Inverter DC Link Pre-Charging for High Voltage PV Connection

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Solution Overview

Problem

Connecting a photovoltaic array to an AC power grid via an inverter poses challenges due to high open circuit voltages, which can overload circuit components, and existing solutions either increase costs or result in power losses, particularly when using higher voltage class components or additional converters.

Innovation Solution

The method involves pre-charging the DC link of the inverter from the AC power grid and adjusting the link voltage to a pre-set value lower than the open circuit voltage of the PV array, allowing direct connection while continuously adjusting the link voltage, thereby avoiding the need for additional voltage adaptation and minimizing power losses.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Adaptability or versatility

If higher voltage class components are used to handle high open circuit voltages, then the inverter can connect to PV arrays with higher output voltages, but the cost increases and power losses occur due to lower efficiency

Engineering Contradiction:

Improveability to connect to PV arrays with higher output voltagesVSAvoidpower losses due to lower efficiency of higher voltage class components

Core Design Contradiction:

Adaptability or versatilityVSLoss of energy

Solution Approach 1:

The inverter connects to the AC power grid before connecting to the PV array, pre-charging the DC link and establishing voltage control capability. This preliminary action enables the inverter to handle the subsequent connection of high voltage PV arrays without requiring higher voltage class components, as the DC link is already prepared to accept the voltage difference through controlled charging currents.

Inventive Principle:

Principle #10Preliminary action

2Ease of operation

If the inverter connects directly to the PV array at high open circuit voltage, then the connection is simple and fast, but excessive currents and voltages may overload the circuitry components

Engineering Contradiction:

Improvesimplicity and speed of connectionVSAvoidrisk of overloading circuitry components

Core Design Contradiction:

Ease of operationVSReliability

Solution Approach 1:

The inverter performs preliminary connection to the AC power grid and pre-charges the DC link before connecting to the PV array. This sequence ensures that when the PV array connects, the inverter is already operational and can immediately control the charging current, preventing excessive currents while maintaining simple direct connection topology.

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The DC link acts as an intermediary energy buffer between the PV array and the inverter bridge. By pre-charging this intermediate storage element from the AC grid, the system creates a controlled transition path that prevents direct high-voltage冲击 to the inverter components, thereby protecting them while enabling direct PV connection.

Inventive Principle:

Principle #24Intermediary (Mediator)

3Reliability

If a DC/DC converter is used to adapt voltage between the PV array and inverter, then the inverter bridge is protected from high open circuit voltages, but the device complexity increases and additional power loss occurs

Engineering Contradiction:

Improveprotection of inverter bridge from high voltagesVSAvoidcomplexity of the apparatus

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The invention extracts the voltage adaptation function from a separate DC/DC converter and integrates it into the inverter's existing voltage control unit. By removing the need for a dedicated DC/DC converter and utilizing the inverter's built-in capabilities for DC link voltage control, the solution protects the inverter bridge from high voltages while reducing overall device complexity.

Inventive Principle:

Principle #2Taking out (Extraction)

Solution Approach 2:

The inverter's voltage control unit, originally designed for MPP-tracking during normal operation, is made multi-functional by enabling it to perform voltage adaptation during the connection phase. This universal approach allows the same hardware to handle both normal power conversion and initial voltage matching, eliminating the need for additional dedicated voltage adaptation equipment.

Inventive Principle:

Principle #6Universality (Multi-functionality)

4Reliability

If a voltage divider with resistors is used to protect the inverter bridge, then the inverter is protected from high open circuit voltages, but additional power loss occurs due to the resistors even in normal operation

Engineering Contradiction:

Improveprotection of inverter bridge from high voltagesVSAvoidpower loss due to resistors

Core Design Contradiction:

ReliabilityVSLoss of energy

Solution Approach 1:

The invention removes the voltage divider resistor configuration from the system entirely. Instead of using resistors to divide and limit voltage, the solution relies on the inverter's active voltage control capability to manage the voltage difference during connection, eliminating the continuous power loss associated with resistive voltage division while maintaining protection functionality.

Inventive Principle:

Principle #2Taking out (Extraction)

Data Source

PatentUS9331564B2Connecting a photovoltaic array at a high open circuit voltage

Publication Date: 2016.05.03 SMA SOLAR TECH AG

AI summary

For connecting a PV array via an inverter to an AC power grid, at first a DC link at the input side of the inverter is pre-charged from the AC power grid. A link voltage of the DC link is adjusted to a pre-set value with the inverter connected to the AC power grid, the pre-set value being lower than an open circuit voltage of the PV array, and then the PV array at its open circuit voltage is directly connected to the DC link, while the link voltage is continuously adjusted to the pre-set value.