Voltage Transformer Ferroresonance Protection via Dynamic Damping
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Solution Overview
Problem
Existing solutions for attenuating ferroresonant oscillations in voltage transformers, such as those using attenuating resistors in open delta circuits, face issues with thermal damage and measurement accuracy errors due to unbalanced supply networks and the need for electromechanical components like relays, which are inefficient and can distort voltage measurements.
Innovation Solution
A protective system for voltage transformers that employs a damping burden connected to open delta secondary windings, controlled by a microcontroller-based switching element, which activates and deactivates the damping burden based on zero-sequence voltage thresholds, using PWM to gradually adjust resistance and avoid thermal damage and measurement errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an attenuating resistor with low resistance value is connected to the open delta circuit to effectively attenuate ferroresonant oscillations, then the attenuation effectiveness is improved, but the risk of thermal damage to the potential transformers or resistor increases
Solution Approach 1:
The patent applies a switching device that dynamically connects or disconnects the attenuating resistor based on detected oscillation conditions. The resistor is only connected when ferroresonant oscillations are detected, and disconnected when they cease, making the attenuation effect dynamic rather than static. This resolves the contradiction by providing effective attenuation only when needed, avoiding continuous thermal stress.
Solution Approach 2:
The system uses the potential transformers themselves to detect the presence of ferroresonant oscillations and automatically control the switching of the attenuating resistor. The detection and control functions are integrated into the existing transformer system, allowing self-monitoring and self-protection without external intervention.
2Reliability
If a burden is connected between open delta terminals to provide damping, then ferroresonant oscillation attenuation is improved, but the measurement accuracy of voltage measurement deteriorates due to zero sequence voltage presence
Solution Approach 1:
The switching device dynamically controls the connection of the attenuating resistor based on real-time detection of oscillation conditions. When ferroresonant oscillations are detected, the resistor is connected for attenuation; when they are not present, the resistor is disconnected to avoid affecting measurement accuracy. This dynamic approach resolves the contradiction between attenuation effectiveness and measurement precision.
Solution Approach 2:
The system periodically monitors the voltage conditions and alternates the connection state of the attenuating resistor based on detected oscillation patterns. This periodic monitoring and switching ensures attenuation is applied only during oscillation events, minimizing impact on normal measurement operations.
3Reliability
If electromechanical relays are used to control the attenuating resistor connection, then the system can respond to network unbalance, but the device complexity and potential for measurement distortion increase
Solution Approach 1:
The patent replaces electromechanical relays with solid-state switching devices controlled by electronic detection circuits. The switching device is controlled by voltage and current signals from the potential transformers, eliminating moving parts and mechanical wear. This substitution reduces device complexity while maintaining or improving reliability of network unbalance protection.
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 effectively prevents thermal damage and maintains measurement accuracy by dynamically controlling the damping burden's connection and disconnection, ensuring timely damping of ferroresonant oscillations without unnecessary transients or cooling time, thus enhancing protection and measurement reliability.
Implementation Method 1
the thermal fuse has the form of a PTC resistor
Implementation Method 2
an attenuating resistor of a resistance of tens of ohms is typically used
Data Source
Figure 1
Figure 2~3
Figure 4
AI summary
The subject of the invention is a protective system for voltage transformers, finding application in the attenuation of ferroresonant states occurring in voltage transformers in high and medium voltage grids. A protecting system for voltage transformers comprising a damping burden (3) connected into an open delta system of three auxiliary secondary windings of three single-phase transformers (VT1, VT2, VT3), which is activated and deactivated by a switching element (2) connected in series with the damping burden (3). The protective system is characterised in that the switching element (2) has a control input (C) connected to an output of control means suitable for controlling a time sequence of deactivating the damping burden (3) by controlling the conductive and nonconductive states of the switching element (2).