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3Transmission system reactive power compensation and control : 

3Transmission system reactive power compensation and control

Slide 2: 

2

Classifications : 

3 Classifications Series versus shunt compensation. Passive versus active compensation. Common forms: Series capacitor banks. Shunt reactors. Shunt capacitor banks. Static VAR Compensators. Under-load tap changers.

NERC/WECC Planning Standards on Voltage Support and Reactive Power : 

4 NERC/WECC Planning Standards on Voltage Support and Reactive Power Both static and dynamic reactive power resources are needed to supply the reactive power requirements of customer demands and the reactive power losses in the transmission and distribution systems, and provide adequate system voltage support and control.

Slide 5: 

5 They are also necessary to avoid voltage instability and widespread system collapse in the event of certain contingencies. Transmission systems cannot perform their intended functions without an adequate reactive power supply. Dynamic reactive power support and voltage control are essential during power system disturbances.

Slide 6: 

6 Synchronous generators, synchronous condensers, and static var compensators (SVCs and STATCOMs) can provide dynamic support. Transmission line charging and series and shunt capacitors are also sources of reactive support, but are static sources.

Transmission system characteristics : 

7 Transmission system characteristics Lines produce and consume reactive power. Line charging generates relatively constant reactive power. Series inductance consumes reactive power. Heavily loaded long lines are trouble. They can not transmit reactive power. In fact, they consume reactive power.

Slide 8: 

8 Transmission line consists of: Shunt capacitance Series resistance and reactance Distributed along length of line Treat as distributed lumped elements Can ignore resistance

Slide 9: 

9 Transmission line Q production=BV2 Transmission line Q Consumption=XI2.

Surge Impedance Loading (SIL) : 

10 Surge Impedance Loading (SIL) Close the breaker at sending end Shunt capacitance charges to ½ CwV2 Close the breaker at receiving end and feed the load Series inductances use energy at ½ wLI2

Surge Impedance Loading : 

11 Surge Impedance Loading Natural surge impedance is obtained for equal production and consumption of Q.

Reactive Support : 

12 Reactive Support At SIL, shunt capacitive energy equals series reactive energy Below SIL there is an excess of shunt capacitive energy Above SIL there is a deficiency of shunt capacitive energy Sending and receiving ends have to supply this energy

St. Clair Curve : 

13 St. Clair Curve Based on empirical knowledge of 1950’s Maximum loading at 50 miles (80 km) 3 x SIL Maximum loading at 300 miles (480 km) 1 x SIL St. Clair recognized relationship not linear and drew a curved line between the two points Assumed infinite VAr supply

St. Clair Curve : 

14 St. Clair Curve Dunlop et al in 1979 supplied analytical development for the loading curve Loading dependent on voltage drop up to approximately 200 miles (320 km) Loading dependent upon Steady State Stability Limit above 200 mile (320 km) Assumed fully developed systems, i.e. high short current or low short circuit impedance

Slide 15: 

15

Line Parameters : 

16 Line Parameters Consider the formulae for inductance and capacitance of lines. Conclude that reducing phase spacing and using bundle conductors reduce L and increase capacitance. Increases SIL and transmission capacity.

Slide 17: 

17

Slide 18: 

18

Ferranti Effect : 

19 Ferranti Effect Open circuit voltage rise at the receiving end Can be worked out from first principles Voltage rise equal to the inverse of the cosine of the line angle Open end voltage rise on 240 kV, 250 km line is:

Cables : 

20 Cables Inductive reactance X is lower than X of OHL. Capacitance C is higher than C of OHL. Critical length (CL) is where charging power equals cable thermal capacity. EHV cables have CL= 25 km.

Series capacitors : 

21 Series capacitors Conventional use for EHV long lines to improve transient stability. Now apply to short lines to improve voltage stability. QC = XI2 when most needed. Inherent self regulation. No effect at light load.

Slide 22: 

22 Series capacitors are connected in the line at line terminals or intermediate points along the line. Series compensation raises SIL. 0.5 series comp increases SIL by 41%

Difficulties with series comp : 

23 Difficulties with series comp For parallel lines, one line outage causes overloading of remaining lines. Quadruple reactive power generation. Take advantage of short time overload capability. Under heavy load, get high voltages at one end of series cap. Sub synchronous resonance concern.

Shunt capacitors and reactors : 

24 Shunt capacitors and reactors Shunt capacitors are bus connected, not line connected. Shunt reactors may be either line or bus connected. Good for voltage stability. With lower voltage, Q drops. Not self regulating.

Slide 25: 

25 Mechanically switched capacitor banks have advantage of lower cost. Fast switching speed. After line outage delay capacitor switching to allow line reclosing. Capacitor needs to be switched before significant load amounts.

Disadvantages of Shunt Caps : 

26 Disadvantages of Shunt Caps For transient voltage instability, switching speed may no e fast enough. Damaging overvoltages can happen after separation of subsystems.

Static var systems : 

27 Static var systems Overcome limitations of switched caps. Fast voltage regulation. Static VAR System = Static VAR Comp+ controlling also shunt cap, reactor mechanical switching. Thyristor controlled reactor (TCR) Thyristor switched capacitor (TSC)

Slide 28: 

28 Long lines are supported at intermediate points. SVC’s have fast response. Close regulation of voltage. Float (inductive) output under normal conditions, with rapid capacitive boost on disturbances.

Slide 29: 

29 Return to set point after situation stabilized. SVC slope setting is important.

Comparisons between series and shunt compensation : 

30 Comparisons between series and shunt compensation Advantages of series compensation: Less expensive. Self regulating. For voltage stability, series capacitors lower the critical or collapse voltage. Significant time-overload capability. Can control loading of parallel lines to minimize losses.

Slide 31: 

31 Series Compensation disadvantages: To reinforce an established grid, may need to series compensate many paralel lines. Capacitors in parallel lines may overload. Overvoltage at one end. May need shunt reactors at light load. Subsynchronous resonance.

SVC Advantages : 

32 SVC Advantages Direct voltage control. Rapid control of temporary overvoltages.

SVC Disadvantages : 

33 SVC Disadvantages Have limited overload capability. The critical or collapse voltage becomes the SVC-regulated voltage. Instability happens once a limit is reached. Expensive

Synchronous condensers : 

34 Synchronous condensers Not competitive with SVC cost wise. In voltage-weak systems SC have advantage of not being voltage dependent.

Transmission network LTC transformers : 

35 Transmission network LTC transformers Provide voltage and reactive power control. Tap changers regulate the low voltage side. Faster load restoration.

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