The document provides information about HVDC transmission, including:
1. It lists two merits of AC transmission (power can be generated at high voltages, maintenance of AC substations is easy and cheaper) and two merits of DC transmission (it requires only two conductors, there is no skin effect).
2. It discusses types of DC link including monopolar, bipolar, and homopolar links.
3. It lists types of power devices used for HVDC transmission including thyristor, IGBT, GTO, LTT, and MCT.
4. It provides advantages and disadvantages of HVDC transmission such as full control over power, reduced transmission lines, and inability to change voltage
The concept of FACTS (Flexible Alternating Current Transmission System) refers to a family of power electronics-based devices able to enhance AC system controllability and stability and to increase power transfer capability.
Nowadays, it is very important to maintain voltage level. Controlling of that voltage is also important. This Presentation contains methods of voltage control.
The concept of FACTS (Flexible Alternating Current Transmission System) refers to a family of power electronics-based devices able to enhance AC system controllability and stability and to increase power transfer capability.
Nowadays, it is very important to maintain voltage level. Controlling of that voltage is also important. This Presentation contains methods of voltage control.
This directional over current relay employs the principle of actuation of the relay....It has a metallic disc free to rotate between the poles of two...
Introduction
Definition of FACTS system
Necessity of facts devices
Shunt connected controllers
Types of facts controllers
Shunt connected controllers
Benefits of FACTS
The concept of FACTS (Flexible AC Transmission System) refers to a family of power electronics based devices able to enhance AC system controllability and stability and to increase power transfer capability.
The design of the different schemes and configurations of FACTS devices is based on the combination of traditional power system components (such as transformers, reactors, switches, and capacitors) with power electronics elements (such as various types of transistors and thyristors).
The significance of power factor correction (PFC) has long been visualized as a technology requirement for improving the efficiency of a power system network by compensating for the fundamental reactive power generated or consumed by simple inductive or capacitive loads. With the Information Age in full swing, the growth of high reliability, low cost electronic products have led utilities to escalate their power quality concerns created by the increase of such “switching loads.” These products include: entertainment devices such as Digital TVs, DVDs, and audio equipment; information technology devices such as PCs, printers, and fax-machines; variable speed motor drives for HVAC and white goods appliances; food preparation and cooking products such as microwaves and cook tops; and lighting products, which include electronic ballasts, LED and fluorescent lamps, and other power conversion devices that operate a variety of lamps. The drivers that have resulted in this proliferation are a direct result of the availability of low-cost switch-mode devices and control circuitry in all major end-use segments: residential, commercial, and industrial.
This directional over current relay employs the principle of actuation of the relay....It has a metallic disc free to rotate between the poles of two...
Introduction
Definition of FACTS system
Necessity of facts devices
Shunt connected controllers
Types of facts controllers
Shunt connected controllers
Benefits of FACTS
The concept of FACTS (Flexible AC Transmission System) refers to a family of power electronics based devices able to enhance AC system controllability and stability and to increase power transfer capability.
The design of the different schemes and configurations of FACTS devices is based on the combination of traditional power system components (such as transformers, reactors, switches, and capacitors) with power electronics elements (such as various types of transistors and thyristors).
The significance of power factor correction (PFC) has long been visualized as a technology requirement for improving the efficiency of a power system network by compensating for the fundamental reactive power generated or consumed by simple inductive or capacitive loads. With the Information Age in full swing, the growth of high reliability, low cost electronic products have led utilities to escalate their power quality concerns created by the increase of such “switching loads.” These products include: entertainment devices such as Digital TVs, DVDs, and audio equipment; information technology devices such as PCs, printers, and fax-machines; variable speed motor drives for HVAC and white goods appliances; food preparation and cooking products such as microwaves and cook tops; and lighting products, which include electronic ballasts, LED and fluorescent lamps, and other power conversion devices that operate a variety of lamps. The drivers that have resulted in this proliferation are a direct result of the availability of low-cost switch-mode devices and control circuitry in all major end-use segments: residential, commercial, and industrial.
The power electronics device which converts DC power to AC power at required output voltage and frequency level is known as inverter. Multilevel inverter is to synthesize a near sinusoidal voltage from several levels of dc voltages. In order to maintain the different voltage levels at appropriate intervals, the conduction time intervals of MOSFETS have been maintained by controlling the pulse width of gating pulses. In this paper single phase to three phase power conversion using PWM technique. The simulation is carried out in MATLAB/Simulink environment which demonstrate the feasibility of proposed scheme.
Fuzzy Logic Controller Based on Voltage Source Converter-HVDC with MMC TopologyIJMTST Journal
This paper presents Modular Multi Level Converters (MMC) are used for high voltage high power DC to AC conversion. The MMCs with increased number of levels offer close to sine wave operation with reduced THD on the AC side. This is a new type of voltage source converter (VSC) topology. The use of this converter in a high-voltage direct current (HVDC) system is called by a MMC-HVDC system. The MMC-HVDC has the advantage in terms of scalability, performance, and efficiency over two-and three-level VSC-HVDC. The proposed HVDC system offers the operational flexibility of VSC based systems in terms of active and reactive power control, in addition to improved ac fault ride-through capability and the unique feature of current-limiting capability during dc side faults. The proposed VSC-HVDC system, in this project assesses its dynamic performance during steady-state and network alternations, including its response to AC and DC side faults. In this project using a fuzzy controller and the proposed topology is implemented in MATLAB/SIMULINK environment and the simulation results are observed.
Design of High-Speed Dynamic Double-Tail ComparatorIJERDJOURNAL
ABSTRACT:- The analog-to-digital converters which are of ultra low-power, area efficient, and high speed converters are made of dynamic regenerative comparators. These comparators can maximize speed and power efficiency. The delay and power dissipation of dynamic comparators are analyzed in this paper. The delays and tradeoff can be explored. The circuit of a conventional double tail comparator in this analysis is modified for fast operation even in different supply voltages. By using power gating technique and adding few transistors, the positive feedback during the regeneration is strengthened in the proposed comparator structure. The delay time can be reduced by providing positive feedback instead of adding few transistors. The analysis results are going to be confirmed on the basis of 0.25-µm CMOS technology. The power consumption and delay time can be significantly reduced based on this analysis. All the simulation are made using TANNER TOOLS, Generic 250nm. The schematic are drawn in the T-SPICE schematic editor.
IJERA (International journal of Engineering Research and Applications) is International online, ... peer reviewed journal. For more detail or submit your article, please visit www.ijera.com
A hybrid converter topologies which can supply simultaneously AC as well as DC from a single DC source. The new Hybrid Converter is derived from the single switch controlled Boost converter by replacing the controlled switch with voltage source inverter (VSI). This new hybrid converter has the advantages like reduced number of switches as compared with conventional design having separate converter for supplying AC and DC loads, provide DC and AC outputs with an increased reliability, resulting from the inherent shoot through protection in the inverter stage. For controlling switches PWM control, based upon unipolar Sine-PWM is described.
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2. HVDC TRANSMISSION
Unit-1 INTRODUCTION
PART-A
1. List out two merits of AC and DC transmission.
DC Transmission.
It requires only two conductors as compared to three for a.c transmission
There is no skin effect in a d.c system.
A d.c line has less corona loss and reduced interference.
AC Transmission.
The power can be generated at high voltages
The maintenance of a.c sub-station is easy and cheaper
2. What are the types of DC link?
Monopolar link
Bipolar link
Homopolar link
3. Draw the block diagram of bipolar link
4. List the types of power devices for HVDC transmission
1. Thyristor
2. Insulated fiats bipolar transistor
3. GTO-gate turn-off thyristor
4. LTT- Light hissered thyrisor
5. Mos-controlled thyristo(MCT)
5. Write the advantages and disadvantages of HVDC transmission?
Advantages
1. Full control over power transmitted
2. The ability to enhance transient and dynamic stability
in associated AC networks
W.VIDYARTHIPLUS.CO
3. Fast control to limit fault current in DC lines
4. Reduced transmission lines.
5. Interconnection of systems operating at different frequencies
3. Disadvantages
1. Inability to use transformer to change voltage levels
2. High cost of converter equipment
3. Generation of harmonics which requires AC and DC
filters, adding to the cost of converters station
4. Complexity of control
6. Mention the some of HVDC projects from abroad?
1. Gotland 1 – 98km, 200kv, 20mw, 1954
2. HVDC Gotland 2 – vastervik (Sweden) to yipne (Sweden) 92.9km,
150kv, 130mw, 1983
3. Nelson river bipole 2 – sundance(Canada) to rosser
(Canada) 937km, ±500kv, 1800Mw, 1985
4. HVDC Tjaereborg - Tjaereborg (Denmark) – Tjaereborg 4.3km,
±9kv, 7.2mw,2000(interconnection of wind power station)
5. HVDC back-to-back station – eagle pan (USA) - eagle
pan (USA)(Texas) ±15.9kv,36mw,2000
6. Caprivi link – Namibia (gerus) to Namibia
zembari 970km,500kv,300mw,2010
7. HVDC valhall- lista(Norway) to valhall (offshore) 292km,
150kv, 78mw, 2009
7. What are the types of power losses in thristor?
1. Forward conduction losses
2. Loss due to leakage current during forward & reverse blocking
3. Switching losses due to ton and toff
4. Gate triggering loss
8. Define Reliability.
The reliability of DC transmission system is quite good on exhaustive record of
existing HVDC lines in the world in available from which the reliability statistics
Can be computed the development of LTT is expected to improve reliability
Because of elimination of high voltage pulse transformers and auxiliary supplies
For turning on the devices.
9. Energy Availability.
It is defined as,
M
Energy Availability=100 (1 − Equivalent outage time)%
Where equivalent outage time is the product of the actual outage time
and The fraction of system capacity lost to outage.
4. 10. Write down any two application DC transmission?
Long distance bulk power transmission
Underground or underwater cables
Asynchronous interconnection of A.C systems operating at
different frequencies.
11. What are the factors to be considered for planning HVDC transmission?
The system planner must consider the factors are,
Cost
Technical performance
Reliability
12. What are the advantages of LTT over ETT?
Infinite gate isolation
Total noise immunity for the control circuits
Faster turn-on time
Elimination if high voltage pulse transformers and auxiliary power supplies.
13. Distinguish between AC & DC transmission.
S.NO AC Transmission DC Transmission
1. It requires three conductors It requires only
for transmission 2conductors
2. There skin effect is present There is no skin effect in
in AC DC Transmission
3. More corona loss Less corona loss
4. Stability problem occurs. No stability problem.
14. What is meant by MOS controlled thyristor?
An MCT is a new device in the field of semiconductor-controlled devices. It is
basically a thyristor with two MOSFETs built into the gate structure one MOSFET
is used for turning on the MCT and other for turning off device. An MCT is
High – frequency, high power, low – conduction drop switching
device.DYARTHIPLUS.COM
15. Define break even distance- give its range of value for overhead line
5. 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.
16. State atleast four HVDC projects in India.
S.NO System/Project
Year of
Supplier
Power Voltage
Commissioned Rating(mw) (kv)
1.
National HVDC
1989 BHEL 100 100
project-stage-I
2. NHVDC-stage-II 2000 BHEL 100 200
3. Rihand-Delhi 1991-92 ABB 750 ±500
4.
Chandrapur-
1998 ABB 1500 ±500
padghe
17. What is meant by an Asynchronous tie?
When two power system are connected through DC ties, there is no
need Of coordinated control. It is called asynchronous fie. The two systems
which Have different nominal frequencies.
18. What is LASCR? How does it differ from a conventional SCR?
Light activated thyristor, also called LASCR. It is turned on by throwing
a Pulse of light on the silicon wafer of thyristor. This is the major difference
to Others.
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19. What are the types of commutation?
20.Why circuit turn off time should be greater than the thyristor turn-off time?
Circuit turn off time should be greater than the thyristor turn-off time for re
liable turn-off, otherwise the device may turn-on at an undesired instant, a process
called commutation failure.
21. What is the turn-off time for converter grade SCRs and inverter grade SCRs?
Turn-off time for converter grade SCRs is 50 – 100 ms turn-off time for converter
grade SCRs and inverter grade SCRs and for inverter grade SCRs is 3 – 50 ms.
6. PART-B
1
Explain in detail about the various components of a HVDC transmission system.
2
Briefly explain the types of DC link in HVDC transmission.
3 Compare the AC and DC transmission and mention the applications of
DC transmission.
4 Write the HVDC projects in India and Abroad?
5 Write the advantages and applications of HVDC TRANSMISSION?
6. Explain the planning of HVDC system.
7. Explain the modern trends in HVDC transmission system.
8.. Comparison between CSC based and VSC based HVDC system.
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7. UNIT -2 ANALYSIS OF HVDC CONVERTERS
1. What is firing angle?
The angle at which thyristor is triggered it is defined as the angle between
the zero crossing of the input voltage and the instant the thyristor is fired
2. Draw the circuit of Graetz CIRCUIT.
3. What is meant by pulse number of a converter?
It is defined as the ratio of the base frequency of the DC voltage ripple
To the fundamental frequency of the AC voltage is called pulse number.
4. List some of the converters in HVDC systems.
Line commutated converter
i. Six pulse converter
ii. 12-pulse converter
Voltage source converter
i. Basic two level converter
ii. Three level voltage source converter
5. Define value rating.
The value voltage rating is specified in terms of peak inverse voltage (PIV) it has to
withstand. The ratio of PIV to the average dc voltage is an Index of the value
utilization. The average maximum dc voltage across the Converter is given by
6. What are the merits of twelve or multibridge pulse converter?
Reduced filtering requirements and Harmonics are eliminated
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7. How can the converter configuration defined?
There are several configuration for a converter of a specified pulse
Number, we have in addition to the graetz bridge, six phase diametric
Connection, cascade of three single phase fall wave converters, cascade
Of two three phase converters.
8. 8. List the assumption made to develop the equivalent circuit of a converter
Bridge used in dynamic simulation.
All the values in a bridge have identical characteristics
A value offers infinite impedance in the reverse direction
The grading and damping circuits across the values are ignored
The current id is assumed to be continuous and non zero.
L/R of each phase of the converter transformer in the same.
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.
11. Write the assumptions for analysis of 6 pulse converter?
To consider the theoretical analysis of a conventional 6-pulse bridge , the
following assumptions are made:
DC current is constant (i.e. the smoothing reactor is infinite),
Valves are ideal switches, and
AC system is infinitely strong (i.e. the 3 phase emfs are balanced
and perfectly sinusoidal).
12. Write the average direct voltage expression for graetz circuit?
Vd = Vd0[ cosα + cos(α+u) and Vd = Vd0 cosα - RcId
Where; Rc = 3/πωLc = 3/πXc = equivalent commutation resistance
13. Mention 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)
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14. Mention the various modes of operation of inverter characteristics.
Mode I : β <60 deg for values of u <60-£ , the characteristics are linear
60deg<β,90deg ; u=60deg - £ = 60deg – gamma = constant
The characteristics are elliptical
Mode II : For u>60deg corresponding to β> 90deg +gamma zero
the characteristics are linear
9. 15. Mention the various modes of characteristics of 12 pulse converter.
Mode I : 4 and 5 valve conduction 0 <u < 30deg Mode
2: 5 and 6 valve conduction; 30deg < u < 60deg
Mode 3: 6 valve conduction 0 < α < 30deg, u = 60deg
Mode 4 : 6 and 7 valve conduction 60deg< u <90deg
Mode 5: 7 and 8 valve conduction, 90deg<u<120deg
16. Draw the equivalent circuit of rectifier.
17. Draw the thyristor voltage waveform for inversion for 6 pulse converter.
18. Draw the a.c current waveform. When star to star connected transformer
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19. When star to delta star connected transformer
10. 20. Draw the circuit diagram of 12 pulse converter.
21. Why series and parallel operation of thyristor in HVDC TRANSMISSION?
For higher voltage rating use series operation of thyristor
For higher current rating use parallel operation of thyristor.
22. What is mean by snubber circuits?
A Snubber circuits consists of a series combination of resistance Rs and
Capacitance es in parallel with the thyristor. A capacitor es in parallel with the
Device is sufficient to prevent unwanted triggering of the SCR.
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PART – B
1 Explain the complete characteristics of converter as Rectifier and inverter
2 Derive the expression for output dc average voltage for converter as rectifier
3 Explain in detail about the choice of Converter configuration with valve
rating and transformer rating
4 Explain the complete characteristics of 12 pulse converter
5 Explain the simplified analysis of rectifier characteristics
6 Explain the complete characteristics of converter as Rectifier and
inverter. And also draw the equivalent circuit of HVDC link
7 Draw the 6 pulse Greatz converter and analysis the circuit with overlap
for mode-1 overlap angle <60deg
8 Explain the operation of three phase fully controlled converter with
source inductance.
9 Explain the parameter and characteristics of three phase fully
controlled converter used as inverter.
10 Explain the detailed analysis of HVDC converter
11. UNIT III- CONVERTER AND HVDC SYSTEM CONTROL
1. What is meant by firing angle control?
The current or extinction angle controller generates a control signal vc,
Which is related to the firing angle required. The firing angle controller
Generates gate pulses in response to the control signal vc.
2. Write the features control?
i) Current order setting can be quickly and reliably changed depending on
the requirement
ii) Power reversal can be done easily and quickly
iii) Fault current levels are limited to rated values.
3. Why the necessity of control in a DC link?
The expression for current through a DC link it can be observed that the
denominator has only resistances, which are small when compared with the
reactance of the AC system. Hence, current is sensitive to change in voltage
resulting in large fluctuations which can damage the thyristors,
4. What is the principal of control in DC link?
The control of power in a DC link can be achieved through the control of current of
voltage. From minimization of loss considerations, it is important to maintain
constant voltage in the link and adjust the current to meet the required power.
5. What is meant by current and extinction control?
The current controller is invariably of feedback type the controller
which Is PI type.
The extinction angle controller can be of predictive type or feedback
type With EPC control. The predictive controller is considered to be less
Prone to commutation failure.
6. State any four important reasons why the current control is desirable in the
Rectifier station under normal operating conditions?
i. The increase of power in the link is achieved by reducing αr,
which improves the power factor at the rectifier.
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ii. The inverter can now be operated at minimum ﬠ thereby
minimize the reactive power consumption.
iii. The operation at minimum extinction angle at the inverter and
current control at the rectifier results in better voltage
12. regulation them the operation with minimum delay angle at the
rectifier & current control at the inverter.
iv. The current during line fault are automatically limited with
rectifier station in current control.
7. How power is reversed in HVDC link?
The power reversal in the link can take place by the reversal of the DC
Voltage. This is done easily by increasing the delay angle at the station
initially operating as the rectifier, while reducing the delay angle at the
station initially operating as the inverter.
8. Define current margin.
The difference between the current controller settings of the two stations is called
current margin(Im). In order to avoid conflict between the two current controllers at
the both ens, the rectifier current controller is provided with a higher current order.
Idi = Idr – Im
Where Idi – current order of the inverter
Idr - current order of the rectifier
Im usually about 10% of rated value
9. Draw backs of constant current control(CCC ).
Increase in the converter valve voltage stress due to the voltages
across the series capacitors
Increase in the magnitude of AC harmonics as the overlap angle is
the reduced for a specified DC current
10.What are the parameters to change current and power transfer in DC link?
i) Control angle of rectifier α
ii) Control angle of inverter β
iii) Tap changer on rectifier side
iv) Tap changer on inverter side
11.What is meant by compounding?
The term compounding a converter implies selection of converter characteristics in
order to meet the requirements of regulation and protection
IDYARTHIPLUS.COM
12. Define required regulation.
Converter valves should be operated strictly within their current rating, since
there is substantial rise of damage if the current is increased beyond the rated value for
a even a short time. Therefore constant current regulation is thus clearly desirable.
13. Draw the characteristics of compounding of inverter.
13. 14.What is the use of transformer tap changer control at the inverter side of an
HVDC system?
The on-load tap changer control at the inverter is used mainly to maintain a constant
DC voltage. The tap changer control at the rectifier is designed to maintain delay
angle within the limits (say 10deg to 20deg) in order to maintain certain voltage
margin for the purpose of current control.
15.What is the use of tap changing transformer in HVDC systems?
The tap changing transformer used to increase the power factor obtained in the ac
side and maintain the operating point at point A or B in the Vd , id characteristics of
rectifier and inverter use tap changing transformer.
16.Draw the communication link of the HVDC transmission.
A communication link is necessary to carry information from the receiving end to the
rectifier regulator and may also be needed for protective purposes, starting purposes
and reversal of the direction of power flow
There are 3 main possibilities:
1. Short wave radio link
2. Pilot wires
3. The use of carrier frequency on the power conductors.
17.What is meant by compounding a converter?
The term compounding a converter implies selection of converter characteristics in
order to meet the requirements of regulation and protection IDYARTHIPLUS.COM
18. What is the need for transformer tap changer control of HVDC converter?
The tap changing transformer used to increase the power factor obtained in the
ac side and maintain the operating point at point A or B in the Vd , id characteristics
of rectifier and inverter use tap changing transformer.
14. PART B
1 Briefly explain the principle of DC link control and the converter control basic
Characteristics.
2 Draw and explain the basic V-I characteristics of HVDC converter control.
3 Principle and necessary of control, compounding and regulation.
4 Write short note on the following i) Tap changer control
5 Explain the operation of current control and constant extinction angle control.
6 Explain how power reversal occurs in DC link.
7 Explain the operation of inverter compounding and uncompounded inverter.
8. Describe the operation of current regulation from the inverter side
9. Explain the operation of rectifier compounding.
10. Explain the operation of transformer tap changing.
11. Explain in detail the rectifier and inverter compounding with appropriate diagram
and expressions
12. Write short notes on the following: a. Transmission characteristics in the rectifier
and inverter compounding b. Necessity of communication link in HVDC power
transmission
13. Draw and explain the combined rectifier and inverter characteristics with current
regulation from both sides
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15. UNIT IV- REACTIVE POWER AND HARMONICS CONTROL
1. Mention the performance criteria for selection of harmonic filter
Harmonic distortion
Telephone influence factor
Telephone Harmonic form factor
IT product
2. Mention the Types of filters
There are basically two types of filters
Passive filters ---- tuned filters and damped filter; single and double
tuned , high pass filters
Active filters
3. Differentiate characteristic and non-characteristic harmonics.
s.no Characteristics harmonics Non-characteristics harmonics
It’s always presents even
1.
under ideal operation, Unbalance and distortion in AC
balanced AC voltages, voltages
equidistant pulses
2.
Equal transformer leakage Unequal transformer leakage
reactance impedances
DC current is assumed to be
DC current is varied
3. constant
h=np
h=np±1
4. State the ill effects of harmonics injected into the AC line?
Telephone interference
Extra power looses & consequent heating in machines
Over voltages due to resonances
Instability of converter controls
Interference with ripple control system used in load management.
5. What is radio interference?
The radio interference is mainly due to the positive conductor. This is
Because of the fact that the corona discharges from the negative
conductor are in the form of trichel pulses which are uniformly distributed
ever the conductor surface
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16. 6.What are the sources of harmonics?
• Magnetization nonlinearities of transformer
Rotating machines and Adjustable speed drives.
• Arcing devices and Electronic and medical test equipment
• PCs and office machines , Induction Heaters
• Semiconductor based power supply system
• Inverter fed A.C. drives , Thyristor controlled reactors
• Phase controllers and A.C. regulators
7. What are the effects of trouble caused by harmonics?
1.Resonance 2. Poor Damping
3. efficiency of motor reduced
4. Overheating of cable
5. Trip of protection
6. Overheating at winding.
7.Increase magnetic losses.
8. Define THD.
The THD is a measure of the effective value of the harmonic components of a
distorted waveform. That is, it is the potential heating value of the harmonics
relative to the fundamental. This index can be calculated for either voltage or
current
9. What are the means to reduce harmonis?
Using filter (passive and active filters)circuit and increasing pulse number
10. List the causes of non-characteristics harmonis.
a) Imbalance in the operation of two bridges forming 12 pulse converter
b) Firing angle errors
c) unbalance and distortion in AC voltage and
d) unequal transformer leakage impedances
11. Write the effects of unbalanced voltages.
The presence of the negative sequence component in the AC voltages shifts the
zero crossing of the commutation voltages.
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12. Mension the criteria for selection of DC filter.
a) Maximum voltage TIF on DC high voltage bus
b) maximum induced noise voltage in mv/km in a particular test line one km away
from the HVDC line
c) maximum permissible noise to ground in dB in telephone lines close to the HVDC
lines.
17. 13. Define short circuit ratio(SCR)
The SCR is defined as the ratio of the short- circuit MVA of the ac system at the ac
bus to the rated dc power at that bus.
14. What are the factors depends commutation failure?
The recovery from a commutation failure depends on the following Factors
The response of the gamma controller at the inverter
The current control in the link
The magnitude of AC voltage
15. Function of smoothing reactor
A sufficiently large series reactor is used on DC side to smooth DC
current and also for protection. The reactor is designed as a linear reactor
and is connected on the line side, neutral side.
18. PART-B
1. Explain the generation of Harmonics in HVDC system
2. Explain detail about the analysis of single and double tuned AC
filters and write the design criteria for AC filters.
3. Explain in detail about the passive and active DC filters with the
design criteria.
4. Write short notes on:
(a) Stability aspects of synchronous link.
(b) Filters for harmonic elimination in HYDC-system
5. Discuss criteria of design of a.c. filters. Also mention various types of
a.c. filters and show their circuit configuration and impedance
characteristics
6. Why are harmonics generated in HVDC converter and what are
the problems associated with the harmonics. Suggest some
remedial measures.
7. Explain the characteristics and non- characteristics harmonics
in HVDC converter system.
8. Write short note on the following terms:
1. Harmonic distortion 2.TIF 3. THFF
2. IT and KIT
9. What are the types of AC filters? Also explain with waveforms
10. Write criteria for design of AC filters.
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19. UNIT V - POWER FLOW ANALYSIS IN AC/DC SYSTEMS
1. What is HVDC simulator?
HVDC simulator is similar to transient network analyzer (TNA) which
is Used to determine over voltages in AC systems due to switching surges
and load refection.
2. What are the requirements of a good simulation tool?
The requirements of good simulation tool are as follows,
Easy maintenance
Accuracy of solution
Flexibility of use
Reduced cost
Real time simulation
Easy monitoring and control
3. List some tools used for the simulation of HVDC simulation.
1. Physical simulator
2. Parity simulator
3. Analog computer
4. Digital computer
5. Hybrid computer
4. Application of HVDC transmission
Long distance bulk power transmission
Underground or under water cables
Asynchronous interconnection of AC systems operating at different
frequencies
Control and stabilization of power flows in AC ties
5. What is parity simulator?
A parity simulator is essentially a synthetic breadboard which
Electronically simulates the physical terminal characteristics of
Each network element rather than its mathematical
input/outputVIDYARTHIPLUS.COM
6. List the assumption made to develop the equivalent circuit of a converter
Bridge used in dynamic simulation.
All the values in a bridge have identical characteristics
A value offers infinite impedance in the reverse direction
The grading and damping circuits across the values are ignored
The current id is assumed to be continuous and non zero.
20. L/R of each phase of the converter transformer in the same.
7. What are the applications of DC simulator?
The application of DC simulator are,
1. Insulation coordination
2. Testing of controllers and their optimization
3. Evaluation of surge arrestor ratings
4. Harmonic analysis
8. State the advantages of parity simulator?
It avoids drawback of an analog computer simulation
The advantages of an analog computer interms of change in
Time scaling are retained while eliminating the drawbacks
The principle of parity simulation permits hybrid structure.
9. Write the system studies required for design of HVDC system.
i) Dc power transfer under various normal and contingency conditions
ii) Reactive power requirement
iii) DC terminal arresters
iv) AC and DC filter design
v) Controller requirements under various normal and faulted system
conditions
10.Write the advantages of digital dynamic simulation.
Easy transportability and maintenance
Reduced cost of simulation
Flexibility interms of representing any components of the system.
11. Write the disadvanges of digital dynamic simulation.
Increased simulation time
Lack of adequate mathematical models
Numerical problems and Lack of interactive capability
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12.Mention the types of valve model.
3 types are:
Time varying imdedance, ideal switch which is controllable and ideal switch in
series with a constant voltage source.
13. What are the approaches used for transient analysis of electrical network?
1. The use of trapezoidal rule of integration to tramsform the energy
storage elements to resistive elements with current sourse across them
which represent the past history
2. Formulation of stage equation for network
21. 14. What are two types of programs used for HVDC system studies?
i) AC/DC load flow analysis
ii) AC/DC transient stability analysis
15. What are advantages of EMPT representation of elements in DC system?
i) Easy of obtaining solutions particularly for piecewice linear components
ii) Inclusion of models for distributed lossless elements such as long transmission
lines.
16. Write the equations representing the equivalent circuit of lumped element.
For Inductor: i(t) = (1/L)ʃ v(t) dt + i(t-h)
Apply trapezoidal rule
I(t) = (h/2L) [ v(t) + v(t-h) + i) t-h)
For capacitor
I(t) = -(2C/h) [ v(t) + v(t-h) + i( t-h)
17. Drawbacks of parity simulation.
The problem of offset voltages and currents of OP-AMP used.
Each component of a parity simulator is electrically isolated, there could
be problems of interconnection and EMC.
18. Mention the some problems studied for DC simulator.
The development of concepts and equipment for control and protection of
HVDC systems.
i) Control of power, current and extinction angle in 2 terminal system
ii) Evaluation of the control performance under AC and DC faults
iii) Evaluation of overcurrent and over voltages stresses in
various components.
iv) Analysis of various AC/DC system interations
v) Anlysis of AC and DC harmonics
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19. Comparison between insulation characteristics of DC and AC cable.
1. In the ac cable case, the radial dielectric stress distribution is dependent upon
the perttivity of the dielectric. Where in the case of dc the stress distribution
determined by insulation resistance.
2. In the case of an ac cable the maximum stress always appears at the conductor
surface, but with dc the maximum sress may appear at the conductor surface
or at the outer boundary of the dielectric, depending on the temperature.
3. The dc strength of a dielectric is much higher than tha ac strength
4. With ac cables the temperature limitations are set by the physical behavious of
the materisl and the method of construction whereas with dc cable the
temperature limitations are set by the physical behavious of the materisl and
the method of construction but also by variations in dielectric stressing arising
from temperature effects.
22. 20. Write practical dielectrics used in HVDC cables.
Impregnated paper and polythene
21. Compare the DC and AC cables from economic point of view.
DC is particularly applicable to long submarine cable transmission where the cost
ratio is high. The cost of DC cable is only a fraction of those for the AC cable
and in addition much less physical space tends to be required in the waterway
involved. IN DC cable the problems such as migration of impregnating
compound, movement of the bales on the sea-bed, under the effects of currents
which may affect the economic considerations.
PART-B
1. Brief about the various types of system studies in design of
HVDC system.
2. Explain the modeling of HVDC systems for digital dynamic simulation.
3. Describe the operation of physical simulator.
4. Explain the various problems studies using DC simulator.
5. Describe the operation of parity simulator
6. Discuss about the advantages and disadvantages of digital dynamic
simulation.
7. Write short on valve model and firing pulse generation.
8. Explain the practical dielectric used in hvdc cables.
9. Explain the operation of dielectric stress considerations.
10 Explain the thermal consideration and losses in dc cables.