Multistring Inverter String Configuration Detection
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Solution Overview
Problem
Multistring-inverters face inefficiencies and potential damage due to incorrect bridging information between input connectors, leading to power losses and reduced lifespan of DC/DC converters, as existing methods require time-consuming determinations that disrupt AC power grid connectivity and stress AC relays.
Innovation Solution
The method involves capturing partial powers through DC/DC converters, balancing or connecting them to determine string configurations, and adjusting operations based on threshold differences in partial powers to differentiate between bridged and unbridged strings, optimizing DC/DC converter usage and reducing unnecessary AC grid attempts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the input voltage at input connectors is varied to determine string connection type, then the string configuration can be determined, but the determination process takes considerable time and causes loss of feedable electric power
Solution Approach 1:
The controller determines the string configuration by comparing partial powers of DC/DC converters before the inverter starts feeding power to the AC grid. This preliminary determination ensures that the inverter is fully configured and ready before grid connection, preventing repeated connection attempts and reducing overall startup time.
Solution Approach 2:
The patent replaces the traditional voltage variation method with a power comparison method. Instead of actively varying voltages and measuring changes (which is time-consuming), the system uses the naturally occurring partial powers during normal operation to determine string connections, significantly reducing determination time.
2Measurement precision
If the input voltage is varied to load strings for determination, then string configuration is identified, but the voltage link is not fed with maximum power and may cause undesired separation from AC power grid
Solution Approach 1:
The patent substitutes the voltage variation loading method with a natural operation power comparison method. The DC/DC converters operate in their normal power conversion mode, and the controller determines string configuration by comparing their partial powers. This eliminates the need to deliberately reduce power feed to the voltage link while maintaining maximum power availability for the AC grid.
3Loss of information
If determination is performed before inverter operation, then string configuration is known, but additional attempts of connecting to AC power grid result, stressing AC relays and reducing their lifetime
Solution Approach 1:
The controller performs string configuration determination by comparing partial powers of DC/DC converters before the inverter attempts to connect to the AC grid. This preliminary action ensures that the inverter has complete and accurate configuration information before grid connection attempts, eliminating the need for additional retry attempts and thereby protecting AC relays from unnecessary stress.
4Ease of operation
If wrong information about bridges between input connectors is used, then the inverter operates with incorrect configuration, but high losses result and lifetime of DC/DC converters is restricted
Solution Approach 1:
The controller continuously monitors the partial powers of DC/DC converters and uses this feedback to determine the actual string configuration. By comparing the measured partial powers with expected values, the controller can accurately identify bridged versus unbridged strings and adjust its operation accordingly, preventing energy losses that would occur with incorrect configuration assumptions.
Data Source
Figure 1
AI summary
In operating an inverter (1 ) comprising input connectors (21 to 25), (i) to which strings (1 1 to 15) of photovoltaic cells are connected, (ii) each of which is connected via a DC/DC converter (41 to 45) to a common DC voltage link (2), and (iii) which are bridgeable, the partial powers flowing through the individual DC/DC converters are captured and for some time at least two DC/DC converters are either operated with the aim of balancing the partial currents (46 to 50) flowing through them or connected through. During this operation or connecting through, the partial powers flowing through the at least two DC/DC converters are compared with each other. If a difference between the partial powers exceeds a threshold value, the DC/DC converters are subsequently operated in a way adjusted to the fact that they connect different strings (1 1 to 15) to the DC voltage link (2).