(mw)
Voltage
(kv)
1989
BHEL
100
100
2000
BHEL
100
200
1991-92
ABB
750
±500
1998
ABB
1500
padghe
6. List the types of power devices for HVDC transmission
1.
2.
3.
4.
5.
7. Mention
Thyristor
Insulated fiats bipolar transistor
GTO-gate turn-off thyristor
LTT- Light triggered thyristor
Mos-controlled thyristor(MCT)
the various modes of operation of rectifier characteristics.
Mode I : 2 and 3 valve conduction (u<60)
Mode II : 3 valve conduction only α < 30 deg, u = 60deg
Mode III : 3 and 4 valve conduction mode α > 30 deg,(60<u<120deg)
8. Draw the circuit of Graetz’s CIRCUIT.
±500
9.
Define overlap and overlap angle.
Overlap is the phenomenon due to the effect of source inductance on the a.c. side. The
current commutation is delayed due to the source inductance which is normally the
leakage reactance of a transformer. The waveforms with commutation period, denoted by
µ during which both the outgoing diode and incoming diode
are conducting. This period is also known as “overlap” period.
Or
The commutation period, when outgoing and incoming thyristors are conducting, is also
known as the overlap period. The angular period both devices share conduction is known
as the commutation angle / overlap angle.
10.(a) Compare the AC and DC transmission system based on economic,
technical performance aspects and reliability
10.(b) Draw the schematic diagram of typical HVDC converter station and
explain the function of various components available.
HVDC converter station (or simply converter station) is a specialized type of substation which
forms the terminal equipment for a high-voltage direct current (HVDC) transmission line. It
converts direct current to alternating current or the reverse. In addition to the converter, the
station usually contains:
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three-phase alternating current switch gear
transformers
capacitors or synchronous condensers for reactive power
filters for harmonic suppression, and
direct current switch gear.
Converter

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Valve hall at Henday converter station, part of the Nelson River DC Transmission
System in Canada.
The converter is usually installed in a building called the valve hall. Early HVDC systems
used mercury-arc valves, but since the mid-1970s, solid state devices such as thyristors are
used. Converters using thyristors or mercury-arc valves are known as line commutated
converters. In thyristor-based converters, many thyristors are connected in series to form a

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thyristor valve and each converter normally consists of six or twelve thyristor valves. The
thyristor valves are usually grouped in pairs or groups of four and can stand on insulators on
the floor or hang from insulators from the ceiling.
Line commutated converters require voltage from the AC network for commutation, but
since the late 1990s, voltage sourced converters have started to be used for HVDC. Voltage
sourced converters use insulated-gate bipolar transistors instead of thyristors and can provide
power to a deenergized AC system.
Almost all converters used for HVDC are intrinsically able to operate with power conversion
in either direction. Power conversion from AC to DC is known as rectification and
conversion from DC to AC is known as inversion.
DC equipment



HVDC cable termination and DC smoothing reactor on the Baltic Cable HVDC link.
The direct current equipment often includes a coil (called a reactor) that adds inductance in
series with the DC line to help smooth the direct current.
Special direct current filters are used to eliminate high frequency interference. Such filters
are required if the transmission line will use power-line communication techniques for
communication and control, or if the overhead line will run through populated areas.
Converter transformer

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A single-phase, three-winding converter transformer.
The converter transformers step up the voltage of the AC supply network. Using a star-todelta or "wye-delta" connection of the transformer windings, the converter can operate with
12 pulses for each cycle in the AC supply, which eliminates numerous harmonic current
components.
Converter transformers operate with high flux Power Steps in the Four Steps of the Converter
per cycle, and so produce more acoustic noise than normal three-phase power transformers.
This effect should be considered in the sitting of an HVDC converter station. Noise-reducing
enclosures may be applied.
Reactive Power


When line commutated converters are used, the converter station will require between 40%
and 60% of its power rating as reactive power.
Voltage sourced converters can generate or absorb reactive as well as real power, and
additional reactive power equipment is generally not needed.
Harmonic filters


Harmonic filters are necessary for the elimination of the harmonic waves and for the
production of the reactive power at line commutated converter stations. At plants with six
pulse line commutated converters.
Voltage sourced converters generally produce lower intensity harmonics than line
commutated converters. As a result, harmonic filters are generally smaller or may be omitted
altogether.


Beside the harmonic filters, equipment is also provided to eliminate spurious signals in the
frequency range of power-line carrier equipment in the range of 30 kHz to 500 kHz. These
filters are usually near the alternating current terminal of the static inverter transformer. They
consist of a coil which passes the load current, with a parallel capacitor to form a resonant
circuit.
In special cases, it may be possible to use exclusively machines for generating the reactive
power. This is realized at the terminal of HVDC Volgograd-Donbass situated on Volga
Hydroelectric Station.
AC switchgear

The three-phase alternating current switch gear of a converter station is similar to that of an
AC substation. It will contain circuit breakers for over current protection of the converter
transformers, isolating switches, grounding switches, and instrument transformers for
control, measurement and protection. The station will also have lightning arresters for
protection of the AC equipment from lightning surges on the AC system.
Location factors

Converter stations produce acoustic noise and radio-frequency interference. Walls may
provide noise protection. As with all AC substations, oil from equipment must be prevented
from contaminating ground water in case of a spill. Substantial area may be required for
overhead transmission lines, but can be reduced if underground cable is used.
11.(a)(i) State the advantages and disadvantages DC transmission.
DC Transmission:
Sometimes ago. The Electric power transmission was in DC due to the following
advantages.
Advantages of DC Transmission
 There are two conductors used in DC transmission while three conductors required in AC
transmission.
 There are no Inductance and Surges (High Voltage waves for very short time) in DC
transmission.
 Due to absence of inductance, there are very low voltage drop in DC transmission lines
comparing with AC (if both Load and sending end voltage is same)
 There is no concept of Skin effect in DC transmission. Therefore, small cross sectional
area conductor required.
 A DC System has a less potential stress over AC system for same Voltage level.
Therefore, a DC line requires less insulation.
 In DC System, There is no interference with communication system.
 In DC Line, Corona losses are very low.
 In High Voltage DC Transmission lines, there are no Dielectric losses.
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In DC Transmission system, there are no difficulties in synchronizing and stability
problems.
DC system is more efficient than AC, therefore, the rate of price of Towers, Poles,
Insulators, and conductor are low so the system is economical.
In DC System, the speed control range is greater than AC System.
There is low insulation required in DC system (about 70%).
The price of DC cables is low (Due to Low insulation)
In DC Supply System, the Sheath losses in underground cables are low.
DC system is suitable for High Power Transmission based on High Current transmission.
In DC System, The Value of charging current is quite low, there fore, the length DC
Transmission lines is greater than AC lines.
Disadvantages of DC Transmission:
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
Due to commutation problem, Electric power can’t be produce at High (DC) Voltage.
For High Voltage transmission, we cannot step the level of DC Voltage (As Transformer
cannot work on DC)
There is a limit of DC Switches and Circuit breakers (and costly too)
Motor generator set is used for step down the level of DC voltage and the efficiency of
Motor-generator set is low than transformer.
so the system makes complex and costly.
The level of DC Voltage cannot be change easily. So we cannot get desire voltage for
Electrical and electronics appliances (such as 5 Volts, 9 Volts 15 Volts, 20 and 22 Volts
etc) directly from Transmission system.
11.(a)(ii) List out the Modern trends in DC transmission.
11.(b). With neat sketches explain the different types of DC links available.
Monopolar Link
Bipolar Link
Homopolar Link
12.(a).Draw the schematic diagram of a 6 pulse Graetz’s circuits and explain
the principle of operation.
12.(b)(i).Elucidate in detail about the choice of converter configuration with
valve rating and transformer rating .
• The peak inverse voltage (PIV) across a valve can be obtained as follows :
• If ‘q’ is even. then the maximum inverse voltage occurs when the valve with a
phase displacement of  radian (l80 ) is conducting and this is given by
PIV =2 ----------------(3)
• If ‘q’ is odd, maximum inverse voltage occurs when the valve with a phase shift
of ∓ is
conducting.
• In this case, PIV =2 cos 2 −−−−−−−−−−−−−−−− − 4
• The value utilization factor is given by
=
=
2
sin
sin
for q even −−−−−−−−− −(5)
2
for q odd −−−−−−−−−− −(6)
2
 TRANSFORMER RATING
• The current rating of a valve is given by
----------- (7)
=
Where , is the DC current which is assumed to be constant.
• The transformer rating on the valve side is given by ,
------------ (8)
• The transformer utilization factor
is only a function of q. The optimum
value do of q which results in maximum utilization is equal to 3. It is a fortunate
coincidence that the AC power supply is 3 phase and the commutation group of 3
valves is easily arranged.
For q = 3,
= 1.481 --------- (9)
• The transformer utilization can be improved further if two valve groups can share
a single transformer winding. In this case, the current rating of the winding can be
increased by a factor of 2 while decreasing the number of windings by a factor of
2.
For this case,
= 1.047 ---------- (10)
12.(b)(ii).Write a short notes on valves.
VALUE AND SWITCHES
A valve can be treated as a (controllable) switch which can be turned on at any
instant, provided the voltage across it is positive.
A diode is an uncontrolled switch which will turn on immediately after the voltage
becomes positive whereas the thyristor switching can be delayed by an angle or
(alpha).
The opening of the switch(both for diode and thyristor) occurs at the current zero
(neglecting the turn-off time).