Voltage Transformer (VT/PT): Authoritative Guide for "Precision Oscilloscope Probe" and Engineering Selection in Power Grid

2025-08-24 16:10:47

1、 Core mission: Building a voltage reference benchmark for secondary systems

Voltage Transformer (PT, commonly referred to as VT in the IEC system) is the reference voltage source of the power system. Its core task is to accurately convert a high voltage (such as 10kV) into a safe low voltage (100V or 100/√ 3V), providing isolated, standardized, and accurate voltage signals for measurement, metering, protection, and control systems.  

>Analogy: Similar to the high-voltage probe of a precision oscilloscope - it safely attenuates thousands of volts of voltage into millivolt level signals that can be processed by the device, with the key value of maintaining high fidelity of signal waveform and phase, and absolutely isolating the danger on the high-voltage side. Any distortion or error will directly lead to measurement errors or protection misjudgments.  

Its design must meet three core requirements:

-Accuracy: Provides indisputable measurement basis for electricity trade settlement (with extremely strict error requirements).  

-Reliability: Stable operation under various system conditions (especially overvoltage), without becoming a source of failure.  

-Insulation safety: Absolute isolation of primary and secondary systems to ensure personal and equipment safety.  

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2、 Working principle and architecture: electromagnetic vs. capacitive

1. Electromagnetic (inductive VT)

-Principle: Based on the principle of transformers, relying on electromagnetic induction between the primary and secondary windings.  

-Advantages: Simple structure, mature technology, high precision (especially near rated voltage).  

-Fatal weakness: ferromagnetic resonance. Its nonlinear excitation characteristics are prone to resonance with the system's ground capacitance, generating overvoltage several times higher than the rated voltage, leading to PT burnout or explosion.  

2. Capacitive VT (CVT)

-Principle: Composed of a capacitive voltage divider and an electromagnetic unit connected in series. First, divide the voltage through a capacitor, and then reduce the voltage through an intermediate transformer.  

-Advantages: It naturally suppresses ferromagnetic resonance, doubles as a carrier communication coupling capacitor, and is cost-effective for ultra-high voltage (110kV+) applications.  

-Disadvantage: The transient response characteristics are slow, which may affect the performance of certain fast protection actions.  

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3、 Selection core: Triangular balance of precision, insulation, and anti resonance

Selection is not just about ratio selection, but a precise balance between measurement, protection, and system safety.  

The rated voltage ratio (10kV/√ 3)/(0.1kV/√ 3)=10000/100 must match the nominal voltage of the system. The rated voltage is usually equal to the system phase voltage (Um/√ 3).               

Accuracy level measurement: 0.2, 0.5, 1.0

Protection: For 3P and 6P metering, 0.2 or 0.5 level must be selected; Protecting against composite errors, with a 3P level tolerance of 3% error.             

Rated Capacity: 10VA, 25VA, 50VA, 100VA Core Parameters! Calculate the power consumption of all secondary equipment (instruments, relays) plus the loss of connecting cables, leaving a 30% margin. Overloading can cause PT overheating and decreased accuracy.

The insulation level of power frequency withstand voltage/lightning impulse withstand voltage (such as 42/75kV) must be higher than the maximum overvoltage that may occur in the system (such as switch operation, lightning strikes).           

The thermal limit output of several hundred VA to several kVA measures the short-term overload capacity of PT under fault conditions, which needs to be greater than the maximum possible secondary load. 

Engineering Design Checklist (Taking Electromagnetic PT as an Example):

1. Determine the network type: Neutral point effective grounding system? Or is it an ineffective grounding (insulation) system? This determines the wiring method (star/delta) and rated voltage of PT.  

2. Calculate the total secondary load: ` S total=∑ (S devices)+I 2 R cable `. This is the key to avoiding accuracy loss of control.  

3. Select accuracy level: Select 0.2 level for trade settlement; Generally, a monitoring level of 0.5 is selected; Choose 3P or 6P level for protection.  

4. Resonance verification: mandatory! For neutral point insulation systems, priority should be given to using anti ferromagnetic resonance PT (such as three-phase five column, single-phase PT primary side neutral point series connection high resistance harmonic eliminator).  

5. Selection of insulation medium:

-Oil immersed: Traditional, inconvenient to maintain.  

-Epoxy resin casting type (dry type): maintenance free, explosion-proof, fireproof, becoming the preferred choice for indoor and environmentally friendly projects.  

-SF6 gas insulation: used for ultra-high voltage or special environments.  

4、 Engineering Traps and Failure Modes: Lessons from the Field

1. Ferroresonance: Systematic 'suicide'

-Scenario: After a single-phase grounding fault is eliminated in the neutral point insulation system, the nonlinear inductance of the PT couples with the system's capacitance to ground, causing sustained resonant overvoltage and resulting in PT explosion and Lightning Arrester rupture.  

-Solution:

-Select harmonic elimination PT (primary neutral point series connected nonlinear Resistor).  

-Connect a damping resistor (microcomputer harmonic elimination device) to the triangular winding on the PT secondary side.  

-Change the operating mode of the system (such as reducing the length of the line to change the capacitance to ground).  

2. Secondary short circuit: a risk that cannot be ignored

-Principle: When PT is working normally, it is approximately unloaded. Once there is a secondary short circuit, the huge short-circuit current will burn out the winding.  

-Protection: A high-voltage fuse (protecting the PT itself) must be installed on the primary side of the PT, and a miniature circuit breaker (MCB) or fast fuse (protecting the secondary circuit) must be installed on the secondary side. It is strictly prohibited to use ordinary Fuses instead!  

3. Accuracy failure

-Scenario: Deviation in electricity metering or abnormal sampling of protective devices.  

-Troubleshooting: Is the secondary load exceeding the limit? Is the cross-sectional area of the connecting cable too small (causing excessive voltage drop)? Has the PT body ever experienced changes in iron core characteristics due to voltage surges?  

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5、 Technological Evolution: From Electromagnetic Induction to Electronic Sensing

-Electronic Voltage Transformer (EVT): Based on resistive voltage division, capacitive voltage division, or optical principles (such as Pockels effect).  

-Advantages: No magnetic saturation, wide frequency band, simple insulation, small size, and direct digital output.  

-Challenge: Long term operational stability and environmental adaptability (temperature, vibration) are still the focus of engineering applications.  

-Intelligent integration: As part of the merging unit (MU), it becomes the core sampling unit of the intelligent substation.  

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Conclusion: The Guardian Anchor Point for System Security and Metrology

When choosing VT/PT, it is important to remember:

It is not only a measuring element, but also a component of the system insulation coordination and possible risk points. Its accuracy is related to trade fairness, and its stability is related to system security.  

The most reliable selection is based on a comprehensive decision of system parameters (grounding method, ground capacitance), accurate calculation of secondary load, and anti resonance design. Never ignore the destructive risks brought by ferromagnetic resonance in order to save a little initial cost.