Modeling and Simulation of Current Source Converter ...

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Modeling and Simulation of Current Source Converter for Proposed India–Sri Lanka HVDC Interconnection Asanka S. Rodrigo Deparment of Electrical Engineering University of Moratuwa Katubedda, Sri Lanka [email protected] Abstract— The proposed 500 MW interconnection is the first DC power transmission link that is planning to connect in between Sri Lankan and Indian AC transmission networks. Proposed AC-DC system integration should determine the asymptotic stability of transmission network in both countries. Reliability can be improved by properly selecting system configurations. Properly selected DC & AC system parameters determine the stable performance at steady state and at perturbed conditions. This paper presents the modelling analysis of proposed HVDC interconnection for selected physical configurations. The interconnection was modelled using PSCAD/EMTDC software and simulated under the steady state conditions and perturbed conditions. The analytical results were verified using time domain simulations. Keywords— CSC, HVDC, ISE, SCR, time domain analysis, VDCL

I. INTRODUCTION Rapid electrification and industrial development in Sri Lanka leads drastically growth in the electric power sector in recent past. This ever growing electric energy demand keens electric power industry to build more power generation plants in the island or import power through the interconnections from neighboring countries. Other than generation plant addition in the country, it is proposed to import power from the Sri Lanka’s neighboring country India. This proposal has got apparent advantages despite in electric energy, economic, social, environmental and political sectors relative to the other available options [1]. According to the prefeasibility study, the primitive objectives of this proposed interconnection are [1], • The interconnection would enhance the reliability and system security of both the countries. • The interconnection could facilitate the development of power exchange between the two countries. It was found that the HVDC technology is preferable with respect to the HVAC technology to link as an interconnection between two independent countries [2]. HVDC interconnections tend towards larger capacity to make interconnection economically feasible. These links could be sized at hundreds of MWs, which is competitive with

Chenuka U. Perera Deparment of Electrical Engineering University of Moratuwa Katubedda, Sri Lanka [email protected] conventional power plants. Consequently, grid integration of such size HVDC links seriously impact the operation and dynamic stability of the interconnected power systems [3]. HVDC link is characterized by independent active and reactive power control and hence is able to provide reactive power/voltage support during grid voltage disturbances. These characteristics are highly depending on the grid characteristics and HVDC control systems. In order to achieve the full advantages of the HVDC interconnection, the grid with HVDC should perform appropriately for different kinds of disturbances and system conditions. Thus, it is essential to study the grid and HVDC link performance under different disturbance scenarios. As the Sri Lankan power system is weak and very small, proper analysis of dynamic behavior of the link is very critical. This paper describes the modeling details of the proposed India-Sri Lanka HVDC interconnection for transient and dynamic stability. Paper proposes a systematic way to ensure that there are no risks of adverse interactions due to the proposed link. The simulation results obtained for different disturbances to compare the system performances with the proposed link are also presented in this paper. II. SYSTEM CONFIGURATION There are two basic converter technologies in modern HVDC transmission systems namely (1) Current Source Converter (CSC) and (2) Voltage Source Converter (VSC). Based on the configurations of three phase converters in conversion process, it has divided as above two technologies [3]. Comparisons of attributes of two technologies are given in Table 1[4], [5]. In this study, the system is modelled as, India feeds proposed 500 MW power to Sri Lankan network through DC interconnection at rated 400 kV DC voltage. There are 4 proposed options for India-Sri Lanka interconnection based on the terminal points of the interconnection and the type of the interconnection according to the prefeasibility study [6]. This study selected Madurai to Anuradhapura mono-polar interconnection out of those four options for the modeling and simulation as it is found as the cost optimal solution [1]. Anuradapura bus is also identified as one of the stronger point for large power integration [7].

TABLE I.

COMPARISON OF TWO CONVERSIO ON TECHNOLOGIES

Attribute

CSC technology classic 6400 MW, ± 800 kV (Overhead lines )

VSC technology Light/Plus 3 MW,± 150 350 k kV(cable)

(wide range) 2.5-4.5%

Narrow range) (N 4 4-6%

Per MW cost

Reliability/Availability proven 40-60 kEUR/MVA

Technology maturity

More than 54 projects by 2010

No formal records N a available present 5 50-70 k kEUR/MVA A About 11 projects by 2 2010

Max converter rating at present (range for converter selection) Typical converter losses Reliability indexes

rectifier side modeling. Thee maximum impedance angle should be in 75-85 degrees raange [15]. Therefore, the paper assumes 84 degrees as the maaximum impedance angle at the rectifier side.

Fig. 1. Equivaleent Indian network

Short circuit ratio (1)

NG III. MATHEMATICAL MODELIN

For the modeling and analysis purpose, thhis study made the following assumptions; • Master control is at the inverter terminaal • 400 kV is inverter terminal DC voltagee • India feeds 500 MW to Sri Lanka • Transformer reactance (Xc) is 0.18 pu. • Tap changer of transformers is avoidedd and has constant transformer ratio • At the steady state operating point, recctifier firing angle is 200 and inverter extinction angle is 180. • Governor action is neglected • Rectifier AC bus is an infinite bus with a frequency dependent impedance

System fault level

All the parameters of the modeled systtem were derived using mathematical differential and alggebraic equations extracted from references [8], [9], [10], [11], [12], [13]. Each equation is mentioned at the relevant modeleed sub system. In the equations, labels i and r denote recctifier and inverter quantities respectively. k denotes r or i for rectifier and inverter respectively. Details of the notatiions are given in Appendix.

Rated power of the converter

A. Indian Network In this paper it was taken the rectifier sidde Indian network coupling bus bar as an infinite bus bar. The rectifier side Indian terminal point was modeled as thevvenines equivalent voltage source, as the paper studies aboutt the inverter side AC-DC interaction. The strength of the nettwork or the short circuit ratio (SCR) of the Indian network was w taken as 5 for steady state modeling purpose. The relationnship between the system strength and the short circuit ratio is shown in Table 2 [9]. Thus, it is assumed here that Inndian network is intermediately strong. The modeled equivalent Indian voltage source s is shown in Fig. 1. The algebraic diagram used to derrive the electrical parameters of this equivalent network shows in Fig. 2. It is proposed that a R-R-L type freqquency dependent equivalent network is much suitable for stability analysis [14]. Thus same type of R-R-L equivalent circuuit is used for the

(2) derived AC-DC interaction equattions [13] √

√2 (3)



| || |

|

|

|

| 2|

||

|

|

Fig. 2. Algebraic Diaggram of Indian Network

TABLE II.

|

(4)

CATEGORIES OF O AC SYSTEMS BASED ON SCR

Category AC system Weak Intermediate Strong

SCR range < 3 3to 5 >5

ESCR range 3

B. Sri Lankan HVAC network Sri Lankan power system was modeled with the 132 kV and 220 kV transmission netw work. Transmission lines were modeled as Bergeron model [116]. The loads were modeled as constant impedance loads for the t stability studies [17]. It was assumed that the transformers are non-saturation type. The

synchronous generators modeled with the simplified exciter characteristics. It was taken that synchronous generators has one field coil on d- axis and one damper coil on q- axis. The governors did not added to synchronous generators since governor effect on transient stability of the AC network is negligible [18]. This research study primarily focused on the transient stability of the AC-DC interaction on inverter side. C. Filters Converters are modeled as 12 pulse converter systems and less harmonic filtration is required. The 12- pulse converter system generates characteristic harmonics of the order of 11th, 13th, 23rd, 25th, 35th, 37th, 47th, 49th and so on. [12] The single tuned low pass filters are used to filter out 11th and 13th harmonics. The high pass filter is used for 23rd and above harmonic filtration. The required quality factor and tuning factor were selected as per the requirement of the design [19]. D. Converter transformer with valve group A 12-pulse converter has 2 six-pulse converter bridges connected in series; one supplied by a star-star transformer and the other by a star-delta transformer. This arrangement provides 300 phase shift for 12 – pulse operation. The transformer reactance is taken as 0.18 p.u. This order reactance limits the fault surge current in the thyristor valves to an acceptable level [20]. Furthermore, the first benchmark model for HVDC control studies has also used transformer reactance as 18% [14]. The Converter transformer equations are shown below. .

(5) √3 E. DC transmission network with DC smoothing reactor The DC line comprises with 150 km long two overhead lines and 50 km long submarine cable. However, due to limitations in the PSCAD version used, the overhead lines and the cables couldn’t model altogether. Therefore, the overhead line was modeled as a simple T network and submarine cable was modeled with the cable block in PSCAD library. The overhead line parameters were derived from ACSR Moose conductor [21]. The submarine cable parameters were derived from mass impregnated cable with 1500 mm2 where used the same cable for the similar interconnection built between Tasmania and Australia [22]. The smoothing reactor is an essential part of the DC transmission network. It is always connects in series with each pole of each converter station. These reactors ensure that overcurrent transient occurring during an inverter commutation failure or a DC line fault is kept within limits acceptable to the valves [20]. DC line equations are given below;

(6) cos cos DC smoothing reactor equations are given below;



2

cos ∆

cos cos ∆

cos 1

2

(7)

∆ ∆

F. Control system Master control system is modeled to manually set the power order. It is assumed that the master control is at inverter terminal. So the manually operated power order values at inverter master control are notified to rectifier side slave control system to change the firing angle as required. The Fig. 4 illustrates the HVDC control characteristic curve used for this study. Rectifier characteristic curve comprises with Constant Ignition Angle (CIA) curve and Constant Current control (CC) curve. For the CIA curve, the rectifier firing angle α was taken as 50 which is the minimum value of α (αmin) for rectifier side converters to ensure that the converter valves have a minimum positive voltage for turning on [2]. The CIA characteristic curve and CC characteristic curve mathematically is shown in the equations 8. U cos (8) U cos Inverter characteristic curve comprises of Constant Extinction Angle (CEA), CC, Voltage Dependent Current Limit (VDCL) and Maximum alpha control mode. CEA curve and CC characteristic curve are mathematical representation in the equation 9. U cos (9) U cos ∆ At the steady state conditions, the rectifier and inverter operate at the interception point in the characteristic curve. At the interception, the rectifier operates at 200 and inverter operates at 180.

Fig. 3. HVDC characteristic curve

+ D Imeas

the assumption due to the tuning problem faced during the system modeling.

Simplex . | X|

2 X

1 sT

obj Optimum Run

1

2 p

i

AOIde

110 100

inverter gamma angle (degrees)

Iorder F

Fig. 4. Rectifier current controler optimizing block diagram

G. Controller optimization There are three controllers in the HVDC system operation, namely rectifier current controller, inverter current controller and inverter gamma controller. Before any test of the system, it is recommended to optimize all these three controllers [3]. For that it is necessary to tune up the controller parameters. In the HVDC control system, PI controller is used as the regulator, because it is the simplest and robust controller for complex nonlinear and dynamic systems [23]. The objective of tuning the PI controller is to find the optimum proportional (P) and integral (I) parameters as it gives the correct response as the set value. The relationship between the angle, P and I parameters are as below. Rectifier firing angle is defined as [13];

90 80 70 60 50 40 30 20 10

Time

0

20

40

60

80

100

120

100

120

100

120

Fig. 5. Steady state DC power Vdrms

400 350

D C vo lta g e (kV )

300 250 200 150 100 50 0 Time

0

20

40

60

80

Fig.6. Steady state DC voltage

(10)

AORdeg

100 rectifier firing angle (degrees)

90

and the inverter firing angle is defined as [13]; (11) There is the facility provided in PSCAD to optimize the parameters by executing different algorithms. This paper used simplex algorithm to find the optimum P and I parameters iteratively. The integral square error (ISE) which represent below are used as the optimum performance index in this study.

80 70 60 50 40 30 20 10 0

time

0

20

40

60

80

Fig. 7. Steady state rectifier firing angle p

600 500 400

T is the simulation time which is longer time than settling time [23]. In the optimizing process, it finds the parameters P and I that minimize the above objective function. The simplex optimum run control block for rectifier side current controller was configured [24] as shown in Fig. 4. The objective function is the square of the error between current order and the measured current at rectifier side. IV. SIMULATIONS A Simulation results for steady state condition The steady state system was designed such that India feeds 500 MW at rated 400 kV DC voltage. At the operating point of the control system, rectifier firing angle is 200 and inverter extinction angle is 180. These parameters for steady state operation were verified by the time domain results obtained in Fig. 5, 6, 7 and 8. Only the DC voltage has a low value than

Active power (MW)

(12)

300 200 100 0 -100

Time

0

20

40

60

80

100

120

Fig. 8. Steady state inverter extinction angle

B

Simulation results for specified disturbances Despite the steady state normal operation, the system should act stably under perturbed conditions as well. Two scenarios were simulated to verify that the AC-DC interaction performs stably under the system disturbances. Inverter side single phase fault for 1 cycle period and 5 cycle period were the disturbances created in this paper to proof the stable operation of the AC-DC interaction at inverter terminal side. Inverter side AC faults Single phase fault was applied at 14 s and let the faults sustained for 1 cycle and 5 cycle periods respectively. DC power, DC voltage, Rectifier firing angle and inverter extinction angle behavior were observed in time domain.

Single Phase fault for 1 cycle period

Main : Graphs p

600 500 400

Active power (MW)

Single phase fault for single cycle in AC-DC interaction is called single commutation failure as one phase voltage drop caused for the converter commutation process to be disturbed until the AC voltage resets. This commutation failure results in drop of DC voltage to zero. This causes to VDCL control to limit the DC current to the minimum value. However the DC power delivery becomes zero as DC voltage drops to zero during the fault period. After the fault is cleared the DC current is ramped up to the pre fault level according to the VDCL control action [3]. Fig. 9 to 12 illustrate the results.

300 200 100 0

Time

30.0

35.0

40.0

300 250 200 150 100 50

200

0

100

Time

10.0

0

15.0

-100 0.0

5.0

10.0

15.0

20.0

25.0

20.0

25.0

30.0

35.0

40.0

Fig. 14. DC voltage at single phase five cycle fault

30.0

Fig. 9. DC power at single phase one cycle fault

Main : Graphs AORdeg

100

Vdrms rectifier firing angle (degrees)

90

350 300 DC voltage (kV)

25.0

400

D C vo lt age (kV)

300

400

20.0

350

400

Time

15.0

Vdrms

450

p

500

Active power (MW)

10.0

Fig. 13. DC power at single phase five cycle fault

Main : Graphs 600

5.0

250 200 150 100

80 70 60 50 40 30 20 10 0

50 Time

time

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

10.0

AORdeg

160

40.0

150

35.0

140

inverter firing angle

rectifier firing angle (degrees)

25.0

30.0

35.0

40.0

45.0

50.0

Main : Graphs

Main : Graphs

30.0 25.0 20.0 15.0

AOIde

130 120 110 100

10.0

90

5.0 time

20.0

Fig. 15. Rectifier firing angle at single phase five cycle fault

Fig. 10. DC voltage at single phase one cycle fault 45.0

15.0

45.0

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

40.0

45.0

Fig 11. Rectifier firing angle at single phase one cycle fault

Time

0.0

5.0

10.0

15.0

20.0

25.0

30.0

35.0

Fig. 16. Inverter firing angle at single phase five cycle fault

Main : Graphs 155.0

AOIde

V. CONCLUSION

inverter firing angle

150.0 145.0 140.0 135.0 130.0 125.0 120.0 115.0 Time

0.0

5.0

10.0

15.0

20.0

25.0

30.0

Fig. 12. Inverter firing angle at single phase one cycle fault

Single phase fault for 5 cycle period The single phase fault for 5 cycle period is known as multiple commutation failures. The fault was applied at 14 S and fault cleared after 5 cycles. According to the reference [18] commutation failure caused the DC voltage and DC power to drop to zero and after certain time to increase and stabilize at the scheduled values. Fig. 13, 14, 15 and 16 are shown below.

In this work, the India –Sri Lanka HVDC-HVAC network was modeled using PSCAD and the steady state and dynamic behavior of the inverter side were investigated. From this study, following conclusions could be made. • From the derived algebraic and differential equations, the HVDC-HVAC network could be mathematically modeled. The model accuracy was verified by the steady state time domain simulations. 500 MW DC power under rated 400 kV DC voltage was delivered from Indian power network to Sri Lankan power network. The steady state operating point of the rectifier was the firing angle of 0 200 while the inverter extinction angle was 18 . • From the time domain simulations for inverter side AC faults, it was emphasized that the inverter side AC

[19] E. W. Kimbark, “Direct current transmission,” Vol 1, Wiley, 1971. [20] S Kamakshaiah; V Kamaraju, “HVDC transmission,” Tata McGraw-Hill Education, [21] Technical Specification of ACSR Conductor for Transmission Line, http://www.gseb.com/Tender/File/GETCO/Corporate%20Office/0specificaiton/12_ACSR_Conductor.pdf [22] Basslink Proposed interconnector linking the Tasmanian and Victorian electricity grids- Final Panel Report, Basslink Joint Advisory Panel, June 2002 [23] F. Yang, Z. Xu, J. Zhang, "An approach to select PI parameters of HVDC controllers," Power Engineering Society General Meeting, IEEE, 2006. [24] D. R. Northcott, S. Filizadeh, A. R. Chevrefils; “Design of a bidirectional buck-boost dc/dc converter for a series hybrid electric vehicle using PSCAD/EMTDC," in Vehicle Power and Propulsion Conference, 2009. VPPC '09. IEEE , pp.1561-1566, 7-10 Sept. 2009

terminal voltage has a huge impact on the AC-DC interaction. It was found that the commutation failures of the converters can happen when the single phase AC voltage drops beyond the limit. This is one drawback of the CSC technology as it thoroughly depends upon the terminal voltage stiffness.



This system was modeled with the basic DC control system without adding any supplementary control systems to stabilize the system under perturbed conditions. Therefore it can be said that, the modeled AC-DC interaction is asymptotic stable as it regain the pre-fault operating state after the fault is cleared as shown in inverter side AC fault simulations. REFERENCES [1]

[2] [3] [4] [5]

[6]

[7]

[8]

[9] [10] [11]

[12] [13]

[14] [15]

[16] [17] [18]

S W A D N Wickramasinghe, “Technical Prefeasibility for Developing a Transmission System Interconnection Between India & Sri Lanka,” 2006. HVDC transmission; S Kamakshaiah; V Kamaraju V. K. Sood, “HVDC and FACTS controllers,” Boston. Kluwer Academic Publishers, 2004, Le Tang, “High Voltage DC technologies,” www.ABB.com “East Coast Transmission Network-Technical feasibility Study,” Crown Estate, 1845 East Coast Transmission Network v 1.0(Public release).doc USAID-SARI/Energy Program, “Viability of Developing a Transmission System Interconnection between India and Sri Lanka: Technical Options and Investment Requirements,”. www.sari-energy.org. A.S. Rodrigo, P.D.C. Wijayatunga, "Pricing of embedded generation: Incorporation of exernalities and avoided network losses," Energy Conversion and Management, Vol 48(8), pp 23322340,August 2007. A. L'Abbate, G. Fulli, “Modeling and Application of VSC-HVDC in the European transmission system,” International Journal of Innovations in Energy Systems and Power , vol. 5, pp. 8-16, April 2010 S Rao, “EHV-AC & HVDC Transmission Engineering & practice,” 2nd ed. Delhi: Khanna Publisher, 1996. P Kundur, Power System Stability and Control” McGraw Hill, Inc 1994 D. Jovcic, N. Pahalawaththa, M. Zavahir,”Analytical modeling of HVDC –HVAC systems” IEEE Trans. on PD, Vol. 14, no 2, pp.506-511, 1999. M. Anup, “Capacitor commutated converters for HVDC transmission system”, 2002. D. Jovcic, Control of High Voltage DC and flexible AC transmission systems, Thesis (PhD--Electrical and Electronic Engineering)--University of Auckland, 1999. First benchmark model for HVDC control studies; M. SZECHTMAN (Convener), T. WESS, C.V. THIO N. G. Hingorani and M. F. Burbery, "Simulation of AC System Impedance in HVDC System Studies," Power Apparatus and Systems, IEEE Transactions on , vol. PAS-89, no.5, pp.820,828, May 1970. Applications of PSCAD® / EMTDCTM, Manitoba HVDC Research Centre Inc K.U Rao,”Computer Techniques and Models in Power Systems,” IK International Pvt Ltd, 2008. K. R. Padiyar, “Power system dynamics. BS publications,” 2008.

APPENDIX Q δ

= quality factor = tuning factor

= angle between Iack and Vack Vack = line-line rms value of AC voltage Yh = admittance of filter to harmonic of order h Yhf = admittance of network to harmonic of order h h = harmonic order = network impedance angle f = maximum impedance angle m Ihf = harmonic current of order h Uc = unit cost of the capacitor UL = unit cost of inductor A = constant (Rs/MVar) B =constant (Rs.Mvar) S = reactive power supplied by the filter ωn = filter tuned frequency Rck = 3Xc equivalent commutating reactance π √

3√2 I0 = current order α = rectifier firing angle ILLk= rms value of secondary line current (valve side) VLLk= line voltage of transformer (Valve side) S= transformer rating ΔVd = rated voltage per bridge √

√3 = peak line voltage on secondary (valve) side of the converter transformer.