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Protective and Measuring Transformers for Medium-Voltage Equipment
Date:2022.09.26 Views:620

Protective and Measuring Transformers for Medium-Voltage Equipment

The task of instrument transformers is to transform high currents and voltages proportionally and in-phase into small current or voltage values for measuring or protection purposes. So they are used either to measure and record the transmitted power or to feed protection devices with evaluable signals, which enable the protection device to trip a switching device depending on the situation. Furthermore, they isolate the connected measuring or protection equipment electrically from live parts of the switchgear.

Current transformers can be regarded as transformers working in short-circuit, with the full normal current flowing through their primary side. Devices connected on the secondary side are series-connected. Current transformers can have several secondary windings with magnetically separated cores of the same or different characteristics. They can, for example, be equipped with two measuring cores of different accuracy class, or with measuring and protection cores with different accuracy limit factors. Due to the risk of overvoltages, current transformers must not be operated with open secondary terminals, but only in short circuit or with the burden of the measuring equipment.

Voltage transformers contain only one magnet core and are normally designed with one single secondary winding. If necessary, earthed (single-phase) voltage transformers are provided with an additional residual voltage winding (earth-fault winding) beside the secondary winding (measuring winding). In contrast to current transformers, voltage transformers must never be short-circuited on the secondary side. The earth-side terminal of the primary winding is effectively earthed in the terminal box, and must not be removed in operation.

Types of construction

Protective and measuring transformers are designed in different types of construction for the multiple installation requirements and operating conditions they are subjected to. They are electrical devices which convert primary electrical values – currents or voltages – into proportional and in-phase values that are adequate for the connected devices such as measuring instruments, meters, protection relays and similar.

Current transformers

•Indoor support-type current transformer in block-type design

•Indoor support-type current transformer in single-turn design (e.g. bar-primary transformer)

•Indoor bushing-type current transformer in single-turn design

•Indoor bar-primary bushing-type current transformer

•Outdoor support-type current transformer

Operating voltage Um: 12/24/40.5kV
Rated short-duration power-frequency withstand voltage Ud: 42/65/95kV
Rated lightning impulse withstand voltage Up: 75/125/185kV
Rated frequency:  50/60Hz
Rated primary current IPN: 5A to 4000A
Multi-ratio 2x20 to 2x600
Secondary current ISN:  1/5A
Rated short time thermal current Ith:  80kA or 100xIPN
Rated dynamic current Idyn:  120kA or 2.5xIth
Protection (Factor) 5P10, 5P15, 5P20, 10P10, 10P15, 10P20
Power Capacity 10VA/15VA/20VA/30VA
Measuring (Accuracy) 0.2/0.2S/0.5/0.5S/1/3
Power Capacity 10VA/15VA/20VA/30VA

For example: Measuring only model 600A/5A, 0.2S, 10VA or multi-ratio 2x500A/5A, 0.2/0.5 10/15VA

Measuring with protection model: 600A/5A 0.2/10P10 10/15VA

Voltage transformers

•Earthed (single-phase) or unearthed (double-phase) indoor transformers in different sizes

•Earthed (single-phase) or unearthed (double-phase) outdoor transformers in different sizes

Operating voltage Um: 3.6/7.2/12/40.5kV
Rated short-duration power-frequency withstand voltage Ud: 25/32/42/95kV
Rated lightning impulse withstand voltage Up: 40/60/75/200kV
Rated frequency:  50/60Hz
Rated thermal limiting output of the residual voltage winding:  230VA/4A
Rated primary voltage maximum VPN:  3kV, 6kV, 10kV, 40.5kV, 3/√3 V, 6/√3 V, 10/√3 V,40.5/√3 V
Secondary voltage USN:  100V, 220V, 100/√3 V, 220/√3 V
Thermal limiting output Sth:  200VA, 300VA, 400VA, 500VA, 600VA, 800VA, 1000VA
Auxiliary residual voltage winding Without or 100/3V, 110V/3V, 120/3V
Rated output of the measuring winding Class 0.2/0.5/1/3/5 
Rated output of the protection winding 3P or 6P
Rated output of the power capacity 10VA, 15VA,20VA, 30VA, 50VA, 60VA, 100VA,150VA, 200VA etc.

For example: JDZ10-10A, 10/0.1KV, 100V, 0.2/30VA, Dual output voltage: JDZ10-10A, 10/0.1KV, 100/100V 0.2/0.5 10/15VA

JDZ10-10A 10/√3 / 100/√3 /100/3V 0.5/3P 15/20VA


Catalogue Download - Protective and Measuring Transformers for Medium-Voltage Equipment, Cast-resin type.


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