Damping of Low-Frequency Interarea

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Damping of Low-Frequency Interarea OscillationsUsing HVDC Modulation and SVC VoltageSupport

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EMP 31(4) #4046

Electric Power Components and Systems, 31:389402, 2003

Copyright c 2003 Taylor & Francis

1532-5008/03 $12.00 + .00

DOI: 10.1080/15325000390112251

Damping of Low-Frequency InterareaOscillations Using HVDC Modulation and

SVC Voltage Support

A. R. MESSINA

J. ARROYO

N. EVARISTO

I. CASTILLO T

Graduate Program in Electrical EngineeringCinvestav, IPNP.O. Box 31-438Guadalajara, Jal. 44550Mexico

This paper discusses the results of preliminary studies conducted to examinethe applicability of high-voltage direct current (HVDC) technology and staticVolt-Amp Reactive (VAR) compensation to aid damping of low-frequency in-terarea oscillations in the Mexican interconnected system (MIS). The study ofinterarea oscillations and the design of controls is done using modal analysisof a linearized AC/DC state space model of the power system at a particularoperating point. The utilized methodology is tested on a planning model of theMIS that includes a proposed HVDC transmission system embedded in an ACnetwork in which several SVCs are used for dynamic voltage support. Detailed

time-domain analyses are performed to complement knowledge on the effect ofFACTS technology on system transient behavior.

Keywords interarea oscillations, HVDC transmission, SVC

1. Introduction

High-voltage direct current (HVDC) transmission systems offer a powerful alter-native to increase the stability of a power system as well as to improve systemoperating flexibility. The application of HVDC technology is of special interest totransfer massive amounts of remote hydroelectric power over large distances [1, 2].In complex interconnected systems, however, the application of this technologyto achieve the most effective stabilization of critical interarea modes requires the

proper assessment of many interacting factors such as the location of the DC link,the control configuration, and the use of modulation controls.

Manuscript received in final form on 4 March 2002.The financial support of the Mexican Council for Science and Technology under Grant

31840-A is acknowledged.Address correspondence to A. R. Messina. E-mail: [email protected]

389


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390 A. R. Messina et al.

This paper reports on the application of HVDC technology and Static VARCompensator (SVC) modulation to aid damping of low-frequency interarea oscil-lations in power systems of essentially longitudinal structure. A planning modelof the Mexican interconnected system (MIS) is adopted to assess the applicabil-

ity of HVDC transmission to enhance transfer capabilities as well as to determinepotential adverse interactions with other controllers.

The analysis of low-frequency oscillations and the development of remedialmeasures based on Flexible ac Transmission Systems (FACTS) technology is donein the modal domain using a linearized state space representation of the powersystem. Time-domain simulations are then performed to validate the accuracy ofthe developed models. System studies focus on the analysis and control of twolightly damped interarea modes associated with the interaction of geographicallywidespread machines.

Study results include the identification of interarea modes that are more con-trollable by the proposed HVDC link, and the use of SVC voltage support andmodulation controls to enhance the damping of critical system modes. The effectsof HVDC modulation and SVC voltage support on system damping are discussedfor various control configurations and input signals. It is shown that the coordi-nated application of HVDC transmission and SVC voltage support can effectivelycontribute to the simultaneous stabilization of critical interarea modes.

2. Description of the Study System

2.1. General Structure of the MIS

The bulk Mexican interconnected system comprises a huge 400/230 kV transmissionsystem stretching from the border with Central America to its interconnection withthe United States. A one-line diagram of the study system is given in Figure 1showing major existing SVCs and the geographical location of the proposed HVDC

system.The MIS consists of six areas designated as the north (N), northeastern (NE),western (W), central (C), southeastern (SE), and peninsular systems. System stud-ies in this research are based on a reduced-order equivalent model of the MIS thatincludes the detailed representation of 49 machines, 191 buses, 183 transmissionlines, and 5 SVCs. Loads are represented as 70% constant current load and 30%constant impedance characteristics for both active and reactive power. In this modelno interconnections with the northwestern and peninsular systems of the Mexicansystem are considered. This system is considered a realistic test system to evaluatethe performance of the proposed HVDC control scheme.

2.2. The Southeastern System

The southeastern network of the MIS was singled out to investigate the capabilityof HVDC transmission of introducing damping to critical interarea modes. Figure 2depicts the general structure of the 400/230 kV SE network, showing the main ma-chines and transmission facilities. The SE network constitutes a typical longitudinalsystem, characterized by sparse, long transmission paths, and remote generationsources [3, 4]. Hydroelectric power is transmitted to the Mexico City metropolitanarea over a distance of about 900 km by a 400 kV network of essentially longitudinal


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Damping of Low-Frequency Interarea Oscillations 391

Figure 1. AC/DC system configuration.

structure. Dynamic voltage support and series capacitive compensation are pro-

vided at several key locations to allow for enhanced steady-state and dynamicsystem performance. The main local generation includes important hydro powerstations at MMT, ANG, MPD, and PEA accounting for about 3,900 MW of in-stalled capacity. In addition, three generating units of 300 MW each are expectedto come into service at MMT by the year 2002 to serve the increasing load in theMexico City metropolitan area [4]. As a consequence, several AC/DC transmis-sion and control alternatives are being currently considered to increase the powertransfer capability of this network as well as to improve system operating security.

In the present study, several operating alternatives to enhance transmissioncapabilities have been considered with an emphasis on the application of multi-FACTS compensation and HVDC technology. The study focuses on the analysis ofthe applicability of these technologies in removing present operational restrictions.

2.3. HVDC Transmission Studies

System studies were directed to assessing the impact of HVDC transmission aug-mented by SVCs on the damping of critical interarea modes. Sites identified aspotential locations for a DC link are close to the AC lines linking hydro machinesin the SE network with substations close to Mexico City. Of special interest, a DClink is considered between MMT 400 kV power station and Temascal, a 400 kV


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Figure 2. Detail of the 400 kV SE network showing the location of the HVDC linksconsidered in feasibility studies.

substation located in the electrical center of the SE region (refer to Figure 2). Ade-quate reactive power support for this alternative is available at the rectifier end bythe 300 MVAr SVC at Temascal.

For the purpose of this analysis, the HVDC system is represented as a two-

terminal, bipolar

500 kV, 1,000 MW DC line model. Each terminal has anindependent control loop. The rectifier is provided with a proportional-plus-integraltype of constant power controller with minimum firing angle control, while voltagecontrol was assumed for the inverter with minimum reactive power consumption. Ageneric control diagram of the HVDC link including the use of supplemental controlfunctions is given in the appendix. In addition to the normal rectifier control,a power system damping control (PSDC) function is included with the controlsystem to modulate the power or current setpoint (Pref, Iref) in response to signalsobtained from the adjacent AC system to improve the dynamic performance of thesystem. Inputs to the PSDC function are chosen to be rotor speed deviation fromlocal hydro machines, AC power and current from adjacent transmission lines, andfrequency difference. Table 1 summarizes the main control characteristics of the DClink considered.

2.4. The SVC at the 400 kV Temascal Substation

The Temascal static VAR compensator consists basically of two thyristor-switchedcapacitor (TSC) groups, one thyristor-switched reactor (TSR), and one thyristor-controlled reactor group (TCR) [5]. Each of these groups has a three-phase ratingof 75 MVAr, thus producing an overall dynamic range of 300 MVAr. A single-


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Table 1

Control characteristics of the DC link

DC substation AC substation Control characteristics/setting

Rectifier MMT Constant power (1,000 MW) and minimumfiring angle control

Inverter Temascal Constant voltage control (500 kV)

line diagram of the SVC is given in Figure 3. The primary voltage control loopincludes a measuring device (MD), a six-pulse rectifier, and a low-pass filter with atime constant of 1.5 ms to reduce fast transients in the bus voltage. No modulationcontrols are presently used.

The SVCs at the TOP and TEX substations have similar control characteristicsand are described elsewhere [3].

3. Small Signal Analysis of the Study System

3.1. Damping Characteristics in the MIS System

The five-area model of the MIS exhibits three critical lightly damped interareamodes at 0.4, 0.5, and 0.7 Hz [6, 7]. Table 2 lists the main characteristics of thethese modes showing their oscillation pattern and most dominant machines.

The 0.4 Hz north-south interarea mode 1 is of the utmost importance to systembehavior as it involves the interaction of most machines in the north systemsswinging against machines in the south systems. Also of concern, the 0.73 Hz east-west interarea mode 3 shows the interaction of hydro-generating plants locatedin the SE network, swinging against those in the W and C systems. Damping

characteristics of these modes are determined by generation patterns, the level ofpower transfers across the SE system, and network structure.

Figure 3. One-line diagram of the SVC at Temascal.


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Table 2

The three slowest oscillation modes

Mode Frequency Oscillation Dominantdesignation Eigenvalue (Hz) pattern machines

North-southinterareamode 1

0.0860 j2.57 0.40 North systemsvs. southsystems

MTYFALRIBMZD

Interareamode 2

0.1877 j3.66 0.58 Machines inthe northsystems

MTYMZDGPLSYC

East-westinterareamode 3

0.0675 j4.61 0.73 W and Csystems vs.SE system

SLMMNZANGMMT

Table 3

Effect of HVDC and SVC modulation on the damping of thecritical interarea modes

HVDC/SVC controlSystem condition control strategy Eigenvalue

SVC at Temascal andno HVDC

SVC in voltage control mode 0.0860 j2.570.1877 j3.660.0675 j4.61

HVDC modulation andSVC at Temascal

HVDC input signal: speeddeviation from MMT

SVC input signal: tie-line powerTemascal-PBD

0.3727 j2.800.2318 j3.661.0470 j4.71

HVDC modulation andSVC at Temascal

HVDC input signal: speeddeviation from MMT

SVC in voltage control mode

0.3143 j2.750.2230 j3.690.1207 j4.83

HVDC power modulationand SVC at Temascal

HVDC input signal: tie-linepower MMT-MPD

SVC in voltage control mode

0.3742 j3.030.2226 j3.72

0.2537

j5.45HVDC power modulationand SVC at Temascal

HVDC input signal: speeddeviation from ANG

SVC input signal: tie-line powerTemascal-PBD

0.3725 j2.800.2317 j3.701.0480 j4.71


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Figure 4. Relative rotor angles following a three-phase fault at PBD with HVDC link

on constant power control mode.

Earlier studies [7] identified several operating conditions under which FACTScompensation might increase overall system damping. In the following sections,small signal analysis is used to assess the effect of dynamic voltage support andHVDC transmission on system damping. Basic AC/DC transmission alternativesconsidered in this study included the following:

1. Two additional 400 kV AC lines with various levels of series compensation2. One 500 kV HVDC transmission line between MMT and the Temascal

substation, augmented by supplemental control functions and SVC voltagesupport

3. A combination of AC transmission and FACTS technology with an emphasison the full utilization of existing SVCs and the addition of series capacitivecompensation on the MMT-Temascal 400 kV lines.

For each transmission alternative, detailed steady-state and dynamic studieswere conducted including the representation of operating limits in machines, SVCs,and the DC link.


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Figure 5. Active power flow following a three-phase fault at PBD with HVDC link onconstant power control mode.

3.2. Effect of HVDC Modulation Control and SVC Voltage Support on

System Damping

To assess the effectiveness of HVDC transmission on system stabilization, detailed

models of SVCs and DC links were developed during the course of this research.Special care was taken to represent the different SVC control configurations usedin the MIS.

The approach in [7] was applied to determine the controllability of the criticalinterarea modes by major control devices. Results of these studies showed thatinterarea mode 1 is strongly controllable by the DC link. Further, the TemascalSVC was revealed to have a strong leverage on interarea mode 3. Based on these


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Figure 6. Relative rotor angles following a three-phase fault at PBD with HVDC mod-ulation.

studies, the control systems of the DC link and the Temascal SVC were designedto provide stabilization for both interarea modes 1 and 3.

Several different classes of measurement signals were investigated to be used asinputs to the supplementary damping control in both the SVC at Temascal and theproposed DC link. These included relative rotor deviations from selected machinesand active power flow in a nearby AC line. Because of reliability considerations,the choice of the controller input signal was limited to local rotor speed and ACtie-line real power.

Table 3 summarizes the effect of some selected AC/DC alternatives on system

damping. The results show that the addition of the HVDC link provides positivedamping on all three critical interarea modes. Of special interest, the HVDClink is seen to have a significant effect on the 0.40 Hz north-south mode whilethe SVC is revealed to influence the damping of the 0.73 Hz east-west interareamode 3. Among the several supplementary signals analyzed, speed-based signalsproved more desirable than other alternatives, which was further confirmed fromobservability studies. It is also noticed that the coordinated application of HVDC


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Figure 7. Active power flow following a three-phase fault at PBD with DC modulation.

control with a speed-based damping controller and SVC modulation, on the otherhand, leads to increased system performance. The addition of series capacitivecompensation was found to have a marginal effect on overall system damping.

Additional studies are now being conducted to investigate possible interactions

between the DC system, the Temascal SVC, and the parallel AC series-compensated400 kV transmission lines.

4. Dynamic Response to Large Disturbances

Extensive nonlinear time-domain simulations were conducted to assess the accuracyof the developed models as well as to verify potential adverse interaction with


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Figure 8. Comparison of Temascal 400 kV bus voltage response with and without HVDCmodulation.

other devices and system modes. The criterion used to assess acceptable transientperformance requires the maintenance of synchronization following the loss of amajor single transmission element.

The critical contingency is a fault at the substation PBD followed by the lossof the PBD-TEX 400 kV line. This disturbance is known to excite both the north-south and the east-west interarea modes and causes loss of synchronism of machines

in the SE system leading to severe postfault voltage swings at critical system buses.Following small signal analyses, a speed-input PSDC was designed for the

HVDC link. In this study the speed deviation from the MMT generating station wastested as the input to the supplemental HVDC modulation control. The parametersof the controller were tuned to aid damping of interarea mode 1.

Simulation results with and without DC modulation and voltage support areshown in Figures 47. The fault is a solid short circuit applied at 20 seconds cleared


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Figure 9. Comparison of Temascal SVC response with and without HVDC modulation.

in 5 cycles by opening one of the circuits. It should be noted that the system becomesunstable without the addition of the HVDC link, leading to severe postfault voltageoscillations and eventually voltage collapse, as shown in Figure 8.

The simulation results indicate that the coordinated application of HVDCtransmission and SVC voltage support allows for a more damped system response.Paramount to system dynamic behavior, Figures 8 and 9 depict the SVC responsefor the fault considered.

In Figure 8 one sees the system dynamic enhancement manifested by improveddynamic voltage regulation throughout the 400 kV SE network. Also of interest,the analysis of the SVCs output in Figure 9, on the other hand, does not reveal anyundesirable effect on the controller response.


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5. Conclusions

In this paper a fundamental investigation into the applicability of HVDC transmis-sion to enhance system dynamic performed has been presented. HVDC transmis-

sion has been found to provide an effective contribution to systemwide dampingparticularly for two critical interarea modes. Such a contribution can be comparedfavorably to other AC transmission alternatives. Application studies on the Mexicaninterconnected system show that the coordinated application of HVDC technologyand SVCs can provide significant benefits in terms of both increase transient sta-bility margins and overall system damping. Experience with the coordination ofcontrollers in HVDC systems seems to suggest that a properly selected controllercould suffice for most operating conditions.

Additional studies are underway to explore the applicability of other FACTScontrollers as well as to determine the potential for interaction with other controllerson the system.

References

[1] R. Rudervall, J. P. Charpentier, and R. Sharma, High voltage direct current (HVDC)transmission system technology review, presented at Energy Week 2000, Washington,D.C., March 78, 2000.

[2] CIGRE JWG 14/37/38/39-24, FACTS technology for open access, Final DraftReport, August 2000.

[3] J. G. Flores, C. F. Estrada, M. A. A. Rosales, and R. Grunbaum, Mexican grid usesFACTS for greater flexibility, Modern Power Systems, pp. 14, June 1999.

[4] H. G. Sarmiento, E. Estrada, J. Naude, C. Tovar, H. Ambriz, M. A. Avila, andC. Fuentes, Preliminary studies to evaluate the application of FACTS controllersin the Mexican electrical system, presented at the VII SEPOPE, Curitiba, Brazil,May 2000.

[5] P. Kundur, Power System Stability and Control, EPRI Power Systems EngineeringSeries, New York: McGraw-Hill, pp. 639655, 1994.

[6] A. R. Messina, J. M. Ramrez, and J. M. Canedo, An investigation on the use of powersystem stabilizers for damping inter-area oscillations in longitudinal power systems,IEEE Trans. Power Systems, vol. 13, pp. 552559, May 1998.

[7] M. Nayebzadeh and A. R. Messina, Advanced concepts of analyzing static VAr com-pensators to damp inter-area oscillation modes, European Transactions on ElectricalPower, vol. 9, pp. 159165, May/June 1999.

Appendix

Figure 10 shows the block-diagram representation of the two-terminal HVDC con-trol used in the studies presented.


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(a)

(b)

Figure 10. Block diagram representation of the HVDC control system: (a) functionaldiagram of converter station control system; (b) power system damping controller (PSDC).


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