Importance of Reactive Power for System
March 21, 2011 34 Comments
- We always in practice to reduce reactive power to improve system efficiency .This are acceptable at some level. If system is purely resistively or capacitance it make cause some problem in Electrical system. Alternating systems supply or consume two kind of power: real power and reactive power.
- Real power accomplishes useful work while reactive power supports the voltage that must be controlled for system reliability. Reactive power has a profound effect on the security of power systems because it affects voltages throughout the system.
- Find important discussion regarding importance about Reactive Power and how it is useful to maintain System voltage healthy
Importance of Reactive Power:
- Voltage control in an electrical power system is important for proper operation for electrical power equipment to prevent damage such as overheating of generators and motors, to reduce transmission losses and to maintain the ability of the system to withstand and prevent voltage collapse.
- Decreasing reactive power causing voltage to fall while increasing it causing voltage to rise. A voltage collapse may be occurs when the system try to serve much more load than the voltage can support.
- When reactive power supply lower voltage, as voltage drops current must increase to maintain power supplied, causing system to consume more reactive power and the voltage drops further . If the current increase too much, transmission lines go off line, overloading other lines and potentially causing cascading failures.
- If the voltage drops too low, some generators will disconnect automatically to protect themselves. Voltage collapse occurs when an increase in load or less generation or transmission facilities causes dropping voltage, which causes a further reduction in reactive power from capacitor and line charging, and still there further voltage reductions. If voltage reduction continues, these will cause additional elements to trip, leading further reduction in voltage and loss of the load. The result in these entire progressive and uncontrollable declines in voltage is that the system unable to provide the reactive power required supplying the reactive power demands
Necessary to Control of Voltage and Reactive Power:
- Voltage control and reactive power management are two aspects of a single activity that both supports reliability and facilitates commercial transactions across transmission networks.
- On an alternating current (AC) power system, voltage is controlled by managing production and absorption of reactive power.
- There are three reasons why it is necessary to manage reactive power and control voltage.
- First, both customer and power system equipment are designed to operate within a range of voltages, usually within±5% of the nominal voltage. At low voltages, many types of equipment perform poorly, light bulbs provide less illumination, induction motors can overheat and be damaged, and some electronic equipment will not operate at. High voltages can damage equipment and shorten their lifetimes.
- Second, reactive power consumes transmission and generation resources. To maximize the amount of real power that can be transferred across a congested transmission interface, reactive power flows must be minimized. Similarly, reactive power production can limit a generator’s real power capability.
- Third, moving reactive power on the transmission system incurs real power losses. Both capacity and energy must be supplied to replace these losses.
- Voltage control is complicated by two additional factors.
- First, the transmission system itself is a nonlinear consumer of reactive power, depending on system loading. At very light loading the system generates reactive power that must be absorbed, while at heavy loading the system consumes a large amount of reactive power that must be replaced. The system’s reactive power requirements also depend on the generation and transmission configuration.
- Consequently, system reactive requirements vary in time as load levels and load and generation patterns change. The bulk power system is composed of many pieces of equipment, any one of which can fail at any time. Therefore, the system is designed to withstand the loss of any single piece of equipment and to continue operating without impacting any customers. That is, the system is designed to withstand a single contingency. The loss of a generator or a major transmission line can have the compounding effect of reducing the reactive supply and, at the same time, reconfiguring flows such that the system is consuming additional reactive power.
- At least a portion of the reactive supply must be capable of responding quickly to changing reactive power demands and to maintain acceptable voltages throughout the system. Thus, just as an electrical system requires real power reserves to respond to contingencies, so too it must maintain reactive-power reserves.
- Loads can also be both real and reactive. The reactive portion of the load could be served from the transmission system. Reactive loads incur more voltage drop and reactive losses in the transmission system than do similar size (MVA) real loads.
- System operation has three objectives when managing reactive power and voltages.
- First, it must maintain adequate voltages throughout the transmission and distribution system for both current and contingency conditions.
- Second, it seeks to minimize congestion of real power flows.
- Third, it seeks to minimize real power losses.
Basic concept of Reactive Power
1) Why We Need Reactive Power:
- Active power is the energy supplied to run a motor, heat a home, or illuminate an electric light bulb. Reactive power provides the important function of regulating voltage.
- If voltage on the system is not high enough, active power cannot be supplied.
- Reactive power is used to provide the voltage levels necessary for active power to do useful work.
- Reactive power is essential to move active power through the transmission and distribution system to the customer .Reactive power is required to maintain the voltage to deliver active power (watts) through transmission lines.
- Motor loads and other loads require reactive power to convert the flow of electrons into useful work.
- 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.”
2) Reactive Power is a Byproduct of AC Systems
- Transformers, Transmission lines, and motors require reactive power. Electric motors need reactive power to produce magnetic fields for their operation.
- Transformers and transmission lines introduce inductance as well as resistance
- Both oppose the flow of current
- Must raise the voltage higher to push the power through the inductance of the lines
- Unless capacitance is introduced to offset inductance
3) How Voltages Controlled by Reactive Power:
- Voltages are controlled by providing sufficient reactive power control margin to supply needs through
- Shunt capacitor and reactor compensations
- Dynamic compensation
- Proper voltage schedule of generation.
- Voltages are controlled by predicting and correcting reactive power demand from loads
4) Reactive Power and Power Factor
- Reactive power is present when the voltage and current are not in phase
- One waveform leads the other
- Phase angle not equal to 0°
- Power factor less than unity
- Measured in volt-ampere reactive (VAR)
- Produced when the current waveform leads voltage waveform (Leading power factor)
- Vice verse, consumed when the current waveform lags voltage (lagging power factor)
5) Reactive Power Limitations:
- Reactive power does not travel very far.
- Usually necessary to produce it close to the location where it is needed
- A supplier/source close to the location of the need is in a much better position to provide reactive power versus one that is located far from the location of the need
- Reactive power supplies are closely tied to the ability to deliver real or active power.
Reactive Power Caused Absence of Electricity -A Blackout
- The quality of the electrical energy supply can be evaluated basing on a number of parameters. However, the most important will be always the presence of electrical energy and the number and duration of interrupts.
- When consumption of electrical energy is high, the demand on inductive reactive power increases at the same proportion. In this moment, the transmission lines (that are well loaded) introduce an extra inductive reactive power. The local sources of capacitive reactive power become insufficient. It is necessary to deliver more of the reactive power from generators of power plants.
- It might happen that they are already fully loaded and the reactive power will have to be delivered from more distant places. Transmission of reactive power will load more the lines, which in turn will introduce more reactive power. The voltage on customer side will decrease further. Local control of voltage by means of auto transformers will lead to increase of current (to get the same power) and this in turn will increase voltage drops in lines. In one moment this process can go like avalanche reducing voltage to zero. In mean time most of the generators in power plants will switch off due to unacceptably low voltage what of course will deteriorate the situation.
- Insufficient reactive power leading to voltage collapse has been a causal factor in major blackouts in the worldwide. Voltage collapse occurred in United States in the blackout of July 2, 1996, and August10, 1996 on the West Coast
- While August 14, 2003, blackout in the United States and Canada was not due to a voltage collapse as that term has traditionally used by power system engineers, the task force final report said that” Insufficient reactive power was an issue in the blackout” and the report also “overestimation of dynamics reactive output of system generation ” as common factor among major outages in the United States.
- Demand for reactive power was unusually high because of a large volume of long-distance transmissions streaming through Ohio to areas, including Canada, than needed to import power to meet local demand. But the supply of reactive power was low because some plants were out of service and, possibly, because other plants were not producing enough of it.”
Problem of Reactive Power:
- Though reactive power is needed to run many electrical devices, it can cause harmful effects on appliances and other motorized loads, as well as electrical infrastructure. Since the current flowing through electrical system is higher than that necessary to do the required work, excess power dissipates in the form of heat as the reactive current flows through resistive components like wires, switches and transformers. Keep in mind that whenever energy is expended, you pay. It makes no difference whether the energy is expended in the form of heat or useful work.
- We can determine how much reactive power electrical devices use by measuring their power factor, the ratio between real power and true power. A power factor of 1 (i.e. 100%) ideally means that all electrical power is applied towards real work. Homes typically have overall power factors in the range of 70% to 85%, depending upon which appliances may be running. Newer homes with the latest in energy efficient appliances can have an overall power factor of 90%.
- Electric companies correct for power factor around industrial complexes, or they will request the offending customer to do so, or they will charge for reactive power. Electric companies are not worried about residential service because the impact on their distribution grid is not as severe as in heavily industrialized areas. However, it is true that power factor correction assists the electric company by reducing demand for electricity, thereby allowing them to satisfy service needs elsewhere.
- Power factor correction will not raise your electric bill or do harm to your electrical devices. The technology has been successfully applied throughout industry for years. When sized properly, power factor correction will enhance the electrical efficiency and longevity of inductive loads. Power factor correction can have adverse side effects (e.g. harmonics) on sensitive industrialized equipment if not handled by knowledgeable, experienced professionals. Power factor correction on residential dwellings is limited to the capacity of the electrical panel (200 amp max) and does not over compensate household inductive loads. By increasing the efficiency of electrical systems, energy demand and its environmental impact is lessened
Effects of Reactive Power in Various elements of Power System:
- An electric power generator’s primary function is to convert fuel into electric power. Almost all generators also have considerable control over their terminal voltage and reactive-power output.
- The ability of generator to provide reactive support depends on its real power production. 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.
- Production of reactive power involves increasing the magnetic field to raise the generator’s terminal voltage. Increasing the magnetic field requires increasing the current in the rotating field winding. Absorption of reactive power is limited by the magnetic-flux pattern in the stator, which results in excessive heating of the stator-end iron, the core-end heating limit.
- The synchronizing torque is also reduced when absorbing large amounts of reactive power, which can also limit generator capability to reduce the chance of losing synchronization with the system.
- The generator prime mover (e.g., the steam turbine) is usually designed with less capacity than the electric generator, resulting in the prime-mover limit. The designers recognize that the generator will be producing reactive power and supporting system voltage most of the time. Providing a prime mover capable of delivering all the mechanical power the generator can convert to electricity when it is neither producing nor absorbing reactive power would result in under utilization of the prime mover.
- To produce or absorb additional VARs beyond these limits would require a reduction in the real power output of the unit. Control over the reactive output and the terminal voltage of the generator is provided by adjusting the DC current in the generator’s rotating field .Control can be automatic, continuous, and fast.
- The inherent characteristics of the generator help maintain system voltage. 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. The voltage regulator will accentuate this behavior by driving the field current in the appropriate direction to obtain the desired system voltage.
2) Synchronous Condensers:
- Every synchronous machine (motor or generator) with a controllable field has the reactive power capabilities discussed above.
- Synchronous motors are occasionally used to provide dynamic voltage support to the power system as they provide mechanical power to their load. Some combustion turbines and hydro units are designed to allow the generator to operate without its mechanical power source simply to provide the reactive power capability to the power system when the real power generation is unavailable or not needed. 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.
3) Capacitors & Inductors:
- Capacitors and inductors (which are sometimes called reactors) are passive devices that generate or absorb reactive power. They accomplish this without significant real power losses or operating expense.
- The output of capacitors and inductors is proportional to the square of the voltage. Thus, a capacitor bank (or inductor) rated at 100 MVAR will produce (or absorb) only 90 MVAR when the voltage dips to 0.95 pu but it will produce (or absorb) 110 MVAR when the voltage rises to 1.05 pu. This relationship is helpful when inductors are employed to hold voltages down.
- The inductor absorbs more when voltages are highest and the device is needed most. The relationship is unfortunate for the more common case where capacitors are employed to support voltages. In the extreme case, voltages fall, and capacitors contribute less, resulting in a further degradation in voltage and even less support from the capacitors; ultimately, voltage collapses and outages occur.
- Inductors are discrete devices designed to absorb a specific amount of reactive power at a specific voltage. They can be switched on or off but offer no variable control.
- Capacitor banks are composed of individual capacitor cans, typically 200 kVAR or less each. The cans are connected in series and parallel to obtain the desired capacitor bank voltage and capacity rating. Like inductors, capacitor banks are discrete devices but they are often configured with several steps to provide a limited amount of variable control which makes it a disadvantage compared to synchronous motor.
4) Static VAR Compensators : (SVCs)
- An SVC combines conventional capacitors and inductors with fast switching capability. Switching takes place in the sub cycle timeframe (i.e. in less than 1/60 of a second), providing a continuous range of control. The range can be designed to span from absorbing to generating reactive power. Consequently, the controls can be designed to provide very fast and effective reactive support and voltage control.
- 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.
5) Static Synchronous Compensators : (STATCOMs)
- 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.
- 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 fast switches, however, the STATCOM uses 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. 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.
6) Distributed Generation:
- Distributing generation resources throughout the power system can have a beneficial effect if the generation has the ability to supply reactive power. Without this ability to control reactive power output, performance of the transmission and distribution system can be degraded.
- Induction generators were an attractive choice for small, grid-connected generation, primarily because they are relatively inexpensive. They do not require synchronizing and have mechanical characteristics that are appealing for some applications (wind, for example). They also absorb reactive power rather than generate it, and are not controllable. If the output from the generator fluctuates (as wind does), the reactive demand of the generator fluctuates as well, compounding voltage-control problems for the transmission system.
- Induction generators can be compensated with static capacitors, but this strategy does not address the fluctuation problem or provide controlled voltage support. Many distributed generation resources are now being coupled to the grid through solid-state power electronics to allow the prime mover’s speed to vary independently of the power-system frequency. For wind, this use of solid-state electronics can improve the energy capture.
- For gas-fired micro turbines, power electronics equipment allows them to operate at very high speeds. Photovoltaic’s generate direct current and require inverters to couple them to the power system. Energy-storage devices (e.g., batteries, flywheels, and superconducting magnetic-energy storage devices) are often distributed as well and require solid-state inverters to interface with the grid. This increased use of a solid-state interface between the devices and the power system has the added benefit of providing full reactive-power control, similar to that of a STATCOM.
- In fact, most devices do not have to be providing active power for the full range of reactive control to be available. The generation prime mover, e.g. turbine, can be out of service while the reactive component is fully functional. This technological development (solid-state power electronics) has turned a potential problem into a benefit, allowing distributed resources to contribute to voltage control.
7) Transmission Side:
- Unavoidable consequence of loads operation is presence of reactive power, associated with phase shifting between voltage and current.
- Some portion of this power is compensated on customer side, while the rest is loading the network. The supply contracts do not require a cosφ equal to one. The reactive power is also used by the transmission lines owner for controlling the voltages.
- Reactive component of current adds to the loads current and increases the voltage drops across network impedance. Adjusting the reactive power flow the operator change voltage drops in lines and in this way the voltage at customer connection point.
- The voltage on customer side depends on everything what happens on the way from generator to customer loads. All nodes, connection points of other transmission lines, distribution station and other equipment contribute to reactive power flow.
- A transmission line itself is also a source of reactive power. A line that is open on the other end (without load) is like a capacitor and is a source of capacitive (leading) reactive power. The lengthwise inductances without current are not magnetized and do not introduce any reactive components. On the other hand, when a line is conducting high current, the contribution of the lengthwise inductances is prevalent and the line itself becomes a source of inductive (lagging) reactive power. For each line can be calculated a characteristic value of power flow.
- If the transmitted power is more than pre define Value, the line will introduce additionally inductive reactive power, and if it is below pre define Value, the line will introduce capacitive reactive power. The pre define Value depends on the voltage: for 400 kV line is about 32% of the nominal transmission power, for 220 kV line is about 28% and for 110 kV line is about 22%. The percentage will vary accordingly to construction parameters.
- The reactive power introduced by the lines themselves is really a nuisance for the transmission system operator. In the night, when the demand is low it is necessary to connect parallel reactors for consuming the additional capacitive reactive power of the lines. Sometimes it is necessary to switch off a low-loaded line (what definitely affect the system reliability). In peak hours not only the customer loads cause big voltage drops but also the inductive reactive power of the lines adds to the total power flow and causes further voltage drops.
- The voltage and reactive power control has some limitations. A big part of reactive power is generated in power plant unites. The generators can deliver smoothly adjustable leading and lagging reactive power without any fuel costs.
- However, the reactive power occupies the generation capacity and reduces the active power production. Furthermore, it is not worth to transmit reactive power for long distance (because of active power losses). Control provided “on the way” in transmission line, connation nodes, distribution station and other points requires installation of capacitors or\and reactors.
- They are often used with transformer tap changing system. The range of voltage control depends on their size. The control may consist e.g. in setting the transformer voltage higher and then reducing it by reactive currents flow.
- If the transformer voltage reaches the highest value and all capacitors are in operation, the voltage on customer side cannot be further increase. On the other hand when a reduction is required the limit is set by maximal reactive power of reactors and the lowest tap of transformer.
Assessment Practices to control Voltage & Reactive Power:
- Transmission and Distribution planners must determine in advance the required type and location of reactive correction.
1) Static vs. Dynamic Voltage Support
- The type of reactive compensation required is based on the time needed for voltage recovery.
- Static Compensation is ideal for second and minute responses. (Capacitors, reactors, tap changes).
- Dynamic Compensation is ideal for instantaneous responses. (condensers, generators)
- A proper balance of static and dynamic voltage support is needed to maintain voltage levels within an acceptable range.
2) Reactive Reserves during Varying Operating Conditions
- The system capacitors, reactors, and condensers should be operated to supply the normal reactive load. As the load increases or following a contingency, additional capacitors should be switched on or reactors removed to maintain acceptable system voltages.
- The reactive capability of the generators should be largely reserved for contingencies on the EHV system or to support voltages during extreme system operating conditions.
- Load shedding schemes must be implemented if a desired voltage is unattainable threw reactive power reserves
3) Voltage Coordination
- The reactive sources must be coordinated to ensure that adequate voltages are maintained everywhere on the interconnected system during all possible system conditions. Maintaining acceptable system voltages involves the coordination of sources and sinks which include:
- Plant voltage schedules
- Transformer tap settings
- Reactive device settings
- Load shedding schemes.
- The consequences of uncoordinated of above operations would include:
- Increased reactive power losses
- A reduction in reactive margin available for contingencies and extreme light load conditions
- Excessive switching of shunt capacitors or reactors
- Increased probability of voltage collapse conditions.
- Plant Voltage Schedule :Each power plant is requested to maintain a particular voltage on the system bus to which the plant is connected. The assigned schedule will permit the generating unit to typically operate:
- In the middle of its reactive capability range during normal conditions
- At the high end of its reactive capability range during contingencies
- “Under excited” or absorb reactive power under extreme light load conditions.
- Transformer Tap Settings :Transformer taps must be coordinated with each other and with nearby generating station voltage schedules.
- The transformer taps should be selected so that secondary voltages remain below equipment limits during light load conditions.
- Reactive Device Settings :Capacitors on the low voltage networks should be set to switch “on” to maintain voltages during peak and contingency conditions. And “Off” when no longer required supporting voltage levels.
- Load Shedding Schemes: Load shedding schemes must be implemented as a “last resort” to maintain acceptable voltages.
4) Voltage and Reactive Power Control
- Requires the coordination work of all Transmission and Distribution disciplines.
- Transmission needs to:
- Forecast the reactive demand and required reserve margin
- Plan, engineer, and install the required type and location of reactive correction
- Maintain reactive devices for proper compensation
- Maintain meters to ensure accurate data
- Recommend the proper load shedding scheme if necessary.
- Distribution needs to:
- Fully compensate distribution loads before Transmission reactive compensation is considered
- Maintain reactive devices for proper compensation
- Maintain meters to ensure accurate data
- Install and test automatic under voltage load shedding schemes
- Samir Aganoviş,
- Zoran Gajiş,
- Grzegorz Blajszczak- Warsaw, Poland,
- Gianfranco Chicco
- Robert P. O’Connell-Williams Power Company
- Harry L. Terhune-American Transmission Company,
- Abraham Lomi, Fernando Alvarado, Blagoy Borissov, Laurence D. Kirsch
- Robert Thomas,
- OAK RIDGE NATIONAL LABORATORY