SHUNT REACTORS- JAVED 1
SHUNT REACTORS & SHUNT REACTOR PROTECTION
SHUNT REACTORS- JAVED 2 INTRODUCTION
Main Power System parameters are System Voltages and Frequency which normally indicate the healthiness level (they indicate the level of Generated active & reactive Powers against the load power requirement). In a healthy system, Voltage and frequency are close to the rated system values. Increased active & reactive power load tends to decrease the system frequency and voltage levels
respectively. It then becomes essential to generate additional active & reactive power. Synchronous Generators respond to the demand by means of Governor/AVR control systems. In order to preserve MVA capability of Generators (stator thermal limit) it is preferred to have reactive load support from other sources of reactive power like fixed Capacitor banks or other FACTS . Shunt Reactors are Inductive device commonly used in HV & EHV Systems for compensating the excess capacitive VArs in a power system. Due to their inductive nature of the Shunt Reactor, it is used whenever there is need for compensation of capacitive reactance. Power System loads are predominantly inductive in nature and Capacitor banks are used to compensate for the inductive loads. During system light load condition, often voltages increase beyond the normal operating levels and such a condition demands additional inductive loads to maintain system voltage levels within the normal range.
Generators have limited capacity with regard to under & over excitation operation. Typical Generators have rated power factor (pf) of between 0.8 to 0.85 (lead & lag) at rated MVA. At different MVA loading, Synchronous Generators can be operated within its limit of stability, stator/rotor thermal capacity as over & under-excited. These limiting values of under & over excitation are given as Generator capability curves by the Generator Manufacturer.
As mentioned above, during light load condition there is risk of system instability due to generated VAr larger than system can absorb. When system VAr generation is higher than the required VAr load, Generators tend to go to under-excitation. Under excitation limit of AVR is used to prevent level of under- excitation below stability limit.
Voltage rise due to increased total system generated VAr is dependent on the source impedance of the system. Source impedances are inductive in nature and weaker system has larger inductive reactance. It is known that inductive reactance and capacitive reactance are opposite in sign. Thus result in Voltage rise in percentage as per equation:
%ΔV = MVar (Capacitive) /MVA (system short-circuit) x100 In Figure-1, three different Shunt Compensations are shown namely:
By Line connected Shunt Reactors (Pos. 2) Bus connected Shunt Reactors (Pos. 1)
Shunt Reactor connected on Transformer Tertiary side bus (Pos. 3)
SHUNT REACTORS- JAVED 3 CAPACITIVE VArs OF EHV LINES
Capacitive VAr produced by Cables & Overhead circuits is proportional to the Voltage square and length of the circuit. Thus, practical HV & EHV cables (which are normally few tens of kilometers) and EHV lines (which are normally few hundreds of miles) result in large VArs generated. Typical values of reactive power produced by a 10km long 132kV, 220kV and 500kV cable may be of the order of 10MVAr, 30MVAr & 100MVAr respectively or more. Typical values of reactive power produced by a 100 mile long 220kV and 500kV OH line may be of the order of 25MVAr & 160MVAr respectively. Long EHV line due to large charging currents (due to high
capacitance) causes rise in receiving end line voltage due to Ferranti effect and are provided by Shunt Reactors (normally at both ends).
For a long EHV Transmission lines, it is clear that the capacitive VAr generation are quite high. To prevent Line end Over Voltage, it is common practice to compensate this line produced Capacitive VArs by means of Inductive VArs. Shunt Reactors are connected to these long EHV Line (normally at both ends since direction of energization in EHV system is normally possible in both ways) as normal practice for providing Inductive VArs. Total degree of Inductive Compensation (called k) by means of Shunt Reactors are typically between 0.6 to 0.8 (Resonance effect prohibits higher degree of compensation). Shunt Reactor inductance is selected based on the required degree of compensation k.
; (for parameters refer to Figure-2 below)
Figure-2 three reactor configuration:
Often Single Pole Auto-Reclosing (SPAR) is practiced for EHV lines due to stability requirement.
Table below indicates acceptable Length of EHV /UHV lines for SPAR implementation without use of Line connected Shunt Reactors. Table:
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Four Shunt Reactors configuration is applied for EHV Over head lines with SPAR if the arc extinction problem is expected due to coupling from healthy phases with faulted phase-reactor ground loop. Four reactor configuration has three phase Shunt reactors in Star form with a Neutral reactor.
If the degree of shunt compensation k is higher, the fourth (neutral reactor) required is small and vice-versa.
Figure-3, four reactor configuration:
; Where;
SHUNT REACTORS- JAVED 5 REACTOR APPLICATIONS:
In addition Shunt Reactors used for Voltage control & VAr compensation, Reactors are also used for various applications in a Power System.
Capacitor banks (without synchronized switching schemes) exhibit high frequency energizing inrush, reverse discharge currents (worst cases can occur when energizing a bank with other banks already connected in parallel). Series air-cored reactors often are used to limit these currents. These are called Transient limiting Series Reactors.
Series Reactors also appear as part of harmonic filters in series with capacitors and tuned for particular harmonic. These are typically used with SVCs & other electronic switching schemes.
Series Reactors are also used to limit the fault current at a bus if the rated fault level is lower than actual fault levels at a particular location.
Earth fault current limiting Neutral Reactors are used in some MV power systems. Other applications include HVDC systems, MV motor starting etc
SHUNT REACTOR TYPES:
Shunt Reactors are classified by means of various differences:
By means of core type: Air-cored & Gapped Iron-cored type. Shunt Reactors are similar to Power Transformers. However, unlike a Power Transformer, a Reactor is not meant power transfer from one voltage system to other and hence differs considerably by design. Thus a Reactor can be complete air cored or iron-core with air gaps (which is not the case in case of a typical power transformer which are always iron-cored without air-gaps).
By means of cooling media: Reactors can be dry type or oil-immersed type. Sometimes have additional external forced air cooling.
By means of connection: Reactors can be star grounded, star ungrounded or delta connected type.
By means of control of Inductance: Shunt Reactors can be fixed type or power electronic switched type (typically found in TCRs, SVC etc)
DRY TYPE & OIL IMMERSED REACTORS:
Dry type Shunt Reactors are normally applied at MV systems but are available even in EHV ranges.
Dry type Air-cored Shunt Reactors are also applied in Static Var Compensators (SVCs) as well as in Thyristor controlled Reactors (TCRs). In a TCR, these are applied on tertiary of EHV/HV Power Transformer. Often tertiary windings of two Transformers are used with TCRs. One Transformer tertiary winding is ungrounded star-connected and second Transformer tertiary winding is delta-connected. TCRs are then used as multi-pulsed switching (12-pulse) to achieve harmonic elimination.
Three phase Dry Air-cored Shunt Reactor banks are normally connected in ungrounded Y or in delta.
These are often natural air cooled and open to atmosphere (used in both Outdoor & Indoor applications) and physically arranged to minimize stray magnetic field effect.
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500kV Air core Reactor:
TYPICAL MV AIR-CORE REACTOR:
Gapped Iron-cored Shunt Reactors are applied at HV & EHV systems. These are normally Star-connected with Neutral solidly grounded. These types of reactors are normally applied as Bus-connected Shunt Reactors. They are designed as bank of three single-phase units
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or as three-phase units similar to a Power Transformers. For long EHV Transmission lines Shunt Reactors are used to limit the line-end Over Voltages due to Ferranti effect. Often Line-connected Shunt Reactors have air-cored reactor in the neutral to facilitate successful single-pole reclosing and arc extinction.
Iron-cored reactors are subject to magnetizing inrush. Iron-cored reactors are similar to an iron-cored power transformer (except that iron-cored reactors have small air-gaps giving them required linearity & reduced remanence.
Both Dry & Oil-immersed Reactors are designed with gapped Iron-core and air-core type.
SHUNT REACTOR & POWER TRANSFORMERS:
Reactors and Transformer both appear similar in construction. Reactors are also often equipped with Fans for cooling similar to Power Transformers. However, there are major differences between the two. While a Power Transformer is designed for efficient power transfer from one voltage system to another, a reactor is intended only to consume reactive VArs (or in other words it can be stated as to produce lagging VArs). Thus, there are more than one winding on a Power Transformer with magnetic core which carry the mutual flux between the two. In reactor there is just one winding. The core is not therefore meant only to provide a low reluctance path for flux of that winding to increase the Inductance.
In case of a Power Transformer, primary Ampere-Turns (AT) is sum of exciting AT and secondary AT. AT loss (in winding resistance, eddy loss & hysteric loss) is kept to as minimum as possible. Exciting AT is small compared with the secondary AT. Rated current is based on the load transfer requirement. Magnetizing current is small and is negligible value when compared with the secondary rated current. Further, since mutual flux is main flux which results in transformation, leakage flux is kept small and will be based on fault current limitation.
In case of a Shunt Reactor due to absence of other windings, all primary AT is equal to the exciting AT. Similar to a Power Transformer, loss in AT (in winding resistance, eddy current & hysteresis) are also kept to minimum by design. Magnetizing AT is major component of a Shunt Reactor. Reactor magnetizing current is its rated current.
Since a Shunt Reactor magnetizing current is large, if it is designed with Iron alone as a Power Transformer, there will be large hysteresis loss. Air gaps in Iron core are provided in a Shunt Reactor to reduce this loss and to minimize the remanent flux in the core. Thus a Shunt Reactor may also be constructed without iron (air-core).
By construction, a Shunt Reactor can be oil immersed or dry type for both with and without iron core.
Dry type Reactors are constructed as single phase units and are thus arranged in a fashion to minimize stray magnetic field on surrounding (in the absence of metallic shielding). When such an arrangement is difficult, some form of magnetic shielding is required and designed with care to minimize eddy current loss and arcing at any joints within the metallic loops. One of the advantages of dry type reactor is absence of inrush current.
Oil immersed reactors can be core-less or with gapped iron core. These are either single phase or three phase design with or without fan cooling. These are installed within tanks which hold oil & act as metallic magnetic shields.
In some cases, a Shunt Reactor may have additional small capacity winding which can provide power for small station power loads. Since Shunt Reactor rating is normally based on MVAr rating, this added station load VA shall be accounted for in designing the Reactor for such applications.
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