Power Converter Pre-Charge Circuit for Grid Transformer Inrush
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Solution Overview
Problem
Electric circuits connecting DC power sources to a power grid face issues with magnetising inrush currents and over-voltages when switchgear is closed, leading to transformer degradation and power quality issues, and there is a need for ensuring continuous power supply to critical components when switchgear is open.
Innovation Solution
An electric circuit with a main power converter, a main transformer, and an auxiliary transformer, where the auxiliary transformer provides continuous power to electrical components when the main transformer is disconnected, and a pre-charge circuit allows the DC link to be pre-charged from the power grid, reducing inrush currents by synchronizing the transformer voltage with the grid voltage before reconnecting the main switchgear.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If the switchgear is closed to reconnect the transformer to the power grid, then the transformer can supply power to the grid, but significant magnetising inrush currents occur which degrade the transformer and affect power quality
Solution Approach 1:
The patent applies preliminary action by pre-charging the DC link before closing the switchgear. The pre-charge circuit is activated first to charge the DC link capacitors to the required voltage level, and only after this preliminary action is complete does the control system close the switchgear to connect the transformer to the grid. This sequencing prevents inrush currents by ensuring the transformer is already energized when connection occurs.
Solution Approach 2:
The patent uses an intermediary approach by introducing a pre-charge circuit as a mediator between the power grid and the main transformer. This intermediate circuit includes pre-charge contactors and resistors that control the charging process, acting as a buffer that prevents direct connection shocks. The pre-charge circuit mediates the energy transfer, allowing gradual charging without subjecting the transformer to harmful inrush currents.
2Loss of energy
If the switchgear is opened to disconnect the transformer from the power grid, then no-load transformer losses are avoided, but the sudden interruption of current leads to over-voltages within the transformer windings
Solution Approach 1:
The patent applies preliminary action by de-energizing the transformer before opening the switchgear. The control system first commands the DC/AC power converter to stop supplying power to the transformer, allowing the transformer to naturally de-energize and its voltage to decay. Only after this preliminary de-energization action is complete does the switchgear open to disconnect the transformer from the grid, preventing over-voltage spikes.
3Device complexity
If the DC link is not pre-charged before connecting to the power grid, then the circuit can be simplified, but the DC/AC power converter suffers significant damage
Solution Approach 1:
The patent applies preliminary action by making DC link pre-charging a mandatory preliminary step before the DC/AC power converter is allowed to operate. The control system monitors DC link voltage and only permits power converter activation after the pre-charge circuit has charged the DC link to the required voltage threshold. This preliminary check protects the power converter from damage while maintaining relatively simple circuit architecture.
Solution Approach 2:
The pre-charge circuit acts as an intermediary protective mechanism between the power grid and the DC/AC power converter. It provides a safe charging path through controlled resistors and contactors, mediating the energy transfer to gradually charge the DC link without exposing the sensitive power converter electronics to damaging current spikes or voltage transients.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively reduces magnetising inrush currents and ensures continuous power supply to critical components, preventing transformer degradation and maintaining power quality by pre-charging the DC link and synchronizing the transformer voltage with the grid voltage before reconnecting the main switchgear.
Implementation Method 1
an auxiliary transformer having a primary winding connectable to the power grid in parallel with the main switchgear and a secondary winding connected to the one or more electrical components to provide power to the one or more electrical components
Implementation Method 2
a main transformer having a primary winding and a secondary winding, the primary winding being connected to the second main terminals of the main power converter; main switchgear connected to the secondary winding of the main transformer and connectable to the power grid
Data Source
Figure 1
Figure 2
AI summary
The invention relates to a power system 1 with an electric circuit connected between a power grid 4 and a power source 16. The electric circuit includes a main power converter 10 having main input terminals connected to the power source 16 by a DC link 14 and output terminals. The main power converter 10 is controlled by a controller 18. The electric circuit includes a main transformer 8 having a primary winding 8a and a secondary winding 8b, the primary winding being connected to the output terminals of the main power converter 10. Main switchgear 6 is connected between the secondary winding 8b of the main transformer 8 and the power grid 4. An auxiliary transformer 24 has a primary winding 24a connected to the power grid 4 in parallel with the main switchgear 4 and a secondary winding 24b connected to the controller 18. A pre-charge circuit 28 is connected between the auxiliary transformer 24 and the DC link 14.