In electrical engineering, a synchronous condenser (sometimes synchronous capacitor or synchronous compensator) compensator) is a device identical to a synchronous motor, whose shaft is not connected to anything but spins freely. Its purpose is not to convert electric power to mechanical power or vice versa, but to adjust conditions on the electric power transmission grid. Its field is controlled by a voltage regulator to either generate or absorb reactive power as needed to adjust the grid's voltage, or to improve power factor. The condenser’s installation and operation are identical to large electric motors. Increasing the device's field excitation results in its furnishing magnetizing power (kvars) to the system. Its principal advantage is the ease with which the amount of correction can be adjusted. The energy stored in the rotor of the machine can also help stabilize a power system during short circuits or rapidly fluctuating loads such as electric arc furnaces. Large installations of synchronous condensers are sometimes used in association with high-voltage direct current converter stations to supply reactive power. Unlike a capacitor bank, the value of reactive power from a synchronous condenser can be continuously adjusted. In addition, reactive power from a capacitor bank decreases with voltage decrease, while a synchronous condenser can increase current as voltage decreases. However, it does have higher losses than a static capacitor bank. Most synchronous condensers connected to electrical grids are rated between 20 Mvar and 200 Mvar and many are hydrogen cooled. As the load on a synchronous motor increases, the stator current Ia increases regardless of excitation. For under and over excited motors, the power factor (p.f.) tends to approach 1 with increase in load. The change in power factor is greater than the change in Ia with increase in load. The magnitude of armature current varies with excitation. The current has large value both for low and high values of excitation. In between, it has minimum value corresponding to a certain excitation. The variations of I with excitation are known as V curves because of their shape. For the same output load, the armature current varies over a wide range and so causes the power factor also to vary accordingly. When over-excited, the motor runs with leading power factor and with lagging power factor when under-excited. In between, the power factor is unity. The minimum armature current corresponds to unity power factor.
V- curves for a synchronous machine. A synchronous condenser operates at nearly zero real power. As the machine passes from under-excited to overexcited, its stator current passes through a minimum Application An over-excited synchronous motor has a leading power factor. This makes it useful for power factor correction of industrial loads. Both transformers and induction motors draw lagging currents from the line. On light loads, the power drawn by induction motors has a large reactive component and the power factor has a very low value. The current flowing to supply reactive power creates losses in the power system. In an industrial plant, synchronous motors can be used to supply some of the reactive power required by induction motors. This improves the plant power factor and reduces supply current. A synchronous condenser provides step-less automatic power factor correction with the ability to produce up to 150% additional Mvars. The system produces no switching transients and is not affected by system electrical harmonics (some harmonics can even be absorbed by synchronous condensers). They will not produce excessive voltage levels and are not susceptible to electrical resonances. Because of the rotating inertia of the condenser, it can provide limited voltage support during short power outages. The use of rotating synchronous condensers was common through the 1950s. They remain an alternative (or a supplement) to capacitors for power factor correction because of problems that have been experienced with harmonics causing capacitor overheating and catastrophic failures. Synchronous condensers
capacitor bank is in direct proportion to the square of its terminal voltage, where a synchronous condenser's reactive power declines less rapidly, and can be adjusted to compensate for falling terminal voltage. This reactive power improves voltage regulation in situations such as starting large motors, or where power must travel long distances from where it is generated to where it is used, as is the case with power wheeling , the transmission of electric power from one geographic region through another within a set of interconnected electric power systems. Synchronous condensers may also be referred to as Dynamic Power Factor Correction systems. These machines can prove very effective when advanced controls are utilized. A PLC based controller with PF controller and regulator will allow the system to be set to meet a given power factor or can be set to produce a specified amount of reactive power. On an electric power system, synchronous condensers can be used to control the voltage on long transmission lines, especially for lines with relatively high ratio of inductive reactance to resistance. Conclusion for effect for field change with constant load on power factor •
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For motor with increased (decreased) excitation power factor becomes leading (lagging). For generator with increased (decreased) excitation power factor becomes lagging (leading). Unloaded overexcited synchronous motors are sometimes used to improve power factor. They are known as synchronous condensers.
A Series SC is an unloaded or lightly loaded synchronous machine that will deliver the required reactive power dynamically. The SC is connected to the power line and is intentionally run in an overexcited condition. The level of excitation is dependent on the amount of power factor correction desired and the amount of power factor sensed by the condenser controls. The condenser will adjust the excitation level automatically to maintain the power factor at the correct setting. Once the Series SC is installed, it continuously monitors the power factor and produces the right amount of VAR needed to correct any power factor without switching transients and is not troubled by harmonic currents produced by solid state motor drives. The Series SC also helps overall power quality by reducing voltage transients and by reducing the problems associated with harmonic distortion found in many manufacturing process. The Series SC is available from 100 kVAR to 10,000 kVAR modules, from 480 volts up to 15 KV. Applying a synchronous condenser for power factor correction provides many advantages with no risks. Correcting power factor with a condenser is much smoother and will not adversely affect a system loaded with current harmonics. The condenser is a low impedance source and appears inductive to loads. In addition the Series SC is easy to maintain (a simple annual lubrication) and adapts easily to a plant’s changing loads.(Dynamic Correction). The SC has many user friendly features such as the automatic restart after power loss. The SC also has a state-of-the-art integrated touch screen display and control interface. The controls are designed to be friendly and easily understood so that the user can control the SC from this panel. The touch screen will display voltage, current, power factor, and power information as a minimum.
Reactive Power compensation devices Synchronous condensers Every synchronous machine (motor or generator) has the reactive power capabilities the same as synchronous generators. Synchronous machines that are designed exclusively to provide reactive support are called synchronous condensers. Synchronous condensers have all of the response speed and controllability advantages of generators without the need to construct the rest of the power plant (e.g., fuel-handling equipment and boilers). Because they are rotating machines with moving parts and auxiliary systems, they require significantly more maintenance than static compensators. They also consume real power equal to about 3% of the machine’s reactive-power rating. Synchronous condensers are used in transmission systems: at the receiving end of long transmissions, in important substations and in conjunction with HVDC converter stations.
Small synchronous condensers have also been used in high-power industrial networks to increase the short circuit power. The reactive power output is continuously controllable. The response time with closed loop voltage control is from a few seconds and up, depending on different factors. In recent years the synchronous condensers have been practically ruled out by the thyristor controlled static VAR compensators, because those are much more cheaper and have regulating characteristics similar to synchronous condensers. Static VAR compensators An SVC combines conventional capacitors and inductors with fast switching capability. Switching takes place in the sub cycle timeframe (i.e., in less than 1/50 of a second), providing a continuous range of control. The range can be designed to span from absorbing to generating reactive power. Advantages include fast, precise regulation of voltage and unrestricted, largely transient-free, capacitor bank switching. Voltage is regulated according to a slope (droop) characteristic.
Static VAR compensator could be made up from: TCR (thyristor controlled reactor); TSC (thyristor switched capacitor); TSR (thyristor switched reactor); FC (fixed capacitor); Harmonic filter. Because SVCs use capacitors they suffer from the same degradation in reactive capability as voltage drops. They also do not have the short-term overload capability of generators and synchronous condensers. SVC applications usually require harmonic filters to reduce the amount of harmonics injected into the power system by the thyristor switching. SVCs provide direct control of voltage (C.W. Taylor, 1994); this is very valuable when there is little generation in the load area. The remaining capacitive capability of an SVC is a good indication of proximity to voltage instability. SVCs provide rapid control of temporary overvoltages. But on the other hand SVCs have limited overload capability, because SVC is a capacitor bank at its boost limit. The critical or collapse voltage becomes the SVC regulated voltage and instability usually occurs once an SVC reaches its boost
Static synchronous compensator (STATCOM) The STATCOM is a solid-state shunt device that generates or absorbs reactive power and is one member of a family of devices known as flexible AC transmission system (FACTS) devices. The STATCOM is similar to the SVC in response speed, control capabilities, and the use of power electronics. Rather than using conventional capacitors and inductors combined with thyristors, the STATCOM uses self-commutated power electronics to synthesize the reactive power output. Consequently, output capability is generally symmetric, providing as much capability for production as absorption. The solid-state nature of the STATCOM means that, similar to the SVC, the controls can be designed to provide very fast and effective voltage control (B. Kirby, 1997). While not having the short-term overload capability of generators and synchronous condensers, STATCOM capacity does not suffer as seriously as SVCs and capacitors do from degraded voltage. STATCOMs are current limited so their MVAR capability responds linearly to voltage as opposed to the voltage-squared relationship of SVCs and capacitors. This attribute greatly increases the usefulness of STATCOMs in preventing voltage collapse. Series capacitors and reactors Series capacitors compensation is usually applied for long transmission lines and transient stability improvement. Series compensation reduces net transmission line inductive reactance. The reactive generation I2XC compensates for the reactive consumption I2X of the transmission line. Series capacitor reactive generation increases with the current squared, thus generating reactive power when it is most needed. This is a self-regulating nature of series capacitors. At light loads series capacitors have little effect. Shunt capacitors The primary purposes of transmission system shunt compensation near load areas are voltage control and load stabilization. Mechanically switched shunt capacitor banks are installed at major substations in load areas for producing reactive power and keeping voltage within required limits. For voltage stability shunt capacitor banks are very useful in allowing nearby generators to operate near unity power factor. This maximizes fast acting reactive reserve. Compared to SVCs, mechanically switched capacitor banks have the advantage of much lower cost. Switching speeds can be quite fast. Current limiting reactors are used to minimize switching transients.
There are several disadvantages to mechanically switched capacitors. For voltage emergencies the shortcoming of shunt capacitor banks is that the reactive power output drops with the voltage squared. For transient voltage instability the switching may not be fast enough to prevent induction motor stalling. Precise and rapid control of voltage is not possible. Like inductors, capacitor banks are discrete devices, but they are often configured with several steps to provide a limited amount of variable control. If voltage collapse results in a system, the stable parts of the system may experience damaging over voltages immediately following separation. Shunt reactors Shunt reactors are mainly used to keep the voltage down, by absorbing the reactive power, in the case of light load and load rejection, and to compensate the
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Other Other equipment can be involved in the provision of reactive power and energy, such as: Unified Power Flow Controllers (UPFC) and other advanced FACTS (flexible ac transmission system) devices; Tap staggering of transformers connected in parallel; Disconnection of transmission lines; Load shedding;
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Investments in reactive compensation This section will try to answer the question: How much does 1 MVar cost to install and to produce? The table below from (B. Kirby, 1997) gives some numbers. These are, of course, approximate and may vary according to equipment producer and type. Capital and operating costs of reactive power compensation equipment ��������� ����
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Conclusions In this work we tried to define the value of reactive power using four different valuation methods: Voltage Sensitivity, PV curve, Equivalent Reactive Compensation and Back-up generation. A simple test system model was build and tested with Power World software. Preliminary results showed different nature of reactive power from the sources. For the same test system model we have tried to find out the advantages and disadvantages of having zero reactive power flow in the system transformers. So the results showed that for every extra MVar towards the zero reactive power flow in the system transformers we need to install 2.5 MVar of reactive power compensation devices. Having zero flow system losses and transfer capacity changes very little. In the introduction part we defined some questions we wanted to answer in this work. So the answers and the final conclusions are like this: •
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Which reactive power source is the most important to the system? What are the criteria? We tried to answer this question with the help of all four proposed methods of reactive power valuation. They give different results, but at the same time showed different nature of reactive power. Voltage Sensitivity and PV curve methods show how generators react with their reactive power output to the changing load in the system. ERC and Back-up generation methods help to realize the importance of the reactive power reserve of the sources. Generator closest to the relatively high load is the most important to the system. How much MVar from other sources would be needed to replace Q from the selected source? Back-up generation method is the answer to this. It describes the importance of the source to system reactive power reserve and shows how much of additional reactive power we have to supply to cover the Q
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shortage from one or other reactive power source to keep bus voltages (at PV buses) at the predetermined values. What does 1 MVar from one or other source do to system losses? Active and reactive power losses sensitivities to additional MVar injected into the system can be obtained using Voltage Sensitivity method. Marginal losses produced by the source depend on the network configuration and location of the source in the system. What does this 1 MVar mean to transfer capacity? With PV curve method we were able to see what influence each generator in our test system had on the transfer limit. During high load condition the transfer limit was obtained when one of the sources reached the limits of reactive power output. So the source closest to the loads and with relatively low reactive power output limits is crucial to the system. How much does this MVar cost to install and to produce? Reactive power sources and sinks were analyzed in the section 2.3 and in table 2 approximate numbers about the installation and production costs of reactive power equipment were given. The most expensive, but the best sources of reactive power support are machinery equipment – generators, synchronous condensers. For further investigation in this area, the results should be checked on the real electric power system to find out if the proposed methods are suitable.
9. Generator reactive power capability 9.1 A generator‘s output capabilities depend on the thermal limits of various parts of the generator and on system stability limits. Thermal limits are physical limits of materials such as copper, iron and insulation, if the generator overheats, insulation begins to degrade and over time this could result in equipment damage. Increasingly real power output of a generator heats up the armature. Increasing reactive power output heats up the field windings and the armature. 9.2 Power Generators shall be able to supply or absorb reactive power according to the reactive power capability curve that is defined in connection agreement and shall perform yearly inspection to ensure that generators can perform according to the reactive power capability curve. 9.3 To supply reactive power, the generator must increase the magnetic field to raise the voltage it is supplying to the power system; this means increasing the current in the field windings, which is limited by the thermal properties of the metal and insulation. The field current is supplied by the generator exciter, which is a DC power supply connected to the generator. The field current can be quickly adjusted by automatic control or with a dial to change the reactive power supplied or consumed by the generator. 9.4 At any given field setting, the generator has a specific terminal voltage it is attempting to hold. If the system voltage declines, the generator will inject reactive power into the power system, tending to raise system voltage. If the system voltage rises, the reactive output of the generator will drop and ultimately reactive power will flow into the generator, tending to lower system voltage. 9.5 The voltage regulator will accentuate this behavior by driving the field current in the appropriate direction to obtain the desired system voltage. Because most of the reactive limits are thermal limits associated with large pieces of equipment, significant short-term extra reactive-power capability usually exists. 9.6 Power-system stabilizers also control generator field current and reactive power output in response to oscillations on the power system. This function is a part of the network-stability ancillary service. 9.7 Stability limits are determined by the ability of the power system to accept delivery of power from the connected generator under a defined set of system conditions including recognized contingencies. All generators
generator loses synchronism with the rest of the system, it will trip offline to protect itself. 9.8 Capacitors supply reactive power and have leading power factors, while inductors consume reactive power and have lagging power factors. The convention for generators is the reverse. When the generator is supplying reactive power, it has a lagging power factor and its mode of operation is referred to as overexcited. When a generator consumes reactive power, it has a leading power factor region and is under-excited. 9.9 The capability-set limits are thermal limits for different parts of the generator, if the generator output approaches these limits, an alarm will notify the generator operator of the problem; if the operator does not bring the generator back to a safe operating point, the generator‘s protection scheme (relays, circuit breakers, fuses) will operate, resulting in disconnection of the generator from the network; finally, if the protection equipment fails and the operator does not act in time, the generator will overheat, potentially causing equipment damage. Because generators are expensive, generator operators generally will not operate the generator in a way that risks damaging the equipment and losing revenue during repair. 9.10 The ability of a generator to provide reactive support depends on its real power production which is represented in the form of generator capability curve or D - curve. Figure 9.1 shows the combined limits on real and reactive production for a typical generator. Like most electric equipment, generators are limited by their current-carrying capability. Near rated voltage, this capability becomes an MVA limit for the armature of the generator rather than a MW limitation, shown as the armature heating limit in the Figure.
Fig 9.1 GENERATOR CAPABILITY CURVE or D CURVE
9.11 At the edges of the D-curve, the opportunity cost of extending generator real or reactive power supply amounts to the millions of rupees that would be needed to replace damaged generator equipment and lost revenue during repair. The characteristics of the generator step-up transformer that connects the generator to the electric transmission system, as well as operational policies of the transmission system, may impose further limits on generator output. 9.12 Generator capability may be extended by the coolant used in the generator. A more efficient coolant allows the generator to dissipate more heat, thereby extending thermal limits. Most large generators are cooled with hydrogen; increasing the hydrogen pressure cools the generator equipment more effectively, increasing the generator‘s capability .
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SYNCHRONOUS CONDENSERS:
9.13.1. Synchronous condenser is another reactive power device, traditionally in use since 1920s. A synchronous Condenser is a synchronous machine running without a prime mover or a mechanical load. Like generators, they can be over-exited or under-exited by varying their field current in order to generate or absorb reactive power, synchronous condensers can continuously regulate reactive power to ensure steady transmission voltage, under varying load conditions. They are especially suited for emergency voltage control under loss of load, generation or transmission, because of their fast short-time response. Synchronous condensers provide necessary reactive power even exceeding their rating for short duration, to arrest voltage collapse and to improve system stability. It draws a small amount of active power (about 3%) from the power system to supply losses. Synchronous machines that are designed exclusively to provide reactive support are called synchronous condensers. Synchronous condensers have all of the response speed and controllability advantages of generators without the need to construct the rest of the power plant (e.g., fuel-handling equipment and boilers). Because they are rotating machines with moving parts and auxiliary systems, they may require significantly more maintenance than static alternatives. They also consume real power equal to about 3% of the machine‘s reactive-power rating. That is, a 50MVAR synchronous condenser requires about 1.5 MW of real power. 9.14 Synonymous terms are synchronous compensator and synchronous
in transmission systems: at the receiving end of long transmissions, in important substations and in conjunction with HVDC inverter stations. Small synchronous compensators have also been installed in high-power industrial networks of steel mills; few of these are in use today. Synchronous compensators in use range in size from a few MVA up to hundreds of MVA. 9.15 Some hydro/gas generators can operate as synchronous condensers. Such gas based units are often equipped with clutches which can be used to disconnect the turbine from the generator when active power is not required from them. In case of hydro, water supply is blocked and units run with loads of only air friction 9.16 A synchronous Compensator has several advantages over static compensators. Synchronous compensators contribute to system short circuit capacity. Their reactive power production is not affected by the system voltage. During power swings (electro mechanical oscillations) there is an exchange of kinetic energy between a synchronous condenser and the power system. 9.17 During such power swings, a synchronous condenser can supply a large amount of reactive power, perhaps twice its continuous rating. Unlike other forms of shunt compensation, it has an internal voltage source and is better to cope with the low voltage conditions. Because of their high purchase and operating costs, they have been largely superseded by static var compensators. 9.17 In recent years the synchronous compensator has been practically ruled out by the SVC, in the case of new installations, due to benefits in cost performance and reliability of the latter. One exception is HVDC inverter stations, in cases where the short-circuit capacity has to be increased. The synchronous compensators can do this, but not the SVC. 9.18 Comparison between Synchronous Condenser and shunt capacitor is explained in the below table:Sl.No
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Synchronous condenser Synchronous condenser can supply kVAR equal to its rating and can absorb up to 100% of its KVA rating This has fine control with AVR The output is not limited by the system voltage condition. This
Shunt capacitor Shunt capacitor should be associated with a reactor to give that performance
This operates in steps The capacitor output is proportional to V2 of the system. Hence its
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For short periods the synchronous condenser can supply KVAR in excess of its rating at nominal voltage The full load losses are above 3% of its capacity These cannot be economically deployed at several locations in distribution The synchronous condenser ratings cannot be modular
The capacitor cannot supply more than its capacity at nominal voltage. Its output is proportional to V 2. The capacitor losses are about 0.2%
The capacitor banks can be deployed at several locations economically in distribution The capacitors are modular. They can be deployed as and when system requirements change A failure in the synchronous A failure of a single fused unit in a condenser can remove the entire bank of capacitors affects only that unit unit ability to produce KVAR. and does not affect the entire bank However failures are rare in synchronous condensers compared to capacitors They add to the short circuit The capacitors do not increase the current of a system and short circuit capacity of the system, as therefore increase the size of their output is proportional to V 2 (11kV etc.) breakers in the neighbor-hood. This is a rotating device. Hence These are static and simple devices. the O&M problems are more Hence O&M problems are negligible
9.19 As per planning philosophy and general guidelines in the Manual on Transmission planning criteria issued by CEA (MOP, India), Thermal / Nuclear Generating Units shall normally not run at leading power factor. However for the purpose of charging unit may be allowed to operate at leading power factor as per the respective capability curve.
9.20 List of synchronous condenser at all India level is given :
