UNIT III
MULTITERMINAL DC SYSTEMS
Introduction – Potential Potential applications of MTDC systems - Types of MTDC systems Control and protection of MTDC systems - Study of MTDC systems.
3.1 Introduction
HVDC transmission systems designed and operated so far are point to point systems with two terminals con!erter stations". # multiterminal DC MTDC" system has more than two con!erter stations$ some of them operating as rectifiers and others as in!e in!ert rters ers.. The The simpl simplest est way way of %uil %uildi ding ng a MTDC MTDC syst system em from from an e&is e&isti ting ng two two terminal system is to introduce tappings. Parallel operation of con!erters and %ipoles can also %e !iewed as multi terminal operation.
'nli(e in #C systems$ the tas( of e&tending two terminal systems to multiterminal syst system emss is not not tri!i ri!ial al.. The com comple& ple&it itie iess of cont contro roll and and prot protec ecti tion on incr increa ease se considera%ly$ and the use of HVDC %rea(ers is generally re)uired in the MTDC systems.
In recent times$ with growth in HVDC transmission$ there could %e more than one con!erter in pro&imity feeding same load area. The operational issues of such Multi-Infeed DC MIDC"system MIDC"systemss are similar to those of MTDC systems. systems. *ith *ith recent ad!ances in the emerging technology of VSC-HVDC transmission$ the application of MTDC systems is %ecoming more attracti!e than %efore.
3.2 Potential alication! o" MTDC !#!te$!
#part from tapping of power from e&isting two terminal systems$ there are three specific areas of applications for MTDC systems. These are listed %elow+
,. ul( ul( powe powerr tran transm smis issi sion on from from se!e se!eral ral remo remote te gene genera rati ting ng stat statio ions ns to se!eral load centres.Here$ each generating plant or unit" is connected directly to a rectifier station there%y dispensing with the #C collector system. Similarly$ a con!erter station at each load centre eliminates the need to %uild additional #C or DC" lines for fle&i%le energy e&change. #n MTDC system has se!eral ad!antages o!er the alternati!e of point to point systems. or e&le$ consider a system of two generating stations and two loads as shown in ig./.,.This is a radial system with two rectifiers and two in!erters. or ensuring the same le!el of fle&i%ility in energy e&change$ three two terminal DC lin(s will %e re)uired in addition to a lin( connecting the two recei!ing systems$ which could %e #C or DC see ig./.0". This would result in e&tra costs for the con!erter stations$ lines and additional power losses in increased num%er of con!ersions.
fig /., MTDC system configuration for %ul( power transmission
fig /.0 ul( power transmission using two terminal lin(s
The elimination of #C collector system at the remote hydro generating stations can result in %etter efficiency in the operation of hydraulic tur%ines which are free to run at speeds independent of the system fre)uency. 0. #synchronous interconnection %etween ad1acent power systems. *hen more than two systems are in!ol!ed$ a MTDC system for interconnection is more fle&i%le and economical than employing se!eral two terminal DC lin(s.
/. 2einforcing of an #C networ( which is hea!ily loaded. Consider an ur%an power system which is fed %y a distant power station. It would %e ad!antageous to arrange the power in1ection at more than one point so that the underlying #C networ( is not o!erloaded. This is easily achie!ed using a MTDC system with one rectifier station and se!eral in!erter stations see ig././".
fig /./ 2einforcing of #C networ( using MTDC lin( 3.3 T#e! o" MTDC !#!te$!
There are two possi%le types of MTDC systems i" Series ii" Parallel The parallel MTDC systems can %e further su%di!ided into the following categories+ a" 2adial %" Mesh
%i& Serie! MTDC S#!te$
This is a natural e&tension of the two terminal system which is a series connected system. # three-terminal MTDC system is shown in ig./.3. This shows a monopolar arrangement4 howe!er$ a homopolar arrangement with two conductors is also possi%le. The system is grounded at only one point which may %e con!eniently chosen. If the line insulation is ade)uate$ the grounding point can %e shifted$ %ased on changes in the operating conditions. 5rounding capacitors may also %e used to impro!e insulation coordination and system performance during transients.
ig /.3 - # series connected MTDC system In a series connected system$ the current is set %y one con!erter station and is common for all the stations. The remaining stations operate at constant angle e&tinction or delay" or !oltage control. In order to minimi6e the reacti!e power re)uirements and the losses in !al!e damper circuits$ the normal operating !alues of firing angles may %e ad1usted using tap changer control. #t all times$ the sum of the !oltages across the rectifier stations must %e larger than the sum of the !oltages across the in!erter station. In case of a drop in the !oltage at the current controlling rectifier station$ the in!erter with the larger current reference ta(es o!er the current control.
The switching in or out of a %ridge is accomplished %y de%loc(ing7%loc( and %ypass in a manner similar to that in a two terminal system. The clearing of a fault in the DC line is also similar. The power re!ersal at a station is also done as in a two terminal system$ %y re!ersing the DC !oltage %y con!erter control.
The power control in a two terminal system is accomplished %y ad1usting the current while trying to maintain a constant !oltage in the system. This is done to minimi6e the losses. Howe!er$ in a MTDC series system$ central control would %e re)uired to ad1ust the current in response to changing loading conditions. The local control of power would imply ad1usting !oltage at the con!erter station using angle and tap controls. 'sing only one %ridge or a ,0 pulse unit for the !oltage control and operating the remaining %ridges at minimum or ma&imum" delay angle can reduce the reacti!e power re)uirements.
%ii&Parallel MTDC S#!te$
Here$ the operating philosophy of constant !oltage #C systems is e&tended to DC systems. The currents in all the con!erter stations e&cept one are ad1usted according to the power re)uirement. 8ne of the terminals operates as a !oltage setting terminal at constant angle or !oltage. #n e&le of / terminal radial system is shown in ig./.9. This shows a monopolar system %ut %ipolar arrangement would %e normally used.
ig /.9 : # parallel connected radial MTDC system # radial system is one in which the disconnection of one segment of transmission would result in interruption of power from one or more con!erter stations. In a mesh system$ the remo!al of one lin( would not result in a disruption$ pro!ided the remaining lin(s are capa%le of carrying the re)uired power with increased losses". ;!idently$ a mesh system can %e more relia%le than a radial system. #n e&le of a 3 terminal mesh system is shown in ig. /.<.
fig /.< : # parallel connected mesh type MTDC system
The power re!ersal in a parallel MTDC system would in!ol!e mechanical switching as the !oltage cannot %e re!ersed. #lso$ loss of a %ridge in one con!erter station would re)uire either the disconnection of a %ridge in all the stations or disconnection of the affected station.
3.3.1 Comparison of Series and Parallel MTDC Systems
The ad!antages and disad!antages of series and parallel MTDC systems are gi!en %elow+ ,. High speed re!ersal of power is possi%le in series systems without mechanical switching.This is not possi%le in parallel systems. 0. The !al!e !oltage rating in a series system is related to the power rating$ while the current rating in a parallel connected system is related to power. This would imply that for small power ratings of the tap$ series connection may %e cheaper e!en though !al!es ha!e to %e insulated for full !oltage to ground. The parallel connection has the ad!antage of staged de!elopment in the con!erter stations %y adding parallel con!erters as the power re)uirements increase. /. There are increased losses in the line and !al!es in series systems$ in comparison to parallel systems. The system operation in series systems can %e optimi6ed %y operating the largest in!erter at rated !oltage. 3. Insulation coordination is a pro%lem in series systems as the !oltage along the line !aries. 9. The permanent fault in a line section would lead to complete shutdown in a series connected system$ while it would lead to only the shutdown of a con!erter station connected to the line section in a radial MTDC system. *ith pro!isions for fast identification and clearing of faults in mesh connected system$ there is no disruption of power transfer.
<. The reduction in #C !oltages and commutation failures in an in!erter can lead to o!erloading of con!erters as current is transferred from other terminals in a parallel system. The pro%lem is se!ere if the rating of the in!erter is relati!ely small. Increased !alues of smoothing reactor and !oltage dependent current limits can reduce the se!erity. Howe!er$ the !al!e ratings would increase$ resulting in increased unit costs. # recent study shows that the cost of a 9== M* DC e)uipment at >9== (V" would %e ?3@ of the cost of a ,=== M* DC e)uipment . It is concluded that a practical limit to une)ual in!erter ratings may %e ?9@ + 09@. ?.The control and protection philosophy in a series MTDC system is a natural e&tension of that in a two terminal system. Howe!er$ e&tension to parallel systems is not straightforward. Increased communication re)uirements and pro%lems in reco!ery from commutation failures are associated with parallel systems. HVDC %rea(ers of appropriate rating may %e re)uired for clearing faults in the DC line or con!erter stations.
rom the relati!e merits and demerits of series and parallel MTDC systems descri%ed a%o!e$it may %e concluded that series connection is appropriate for taps of rating less than 0=@ of the ma1or in!erter terminal. Parallel connection is more !ersatile and is e&pected to %e widely used as in #C systems. The first application of a MTDC system is the Sardinia-Corsica-Italy lin( where an e&isting lin( %etween Sardinia and Italy is tapped at Corsica. This is a 9= M* parallel connected tap with two ,== (V si& pulse thyristor %ridges connected in series. # series tap was re1ected for two reasons i" the operating current due to fre)uency control can %e as low as ,=@ of the rated current. This increases the !oltage rating of the series tap. ii" # series tap in in!erter operation reduces !oltage at the main in!erter terminal$ re)uiring increased e&tinction angles. This is harmful to mercury arc !al!es as the pro%a%ility of arc-through increases.
Commutation failure at Corsica can result in o!ercurrents of ? p.u. Smoothing
reactors of 0.9H are chosen to limit the o!ercurrents due to distur%ances in the #C system.
Aue%ec-Bew ;ngland lin( from 2adisson in Aue%ec to Sandy Pond in Massachusetts with a con!erter at Bicolet %ecame operational in ,0 with two in!erters and one rectifier of capacity 009= M*. *hile it was planned to integrate an e&isting two terminal HVDC lin( from Des Canton to Comerford" with this system to form a 9 terminal MTDC system$ this plan was dropped. 8ther plans to introduce MTDC system was also shel!ed. The set%ac( to planning for MTDC was primarily due to the inade)uate preparation in the de!elopment of control and protection concepts re)uired for relia%le system operation.
3.' Control and Protection o" MTDC !#!te$!
There are se!eral methods of control in MTDC systems. 8nly parallel MTDC systems areconsidered as these in!ol!e comple&ities in e&tending the e&isting control methods. The !arious methods suggested are re!iewed %elow.
%i&Current Mar(in Met)od
This is most widely considered and the natural e&tension of control philosophy in a two terminal system. 8ne of the con!erter station which is operating at the angle limit minimum or minimum E" determines the DC !oltage dependent on the #C !oltage and the tap ratio". The remaining terminals operate as current controlling terminals. The current through the !oltage setting terminal say n" is gi!en %y
where n is the num%er of terminals. In the a%o!e e)uation$ the in!erter currents are treated as negati!e$ while the rectifier currents are treated as positi!e. The current controlling terminals operate with a !oltage margin which may %ecome 6ero or negati!e during distur%ances in the #C system. #s e!en small distur%ances can affect
the !oltage margin$ it is necessary to maintain the current and power distri%ution in the system with minor changes$ during the distur%ances. This is possi%le if current control is also pro!ided at the !oltage setting terminal or slac( terminal" such that it tries to maintain the same current as %efore. ecause of measurement errors and the re)uirements of a smooth transition from angle or !oltage" control to current control$ the current reference at the !oltage setting terminal VST" is chosen to satisfy the following e)uation+
The con!erter with the lowest !oltage ceiling always acts as a !oltage setting terminal. The changes in the !oltage setting terminal due to distur%ances in the #C system are called mode shifts. 'ncontrolled mode shifts can %e minimi6ed %y selecting a terminal with highest short circuit ratio as the !oltage setting terminal. Due to the negati!e resistance characteristics of the constant e&tinction angle control$ it would %e ad!isa%le to choose a rectifier terminal as VST. The magnitude of the current margin is critical as con!erters of lower ratings can %e o!erloaded when operating at angle limit.
The central controller that regulates the current orders at all the con!erter stations is termed as Current 2eference alancer C2" and is shown in the analog !ersion in ig./.?. Here$ the current orders calculated from local power controllers are ad1usted in order to satisfy ;). 0". The limits on the current orders are ta(en into account in %alancing current references.The actual implementation of C2 can %e performed %y using microprocessors.
Satisfactory operation of MTDC systems re)uires a relia%le central C2 that operates at all times. This re)uires relia%le two way communication %etween a central station and each con!erter station. If there is loss of a station and this information is
not communicated$ the system operation is ad!ersely affected. In case of loss of a rectifier station$ the power transfer is interrupted %y !oltage collapse. In case of loss of an in!erter station$ other stations will %e o!erloaded.
ig /.? Current reference %alancer In the current margin method$ the change in the !oltage setting terminal re)uires the operation of the tap changer in con!erter transformer to modify the !oltage margin. This can %e slow and results in less fle&i%le control to deal with mode shifts. #n impro!ement has %een suggested %y using a modified control scheme termed as !oltage margin control method. In this method$ all con!erter stations are pro!ided with automatic !oltage regulators #V2" along with automatic current regulators #C2". In the !oltage setting terminal$ #V2 reference !oltage is set to the rated !oltage and in other stations$ #V2 reference !oltage is set higher %y an amount F;.
The operation of this control techni)ue is illustrated %y ig./.G which represents the control characteristics for a MTDC system with two rectifiers and two in!erters. *hen the !oltage setting terminal is shifted from IBV0 to 2;C,$ the !oltage margin F; is added to the reference !alue at IBV0 and su%tracted from
2;C,. The !oltage margin control method is also not free from the re)uirements of the centrali6ed control and fast communication.
ig /.G 8peration with !oltage margin control method a"VST IBV, %" VST 2;C ,
In order to facilitate the operation of MTDC systems e!en when there is failure of communication system$ the following modifications to the %asic current margin method of control ha!e %een suggested+ ,. Voltage imiting Control 0. 'se of Decentrali6ed Current 2eference alancer DC2" /. Two #C2 #utomatic Current 2egulator" method These are discussed %elow.
%ii& *olta(e Li$itin( Control
In the !oltage limiting control method$ the rectifier and in!erter characteristics are arranged as shown in ig./.. The loss of a rectifier station does not ha!e any significant effect on the system operation e!en in the a%sence of communication failure. The in!erter currents are reduced in order to pre!ent !oltage collapse. #lso$ the loss of an in!erter station operating on current control" would not o!erload the !oltage controlling in!erter %ecause of its !oltage reser!e.
ig /. 8peration with !oltage limiting method The draw%ac(s of this scheme are i" the distur%ance in the #C system connected to an in!erter station can result in other in!erters getting unloaded due to drop in the DC !oltage". This may cause ad!erse effect on the #C systems supplied %y healthy in!erter stations$ ii" Two or more terminals can operate in the !oltage controlling mode forci%ly$ in the case of loss of a terminal resulting in indeterminate distri%ution of currents in those terminals$ iii" Currents during DC line fault or commutation failure are li(ely to %e higher without additional measures.
%iii& Decentrali+ed Current Re"erence ,alancin(
Decentrali6ed Current 2eference alancing operates on the current and !oltage characteristics. DC2 is designed to allow rapid reco!ery of MTDC system operation following se!ere distur%ances which re)uire a momentary rundown of the DC system. The DC2 is supposed to permit the system reco!ery without coordinated centrali6ed control of con!erters and e!en if one or more of the terminals is remo!ed from ser!ice due to the distur%ance.
The operation of the DC2 is illustrated with the help of a / terminal system with two rectifiers and one in!erter. In the case of loss of one rectifier station$ the characteristics of the remaining two con!erter stations do not intersect see ig.,=".
The o%1ect of the DC2 is to ad1ust the con!erter control characteristics
around a preselected !oltage le!el Vo in the manner indicated in ig./.,=. The slopes of the %alancing characteristics Jr and Ji determine the relati!e ad1ustments that must %e made to the current orders. In DC2$the minimum and ma&imum current limits are preselected. The intersection of the %alancing characteristics determines the steady state operation of the con!erters following a distur%ance.The normal postdistur%ance characteristics are also shown in ig. ,= %y solid lines. The current order re)uired for this is locally determined %y the %alanced current and the specified current margin. In case of a rectifier$ a fraction of the current margin would %e added to the %alanced current$ while in the case of an in!erter$ a fraction of the margin would %e su%tracted.
ig /.,= Decentrali6ed current reference %alancing method The slopes of the %alancing characteristics along with the !oltage le!el Vo are chosen$ %ased on the considerations of !oltage limits and magnitude of the !ariations in the DC networ( !oltages during the %alancing period.
Simulator studies ha!e %een carried out to demonstrate the functioning of DC2 and it is claimed that this techni)ue is )uite satisfactory in rapid restarting of the system with communication failure.
%i-&To ACR Met)od
In this method$ each-con!erter is pro!ided with two automatic current regulators #C2" and one !oltage regulator. The rectifier and in!erter control characteristics are shown in ig./.,,.
ig /.,, : Static Characteristics of a acon!erter in two #C2 method a" 2ectifier$ %" In!erter
During the normal mode of operation$ the rectifier station with the lowest !oltage reference or the in!erter station with highest !oltage reference acts as !oltage setting terminal. see ig./.,0". The remaining con!erter stations operate under the control of #C2,. Iref, of the !oltage setting terminal is set larger than the operating current determined %y other con!erters" %y the current margin. The #C20 operates during a contingency and its order can %e fi&ed at the minimum operating current of the in!erter.
ig /.,0 Static V c - Id characteristics of a 3 terminal system *ithout additional measures$ the in!erter may not %e a%le to reco!er from a commutation failure. Voltage dependent current order limit VDC8" is introduced to o!ercome this pro%lem. The operation of a 3 terminal system %oth during normal conditions and when one of the con!erter stations is shutdown$ is shown in ig./.,/. The rectifier with the highest !oltage order and in!erter with lowest !oltage order are always controlled %y #C2,$ thus resulting in small power de!iations during a distur%ance. This can %e utili6ed to pre!ent shutdown of a %ase station such as a nuclear power station" or pre!ent interruption of supply to a critical #C system. It is claimed that the two #C2 method permits the use of DC circuit %rea(ers to isolate a line fault or con!erter fault without coordination with control. The system operation following a distur%ance is optimi6ed %y modifying the current orders from central controller.
The performance of this control techni)ue has %een studied using digital dynamic simulation and HVDC simulator.
ig /.,/ a" 8perating under normal conditions %" 8perating without 2;C , C" 8perating point without 2;C 0 d" 8perating point without IBV , e" 8perating point without IBV 0
Protection o" MTDC S#!te$!
8ne of the issues under discussion regarding MTDC system operation is the e!aluation of protection re)uirements and coordination with control. Conceptually$ the system can %e shut down following a fault in DC line or con!erter station and the faulted component isolated using high speed disconnect switches. The system can %e restarted after ade)uate time for deioni6ation of the arc path in case of short circuits". System relia%ility considerations dictate the need for fast clearing of fault with minimum distur%ance to the healthy parts of the system. The DC %rea(er ratings can %e minimi6ed %y utili6ing the inter!ention of fast current controls to reduce the magnitude of the fault currents. The detection of DC line faults gets complicated in a mesh system. # ma1or pro%lem is the large drop in the DC !oltage e!en for distance faults. This re)uires fast detection and clearing of faults to maintain power transfers. Differential type of protection or directional sensiti!e measurements of the currents can %e used to locate the fault. Communication would %e re)uired in %oth methods. DC switches would %e re)uired primarily for parallel MTDC systems and particularly in mesh systems to utili6e the impro!ed security pro!ided %y such configurations.
3./ Stud# o" MTDC S#!te$!
The planning$ design and operation of MTDC systems re)uire detailed study using simulation tools. The new control and protection concepts ha!e %een formulated using HVDC simulators. The performance of the controllers are 1udged not only under normal conditions with changing load conditions$ %ut also under distur%ances caused %y faults in the #C system and DC lines.
Some of the typical pro%lems that ha!e %een considered for study are as follows+ ,. 8peration of small in!erter taps connected to wea( #C systems 0. Integration of e&isting HVDC con!erter stations in MTDC systems without ma1or modifications in control /. ;!aluation of communication$ reacti!e power and filtering re)uirements 3. Security of power transfer without fast communication lin(s 9. Power and reacti!e power modulation strategies in MTDC systems.
The type of pro%lems that can %e anticipated in the operation of MTDC systems connected to wea( #C systems. ,. arge smoothing reactor may %e re)uired to help in the reco!ery of a small in!erter from #C faults. 0. The speed of reco!ery of the entire MTDC system depends upon the reco!ery of the small tap$ if it is still connected to the system following clearance of a fault. /. It is necessary to consider the effect of mode shifts and de!elop additional protection se)uences$ particularly for faults in DC line. 3. The pro!ision of VDC8 at each rectifier is %eneficial and its characteristics ha!e to %e ad1usted suita%ly. *ith a good (nowledge of HVDC controls and their accurate modeling$ HVDC simulators %oth physical and digital" can %e used to analy6e pro%lems and search for solutions. or e&le$during the tests for parallel operation of the two %ipoles in Belson 2i!er HVDC transmission system in Manito%a$ Canada$ in ,G9$ it was o%ser!ed that there was a current oscillation of < H6 in the two in!erters although the current in the two rectifiers was steady. The Vd:Id characteristics for the rectifiers and the in!erters are shown in ig./.,3 a" and %" respecti!ely.The operating !oltage is also shown here$ which indicates that the in!erter 0 operates in the mode of current error dependent gama E" control mode. In simulating the system using ;MTDC program with a generic modeling of the controls$ it was not possi%le to identify the
source of the oscillation" pro%lem. Howe!er$ from the (nowledge of the actual !al!e group controller pro!ided %y the manufacturer" it was possi%le to identify the source of the pro%lem to a particular K7, L sT" type delay %loc( in the actual controller. y decreasing the time constant T from 00 ms to / ms$ it was possi%le to eliminate the current oscillation pro%lem in the real system. Incidentally$ the digital program also allows one to e&periment and determine how the oscillations are initiated from a sta%le operating point. In this case$ changing the tap ratios from the set point initiated the low fre)uency current oscillations .
ig /.,3 Multiterminal Characteristics # HVDC simulator study of a four terminal MTDC system indicated the possi%ility of ad!erse interaction %etween the control and DC networ( dynamics. This can result in multiple mode shifts in rectifiers following a remote three phase fault at a rectifier . # commutation failure at the small in!erter results in the total DC current from the rectifiers" di!erted into the faulted in!erter due to !oltage drop in the in!erter caused %y the commutation failure. The transient current is e!en higher due to the discharge of line capacitances.The pea( current can %e limited partly %y the use of higher smoothing reactor. Howe!er$ it is difficult for the in!erter to reco!er e!en when the #C system is strong. The system reco!ery is made possi%le %y
initiating orce-2etard 2" on the rectifiers increasing to !alues a%o!e =$ say ,39" as done in the case of DC line faults in a two terminal system. 2 can %e initiated without telecommunications since the collapse of the DC !oltages at the rectifiers can %e used as signals forthe protecti!e action. #fter a pre-determined time$ the con!erter firing angles are returned to their normal !alues at a controlled speed. Howe!er$ there is still a danger of a su%se)uent commutation failure at the small in!erter due to current o!ershoots. The characteristics of a VDC8 at the in!erter ha!e to %e carefully chosen to a!oid this pro%lem.
