Reactive Power Document and Voltage Control of North Eastern Region

Reactive Power Document and Voltage Control of North Eastern Region December-2018 Edition-10 EHV Shunt Reactor North Eastern Regional Load Despatch Centre Shillong Power System operation Corporation Limited (A Government of India Enterprise)

CONTENTS CONTENTS...1 List Details....2 List of Figures:...2 List of Tables:...3 1 REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL...6 1.1 Introduction... 6 1.2 Analogy of Reactive Power... 8 1.3 Understanding Vectorally... 10 1.4 Voltage Stability... 11 1.5 Voltage Collapse... 12 1.6 Proximity to Instability... 14 1.7 Reactive Reserve Margin... 15 1.8 NER GRID Overview... 18 1.9 Reliability Improvement Due to Local Voltage Regulation... 21 2 TRANSMISSION LINES AND REACTIVE POWER COMPENSATION... 22 2.1 Introduction... 22 2.2 Surge Impedance Loading (SIL)... 23 2.3 Shunt Compensation in Line... 23 2.4 Line loading as function of Line Length and Compensation... 24 3 SERIES AND SHUNT CAPACITOR VOLTAGE CONTROL... 30 3.1 Introduction... 30 3.2 MeSeb Capacity Building And Training Document Suggest (Sub Title As Given In The PFC Document For Corporatization Of MeSeb):... 31 3.3 As Per The Assam Gazette, Extraordinary, February 10, 2005... 31 4 TRANSFORMER LOAD TAP CHANGER AND VOLTAGE CONTROL... 33 4.1 Introduction...33 4.2 As Per The Assam Gazette, Extraordinary, February 10, 2005... 35 5 HVDC AND VOLTAGE CONTROL... 37 5.1 Introduction... 37 5.2 HVDC Configuration... 37 5.3 Reactive Power Source... 40 5.4 ±800 kv HVDC Bi-Pole... 40 5.5 Technical details of Biswanath Chariali Alipurduar-Agra HVDC:... 41 5.6 Impact of Largest Filter Switching Under Different HVDC Power Order.... 43 6 FACTS AND VOLTAGE CONTROL... 44 6.1 Introduction... 44 6.2 Static Var Compensator (SVC)... 44 Page 1 of 60

6.3 Converter-based Compensator... 45 6.4 Series-connected controllers... 46 7 GENERATOR REACTIVE POWER AND VOLTAGE CONTROL... 47 7.1 Introduction... 47 7.2 Synchronous Condensers... 49 8 CONCLUSION... 50 9 SUMMARY... 51 10 STATUTORY PROVISIONS FOR REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL... 54 10.1 Provision in the Central Electricity Authority (Technical Standard for connectivity to the grid) Regulations 2007 [8]:... 54 10.2 Provision in The Indian Electricity Grid Code (IEGC), 2010:... 54 11. BIBLIOGRAPHY:... 59 List Details. List 1 International connectivity of NER at 400kV (Charged at 132kV)... 25 List 2 International Connectivity of NER at 132kV... 25 List 3 +/- 800 kv HVDC Lines Agra-BNC... 25 List 4: Fixed, Switchable and Convertible Line reactors in North Eastern Region... 26 List 5: Bus Reactors in North Eastern Region... 28 List 6: List of Upcoming Bus Reactors in North Eastern Region. 29 List 7: Tertiary Reactors on 33kV side of 400/220/33 kv ICTs in North Eastern Region... 29 List 8: Shunt Capacitors details in North Eastern Region... 32 List 9: Transmission/Transformation/VAR Compensation Capacity of North Eastern Region... 36 List of Figures: Figure 1 Voltage and Current Waveform... 6 Figure 2 Power Triangle... 7 Figure 3 Boat Pulled by Horse... 8 Figure 4 Direction of Pull... 8 Figure 5 Vector Representation of Analogy... 8 Figure 6 Labyrint Spel... 9 Figure 7 Vector Representation... 10 Figure 8 Time frames for voltage stability phenomena... 13 Figure 9 PV curve and Voltage stability margin under different conditions... 14 Figure 10 Average cost of Reactive power technologies... 17 Figure 11 NER Grid map... 18 Figure 12 Switching principle of LTC...33 Page 2 of 60

Figure 13: An example of voltage scatter plot 35 Figure 14 HVDC Fundamental components... 39 Figure 15: Schematic Diagram of HVDC-BNC... 41 Figure 16 Static VAR Compensators (SVC): TCR/TSR, TSC, FC and Mechanically Switched Resistor... 45 Figure 17 STATCOM topologies:(a) STATCOM based on VSI and CSI (b) STATCOM with storage... 45 Figure 18 Series-connected FACTS controllers: (a) TCSR and TSSR; (b) TSSC; (c) SSSC... 46 Figure 19 D-Curve of a typical Generator... 47 List of Tables: Table 1: Reactive power compensation sources... 16 Table 2 : Line Parameters & Surge Impedance Loading of Different Conductor Type... 24 Table 3: Equipment preference... 30 Table 4: AC Filter Bank at HVDC Agra... 42 Table 5: AC Filter Bank at HVDC BNC.... 42 Table 6: Impact of Largest Filter Switching under different HVDC Power order.... 43 Table 7: List of units in NER required to be normally operated with free governer action and AVR in service... 49 Table 8: IEGC operating voltage range... 56 ANNEXURES Annexure I: Fault levels of major substation in NER Annexure II: List of Lines in North Eastern Region Annexure III: List of ICTs in North Eastern Region Annexure IV: Substations in North Eastern Region Annexure V: Voltage Deviation Index of 400 kv Substations of North Eastern Region for the Month of November 2018 Annexure VI: Over-Voltage relay settings of lines connected at 400 kv Substations of NER Grid Annexure VII: Capability curves of various generators in NER GRID Annexure VIII: The AEGCL Gazette, Extraordinary, February 10, 2005 Annexure IX: NERLDC SCADA display for Real time Reactor status Annexure X: NERLDC URTDSM (WAMS) display for real time voltage contour view Page 3 of 60

EXECUTIVE SUMMARY Quality of power to the stakeholders is the question of the hour worldwide. Enactment of several regulations viz. IE act 2003, ABT, Open access regulations, IEGC, DSM and several other amendments are in the direction towards improvement of system reliability and power quality. It is also significant to mention that due to the massive load growth in the country, the existing power networks are operated under greater stress with transmission lines carrying power near their limits. Increase in the complexity of network and being loaded non-uniformly has increased its vulnerability to grid disturbances due to abnormal voltages (High and Low). In the past, reason for many a black outs across the world have been attributed to this cause. Three objectives dominate reactive power management. Firstly, maintaining adequate voltage throughout the transmission system under normal and contingency conditions. Secondly, minimizing congestion of real power flows. Thirdly, minimizing real power losses. Also with dynamic ATCs, var compensation, congestion charges, if not seriously thought, it may have serious commercial implications in times to come due to the amount of bulk power transfer across the country. Highlights of rolling year of NER grid include commercial operation of Unit 1 & 2 of Pare HEP of 55 MW each, 400/132 / 33 kv, 315 MVA ICT 3 at Silchar, 400 kv Silchar Melriat D/C line (charged at 132 kv), 132 kv Ranganadi Itanagar S/C line, 132 kv Ranganadi Pare II, 132 kv Pare Itanagar S/C line, 132 kv Teju Namsai S/C, LILO of 132 kv Aizwal Jiribam at Tipaimukh S/S of Manipur, LILO of 132 kv Ranganadi Lekhi at Pare S/S. Commissioning of 110 MW Pare HEP has increased the generation availability of NER Region. The reactive power generation/absorption capability of both the units of Pare HEP is also being used for voltage management. The 400/132/33 kv ICT 3 at Silchar has fulfilled the N-I-I contingency requirement of existing 400/132 kv 2 x 200 MVA ICT at Silchar. The N-I contingency requirement of the existing 132 kv Aizawl Melriat T/L has been fulfilled with the commissioning of 132 kv Silchar Melriat D/C lines. This has increased the reliability of Melriat S/S which is a major connectivity of the Mizoram system with the rest of NER Grid. The 132 kv Itanagar S/S of Arunachal Pradesh which was earlier radially connected from 132 kv Lekhi S/S, has been connected from 132 kv Ranganadi S/S and Pare S/S through 132 kv Ranganadi Itanagar S/C line and 132 kv Pare Itanagar S/C line, thus increasing the reliability of Itanagar S/S, which serves a major portion of the capital load of Arunachal Pradesh. Page 4 of 60

Other major elements commissioned during current year were 420 kv 125 MVAR Bus Reactor at Bongaigaon S/S, 420 V 125 MWAR Bus Reactor at Balipara S/S, 420 V 125 MVAR Bus Reactor at Silchar S/S and 420kV 80 MVAR Bus Reactor at Imphal S/S. These reactors have helped in controlling the problem of high voltage in the 400 kv System in the Northern part of NER Grid which mainly occurs as the 400 kv lines are very lightly loaded during the off-peak hours. The 132 kv Tezu Namsai S/C line, 132/ 33 kv 15 MVA ICT-I & ICT-II at Namsai S/S, and 145 kv 20 MVAR Bus Reactor at Namsai S/S of Arunachal Pradesh were also commissioned during the current year. Commissioning Namsai S/S has improved the connectivity of Arunachal Pradesh system with the rest of the NER Grid. The load of Namsai area of Arunachal Pradesh has been connected with the rest of NER grid through the Namsai S/S. With the increase in controllability compared to earlier years, grid operation has been smooth and grid parameters were maintained within the prescribed IEGC limits. This manual is in continuation to the previous edition for understanding the basics of reactive power and its management towards voltage control, its significance and consequences of inadequate reactive power support. It also includes details of reactive power support available at present and efforts by planners from future perspective in respect of NER grid. New inclusions and updations in the current edition of the manual are as follows: Details of total 498 ckm of transmission lines which were commissioned in NER Grid during the year 2018 have been included. Details of total 507 MVARs of Shunt Reactors which were commissioned in NER Grid during the year 2018 have been included. Details of total 345 MVA of Transformers which were commissioned in NER Grid during the year 2018 have been included. Fault level of major substations of NER Grid has been updated as on December 2018. Tap Settings of ICTs have been updated as on May 2018 Voltage Deviation Index of 400 kv Substations of North Eastern Region for the Month of November 2018 have been added. Over-Voltage relay settings for all lines connected at 400 kv Substations of NER Grid as on August 2018 have been added. Capability curve for Unit I and Unit II (55 MW each) of Pare HEP has been included. NERLDC SCADA display for Reactor Status and NERLDC URTDSM display for Voltage Contour has been added. Page 5 of 60

1 REACTIVE POWER MANAGEMENT AND VOLTAGE CONTROL 1.1 Introduction W 1.1.1 hat is Reactive Power? Reactive power is a concept used by engineers to describe the background energy movement in an Alternating Current (AC) system arising from the production of electric and magnetic fields. These fields store energy which changes through each AC cycle. Devices which store energy by virtue of a magnetic field produced by a flow of current are said to absorb reactive power (viz. Transformers, Reactors) and those which store energy by virtue of electric fields are said to generate reactive power (viz. Capacitors). 1.1.2 Power flows, both actual and potential, must be carefully controlled for a power system to operate within acceptable voltage limits. Reactive power flows can give rise to substantial voltage changes across the system, which means that it is necessary to maintain reactive power balances between sources of generation and points of demand on a 'zonal basis'. Unlike system frequency, which is consistent throughout an interconnected system, voltages experienced at points across the system form a "voltage profile" which is uniquely related to local generation and demand at that instant, and is also affected by the prevailing system network arrangements. 1.1.3 In an interconnected AC grid, the voltages and currents alternate up and down 50 times per second (not necessarily at the same time). In Figure 1 Voltage and Current Waveform that sense, these are pulsating quantities. Because of this, the power being transmitted down a single line also pulsates - although it goes up and down 100 times per second rather than 50. Page 6 of 60

1.1.4 To distinguish reactive power from real power, we use the reactive power unit called VAR - which stands for Volt-Ampere-Reactive (Q). Normally electric power is generated, transported and consumed in alternating current (AC) networks. Elements of AC systems supply (or produce) and consume (or absorb or lose) two kinds of power: real power and reactive power. 1.1.5 Real power accomplishes useful work (e.g., runs motors and lights lamps). Reactive power supports the voltages that must be controlled for system reliability. In AC power networks, while active power corresponds to useful work, reactive power supports voltage magnitudes that are controlled for system reliability, voltage stability, and operational acceptability. 1.1.6 VAR Management? It is defined as the control of generator voltages, variable transformer tap settings, compensation, switchable shunt capacitor and reactor banks plus allocation of new shunt capacitor and reactor banks in a manner that best achieves a reduction in system losses and/or voltage control. 1.1.7 Although active power can be transported over long distances, reactive power is difficult to transmit, since the reactance of transmission lines is often 4 to 10 times higher than the resistance of the lines. When the transmission system is heavily loaded, the active power losses in the transmission system are also high. Reactive power (vars) is required to maintain the voltage to deliver active power (watts) through transmission lines. Figure 2 Power Triangle When there is not enough reactive power, the voltage sags down and it is not possible to push the power demanded by loads through the lines. Reactive power supply is necessary in the reliable operation of AC power systems. Several recent power outages worldwide may have been a result of an inadequate reactive power supply which subsequently led to voltage collapse. 1.1.8 Voltage and current may not pulsate up and down at the same time. When the voltage and current do go up and down at the same time, only real power is transmitted. When the voltage and current go up and down at different times, reactive power is also gets transmitted. How much reactive power and which direction it is flowing on a transmission line depend on how different these two items are. Although AC voltage and current pulsate at the same frequency, they peak at a different time. Power is the algebraic product of voltage and current. Over a cycle, power has an average value, called real power (P), measured in voltamperes, or watts. There is also a portion of power with zero average value that Page 7 of 60

is called reactive power (Q), measured in volt-amperes reactive, or vars. The total power is called apparent power or Complex power, measured in voltamperes, or VA. 1.2 Analogy of Reactive Power 1.2.1 Why an analogy? Reactive Power is an essential aspect of the electricity system, but one that is difficult to comprehend by a lay man. The horse and the boat analogy best describe the Reactive Power aspect. Visualize a boat on a canal, pulled by a horse on the bank of the canal. Figure 3 Boat Pulled by Horse Figure 4 Direction of Pull The horse is not in front of the boat to do a meaningful work of pulling it in a straight path. Due to the balancing compensation by the rudder of the boat, the boat is made to move in a straight manner rather deviating towards the bank. This is in line with the understanding of the reactive power. Figure 5 Vector Representation of Analogy Page 8 of 60

1.2.1 In the horse and boat analogy, the horse s objective (real power) is to move the boat straight. The fact that the rope is being pulled from the flank of the horse and not straight behind it, limits the horse s capacity to deliver real work of moving straight. Therefore, the power required to keep the boat steady in navigating straight is delivered by the rudder movement (reactive power). Without reactive power there can be no transfer of real power, likewise without the support of rudder, the boat cannot move in a straight line. 1.2.2 Reactive power is like the bouncing up and down that happens when we walk on a trampoline. Because of the nature of the trampoline, that up-down bouncing is an essential part of our forward movement across the trampoline, even though it appears to be movement in the opposite direction. 1.2.3 Reactive power and real power work together in the way that s illustrated very well by the labyrinth puzzle, LABYRINTSPEL: The description of the puzzle begins to show why this game represents the relationship between real and reactive power: The intent is to manipulate a steel ball (1.2cm in diameter) through the maze by rotating the knobs without letting the ball fall into one of the holes before it reaches the end of the maze. If a ball does fall prematurely into a hole, a slanted floor inside the box returns the ball to the user in the trough on the lower right corner of the box. Figure 6 Labyrint Spel 1.2.4 The Objective is to twist the two knobs to adjust the angle of the platform in two directions, in order to keep the ball rolling through the maze without falling into any holes. Those twists are REACTIVE POWER, which helps propel the real power through to its ultimate goal, which is delivery to the user. Without reactive power, ball falls into holes along the way, which are NETWORK failures. 1.2.5 Both of these examples illustrate how important it is to understand the system and how it works in order to meet our objectives effectively. In the LABYRINTSPEL game, if the structure of the system is not taken into account, winning would be really easy because one knob would be turned all the way in Page 9 of 60

one direction, and the other knob all the way in the other direction, and the ball would merely roll across the platform. If that s the model how electricity works, then that would deliver the electrons to the end user in the form of real power. But in the game, on the trampoline, and in the electric power network, the system has more going on that means it s essential to do things that seem counterintuitive, like bouncing up and down on the trampoline or turning the platform in the game towards west to avoid the hole to the east, even though we have to go east to win. 1.2.6 In electric power, the counterintuitive thing about reactive power is to use some power along the path to balance the flow of electrons and the circuits. Otherwise, the electricity just flows from the generator to the largest consumer (that s Kirchhoff s law, basically). In this sense, reactive power is like water pressure in a water network. 1.2.7 LABYRINTSPEL game and the trampoline are good examples that they capture the fact that mathematically, real power and reactive power are pure conjugates. 1.3 Understanding Vectorally 1.3.1 In practice circuits are invariably combinations of resistance, inductance and capacitance. The combined effect of these impedances to the flow of current is most easily assessed by expressing the power flows as vectors that show the angular relationship between the powers waveforms associated with each type of impedance. Figure 7 shows how the vectors can be resolved to determine the net capacity of the circuit needed to transfer the power requirements of the connected equipment. 1.3.2 The useful power that can be drawn from the electricity distribution system is represented by the vertical vector in the diagram and is measured in kilowatts (kw).the reactive or wattless power that is a consequence of the inductive load in the circuit is represented by the horizontal vector to the right and the reactive power attributable to the circuit capacitance by Page 10 of 60 Figure 7 Vector Representation

the horizontal vector to the left. These are measured in kilovars (kvar). 1.3.3 The resolution of these vectors, which is the diagonal vector in the diagram is the capacity required to transmit the active power, and is measured in kilovoltsampere (kva). The ratio of the kw to kva is the cosine of the angle in the diagram shown as theta, and is referred to as the power factor. 1.3.4 When the net impedance of the circuit is solely resistance, so that the inductance and capacitance exactly cancel each other out, then the angle theta becomes zero and the circuit has a power factor of unity. The circuit is now operating at its highest efficiency for transferring useful power. However, as a net reactive power emerges the angle theta starts to increase and its cosine falls. 1.3.5 At low power factors the magnitude of the kva vector is significantly greater than the real power or kw vector. Since distribution assets such as cables, lines and transformers must be sized to meet the kva requirement, but the useful power drawn by the customer is the kw component, a significant cost emerges from having to over-size the distribution system to accommodate the substantial amount of reactive power that is associated with the active power flow. 1.4 Voltage Stability 1.4.1 Power flows, both actual and potential, must be carefully controlled for a power system to operate within acceptable voltage limits and vice versa. Not only is reactive power necessary to operate the transmission system reliably, but it can also substantially improve the efficiency with which real power is delivered to customers. Increasing reactive power production at certain locations (usually near a load center) can sometimes alleviate transmission constraints and allow cheaper real power to be delivered into a load pocket. 1.4.2 Voltage control (keeping voltage within defined limits) in an electric power system is Important for proper operation of electric power equipment and saving it from imminent damage, to reduce transmission losses and to maintain the ability of the system to withstand disturbances and prevent voltage collapse. In general terms, decreasing reactive power causes voltages to fall, while increasing reactive power causes voltages to rise. A voltage collapse occurs when the system is trying to serve much more load than the voltage can support. Page 11 of 60

1.4.3 As voltage drops, current must increase to maintain the power supplied, causing the lines to consume more reactive power and the voltage to drop further. If current increases too much, transmission lines trip, or go offline, overloading other lines and potentially causing cascading failures. If voltage drops too low, some generators will automatically disconnect to protect themselves. 1.4.4 Usually the causes of under voltages are: Overloading of supply transformers Inadequate short circuit level in the point of supply Excessive voltage drop across a long feeder Poor power factor of the connected load Remote system faults, while they are being cleared Interval in re-closing of an auto-reclosure Starting of large HP induction motors 1.4.5 If the declines continue, these voltage reductions cause additional elements to trip, leading to further reduction in voltage and loss of load. The result is a progressive and uncontrollable decline in voltage, all because the power system is unable to provide the reactive power required to supply the reactive power demand. 1.5 Voltage Collapse 1.5.1 When voltages in an area are significantly low or blackout occurs due to the cascading events accompanying voltage instability, the problem is considered to be a voltage collapse phenomenon. Voltage collapse normally takes place when a power system is heavily loaded and/or has limited reactive power to support the load. The limiting factor could be the lack of reactive power (SVC and generators hit limits) production or the inability to transmit reactive power through the transmission lines. 1.5.2 The main limitation in the transmission lines is the loss of large amounts of reactive power and also line outages, which limit the transfer capacity of reactive power through the system. 1.5.3 In the early stages of analysis, voltage collapse was viewed as a static problem but it is now considered to be a non linear dynamic phenomenon. The dynamics Page 12 of 60

in power systems involve the loads, and voltage stability is directly related to the loads. Hence, voltage stability is also referred to as load stability. 1.5.4 There are other factors which also contribute to voltage collapse, and are as below: Increase in load Action of tap changing transformers Load recovery dynamics All these factors play a significant part in voltage collapse as they effect the transmission, consumption, and generation of reactive power. Usually voltage stability is categorized into two parts Large disturbance voltage stability Small disturbance voltage stability Figure 8 Time frames for voltage stability phenomena 1.5.5 When a large disturbance occurs, the ability of the system to maintain acceptable voltages falls due to the impact of the disturbance. Ability to maintain voltages is dependent on the system and load characteristics, and the Page 13 of 60

interactions of both the continuous and the discrete controls and protections. Similarly, the ability of the system to maintain voltages after a small perturbation i.e. incremental change in load is referred to as small disturbance voltage stability. It is influenced by the load characteristics, continuous control and discrete controls at a given instant of time. 1.6 Proximity to Instability 1.6.1 Static voltage instability is mainly associated with reactive power imbalance. Thus, the loadability of a bus in a system depends on the reactive power support that the bus can receive from the system. As the system approaches the maximum loading point or voltage collapse point, both real and reactive power losses increase rapidly. 1.6.2 Therefore, the reactive power supports have to be locally adequate. With static voltage stability, slowly developing changes in the power system occur that eventually lead to a shortage of reactive power and declining voltage. 1.6.3 This phenomenon can be seen from a plot of power transferred versus voltage at the receiving end. These plots are popularly referred to as P V curves or Nose curves. As power transfer increases, the voltage at the receiving end decreases. In the fig(9) eventually, a critical (nose) point, the point at which the system reactive power is out of usage, is reached where any further increase in active power transfer will lead to very rapid decrease in voltage magnitude. v Knee point Figure 9 PV curve and Voltage stability margin under different conditions 1.6.4 Before reaching the critical point, a large voltage drop due to heavy reactive power losses is observed. The only way to save the system from voltage collapse is to reduce the reactive power load or add additional reactive power prior to reaching the point of voltage collapse. Page 14 of 60

These are curves drawn between V and P of a critical bus at a constant load power factor. These are produced by using a series of power flow solutions for different load levels. At the knee point or the nose point of the V-P curve, the voltage drops rapidly with an increase in the load demand. Power flow solution fails to converge beyond this limit which indicates the instability. 1.7 Reactive Reserve Margin 1.7.1 The amount of unused available capability of reactive power static as well as dynamic in the system (at peak load for a utility system) as a percentage of total capability is known as Reactive reserve margin. 1.7.2 Voltage collapse normally occurs when sources producing reactive power reach their limits i.e. generators, SVCs or shunt reactors, and there is not much reactive power to support the load. As reactive power is directly related to voltage collapse, it can be used as a measure of voltage stability margin. 1.7.3 The voltage stability margin can be defined as a measure of how close the system is to voltage instability, and by monitoring the reactive reserves in the power system, proximity to voltage collapse can be monitored. 1.7.4 In case of reactive reserve criteria, the reactive power reserve of an individual or group of VAr sources must be greater than some specified percentage (x %) of their reactive power output under all contingencies. The precincts where reactive power reserves were exhausted would be identified as critical areas. 1.7.5 Reactive power requirements over and above those which occur naturally are provided by an appropriate combination of reactive source/devices which are normally classified as static and dynamic devices. STATIC SOURCES: Static sources are typically transmission and distribution equipments such as Capacitors and Reactors that are relatively static and can respond to the changes in voltage support requirements only slowly and in discrete steps. Devices are inexpensive, but the associated switches, control, and communications, and their maintenance, can amount to as much as one third of the total operations and maintenance budget of a distribution system. Page 15 of 60

DYNAMIC SOURCES: It includes pure reactive power compensators like synchronous condensers, Synchronous generators and solid-state devices such as FACTS, SVC, STATCOM, D-VAR, and SuperVAR which are normally dynamic and can respond within cycles to changing reactive power requirement. These are typically considered as transmission service devices. 1.7.6 Static devices typically have lower capital costs than dynamic devices, and from a system point of view, they are used to provide normal or intact-system voltage support and to adapt to slowly changing conditions, such as daily load cycles and scheduled transactions. By contrast, dynamic reactive power sources must be deployed to allow the transmission system to respond to rapidly changing conditions on the transmission system, such as sudden loss of generators or transmission facilities. An appropriate combination of both static and dynamic resources is needed to ensure reliable operation of the transmission system at an appropriate level of costs. 1.7.7 Reactive power absorption occurs when current flows through an inductance. Inductance is found in transmission lines, transformers, and induction motors etc. The reactive power absorbed by a transmission line or transformer is proportional to the square of the current. Sources of Reactive Power Static: Shunt Capacitors Filter banks Underground cables Transmission lines (lightly loaded) Dynamic: Synchronous Generators/Synchronous Condensers FACTS (e.g.,svc,statcom) Sinks of Reactive Power Transmission lines (Heavily loaded) Transformers Shunt Reactors Synchronous Generators FACTS (e.g.,svc,statcom) Induction generators (wind plants) Loads Induction motors (Pumps, Fans etc) Inductive loads (Arc furnace etc) Table 1: Reactive power compensation sources Page 16 of 60

1.7.8 A transmission line also has capacitance. When a small amount of current is flowing, the capacitance dominates, and the lines have a net capacitive effect which raises voltage. This happens at night when current flows/load is low. During the day, when current flow/load is high, inductive effect is greater than the capacitance, and the voltage sags. Figure 10 Average cost of Reactive power technologies Page 17 of 60

1.8 NER GRID Overview 1.8.1 NER grid with a maximum peak requirement of around 2900 MW and installed capacity of 3912 MW caters to the seven north eastern states (namely Arunachal Pradesh, Assam, Manipur, Meghalaya, Mizoram, Nagaland and Tripura). It is synchronously connected with ER Grid through 400 kv BONGAIGAON NEW SILIGURI D/C, 400 kv BONGAIGAON ALIPURDUAR D/C, 220 kv BIRPARA ALIPURDUAR D/C and internationally through 132 kv SALAKATI GELYPHU(Bhutan), 132 kv RANGIA DEOTHANG (Bhutan), 132 kv D/C SURAJMANINAGAR COMILLA (Bangladesh) and 11 kv MOREH TAMU(Myanmar). Also, it is connected to NR grid through ±800kV HVDC Bipole Biswanath Charali-Agra link. The bottle neck of operating the NER grid arises because of the brittle back bone network of about 8559 Ckt Kms of 132 KV lines, 3410 Ckt Kms of 220 KV lines and 4409 Ckt Kms of 400 KV lines compared to other regional grids. 1.8.2 With Commissioning of first 800kV multi-terminal HVDC between Biswanath Charali, Alipurduar and Agra, NER grid is directly connected with NR grid by this HVDC link. The capacity of each terminal at Biswanath Charali(NER) and Alipurduar (ER) is 3000 MW and at Agra it is 6000 MW, at ± 800 kv voltage level. Figure 11 NER Grid map Page 18 of 60

1.8.3 Highlights of NER grid for current year include commercial operation of Unit 1 & 2 of Pare HEP of 55 MW each, 400/132 / 33 kv, 315 MVA ICT 3 at Silchar, 400 kv Silchar Melriat D/C line (charged at 132 kv), 132 kv Ranganadi Itanagar S/C line, 132 kv Ranganadi Pare II, 132 kv Pare Itanagar S/C line, 132 kv Teju Namsai S/C, LILO of 132 kv Aizwal Jiribam at Tipaimukh S/S of Manipur, LILO of 132 kv Ranganadi Lekhi at Pare S/S. Commissioning of 110 MW Pare HEP has increased the generation availability of NER Region. The reactive power generation/absorption capability of both the units of Pare HEP is also being used for voltage management. The 400/132/33 kv ICT 3 at Silchar has fulfilled the N-I-I contingency requirement of existing 400/132 kv 2 x 200 MVA ICT at Silchar. The N-I contingency requirement of the existing 132 kv Aizawl Melriat T/L has been fulfilled with the commissioning of 132 kv Silchar Melriat D/C lines. This has increased the reliability of Melriat S/S which is a major connectivity of the Mizoram system with the rest of NER Grid. The 132 kv Itanagar S/S of Arunachal Pradesh which was earlier radially connected from 132 kv Lekhi S/S, has been connected from 132 kv Ranganadi S/S and Pare S/S through 132 kv Ranganadi Itanagar S/C line and 132 kv Pare Itanagar S/C line, thus increasing the reliability of Itanagar S/S, which serves a major portion of the capital load of Arunachal Pradesh. Other major elements commissioned during current year were 420 kv 125 MVAR Bus Reactor at Bongaigaon S/S, 420 kv 125 MWAR Bus Reactor at Balipara S/S, 420 V 125 MVAR Bus Reactor at Silchar S/S and 420kV 80 MVAR Bus Reactor at Imphal S/S. These reactors have helped in controlling the problem of high voltage in the 400 kv System in the Northern part of NER Grid which mainly occurs as the 400 kv lines are very lightly loaded during the off-peak hours. The 132 kv Tezu Namsai S/C line, 132/ 33 kv 15 MVA ICT-I & ICT-II at Namsai S/S, and 145 kv 20 MVAR Bus Reactor at Namsai S/S of Arunachal Pradesh were also commissioned during the current year. Commissioning Namsai S/S has improved the connectivity of Arunachal Pradesh system with the rest of the NER Grid. The load of Namsai area of Arunachal Pradesh has been connected with the rest of NER grid through the Namsai S/S. 1.8.4 Almost 50% of the total NER load is spread out in 132 kv pocket of southern part of NER which were without the direct support of major EHV trunk lines. This part of the network was highly sensitive and was susceptible to grid disturbance in the past and demanded more operational acumen. Increase in the loading of major 132 kv trunk lines, in particular 132 kv DIMAPUR IMPHAL S/C,132 kv JIRIBAM LOKTAK S/C and 132 kv BADARPUR KHLIEHRIAT S/C in peak hours has led to many a grid incidents in the past in the form of cascade tripping accompanied by voltage sag. Page 19 of 60

However, with system augmentation grid incidence in this part of the grid has become a matter of past. 1.8.5 Relationship between frequency and voltage is a well-known fact. Studies have revealed that though voltage is a localized factor, it is directly affected by the frequency which is a notional factor. Any lopsidedness in the demand/generation side leading to fluctuations in NEW grid frequency affects NER grid immensely, in particular the voltage profile of the grid, leading to sagging and swelling of voltage heavily during such occasions. Ironically, NER was synchronously connected with NEW grid for stretching the transmission capability to reduce the load generation mismatch of the country. 1.8.6 FSC s have been integrated with the NER system in the 400 kv Balipara Bongaigaon III & IV at Balipara end. 1.8.7 Presently NER Grid is supported by 3562 MVAr from shunt reactors and 273 MVAr from shunt capacitors spread across the region. 1.8.8 Skewness in the location of hydro stations and load centers in NER is another obstacle which aggravates the voltage problem further. Lines are long and pass through difficult terrains to the load centers. Northern part of NER grid which is well supported by some strong 400 KV and 220 KV network faces high voltage regime during lean hydro period as the corridor is not fully utilized and is usually lightly loaded. Supports from hydro stations in condenser mode are not available for containing low voltage conditions. D curve optimization is yet to be realized fully due to technical glitches. 1.8.9 Reactive power management and voltage control are two aspects of a single activity that both supports reliability and facilitates commercial transaction across transmission network. Controlling reactive power flow can reduce losses and congestion on the transmission system. 1.8.10 Operationally in NER, Voltage is normally controlled by managing production and absorption of reactive power in real time : a. By Switching in and out of Line reactance compensators such as capacitors and shunt reactors (Line/Bus Reactors) as and when system demands in cooperation with the constituents and the CTU. b. By using automatic voltage regulators (AVR), the generating units control field excitation to maintain the scheduled voltage levels at the terminals of the generators. In real time operation, the generation/consumption of reactive power must be within the capability curve of generator. Page 20 of 60

c. By generation re-dispatch/rescheduling. d. By regulating voltage with the help of OLTC s. e. By load staggering/shedding. f. By Circuit switching: Mostly one circuit of the lightly loaded d/c line is kept open keeping in mind the n-1 criterion during high voltage and high frequency period. Voltage differences as well as fault level of stations are taken into account before any switching operation of circuits. Fault Level at important substations of NER is mentioned in Annexure I. All constituents must maintain the voltage of their periphery inside the IEGC band at all times as reactive power management/voltage control is a local phenomenon. 1.9 Reliability Improvement Due to Local Voltage Regulation 1.9.1 Local voltage regulation to a voltage schedule supplied by the utility can have a very beneficial effect on overall system reliability, reducing the problems caused by voltage dips on distribution circuits such as dimming lights, slowing or stalling motors, dropout of contactors and solenoids, etc. 1.9.2 In past years a voltage drop would inherently reduce load, helping the situation. Light bulbs would dim and motors would slow down with decreasing voltage. Dimmer lights and slower motors typically draw less power, so the situation was in a certain sense self-correcting. With modern loads, this situation is changing. 1.9.3 Today many incandescent bulbs are being replaced with compact fluorescent lights, LED lamps that draw constant power as voltage decreases, and motors are being powered with adjustable-speed drives that maintain a constant speed as voltage decreases. In addition, voltage control standards are rather unspecific, and there is a tremendous opportunity for an improvement in efficiency and reliability from better voltage regulation. Capacitors supply reactive power to boost voltage, but their effect is dramatically diminished as voltage dips. 1.9.4 Capacitor effectiveness is proportional to the square of the voltage, so at 80% voltage, capacitors are only 64% as effective as they are at normal conditions. As voltage continues to drop, the capacitor effect falls off until voltage collapses. The reactive power supplied by an inverter is dynamic, it can be controlled very rapidly, and it does not drop off with a decrease in voltage. Distribution systems that allow customers to supply dynamic reactive power to regulate voltage could be a tremendous asset to system reliability and efficiency by expanding the margin to voltage collapse. Page 21 of 60

2 TRANSMISSION LINES AND REACTIVE POWER COMPENSATION 2.1 Introduction 2.1.1 In moving power from generators to loads, the transmission network introduces both real and reactive losses. Housekeeping loads at substations (such as security lighting and space conditioning) and transformer excitation losses are roughly constant (i.e., independent of the power flows on the transmission system). Transmission-line losses, on the other hand, depend strongly on the amount of power being transmitted. 2.1.2 Real-power losses arise because aluminum and copper (the materials most often used for transmission lines) are not perfect conductors; they have resistance. The consumption of reactive power by transmission lines increases with the square of current i.e., the transmission of reactive power requires an additional demand for reactive power in the system components. 2.1.3 The reactive-power nature of transmission lines is associated with the geometry of the conductors themselves (primarily the radius of the conductor) and the geometry of the conductor configuration (the distances between each conductor and ground and the distances among conductors). 2.1.4 The reactive-power behavior of transmission lines is complicated by their inductive and capacitive characteristics. At low line loadings, the capacitive effect dominates, and generators and transmission-related reactive equipment must absorb reactive power to maintain line voltages within their appropriate limits. On the other hand, at high line loadings, the inductive effect dominates, and generators, capacitors, and other reactive devices must produce reactive power 2.1.5 The thermal limit is the loading point (in MVA) above which real power losses in the equipment will overheat and damage the equipment. Most transmission elements (e.g., conductors and transformers) have normal thermal limits below which the equipment can operate indefinitely without any damage. These types of equipment also have one or more emergency limits to which the equipment can be loaded for several hours with minimal reduction in the life of the equipment. 2.1.6 If uncompensated, these line losses reduce the amount of real power that can be transmitted from generators to loads. Transmission-line capacity decreases as the Page 22 of 60

line length increases if there is no voltage support (injection or absorption of reactive power) on the line. 2.2 Surge Impedance Loading (SIL) 2.2.1 Transmission lines and cables generate and consume reactive power at the same time. The reactive power generation is almost constant, because the voltage of the line is usually constant, and the line s reactive power consumption depends on the current or load connected to the line that is variable. So at the heavy load conditions transmission lines consume reactive power, decreasing the line voltage, and in the low load conditions generate, increasing line voltage. 2.2.2 The case when line s reactive power produced by the line capacitance is equal to the reactive power consumed by the line inductance is called natural loading or surge impedance loading (SIL), meaning that the line provides exactly the amount of MVAr needed to support its voltage. The balance point at which the inductive and capacitive effects cancel each other is typically about 40% of the line s thermal capacity. Lines loaded above SIL consume reactive power, while lines loaded below SIL supply reactive power. 2.2.3 A 400 kv, line generates approximately 55 MVAR per 100 km/ckt, when it is idle charged due to line charging susceptance. This implies a 300 km line generates about 165 MVAR when it is idle charged. 2.3 Shunt Compensation in Line 2.3.1 Normally there are two types of shunt reactors; Line reactor and bus reactor. Line reactors are normally used to control over voltage due to switching and load rejection whereas Bus reactors are normally used to control the steady state over voltages during light load conditions. 2.3.2 The degree of compensation is decided by an economic point of view between the capitalized cost of compensator and the capitalized cost of reactive power from supply system over a period of time. In practice a compensator such as a bank of capacitors (or inductors) can be divided into parallel sections, each Switched separately, so that discrete changes Page 23 of 60

in the compensating reactive power may be made, according to the requirements of the load. 2.3.3 In general, shunt compensation is provided in all lines of length 120 km and above. 2.3.4 Reasons for the application of shunt capacitor units are : Increase voltage level at the load Improves voltage regulation if the capacitor units are properly switched. Reduces I 2 R power loss in the system because of reduction in current. Increases power factor of the source generator. Decrease kva loading on the source generators and circuits to relieve an overloaded condition or release capacity for additional load growth. By reducing kva loading on the source generators additional kilowatt loading may be placed on the generation if turbine capacity is available. 2.4 Line loading as function of Line Length and Compensation 2.4.1 The operating limits for transmission lines may be taken as minimum of thermal rating of conductors and the maximum permissible line loadings derived. SIL given in table below is for uncompensated line. Table 2 : Line Parameters & Surge Impedance Loading of Different Conductor Type Page 24 of 60

List 1: International connectivity of NER at 400kV (Charged at 132kV) SR. NO FROM TO UTILITY KM CKT CONDUCTOR. 1 Comilla Surajmani Nagar POWERGRID 47 1 ACSR Twin Moose 2 Comilla Surajmani Nagar POWERGRID 47 2 ACSR Twin Moose List 2 : International Connectivity of NER at 132kV SR. NO. FROM TO UTILITY KM CKT CONDUCTOR 1 Gelyphu(BH Salakati(IND U) ) POWERGRID 49.2 1 ACSR Panther 2 Motonga(BH U) Rangia(IND) AEGCL 49 2 ACSR Panther List 3 : +/- 800 kv HVDC Lines Agra-BNC SR. NO. FROM TO UTILITY KM CKT CONDUCTOR 1 Agra Biswanath Hexa Lapwing POWERGRID 1728 1 Charali 2 Agra Biswanath Hexa Lapwing POWERGRID 1728 2 Charali List of Transmission Lines in NER GRID along with their line length and conductor type is given in ANNEXURE II Page 25 of 60

List 4 : Fixed, Switchable and Convertible Line reactors in North Eastern Region SR. NO. UTILITY FROM TO INSTALLED AT (STATION) KV MVAR KM CONVERTIBLE SWITCHABLE FIXED 1 POWERGRID BONGAIGAON BALIPARA BONGAIGAON 400 50 289.9 TRUE.. 2 POWERGRID BONGAIGAON BALIPARA BONGAIGAON 400 50 289.9 TRUE.. 3 POWERGRID BONGAIGAON BALIPARA BALIPARA 400 63 289.9 TRUE.. 4 POWERGRID BONGAIGAON BALIPARA BALIPARA 400 63 289.9 TRUE.. 5 POWERGRID BONGAIGAON BALIPARA BONGAIGAON 400 63 305 TRUE.. 6 POWERGRID BONGAIGAON BALIPARA BONGAIGAON 400 63 305 TRUE.. 7 POWERGRID BONGAIGAON BALIPARA BALIPARA 400 63 305 TRUE.. 8 POWERGRID BONGAIGAON BALIPARA BALIPARA 400 63 305 TRUE.. 9 POWERGRID BONGAIGAON BINAGURI(ER) BONGAIGAON 400 63 218.... TRUE 10 POWERGRID BONGAIGAON BINAGURI(ER) BONGAIGAON 400 63 218.... TRUE 11 POWERGRID MISA NEW MARIANI MISA 220 50 222.7.... TRUE 12 POWERGRID MISA MARIANI MISA 220 50 220.. TRUE 13 POWERGRID PALATANA SILCHAR SILCHAR 400 50 247. TRUE. 14 POWERGRID PALATANA SILCHAR SILCHAR 400 50 247. TRUE. 15 OTPC PALATANA SILCHAR PALLATANA 400 63 247. TRUE. 16 OTPC PALATANA SILCHAR PALLATANA 400 63 247. TRUE. BISWANATH.. 17 NEEPCO RANGANADI CHARALI RANGANADI 400 50 131 TRUE BISWANATH.. 18 NEEPCO RANGANADI CHARALI RANGANADI 400 50 131 TRUE BISWANATH. 19 POWERGRID CHARALI BALIPARA BALIPARA 400 50 65 TRUE... BISWANATH. 20 POWERGRID CHARALI BALIPARA BALIPARA 400 50 65 TRUE... 21 POWERGRID SILCHAR BYRNIHAT SILCHAR 400 63 217.14. TRUE. Page 26 of 60

22 MeECL SILCHAR BYRNIHAT BYRNIHAT 400 63 217.4 TRUE.. 23 POWERGRID SILCHAR AZARA SILCHAR 400 63 256. TRUE. 24 AEGCL SILCHAR AZARA AZARA 400 63 256.. TRUE 25 POWERGRID BANGAIGAON BYRNIHAT BANGAIGAON 400 63 167 TRUE.. 26 POWERGRID BANGAIGAON AZARA BANGAIGAON 400 63 118 TRUE.. 27 NEEPCO NEW MARIANI AGBPP AGBPP 220 20 160.54 TRUE.. NOTE: CONVERTIBLE: LINE REACTORS WHICH CAN BE OPERATED UPON ONLY WHEN LINE IS IN OUT CONDITION. SWITCHABLE: LINE REACTORS WHICH CAN BE OPERATED EVEN WHEN LINE IS IN SERVICE. FIXED: LINE REACTORS WHICH ARE FIXED AND CANNOT BE OPERATED UPON AS A BUS REACTOR. Page 27 of 60

List 5: Bus Reactors in North Eastern Region SR. NO. UTILITY INSTALLED AT RATING KV (STATION) MVAR MAKE STATUS 1 POWERGRID BALIPARA 400 50 BHEL IN SERVICE 2 POWERGRID BALIPARA 400 80 BHEL IN SERVICE 3 POWERGRID BONGAIGAON 400 2 X 50 BHEL IN SERVICE 4 POWERGRID BONGAIGAON 400 2 X 80 BHEL IN SERVICE 5 POWERGRID MISA 400 50 BHEL IN SERVICE 6 POWERGRID SILCHAR 400 2 X 63 CGL IN SERVICE 7 POWERGRID BISWANATH IN SERVICE 400 2 X 80 CHARALI 8 OTPC PALATANA 400 80 BHEL IN SERVICE 9 ASSAM MARIANI 220 2 X 12.5. IN SERVICE 10 ASSAM SAMAGURI 220 2 X 12.5. IN SERVICE 11 POWERGRID AIZWAL 132 20. IN SERVICE 12 POWERGRID KUMARGHAT 132 20. IN SERVICE 13 TRIPURA DHARMANAGAR 132 2 X 2. IN SERVICE 14 POWERGRID ZIRO 132 20. IN SERVICE 15 POWERGRID IMPHAL 132 20. IN SERVICE 16 POWERGRID NEW MARIANI 132 20. IN SERVICE 17 ASSAM SAMAGURI 132 2X12.5. IN SERVICE 18 ASSAM AZARA 400 63 BHEL IN SERVICE 19 MEGHALAYA BYRNIHAT 400 63 CGL NOT IN SERVICE To be replaced with 80 MVAR B/R. Sent to CEA for approval 20 POWERGRID ROING 132 20. IN SERVICE 21 POWERGRID TEJU 132 20. IN SERVICE 22 POWERGRID BISWANATH. IN SERVICE 400 2 X 63 CHARALI 23 POWERGRID NAMSAI 132 20. IN SERVICE 24 POWERGRID BONGAIGAON 400 125. IN SERVICE 25 POWERGRID BALIPARA 400 125. IN SERVICE 26 POWERGRID MOKOKCHUNG 220 31.5. IN SERVICE 27 POWERGRID IMPHAL 400 80. IN SERVICE 28 POWERGRID SILCHAR 400 125. IN SERVICE Page 28 of 60

List 6: List of Upcoming Bus Reactors in North Eastern Region SR. NO. UTILITY TO BE INSTALLED AT (STATION) KV RATING (MVAR) 1 AEGCL Rangia 400 80 2 AEGCL Sonapur 400 80 3 POWERGRID Misa 400 80 4 POWERGRID New Mariani 400 125 5 POWERGRID Imphal 400 125 6 STERLITE PK Bari 400 125 7 STERLITE Surjamaninagar 400 125 List 7: Tertiary Reactors on 33kV side of 400/220/33 kv ICTs in North Eastern Region SR. NO. UTILITY INSTALLED RATING INSTALLED AT ON MVAR MAKE (STATION) STATUS 1 POWERGRID BALIPARA 33 KV SIDE IN 4 X 25 BHEL OF ICT I SERVICE 2 POWERGRID BONGAIGAON 33 KV SIDE IN 2 X 25 BHEL OF ICT I SERVICE 3 POWERGRID MISA 33 KV SIDE IN 4 X 25 BHEL OF ICT I SERVICE Page 29 of 60