cvkr_modelling of hvdc

7/25/2019 Cvkr_modelling of Hvdc

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CONTENTS

MODELLING OF HVDC SYSTEMS

Economics of Power Transmission

Technical Performance

Stability Limits

Voltage Control

Line CompensationProblems of AC Interconnection

Ground Impedance

Disadvantages of DC Transmission

Reliability

Energy Availability

Transient Reliability


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CONTENTS

Application of DC Transmission

Component Models for the Analysis of AC/DC Systems

Converter Model

Simplified Continuous Time Model

Converter Control

Modeling of DC NetworkModeling of AC Network

Control of HVDC Systems

Basic Principles of Control

Basic Means of Control

Basic for Selection of Controls


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MODELLING OF HVDC SYSTEMS

The relative merits of the two modes of transmission (AC and

DC) which need to be considered by a system planner are

based on the following factors:1. Economics of transmission

2. Technical performance

3. Reliability

Economics of Power Transmission

The cost of a transmission line includes the investment and

operational costs. The investment includes costs of Right of

Ways (RoW), transmission towers, conductors, insulators and

terminal equipments. The operational costs include mainly the

cost of losses.

For a given power level, DC line requires less RoW, simpler and

cheaper towers and reduced conductor and insulator cost.


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Economics of Power Transmission Cont

The power losses are also reduced with DC as there are only

two conductors (about 67% of that for AC with same current

carrying capacity of conductors). The absence of skin effect with DC is also beneficial in reducing

power losses marginally.

The corona effects tend to be less significant on DC conductors

than for AC and this also leads to the choice of economic size of

conductors with DC transmission. The other factors that influence the line costs are the costs of

compensation and terminal equipment.

DC lines do not require compensation but the terminal

equipment costs are increased due to the presence of converterand filters.


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d*=break even distance

Fig 1: Variation of costs with line length

The break even distances can vary from 500 to 800 km in

overhead line depending on the per unit line costs.


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Technical performance

The DC transmission has some positive features which are

lacking in AC transmission.

These are mainly due to the fast controllability of power in DClines through converter control.

1. Full control over power transmitted.

2. The ability to enhance transient and dynamic stability in

associated AC network.

3. Fast control to limit fault currents in DC lines. This makes it

feasible to avoid DC breakers in two terminals DC links

In addition, the DC transmission overcomes some of the problems ofAC transmission.


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Stabili ty Limits

The power transfer in AC line is dependent on the angle

difference between the voltage phasors at the two ends.

The maximum power transfer is limited by the considerations ofsteady state and transient stability.

Fig: 2 Power transfer capability vs. Distances


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Voltage Control

The voltage control in AC line is complicated by the line charging and

inductive voltage drop.

The voltage profile varies with the line loading.

Fig:3 Variation of voltage along the line

The maintenance of constant voltages at the two ends requires reactive

power control from inductive to capacitive as the line loading is increased.


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Although DC converter stations require reactive power related to the

line loadings, the line itself does not require reactive power.

Line Compensation

In AC cable transmission, it is necessary to provide shuntcompensation at regular intervals. This is a serious problem in

underwater cables.

Problems of AC Interconnection

The operation of AC ties can be problematic due to (i) thepresence of large power oscillations which can lead to frequent

tripping (ii) increase in fault level (iii) the transmission of

disturbances from one system to the other.

The controllability of power flowing DC lines eliminates all theabove problems.

In addition, for asynchronous DC ties, there is no need of

coordinated control.


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Ground impedance

In AC transmission, the existence of ground (zero sequence) currentcan not be permitted in steady-state due to high magnitudes of ground

impedance which will not only affect efficient power transfer, but also

result in telephone interference.

The ground impedance is negligible for DC currents and a DC link canoperate using one conductor with ground return (monopolar operation).

Disadvantages of DC transmission

The scope of application of DC Transmission is limited by the fowlingfactors:

1. The difficulty of breaking DC currents which results in high

cost of DC breakers

2. Inability to use transformers to change voltage levels

3. High cost of conversion equipment4. Generation of harmonics which require AC and DC filters,

adding to the cost of converter stations

5. Complexity of control


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Over the years, there have been significant advances in DC

technology, which have tried to overcome disadvantages listedabove except for (2).

1. Development of DC breakers

2. Modular construction of thyristor valves

3. Increase in ratings of thyristor cells that make up a valve

4. Twelve pulse operation of converters

5. Use of metal oxide , gapless arrestors

6. Application of digital electronics and fiber optics in control of

converters

Complexity of control does not pose a problem and can actually be

used to provide reliable and fast control of power transmission not

only under normal conditions but also under abnormal conditions

such as line and converter faults.

ReliabilityThe reliability of DC transmission systems is quite good and

comparable to that of AC systems.


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Energy availabil ity

Both energy availability and transient reliability of existing DC

systems with thyristor valves is 95% or more.

In comparing the reliability of various alternatives, it must be keptin mind that bipolar DC line can be as reliable as a double circuit AC

line with the same power capability.

Transient reliability

The detailed comparison of AC and DC transmission in terms ofeconomics and technical performance leads to the following areas of

application for DC transmission.

Long distance bulk power transmission

Underground or underwater cables

Asynchronous interconnection of AC systems operating at different

frequencies or where independent control of systems is desired.

Control and stabilization of power flows in AC ties in an integrated

power system

Application of DC Transmission


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The prediction of the system performance under various conditions

helps in assessing the stresses on the various system components and

preparing the specifications of the equipment.

Component Models for the Analysis of AC/DC Systems

The technical superiority of DC transmission dictates its use for

asynchronous interconnection, even when the transmission distancesare negligible.

Actually there are many back to back DC links in existence where

the rectification and inversion are carried out in the same converter

station with no DC lines.

Fig.4: The Schematic of a converter transformer with Bridge

Converter model

Simplified continuous time model


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Ed = Vdo Cos & Vdo = aVWhere a= (3/) 2 Ns /(Np T)Ns /Np = nominal turns ratios of the three phases transformer,

T=off nominal ratio, V=line to line voltage at the primary.

In Fig.5 , Rc is the commutation resistance given byRc = (3/) Xcwhere Xc is the leakage reactance of the converter transformer,

Lc is the average inductance given by

Lc = (Xc /o) [2(1-k) +1.5k]

wherek=3u/ , u=overlap angleo= system frequency in rad/sec.

The equivalent circuit of fig. 5 is based on assumptions:

Fig.5: Simplified Continuous Time Equivalent Circuit of Bridge


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Power control, auxiliary control and voltage dependent currentorder limiter (VDCOL). The output of this block is the current order.

Constant Current (CC) and Constant Extinction Angle (CEA)

controls. These are usually of feedback type. However, the

extinction angle control can also be of predictive (open loop) type.The output of these controllers is a control voltage that determines

the instant of gate pulse generation. The input is taken as the

current order (generated locally or at the remote station) or the

extinction angle reference (generated locally). The communication

delay in transmitting the current order may have to be represented.

Gate pulse generator which has input from the CC or CEA

controller and determines the instant of gate pulse generation for

each valve. There are basically two types of firing control schemes.

Individual phase control (IPC) and (ii) Equidistant Pulse

Control (EPC). The latter can be of pulse frequency

control (PFC) or pulse phase control (PPC).

Converter control


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Fig.6: Power and auxiliary controller block diagram

The DC network is assumed to consist of smoothing reactor, DC filters

and the Transmission line.

Modelling of DC Network:

For some types of analyses, the AC network can be assumed to be in

steady-state (say for load flow analysis or long term stability analysis).

Modelling of AC networks:


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An HVDC transmission system is highly controllable.

Control of HVDC Systems

It represents a monopolar link or one pole of a bipolar link.

Basic Principles of Control

(a) Schematic diagram

(b) Equivalent circuit.


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(c) Voltage Profile

Fig.7: HVDC transmission link

The direct current flowing from the rectifier to the inverter isVdorCos VdoiCos

Id = --------------------------------------

Rcr + RL RciThe power at the rectifier terminal is

Pdr= VdrIdand at the inverter terminal is

Pdi = Vdi Id = Pdr- RL Id2


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The direct voltage at any point on the line and current (or power) canbe controlled by controlling the internal voltages (VdorCos) and (VdoiCos). Power reversal is obtained by reversal of polarity of direct voltages

at both ends.

Basic Means of Control

Following considerations influence the selection of control

characteristics:1. Prevention of large fluctuations in direct current due to variations

in ac system voltage.

2. Maintaining direct voltage near rated value.

3. Maintaining power factors at the sending and receiving end thatare as high as possible.

4. Prevention of commutation failure in inverters and arc-back in

rectifiers using mercury-arc valves.

Basis for Selection of Controls


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There are several reasons for maintaining the power factor high

To achieve high power factor, for a rectifier and for an invertershould be kept as low as possible.

The rectifier, however, has a minimumlimit of about 50 to ensureadequate voltage across the valve before firing.

The rectifier normally operates at a value ofwithin the range of 150

to 200 so as to leave some room for increasing rectifier voltage to

control dc power flow.

In the case of an inverter, it is necessary to maintain a certain

minimum extinction angle to avoid commutation failure.

Typically, the value of with acceptable margin is 150 for 50 Hzsystems and 180 for 60Hz system.

[ 1 ] Prabha Kundur : Power System Stability and control , The EPRI PowerSystem Engineering Series, McGraw-Hill, Inc., 1994.

[ 2 ] K. R. Padiyar : HVDC Power Transmission Systems : Technology and

System Interaction , New Age International (P) Limited, Publishers, 1996.

REFERENCES


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cvkr_modelling of hvdc

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