The amount of fault current which flows depends on the reactance in the circuit. Reactors are therefore used in addition to limit the short circuit current and protect the system against the fault current as shown in fig. below,
Reactors are connected in series with the line or feeder. Typical uses are fault-current reduction, load balancing in parallel circuits, limiting inrush currents of capacitor banks, etc.
Current Limiting Reactors reduce the short-circuit current to levels within the rating of the equipment on the load side of the reactor. Applications range from the simple distribution feeder reactor to large bus-tie and load-balancing reactors on systems rated up to 765 kV / 2100 kV BIL.
Main Functions of Reactor: –
- They limit the fault current to a safe value.
- These are also used to protect the CB’s.
- They restrict the fault by limiting the current that flows into the faulty part through the other healthy feeders which ensures that the fault doesn’t spread
- Insertion of reactors doesn’t not affect much on the power loss as their reactance is comparativelynegligible.
The CLR design is based on two parameters. The first is the actual inductance of the CLR,expressed in millihenries. The second is the current rating of the reactor. The impedance ofthe reactor can be calculated by:
Where Z = impedance in ohms
L = inductance in henries
f = frequency in hertz
The amount of protection that a CLR gives a specific T/R set is dependent upon the percentimpedance that the reactor provides for the T/R set. The impedance of the T/R set is definedby:
The percent impedance that the CLR provides is calculated by:
Working backwards, the inductance can be calculated for a desired percent impedance of a T/Rby the following formula:
When an arc occurs the impedance of the T/R set becomes very small (The short is reflected tothe primary of the T/R). The primary voltage is across the CLR only. The amount of faultcurrent that will flow through the system is dependent upon the percent impedance that the CLRprovides.
The CLR is designed with a core that will handle several times the current rating withoutsaturating. The voltage drop across the CLR will be linear with respect to the current through it.If a CLR were to saturate, excess current would pass through it without any additional voltagedrop. At this point it would cease to be linear.
Types of Reactors: –
- Iron cored type
- Air cored type
In case of iron cored type, the reactance of the reactor will not limit the current, if at all the iron core becomes saturated due to the faulty conditions. It must always operate on the straight portion of the B -H curve. For achieving this there are several ways
- Iron core can be used with air gap
- Reactors can be built with nonmagnetic cores that are air cored.
It should also be kept in mind that the iron core introduces eddy current lossand also hysteresis lossin reactor. Thus, an iron core reactor introduces more power loss as compared to the air cored reactor.
- Reactors are not used inmodern alternators as they have already sufficient reactance to limit the short circuit current to a safe value.
- However, reactors are used in old alternators.
- Reactors are also used in feeders.
- Mostly, they aren’t used in individual feeder circuit as they become expensive.
- However, these are used in group feeders, CB or busbar.
Reactors also have some disadvantages: –
- By increasing the reactors, the total reactance of the circuit increases which increases the reactive drop voltage causing the power factor to lag. Hence poor regulation.
- The reactor must be chosen such that it doesn’t interfere the normal transfer of load.
- The CLR can also be used to increase the conduction angle of the SCRs under certain operating conditions.
- If the conduction angle is low due to sparking within the precipitator, increasing the inductance of the CLR will increase the conduction angle and the average voltage and current levels.
- Since, sparking will occur at a fixed peak voltage that is defined by the precipitator, lowering the peak available output voltage of the T/R will allow the spark over to occur at a later portion of the half cycle, thereby increasing conduction.
- Increasing the inductance of the CLR will increase the voltage drop across it, thereby decreasing the voltage available to the T/R set.
- It should be noted that the voltage drop across the CLR and the primary voltage of the T/R set will not directly add up to the system line voltage.
- Because the CLR is reactive there is a phase shift between the voltages.
The relationship is as follows:
(Line Voltage)2 = (CLR Voltage)2 + (Primary Voltage)2
METHOD FOR TESTING THE INDUCTANCE OF A CLR
- Apply power to the system to obtain the maximum attainable primary current without spark-over. Use a true RMS meter for measuring the current.
- Measure the voltage-across the CLR, at the above current, using a true RMS meter.
- Calculate inductance – by using the following formula.
Inductance = (Voltage/Current) x 377
- ELECTRICAL POWER SYSTEMS BY SUNIL S RAO, UPPAL