Series and Shunt Compensation.pptx

Power system

1. Surge Impedance Loading

2. Surge Impedance Loading (SIL )  is the most important parameter for determining the maximum loading capacity (MW loading) of transmission lines. Before understanding SIL in detail, first, we have to understand the concept of Surge and  Surge impedance (Zs)  and its physical significance. So let’s discuss the topic in detail .

3. What is Surge Impedance (Zs )? Surge impedance is nothing but the characteristic impedance ( Zc ) of the lossless transmission line. It is also known as the Natural impedance of the line. As we all know a long transmission line (length > 250 km) is represented by a distributed parameter model. In the Distributed parameter model of the long transmission line, resistance (R), inductance (L), capacitance (C), and conductance (G) are uniformly distributed over the whole length of the line (As shown in the below figure ).

4. Let us assume that the line has shunt admittance (y)   per unit length   series impedance (z) per unit length. Then the  Characteristic impedance ( Zc ) of any lossless transmission line is defined as the square root of (z/y ). Where ,   z = R + jwL   and  y = G + jwC . If we put the value of  z  and  y  in the definition of ( Zc ), then we find that  Characteristic Impedance is a complex quantity. However, for lossless transmission lines (R=0 and  G= 0 ) z = jwL   and  y = jwC .

5. According to the definition, Characteristic Impedance ( Zc ) is calculated as : Characteristic Impedance ( Zc ) = square root of ( jwL / jwC ). On simplifying it we got the result : Zs = Zc = square root of (L/C ). The above quantity has a dimension of resistance known as the  Surge impedance  of the line. When a purely resistive load of value equal to surge impedance is connected at the receiving end of the line, then the reactive power generated by the shunt capacitor will be completely absorbed by the series inductor of the transmission line . The value of Surge impedance for overhead transmission line is around 400 ohm, whereas surge impedance value for underground cable is around 40 ohm.

6. Significance of Surge impedance:- The significance of surge impedance is that if a pure resistance load that is equal to the surge impedance is connected to the end of the line with no resistance, a voltage surge introduced by the shunt capacitor to the sending end of the line would be completely absorbed by the series inductance at the receiving end of the transmission line . In this case, the voltage at the receiving end would have the same magnitude as the sending end voltage and also have a phase angle lagging with respect to the sending end by an amount equal to the time required to travel across the line from the sending end to the receiving end.  Surge impedance (Zs)  is a technical term that is used mostly in Electrical science in connection with the  Surges on transmission lines  which may appear due to switching or lightning operation in our Electrical power system.

7. What happens if the line terminates in surge impedance ? If a lossless transmission line terminates in its surge impedance (i.e. if the load is a pure resistance of value equal to the characteristic impedance of the line), then that transmission line is known as the  infinite line or flat line . So, in that case, many interesting phenomena happen in such a line : There will not be any reflection of forwarding traveling waves and hence there will be no standing wave in the line. Therefore, the voltage will be the same throughout the line. Hence, in this case, the receiving end and sending end voltage will be the same. The line will compensate itself. That is, the reactive power demanded by the series inductance of the line will be supplied by the shunt capacitance. That’s why there will be no voltage drop (due to series inductance) and also no voltage boost (due to shunt capacitance ). The load, as seen by the generator, is a pure resistance that will be equal to characteristic impedance. Hence the line is observed as equivalent to a pair of wires with zero resistance . Now come to our main topic  Surge impedance loading (SIL) and its significance .

8. What is Surge impedance loading (SIL )? In our power system, there are some limitations of loading on the transmission line network. Generally, the loading of any transmission line depends on some factors like : Thermal limitation (I 2 R Limitation) Voltage regulation Stability limitation So in context to these limitations  Surge impedance loading (SIL)  is an important parameter in electrical science to predict the maximum loading capacity of any transmission line. It is the maximum MW loading of the transmission line at which  reactive power  balance occurs.

9. SIL is defined as the maximum load (at unity  power factor ) that can be delivered by the transmission line when the loads terminate with a value equal to surge impedance (Zs) of the line.  Simply if any line terminates with surge impedance then the corresponding loading in MW is known as Surge Impedance Loading (SIL).   In other words, we can define surge impedance loading (SIL) as  SIL is the maximum load connected in the transmission line for which total reactive power generated (Capacitive VAR) is equal to total reactive power consumed (Inductive VAR) . So as to maintain an exact balance of reactive power consumption (by series inductance of line) and generation (by shunt capacitance of line). That’s why the net flow of reactive power in the transmission line will be zero and hence transmission line is assumed to be loaded as purely resistive load.

10. SI unit of surge impedance loading (SIL) is Mega-Watt (MW ). Mathematically SIL is expressed as: SIL (in MW) = (Square of line voltage in kV)/(Surge impedance in ohm ) Hence the formula for SIL will be: The above expression gives the maximum power limit that can be delivered by any transmission line which is very useful in designing the transmission line. SIL can be used for the comparison of loads that can be transmitted through the overhead transmission lines at different line voltages.

11. Calculation of Surge impedance loading (SIL ):- As we know long transmission lines (length > 250 km) are represented by the distributed parameter model. In this model, the capacitance and inductance are distributed uniformly along the line. When the line is charged then the shunt capacitance generates reactive power and feeds to the line while the series inductance absorbs the  reactive power . Hence voltage drop occurs in line due to series, inductance is compensated by the shunt capacitance of the line . If we take a balance of reactive powers due to inductance and capacitance then we get an expression as:

12. To simplify we got the following : Here the quantity having a dimension of resistance is surge impedance denoted by the symbol Zs. It is considered as a purely resistive load which when connected at the receiving end of the transmission line, then the reactive power generated by shunt capacitance will be completely absorbed by the series inductance of the line . Now the exact value of SIL can be calculated by putting the  surge impedance  (Zs) value in the above mathematical formula of SIL is expressed as : SIL (in MW) = (Square of line voltage in kV)/(Surge impedance in ohm )

13. Effect of Surge Impedance Loading (SIL ):- From the above expression of SIL we observed that SIL depends on the line voltage at the receiving end. Normally a line is loaded above SIL for better utilization of the conductor. In other words, we can say that SIL should always be less than the maximum loading capacity of the line . When the line is loaded less than its SIL, then it acts like a shunt capacitor which means it will supply MVAR to the system. In this case, the receiving end voltage will be greater than sending end voltage. In such a case line has to be compensated with an inductor to bring down the voltage to a normal level. However when the line is loaded above its SIL, then it acts like a shunt reactor that will absorb MVAR from the system. In such a case voltage drop occurs in the line, due to this receiving end voltage will be smaller than the sending end voltage. Hence a compensator is required to maintain the voltage level .

14. The below figure contains a graphic of the effect of SIL.  For a particular line of SIL value 450 MW. So if the line is loaded to 450 MW, then MVAR produced by the line will exactly balance the MVAR absorbed by the line. Hence there will be no flow of reactive power in the line.

15. Also when we observed the line voltage vs. length curve of the transmission line ( as shown in the below figure), we concluded different voltage profiles for loading the line in different conditions . If the loading is equal to SIL, then the voltage profile of the line is Flat. If the loading is greater than SIL, then the line has an inductive nature. If the loading is less than SIL, then the line has a capacitive nature .

16. How to improve surge impedance loading ? From the above expression of SIL, we observe that the transmitted  Electrical power   through a transmission line can be either increased by increasing the value of the receiving end line voltage (V LL ) or by reducing the value of surge impedance (Z s ). Since Voltage transmission capability is increasing day by day. So the most commonly adopted method for increasing the power limit of the heavily loaded transmission line is by increasing the voltage level. However, there is a limit beyond which it is neither economical nor practical to increase the receiving end line voltage of the power network. Another option is by reducing the value of  surge impedance (Z s )  or characteristics impedance of the transmission line, we can easily improve its surge impedance loading (SIL). Since surge impedance is directly proportional to inductance and inversely proportional to the capacitance. Hence the value of surge impedance can be reduced  either  by increasing the capacitor (C) of the line  or   by decreasing the inductance (L) of the line. However, the inductance of the line cannot be reduced easily.

17. Further , the capacitance value can be increased in two ways either by using a series capacitor or by using a shunt capacitor. Hence there are two methods to improve the surge impedance loading of transmission lines : Using series capacitor:  By the use of a series capacitor surge impedance and phase shift get reduced due to a decrease in inductance value (L). It also improves system stability. This capacitor also helps in reducing the line voltage drop. But the main problem with this method is It causes difficulty under the short circuit condition as a series capacitor will get damaged . Using a shunt capacitor:  Also by the use of a shunt capacitor surge impedance is reduced but the phase shift of the system increases. This affects the poor stability of the system especially when the synchronous machines are present in the load. So this method is not feasible where the stability limit is the main concern in the  power system . Hence surge impedance loading of line is increased by using either series capacitor or shunt capacitor in the transmission line. But practically series capacitor is a more feasible and effective method for improving the SIL of a line.

18. Conclusion:-

19. Series and Shunt Compensation of Transmission lines

20. Introduction Transmission lines involve heavy voltages during the transmission of power from the sending end to the receiving end. The power transferred consists of real power and the reactive power. The reactive power may be higher or less due to the reactance parameter of the transmission lines and the load impedance at the receiving end. Reactive power generated by the a.c power source is stored in a capacitor or reactor during a quarter of a cycle and in the next quarter of the cycle it is sent back to the power source. Therefore the reactive power oscillates between the a.c source and the capacitor or reactor at a frequency of twice the value(50 or60). So to avoid the circulation between the load and source, the reactive power needs to be compensated. Therefore, series compensation is used to modify the reactance parameter of the lines or power system while shunt compensation is to change the equivalent load impedance. In both cases, the line reactive power can be effectively controlled thereby improving the performance of overall electric power system.

21. Series Compensation Series Compensation is basically used to improve performance of extra high voltage transmission lines. It consists of a device connected in series with the line at suitable locations. There are two modes of operation: - capacitive mode of operation and the inductive mode of operation. A simplified model of transmission system with series compensation is shown below, the magnitude of the two buses is assumed to equal as V and δ is the phase angle between the two buses.

22. Series Compensation (cont.…..) By inserting reactive power in series with transmission line, the line impedance is reduced which improves the power transfer capability of the line. Series compensation can be achieved by using fixed or switched capacitors or by using a thyristor-controlled reactor. Fixed capacitors provide constant level of compensation while switched capacitors can be turned on or off to adjust the level of compensation as needed. TCRs can provide continuous and adjustable series compensation by using thyristors to control the flow of current through the reactor.

23. Advantages of series compensation Increases power transfer capacity : by reducing the impedance of the transmission line, series compensation can increase the power transfer capacity of the line, allowing it to transmit more power. Power transfer capacity of the line is given by; P = sin δ , where E= sending voltage V= receiving voltage X= reactance of the line δ = phase angle between E and V  

24. Advantages….. Power transfer without and with compensation P 1 = sin δ P 2 = sin δ = = = Where k is the degree of compensation The economic degree of compensation lies in the range of 40-70%  

25. Advantages…. Improvement of system stability : series compensation can help to improve the stability of transmission system by reducing voltage drop along the line and providing a stable source of reactive power. For the same amount of power transfer and same value of E and V, the δ in the case of compensated line is less than that of uncompensated line. P= sin δ 1 , P= = sin δ 2 , = A lower δ means better system stability. Series compensation offers most economic solution for the system stability as compared to other methods (reducing generator, x- mer reactance, bundled conductors, increase no. of parallel circuits).  

26. Advantages….. Less installation time : the installation time of the series capacitor is smaller( 2 years approx.) as compared to the installation time of the parallel circuit line(5 years approx.) This reduces risk factor. Hence used to hit the current thermal limit. The life of transmission line and capacitor is generally 20-25years. Load division between parallel circuits : When a system is to be strengthened by the addition of a new line or when one of the existing circuit is to be adjusted for parallel operation in order to achieve maximum transfer of or minimise losses, series compensation can be used.

27. Improves voltage transmission : By injecting voltage in to the transmission line, series compensation helps to improve the voltage profile at the load end and reduces the voltage drop along the line. This can help to improve the performance of equipment and reduce power loss. Reduced transmission losses : Series compensation can help to reduce transmission losses by improving the transmission of power along the distances Easy to maintain : Series compensation devices, such as fixed capacitors, are generally self-regulating and require little or no control equipment, which makes them easy to maintain.

28. Disadvantages of Series Compensation Increase in fault calculation. Mal operation of distance relay- if the degree of compensation and location is not proper. High recovery voltage of lines- across the circuit breaker contacts and is harmful. Limited effectiveness :   Series compensation is most effective during heavy load conditions, when the voltage drop along the transmission line is significant. During light load conditions, shunt compensation may be more effective in improving the power factor of the system.

29. Outage issues:  When an outage occurs on a transmission line with series compensation, the series compensation must be removed to prevent overloading of the other parallel lines. This can be a complex process and may require additional protection and control measures. Parallel line issues:  If series compensation is added to an existing transmission system, it may be necessary to have it on all lines in parallel to ensure that the system is balanced. This can be a complex and expensive process. High voltage issues:  During system outages, the series capacitors in the transmission line may be subjected to high voltage, which can lead to damage or failure. Sub-synchronous resonance:  Series compensation can cause sub-synchronous resonance (SSR) in some systems, which can lead to instability and damage to equipment. Additional expenses may be needed to address this issue. Problem of ferro-resonance ; occurring between the iron created inductance and the compensated line, leading to flow of high current. Disadvantages of series compensation….

30. Location of series compensating element The choice of the location of the series compensating element depends on many technical and economical considerations. In each case, a special system study concerning load flow, stability, transient overvoltage, protection requirement, system voltage profile, etc is necessary before the optimal location is chosen. Location along the line; capacitor bank located at middle of the line(if 1 bank) and at 1/3 th distance along the line(if 2 banks). This has advantage of better voltage profile along the line, lesser SC current the capacitor in even of fault and simpler protection of capacitor The capacitor stations are generally unattended.

31. Location of series compensating element…. Location at one or both ends of the line section on the line side in the switching station The main advantage of this location is that the capacitor installation is near the manned sub stations. However, requires more advanced line protection. For the same degree of compensation, more MVAr capacity is needed as compared method 1

32. Degree of series compensation We know that the surge impedance Z c = = = Suppose C se is the series capacitance per unit length for series compensation, Then the total series reactance will be j = j L . J ( ) ) Where is known as the degree of series compensation. Therefore, virtual surge impedance  

33. Shunt compensation Shunt compensation involves the use of a capacitor or reactor in parallel with a transmission line to improve its reactive power transmission characteristics. Shunt compensation is used to improve the power factor of the transmission system by providing a source of reactive power to the transmission line. For high voltage transmission line the capacitance is high and plays a significant role in voltage conditions of the receiving end. When line is loaded then the reactive power demand of the load is partially met by the reactive power generated by the line capacitance and the remaining reactive power flow through the line from the sending end to the receiving end.

34. Shunt compensation…. When load is high (more than SIL) then a large reactive power flows from the sending end to the receiving end resulting in large voltage drop in the line. To improve the voltage at the receiving end, shunt capacitors may be connected at the receiving end to generate and feed the reactive power to the load so that reactive power flow through the line and consequently the voltage drop in the line is reduced. To control the receiving end voltage, a bank of capacitors (large number of capacitors connected in parallel) is installed at the receiving end and a suitable number of capacitors are switched in during the high load condition depending upon the load demand.

35. Shunt compensation…… Thus the capacitors provide leading VAr to partially meet reactive power demand of the load to control the voltage. If be the reactance of the shunt capacitor then the reactive power generated of leading VAr supplied by the capacitor. , where is the magnitude of receiving end voltage.  

36. Shunt compensation…… When load is small(less than SIL) then the load reactive power demand may even be lesser than the reactive power generated by the line capacitor. Under these conditions the reactive power flow through the line becomes negative, i.e. the reactive power flows from receiving end to sending end, and the receiving end is higher than the sending end voltage (Ferranti effect). To control the voltage at the receiving end it is necessary to absorb or sink reactive power. This is achieved by connecting shunt reactors at the receiving end.

37. Shunt compensation….. If be the reactance of the shunt reactor (inductor) then the reactive VAr absorbed by the shunt reactor. is the magnitude of receiving end voltage. To control the receiving end voltage generally one shunt reactor is installed and switched in during the light load condition. To meet the variable reactive power demands requisite number of shunt capacitor are switched in, in addition to the shunt reactor, which results in adjustable reactive power absorption by the combination.  

38. Advantages of shunt compensation Improves voltage profile:  this done by providing a source of reactive power to the transmission line. This can help to reduce voltage drop and improve the performance of equipment. Effective at all load levels:  Unlike series compensation, shunt compensation is effective at all load levels. This makes it a useful tool for improving the power factor of a transmission system regardless of the load on the system. Fast control of over voltages:  Shunt compensation can provide fast control of temporary overvoltage that may occur in a transmission system. This can help to protect equipment and improve the stability of the system. Cost-effective:  Shunt compensation is generally less expensive than series compensation, making it a cost-effective option for improving the transmission of power and correcting the power factor of a system. Easy to maintain:  Shunt compensation devices, such as fixed capacitors, are generally self-regulating and require little or no control equipment, which makes them easy to maintain.

39. Disadvantages of shunt compensation Higher cost:  Shunt compensation is generally more expensive than series compensation, particularly when large amounts of reactive power are required. Limited overload capability:  The overload capability of shunt compensation is limited, as the capacitors can only provide a certain amount of reactive power before they become overloaded. Complex control:  Shunt compensation requires complex control systems to ensure that the correct amount of reactive power is provided to the transmission line. This can increase the cost and maintenance requirements of the system. Limited power transfer capability:  Shunt compensation does not directly improve the power transfer capability of a transmission line. To increase the power transfer capacity of the line, other measures, such as series compensation or upgrading the line, may be needed. Risk of overvoltage:  If the shunt compensation is not properly coordinated with the rest of the transmission system, it can lead to overvoltage and instability in the system.

40. Differences between series and shunt compensation Series compensation Lower cost They used to control loading of parallel lines They are self regulating and increases the system stability Series reactor used to limit SC current. Series capacitor used to increase transmission capacity and improve stability Shunt compensation Higher cost compared to series compensation They are used to regulate the grid voltage It has greater voltage control and stability Shunt reactor used to avoid Ferranti effect Shunt capacitor used to improve power factor

41. Conclusion Following the above discussion, we can conclude that the series compensation is used in long transmission lines to improve the voltage profile at the receiving end, while shunt compensation is used to provide reactive power compensation and improve power factor of the transmission system. The capacity of the shunt compensation device depends on the reactive power requirement of the transmission system and the size of the lagging load.