Selection of Reactor for Different Applications
Air Core or Coreless Reactor without Magnetic ShieldThis type of reactor is constructed by wrapping insulated copper or aluminum conductor of suitable cross section on a ceramic, concrete, fiber glass or glass polyester made core. Sometime it may also be constructed without any core just like air cored solenoid. Such core is nonmetallic in nature. As there is no iron core in the air core or coreless reactor, there will be no magnetic saturation occurs in the core. That is why this reactor can be operated in linear region of magnetic characteristic throughout its normal range of operation. To limit the inrush current through a capacitor bank during switching on, this reactor is techno commercially suitable for the purpose.
Air Core or Coreless Reactor with Magnetic ShieldThere is significant amount of leakage flux in the air core or core less reactor, because of absence of any magnetic core inside the winding. This large leakage flux creates stray magnetic field surround the reactor. When there is any fault or any switching operation, the current through the reactor becomes quite a high, which significantly increases the stray magnetic field surround the reactor. This increased stray magnetic field may induce emf in the iron and other conducting parts nearby the reactor winding. Because of the induced emf there is local circulating current in those parts and thereby generation of unwanted heat in them. This causes overall power loss of the system. The inductance of the reactor cannot remain constant at higher current situation because for high reactor current, magnetic fields created due to local circulating current at different conducting parts of the reactor body will oppose the main field. Not only that, the stray magnetic field may affect the performance of the relays and metering devices if installed nearby the reactor. To overcome these difficulties, the coreless reactor is covered by magnetic frame. This frame is made of tightly clamped together laminated steel sheets. This frame is referred as Faraday’s cage.
The magnetic shield confines the space magnetic field inside it. But due to magnetic shield the reactor suffers from magnetic saturation. The reactor is generally designed not to be saturated for the current less than 150% of its rated value. The magnetically shield coreless reactor are used for same purposes of coreless reactor but for higher current rating. It is not suitable to use for system with high harmonic currents and linearity of operation required at current above 150% of rated current.
Air Core or Coreless Reactor with Non - magnetic ShieldSometime an air core or coreless reactor is shield with non-magnetic electrical conducting material such as copper or aluminum. In that case the inductance of the reactor may be decreased when current through it increases. This is because, the alternating magnetic field of the reactor links with the conducting shield, thereby induced emf in the shielding body. The current in the shielding body produced due to this emf will create counter field which opposes the main field. As in coreless non-magnetically shield reactor, there is no saturation, the V – I characteristics of the reactor remains almost linear. This type of reactor can be effectively used as current limiting device and also as harmonic suppresser. This can also be suitably used in filter circuit due to its linear characteristic.
Gapped Core Shunt ReactorShunt reactors are generally used in ultra or extra high voltage electrical transmission system to compensate capacitive reactive power generated in the system due to lightly loaded or no loaded long transmission lines connected to that system. That means the reactors neutralize the capacitive effect of the lightly loaded or no loaded long transmission lines. Thus these shunt reactors stabilize the system voltage. The shunt reactors are generally connected between electrical bus and ground or sometimes between transmission line and ground. The reactors are not continuously connected to the system, because it contributes reactive power to the system even when it is not required. The reactors are switched on or off on daily basis depending upon the system condition.
The core of the shunt reactor is specially designed with integrated air gap, so that it cannot be saturated significantly and thereby will have a reasonably linear I - Φ characteristic. In UHV three phase system multiunit reactor is often used, where three separate single phase units are used as one for each phase. But in three phase EHV system, normally single unit three phase reactor is used. The core of the single unit three phase reactor is either of core type or shell type. In core type design there are three legs and in the shell type design there are five legs in the core. In transmission system, mainly shell type design is used.