# Inrush current limiting circuit for a 1.5 MW transformer

I am looking for help with an inrush current limiting circuit for a 1.5 MW transformer as of now the current can reach as high as 10 - 15 kA.

I would like to keep the current under 1800 A per phase and each phase is 480 VAC at 60 Hz.

The input will be 2.5 MVA I’ve been researching various methods but have yet to come up with a solution.

I do not have knowledge on harmonics so I’m trying to keep it simple, but yet effective,I would like to build the circuit myself and will be doing the testing myself as well.

• Switch on at peak voltage, that keeps the core out of saturation, and prevents inrush. If you switch on near zero crossing, that will result in highest inrush. Not sure if that's possible with three phase. – Neil_UK Jul 23 '19 at 15:34
• Also, the inrush can’t be more than the available fault current. What is source impedance of the supply? – relayman357 Sep 19 '19 at 1:38

# Sequential phase energization with resistor in neutral supply

From my own hands on experience with a 80 kVA 400 VAC 50 Hz transformer with a specified 1216 Amps of maximum inrush current, I can tell that switching on phases L1-L2-L3 sequentially with a resistor in the neutral line works better then expected. 2 relays, 3 timer relays and 1 resistor was all I needed beside the contactor already in place. The contactor is there to disconnect at night and to short-circuit the bypass resistor.

### Timing of sequential phase delay

Somewhere between 0.015 and 0.5 seconds is a good delay for L2 and L3 at 50 hertz.

By trial and resistive inrush current measurements at 100 ksamples per second, I can measure less frequent high peaks at longer delays, though high inrush current peaks do still occur at 0.55s.

L2+L3 delay [s]    L1 peak [A]    L2 peak [A]    #tests
---------------    -----------    -----------    ------
0.015...0.025      83             42             247
0.1                56             56             459
0.25               26             41             19
0.5                50             62             59
0.55               99             99             205
---------------    -----------    -----------    ------


### Timer protection

Because the MTR17-TAB-U240-208 timer relay is rated at 8 amps and magnetic circuit breakers in D-8A are expensive to source, a lower value D-6A breaker was used. Even with 0.55 seconds delay the L3 D-6A breaker tripped once where L1 and L2 inrush measurements were at 99 amps.

For improved uptime I'd rather accept 99 amp peaks over 45 amps, when the circuit can be protected with 32A breakers instead of 6A breakers. Therefore I chose to replace 100 ms delays of the MTR17-TAB-U240-208 with 15...25 milliseconds make delays of the IKA232-20/230V.

Note: 0.6 seconds was used as delay for a DILMP125(RAC240) contactor.

### Ohmic value grounding resistor

In our case the primary side no load phase average measured around 2.2 Amps. Optimal neutral line resistance was calculated according to part II to be 8.97 Ohms:

U ≈ 235 volts
Z_open = U/I_no-load_phase_average ≈ 106.33 ohms
R_open = P_loss / (3 · I_no-load² ) ≈ 12.727 ohms
X_no-load = SQRT(11307.1-162) ≈ 105.57 ohms

R_N_optimal ≈ 0.085 · X_no-load ≈ 8.97 ohms


### Include or exclude neutral to ground resistance?

No clue whether that resistor value should include or exclude the line to ground resistance of the earth rod. Although my measurements suggests the calculated resistance value includes the full circuit including star point earth rod resistance. With 8.5 Ohms inserted in the circuit the maximum inrush phase current was measured at 99 Amps and a D-6A breaker tripped (once during 60 test runs). Lowering the resistance to 4.5 Ohms reduced the peak values and didn't trip the D-6A breaker during 697 test runs. Lowering to 3.644 Ohms seems to slightly increase inrush L2-L3 current peak values.

Resistance [Ω]    L1 peak [A]    L2 peak [A]    #tests
--------------    -----------    -----------    ------
3.644             56             62             138
4.0               52             50             197
4.5               45             49             118
5.0               42             48             87
6.0               37             47             98
7.061             31             42             21
8.0               83             43             241
8.5               99             99             60
--------------    -----------    -----------    ------


My first thought is to exclude/ignore neutral to earth resistance. Though the calculated optimal value seems a too high for L3 due to the breaker tripping once. Lowering neutral resistance with 1.5 ohms to around 7 ohms seems optimal in this case. Choosing between fixed value resistors, better use 4 ohms over 8 ohms for lowest peak values (52 versus 83).

Note: I didn't measure neutral to earth circuit resistance, though suspect 5 Ohms of resistance in the earthing rod at the utility company transformer according to that company its public documents for contracts, which states "...stop adding earth rods as soon as the resistance is lower then 5 Ohms". At the time of the measurements soil is dry to a lack of rain and ground water level is rather low at 3.3 meters below surface.

### Resistor power rating

When switching 6 times per minute the 8 ohms 50 watt NHS50 resistor increased from 17º to 45ºC at most. With 4 ohms 50 watt the temperature hardly reaches 28ºC. Choosing a power rating of 10 watts seems safe for 4 ohms. For 8 ohms 25 watts leaves a lot of margin.

### Schematic with power saving

In this schematic L2 + L3 are operated with timer relays. These timer relays can be exchanged with breakers, like N + L1.

This energy saving design powers down 4 (timer) relays N-L1-L2-L3 after enabling the contactor. This is done using a make-before-break H3DT-A2 timer relay and DILM150-XHI11 auxiliary its NC terminals on the contactor. Thus after switching, the only power consumed is by the contactor (2.4W) and the H3DT-A2 timer relay (0.6W). Start is issued by making an external relay. In this case the relay of a solar DC to AC inverter.

### DIN rail with phase delay modules

Tricky with 3 phases because you have to sequence the application of voltage on each phase to correspond with the peak voltage of that phase. However, this IEEE document suggests a sequencing method. It's called: -

Elimination of Transformer Inrush Currents by Controlled Switching


And it discusses how this can be achieved using contactors and mentions some limitations: -

In theory, transformer inrush transients can be eliminated using controlled energization. In practice, however, a number of factors can prevent achieving the goal of complete elimination. These factors include:

• Deviations in circuit breaker mechanical closing time.

• Effects of circuit breaker prestrike.

• Errors in the measurement of residual flux.

• Transformer core or winding configurations that prevent an optimal solution.

I'd encourage you to read it. The theory why inrush current is minimized by applying voltage at the peak of the sine wave is well understood despite a lot of engineers believing intuitively that zero-crossing switching is the right way.