multi-infeed hvdc with upfc as ancillary controller · 978-1-5386-6159-8/18/$31.00 ©2018 ieee...

978-1-5386-6159-8/18/$31.00 ©2018 IEEE

Multi-Infeed HVDC with UPFC as Ancillary Controller

R. K. Pandey, Senior Member IEEE Department of Electrical Engineering

Indian Institute of Technology (BHU), Varanasi [email protected]

Abstract--This paper evaluates critically the operational aspects of multi-infeed HVDC network during various conditions such as ESCR variation and faults at any one of the remote converter terminals. The study reveals that FACTS controllers may significantly improve operational reliability under dynamical changes. It has been demonstrated through extensive simulation studies that voltage and power in the multi-infeed network is regulated effectively with Unified Power Flow Controller (UPFC). During course of investigation, this has been found that an appropriate integration of a tuned UPFC in multi-infeed network greatly helps not only to optimize the power flow in the AC transmission lines maintaining voltage and power stability during sudden loss of generator or fault in the network but also assists in smooth multi-infeed converter operation with least excursions on converter controls. A sample multi-infeed HVDC network with UPFC is modelled in PSCAD/EMTDC environment and the simulation results demonstrate significant contribution of UPFC controller for effective oscillation damping even in weak AC system conditions and also recovery after fault is cleared.

Keywords— Multi- infeed HVDC system, Effective Short Circuit Ratio (ESCR), Fault, UPFC, FACTS controllers.

I. INTRODUCTION

he electrical power demand is increasing rapidly and in order to match this, it is becoming difficult to add generating plants very near to the load centers due to

environment constraints. Power generated at the remote locations (may be hydro, nuclear, non-conventional and/or coal based) should be transmitted efficiently to respective load centers. The multi-infeed HVDC system will be more appropriate to evacuate bulk power due to operational flexibility. However, the experiences of such systems necessitate the proper control initiation under variety of operational change in interconnected AC system. The operational reliability may be improved with FACTS controllers, if properly planned after careful study. The concept of multi-infeed HVDC systems are primarily important due to the configuration of many converters (rectifiers/inverters) connectivity to a large load centers initially in point to point mode where most of the inverters are in close proximity. The variation in AC system strength or faults even at one location in close proximity may affect the remaining nearby converter operation significantly [8]. This scenario may be very detrimental. Multi-infeed HVDC systems have constraints in carrying power over long distance to different locations [2]. Special attention should be given to network with weak AC

system interconnection and fault condition as it affects the performance of all the connected links in the multi-infeed network. Point to point HVDC links are easier to operate and control than the multi-infeed network as variation in voltage and power due to faults are well within converter station control range. In today’s evolving power systems, especially in open access regime, many HVDC links are connected in a large network and due to commercial requirements; power becomes necessary at some strategic locations which can only be arranged with many point to point HVDC systems, forming inverters connectivity in close proximity. In situations of fault or load variations at one location, the network will have adverse effect on all the connected system (nearby inverter stations) resulting variation in power and voltage that may not be controlled [2]. Multi-infeed network operation depends mainly on AC system strength which is represented using ESCR (Effective short circuit ratio) value, if the AC system strength is strong then the system will effectively feed the power to the load centers but if it is weak then the system will lead to operational instabilities. Major concern has been instability during commutation failure and faults. Since many links are connected in the multi-infeed system, oscillation in one link due to fault or load variation will deteriorate and introduce oscillations in all the connected links and may aggravate.

System voltage and power should be maintained within acceptable limits before and after the occurrence of fault or load changes. Islanding may be one way of avoiding instability but it is not preferred in normal operation, however this may be a mechanism to avoid major instability at some cost. FACTS controllers if properly planned in power network may prove the best in handling power swing [1-6]. The weak AC system connectivity in multi-infeed framework is detrimental as a fault in weak system will introduce oscillation in adjacent network which can be effectively damped with UPFC if properly tuned.

II. MULTI-INFEED HVDC SYSTEM REPRESENTATION IN

PSCAD/EMTDC ENVIRONMENT

This section deals with the sample model of multi-infeed HVDC network in PSCAD/EMTDC environment. The configuration comprising of four point to point (bipolar) HVDC links which are connected at inverter side AC buses using AC lines are represented using CIGRE benchmark model [7] for system study and analysis. Bipolar links are represented as BP throughout this paper for convenience and the no. represents the corresponding link (BP1-biploar link 1, BP2- bipolar link 2,

T

Proceedings of the National Power Systems Conference (NPSC) - 2018, December 14-16, NIT Tiruchirappalli, India

978-1-5386-6159-8/18/$31.00 ©2018 IEEE

BP3-bipolar link 3 and BP4- bipolar link4). BP1 and BP2, BP2 and BP3, BP3 and BP4 are connected at inverter terminal with 150km AC line and BP1 and BP4 are connected with a long 500km AC line to form a sample multi-infeed network (Fig. 1). The distance assumed signifies that the inverter terminals are connected in close proximity forming multi-infeed HVDC system. First set of analysis is carried out without UPFC and the second set of analysis is done with UPFC in network to demonstrate the usage of FACTS controller.

Fig. 1 Sample Multi-infeed HVDC network model with inverter terminal connectivity without UPFC

III. UPFC AS REAL AND REACTIVE POWER COMPENSATOR

UPFC is multi-purpose FACTS controller which combines the features of both STATCOM (Static Synchronous Compensator) and SSSC (Static Synchronous Series Compensator). These two converters are connected in network, one converter in shunt and the other in series with the AC line with common capacitor in between them to exchange real power. Series connected converter will inject controllable voltage to make the desired power change in the network as this exchange both real and reactive power in the network whereas shunt converter can exchange only reactive power to regulate the voltage at the connected AC bus but its primary duty is to supply the real power demand of series converter [4]. The UPFC can inject real and reactive power in multi-infeed HVDC

network for maintaining the network parameters in order to regulate the power flow. This has four control parameters (Vref, DCgain, Thetaord and Gconst) which should be adjusted properly to have effective system operation. Out of these four control parameters, shunt converter parameters (Vref and DCgain) are identified as most contributory parameters and these parameters are tuned accordingly to exchange real and reactive power in network under variety of system conditions [4]. Since power injection capacity of UPFC is linked with control, the range of control precisely regulates power flow.

Fig. 2 UPFC controller for real and reactive power control

Mere connection of UPFC in the network won’t help to compensate the demand required by the system. Improper tuning will create more operational problem and so proper selection of control parameter and tuning are essential to precisely compensate the real and reactive power in the multi-infeed network. This has been studied at length in this paper.

IV. CASE STUDIES

The detailed analysis has been done for multi-infeed network model shown in Fig. 1 with various ESCR values and fault condition (1p-f: single phase to ground fault, 3p-f: three phase to ground fault) at both inverter and rectifier ends. The impedance angles at rectifier and inverter ends are maintained as 840 and 740 respectively. The simulation results are designated as case 1 (without UPFC) and case 2 (with UPFC). Case1 represents the analysis of Bipolar link (BP1) inverter terminal as weak along with 1p-f, BP1 rectifier end and all other bipolar terminals (both inverter and rectifier) are strong, without faults and no UPFC, whereas Case2 represents the analysis of Bipolar link (BP1) inverter terminal as weak along with 1p-f, BP1 rectifier end and all other bipolar terminals (both inverter and rectifier) are strong and without faults and with UPFC connected between BP2 and BP3 inverter terminal. The results of BP1-BP4 consist of AC voltages (both instantaneous and RMS in per unit), DC voltage and current (positive and negative pole of both DC converters in per unit) and power (in MW) at switchyard at both rectifier and connected inverter end of overall system. A rigorous analysis has been carried out for different ESCR values and fault condition (fault is introduced at 0.3 sec and cleared at 0.4 sec) at both inverter and rectifier terminals without UPFC in the sample model shown in Fig. 1. Once the ESCR values are lowered and some fault is introduced in the network, large power swing and oscillation are observed in all the connected links. In second set of analyses, same multi-infeed network model (Fig. 1) and conditions are considered

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with a 100 MVA, 230kV UPFC connected in between BP2 and BP3 inverter end AC buses. Multi-infeed network showed improved performance and the oscillations were suppressed as UPFC helps to precisely compensate real and reactive power required by the network. Detailed analysis has been carried out in this section without and with UPFC to demonstrate the importance of UPFC in multi-infeed HVDC network.

A. Multi-Infeed Network Analysis without UPFC

Four bipolar links (BP1, BP2, BP3 and BP4) are connected at inverter terminal using AC lines and each bipolar link is modeled to carry 2000-MW and for each system reactive power compensation is done using filters and shunt capacitors at AC buses under normal operation. The PSCAD/EMTDC model of multi-infeed network shown in Fig. 1 has been considered with varying ESCR at all four-bipolar links (both at inverter and rectifier end) and various analysis have been carried out. ESCR values for strong system at both inverter and rectifier end is considered as 3.4 and for weak as 1.9. Multi-infeed system behavior without UPFC for BP1 inverter end weak with 1p-f at same end, while BP1 rectifier end and all other bipolar links are strong (referred as case 1) is shown in Figs. 3, 4, 5 and 6 respectively.

B. Multi-Infeed Network Analysis with UPFC in between BP2 and BP3

The sample network shown in Fig. 1 is again considered for analysis to show the effect of UPFC under varying ESCR at bipolar rectifier and inverter terminals with different faults. The bipolar link 1 (BP1) inverter end is made weak (ESCR=1.9) and 1p-f is introduced at inverter end, while BP1 rectifier end and all other bipolar terminals are strong and without any fault. Multi-infeed system behavior, with UPFC between inverter terminals of BP2 and BP3 (referred as case 2) when BP1 inverter end is weak with 1p-f at same end, while BP1 rectifier end and all other bipolar links are strong is shown in Figs. 7-10 respectively. Comparing Figs (3-6) and (7-10), it is evident that UPFC exchanges the real and reactive power in the network and helps to maintain system operation smoothly. It can be seen that UPFC quickly regulates real and reactive power in the network in changing requirements and damps power swing. It has also been noticed that with proper tuning of UPFC control parameters, oscillations can be suppressed.

C. Analysis of Results

The operational behavior of multi-infeed HVDC network for different conditions has been observed and properly analyzed. The factors that need to be considered are strength of the AC system at interface bus, behavior during fault conditions and integration of UPFC between identified location for effective real and reactive power injection. It is clear from Figs. 3-6 (multi-infeed network without UPFC and with low ESCR value and fault) and Figs. 7-10 (multi-infeed network with UPFC and with low ESCR value and fault) that AC system strength and UPFC plays an effective role in stable system operation along with required power delivery at respective locations.

It is clear from the above results (Figs. 3-6) that while feeding power to load centers with one of the AC systems as weak-low ESCR value (BP1 inverter end is weak with 1p-f fault), while BP1 rectifier end and all the other bipolar links (BP2, BP3 and BP4) are strong, still the network is not in a position to feed reliable power (even though three out of four bipolar links are strong at both the ends) to the network and oscillations are introduced which not only exists in adjacent links but are transmitted to all the links in the multi-infeed system. Multi-infeed HVDC systems should be planned either with strong AC system interface or with UPFC under weak AC system interface. Since the prior knowledge of system strength may be obtained at given point of time (may be planning multi-infeed HVDC) but the actual system strength is dependent on various other factors such as load addition/network expansion/generation deficit/generation excess over the next time period (may be decade), and therefore, the existing multi-infeed HVDC system should be equipped with UPFC for regulated power flow to ensure interface AC system stability first and global network stability in general. Table I shows the change in real power (in MW) during power swing due to ESCR changes and different fault conditions at inverter and rectifier end, 20 different cases are simulated and analyzed with different ESCR values (strong ESCR = 3.4, weak ESCR =1.9) and faults (1p-f and 3p-f) at inverter and rectifier end in the multi-infeed network. Except two cases (4x4 and 4x5 in Table I) all other cases show oscillation and dip in power which signifies usage of UPFC power system. Same configuration (Fig. 1) is considered with a 100 MVA, 230kV UPFC in between BP2 and BP3 at inverter end AC buses to compensate the required real and reactive power for stabilization. From Figs. 7-10, it can be observed that the multi-infeed network shows improved performance with UPFC. The UPFC has four control parameters [4], two each for UPFC converters (shunt and series converter) which should be tuned for individual case in order to have proper exchange of real and reactive power in the network.

During course of study it has also been found that control parameters differ from case to case, it is clear that mere connection of UPFC in the network will not help as the control parameters should be tuned for every case depending on system configuration. An improper tuning will result adversely. It has been observed that UPFC control parameters (Vref, DCgain, Thetaord and Gconst) need to be properly adjusted for different system conditions [4]. Two control parameters in UPFC (DCgain and Thetaord) which are related to shunt and series inverter part can operate both in capacitive and inductive mode based upon the requirement. In this way UPFC can supply or absorb real and reactive power to or from the network, once its control parameters are properly tuned. UPFC shunt inverter control parameters (Vref and DCgain) are taken as most contributory parameters among the four UPFC control parameters for ensuring system stability [4].


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Fig. 3 BP1 output considering case1




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1.0

2.0

3.0

(pu)

DC Volts DC Current


x 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

-1.0

0.0

1.0

2.0

3.0

(pu)

DC Volts DC Current


x 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

-1.0

0.0

1.0

2.0

3.0

(pu)

DC Volts DC Current


x 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

-1.0

0.0

1.0

2.0

3.0

(pu)

DC Volts DC Current


x 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

0.0

0.3k

0.5k

0.8k

1.0k

1.3k

1.5k

1.8k

2.0k

(MW)

Active PowerRectifier3ActivePower

x 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

0.0

0.3k

0.5k

0.8k

1.0k

1.3k

1.5k

1.8k

2.0k

(MW)

Active Power

Rectifier4Voltage

x 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

(pu)



x 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

-1.0

0.0

1.0

2.0

3.0

(pu)

DC Volts DC Current


x 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

-1.0

0.0

1.0

2.0

3.0

(pu)

DC Volts DC Current


x 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

0.0 0.3k

0.5k

0.8k

1.0k1.3k1.5k

1.8k

2.0k

(MW)

Active Power

Inverter4Voltage

x 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

-1.50

-1.00

-0.50

0.00

0.50

1.00

1.50

(pu)



x 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

-1.0

0.0

1.0

2.0

3.0

(pu)

DC Volts DC Current


x 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

-1.0

0.0

1.0

2.0

3.0

(pu)

DC Volts DC Current


x 0.00 0.25 0.50 0.75 1.00 1.25 1.50 1.75 2.00

0.0 0.3k

0.5k

0.8k

1.0k1.3k1.5k

1.8k

2.0k

(MW)

Active Power

Time (sec)


978-1-5386-6159-8/18/$31.00 ©2018 IEEE

TABLE I Sample System (Fig.1) without UPFC-Swing in Real power (MW dip) as ESCR changes at inverter /rectifier during normal / fault condition, suffix R and I represents the corresponding rectifier and inverter terminals and --- represent no change

ESCR Strong =3.4,

ESCR Weak =

1.9

ESCR changes made at BP1 and

all other(BP2,BP3,BP4) terminals are strong

ESCR changes

made at BP2 and all other

(BP1,BP3, BP4)

terminals are strong

ESCR changes

made at BP3 and all other

(BP1,BP2, BP4)

terminals are strong

ESCR changes made at BP4 and all

other (BP1,BP2,

BP3) terminals are

strong

Inverter Weak

BP1R=180, BP1I=150 BP2R=210, BP2I=210 BP3R=190, BP3I=180 BP4R=30, BP4I=50




Inverter Weak (1p-f

at Inverter

end)





Inverter Weak (3p-f

at Inverter

end)





Rectifier Weak (1p-f

at Inverter

end)


BP1R=200, BP1I=175 BP2R=---, BP2I=10

BP3R=200, BP3I=250 BP4R=200, BP4I=250

BP1 R=---, BP1I=---

BP2 R=---, BP2I=---

BP3 R=---, BP3I=---

BP4 R=---, BP4I=---

BP1R=---, BP1I=--- BP2R=---, BP2I=--- BP3R=---, BP3I=--- BP4R=---, BP4I=---

Rectifier Weak

(3p-f at Inverter

end)


BP1R=150, BP1I=150 BP2R=---, BP2I=--- BP3R=---, BP3I=--- BP4R=---, BP4I=---

BP1 R=---, BP1I=---

BP2 R=---, BP2I=---

BP3 R=---, BP3I=---

BP4R=200, BP4I=200


V. CONCLUSIONS

This paper presents an analysis of sample multi-infeed HVDC system under different AC system strength and fault conditions at rectifier and inverter. Multi-infeed HVDC network should be planned and operated with care as fault in remote terminal may deteriorate the operation and performance of all the connected links in the network which may lead to

temporary power shutdown due to control sensitivity related oscillations. A properly tuned UPFC may be more beneficial to maintain network stability, if adequately embedded in multi-infeed HVDC system. The system studies should be thoroughly carried out for effective damping strategy and UPFC location along with rating. The parameters of UPFC may be tuned once at the design stage and moreover, this will perform better normally but as and when the perturbation exceeds the control settings the same performance may not be achieved. In such situations an intelligent control strategy is being researched to cover wider range of operational domain with effective damping and will be reported soon. The proposed concept may be of a great value to the most of utilities which are under expansion due to many upcoming generating plants/load centres. Since the dynamics of entire network may change depending upon the augmentation of network parameters which in turn affect the ESCR at interface buses, UPFC may prove as supplementary control for precise regulation of power flow. The most benefit which utilities may observe may be in form of the least outage of existing HVDC links and also extended life of converters and associated controls thus saving a huge outage cost and adds revenue to the utilities indirectly and services to emergent consumers with quality power.

ACKNOWLEDGMENT

The author thanks Ministry of Power, Govt. of India for providing the infrastructural facilities through CPRI for RSOP Project P-40-28 titled “Stabilization of AC/DC network with UPFC”.

REFERENCES [1] R. K. Pandey and N. K. Singh, “Small Signal Model for Analysis and

Design of FACTS controllers”, IEEE PES GM 2009.

[2] E. Rahimi, A. M. Gole, J. B. Davies, I. T. Fernando and K. L. Kent, “Commutation Failure Analysis in Multi-Infeed HVDC Systems”, IEEE Transactions on Power Delivery, Vol.26, No. 1, pp. 378-384, 2011.

[3] S. Tara Kalyani and G. Tulasiram Das, “Control and Performance of UPFC connected a Transmission Line”, The 8th International Power Engineering Conference (IPEC 2007).

[4] R. K. Pandey and N. K. Singh, “UPFC Control Parameter Identification for Effective Power Oscillation Damping”, Vol. 31, pp. 269-276, IJEPES 2009.

[5] X. Lei, W. Braun, B. M. Buchholz, D. Povh, D. W. Retzmann and E. Teltsch, “Coordinated Operation of HVDC and FACTS”, Proceedings of International Conference on Power System Technology, Vol.1, pp. 529-534, Dec. 2000.

[6] L. A. S. Pilotto, W. W. Ping, A. R. Carvalho, A. Wey, W. F. Long, F. L. Alvarado and A. Edris, “ Determination of Needed FACTS Controllers that Increase Asset Utilization of Power Systems”, IEEE Trasactions on Power Delivery, Vol 12, No. 1, pp. 364-371, Jan 1997.

[7] M. Szechtman, T. Wess, C.V. Thio, “A Benchmark Model for HVDC System Studies”, International conference on AC and DC power transmission, September 1991.

[8] R. K. Pandey, S. K. Soonee, L. Hari, P. Mukherjee, Surajit Banerjee, “Study of HVDC System in Open Access -An Indian System Experience”, 16thNational Power System Conference, December 2010.


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