Virtual Ground Sensing Circuit for High Impedance Voltage Measurement
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Solution Overview
Problem
High impedance voltage sensors used in power distribution systems face significant measurement errors due to cable capacitance, signal attenuation, phase shift, and crosstalk, especially in shielded cables, and are prone to errors from parasitic capacitances and leakage currents, particularly in outdoor applications with high humidity and temperature variations.
Innovation Solution
A virtual ground sensing circuit is employed, utilizing an amplifier with a high impedance element that converts the high voltage signal into a low current signal with substantially zero voltage, which is then amplified to produce an output voltage proportional to the original voltage, minimizing capacitive and resistive errors, and includes surge protection and frequency compensation mechanisms.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of moving object
If a high impedance voltage sensor is used to measure voltage at a remote location, then the measurement range is extended, but cable capacitance causes significant measurement errors and signal attenuation
Solution Approach 1:
The patent introduces a virtual ground node as an intermediary that converts the high impedance voltage sensing problem into a low impedance current measurement problem. The virtual ground node (node 20) acts as a mediator that maintains a fixed potential while allowing current to flow through the cable capacitance without creating voltage drops, thereby eliminating the measurement errors caused by cable capacitance.
Solution Approach 2:
The patent changes the measurement parameter from voltage to current. Instead of measuring voltage directly at the remote location (which is affected by cable capacitance), the system measures the current flowing through the impedance element. This parameter transformation eliminates the harmful effects of cable capacitance on measurement accuracy.
2Object-affected harmful factors
If a shielded cable is used to reduce crosstalk and interference, then electromagnetic interference is reduced, but cable capacitance increases causing greater signal attenuation
Solution Approach 1:
The virtual ground node serves as an intermediary that allows the use of shielded cables for EMI protection while eliminating the signal attenuation problem. By converting the measurement to a current measurement at virtual ground, the system can tolerate the increased capacitance of shielded cables without suffering from signal attenuation.
3Device complexity
If the voltage sensor is made resistive to simplify the circuit, then circuit complexity is reduced, but capacitive reactance introduces significant phase shift in the signal
Solution Approach 1:
The virtual ground node acts as an intermediary that decouples the resistive sensor element from the capacitive cable effects. The current measured at virtual ground is directly proportional to the input voltage without phase shift, even though current does flow through capacitive elements, because the virtual ground maintains a fixed potential.
4Reliability
If voltage clamp devices are added to protect against voltage surges, then surge protection is improved, but parasitic capacitance of the clamp devices causes leakage current and signal attenuation
Solution Approach 1:
The virtual ground node serves as an intermediary that allows voltage clamp devices to be placed on both sides of the cable for surge protection while eliminating their harmful capacitive effects. The clamp devices' parasitic capacitance no longer creates leakage current or signal attenuation because the virtual ground maintains a fixed potential, preventing voltage development across capacitive elements.
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
The virtual ground sensing circuit effectively reduces measurement errors by eliminating capacitive and resistive leaks, while providing surge protection and frequency compensation, ensuring accurate voltage measurement across long distances with minimal signal attenuation and phase shift.
Implementation Method 1
an amplifier having a first input and an amplifier output, the first input being coupled to a node comprising a virtual ground
Implementation Method 2
the first impedance element is structured to cause a current signal having a current directly proportional to the voltage on the conductor and a substantially zero volt voltage to be provided to the first input of the amplifier
Implementation Method 3
the amplifier is structured to cause an output voltage that is directly proportional to the current of the current signal to be provided at the amplifier output
Data Source
AI summary
A voltage sensing circuit includes an amplifier having a first input and an amplifier output, the first input being coupled to a node comprising a virtual ground, and a first impedance element having an input structured to be coupled to the conductor and an output coupled to the first input of the amplifier through the node. The first impedance element is structured to cause a current signal having a current directly proportional to the voltage on the conductor and a substantially zero volt voltage to be provided to the first input of the amplifier through the node responsive to the voltage of the conductor being provided to the input of the first impedance element. The amplifier is structured to cause an output voltage that is directly proportional to the current of the current signal to be provided at the amplifier output.