Switching on a powerful transformer is a critical moment for the supply grid and all connected consumers. Fortunately, there are two techniques for effectively dampening the inrush current.
Unless zadditional measures are in place, switching on transformers without timing control always leads to problems, which can even be felt on a physical level—the air vibrates, elementary particles align, and a loud buzz can be heard. For the supply grid, the connected equipment, and the transformer itself, switch ing on is an ordeal. Controllers indicate faults, lights flicker, protective relays trip in error, and entire processes are affected, putting them at risk of breaking down. The term for this is “inrush” or the “rush effect”.
TEN TO FIFTEEN TIMES THE NOMINAL CURRENT
The surge of inrush current is caused by the iron core becoming electromagnetically saturated when the transformer is switched on. The inductive resistance goes towards zero. This means that only the effective resistance of the transformer and the line impedance are available to limit the inrush current. The reason for this is the nonlinear magnetizing impedance of the transformer. The level of inrush current depends on the switching time, the line impedance (short-circuit capacity), the transformer remanence (residual magnetism), and the connected load.
Inrush currents of 10 to 15 times the nominal current are not uncommon. Loads connected to the transformer can reduce inrush current, but it depends on the type of loads. Some loads can even considerably increase the inrush current; for example, in situations that use a connected frequency converter with an intermediate voltage circuit and a line-side diode converter. In this case, when the transformer is switched on without a precharging unit for the intermediate circuit, the intermediate circuit capacitors are also charged. This effectively results in a higher inrush current.
LIMIT VALUES FOR ACCEPTABLE VOLTAGE DROPS
In line with the applicable standards, there are a number of different requirements and recommendations for acceptable voltage drops in the grid depending on the connection grid level and the connection points. In accordance with the technical regulations on assessing grid feedback effects (in Germany, Austria, Switzerland, and the Czech Republic), the voltage drop must not exceed 2 percent or 3 percent, depending on the repetition rate for medium-voltage grids. The technical connection conditions, which are soon to be established as technical connection regulations, specify relative voltage changes with a maximum of 2 percent to 5 percent depending on the repetition rate for medium-voltage grids and 0.5 percent to 2 percent for high-voltage grids.
NEW REGULATIONS EXACERBATE THE PROBLEM
The consequences of the inrush effect can be severe, as it causes a corresponding temporary voltage drop in the grid. This can lead to faults for parallel-connected consumers. In addition, the inrush current is asymmetrical and contains harmonics that degrade the voltage quality further. The time taken for the inrush current to dissipate increases in proportion to the size of the transformer and ranges from 100 milliseconds to several minutes. With the new Ecodesign Directive in place, the need to find effective ways of combating the inrush effect has become even more pressing. This is because low-loss transformers of the kind specified in the Directive have lower effective resistance resulting in correspondingly higher inrush currents. This makes the transformers particularly susceptible to the problematic rush effect.
TWO FEASIBLE SOLUTIONS
How can we tackle the problem and reduce inrush current? Theoretically, the obvious approach would be to switch on the phases in a controlled manner, but this fails in practice, normally because the associated expense is too great or because the technical conditions of the specific installation would render it impossible. There are, however, two other solutions which are more practical and effective. First solution: Short-term series resistance. This involves increasing the effective resistance when starting the transformer; for example, by using an inrush resistor that significantly limits the inrush current. Second solution: Premagnetization on the low-voltage side.
The premagnetization unit is connected to the secondary or tertiary winding of the transformer and magnetizes the transformer before start-up. This reduces the inrush current considerably so that it is around the range of the magnetization current (approximately 0.1–0.5 percent of the nominal current). This also practically eliminates the occurrence of voltage drops. However, premagnetization can only be used for transformers that are free from load.
SOLUTION IN THE PLANNING PHASE
The best approach is to develop appropriate solutions for countering the inrush effect in the initial planning phase. The grid engineers from MR Power Quality (PQ) conduct grid studies that show the repercussions on a grid when a transformer is switched on—including existing grids. The experts at MR then use these studies to design systems such as inrush resistors or premagnetization units and carry out simulations. Once this is complete, the results are shared with the customer.
The Power Quality division
The Power Quality (PQ) division at MR has been providing filter and compensation systems for clean low-voltage and medium-voltage grids around the world for 20 years. The division’s work focuses on reducing harmonics and compensating for reactive power in public and industrial distribution grids.
When an order has been placed, PQ arranges for the in-house designs to be manufactured to MR specifications at QA-certified production sites, after which the components are installed at the designated site. The inrush resistors with integrated bypass circuit breakers and/or premagnetization units then contain the inrush current when the transformer is switched on. The PQ specialists have demonstrated the effectiveness of both systems by delivering them to various grids around the world, providing solutions for transformers with outputs ranging from 1 to 100 MVA and nominal voltages between 6 kV and 230 kV.
YOUR CONTACT 
Would you like to prepare for inrush effects? Thomas Brückner of the Power Quality division is here to help:
t.brueckner@reinhausen.com