Power Flow Participation by an Embedded.pdf

8/9/2019 Power Flow Participation by an Embedded.pdf

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AbstractRenewable electric energy generation is

continuously increasing worldwide. Because of its climatic basic

requirements for efficient operation there will only be a few

spots for renewable bulk power generation. While locations of

load centers will not change significantly and power generation

will likely be decentralized, the average distance between them

will considerably increase. Simultaneously, the demand for

bulk power long distance transmission grid is increasing. One

possible solution is an HVDC based meshed overlay (MTDC)

grid. An essential inherit capability of MTDC grids is

controllability of its ac/dc coupling points. Thus ancillary

services as load frequency control have to be actively sustained

by pro rata load flow participation of the MTDC grid to

continuously ensure system stability. To participate in those

and other load flows, a method for MTDC grids is presented.

Numerical case studies show that a MTDC overlay grid with

the developed operating scheme relieves the ac grid by an

adjustable amount of power flows and improves the load

frequency control e.g. in terms of faster fault clearing and

smaller frequency deviations.

Index Terms--Load frequency control, HVDC, MTDC,

Phasor Measurement Unit, Wide Area Measurement System

I. INTRODUCTION

HE efforts of reducing carbon dioxide emissions and therisks of nuclear power plants (PP) shown by the accident

of Fukushima in 2011 result in installations of new large-scale PP powered by renewable energies. In addition, moreand more conventional PPs in Europe are being de-energized. In particular, offshore wind farms in the north andwest of Europe will be built while the centers of load remainunchanged. The mean distance between generation andconsumption increases due to this tendency. The existingtransmission grid was not designed for bulk power longdistance transportation and has to be adapted to theaforementioned challenge. One reasonable solution is a highvoltage direct current (HVDC) overlay grid taking powertransmission pro rata. Voltage Source Converters (VSC)represent coupling stations between the existing ac and anHVDC overlay grid [1]. A dc grid does not participate innatural branch power flow distribution as an ac grid does.Hence for integration of VSCs into an ac grid operation,they have to be controlled. In particular this affects ac

A.-K. Marten is with the Power System Department, IlmenauUniversity of Technology, Ilmenau Germany ([email protected]).

D. Westermann is with the Power System Department, IlmenauUniversity of Technology, Ilmenau Germany ([email protected]).

ancillary services as load frequency control ininterconnected ac grids which guarantees power equilibriumat every point in time. An HVDC overlay grid integrationinto ac operating scheme is the only possibility for theMTDC to take part in ac system ancillary services such asload frequency control. A new method for participation of anHVDC grid in load frequency control caused power flows ofan interconnected underlaying ac grid is presented in thispaper.

II. OVERLAY GRID

An electric power network is called an overlay grid whenit is designed to feed bulk power over long distances. Toavoid preventable losses it has to be operated on a highernominal voltage level than the underlying grid which isdesigned for smaller distance power transmission and issupposed to be built in terms of system hierarchy in top ofan existing bulk power transmission grid.

Generally there are two technologies to realize an overlay

transmission grid: ac and dc. It is necessary since there is nodirect infeed to control the coupling points between theexisting and the overlaying transmission grid. Thischaracteristic is inherent to the dc technology but would benecessary to add with additional and expensive equipment incase of an ac overlay grid. In addition to other pros (someare described below), this active converter controllabilitymakes the dc solution the better one in this essentialcharacteristic.

Furthermore, a meshed pan-European HVDC grid withrequired node-to-node-distances of some hundred kilometershas additional advantages if the lines are directly in the

ground. The capable transmission distance for undergroundac lines is very short, or those transmission lines needreactive power compensation. Another ac undergroundtransmission technology is given by gas insulated lines(GIL). They have a much lower reactive power demand thanac cables, but their maximum ac transmission distance ofabout 300 km [2] will not meet the requirements for longdistance underground power transmission. However, thislimitation does not exist when utilizing dc GIL.

Ac and dc transmission overhead lines offer the highestvoltage rating as state of the art with almost 1100 kV.However, the absence of public acceptance makes an

overhead line solution in ac or dc technology almostimpossible, even when this solution turns out to be the mosteconomic one.

In conclusion, a dc solution has the most advantages atleast with partial underground transmission. A dc grid

Power Flow Participation by an EmbeddedHVDC Grid in an Interconnected Power System

A.-K. Marten, Student Member, IEEE, and D. Westermann, Senior Member, IEEE

T

2012 3rd IEEE PES Innovative Smart Grid Technologies Europe (ISGT Europe), Berlin

978-1-4673-2597-4/12/$31.00 2012 IEEE


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consisting of more than two converter stations is called amulti terminal direct current (MTDC) grid. For this kind ofdc grid topology voltage source converters (VSC) areneeded. An example for a MTDC grid topology, forGermany and its neighboring countries is given in figure 1.This grid topology is described in more detail in chapter III.

This paper presents an operating scheme for a meshedHVDC grid. It is subdivided into two sub-functions:

Control method for participation of a MTDC grid inpower flows which can be caused, among others, by loadfrequency control

DC voltage control without need of a slack converter

The proposed control method is embedded into anoverall control regime for MTDC grid control. The dc nodevoltage control is essential to warrant power equilibrium ofthe entire dc grid and normal operating conditions. The dcvoltage stability is analog to the frequency in ac grids.

A. Automatic Participation in Load Flows Voltage AngleGradient Method

The necessity for participation of an HVDC scheme inload frequency control caused power flows was alreadyidentified. Hence some approaches for this challenge havebeen described in literature [3]-[8]. All existing methods arebased on frequency deviation at the converters ac couplingpoint as a controlled variable and do not consider aninterconnected ac grid which can be relieved by a MTDCgrid. By using this method alone, the participation inprimary load frequency control can be achieved. The amountof secondary control caused power flows which aretransmitted through the MTDC grid decreases as the

frequency deviation becomes smaller. Since this methoddoes not provide any information about load flow direction,tertiary control caused and other power flows will not betransmitted through the MTDC grid without extra provisionsin the control system of the converter stations.

Beside frequency control caused power flows, an actransmission grid is naturally loaded e.g. by transportation ofelectric energy to centers of load. A possible approach torealize a participation of MTDC grid in power flows of anyorigin is to detect general power flow directions and toengage MTDC in those. The basis is given by equation (1)describing the power flow/angle relationship in transmission

grids with R/X


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Fig. 2. HVDC grid management system including the participation in load frequency control caused power flows realized by AGM.

power equilibrium and the desired dc voltage concurrently as aslack within power flow calculations.

The main disadvantages of having a slack is that the slackhas to be fed by a very strong ac grid and the slack always hasthe same locally fixed impact on the underlaying ac grid powerflow pattern. Furthermore, there is no need for having a dcnode (slack) with a defined voltage of 1 p.u.. When no slack isdefined, all converters of a MTDC grid balance power in-feedat once in order to keep the dc node voltage in a defined range.The node voltage can be controlled by the converters, each

having a voltage-power-characteristic according to figure 3.

III. CASE STUDIES

To show the effect of the proposed HVDC systemmanagement method (figure 2) on the ac grid, its power flowsand the load frequency control in general, a reference gridbased on [11] and [12] has been chosen for numerical casestudies. It is an example of a grid representing Germany and itsadjacent states. Germany is the biggest load in the ENTSO-Ecentral Europe area, is geographically located in the middle ofEurope, and increases power generation nowadays fromrenewable sources in a unique way. The German part of the

MTDC grid is represented by six and the neighboring states byfour VSC converter stations, as shown in figure 1. The existingac grid is reduced to 6 German PP and 7 PP of the adjacentstates. To a certain extent the location and number of the VSCstations reflect the geographical location of load centers inGermany, where strong ac network nodes can be expected aswell. However, an HVDC overlay network will never be a soleGerman solution. With respect to the required transmissioncapacity (e.g. two digit GW range) of each branch, it is clearthat the MTDC grid must be built all over Europe. Thereference network presented here refers to a nucleus for a Pan-European overlay network. The scenarios are valid for all

potential future stages of a MTDC grid, since

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-4-2

02

4

6

8

converterpower[GW]

dc node voltage [kV]

P_ref = 4.5GW P_ref = 2.5GW P_ref = 0GW

P_ref = -2.5GW P_ref = -4.5GW

Fig. 3. P-V-converter characteristic for dc voltage control.

they address the very basic problem of how to control theconverter stations in order to automatically participate in loadfrequency control caused power flows. This can be seen as onemajor function that enables the seamless integration of anoverlay MTDC grid into an interconnected ac grid, namely theENTSO-E central Europe area.One of the numerical case studies demonstrates the systembehavior in case of a PP trip and the proximate load frequency

control. This scenario reflects a station trip of PP 6 and itrepresents a standard scenario of load frequency control. Ascan be seen in figure 4, the left PPs of the disturbed controlarea (Germany) provide the power deficit caused by the trip.PPs of other control areas only take part in primary loadfrequency control. One of the main duties of the proposedHVDC management system is to let the MTDC grid activelyparticipate in load frequency control caused power flowtransmission. Consequently, converters inside the disturbedcontrol area participate in resulting power flow changes pro

rata while the others are only supporting primary controlpower flows (figure 5).


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Fig. 4 Scenario 1: Conventional PP trip PP.

Fig. 5 Scenario 1: Conventional PP trip converters.

With a look at frequency deviations, it can be seen that theyare eliminated in a very short time after the disturbance (figure6; single frequencies of all PP). That is faster than without thedc grid and its control, due to the higher transmission capacitygiven by the MTDC grid and its fast reaction to power flowdirection changes.

The second scenario has been chosen in order todemonstrate the management systems capabilities in case of aline trip. This multiple line trip causes an isolated network thatcan be fed by the overlay grid (figure 7). It means that all lineflows of disconnected lines should be taken by the MTDCgrid. This would prevent the voltage angle differences from

Fig. 6 Scenario 1: Conventional PP trip node frequencies of all PP andtrumpet curve defined by UCTE operation handbook [13] for maximumfrequency deviation at PP6 blackout node (yellow dotted line).

Fig. 7 Scenario 2: Isolated network connected with the interconnectednetwork by the overlay grid.

Fig. 8 Scenario 2: Isolated network connected with the interconnectednetwork by the overlay grid PP.

Fig. 9 Scenario 2: Isolated network connected with the interconnectednetwork by the overlay grid voltage angles over affected lines.

Increasing as much as they would in case of no connection viathe overlay grid. Hence correction of disturbances in theisolated network can be made by all power plants/converters ofthe whole interconnected network compared to operationswithout an overlay network.This scenario shows the ability of the proposed method tocarry ac load flows in case of ac disturbances with the effectthat no further negative impacts occur for the interconnectedac grid. Figure 8 shows that in case of a PP blackout in theisolated area, which occurs simultaneous to the area isolation,no PP of the other control areas is affected, except a shortperiod during primary control. Figure 9 shows the positiveimpact of the proposed method toward the voltage angles overthe affected lines. All voltage angles are limited anddisturbance caused peaks of voltage angles are removed by the


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MTDC control due to corresponding dc line flows. This can beseen in figure 10, where the relevant converter power valuesare shown.

To show the ability of the MTDC control to keep the effectof a power flow change on a single ac line and to relieve this

line, scenario 3 is defined. It is a change in the PP schedule oftwo neighboring PP (figure 11). To control the resulting powerflow to be on a dedicated line or corridor, a phase shiftingtransformer is used. Figure 12 shows that the phase shiftingtransformer keeps the resulting power flow between those PPon this dedicated line. Figure 13 illustrates the support by thedc grid/converter stations in this situation. Onlythe two closest converter stations to the PP schedule changesupport the extra loaded line. Hence change in PP scheduleonly affects the dedicated ac line with the phase shiftingtransformer and only those converters participate in thoseadditional power flows which are directly connected to this acline.

ConverterPowerG

]

Fig. 10 Scenario 2: Isolated network connected with the interconnectednetwork by the overlay grid converter power values.

Fig. 11 Scenario 2: Power flows caused by a change of PP schedule PP.

Power flow between PP4

and PP5 with integrated

phase shifting transformer

300

50

00

!50

!00

50

0

"ransmissionCapa#it$[%W]

!000 !500 000 500"ime [s]

&vergae of remaining

power flows

Fig. 12 Scenario 2: Power flows caused by a change of PP schedule.

Fig. 13 Scenario 2: Power flows caused by a change of PP schedule converter power.

IV. CONCLUSION

Efforts concerning climate protection and reduction ofcarbon dioxide emission are leading to an increasingpercentage of renewable energies in power generation. At thesame time, conventional PP are shut down and averagedistances for energy and control power transportation areincreasing. To meet that challenge, a pan-European griddevelopment is necessary. A possible technological solutionwith economical and technical benefits is an HVDC basedoverlay grid. The inherent controllability of the VSC isnecessary for the operation of an overlay grid to distributepower from renewable energies in a definite way. Its couplingstations to the existing ac grid also have to be controlled inorder to realize a participation of the MTDC grid in power andcontrol power transmission. The controlled variable of existing

methods for participation of an overlay grid in control powertransportation is the frequency deviation at ac connectingpoints of converters. This method only contributes toparticipation in power flows caused by primary control. Powerflows of other load frequency control levels or even othercauses are not considered since there is no information aboutload flow direction. In this paper a novel method is proposed,which is based on voltage angle gradients 'and named theangle gradient method. ' can be measured by PMUs andcalculated by a WAMS. The approach presented here utilizesthe strong angle/active power relationship over a transmissionline. Consequently, power flows of any cause can be detected,

and the MTDC grid can be controlled in order to transport afraction of these flows via the overlay network. Case studiesshow functionality, performance and stability of this methodnot only during load frequency control. It leads to a loadreduction for the ac grid and a better performance of the loadfrequency control.

V. REFERENCES

[1] CIGR WG B4.52, HVDC Grid Feasibility Study - Interim Report,10/2011.

[2] R. Woschitz, Hchstspannungsbertragungsleitungen fr dieVerlegung in langen Tunneln, 10. Symposium Energieinnovation, Graz,2008.

[3] T. Vrana, R. Torres-Olguin, B. Liu, and T. M. Haileselassie, The North

Sea Super Grid - A Technical Perspective, 9thIEEE IET InternationalConference on AC and DC Power Transmission, London, 2010.


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[4] L. Fan, Z. Miao, and D. Osborn, Wind Farms With HVDC Delivery inLoad Frequency Control,IEEE Transactions on Power Systems, 2009.

[5]

C. Zhao, L. Li, G. Li, and C. Guo, A Novel Coordinated ControlStrategy for Inproving the Stability of Frequency and Voltage Based onVSC-HVDC, IEEE Thirt International Conference on Electric Utility

Deregulation and Restructuring and Power Technologies, Nanjuing,2008.

[6]

L. Xu, L. Yao, M. Bazargan, and Y. Wang, The Role of MultiterminalHVDC for Wind Power Transmission and AC Network Support, IEEEPower and Energy Engineering Conference - Asia-Pacific, 2010.

[7]

G. Adam, O. Anaya-Lara, and K. Lo, Grid Integration of OffshoreWind Farms using Multi-Terminal DC Transmission Systems(MTDC), IEEE 5th IET International Conference on PowerElectronics, Machines and Drives, Glasgow, 2010.

[8] T. M. Haileselassie, and K. Uhlen, Primary Frequency Control ofRemote Grids Connected by Multi-terminal HVDC, IEEE PESGeneral Meeting, 2010.

[9] A. G. Phadke, J. Thorp, Synchronized Phasor Measurements and TheirApplications, Springer Science+Business Media, ISBN: 978-0-387-76535-8, New York, 2008.

[10] H. Kuehn, M. Wache, and R. Krebs, Wide area monitoring withsynchrophasors German experiences, IEEE PES General Meeting,Minneapolis, 2010.

[11]

ETG Task Force Infrastruktur, VDE-Studie: Strombertragung fr denKlimaschutz - Potentiale und Perspektiven einer Kombination vonInfrastrukturen, 05/2011.

[12]

Asplund, G.; Jacobson, B.; Berggren, B.; Linden, K., ContinentalOverlay HVDC-Grid, CIGR Conference, Paris, 2010.

[13] UCTE (ENTSO-E), UCTE Operation Handbook, Policy 1 & AppendixA1, 2009.

VI. BIOGRAPHIES

Anne-Katrin Marten (M11) received her M.Sc. inElectrical Power and Control Engineering in 2011

at the Ilmenau University of Technology, Germany.In 2011 she joined the power system department atthe Ilmenau University of Technology as a scientificassistant. She is working on research projectsrelated to design, control and operation of futurepower systems with special interest on HVDC grids.

Dirk Westermann (M94SM05) received hisdiploma degree in Electrical Engineering in 1992and his Ph.D. in 1997 at the University ofDortmund, Germany. In 1997 he joined ABBSwitzerland Ltd. (Power Systems) where he heldseveral positions in R&D and TechnologyManagement. He became a full time professor andhead of the power system department at theIlmenau University of Technology in 2004. Therehe has been the director of the Institute of

Electrical Power and Control Technologies since 2005. His current researchinterests are related to design, control and operation of future power systems.He is an active member of IEEE and CIGRE and author of variousinternational scientific publications.

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