Grounding Current Measurement via Current Mirror Circuit
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Solution Overview
Problem
Current methods for measuring grounding currents in photovoltaic power systems are expensive and prone to measurement errors due to potential differences between the measurement point and the evaluation unit, especially when dealing with direct currents and varying grounding configurations.
Innovation Solution
The use of a current mirror circuit with transistors and shunts, where the emitter terminals are connected to the shunt directly or via resistors, and the base terminals are joined together, allowing for asymmetry in the current mirror to be evaluated at the potential of the evaluation unit, enabling low-cost measurement of currents at significantly different potentials without the need for additional voltage supply or magnetic converters.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a shunt resistor is used for current measurement, then the current can be measured through voltage measurement, but the measurement voltage becomes too low and common mode faults strongly falsify the measurement when potential difference exceeds 50V
Solution Approach 1:
The patent introduces an intermediary circuit (difference amplifier with high common mode rejection ratio or instrumentation amplifier) between the shunt resistor and the evaluation unit. This intermediary circuit actively rejects common mode voltages while preserving the differential measurement signal, allowing accurate current measurement even when the potential difference between measurement point and evaluation unit exceeds 50V.
2Measurement precision
If a current transformer is used to measure current, then the shunt can be set to evaluation unit potential, but this solution is only possible for pure alternating currents and cannot measure direct currents
Solution Approach 1:
The patent creates a universal current measurement solution that works for both alternating and direct currents by combining a shunt resistor with a difference amplifier circuit. The shunt resistor handles the current conversion to voltage, while the difference amplifier circuit universally processes both AC and DC voltage signals, eliminating the limitation of current transformers that can only handle AC currents.
3Adaptability or versatility
If current converters with Hall sensors are used to measure direct currents, then both AC and DC currents can be measured, but the devices are very expensive
Solution Approach 1:
The patent replaces expensive Hall sensor-based current converters with a inexpensive shunt resistor and difference amplifier circuit. The shunt resistor is a simple, cheap component that converts current to voltage, and the difference amplifier is a standard electronic circuit that can be implemented with ordinary operational amplifiers, dramatically reducing the cost while maintaining the ability to measure both AC and DC currents.
4Adaptability or versatility
If separate measurement channels are provided for each grounding point, then currents at different points can be measured, but it is necessary to constantly sample several channels and increases device complexity
Solution Approach 1:
The patent implements a dynamic measurement system where a single measurement channel can adaptively measure currents from different grounding points by switching the measurement location. The system dynamically reconfigures the measurement circuit to connect to different grounding points (positive pole or negative pole) based on the actual fault condition, eliminating the need for multiple fixed measurement channels and reducing overall system complexity.
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 approach allows for sensitive and accurate measurement of grounding currents and insulation resistance at reduced costs, effectively addressing the challenges of potential differences and varying grounding configurations, while maintaining stability across temperature changes.
Implementation Method 1
The measurement current flows through these shunt resistors so that a voltage, which is proportional to the current, can be measured at the resistance.
Implementation Method 2
Two of these energizable semiconductor component parts, in particular transistors, are connected to the shunt Rs so that a voltage difference occurs at the semiconductor component parts T1, T2 or at their collector resistors R3, R4, the magnitude of which is proportional to the measurement current I1.
Data Source
AI summary
The invention is a method and electric circuitry for measuring a grounding current of a photovoltaic power system incorporating a photovoltaic inverter, a measurement current being led through two shunts disposed at different points. The measurement current is a current that may flow alternatively at two different points at different electric potentials, the current flowing at a potential that differs by at least 50 V from the potential of the point of evaluation. The shunts are inserted in such a manner in a current mirror circuit that the voltage drop caused by the measurement in the shunts generates an asymmetry in the current mirror the magnitude of which is proportional to the measurement current. The measurement current is the grounding current of the photovoltaic power system with optional positive or negative pole grounding.