This document gives an overview of the HVDC Control System.

The key for understanding how an HVDC transmission operates is to knowthe basic principles of the HVDC Control system functions.

The basic principles highlighted in this report are:

the DC current / DC voltage characteristics of an HVDC transmission

the cooperation between the HVDC transmission converter terminals

2Table of Contents

1. INTRODUCTION .................................................................. 3

2. BASIC CONTROL PRINCIPLES......................................... 32.1. The exchange of power between rectifier and inverter .......... 4

3. Ud/Id CHARACTERISTICS FOR THE HVDCCONVERTER ........................................................................ 5

3.1. The Ud/Id characteristics......................................................... 53.2. Combined characteristics for a rectifier and an inverter

cooperating on a DC line........................................................ 73.3. Power reversal by changing the relation between the

current orders......................................................................... 83.4. The principle of current margin control .................................. 93.5. Influence from the DC line resistance when considering

the static Ud/Id characteristics ............................................. 103.6. Dynamic aspects on the Ud /Id characteristics.................... 10

4. THE BASIC HVDC CONTROL SYSTEM.......................... 114.1. Converter Firing Control System.......................................... 114.1.1. Voltage dependent current order limitation (VDCOL) .......... 114.1.2. The current control amplifier (CCA) ..................................... 124.1.3. Firing Control ........................................................................ 124.1.4. Control Pulse Generator (CPG) ........................................... 124.2. Converter firing control Operation modes............................ 13

5. OTHER HVDC CONTROL FUNCTIONS.......................... 145.1. Voltage and angle reference calculation (VARC)................. 145.2. Tap Changer Control (TCC)................................................. 145.3. Reactive Power Control (RPC)............................................ 15

31. INTRODUCTIONThe major advantage of an HVDCtransmission is its built in ability to controlthe transmitted power between the sendingand the receiving converter terminals, theformer called rectifier and the latterinverter.

This controllability can be utilized for thestabilization of the connected AC network,to control the frequency of a receiving,islanded network and to assist thefrequency control of a generator,connected to the HVDC transmissionrectifier.

The reactive power, that the HVDCconverter consumes, is depending on thevalues of the control angles. Thus, theexchange of the reactive power betweenthe converter and the AC network can becontrolled and the AC voltage can bestabilized.

Also, combined active and reactive powergeneration can be applied when foundadvantageous.

This report will cover the controlfundamentals for the HVDC convertersand the transmission itself.

2. BASIC CONTROLPRINCIPLESThe basic concept to control an HVDCtransmission is the possibility to set the DCvoltage across the converter valve bridgeand the transmitted power by varying thephase position of the gate control pulses tothe converter valves.

Figure 1 shows a simple diagram for amonopolar HVDC transmission. Thevoltage of the rectifier is indicated by Udland the inverter by Ud2. The polarity of thevoltage Ud across the bridge is defined andit should be noted, that to make the invertervalves conduct the rectifier must set up ahigher voltage than the inverter.

Simple block diagram for monopolarHVDC transmission

Id Rd

Rectifier Inverter

+

-

Ud1 Ud2

+

-

Figure 1

42.1. The exchange of powerbetween rectifier and inverterThe normal configuration used for HVDCis a 12 pulse bridge converter. In a 12 pulseconverter two 6 pulse converters areconnected in series with one valve bridgebeing provided, with a Y/Y connectedtransformer and the other with a Y/Dconnected transformer. Figure 2 presentsthe 12 pulse converter bridge and its singleline diagram symbol.

12 pulse converter and its single line diagram

Y / YId

Ud

+

-Y / D

1 3 5

4 6 2

1 3 5

4 6 2

Y / Y

Y / D=

u:/hvdckurs/lk/kontroll/oh/twepuls

Figure 2

The power delivered to the DC circuitfrom the rectifier is

P U IdR d d= 1 (1)

The DC current Id is determined bythe voltage difference betweenrectifier and inverter and by the lineresistance Rd

IU U

Rd

d d

d=

-1 2(2)

and ( )

PU U U

RdR

d d d

d=

-1 1 2(3)

The DC line has normally a lowresistance Rd and accordingly Id isvery sensitive to the converter voltagevariations. Only a small change involtage ( Ud1 - Ud2 ) is needed to causean increase or a decrease in the DCcurrent.Equation (3) gives the power deliveredby the rectifier. The power absorbedby the inverter is obtained by replacingthe first Ud1 in (3) by Ud2

( )P

U U UR

dId d d

d=

-2 1 2(4)

The difference between PdR and PdI isequal to the losses in the DC circuit,i.e. mainly the line losses.

P P P R IdI dR d d d= - = 1 2 (5)

By establishing a DC current feedbackloop in one of the converter stations,normally in the rectifier and by making theinverter determine the DC voltage, a basicHVDC transmission system is obtained.

Powercontrol

Iorder Current

controlamplifier

Converterunitfiring

control Id

Iresponse

Voltagemeasuring

system

Porder

P mod

Ud response

To inverter

HVDC control system

Figure 3

By the feedback current control system inthe rectifier the DC voltage difference(Ud1 - Ud2 ) is automatically kept at such a

5level, that a preset DC current can bedelivered to the inverter.

The current control can be performed byeither the rectifier or the inverter with aprinciple, that the current is alwayscontrolled by one of the convertersand the voltage determined by theother.

3. Ud/Id CHARACTERISTICSFOR THE HVDCCONVERTER

3.1. The Ud/Id characteristics

Assuming that the rectifier in Figure 1 ishaving the current control and the currentorder set to the value I0 .

The Ud/Id characteristics is nowdetermined by the equation

( )U U d d II

Ud dio x rd

dNdioN= - + cosa

(6)

The firing angle a is a parameterinfluenced by the control system tokeep the DC current equal to thepreset reference.The second term in the right memberis the voltage drop in the convertertransformer with (dx + dr) directlyrepresenting the impedance of thetransformer.Udio, with rated value UdioN , is the no-load DC voltage obtained witha = 0.

For the discussion below we assume thesteady state conditions, meaning that theAC bus voltages Uac1 and Uac2 areconstant.

Supposing, that the inverter voltage Ud2 ishigh, the rectifier will try to force theordered current through the DC circuit bydecreasing the firing angle a , which mayin turn reach its minimum limit in the regionof 5 degrees. If Ud2 is very high, themaximum available value of Ud1 willperhaps not be high enough, even with an a= a min to make the inverter valves toconduct. No DC current will flow and weare somewhere above point A on the Udaxis in the following diagram.

UdA

B

const. Id

C

I0

D constant extinction angle

E

Id

Static Ud/Id characteristic of a converter

= min

s t a t c h a r . v s d

Figure 4

It should be noted that the sign of thevoltage of the characteristics shown inFigure 4 is so defined that it is positivewhen the cathode side of the valve bridgeis positive. When discussing thecooperation between the rectifier and theinverter, the sign must be changed so that itis referred to the line instead of the bridge.

Decreasing Ud2 means that we will sooneror later have a current flowing. At first, thecurrent will be lower than the preset value,although the control system orders aminimum a .

We are moving now on section A-B in theUd/Id characteristics above. The term Udio*cos a in equation (6) is now constant andthe slope of the characteristics withinsection A-B is determined by the convertertransformer.

Reaching point B means that the current isequal to the preset current order I0.

6Without a current control system and witha further decreasing inverter voltage Ud2the current would have exceeded thecurrent order I0.

With a decreasing Ud2 the rectifier voltageUd1 is now adapted to it by increasing aand we are now moving on the vertical lineB-C-D in the Ud/Id characteristics.

When Ud2 = 0, the firing angle a in therectifier is around 90 degrees and nopower is transmitted. When Ud2 decreasesfurther into the negative region no dramaticchange in operation will occur in thecorresponding converter. As long as a canbe increased, the rectifier can control thecurrent also with an opposite polarity on theDC circuit. Now the rectifier is absorbingpower from the DC circuit and theinverter.

The current control system can stillincrease a and we can move downwardsalong the line B-C-D until we reach a

maximum at point D. This a max value isdetermined by the minimum permittedvalue for the extinction angle g and thepresent overlap angle u.

If the current through the rectifier shouldincrease further, a must decrease as aconsequence of that the overlap angle mustincrease. However, the extinction angle iskept constant by the converter firingcontrol system (CFC). Thus, when thecurrent increases further, we move alongthe line D-E which is the focus for theconstant extinction angle g in the Ud /Idcharacteristics.

The characteristics A-B-C-D-E is thebasic form of the Ud/Id characteristicsfor an HVDC converter for both therectifier and the inverter operation.

73.2. Combined characteristics for a rectifier and an invertercooperating on a DC line

CFC CPG CPG CFC

CCA CCA

Rectifier Inverter

Uac1 Uac2

12 12

Id1 Id1

Iresp. Iresp.

Io2Io1

Ud1 Ud2

Monopolar HVDC transmission

Figure 5

The definition criteria for the converter tooperate either as rectifier or inverter is, thatthe converter with the highest DCcurrent will operate as rectifier and theother as an inverter.

The rectifier has been provided with acurrent feedback control system consistingof a current control amplifier (CCA), aconverter firing control system (CFC) anda control pulse generator (CPG). Theinverter is also supplied with a controlsystem of a similar kind as shown in Figure5, even if the inverter current control is notactivated in this basic mode of operation.However, a CFC and a CPG are neededto apply some form of control which can beused to obtain a fixed DC voltage.

Assume now, that the two converters withidentical control systems are having thetransformer winding ratios chosen so, thatthe current order for the inverter is slightlylower than for the rectifier. The two Ud/Idcharacteristics will result now as shown inFigure 6 below.

The current control system corrects thefiring angle a by increasing it when the DCcurrent exceeds the current order I0. Thecorrection is equal for both the rectifier andthe inverter.

If both the rectifier and the inverter areenergized on the AC side and deblocked,meaning that they are started, we willfind that the rectifier delivers more currentto the inverter than it requests.

Ud

I01

Id

Static Ud/Id characteristics

I02

RectifierInverter

s t a t c h 1 . v s d

Figure 6

8The current order relation will be now I01> I02. The inverter will resist the DCcurrent increase by increasing the firingangle a to reach the area of a > 90. Theinverter voltage Ud2 will now be negativeon the cathode side of the 12 pulse bridgeand will further increase in that directionmeaning, that the inverter will generate apositive voltage on the DC line and is thusoperating as an inverter. To force theordered current I01 through the DC line andto maintain the rectifier operation therectifier must establish a higher positivevoltage, the cathode side of the converterbeing positive.

To illustrate the cooperation between therectifier and the inverter in the static Ud/Idcharacteristics, let us redefine the sign ofthe voltage across the inverter Ud2 so thatit is positive when the line is positive. Thismeans that the characteristics for theinverter shown in Figure 6 should be turnedaround the Id -axis.By doing so, the diagram shown in Figure 7is obtained and if we disregard the DC linevoltage drop (Ud1 - Ud2 ) we notice that anoperation point A has been established.Here the rectifier controls the currentand the inverter determines the voltageby the constant extinction angle g control.

Ud

Id

Static Ud/Id characteristics

I01I02

RectifierInverter

I01'

B

A

s t a t c h 2 . v s d

Figure 7

3.3. Power reversal bychanging the relation betweenthe current ordersThe Figure 7 is also used to illustrate thepower reversal of an HVDC transmission.The current order for the rectifier isdecreased from I01 to I01 ' so that I01 '

U

ID

ORDER

UMIN

ALPHA MIN

= f(t)

t

AMIN

IO

UD

HVDC Training (cfc.pre)

Figure 13

- The AC voltage has to reach acertain level (UMIN, correspondingto a approximately 5 degrees atnormal voltage) across the thyristorvalve to enable firing. For inverteroperation, the value (ALPHA MIN)is set to approximately 90 degrees,to prevent reversed voltage andthereby reversed power.

- Predictive extinction angle(AMIN)control ensures that the extinctionangle g is kept above the minimumvalue, normally 17 degrees, tominimize the risk of commutationfailures.

4.1.4. Control Pulse Generator(CPG)The firing control orders the control pulsegenerator (CPG) to generate the gatecontrol pulse signals individually to everyvalve in the converter.

In normal current control the a order fromthe current control amplifier CCA isdirectly turned into the correct phaseposition of the gate control pulsesignals.The CPG distributes the controlpulses to the correct thyristor valves. Foreach valve, one control pulse is sent, i.e.for a twelve pulse bridge, 12 pulses aresent per cycle. Furthermore, block, blockwith the bypass pairs, deblock and selectionof the bypass pairs are also performed in

13

this system. The orders are received fromeither the pole sequences or from theprotections.

4.2. Converter firing controlOperation modesAs discussed earlier, the current order I0 tothe inverter is lower by the current marginD I than that in the rectifier. Thus, thecurrent delivered by the rectifier is higherthan the current required by the inverter.The inverter tries to counteract that byincreasing the firing angle a . If the rectifieris capable of forcing the ordered currentthrough the inverter, the a will reach itsmaximum value, determined by theminimum extinction angle g and the CFCwill operate in the extinction angle gcontrol mode of operation.

If the rectifier is not having sufficiently highmaximum voltage, for instance caused by alow AC voltage, the inverter will take overthe current control. The current through therectifier will now be lower than the current

order, by the current margin D I. To beable to deliver the ordered current theintent is to decrease a as far as possible.Thus, the minimum delay angle a modeof operation is obtained from the currentcontrol amplifier output.

The CFC is thus able to operate inwhichever of the following modesdetermined by the CCA:

minimum delay a angle control

constant DC current control

minimum extinction angle g control

constant DC voltage control

14

5. OTHER HVDC CONTROL FUNCTIONS

Overview of Voltage and Angle ReferenceCalculation (VARC)

CFC CFCTCC

VARC

TCC

VARC

ld Rd

UdR Udl

ld ld

ALPHA/UDREFTCOM

GAMMA/UDREF

GAMMA/UDREF

GAMMAALPHA

ALPHA/UDREF

traoverw.vsd

Figure 14

5.1. Voltage and anglereference calculation (VARC)The objective of the Voltage and AngleReference Calculation function (VARC) isto ensure that the DC voltage Ud, theextinction angle gamma g and the firingangle alpha a will be within designlimitations during the steady stateconditions. This is done by calculatingreference values for the DC voltage,gamma g and alpha a , which are then sentto the Tap Changer Control (TCC). TheDC voltage and the gamma referencevalues will also be sent to the converterfiring control (CFC).

The Ud, Udi0 and angles are coordinatedbetween the two stations for variousoperation modes and power levels.

The DC voltage, which is the same in bothstations, except for the voltage drop acrossthe DC line resistance, is controlled by the

(VARC). Telecommunication is used tocalculate the resistance of the line. Sincethe DC voltage is controlled in one station,the resistance is used to ensure a propervoltage control. During telecommunicationoutages the calculated resistance value isfrozen.

5.2. Tap Changer Control (TCC)The Tap Changer Control, (TCC) systemis designed to control the Load TapChangers of the converter transformers.The objective of the (TCC) is to keep theordered alpha a , gamma g and DC voltageat the preset values determined by the(VARC).

The tap changers operate much slowerthan the basic control function, acting onthe control angle a. One tap changer steptakes some seconds to execute. Thus,there is no risk for interference betweenthe basic converter control function and thetap changer control systems. One step

15

gives a change of 1-1.5% of rated value inthe valve side voltage.

The tap changers in the inverter in theextinction angle g control are normally usedto control the voltage on the DC line. Thevoltage response measured by a voltagedivider is compared to an order, Udi0. For asignificant difference between the twosignals, the tap changer control system(TCC) orders increase or decrease of thevalve side voltage. Because of thestepwise character of the tap changer, thecontrol system must be provided with adead band. To bring the voltage back to thereference value and thus avoid hunting, thedead band should have a width of at leastone tap changer step.

When the inverter takes over the currentcontrol, the DC voltage tap changer controlmust be locked.

The voltage in the rectifier end of the linecan be controlled by adding a proportionalamount of the line voltage drop, Rd *Id tothe measured inverter end voltage.

In the rectifier the (TCC) is normally usedto keep the firing angle a as close aspossible to the rated value, which normallyis chosen to 15 degrees. To keep the termUdio *cos a in the expresssion (6)constant, at varying rectifier AC voltage,the current control system responds to it bychanging a . The (TCC) system in therectifier compares an a response signal toa reference value and at a significantdeviation it orders the tap changer to stepand change Udio. The controlcharacteristics must include a dead bandalso here and step of 1.25% from ratedvalue corresponds to a region from 12 to17.5 in a .

5.3. Reactive Power Control(RPC)The purpose of the Reactive PowerControl (RPC) is to control properties inthe AC network connected to theconverter station. The quantity to be

controlled is either the AC bus voltage (U-control) or the reactive power exchangewith the AC system (Q-control). Toprevent excessive harmonics to enter intothe AC system the (RPC) should alsomake sure that a sufficient amount offilters is connected. These tasks areperformed by switching on and off the ACfilters and shunt banks.

The harmonic filtering properties will besupervised by a function (Min filter) in the(RPC), that monitors the number of theconnected filters. This number is comparedto the number of filters, that is required forthe present operating situation. To makesure, that the filtering performancerequirements are met, the necessaryamount and type of filters will depend onthe operation mode and on the DC powerlevel. To avoid overloading the filter, ahigher priority function (Abs Minfilt) isincluded.

To avoid overvoltage, two functions areimplemented in the (RPC) called Q-maximum and U-maximum. Thesefunctions disconnect the AC filters and theshunt capacitors to prevent a protectiveaction at overvoltage. The functions arealso active in the manual mode. The Q-maximum acts to prevent an overvoltage tooccur, at for instance a pole trip, while theU-maximum disconnects AC filters toreduce an already existing high ACvoltage.

In Q-control, the RPC will switch the ACfilter banks to keep the reactive powerexchange with the AC network to a presetvalue and within specified limits.

In U-control, the AC voltage will becontrolled directly by the RPC AC filterswitchings.

The Abs Minfilt has the highest priority ofthe RPC functions, since it prevents fromoverloading the filters and thereby alsofrom filter and pole trips. The U- and Q-maximum functions have second and thirdpriority, since they protect against the

16

overvoltage. The fourth priority is the Minfilter function and finally, the controlfunctions have the lowest priority.