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Simulation of Transformer Inrush Currents and Their Impact on the Grid

de la Peña Toledo, Juan LU (2022) In CODEN:LUTEDX/TEIE EIEM01 20221
Industrial Electrical Engineering and Automation
Abstract
This MSc Thesis’s main objective is to determine the impact of transformer inrush
currents in the grid and develop a tool that helps E.ON Energidistribution AB evaluate
transformer sizing. The motivation of E.ON is to have a more precise way to estimate the
energization consequences of increasing the transformer size in the grid, since currently
is being used a rule of thumb that relates the transformer rated power to the short-circuit capacity of the bus at which the transformer is connected. This rule of thumb
is established because they have the concern that large power transformers can cause
serious inrush currents.

An adequate study of the causes and consequences of the inrush current phenomenon is
conducted to determine the... (More)
This MSc Thesis’s main objective is to determine the impact of transformer inrush
currents in the grid and develop a tool that helps E.ON Energidistribution AB evaluate
transformer sizing. The motivation of E.ON is to have a more precise way to estimate the
energization consequences of increasing the transformer size in the grid, since currently
is being used a rule of thumb that relates the transformer rated power to the short-circuit capacity of the bus at which the transformer is connected. This rule of thumb
is established because they have the concern that large power transformers can cause
serious inrush currents.

An adequate study of the causes and consequences of the inrush current phenomenon is
conducted to determine the duration and magnitude of undesired disturbances. Python
is used to design simplified transformer models based on banks of three single-phase
units due to substantial limitations of more advanced alternatives. PSCAD and Matlab
Simscape Electrical, two superior transient analysis software, are used to verify the
results of test simulations. The non-linearity of the differential system of equations
required numerical methods and the utilization of the Trapezoidal Rule of Integration
and the Newton Raphson Method.

Then, different strategies are considered as potential alternatives to estimate the voltage dip at the surrounding grid, which is the primary concern for E.ON. For the grid
analysis part, PSS/E Xplorer 35.3 (which is a free limited version) is utilized because
PSS/E is the preferred tool at E.ON for steady-state and short-circuit calculations. All
the strategies are compared and evaluated for a hypothetical energization of a distribution transformer in a reduced network correspondent to the Oland island. Some results
coincide with historical data from a similar distribution network, but on-site measurements or dynamic grid studies must validate the results. The in-built Python API of
PSS/E allows automating the process and incorporating it with the rest of the program.


As a result of this MSc Thesis, E.ON engineers will be able to use a friendly interactive tool that simulates with high accuracy the energization of a simplified three-phase
transformer model and estimates the possible effect on the surrounding grid. However,
they must be aware of the limitations and assumptions to make suitable sizing decisions. (Less)
Popular Abstract
When a transformer (an element with the ability to adapt voltage to the individual requirements of the different parts of the system) is switched in, a current inrush is drawn from the grid. The magnitude and duration of the inrush current depend on several factors, such as the size of the transformer and the network strength to which the transformer is being connected. To limit the disturbances that the energization of transformers may cause, their size is often constrained using a rule of thumb that relates the maximum transformer size to the network strength. This restriction comes from the fact that the larger the transformer is, the greater the inrush current and the deeper the voltage in the surrounding grid goes. This MSc Thesis... (More)
When a transformer (an element with the ability to adapt voltage to the individual requirements of the different parts of the system) is switched in, a current inrush is drawn from the grid. The magnitude and duration of the inrush current depend on several factors, such as the size of the transformer and the network strength to which the transformer is being connected. To limit the disturbances that the energization of transformers may cause, their size is often constrained using a rule of thumb that relates the maximum transformer size to the network strength. This restriction comes from the fact that the larger the transformer is, the greater the inrush current and the deeper the voltage in the surrounding grid goes. This MSc Thesis aims to optimize the transformer sizing decision by precisely simulating the voltage dip at the energizing bus and the surrounding area. For this purpose, an adequate study of the causes and consequences of the inrush current phenomenon is conducted to design a Python model that can accurately represent these kinds of transient episodes. Afterwards, a steady-state software for power system analysis called PSS/E is used to estimate the impact on the surrounding area with limited fidelity. In conclusion, considering the limitations and assumptions made, it is possible to have some intuition on the expected impact of energizing a transformer in the grid in terms of the voltage dip in a more precise way than using the actual rule of thumb. However, some effort into verification is necessary. (Less)
Popular Abstract
Inrush currents during transformer energization are a well-known issue extensively described in the literature. Hence, modelling the equations that define its dynamics is a matter of understanding the different phenomena involved and making suitable assumptions. There are two types of transformers, but when it comes to the generation, transmission, distribution and industrial use of electrical power, three-phase transformers are dominant with respect to single-phase transformers. Still, because single-phase transformers are easier to comprehend, they are analysed first, and afterwards, the model is expanded to represent some classes of three-phase transformers. The result has excellent accuracy in comparison with other tools with superior... (More)
Inrush currents during transformer energization are a well-known issue extensively described in the literature. Hence, modelling the equations that define its dynamics is a matter of understanding the different phenomena involved and making suitable assumptions. There are two types of transformers, but when it comes to the generation, transmission, distribution and industrial use of electrical power, three-phase transformers are dominant with respect to single-phase transformers. Still, because single-phase transformers are easier to comprehend, they are analysed first, and afterwards, the model is expanded to represent some classes of three-phase transformers. The result has excellent accuracy in comparison with other tools with superior transient analysis simulation such as Matlab Simscape or PSCAD.

The next step is to find out the effect on the connected network, which relies on the PSS/E software because it is the main tool utilized at E.ON. The fact that is going to be used a steady-state program to estimate a transient effect is one of the challenges of this MSc Thesis and a limitation imposed by E.ON as they do not provide other software licenses.

After developing different strategies to couple the time-simulation results with steady-state PSS/E variables and integrating everything in a Python tool, an energization study is carried out in a reduced grid. This allows us to evaluate the different alternatives and consider the most trustful ones, but the lack of measurements makes this task doubtful and non-verified.

Finally, with the information that E.ON engineers can extract from the tool designed for energization studies, it is possible to have an intuition about the voltage dip at different buses in the grid and make a transformer sizing decision given some boundary conditions imposed by grid standards or special customer requirements. Figure 2 shows the minimum voltage simulated at different buses when varying the size of the transformer being energized. (Less)
Please use this url to cite or link to this publication:
author
de la Peña Toledo, Juan LU
supervisor
organization
course
EIEM01 20221
year
type
H2 - Master's Degree (Two Years)
subject
keywords
Transformer, Energization, PSS/E, PSCAD, Simulation, Magnetizing current, Inrush current
publication/series
CODEN:LUTEDX/TEIE
report number
5477
language
English
id
9095388
date added to LUP
2022-07-07 11:29:05
date last changed
2022-07-07 11:29:05
@misc{9095388,
  abstract     = {{This MSc Thesis’s main objective is to determine the impact of transformer inrush
currents in the grid and develop a tool that helps E.ON Energidistribution AB evaluate
transformer sizing. The motivation of E.ON is to have a more precise way to estimate the
energization consequences of increasing the transformer size in the grid, since currently
is being used a rule of thumb that relates the transformer rated power to the short-circuit capacity of the bus at which the transformer is connected. This rule of thumb
is established because they have the concern that large power transformers can cause
serious inrush currents.

An adequate study of the causes and consequences of the inrush current phenomenon is
conducted to determine the duration and magnitude of undesired disturbances. Python
is used to design simplified transformer models based on banks of three single-phase
units due to substantial limitations of more advanced alternatives. PSCAD and Matlab
Simscape Electrical, two superior transient analysis software, are used to verify the
results of test simulations. The non-linearity of the differential system of equations
required numerical methods and the utilization of the Trapezoidal Rule of Integration
and the Newton Raphson Method.

Then, different strategies are considered as potential alternatives to estimate the voltage dip at the surrounding grid, which is the primary concern for E.ON. For the grid
analysis part, PSS/E Xplorer 35.3 (which is a free limited version) is utilized because
PSS/E is the preferred tool at E.ON for steady-state and short-circuit calculations. All
the strategies are compared and evaluated for a hypothetical energization of a distribution transformer in a reduced network correspondent to the Oland island. Some results
coincide with historical data from a similar distribution network, but on-site measurements or dynamic grid studies must validate the results. The in-built Python API of
PSS/E allows automating the process and incorporating it with the rest of the program.


As a result of this MSc Thesis, E.ON engineers will be able to use a friendly interactive tool that simulates with high accuracy the energization of a simplified three-phase
transformer model and estimates the possible effect on the surrounding grid. However,
they must be aware of the limitations and assumptions to make suitable sizing decisions.}},
  author       = {{de la Peña Toledo, Juan}},
  language     = {{eng}},
  note         = {{Student Paper}},
  series       = {{CODEN:LUTEDX/TEIE}},
  title        = {{Simulation of Transformer Inrush Currents and Their Impact on the Grid}},
  year         = {{2022}},
}