Instrument Transformer Characteristic Tester Purchasing Guide: Two Main Technical Approaches
The Instrument Transformer Characteristic Tester is the “health check device” used in power systems to ensure measurement accuracy and protective relaying reliability. It is primarily used to conduct comprehensive electrical performance testing on Current Transformers (CT) and Voltage Transformers (PT), ensuring that these “eyes of the power grid” can accurately reflect the current and voltage information on the primary side.
With technological development, especially between 2025-2026, these instruments have evolved from early manual voltage-regulating devices into fully automatic, frequency-conversion technology, multi-function integrated intelligent terminals.
⚙️ Core Functions: What Can It Do?
The core mission of the Instrument Transformer Tester is to determine whether the instrument transformer is healthy and accurate through a series of tests. The main functions include:
Volt-Ampere Characteristic Testing (Excitation Characteristic)
This is the most core function. The instrument applies voltage to the secondary side of the instrument transformer, measures the excitation current, and plots the volt-ampere characteristic curve. Through this curve, the core quality of the instrument transformer can be evaluated, the knee point voltage/current can be calculated, and it can be assessed whether premature saturation will occur under fault current, which would affect the correct operation of protective relaying.
Ratio and Polarity Testing
Quickly measure the actual transformation ratio of the instrument transformer (such as 1000/5) and compare it with the nameplate value to ensure measurement accuracy. At the same time, determine the polarity (subtractive or additive polarity) to prevent protection device misoperation or failure due to incorrect wiring.
Error Curve and Parameter Calculation
Based on national standards (such as GB1208), automatically calculate and plot 5% and 10% error curves, which are key criteria for verifying whether protective class CTs are qualified. High-end models can also automatically calculate complex parameters such as the Accuracy Limit Factor (ALF), Instrument Security Factor (FS), and secondary time constant.
Secondary Circuit Inspection and Withstand Voltage Testing
Some models feature a primary current injection function capable of outputting high current (such as 600A) to check the continuity of secondary circuits; they also feature power frequency withstand voltage testing to assess the insulation strength of instrument transformers.
