Instrument transformers – current and voltage transformers – are specially designed transformers that step down high currents or voltages to measurable and safe values. They are the eyes and ears of every electrical protection and billing system. The quality of the instrument transformers determines how precisely metering, diagnostics, and protection tripping function within an installation.
What tasks do instrument transformers perform?
Current transformers typically step down a high primary current to 1 A or 5 A. Voltage transformers reduce medium or high voltage to typically 100 V or 110 V. These secondary values are used for metering, billing, and protection functions. Depending on the accuracy class, instrument transformers can be designed for highly precise energy billing or for robust protection applications.
Key Characteristics at a Glance
- Current and voltage transformers in bushing or pedestal-mounted designs
- Accuracy classes typically 0.1 to 5P
- Protection transformers with a high accuracy limit factor
- Metering transformers for billing (class 0.2 or 0.5)
- Available for low voltage, medium voltage, and high voltage
- Optical current and voltage transformers as a modern alternative
- Rogowski coils for flexible current sensing in confined spaces
Frequently Asked Questions
What is the difference between metering and protection transformers?
Metering transformers are designed for high accuracy within the normal operating range and saturate relatively early during overcurrents to protect the connected measuring equipment. Protection transformers, on the other hand, are designed to still deliver linear values even during short-circuit currents, so that protection relays trip correctly.
How is a transformer’s accuracy class specified?
For current transformers, for example, class 0.5 means a maximum deviation of ±0.5% within the rated range. Protection classes are indicated with the suffix P (e.g. 5P10), where the number after the P indicates the accuracy limit factor during overcurrent.
What advantages do optical instrument transformers offer?
They are extremely compact, free from magnetic saturation, accurately calibrated over a very wide measurement range, and have excellent dynamic properties. Due to their high cost, they have so far mainly been used in extra-high-voltage installations and special applications.
How are instrument transformers tested at the factory?
Standard tests include verification of the turns ratio, accuracy measurement, withstand voltage test, partial discharge measurement, and thermal tests. For protection transformers, the saturation curve is additionally recorded. All values are documented in the test report and form the basis for the conformity certification.
Do you need instrument transformers in special designs or accuracy classes? We supply units tailored to your application – contact our engineering team.