Improving Thailand Power System Dynamic Performance With Power Swing Damping Function Of The Hvdc Transmission System For Ieej Eit 2007

IEEJ-EIT Joint Symposium on Advanced Technology in Power System Rama Garden Hotel, Thailand, November 19-20, 2007

Improving Thailand Power System Dynamic Performance with Power Swing Damping Function of the HVDC Transmission System 1

Nitus Voraphonpiput1, Kittipon Chuangaroon1 and Somchai Chatratana2 Electricity Generating Authority of Thailand, Email: [email protected], [email protected] 2 National Science and Technology Development Agency, Email: [email protected] cycle plant. Krabi power plant is the largest thermal plant in this area with a capacity of 340 MW. The largest hydro plant in the Southern system is the Rajjaprabha (RPB-H) which produces approximately 240 MW depending on water head of the reservoir. In 2006, demands of the central system and the Southern system are approximately 20,000 MW and 1,900 MW while generation in the southern system is 1,700 MW. Therefore, it is necessary to transfer power from the central system through the tie transmission line. Power oscillations on the 230 kV tie transmission line were observed many times at different power transfer levels and for both directions of power flow. The oscillation frequency was 0.45-0.5 Hz, which was classified as inter-area oscillation between the central system and the southern system. To prevent the interarea oscillation, power transfer via the tie transmission line was limited to be less than 350 MW. The inter-area oscillation was not only observed during high power flow on the line, but also found when faults occurred in the distribution system or during tripping of generation or load in the southern system as described in [1].

Abstract - This paper presents utility experience on improving Thailand power system with Power Swing Damping (PSD) function of the EGAT-TNB High Voltage Direct Current (HVDC) transmission system. The EGAT-TNB HVDC system is the only interconnection between Thailand and Malaysia. Due to geographical constraint and power generation constraint of the southern system, inter-area oscillations were observed at various load conditions in the tie transmission line between the central system and southern system of Thailand. The PSD function is provided in the HVDC to mitigate the power oscillation. Analysis of the PSD function is presented with system parameters consideration. Simple single machine model is adopted for the swing equation of the southern system. Design methodology of the PSD function is based on the system behavior obtained from field tests. The power system dynamic performances are presented by field tests without PSD function and with PSD function for both directions of power flow. Responses to disturbances, which are manual one line tripping and one line reclosing, confirm that the PSD function can effectively mitigate the inter-area oscillations on the tie transmission line. Index Terms-Small Signal Stability, Power Swing Damping, EGAT-TNB HVDC, Inter-area Oscillation. I. INTRODUCTION Thailand power system can be considered as two areas which are connected via long transmission lines. The central system is a large system whereas the southern system is a remote system. The southern part of Thailand, which is made of 14 provinces, connects to the central system through the 115 kV and 230 kV tie transmission lines as shown in Fig.1. The 230 kV tie transmission line plays an important role in transferring energy from the central system to the southern system during high demand periods and the power flow direction can be reversed during light load. To increase power transfer of the 230 kV tie transmission line, a +300/-50 Mvar Static Var Compensator (SVC) was installed at Bang Saphan 230 kV substation. The base power plant in the Southern system is Khanom power plant (KECO), which generates approximately 700 MW. This plant consists of two thermal power plants and one gas turbine-combined

Fig.1. The Southern System Map

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IEEJ-EIT Joint Symposium on Advanced Technology in Power System Rama Garden Hotel, Thailand, November 19-20, 2007

The FLC is provided to recover system frequency from excursion. Controller of the FLC is tuned to achieve optimum additional DC power to minimize the frequency deviation. Details of the FLC and design methodology are presented in [2]. The PSD function is provided to damp inter-area oscillation on the 230 kV tie transmission line. The PSD function is one part of the Power Order Setting Functions for Transient Stability (POAC) of the HVDC pole control [3]. The overview of POAC is shown in Fig.2. Outputs of the four functions, i.e. Run-up, Run-Back, Frequency Limit Control (FLC) and Power Swing Damping (PSD), in POAC are fed into a summing function to obtain the power reference (PrefAC). The PrefAC is divided by DC voltage signal (UDC) to form the AC current order (IrefAC). This current order signal is a modulation signal that is added to the main current order (IrefDC) from operator and the result is the current order signal of the converter (Idref).

In 2001, The Tenaga Nasional Berhad (TNB, Malaysia) and Electricity Generating Authority of Thailand (EGAT) have installed a Monopolar 300 kV, 300 MW HVDC system as a connection between the southern part of Thailand and the northern part of Malaysia. The converter stations were erected at Khlong Ngae (KNE) substation, Thailand and at Gurun substation, Malaysia. This HVDC is an only link between two countries. The main purpose of HVDC project is to exchange the energy between two countries with additional benefit of ac system stabilization. This is the reason that stability functions were installed in the pole control of both sides. The stability functions, which are Run-up, Run-Back, Frequency Limit Control (FLC) and Power Swing Damping (PSD), are provided in the HVDC pole control system. Particularly, Power Swing Damping (PSD) function was implemented in 2006. In this paper, analysis of the control functions and design method of PSD are described. Parameters setting and system tuning are illustrated. The power system dynamic performances are presented by the responses of transmission line tripping and reclosing for both directions of power flow. II. POWER SWING DAMPING FUNCTION The EGAT-TNB HVDC provides four enhanced stability functions for ac system, which are Run-up, Run-Back, Frequency Limit Control (FLC) and Power Swing Damping (PSD). Run-up and Run-back functions are employed to reduce the effect of system disturbance such as generation tripping or line tripping, which may cause unacceptable voltage level in the southern system. These functions will increase or decrease the DC power from the HVDC to a setting value, which was previously assigned based on the system studies.

Fig.3. Power Swing Damping Function The block diagram of Power Swing Damping is shown in Fig. 3. It consists of two scaling functions (C1 and C2), a washout function (band pass filter with three time constants T1, T2 and T3), a nonlinear gain, a limiter function and a multiplier function. The input signal for this function is the frequency of the ac voltage at the Klong Ngae (KNE) substation. The actual line frequency fKNE is subtracted by the system frequency of 50 Hz to determine the frequency deviation ∆f. The frequency deviation is then amplified by C1. For this system C1 is 5, thus ∆f of 10 Hz will be scaled up to be 50 Hz at R(s). The scaled frequency deviation is then fed to the washout function. The output from the washout function can be shaped by three time constants. The output signal of the washout function is fed to the nonlinear gain. The dead band of the nonlinear gain prevents the system from the disturbances caused by the noises in the frequency signal. Outside dead band the signal is amplified by a gain with constant slope. The output signal from the nonlinear gain is scaled again by the second scaling function (C2) to obtain the

Fig.2. Overview of POAC

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IEEJ-EIT Joint Symposium on Advanced Technology in Power System Rama Garden Hotel, Thailand, November 19-20, 2007

Since the electrical power PE is directly proportional to sin δ, the linearized power deviation is

per-unit value. The output is limited in range of the values xhi and xlow, with one per-unit equals to 600 MW. The xhi and xlow will be determined from the DC power allowance of the HVDC. The modulation signal must be conformed to the direction of DC power flow. Hence, the multiplying function provides a multiplication factor of +1.0 or -1.0 to correct the phase angle of the output signal. This signal is fed into a switching block, which will block the modulation power signal (∆PA) when enable signal is not present. Finally, the modulation power signal (∆PB) of the PSD function of the opposite converter station (Station B) is added to the output of the PSD function to coordinate PSD functions of both stations when they are operating simultaneously.

∆PE = K S ∆δ

(2)

The damping power PDC is derived from the modulation power of the HVDC which is related to the output of the PSD function. The input signal for PSD function is the actual frequency of the ac voltage. Therefore, the linearized power deviation equation can be rewritten as ∆PDC = 2πK D ∆f = K D

d∆ δ dt

(3)

where ∆f is the frequency deviation of the southern system

III. DESIGN METHODOLOGY FOR PSD The power system within the scope of this study comprises the central system and the southern system with the EGAT-TNB HVDC transmission system as an isolated interconnection. According to [4] the power system with similar configuration can be represented by the simplified model. Therefore, a model with single generator connected to an infinite bus with an isolated HVDC system is proposed for this study as shown in Fig.4.

Substituting (2) and (3) into (1), the result is H d 2 ∆δ d∆ δ = − K S ∆δ − K D π f 0 dt 2 dt

(4)

where KS is the synchronizing coefficient KD is the damping coefficient Equation (4) can be represented by the block diagram in Fig. 5. It can be seen that the damping coefficient (KD) is represented by the time delay of the pole control (converter delay) and PSD function. From (4) the damping ratio of the system is

ξ= Fig. 4. Single Machine and HVDC Transmission with an Infinite Bus

π f0 HK S

(5)

where ξ is the system damping ratio It can be seen from (5) that system damping ratio is directly proportional to the damping coefficient (KD) of the modulation power of the HVDC.

Assuming that ∆Pm = 0, the per-unit swing equation for a small deviation of rotor angle is H d 2 ∆δ = − ∆PE − ∆PDC π f 0 dt 2

KD 2

(1)

where H is the per-unit initial constant of the Southern system [sec] f0 is the system frequency [Hz] ∆δ is the small deviation of the rotor angle [radian] ∆PE is the electrical power of the machine ∆PDC is the modulation power of the HVDC Fig. 5. Block diagram of linearized small perturbation

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IEEJ-EIT Joint Symposium on Advanced Technology in Power System Rama Garden Hotel, Thailand, November 19-20, 2007

IV. SYSTEM PARAMETERS CONSIDERATION The power flowing into the HVDC is transferred from the ac system at the rectifier end and the power flowing from the HVDC system into the ac system can also controlled at the inverter end of the HVDC. When the inter-area oscillation occurs in the AC system, the power oscillation must be transferred through the HVDC with proper frequency and phase angle. In this case, the proper frequency and phase angle are the oscillation frequency and zero phase angle. From Fig. 5, the frequency deviation signal ∆ω passes through the block of PSD function and a block of first order delay (converter delay). The time delay represents actual delay of the converter current with respect to the current command. Normally, dynamic model of the HVDC provided by vendor is not accurate enough for the calculation of this time delay for various operating conditions. Thus, field test must be conducted to determine the time delay of the converter. Test result indicates that phase delay is approximately 50 degrees when a 0.5 Hz input signal is applied to the pole controller. The phase delay of the converter can be compensated with the washout function in the PSD function. Input signal of the PSD function is frequency at the converter station which may have noise and may be sensitive with sudden change of frequency caused by system load. Thus, the washout function is set as a band pass filter. The transfer function of the washout function is expressed as (6). T3 s Ts 1 C (s) = 1 R ( s ) 1 + T1 s 1 + T2 s 1 + T3 s

Fig. 6. Frequency response characteristic of the washout function 10% 8% 6% 4% 2% 0% -2% -4% -6% -8% -10% -1.0%

-0.8%

-0.6%

-0.4%

-0.2%

0.0%

0.2%

0.4%

0.6%

0.8%

1.0%

Fig. 7. Gain characteristic of the nonlinear gain

(6)

The power modulation of the HVDC has to be transferred from the ac system through HVDC to the opposite converter station. Thus, maximum allowance of the modulation power is limited at 18 MW. The frequency deviation of 27 mHz was assigned to achieve maximum DC power allowance. Thus, the gain C2 was set to 6.5. Nonlinear gain was reset as mentioned before. (With the new settings of minimum frequency deviation and dead band, the frequency deviation of 33 mHz will achieve the maximum DC power allowance. Moreover, inter-area oscillation below 30 MW will not activate the PSD function to avoid unnecessary operation.)

Due to fixed structure of the washout function, the phase advance can not be performed without compromising with magnitude shape of the frequency response. In order to achieve phase advance about 50 degrees at 0.45 Hz with narrow bandwidth, three time constants are tuned at T1 = T2 = T3 = 350 ms. Frequency response characteristic of the washout function is presented in Fig. 6. To prevent unnecessary operation caused by noise in the frequency signal, the dead band was set during field tests to block frequency deviation below 21.5 mHz. (The setting was later modified to 30 mHz. This value was determined by observations that the power oscillation below 30 MW caused frequency deviation below 30 mHz.) Gain characteristic of the nonlinear gain during the field test is presented in Fig.7. The dead band was assigned at ±0.075%, and the slope of the gain is approximately 25. (The dead band was later modified to ±0.1 %.)

V. FIELD TESTS The first field test was conducted on Monday 31 July 2006 during light load (1.30 a.m. to 4.00 a.m.) so that the load variation would be low. Three steps of test are: 1. Testing of internal operation of PSD function with initial settings, 2. Resetting of PSD parameters according to the new system analysis and design, and

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IEEJ-EIT Joint Symposium on Advanced Technology in Power System Rama Garden Hotel, Thailand, November 19-20, 2007

oscillation frequency changed from 0.5 Hz to 0.55 Hz when tie transmission line configuration was changed from one circuit to two circuits.

3.Testing the influence of the PSD function on the dynamic performance of the power system. The effect of PSD function on the dynamic performance of the power system was investigated again on Tuesday 29 August 2006. In both field tests on dynamic performance of the power system, the power flow in the tie transmission line was regulated by the generation control of the thermal power plants and the hydro power plant in the southern system. The Power System Stabilizer (PSS) of the hydro power plant of Rajjaprabha (RHB-H plant) must be off during the tests to prevent the interference operation of the PSS. The sequence of dynamic performance test started with the regulation of power flow on the tie transmission line at 150 MW with the power flow from the central system to the southern system through two circuits of 230 kV line. After the system reached steady state, the PSD function was enabled. The disturbance was introduced by manual tripping of one circuit of the transmission line (With PSD). The responses of the power system to line tripping with PSD were recorded by Phasor Measurement Units (PMUs) at Bang Saphan 230 kV substation. After the system responses reached new steady states, second disturbance was introduced by manual reclosing of the trip-out line while PSD function was still enabled. The responses to line reclosing were recorded. The same sequence was carried out without PSD function. The complete set of field tests with manual tripping and manual reclosing, without PSD function and with PSD function were conducted again, but the power was forced to flow from the southern system to the central system. For the sake of comparison, the record of responses were rearranged to compare the response of the same variable without the PSD function and with PSD function as shown in Fig. 8 to Fig.11. In Fig.8, the power flow from the central system to the southern system was approximately 160 MW before disturbance in both cases. Power oscillation with frequency of 0.455 Hz was observed after one line was opened without the PSD function (W/O PSD). Magnitude of oscillation is about 22 MW and the oscillation took approximately 10 seconds to die out. With PSD function, the oscillation magnitude was reduced to 15 MW and it took only 4 seconds to settle (With PSD). Fig.9 shows the responses in the case of power flow from the central system to the southern system. After the line reclosing, the power oscillated with the magnitude of 30 MW and died out within 8 seconds without the PSD function. With PSD function, the oscillation magnitude was reduced to 20 MW and died out within 4 seconds. It could be seen from Fig. 8 and Fig.9 that

P ower Flow [MW] 220 210 200 190

2.0 sec

180 170 160 150 140 130 120

W/O P S D

With P S D

110 100 0

2

4

6

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10

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14

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18

20

T ime [sec]

Fig. 8. Dynamic response caused by one tie line open with power flowed from the central system to the southern system P ower Flow [MW] 240 230 220 210 200

1.8 sec

190 180 170 160 150 140

W/O P S D

With P S D

130 120 0

2

4

6

8

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T ime [sec]

Fig. 9. Dynamic response caused by one tie line reclose with power flowed from the central system to the southern system In Fig.10, the power flowed from the southern system to the central system at approximately 165 MW. After the introduction of disturbance by manual one line tripping, the frequency of oscillation was 0.455 Hz and it took more than 20 seconds to die out without PSD function. Significant improvement can be observed with PSD function, where the power oscillation died out within 6 seconds. Fig. 11 shows that the power flowed from the southern system to the central system at the level of 150 MW. After manual line reclosing, the power oscillation took more than 20 seconds to settle without PSD

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IEEJ-EIT Joint Symposium on Advanced Technology in Power System Rama Garden Hotel, Thailand, November 19-20, 2007

function. However, it took only 8 seconds to die out with PSD function. Moreover, it can also be seen that magnitude of oscillation was reduced because the HVDC transferred excess energy to the ac system of TNB. It can also be seen from Fig. 10 and Fig.11 that, oscillation frequency changed from 0.455 Hz to 0.5 Hz when tie transmission line configuration was changed from one circuit to two circuits. P ower Flow [MW]

Fig. 12. HVDC action during flow down and one line trip

-100 -110

W/O P S D

2.2 sec

With P S D

-120 -130 -140 -150 -160 -170 -180 -190 -200 -210 -220 0

2

4

6

8

10

12

14

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20

Fig. 13. HVDC action during flow down and one line reclose

T ime [sec]

Fig. 10. Dynamic response caused by one tie line trip with power flowed from the southern system to the central system P ower Flow [MW] -100

2.0 sec

-110

W/O P S D

With P S D

-120 -130 -140 -150 -160

Fig. 14. HVDC action during flow up and one line trip

-170 -180 -190 -200 -210 -220 0

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T ime [sec]

Fig. 11. Dynamic response caused by one tie line reclose with power flowed from the southern system to the central system Actions of the HVDC during the field tests are shown in Fig. 12 to Fig. 15. The HVDC was assigned to supply 300 MW to Thailand during the test. The HVDC actions during power flowed from the central system to the southern system (Flow Down) with one line trip is presented in Fig.12 and with one line re-close is presented in Fig. 13.

Fig. 15. HVDC action during flow up and one line reclose Fig.12 presents DC power swing to mitigate power oscillation on the tie transmission line when one line was open. The PSD function was activated in less than 3 seconds and the magnitude of DC power swing was

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IEEJ-EIT Joint Symposium on Advanced Technology in Power System Rama Garden Hotel, Thailand, November 19-20, 2007

approximately 15 MW (The power swing started from 290 MW to 305 MW). The action of the HVDC during line re-close is presented in Fig. 13. The PSD function was activated in less than 3 seconds and magnitude of the DC power swing was approximately 18 MW. Fig. 14 and Fig.15 present HVDC actions when power flowed from the southern system to the central system (flow up). Fig.14 presents HVDC action during one line tripping. The PSD function was activated in less than 3 seconds and magnitude of power swing was 18 MW. In Fig.15, the PSD function was activated within approximately 3.5 seconds and the magnitude of DC power swing was 18 MW during one line reclose. It can be seen from Fig. 12 to Fig 15 that the PSD function produces DC power swing to mitigate power oscillation in the tie transmission line. The PSD function was operated approximately within 3 seconds and maximum magnitude of DC power swing was limited to 18 MW which was according to the design criteria.

REFERENCES [1] Tormit Junrussameevilai and Nitus Voraphonpiput, “Characterization of Dynamic Phenomena on EGAT Tie Transmission Line Caused by Major Disturbance”, CEPSI 2006, Mumbai, India. [2] Nitus Voraphonpiput, Kittipon Chuangaroon and Somchai Chatratana, “Parameter Optimization for Frequency Limit Controller of EGAT-TNB HVDC Interconnection System”, CEPSI 2004, Shanghai, China. [3] Siemens, “Pole Control Design Specification of TNB-EGAT 300kV 300 MW HVDC”, 2001. [4] T. Sawa, Y.Shirai, T.Michigami, Y.Sakanaka and Y.Uemura, “A Filed Test of Power Swing Damping by Static Var Compensator”, IEEE Transaction on Power Systems, vol. 4, No.3, August 1989. Nitus Voraphonpiput received his B.Eng., M.Eng. and Ph.D. in Electrical Engineering from King Mongkut’s Institute of Technology North Bangkok (KMITNB), Thailand in 1993, 1998 and 2007 respectively. He joined the Electricity Generating Authority of Thailand (EGAT) in 1993 where he worked with Power System Operation Division. In 2003, he worked with Power System Analysis Department. Presently, he is an engineer level 8 in Technical Analysis-Foreign Power Purchase Branch, Power Purchase Agreement Division, EGAT. His fields of interest are Power System Operation and Control, Electromagnetic Transients in Power System, FACTS and HVDC.

VI. CONCLUDSIONS Power oscillation is one of the most important problems in large power system. It is necessary to mitigate this problem to achieve security of supply. Due to incomplete information of generator and system parameters, the design methodology presented in this paper was based on basic swing equation. All limits and criteria were thoroughly considered and compromises were made to achieve the objective of the design. Field tests were performed to investigate the performance of the PSD function under various disturbance conditions. Power oscillation on the tie transmission line was created by manual one line tripping and re-closing. Comparisons of results indicate that the PSD function can mitigate inter-area oscillation on the tie transmission line with satisfactory dynamic response. Thus, PSD function of the EGAT-TNB HVDC can effectively improve dynamic performance of the power system in Thailand.

Kittipon Chuangaroon was born in Yala, Thailand. He received his Bachelor’s degree in Electrical Engineering from the Institute of Technology and Vocation in Thailand. He joined the Electricity Generating Authority of Thailand in 1977 and has been engaged mainly in power system analysis since that time. He is currently a Senior Engineer in Control and Protection System Division at EGAT, Thailand.

ACKNOWLEDGEMENTS The authors would like to thank National Control Center (NCC), Central Area Control Center (CAC), Southern Area Control Center (SAC) and Tenaga Nasional Berhad (TNB) for conducting the field tests. The authors would like to thank K. Damrong, J.Tormit, M.Virat, N.Apimuk and operators of KNE Converter Station for relevant comments and contribution.

Somchai Chatratana received his B.Eng. (Honors) in Electrical Engineering from Kasetsart University in 1974, M.Sc. D.I.C. and Ph.D. from Imperial College, London University, U.K. in 1978 and 1982 respectively. He was Associate Professor at EE Department, King Mongkut’s Institute of Technology North Bangkok (KMITNB) until 2003. He is currently an Assistant to the President of National Science and Technology Development Agency (NSTDA). His current research interests include control of electrical drives, power quality and FACTS.

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