DHANALAKSHMI COLLEGE OF ENGINEERING, CHENNAI DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING EE6010– HIGH VOLTAGE DIRECT CURRENT TRANSMISSION UNIT – I : INTRODUCTION PART – A (2 Marks) 1.

List out any two merits of AC and DC transmission. AC Transmission: i. It requires only two conductors as compared to three for AC transmission. ii. There is no skin effect in a DC system. DC Transmission: i. The power can be generated at high voltages. ii. The maintenance of AC sub-station is easy and cheaper.

1. What are the different types of DC link? i. Monopolar link ii. Bipolar link iii. Homopolar link 2. List some of the advantages and disadvantages of HVDC transmission. Advantages: i. Full control over power transmitted ii. The ability to enhance transient and dynamic stability in associated AC network. Disadvantages: i. Inability to use transformer to change voltage levels ii. High cost of converter equipment 3. What are the types of power losses in thyristor? i. Forward conduction losses ii. Loss due to leakage current during forward & reverse blocking iii. Switching losses due to Ton and Toff iv. Gate triggering loss 4. 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 from the development of LTT.

5. Define - Energy Availability It is defined as, Energy Availability=100 (1 - equivalent outage time) Where equivalent outage time is the product of the actual outage time and the fraction system capacity lost to outage.

of

9. List out any two application DC transmission. i. Long distance bulk power transmission ii. Underground or underwater cables iii. Asynchronous interconnection of A.C systems operating at different frequencies. 10. What are the factors to be considered for planning HVDC transmission? The factors to be considered are, i. Cost ii. Technical performance iii. Reliability 11.

What are the advantages of LTT over ETT? i. Infinite gate isolation ii. Total noise immunity for the control circuits iii. Faster turn-on time iv. Elimination if high voltage pulse transformers and auxiliary power supplies.

12. Distinguish between AC and DC transmission. Sl.No. 1. 2. 3. 4.

AC Transmission It requires three conductors for transmission The skin effect is present in AC transmission More corona loss Stability problem occurs.

DC Transmission It requires only 2conductors There is no skin effect in DC Transmission Less corona loss No stability problem.

13. 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 MOSFET’s built into the gate structure one MOSFET is used for turning on the MCT and other for turning off device. An MCT is a High – frequency, high power, low – conduction drop switching device.

14. List any two HVDC projects in India. Sl.No.

1. 2.

System/Project National HVDC project-stage-I NHVDC-stage-II

Year

Supplier

Power

Voltage

Rating (MW)

(kV)

1989

BHEL

100

100

2000

BHEL

100

200

PART – B : (16 Marks) 1. Explain in detail, the economic choice of voltage level selected in DC transmission system. 2. Explain in detail, the technological development in control and protection for better performance and reliability of dc transmission system. 3. Explain in detail, the different applications of dc transmission system. 4. Explain the different types of DC links. 5. Explain in detail, the comparison of AC and DC transmission. 6. Explain in detail, the types and applications of MTDC systems. 7. Explain in detail, the planning of HVDC transmission systems. 8. Explain in detail, the modern trends in DC transmission. 9. Explain in detail, the major components of a HVDC transmission in converter station. 10. Explain in detail, the HVDC transmission based on voltage source converters.

UNIT – II : ANALYSIS OF HVDC CONVERTERS 1.

What is meant by 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 model 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 used in DC transmission. 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 - Valve Rating The valve voltage rating is specified in terms of peak inverse voltage (PIV). The ratio of PIV to the average dc voltage is an Index of the value utilization. 6. Define – Converter Configuration There are several configuration for a converter of a specified pulse number, and in addition to the Graetz bridge, six phase diametric connection, cascade of three single phase fall wave converters, cascade of two three phase converters.

7. List some of the assumptions made to develop the equivalent circuit of a converter. i. All the values in a bridge have identical characteristics ii. A value offers infinite impedance in the reverse direction iii. The grading and damping circuits across the values are ignored iv. The current id is assumed to be continuous and non - zero. v. L/R of each phase of the converter transformer in the same. vi. 8.

Define - Overlap and Overlap Angle Overlap is the phenomenon due to the effect of source inductance on the AC side. The current commutation is delayed due to the source inductance which is normally the leakage reactance of a transformer. This period is also known as “overlap” period. 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.

9.

What are the assumptions made for analysis of 6 pulse converter? i. DC current is constant (i.e. the smoothing reactor is infinite) ii. Valves are ideal switches, and iii. AC system is infinitely strong (i.e. the 3 phase emf’s are balanced and perfectly sinusoidal).

10. 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▫) Mode III : 3 and 4 valve conduction mode (α > 30▫) (60▫
PART – B (16 Marks)

1. For a 3 phase 6 pulse gratez's circuit draw the timing diagram considering overlap angle is less than 60 degree and without overlap for the following: a. Voltage across load b. Voltage across any two pair of conduction valves 2. Explain in detail, the individual characteristics of a rectifier and an inverter with sketches.( 3. Explain in detail, the principle of operation of a 6 pulse gratez's circuit. 4. Explain in detail, the effect of source inductance on HVDC system. 5. Explain in detail, the converter bridge characteristics. 6. Explain the effect of overlap angle on the performance of converter circuit. 7. Explain the choice of converter configuration for any pulse number. 8. Explain the analysis of 12 pulse converter with bridge rectifier. 9. Explain the analysis of two and three valve conduction mode. 10. Explain in detail, the analysis of VSC topologies and firing schemes.

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 V c, Which is related to the firing angle required. The firing angle controller generates gate pulses in response to the control signal Vc. 2. What are the main features of HVDC system control? i. Current order setting can be quickly and reliably changed ii. Power reversal can be done easily and quickly iii. Fault current levels are limited to rated values. 3. What is the principal of DC link control? The control of power in a DC link can be achieved through the control of current or voltage. It is important to maintain constant voltage in the link and adjust the current to meet the required power to minimize loss. 5. What is meant by current and extinction control? The current control is invariably of feedback type. The extinction angle controller can be of predictive type or feedback type with IPC control. 6. Why is current control used in the rectifier station under normal operating conditions? i. The inverter can now be operated at minimum value thereby to minimize the reactive power consumption. ii. The operation at minimum extinction angle at the inverter and current control at the rectifier results in better voltage regulation them the operation with minimum delay angle at the rectifier and current control at the inverter. iii. The current during line fault are automatically limited with rectifier station in current control. 7. How is power reversal achieved 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 ends, the rectifier current controller is provided with a higher current order. Idi = Idr – Im Where, Idi is the current order of the inverter Idr is the current order of the rectifier Im is usually 10% of rated value

9. What are the draw backs of constant current control scheme? i. ii.

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 is the need for transformer tap changer control of HVDC converter? The tap changing transformer is 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 by the use tap changing transformer. 11. Define – Equidistant Pulse Control In the equidistant pulse control, the firing pulses are generated in steady state at equal intervals through a ring counter. This control was first suggested by Ainsworth using a phase locked oscillator to generate the firing pulses. 12. Define – Individual Phase Control In the individual phase control scheme, the firing pulse generation for each phase is independent of each other and the firing pulses are rigidly synchronized with the commutation voltages. 13. What are the different types of individual phase control? There are two different types of individual phase control. They are, i. Constant alpha control ii. Inverse cosine control 14. What is the function of voltage dependent current order limiter? The function of voltage dependent current order limiter is to prevent individual thyristors from carrying full current for long periods during commutation failures.

PART – B : (16 Marks) 1. Explain in detail, the control of VSC based HVDC systems. 2. Explain in detail, the current and extinction angle control. 3. Explain in detail, the converter control characteristics of HVDC systems. 4. Explain in detail, different types of firing angle control schemes. 5. Explain the system control hierarchical structure for a DC link. 6. Explain in detail, the principles of DC link control. 7. Explain in detail, the different functions of higher level controllers. 8. Explain the principle of operation of a basic power controller. 9. Explain in detail, the starting and stopping of DC link.

UNIT – IV : REACTIVE POWER AND HARMONICS CONTROL

1. What are the drawbacks in the operation of weak AC systems? The operation of weak AC systems can be problematic due to voltage instability and dynamic over voltages. 2. What are the various sources of reactive power? The various sources of reactive power are: I. AC system II. AC filters III. Shunt capacitors IV. Synchronous condensers V. Static var system 3. Why is control of reactive power necessary during transients? The control of reactive power is necessary during transients due to the following reasons: I. Control of dynamic over voltages caused by load rejection II. Speed recovery of power followed by a fault in the inverter section III. Control of instability 4. What are the different types of SVS systems? There are three basic types of SVS schemes: I. Variable impedance type II. Current source type III. Voltage source type 5. What are the different factors to be considered for selection of harmonic filter? I. Harmonic distortion II. Telephone influence factor III. IT product IV. Telephone harmonic form factor 6. List the various types of filters. There are basically two types of filters: I. Active filters II. Passive filters

7. What are the various problems associated with the injection of harmonics? The various problems associated with the injection of harmonics are: I. Telephone interference II. Extra power losses III. Over voltages due to resonance IV. Instability of converter control V. Interference with ripple control systems used in load management 8. Define – Characteristic Harmonics The characteristic harmonics are the harmonics of order which is always present even under the ideal operation of balanced AC voltages, symmetric three phase network and equidistant pulses. 9. What are the reasons for the non- characteristic harmonics? The non- characteristic harmonics is due to: I. Imbalance in the operation of two bridges forming a 12 pulse converter II. Firing angle control III. Unbalance and distortion in the AC voltages IV. Unequal transformer leakage impedances 10. What is the effect of unbalanced voltages in the HVDC system? The presence of the negative sequence component in the AC voltages shifts the zero crossing of the commutation voltages. 11. What are the various types of AC filters? The various types of AC filters are: I. Single tuned filter II. Double tuned filter III. High pass filter 12. What are the factors to be considered for selection of DC filter? I. Maximum voltage TIF on DC high voltage bus II. Maximum induced noise voltage in mv / km in a particular test line of 1km away from the HVDC line Maximum permissible noise to ground in dB in telephone lines close to the HVDC lines. 13. Define – Telephone Harmonic Form Factor It is defined as, ⁄

(

)

Where, Wn is the psophometric weight at the harmonic of order n. 14. Define – KIT product It is defined as, ⁄

PART – B : (16 Marks) 1. Explain in detail, the reactive power requirements in steady state conditions. 2. Explain in detail, the principle of operation and V-I characteristics of SVC. 3. Explain in detail, the principle of operation and V-I characteristics of STATCOM. 4. Explain in detail, the generation of harmonics. 5. Explain in detail, the design of AC and DC filters. 6. Explain in detail, the design of single tuned filters. 7. Explain in detail, the design of double and triple tuned filters.

UNIT - V : POWER FLOW ANALYSIS IN AC/DC SYSTEMS 1. Why is power flow analysis most important in AC/DC systems? Power flow analysis is an essential component of system studies which is carried out for planning, design and operation of power systems. It is basically simulation of the systems in steady state and determines the operating point which is later used for initializing variables in transient and dynamic system simulation. 2. Write the necessary equations for the bus voltage estimates used in fast decoupled power flow method. The necessary equations for the bus voltage estimates used in fast decoupled power flow method are: [ ] ⁄ ⁄

[

]

3. What are the different components of DC network? The DC network consists of DC links, smoothing reactors, and converters can be viewed as a resistive network excited by current or voltage sources in the steady state. 4. What are the elements of the DC network? The elements of the DC network can be separated into tree branches and links. In radial DC networks, there is no loss of generality in assuming that all the resistances forms a subset of tree branches. 5. Write the equations describing the DC network. The equations describing the DC network are, [ ]

6. What is meant by DC converter? The DC network is a network which consists of N converters which can be put into m groups such that all the converters are in groups. It is governed by an equation, ∑

7. Write the voltage equation for the converter. The voltage equation of the converter is, (

⁄ )(



)( ⁄ )

( ⁄ )

8. Write the real power and reactive power equations for the AC bus. The real power and reactive power equations of the AC bus are, ∑ ∑ 9. What are the different types of control used for the controller? The different types of control used for the controller are: I. Current control II. DC voltage control III. Power control IV. Reactive power control 10. Write the control equations for the converter. The control equations for the converter are, ∑

=1

11. Write the equation for the base resistance of the converter. The equation of the base resistance of the converter is, ⁄ 12. What are the different methods to solve AC/DC power flow? The different methods to solve AC/DC power flow are: I. Simultaneous or unified method II. Sequential or alternating method 13. What is meant by simultaneous method? The simultaneous method is the method in which both AC and DC equations are solved together. The equations are combined into single set of non – linear algebraic equations. 14. What is meant by sequential method? The sequential method is the method in which the AC and DC quantities are solved separately and sequentially. The DC is solved using a simplified representation of the system.

PART – B (16 Marks) 1. Explain in detail, the modeling of DC links. 2. Explain in detail, the per unit system of DC quantities and inclusion of constraints. 3. Explain in detail, the case study of a five terminal DC system. 4. Explain in detail, the power flow analysis of VSC based HVDC system. 5. Explain in detail, the power flow analysis under dynamic conditions.

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