Modular Power Converter Circuitry for Medium-Voltage Networks
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Solution Overview
Problem
Existing medium-voltage power converters require large and costly transformers due to the limited voltage endurance of switch-type power semiconductor components, leading to inefficiencies and high power losses, especially when dealing with different frequency and voltage levels between networks.
Innovation Solution
A converter circuitry using a power bus to connect low-voltage power cells via transformers, with a high-frequency transformer connection and a low-voltage power unit generating a rectangular AC voltage for pre-charging intermediate DC links, allowing for modular assembly and efficient power transmission between AC and DC networks with bidirectional or unidirectional capabilities.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a transformer is used between medium-voltage network and load network, then voltage transformation and galvanic isolation are achieved, but the transformer size and cost become high
Solution Approach 1:
The patent divides the power conversion system into multiple low-voltage power cells (PC11U...PCNW) connected in series, each handling a portion of the voltage transformation. This segmentation allows each cell to use a smaller transformer optimized for lower voltage, avoiding the need for one large high-voltage transformer, thus reducing overall transformer size and cost while maintaining the required voltage transformation capability.
Solution Approach 2:
The patent changes the operating parameters by using low-voltage power cells operating at higher frequency instead of a single medium-voltage transformer operating at 50/60 Hz. This parameter change allows the use of smaller transformers with optimized frequency characteristics, reducing transformer size and cost while achieving the same voltage transformation function.
2Weight of stationary object
If multiple low-voltage power cells are connected in series, then medium-voltage output is achieved with reduced transformer size, but the voltage endurance of power semiconductor components becomes a limiting factor
Solution Approach 1:
The patent segments the medium-voltage output into multiple low-voltage contributions from series-connected power cells. Each power cell handles a manageable voltage level that matches the voltage endurance capabilities of available power semiconductor components, while the series connection accumulates these to achieve the required medium-voltage output, thus resolving the conflict between reduced transformer size and component reliability.
3Object-affected harmful factors
If a cascade circuit with series-connected power cells is used, then multistep voltage pattern is produced reducing voltage spikes, but the device complexity increases
Solution Approach 1:
The patent uses series-connected power cells to generate a multistep voltage pattern, where each cell contributes a voltage step. This segmentation approach reduces voltage spikes through the stepped waveform while keeping each individual cell relatively simple. The modular nature of the power cells means the complexity is distributed and manageable, rather than concentrated in a single complex circuit.
4Weight of stationary object
If low-voltage power cells are used with high-frequency transformer connection, then transformer size is reduced, but the need for pre-charging intermediate DC links increases system complexity
Solution Approach 1:
The patent implements pre-charging of intermediate DC links as a preliminary action before normal operation. This is achieved through controlled activation of pre-charging circuits that charge the DC links in a predetermined sequence, ensuring readiness for high-frequency operation. This preliminary action reduces transformer size while managing the added complexity through systematic control procedures.
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 enables efficient power transmission with reduced transformer size and cost, achieving a multistep voltage pattern that mimics a sine wave, while allowing for voltage transformation and frequency conversion between various network configurations, including medium-voltage and low-voltage networks, and supporting active and reactive power transfer.
Implementation Method 1
a plurality of low-voltage power cells both at the first and at the second electricity network sides such that each power cell is connected to a power bus via a transformer
Implementation Method 2
The voltage endurance of the switch-type power semiconductor components used in frequency converters is, for reason of manufacturing technology, so small that at medium-voltage a number of them must be connected in series
Implementation Method 3
which reduces dangerous voltage spikes at the load, caused by the known reflection phenomenon that occurs with long cables
Data Source
AI summary
A converter circuitry between a first network, which may be either a poly-phase medium-voltage alternating current (AC) network, at least one polyphase low-voltage AC network or at least one direct-current (DC) network, and a second network, which may be either a polyphase medium-voltage AC network or a medium-voltage DC network, wherein the converter circuitry comprises at least one power bus and low-voltage power cells both at the first and at the second network side such that each power cell is connected to a power bus via a transformer. Each power bus is connected to a low-voltage power unit, which is able to supply pre-charging power via the power bus to all power cell intermediate DC-link filtering capacitors before the converter is started. The low-voltage power unit is also able to take care of a resistor braking in case the first network cannot take the power supplied by the load connected to the second network.