i m.tech hvdc transmission part a a)

WWW.VIDYARTHIPLUS.COM V+ TEAM

WWWWWW.VIDYARTHIPLUS.COM.VIDYARTHIPLUS.COM

I M.Tech – I Semester Regular Examinations

HVDC Transmission

PART – A

Q 1.

a)

1. Long distance high Power transmission by overhead lines.

2. Medium and long high power submarine or underground cables.

3. System interconnection by means of overhead lines or underground/submarine cables

or back to back HVDC coupling stations.

4. Multi-Terminal HVDC System for interconnecting three or more 3 phase AC systems.

5. Frequency conversion (6O Hz —50 Hz ; 50 Hz —25 ‘Hz)

6. Incoming lines in megacities.

b)

1. Monopolar HVDC system

2. Bipolar HVDC Transmission

3. Homopolar HVDC System

c)

HVDC converter produces harmonics of all the orders and some DC component on the volve

winding of transformer.

Harmonics other than np+/-1 & np are known as non-characteristics harmonics.

d)

1 no. star-star and 2 no. star-delta transformer configuration are used to have 300 phase shift.

e)

Steady-state Ud/Id Characteristic of an HVDC Convertor

• The horizontal segment RS has certain slope

representing voltage drop in the DC line resistance (Id.

R).

• The slope of vertical segment Ids is due to actual

characteristic of constant current controller.



f) AC-DC interaction

1. Neighboring independently controlled AC Networks are interconnected by system

interconnections.

2. System interconnection is either by EHV-AC/HVAC or HVDC.

3. The basic function of an interconnection is to transfer energy from surplus zone to

deficit zone.

g)

1. Bulk power transmission from several remote generating stations to several load

centres.

2. Asynchronous interconnection between adjacent power systems.

3. Reinforcing of an AC network which heavily loaded.

h)

1. Types of over voltages,

2. Switching over voltages

3. Temporary over voltages

4. Steep front over voltges

5. Disturbances causes on AC and DC side due to Lightning strokes

i)

Considering N series connected bridges, if one of the bridges is to be taken out of the service,

it is required to block and bypass the bridge. Otherwise, the continuing conduction of pairs

may inject AC voltage into the link which may leads to the current and voltage oscillations due

to lightly damped oscillatory circuit formed by smoothing reactor & line capacitance.

j)

Surge arresters are used for protection of the system against Lightning and surge voltages.

These are basically zinc-oxide metal discs, utilises non-linear resistors which provides least

resistance path to the ground during high voltages and open circuit path during normal

operating conditions.



PART – B

Q 2) Merits and demerits of HVDC transmission,

a) Economics of transmission:

• Capital cost of transmission systems:

• Cost of line conductors, towers, insulators, installation land/right of way.

• Capital cost of substations, intermediate substations, compensating substations,

conversion substations, substation equipment like transformers, switchgear; substation

area, buildings.

• Cost of energy losses, maintenance.



•

b) Technical performance:

1. full control over power transmitted

2. ability to enhance transient and dynamic stability in associated AC networks

3. fast control to limit fault currents in DC lines

4. stability limits

5. better voltage control

6. line compensation is good

c) Reliability:

Reliability of DC transmission system is good.

Q 3)

Analysis of Graetz circuit with overlap angle less than 600



Q 4) Individual Phase Control (IPC) and Equidistant Pulse Control (EPC) schemes in

HVDC projects.

There are two basic firing schemes, namely:

• Individual phase control (IPC)

• Equidistant pulse control (EPC)

IPC was used in the past and has now been replaced by EPC

Individual phase control (IPC)

1. Current control, unit amplifier

2. Valves firing units pulse generator

3. Pulse distribution unit

4. Pulse transmission system

5. Current feed back

6. 6 pulse convertor unit

• This principle is applied for individual valve. Normally with zero delay angle, the

valves will start conducting at respective zero crossing in a sequence.

• However by delaying the instant of firing pulse by delay angle α, the start of

conduction of individual valve is delayed with reference to phase angle of zero

crossing.

• The control pulses are given to each valve at definite phase angle α with respect to

earlier zero crossing.



• In individual phase control the control function for initiating the control pulse is

derived from commutation voltage.

• Three phase alternating voltage UAC is supplied to valve firing control unit.

Drawbacks of IPC Scheme

• Any distortion in the system voltage leads perturbations in the zero crossings which

affect the instants of firing pulses in scheme.

• This implies that even when the fundamental frequency voltage component balanced,

the firing pulses are not equidistant in steady-state. This in turn leads to generation of

non-characteristic harmonics.

Equidistant Pulse Control (EPC)

• The pulses derived from control pulse generator are of nominal frequency fc

proportional to (6f0) or (12f0) for 6 pulse and 12 pulse convertor unit respectively.

Where f0 is the fundamental frequency of AC Network.

• The pulses of (6f0) or (12f0) are separated in pulse distribution unit and are supplied to

individual valves.

• As frequency f0 of AC system is always constant, the control pulses are with constant

frequency and equidistant with respect to timing.

• The voltage controlled oscillator gives

a train of control pulses of frequency (fc). The frequency of output pulses is by control

function (Uc).

Q 5)

Comparison of series and parallel MTDC system

1. High speed reversal is possible in series systems without mechanical switching, which

is not possible in parallel systems.



Q 6)

Protection against over currents:



Q 7) Double tuned filters:



Q 8)

Over voltages excited by disturbances on the DC side.

i m.tech hvdc transmission part a a)

Documents