CLASSIFICATION OF COMPENSATING DEVICES
The devices used for this purpose are classified as:
CLASSIFICATION 1
A. STATIC COMPENSATOR, using slow switching devices eg. Mechanically switched capacitor(using CB), Switched Reactors
Static Compensation is ideal for second and minute responses. (capacitors, reactors, tapchanges).
B. DYNAMIC COMPENSATOR using fast acting switching devices having response less than 10 sec. eg. FACTS Controllers
Dynamic Compensation is ideal for instantaneous responses. (condensers, generators)
CLASSIFICATION 2
A. passive compensation
Shunt capacitors and reactors, and series capacitors provide passive compensation.
They are either permanently connected to the transmission and distribution system, or switched.
They contribute to voltage control by modifying the network characteristics.
B. ACTIVE COMPENSATION
Synchronous condensers and SVCs provide active compensation; the reactive power absorbed/supplied by them is automatically adjusted so as to maintain voltages of the buses to which they are connected.
CLASSIFICATION 3
A. Sources or sinks of reactive power, such as shunt capacitors, shunt reactors, synchronous condensers, and static Var compensators (SVCs).
B. Line reactance compensators, such as series capacitors.
C. Regulating transformers, such as tap-changing transformers and boosters.
CLASSIFICATION 4
A. TCR( Thyristor controlled Reactor) Based(variable impedance type FACTS controllers) include:
(i) Static Var Compensator (SVC), (shunt connected)
(ii) Thyrister Controlled Series Capacitor or compensator (TCSC), (series connected)
(iii) Thyristor Controlled Phase Shifting Transformer (TCPST) of Static PST (combined shunt and series)
B. VSC ( Voltage Source Converter)based FACTS controllers are:
(i) Static synchronous Compensator (STATCOM) (shunt connected)
(ii) Static Synchronous Series Compensator (SSSC) (series connected)
(iii) Interline Power Flow Controller (IPFC) (combined series-series)
(iv) Unified Power Flow Controller (UPFC) (combined shunt-series)
CLASSIFICATION 5
A. Series controllers such as Thyristor Controlled Series Capacitor (TCSC), Thyristor Controlled Phase Angle Regulators (TCPAR or TCPST), and Static Synchronous Series Compensator (SSSC)
B. Shunt controllers such as Staic Var Compensator (SVC), and Static Synchronous Compensator (STATCOM)
C. Combined series-shunt controllers such as Unified Power Flow Controller (UPFC)
D. Combined series-series controllers such as Interline Power Flow Controller (IPFC)
COMPENSATION DEVICES /TECHENIQUES
Due to system expansion without proper and adequate planning and financial provision for the works in time, a large number of distribution systems have run into problems such as poor voltage regulation, poor power factor, high losses and poor efficiency, over loading and less reliability for continuity of supply. The causes for high losses and poor voltage regulation in the distribution and sub transmission system are:
1. Low power factor of the consumer installations.
2. Long and over loaded L.T lines.
3. Distribution transformers’ centers located away from the load centers.
4. Long and overloaded 11kV and sub transmission lines.
5. Poor voltage regulation on 11 kV and L.T lines, voltage drops being extended beyond permissible.
6. Under loading of distribution transformers.
7. Absence of shunt compensation in the sub transmission and distribution system;
Therefore, necessary to improve the working of the power distribution systems to reduce the unfavorable conditions and thereby reduce losses, improve voltage regulation, etc. The system improvement has to be planned properly with the following objectives in mind.
1. To reduce losses in the distribution and subtransmission system.
2. To improve the voltage regulation so as to bring it within the prescribed limit.
3. To improve the power factor in the subtransmission and distribution system so as to get optimum utilization of /subtransmission/distribution capacities.
Application of Tap-Changing Transformers to Transmission Systems
• Transformers with tap-changing facilities constitute an important means of controlling voltage throughout the system at all voltage levels.
• The taps on transformers provide a convenient means of controlling reactive power flow between subsystems.
• Coordinated control of the tap changers of all the transformers interconnecting the subsystems is required if the general level of voltage is to be changed.
• During high system load conditions, the network voltages are kept at the highest practical level to minimize reactive power requirements and increase the effectiveness of shunt capacitors and line charging.
• During light load conditions, it is usually required to lower the network voltages to reduce line charging and avoid under excited operation of generators.
• Transformers with off-load tap-changing facilities can also help maintain satisfactory voltage profiles.
• While transformers with OLTC can be used to take care of daily, hourly, and minute-by-minute variations in system conditions, settings of off-load tap-changing transformers have to be carefully chosen depending on long term variations due to system expansion, load growth, or seasonal changes.
Synchronous Condensers
• A synchronous condenser is a synchronous machine running without a prime mover or a mechanical load. By controlling the field excitation, it can be made to either generate or absorb reactive power.
With a voltage regulator, it can automatically adjust the reactive power output to maintain constant terminal voltage.
• Synchronous compensators contribute to system short-circuit capacity. Their reactive power production is not affected by the system voltage.
• During power swings (electromechanical oscillations) there is an exchange of kinetic
PASSIVE REACTIVE POWER COMPENSATION
(Refer : Fundamental theory of reactive power compensation)
Fixed or mechanically switched capacitors
Shunt capacitors were first employed for power factor correction in the year 1914. The leading current drawn by the shunt capacitors compensates the lagging current drawn by the load. The selection of shunt capacitors depends on many factors, the most important of which is the amount of lagging reactive power taken by the load. In the case of widely fluctuating loads, the reactive power also varies over a wide range. Thus, a fixed capacitor bank may often lead to either over-compensation or under-compensation. Variable VAR compensation is achieved using switched capacitors. Depending on the total VAR requirement, capacitor banks are switched into or switched out of the system. The smoothness of control is solely dependent on the number of capacitors switching units used. The switching is usually accomplished using relays and circuit breakers. However, these methods based on mechanical switches and relays have the disadvantage of being sluggish and unreliable. Also they generate high inrush currents, and require frequent maintenance.
Series Capacitors (SC)
Series compensation with capacitors is the most common strategy. Series Capacitor are installed in series with a transmission line as shown in Fig.3, which means that all the equipment must be installed on a platform that is fully insulated for the system voltage (both the terminals are at the line voltage). On this platform, the main capacitor is located together with overvoltage protection circuits. The overvoltage protection is a key design factor as the capacitor bank has to withstand the throughput fault current, even at a severe nearby fault. The primary overvoltage protection typically involves non-linear metal-oxide varistors, a spark gap and a fast bypass switch. Secondary protection is achieved with ground mounted electronics acting on signals from optical current transducers in the high voltage circuit.
A series capacitor is not just a capacitor in series with the line. For proper functioning, series compensation requires control, protection and supervision facilities to enable it to perform as an integrated part of a power system. Also, since the series capacitor is working at the same voltage level as the rest of the system, it needs to be fully insulated to ground. The main circuit diagram of a state of the art series capacitor is shown in Fig. 4. The main protective device is a varistor, usually of ZnO type, limiting the voltage across the capacitor to safe values in conjunction with system faults giving rise to large short circuit currents flowing through the line. A spark gap is utilized in many cases, to enable by-pass of the series capacitor in situations where the varistor is not sufficient to absorb the excess current during a fault sequence. There are various bypass solutions available today like spark gap, high power plasma switch, power electronic device, etc.
Finally, a circuit breaker is incorporated in the scheme to enable bypassing of the series capacitor for more extended periods of time as need may be. It is also needed for extinguishing the spark gap, or, in the absence of a spark gap, for by-passing the varistor in conjunction with faults close to the series capacitor (so-called internal faults).
Points…..
• Series capacitors are connected in series with the line conductors to compensate for the inductive reactance of the line.
• The reactive power produced by a series capacitor increases with increasing power transfer; a series capacitor is self-regulating in this regard.
• Series capacitors are self- regulating.
• The reactive power supplied by series capacitors is proportional to square of the line current and is independent of the bus voltages.
• This has a favourable effect on voltage stability.
Application to EHV Transmission System
• They have been primarily used to improve system stability and to obtain the desired load division among parallel lines.
• High compensation levels also increase the complexity of protective relaying and the probability of sub-synchronous resonance.
• A practical upper limit to the degree of series compensation is about 80%.
Following are some of the key considerations in the application of series capacitor banks:
• Voltage rise due to reactive current: Voltage rise of on one side of the capacitor may be excessive when the line reactive-current flow is high, as might occur during heavy power transfers.
• Bypassing and reinsertion: provision is made for bypassing the capacitor during faults and reinsertion after fault clearing. Speed of reinsertion may be an important factor in maintaining transient stability.
• Present trend is to use nonlinear resistors of zinc oxide which have the advantage that reinsertion is essentially instantaneous.
• Location: Factors influencing choice of location include cost, accessibility, fault level, protective relaying considerations, voltage profile and effectiveness in improving power transfer capability.
The following are the usual locations considered:
Location along the line
• Midpoint of the line
• Line terminals
• ⅓ or ¼ point of the line
Benefits
Ø Less SC Current
Ø Better Voltage Profile
Ø Simple Protection is needed
One or both ends of a line section on line sides in switching station.
Ø More protection is needed
Ø More KVar is need for same compensation compared to other locations
Location Between busbar in switching stations
Ø More protection is needed
POWER FLOW
Series compensation modify line impedance: X is decreased so as to increase the transmittable active power. However, more reactive power must be provided.(applicable for Facts also)
Befor Compensation
P1=lVsl l Vrl Sind/ XL
Befor Compensation
P1=lVsl l Vrl Sind/ XL-Xc
Degree of Compensation =P2/P1
SHUNT CAPACITORS
Shunt capacitors can be used on the power system to improve the voltage regulation of the system. The shunt capacitors, if connected to utilization equipment and switched on in accordance with the load, reduce the voltage drop in the power system and thus help in obtaining better voltage regulation. If the utilization equipment draws a current which is fairly constant, the voltage regulation by the shunt capacitoris more effective. Shunt capacitors installed on a system reduce energy losses in every part of the system between capacitors and generators. The use of shunt capacitors improve the voltage regulation of the system, (The size of the shunt capacitor banks varies from individual units of 5 to 25 kVA connected to the secondary or primary circuits of a distribution system to a bank of capacitors of large-size kVA connected to the bus of substation at the primary voltage side.)
Shunt capacitors are relatively inexpensive to install and maintain. Installing shunt capacitors in the load area or at the point that they are needed will increase the voltage stability. However, shunt capacitors have the problem of poor voltage regulation and, beyond a certain level of compensation; a stable operating point is unattainable. Furthermore, the reactive power delivered by the shunt capacitor is proportional to the square of the terminal voltage; during low voltage conditions Var support drops, thus compounding the problem.The characteristic of the shunt capacitor is shown in Fig. 1.
Points…..
• Shunt capacitors supply reactive power and boost local voltages. They are used throughout the system and are applied in a wide
• The principal advantages of shunt capacitors are their low cost and their flexibility of installation and operation.
• The principal disadvantage of shunt capacitors is that their reactive power output is proportional to the square of the voltage.
The reactive power output is reduced at low voltages when it is likely to be needed most.
POWER FLOW
Reactive current is injected into the line to maintain voltage magnitude. Transmittable active power is increased but more reactive power is to be provided. (Applicable for Facts also)
Application to transmission system
•Shunt capacitors are used to compensate for the XI2 losses in transmission system and to ensure satisfactory voltage levels during heavy loading conditions.
•Switching of capacitor banks provides a convenient means of controlling transmission system voltages.
• The principal advantages of shunt capacitors are their low cost and their flexibility of installation and operation.
• The principal disadvantage of shunt capacitors is that their reactive power output is proportional to the square of the voltage. The reactive power output is reduced at low voltages when it is likely to be needed most.
The following are the usual locations considered:
• Midpoint of the line
• Line terminals
• ⅓ or ¼ point of the line
(a) Tertiary connected capacitor (b) HV capacitor bank
REACTORS
Reactors, like capacitors, are basic to and an integral part of both distribution and transmission power systems. Depending on their function, reactors are connected in shunt or in series with the network; singularly (current limiting reactors, shunt reactors) or in conjunction with other basic components such as power capacitors (shunt capacitor switching reactors, capacitor discharge reactors filter reactors). Reactors are utilized to provide inductive reactance in power circuits for a wide variety of purposes. These include fault current limiting, inrush current limiting for capacitors and motors, harmonic filtering, VAR compensation, reduction of ripple currents. Reactors may be installed at any industrial, distribution, or transmission voltage level. Shunt reactor compensation is typically required under conditions that are the opposite of this requiring shunt capacitor compensation.
POINTS…….
• Shunt reactors are used to compensate for the effects of line capacitance, particularly to limit voltage rise on open circuit or light load. They are usually required for EHV overhead lines longer than 200Km. A shorter overhead line may also require shunt reactors if the line is supplied from a weak system (low short-circuit capacity).
• A shunt reactor of sufficient size must be permanently connected to the line to limit fundamental-frequency temporary over voltages to about 1.5 pu for a duration of less than 1 second.
• Additional shunt reactors required to maintain normal voltage under light-load conditions may be connected to the EHV bus. During heavy loading conditions some of the reactors may have to be disconnected.
