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

Outline•  Introduction to the Problem•  Steady State Stability Limit of

Transmission Lines•  Selecting optimum Series Compensation

Limit •  Conclusion & Recommendations

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

The Map of Sri Lanka Transmission System in Year 2022

Construction of 400kV Zebra transmission line to connect New Habarana – Sampoor

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)

Vector Diagram of A Series Compensated Line

Theoretical Analysis of Series

Compensation Technique

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

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

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

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

Assessment of Practical Line Loadability

•  PSS/E (Power System Simulator for Engineering) V.33.5.2 power flow software package was used

400kV Option

220kV Option

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

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

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

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

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

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.

Analysis of Dynamic Stability

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)

Voltage Stability – New Habarana 220kV

Voltage Stability – 400kV

No Voltage violations observed during the transient period considered

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.

Thank You