Different inrush current mitigation methods

Previously, Importance of protection against inrush current in stationary device is determined. In this post, different mitigation methods are addressed. 

• Pre-Magnetizing Transformer:

Function: To magnetize the transformer core at starting with regulated voltage source before connecting with main supply and prevent from saturation of core.

Fig. Medium voltage side pre-magnetizing transformer

Fig. Low voltage side pre-magnetizing transformer

                                                                                         

Application: Marine transformer

Limitation: No. of components are more, require external small size auxiliary transformer, cost is more.

•   Pre-Insertion Resistor/Rheostat with controlled switching:

Function: To mitigate the inrush current by providing external resistance at starting and switching control is used to bypass that rheostat after initial few cycles .

Application: MV transformer energization

Fig. Pre-insertion Resistor

Limitation: no-load loss increases, problems in frequent switching.

•   Pre-fluxing capacitor with controlled switching:

      Function: To induce certain defined amount of residual flux in each limb of core with the help of  capacitor discharge (which was initially charged by dc power source) before connecting main supply. It will also prevent the core from the saturation .  

                Application: MV transformer (both oil and dry type) energization.

                Limitation: circuit complexity, optimal determination of capacitance value.

• Controlled switching of circuit breaker:

        Function: To find out the optimal switching angle at which magnetic flux is equal to  residual flux .

• It will reduce the inrush current by the inserting neutral resistor during energizing period.
• It is done by delay energization of each phase in which optimal instance has been detected by the controller and then it will signal to the CB considering its constraints like mechanical operating time, pre-arcing etc.

Application: Low kVA rating transformer

Limitation: power loss due to external resistor, inrush current suppression dependent on resistor value.

• Thyristor based inrush current limiter:

 Function: To energize the transformer core by applying regulated voltage source by controlling the firing angle of anti-parallel thyristors from 180˚ to 0˚.

Fig. thyristor based inrush limiter

              Application:

•      Electric arc furnace transformer

•      Converter duty transformer

   Components:

•      Anti-parallel thyristors

•      Bypass switch

•      Phase lock loop (PLL)

•      Pulse generation circuit

•  Design stage modification:

There are certain modifications available at the design stage that if it is implemented then it will reduce the magnitude of the inrush current to a certain extent .

•      Selection of lower flux density core material and higher no. of turns in each winding to get constant MMF in the windings.

MMF= N * I

•      By increasing the source impedance of the system to decrease the inrush current value during starting.

Limitation: Cost is more, increment in voltage drop due to permanent increment in impedance.