MODELLING OF HVDC SYSTEMS

ByProf. C. Radhakrishna

10/17/2010 12:04 AM 2

CONTENTS

MODELLING OF HVDC SYSTEMS

Economics of Power Transmission

Technical Performance

Stability Limits

Voltage Control

Line Compensation

Problems of AC Interconnection

Ground Impedance

Disadvantages of DC Transmission

Reliability

Energy Availability

Transient Reliability

10/17/2010 12:04 AM 3

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 Network

Modeling of AC Network

Control of HVDC Systems

Basic Principles of Control

Basic Means of Control

Basic for Selection of Controls

10/17/2010 12:04 AM 4

MODELLING OF HVDC SYSTEMS

The relative merits of the two modes of transmission (AC andDC) which need to be considered by a system planner arebased on the following factors:

1. Economics of transmission2. Technical performance3. Reliability Economics of Power Transmission

The cost of a transmission line includes the investment andoperational costs. The investment includes costs of Right ofWays (RoW), transmission towers, conductors, insulators andterminal equipments. The operational costs include mainly thecost of losses.

For a given power level, DC line requires less RoW, simpler andcheaper towers and reduced conductor and insulator cost.

10/17/2010 12:04 AM 5

Economics of Power Transmission Cont……

• The power losses are also reduced with DC as there are onlytwo conductors (about 67% of that for AC with same currentcarrying capacity of conductors).• The absence of skin effect with DC is also beneficial in reducingpower losses marginally.• The corona effects tend to be less significant on DC conductorsthan for AC and this also leads to the choice of economic size ofconductors with DC transmission.• The other factors that influence the line costs are the costs ofcompensation and terminal equipment.• DC lines do not require compensation but the terminalequipment costs are increased due to the presence of converterand filters.

10/17/2010 12:04 AM 6

d*=break –even distanceFig 1: Variation of costs with line length

• The break even distances can vary from 500 to 800 km inoverhead line depending on the per unit line costs.

10/17/2010 12:04 AM

7

Technical performance

• The DC transmission has some positive features which arelacking 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.

10/17/2010 12:04 AM 8

Stability Limits

• The power transfer in AC line is dependent on the angledifference 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

10/17/2010 12:04 AM 9

Voltage Control

• The voltage control in AC line is complicated by the line charging andinductive 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 reactivepower control from inductive to capacitive as the line loading is increased.

10/17/2010 12:04 AM 10

Although DC converter stations require reactive power related to theline 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 inunderwater 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 frequenttripping (ii) increase in fault level (iii) the transmission ofdisturbances 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 ofcoordinated control.

10/17/2010 12:04 AM 11

Ground impedanceIn AC transmission, the existence of ground (zero sequence) currentcan not be permitted in steady-state due to high magnitudes of groundimpedance which will not only affect efficient power transfer, but alsoresult 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 highcost of DC breakers

2. Inability to use transformers to change voltage levels3. High cost of conversion equipment4. Generation of harmonics which require AC and DC filters,

adding to the cost of converter stations5. Complexity of control

10/17/2010 12:04 AM 12

Over the years, there have been significant advances in DCtechnology, which have tried to overcome disadvantages listedabove except for (2).1. Development of DC breakers2. Modular construction of thyristor valves3. Increase in ratings of thyristor cells that make up a valve4. Twelve pulse operation of converters5. Use of metal oxide , gapless arrestors6. Application of digital electronics and fiber optics in control of

converters

Complexity of control does not pose a problem and can actually beused to provide reliable and fast control of power transmission notonly under normal conditions but also under abnormal conditionssuch as line and converter faults.

ReliabilityThe reliability of DC transmission systems is quite good andcomparable to that of AC systems.

10/17/2010 12:04 AM 13

Energy availability

• Both energy availability and transient reliability of existing DCsystems 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 ACline 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 ofapplication for DC transmission.• Long distance bulk power transmission• Underground or underwater cables• Asynchronous interconnection of AC systems operating at differentfrequencies or where independent control of systems is desired.• Control and stabilization of power flows in AC ties in an integratedpower system

Application of DC Transmission

10/17/2010 12:04 AM 14

The prediction of the system performance under various conditionshelps in assessing the stresses on the various system components andpreparing the specifications of the equipment.

Component Models for the Analysis of AC/DC Systems

• The technical superiority of DC transmission dictates its use forasynchronous interconnection, even when the transmission distancesare negligible.• Actually there are many ‘back to back’ DC links in existence wherethe rectification and inversion are carried out in the same converterstation with no DC lines.

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

Converter modelSimplified continuous time model

10/17/2010 12:04 AM 15

Ed = Vdo Cos α & Vdo = aV Where a= (3/π) √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 by

Rc = (3/π) Xcwhere Xc is the leakage reactance of the converter transformer, Lc is the average inductance given by

Lc = (Xc /ωo) [2(1-k) +1.5k]where

k=3u/ π, u=overlap angle ωo= system frequency in rad/sec.

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

Fig.5: Simplified Continuous Time Equivalent Circuit of Bridge

10/17/2010 12:04 AM 16

• 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, theextinction angle control can also be of predictive (open loop) type.The output of these controllers is a control voltage that determinesthe instant of gate pulse generation. The input is taken as thecurrent order (generated locally or at the remote station) or theextinction angle reference (generated locally). The communicationdelay in transmitting the current order may have to be represented.

• Gate pulse generator which has input from the CC or CEAcontroller and determines the instant of gate pulse generation foreach valve. There are basically two types of firing control schemes.

Individual phase control (IPC) and (ii) Equidistant PulseControl (EPC). The latter can be of pulse frequencycontrol (PFC) or pulse phase control (PPC).

Converter control

10/17/2010 12:04 AM 17

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

Modelling of AC networks:

10/17/2010 12:04 AM 18

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.

10/17/2010 12:04 AM 19

(c) Voltage ProfileFig.7: HVDC transmission link

The direct current flowing from the rectifier to the inverter is Vdor Cos α – Vdoi Cos γ

Id = --------------------------------------Rcr + RL – Rci

The power at the rectifier terminal is Pdr = Vdr Id

and at the inverter terminal is Pdi = Vdi Id = Pdr - RL Id2

10/17/2010 12:04 AM 20

• The direct voltage at any point on the line and current (or power) canbe controlled by controlling the internal voltages (Vdor Cosα) and (VdoiCosγ).• Power reversal is obtained by reversal of polarity of direct voltagesat both ends.

Basic Means of Control

Following considerations influence the selection of controlcharacteristics:

1. Prevention of large fluctuations in direct current due to variationsin ac system voltage.

2. Maintaining direct voltage near rated value.3. Maintaining power factors at the sending and receiving end that

are 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

10/17/2010 12:04 AM 21

There are several reasons for maintaining the power factor high• To achieve high power factor, α for a rectifier and γ for an invertershould be kept as low as possible.• The rectifier, however, has a minimum α limit of about 50 to ensureadequate voltage across the valve before firing.• The rectifier normally operates at a value of α within the range of 150

to 200 so as to leave some room for increasing rectifier voltage tocontrol dc power flow.• In the case of an inverter, it is necessary to maintain a certainminimum extinction angle to avoid commutation failure.• Typically, the value of γ 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 andSystem Interaction” , New Age International (P) Limited, Publishers, 1996.

REFERENCES

10/17/2010 12:04 AM 22

THANK YOU