PSA_Unit-6 electrical engineering subject.pptx

Psa

1. Unit 6 Security Analysis Prepared by Dr. B. M. Nayak

2. Samarth Educational Trust Arvind Gavali College of Engineering, Satara   Institute Vision To be an institute of excellence, developing skilled engineers to serve the industry and society. Institute Mission M1: To provide quality education through effective teaching learning process. M2: To develop professional skills and promote innovation among students by providing a conducive atmosphere M3: To inculcate ethical values, respect for the environment, and social responsibility

3. Department of Electrical Engineering   Vision of the Department To be the center for developing professional and skilled electrical engineers, by providing quality education to solve industrial and social problems. Mission of the Department The department is committed to imbibe and empower its faculty and aspiring engineers with: M1: To impart quality education in electrical engineering using effective teaching learning process. M2: To develop skills & attitude to achieve a successful career. M3: To inspire students for becoming socially committed professionals with ethical values.

4. What is power system security Power system security may be looked upon as the probability of the system’s operating point remaining within acceptable ranges, given the probabilities of changes in the system (contingencies) and its environment.

5. A power system may be identified to be operating in a number of states.

6. Preventive state The preventive state is actually the normal state. The term ‘preventive’ was used to stress the ‘Security’ aspect of the normal operation. Normal operating condition usually means that all the apparatus are running within their prescribed limits, and all the system variables are within acceptable ranges. The system should also continue to operate ‘normally’ even in the case of credible contingencies. The operator should ‘foresee’ such contingencies (disturbances) and take preventive control actions (as economically as possible) such that the system integrity and quality of power supply is maintained.

7. Emergency state The power system enters an emergency state when some of the components operating limits are violated; some of the states wander outside the acceptable ranges, or when the system frequency starts to decrease. The control objective in the emergency state is to relieve system stress by appropriate actions. Economic considerations become secondary at this stag

8. Restorative state Restorative state is the condition when some parts (or whole) of the system has lost power. The control objective in this state is to steer the system to a normal state again by taking appropriate actions.

9. Operating under stress and strain

10. State transition diagram,... contd. Power system operation can be described by three sets of generic equations : one differential , and two algebraic . Of the two sets of algebraic equations, one comprises of equality constraints(E ), which is the balance between generation and load demand . The other set consists of inequality constraints (I) which ensure that the various components in the system and the states (e.g. voltages and currents ) remain within safe or acceptable limits

11. State transition diagram,... contd. If the generation falls below certain threshold , load increases beyond some limit, or a potentially dangerous disturbance becomes imminent, the system is said to enter the alert state . Though the equality (E) and inequality (I) constraints are still maintained , preventive controls should be brought into action to steer the system out of the alert state. If preventive control fails , or the disturbance is reverse , the system may enter into an emergency state, though the demand is still met by the generation, one or more component or state violate the prescribed operation limits. Emergency control actions should immediately be brought into action to bring the system back to the normal state .

12. State transition diagram... contd. If the emergency control actions also fail , the system may enter extremis state which is characterized by disintegration of the entire system into smaller islands, or a complete system blackout . It may take anywhere between few seconds to few minutes for a system to enter an extremis state from a normal state. The restoration process however is very slow . It may take several hours or even days to bring the system back to normal.

13. Power system security – Introduction Practically, the power system needs to be secured . We need to protect it from the black out or any internal or external damage . The operation of the power system is set to be normal only when the flow of power and the bus voltages are within the limits even though there is a change in the load or at the generation side . From this we can say that the security of the power system is an important aspect with respect to the continuation of its operation.

14. A very important aspect of the power system security is its ability to withstand the effect of contingency which is actually an output of either a generator, bus bars, transmission line, transformer etc. The contingency analysis technique is being widely used to predict the affect of the failures in the equipment used in power system. It is quite necessary task so as to keep the power system safe and secured . Though maintaining the security in power system is a challenging work for the engineers but it is even equally important to maintain the state of operation.

15. Power System Security Analysis: Power System Security Analysis can be broken down into two major functions that are carried out in an operations control centre: Security assessment, and Security control. The former gives the security level of the system operating state. The latter determines the appropriate security constrained scheduling required to optimally attain the target security level.

16. The security functions in an EMS can be executed in ‘real time’ and ‘study’ modes. Real time application functions have a particular need for computing speed and reliability. The static security level of a power system is characterised by the presence or otherwise of emergency operating conditions (limit violations) in its actual (pre-contingency) or potential (post-contingency) operating states. Power System Security Analysis assessment is the process by which any such violations are detected.

17. System assessment involves Two Functions: System monitoring Contingency analysis Preventive and corrective actions System monitoring provides the operator of the  power system  with pertinent up-to-date information on the current conditions of the power system . In its simplest form, this just detects violations in the actual system operating state. System monitoring The prerequisite for security assessment of a power system is the knowledge of the system states. Monitoring the system is therefore the first step.

18. Measurement devices dispersed throughout the system help in getting a picture of the current operating state. The measurements can be in the form of power injections, power flows, voltage, current, status of circuit breakers, switches, transformer taps, generator output etc ., which are telemetered to the control centre. Usually a state estima tor is used in the control centre to process these telemetered data and compute the best estimates of the system states. Remote control of the circuit breakers , disconnector switches, transformer taps etc. is generally possible. The entire measurement and control system is commonly known as supervisory control and data acquisition (SCADA) system.

19. Contingency For the power system to be secured there must have continuity in the supply without any loses. Whenever the operating variable are out from the specified limits the power system comes into the emergency system. These violation of the operating variable result into the contingency occurring into the system. Thus an important of the security analysis moves around the power system to with stand affect of  contingency. The  contingency analysis is time consuming as it involves computation of load flow calculation followed by the outages from the transmission line, generator, transformer etc.

20. Contingency analysis Contingency analysis is much more demanding and normally performed in three distinct states , i.e. contin­gency definition , selection and evaluation . The  contingency analysis basically involves the simulation of ever contingency of the power system. But this analysis involves three major difficulties 1. Difficulty to develop the appropriate power system model. 2. Confusion to choose contingency case. 3. Difficulty in computing the power flow and the bus voltages which leaves to high time consumption. The contingency analysis is divided into three different stages

21. 1 . Contingency definition  – It comprise of set of contingency that occur in the power system. 2.  Selection  – It is the process of selecting the most severe contingencies from the contingency list. Thus this process removes the  unimportant contingencies and hence the contingency list is shortened. 3 . Evaluation  – In this process it involves the necessary security action or control to function in order to remove the affect of contingency. contingency analysis using sensitivity factor It is  one of the easiest calculation way to provide quick calculation of the possible overloads. These factors show the changes in generation on the network configuration and are derived from dc load flow. The system security assessment is carried out by calculating system operating limits in the  pre contingency and post contingency  operating states . Pre contingency  – It is the state of the power system before the contingency has occurred. Post contingency – It is the state of the power system after the contingency has occurred.It is assumed that this type of the contingency has the security violations such as the line or transformer are beyond its flow limit or the bus voltage is within its limit.

22. Related question and answers 1. What is meant by power system security? Ans :   The power system needs to be secured, we need to protect it from the black out or any internal or external damage.The operation of the power system is set to be normal only when the flow of power and the bus voltages are within the limits even though there is a profitable change in the load or at the generation side. From this we can say that the security of the power system is an important aspect with respect to the continuation of its operation. 2. What are the two functions of security? Ans :  Security function are of two type as follows: Security control  :- It determines the exact and proper security constraint scheduling which is required to obtain the maximized security level. Security assessment  :- It gives the security level of the system in the operating state. 3. Name the various security level. Ans :   The levels of power system security are classified into 5 states:- 1. Normal 2. Alert 3. Emergency 4. Extreme emergency 5. Restorative

23. 4. What is contingency analysis? Ans :  The  contingency analysis basically involves the simulation of ever contingency of the power system. But this analysis involves three major difficulties 1. Difficulty to develop the appropriate power system model. 2. Confusion to choose contingency case. 3. Difficulty in computing the power flow and the bus voltages which leaves to high time consumption 5.What is meant by pre and post contingency ? The system security assessment is carried out by calculating system operating limits in the  pre contingency and post contingency  operating states . Pre contingency  – It is the state of the power system before the contingency has occurred. Post contingency – It is the state of the power system after the contingency has occurred.It is assumed that this type of the contingency has the security violations such as the line or transformer are beyond its flow limit or the bus voltage is within its limit 6.what is meant by corrective rescheduling?? It is basically defined as the measures taken to avoid the the faults after it is occurred i.e.to isolate the defective fault from the non-faulty part of the power system and then further rectify the fault.After the rectification of the fault in the power system it is then restored in the power system from where it was isolated. 7.explain the term economic load dispatch. Economic load dispatch means that generator’s real and reactive power are allowed to vary within certain limits so as to meet particular load demand with minimum fuel cost. It is the process of finding out the maximum output from the generation facilities to meet the load demand while serving the power in reliable manner at lowest possible cost.

24. Contingency Selection: There are two main approaches: Direct methods:  These involve screening and direct ranking of contingency cases. They monitor the appropriate post-contingent quantities (flows, voltages). The severity measure is often a performance index. Indirect methods:  These give the values of the contingency case severity indices for ranking, without calculating the monitored contingent quantities directly.

25. Contingency evaluation is then performed (using AC power flow) on the successive individual cases in decreasing order of severity. The evaluation process is continued up to the point where no post-contingency violations are encountered. Hence, the purpose of contingency analysis is to identify the list of contingencies that, if occur, would create violations in system operating states . They are ranked in order of severity.

26. The second major security function, security control , allows operating personnel to change the power system operation in the event that a contingency analysis program predicts a serious problem, should a certain outage occur. Normally the security control is achieved through SCO (security constrained optimization) program.

27. Modelling for Contingency Analysis : The power system limits of most interest in contingency analysis are those on line flows and bus voltages. Since these are soft limits, limited-accuracy models and solutions are justified. The most fundamental approximate load flow model is the NR model. The DC load flow model in its incremental version is normally preferred.

28. This model assumes voltages to remain constant after contingencies. However, this is not true for weak systems. The utility has to prespecify whether it wants to monitor post-contingency “steady-state” conditions imme­diately after the outage (system inertial response) or after the automatic controls. (governor, AGC, ED) have responded. Depending upon this decision, different participation factors are used to allocate the MW generation among the remaining units. The reactive problem tends to be more non-linear and voltages are also influenced by active power flows.

29. Preventive and corrective actions Preventive and corrective actions are needed to maintain a secure operation of a system or to bring it to a secure operating state . Corrective actions such as switching of VAR compensating devices , changing transformer taps and phase shifters etc. are mainly automatic in nature, and involve short duration . Preventive actions such as generation rescheduling involve longer time scales . Security-constrained optimal power flow is an example of rescheduling the generations in the system in order to ensure a secure operation .

30. ELECTRICAL POWER QUALITY Power quality refers to maintaining a sinusoidal waveform of bus voltages at rated voltage and frequency . The waveform of electric power at generation stage is purely sinusoidal and free from any distortion . Many devices that distort the waveform. These distortions may propagate all over the electrical network. Classification of power quality: Areas may be made according to the source of the problem such as, Converters Magnetic circuit non linearity Arc furnace or by the wave shape of the signal such as, harmonics Flicker or by the frequency spectrum (radio frequency interference).

31. CAUSES OF POWER QUALITY DETERIORATION 1. Natural causes : Faults or lighting strikes on transmission lines or distribution feeders Falling of trees or branches on distribution feeders during stormy conditions, equipment failure etc. 2. Due to load or transmission line / feeder operation : Transformer energisation Capacitor or feeder switching Power electronic loads (UPS, ASD, converters etc.) Arc furnaces and induction heating systems Switching on or off of large loads etc.

32. POWER QUALITY DEFINITION Whole of power engineering, in one way or other is related to power quality. There is no universal agreement for the definition of power quality. A Utility may define power quality as reliability and show statistics demonstrating that its system is 99.98 percent reliable. A manufacturer of load equipment may define power quality as those characteristics of the power supply that enable the equipment to work properly. These characteristics can be very different for different criteria.

33. Transients : undesirable momentary deviation of the supply voltage or load current. Also known as surges or spikes. Transients can be classified into two categories, 1.Impulsive 2.Oscillatory Impulsive transient : It is a sudden, non–power frequency change in the steady-state condition of voltage, current, or both. It is unidirectional in polarity ( primarily either positive or negative). Impulsive transients are normally characterized by their rise and decay times, which can also be revealed by their spectral content.

34. For example, 1.2 *50-μs 2000-volt (V) impulsive transient nominally rises from zero to its peak value of 2000 V in 1.2μs and then decays to half its peak value in 50μ s . The most common cause of impulsive transients is lightning. Impulsive transient

35. Oscillatory Transient: It is a sudden, non–power frequency change in the steady-state condition of voltage, current, or both . It includes both positive and negative polarity values . It consists of a voltage or current whose instantaneous value changes polarity rapidly. It is described by its spectral content (predominate frequency), duration, and magnitude . The spectral content subclasses defined in Table are High Medium Low frequency

36. HF: Primary Freq component > 500khz measured in Micro-Sec duration - Local system response to Imp. Transmission. Med Freq: Primary Freq component 5-500khz measured in Micro-Sec duration - Back-to-back capacitor energization Low Freq: Primary Freq component<5khz measured in Micro-Sec duration 0.3 to 50 ms - Cap Bank energization (T&D). Oscillatory transient current caused by back-to-back capacitor switching.

37. Oscillatory transient

38. Long-Duration Voltage Variations: Long-duration variations encompass root-mean-square ( rms ) deviations at power frequencies for longer than 1 min . It can be either over-voltages OR under-voltages. AND Sustained interruption . Over-voltages and under-voltages generally are not the result of system faults, but are caused by load variations on the system and system switching operations. Long-duration variations are typically displayed as plots of rms voltage versus time. OVERVOLTAGES: Increase in the rms ac voltage greater than 110 percent at the power frequency for a duration longer than 1 min.

39. CAUSES: 1. Load switching (e.g., switching off a large load or energizing a capacitor bank) 2. Incorrect tap settings on transformers can also result in system over-voltages. EFFECT: The over-voltages -result because either the system is too weak for the desired voltage regulation or voltage controls are inadequate. UNDERVOLTAGES: Decrease in the rms ac voltage to less than 90 percent at the power frequency for a duration longer than 1 min. Due to switching events that are the opposite of the events that cause over-voltages.

40. CAUSES: 1.A load switching on or a capacitor bank switching off can cause an under voltage until voltage regulation equipment on the system can bring the voltage back to within tolerances. 2.Overloaded circuits can result in under-voltages. SUSTAINED INTERRUPTION: When the supply voltage has been zero for a period of time in excess of 1 min, the long-duration voltage variation is considered a sustained interruption. This term has been defined to be more specific regarding the absence of voltage for long periods.

41. SHORT DURATION VARIATIONS: This category encompasses the IEC category of voltage dips and short interruptions. Each type of variation can be designated as, 1.Instantaneous, 2.Momentary, 3.Temporary, depending on its duration as defined in Table 2.2. CAUSES: 1.Fault conditions 2.The energization of large loads which require high starting currents 3.Intermittent loose connections in power wiring. Depending on the fault location and the system conditions, the fault can cause either temporary voltage drops (sags), voltage rises (swells), or a complete loss of voltage (interruptions).

42. INTERRUPTION: An interruption occurs when the supply voltage or load current decreases to less than 0.1 pu for a period of time not exceeding 1 min. CAUSES 1.Power system faults 2.Equipment failures 3.Control malfunctions interruption

43. The interruptions are measured by their duration since the voltage magnitude is always less than 10 percent of nominal. The duration of an interruption due to a fault on the utility system is determined by the operating time of utility protective devices. Instantaneous reclosing generally will limit the interruption caused by a nonpermanent fault to less than 30 cycles. Delayed reclosing of the protective device may cause a momentary or temporary interruption. The duration of an interruption due to equipment malfunctions or loose connections can be irregular. Figure shows such a momentary interruption during which voltage on one phase sags to about 20 percent for about 3 cycles and then drops to zero for about 1.8 s until the recloser closes back in.

44. Voltage sags: A sag is a decrease to between 0.1 and 0.9 pu in rms voltage or current at the power frequency for durations from 0.5 cycle to 1 min. Causes of Voltage sags Associated with system faults Energization of heavy loads Starting of large motors. Figure shows typical voltage sag that can be associated with a single- line-to-ground (SLG) fault on another feeder from the same substation.

45. Voltage sag ( a) RMS waveform for voltage sag event. ( b) Voltage sag waveform.

46. Swells: A swell is defined as an increase to between 1.1 and 1.8 pu in rms voltage or current at the power frequency for durations from 0.5 cycle to 1 min. Swells are characterized by their magnitude ( rms value) and duration. Causes of Voltage Swell Associated with system faults Energization of large Capacitor bank Switching off large load The severity of a voltage swell during a fault condition is a function of Fault location System impedance Grounding

47. Instantaneous voltage swell caused by a SLG fault.

48. Mitigation of Voltage Sags:   Different power quality problems would require different solution . It would be very costly to decide on mitigate measure that do not or partially solve the problem.   These costs include lost productivity, labor costs for clean up and restart, damaged product, reduced product quality, delays in delivery and reduced customer satisfaction.   When a customer or installation suffers from voltage sag, there is a number of mitigation methods are available to solve the problem.   These responsibilities are divided into three parts that involves utility, customer and equipment manufacturer.

49.   Different mitigation methods are    Dynamic voltage restorer     Active series Compensators    Distribution static compensator (DSTATCOM)   Solid state transfer switch (SSTS)     Static UPS with energy storage    Backup storage energy supply (BSES)   Ferro resonant transformer     Flywheel and Motor Generator set      Static Var Compensator (SVC)

50. 1. Dynamic Voltage Restorer: (DVR)    Dynamic Voltage Restorers (DVR) are complicated static devices which work by adding the ‘missing’ voltage during a voltage sag. Basically this means that the device injects voltage into the system in order to bring the voltage back up to the level required by the load.    Injection of voltage is achieved by a switching system coupled with a transformer which is connected in series with the load. There are two types of DVRs available; those with and without energy storage. Devices without energy storage are able to correct the voltage waveform by drawing additional current from the supply.   Devices with energy storage use the stored energy to correct the voltage waveform. The difference between a DVR with storage and a UPS is that the DVR only supplies the part of the waveform that has been reduced due to the voltage sag, not the whole waveform.

51. In addition, DVRs generally cannot operate during interruptions. Figure shows a schematic of a DVR. As can be seen the basic DVR consists of an injection/booster transformer, a harmonic filter, a voltage source converter (VSC) and a control system. DVR systems have the advantage that they are highly efficient and fast acting. It is claimed in that the DVR is the best economic solution for mitigating voltage sags based on its size and capabilities. In the case of systems without storage, none of the inherent issues with storage are relevant. Another advantage of DVR systems is that they can be used for purposes other than just voltage sag mitigation.

52. Dynamic voltage restorer (DVR) Primary function: to minimize the voltage sags on lines that cater to sensitive equipment. Consists of VSC placed in series with the load/distribution feeder by means of an injection transformer. It can inject voltages of controllable amplitude, phase angle and frequency into the feeder, thus restoring voltage to critical load during sag. DVR functions as a filter between the transmission line and the facility, thus enabling the facility to continuously receive clean power . It is primarily responsible for restoring the quality of voltage delivered to the end user when the voltage from the source is not appropriate to be used for sensitive loads. Usage of DVR enables consumers to isolate and protect themselves from transients and disturbances caused by sags and swells on the transmission lines or distribution network.

53. Operating Principle: The main operation of the DVR is to inject voltage of required magnitude and frequency when desired by the power system network. During the normal operation, the DVR will be in stand-by mode. During the disturbances in the system, the nominal or rated voltage is compared with the voltage variation and the DVR injects the difference voltage that is required by the load. The equivalent circuit of a DVR connected to the power network is shown in Fig. Here Vs is the supply voltage, Vinj is the voltage injected by the DVR and V L is the load voltage.

54. Equivalent Circuit Diagram of DVR

55. Applications of DVR: There are many applications of DVR in addition to mitigate voltage sag. They are DVR can be used to compensate the load voltage harmonics and improves the power quality of the system. Used under system frequency variations to provide the real power compensation. It is connected shunt to distribution system. can also protect the system against voltage swell or any other voltage imbalances that occur in the power system.

56. 2. Ferro Resonant Transformer: A Ferro resonant transformer, also known as a constant voltage transformer (CVT), is a transformer that operates in the saturation region of the transformer B-H curve . Voltage sags down to 30 % retained voltage can be mitigated through the use of Ferro resonant transformers.   Figure shows a schematic of a Ferro resonant transformer. The effect of operating the transformer in this region is that changes in input voltage only have a small impact on the output voltage.

57. Ferro resonant transformers are simple and relatively maintenance free devices which can be very effective for small loads. Ferro resonant transformers are available in sizes up to around 25 KVA. On the down side, the transformer introduces extra losses into the circuit and is highly inefficient when lightly loaded. In some cases they may also introduce distorted voltages. In addition, unless greatly oversized, Ferro resonant transformers are generally not suitable for loads with high inrush currents such as direct-on-line motors.

58. 3. Uninterrupted Power Supply (UPS): Uninterruptible power supplies (UPS) mitigate voltage sags by supplying the load using stored energy. Upon detection of voltage sag, the load is transferred from the mains supply to the UPS.

59. What is an uninterruptible power supply? An uninterruptible power supply (UPS) is a device that has an alternate source of energy that can provide power when the primary power source is temporarily disabled The switchover time must be small enough to not cause a disruption in the operation of the loads

60. Obviously, the capacity of load that can be supplied is directly proportional to the amount of energy storage available.   UPS systems have the advantage that they can mitigate all voltage sags including outages for significant periods of time (depending on the size of the UPS).   There are 2 topologies of UPS available; on-line and off-line . Comparison of the figures shows that the difference between the two systems is that for an on-line UPS the load is always supplied by the UPS, while for off-line systems; the load is transferred from the mains supply to the UPS by a static changeover switch upon detection of voltage sag.

61. LINE INTERACTIVE UPS BATTERY CHARGER AC SUPPLY INVERTER DC TO AC CRITICAL LOAD (AC) BATTERY AUTOMATIC SWITCH AC FAIL

62. 63 ON-LINE UPS CONVERTER AC TO DC AC SUPPLY INVERTER DC TO AC CRITICAL LOAD (AC) BATTERY AUTOMATIC SWITCH INVERTER FAIL

63. Block Diagram of Offline UPS: Block Diagram of Online UPS :

64. D-STATCOM A Distribution Static Compensator is in short known as D-STATCOM. It is a power electronic converter based device used to protect the distribution bus from voltage unbalances. It is connected in shunt to the distribution bus generally at the PCC. Basic Structure: D-STATCOM is a shunt connected device designed to regulate the voltage either by

65. As in the case of DVR, the VSI generates voltage by taking the input from the charged capacitor. It uses PWM switching technique for this purpose. This voltage is delivered to the system through the reactance of the coupling transformer. The voltage difference across the reactor is used to produce the active and reactive power exchange between the STATCOM and the transmission network. This exchange is done much more rapidly than a synchronous condenser and improves the performance of the system.

66. Operating Principle: A D-STATCOM is capable of compensating either bus voltage or line current. It can operate in two modes based on the parameter which it regulates. They are- • Voltage Mode Operation: In this mode, it can make the bus voltage to which it is connected a sinusoid. This can be achieved irrespective of the unbalance or distortion in the supply voltage. • Current Mode Operation: In this mode of operation, the D-STATCOM forces the source current to be a balanced sinusoid irrespective of the load current harmonics.

67. The basic operating principle of a D-STATCOM in voltage sag mitigation is to regulate the bus voltage by generating or absorbing the reactive power. Therefore, the DSTATCOM operates either as an inductor or as a capacitor based on the magnitude of the bus voltage. Inductive Operation: If the bus voltage magnitude (VB) is more than the rated voltage then the D-STATCOM acts as an inductor absorbing the reactive power from the system. The circuit and phasor diagram are shown in Fig.

68. Capacitive Operation: If the bus voltage magnitude (VB) is less than the rated voltage then the D-STATCOM acts as a capacitor generating the reactive power to the system. The circuit and phasor diagram of this mode of operation are shown in Fig.

69. Control Strategy: The main aim of the control strategy implemented to control a D-STATCOM used for voltage mitigation is to control the amount of reactive power exchanged between the STATCOM and the supply bus. When the PCC voltage is less than the reference (rated) value then the D-ATACOM generates reactive power and when PCC voltage is more than the reference (rated) value then the D-ATACOM absorbs reactive power.

70. To achieve the desired characteristics, the firing pulses to PWM VSI are controlled. The actual bus voltage is compared with the reference value and the error is passed through a PI controller. The controller generates a signal which is given as an input to the PWM generator. The generator finally generates triggering pulses such that the voltage imbalance is corrected. The block diagram of the control circuit is shown in Fig.

71. Block Diagram of the Control Circuit of D-STATCOM Applications of D-STATCOM: The applications of the D-STATCOM are- Stabilize the voltage of the power grid Reduce the harmonics Increase the transmission capacity Reactive power compensation Power Factor correction

72. 4. Static Var Compensator( SVC ): A SVC is a shunt connected power electronics based device which works by injecting reactive current into the load, thereby supporting the voltage and mitigating the voltage sag. SVCs may or may not include energy storage, with those systems which include storage being capable of mitigating deeper and longer voltage sags.

73. Block Diagram of SVC: Motor Generator Set:   Motor-generator (M-G) sets come in a wide variety of sizes and configurations.   This is a mature technology that is still useful for isolating critical loads from sags and interruptions on the power system.

74. Active series compensators Advances in power electronic technologies and new topologies for these devices have resulted in new options for providing voltage sag ride through support to critical loads. One of the important new options is a device that can boost the voltage by injecting a voltage in series with the remaining voltage during a voltage sag condition.