Transformer Pre-Charge for Medium-Voltage Grid Inrush Current

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Solution Overview

Problem

High power rectifiers in medium-voltage grid applications face issues with excessive over-voltage stress and in-rush current due to lightly damped input filters, leading to inefficiencies and increased costs from bulky pre-charge resistors and additional circuit components.

Innovation Solution

A transformer-based pre-charge system with a converter that adjusts modulation index to provide a constant and controlled pre-charge voltage, eliminating the need for resistors and ensuring zero in-rush current, allowing for a smooth voltage rise and optimized system efficiency.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If series-connected resistors are used to reduce over-voltage and in-rush current, then the over-voltage stress and in-rush current are reduced, but the system efficiency degrades and bulky pre-charge resistors are required

Engineering Contradiction:

Improveover-voltage stress and in-rush currentVSAvoidsystem efficiency

Core Design Contradiction:

Object-affected harmful factorsVSLoss of energy

Solution Approach 1:

A pre-charge transformer is introduced as an intermediary device between the medium-voltage grid supply and the input filter capacitor. The transformer provides galvanic isolation and enables controlled voltage transfer through magnetic coupling, eliminating the need for dissipative resistor-based pre-charge while maintaining energy efficiency

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The patent replaces the conventional resistor-based pre-charge mechanism with a transformer-based electromagnetic coupling system. This substitution transitions from a dissipative electrical resistance mechanism to a reactive magnetic coupling mechanism, achieving pre-charge functionality without energy loss through heat dissipation

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Object-affected harmful factors

If additional pre-charge circuit components such as pre-charge resistor and contactors are employed, then over-voltage and in-rush current are controlled, but system costs increase and overall system efficiency diminishes

Engineering Contradiction:

Improveover-voltage and in-rush current controlVSAvoidsystem costs and component count

Core Design Contradiction:

Object-affected harmful factorsVSDevice complexity

Solution Approach 1:

The pre-charge transformer serves multiple functions simultaneously: it provides galvanic isolation, enables controlled pre-charge of the input filter capacitor, and eliminates the need for separate pre-charge resistors and contactors. This multi-functionality reduces both component count and system complexity while maintaining effective pre-charge control

Inventive Principle:

Principle #6Universality (Multi-functionality)

Solution Approach 2:

The patent merges the pre-charge function with the main power transfer function by using the same transformer for both purposes. The transformer is designed to handle the full power rating while providing controlled voltage transfer during pre-charge, eliminating the need for separate pre-charge circuitry and reducing overall system complexity

Inventive Principle:

Principle #5Merging (Combining)

3Quantity of substance

If AC side pre-charge is used instead of DC side pre-charge, then the filter capacitor value can be reduced, but the in-rush current and over-voltage are not eliminated due to small impedance

Engineering Contradiction:

Improvefilter capacitor valueVSAvoidin-rush current and over-voltage

Core Design Contradiction:

Quantity of substanceVSObject-affected harmful factors

Solution Approach 1:

The transformer acts as an intermediary that provides galvanic isolation and controlled voltage transfer during pre-charge. This isolation allows the use of smaller filter capacitors without directly exposing the grid supply to in-rush current, as the transformer limits current transfer through its magnetic coupling characteristics

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The transformer enables preliminary pre-charge action by transferring voltage from the primary to secondary side before direct grid connection. This preliminary voltage establishment on the capacitor allows subsequent full-power operation without dangerous in-rush currents, even with reduced capacitor values

Inventive Principle:

Principle #10Preliminary action

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

This solution reduces system costs, improves efficiency by minimizing pre-charge power, and eliminates in-rush current and over-voltage stress across input filter capacitors, enabling a seamless connection to the medium-voltage grid supply without bulky components.

Implementation Method 1

a transformer configured to supply a pre-charge voltage to a capacitor

Methodology Applied

Scientific EffectElectromagnetic induction: Electromagnetic Induction

Data Source

PatentUS9748860B2Input filter pre-charge fed by a medium-voltage grid supply

Publication Date: 2017.08.29 GE ENERGY POWER CONVERSION TECHNOLOGY LTD(GB)

AI summary

A device includes a transformer configured to supply a pre-charge voltage to a capacitor and a converter configured for coupling to the transformer and responsive to an increasing modulation index. The modulation index increases for a time quanta after the capacitor becomes substantially fully charged and the pre-charge voltage is substantially constant during the time quanta.