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Incorporating Line Series Compensation Technique to Enhance the Power Transfer
Capability of Future Transmission System in Sri Lanka
P. A . G. S . A B E Y N A Y A K EE l e c t r i c a l E n g i n e e r
Tr a n s m i s s i o n P l a n n i n gC E Y L O N E L E C T R I C I T Y B O A R D
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Outline• Introduction to the Problem• Steady State Stability Limit of
Transmission Lines• Selecting optimum Series Compensation
Limit • Conclusion & Recommendations
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Introduction• Long Term Generation Expansion Plan (2015-2034) in
compliance with Long Term Transmission Development Plan (LTTDP) (2015 – 2024) published by Ceylon Electricity Board, in year 2022 it has proposed a 95km long 4xZebra 400kV transmission Line from proposed Sampoor Coal fired generating station to New Habarana Switching Station
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The Map of Sri Lanka Transmission System in Year 2022
Construction of 400kV Zebra transmission line to connect New Habarana – Sampoor
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Series Compensation• Series compensation is a well established and proven
technology • It has been in commercial use since the early 1960s • It enables increase of ;
a) Power Transmission Capacity b) Steady-State stability c) Dynamic Stability
𝑃= 𝑉↓𝑠 𝑥𝑉↓𝑟 /𝑋↓𝑙 − 𝑋↓𝑐 Sin δ (1)𝑘= 𝑋↓𝐶 /𝑋↓𝑙 (2)
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Vector Diagram of A Series Compensated Line
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Theoretical Analysis of Series
Compensation Technique
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Transmission Line Loadability Curve
Ø The loadability of short transmission lines (<80km) is limited by the thermal rating of the conductors
Ø For Medium (80 km < x < 250 km) and Long (>250km) transmission lines, line loadability is much lower than it’s thermal limit
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MajorLineLoadinglimits1. Thermal Limit – This depends on line loss and the maximum
temperature of the conductor. If the line temperature is increasingthenlinestartstretchingandgroundclearancewillreduce
2. Voltagedroplimit–VoltagedropistypicallymaintainedsuchthatVR/VS≥0.95
3. Steady State Stability Limit – Themaximum power that the line cansupplied is called the steady state stability limit (SSSL). If we try totransmit power more than SSSL, then synchronous machines at thesendingendwouldlosesynchronismwiththoseatthereceivingend
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PowerTransferCapabilityofLines
𝑃↓𝑅 = 𝑉↓𝑅 𝑉↓𝑆 /𝑍 𝐶𝑂𝑆(𝜃↓𝑍 −𝛿)− 𝐴𝑉↓𝑅↑2 /𝑍 𝐶𝑂𝑆(𝜃↓𝑍 − 𝜃↓𝐴 ) (1)
q Theoreticalmaximumpower(PR)deliveredforalossyline
q TheSteady-StateStabilityLimitoccurswhen 𝜃↓𝑍 =𝛿
𝑆𝑆𝑆𝐿= 𝑉↓𝑅 𝑉↓𝑆 /𝑍 − 𝐴 𝑉↓𝑅↑2 /𝑍 𝐶𝑂𝑆( 𝜃↓𝑍 − 𝜃↓𝐴 ) (2)
Conductor TypeSSSL1
(Vr=Vs)SSSL2 (Vr=0.95Vs)
SSSLpr (with Vr=0.95Vs and
δ = 350)(MW)
2xZebra 1447.70 1375.32 868.11
4xZebra 1960.65 1862.61 1103.03
Lineloadabilityforuncompensated220kVZebraConductor
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SSSLfor220kVZebraConductorwithSeriesCompensation
Conductor Type
SSSL (MW) with Vr=VsCompensation
10% 15% 20% 25%
2xZebra 1579.92 1655.08 1737.29 1827.51
4xZebra 2155.22 2267.51 2391.97 2530.54
Conductor Type
SSSL (MW) with Vr=VsCompensation
30% 35% 40% 45% 50%
2xZebra 1926.78 2036.33 2157.35 2291.15 2438.83
4xZebra 2685.79 2860.65 3059.05 3285.58 3546.28
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Assessment of Practical Line Loadability
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• PSS/E (Power System Simulator for Engineering) V.33.5.2 power flow software package was used
400kV Option
220kV Option
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P-V Analysis in PSS/E
Bus
Volta
ge (V
) /pu
Load real power (P) /MW
Voltage collapse stability margin/loadability margin
Critical Voltage
Maximum Transfer
Source:VoltageStabilityUsingPVCurves,2014PowerWorldCorporation,slide6
Thevoltagecollapsestabilitymargin(VCSM)otherwisecalledasloadabilitymarginisthemaximumactiveloadatthatbusbarforthetotalcriticalloadinthepowersystem
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0.950
0.960
0.970
0.980
0.990
1.000
1.010
1.020
650.000 675.000 700.000 725.000 750.000 775.000 800.000 825.000 850.000 875.000 900.000
Bus
Volta
ge /
pu
Branch Flow / MW
No comp 10% 15% 20% 25% 30% 35% 40% 45% 50% 400kV
P-VAnalysisResultsNormal Operating Condition
New Habarana Bus Voltage with different line compensation ratios - 4xZebra 220kV
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0.940
0.950
0.960
0.970
0.980
0.990
1.000
1.010
1.020
645.000 670.000 695.000 720.000 745.000 770.000 795.000 820.000 845.000 870.000 895.000
Bus
Volta
ge /
pu
Branch Flow / MW
No comp 10% 15% 20% 25% 30%
35% 40% 45% 50% 400kV
New Habarana Bus Voltage with different compensation ratios - 2xZebra 220kV
P-VAnalysisResultsNormal Operating Condition
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P-VAnalysisResultsSingle Contingency Condition
0.920
0.930
0.940
0.950
0.960
0.970
0.980
0.990
1.000
1250.000 1300.000 1350.000 1400.000 1450.000 1500.000 1550.000 1600.000 1650.000 1700.000
Bus
Volta
ge /
pu
Branch Flow / MW
20% 25% 30% 35% 40% 45% 50% 400kV
1393.4
New Habarana Bus Voltage under One line outage (N-1) with diffrerent line compensation ratios - 4xZebra
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0.920
0.930
0.940
0.950
0.960
0.970
0.980
0.990
1.000
1200.0001250.0001300.0001350.0001400.0001450.0001500.0001550.0001600.0001650.0001700.000
Bus
Volta
ge /
pu
Branch Flow / MW
50% 400kV
New Habarana Bus Voltage under One line outage (N-1) with diffrerent compensation ratios - 2xZebra
P-VAnalysisResultsSingle Contingency Condition
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Analysis of Simulation Results q The allowable Steady-State voltage range for 220kV is ±5% and ±10% under single
contingency condition. Operation beyond this limit will violates the planning criteria
q The selected conductor among above (with or without series compensation), should be able to carry-out 1393.4 MW amount of power under normal and contingency condition
q In order to select a suitable compensation ratio and a conductor (i.e 2xZebra, 4xZebra) the scenario should not violate the allowable voltage limits
q Under single contingency operation q 220kV 4xZebra – network converged above 20% series compensation ratioq 220kV 2xZebar – network converged only for 50% series compensation ratioq 400kV – 4xZebar – network converged
q From 220kV 4xZebra; conductors (with series compensation) which can be loaded above 1393.4 MW should be selected. Under this condition above 40% compensation can be selected as the suitable series compensation ratio
q It is worth to note that voltage drop of 400kV line is superior compared to 220kV compensated lines.
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Analysis of Dynamic Stability
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Voltage Stability - Sampoor 220kVPSS/E dynamic simulation was carried out to observe the voltage stability with series compensation by tripping of one Sampoor – New Habarana 220kV line (for 40% and 50% compensation)
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Voltage Stability – New Habarana 220kV
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Voltage Stability – 400kV
No Voltage violations observed during the transient period considered
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CONCLUSION • Technical analysis suggests that over 40% seriescompensation (200kV 4xZebra conductor) can beused instead of 400kV 4xZebar conductor totransmit bulk power generated at Sampoorswitchingstation.
• However,intermsofpowertransfercapabilityandthe steady state voltage stability, 400kV option issuperiortootheroptionsconsidered.
• Adetaileconomicanalysisisrequiredtoselectthemostpromisingtechno-economicoption.
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Thank You