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Instrument transformers are classified into current transformers (TA, CT) and voltage transformers (TV, PT), serving as the interface between the primary and secondary systems. They convert the high voltage and large current of the primary system into low voltage and small current, which are then used for measurement, monitoring, control, and relay protection.

Built-in (sleeve-type) current transformer:

· In accordance with transformer standards or technical specifications, one or more current transformers are installed in each tap changer housing. All such current transformers are fully immersed in transformer oil to ensure that the product’s insulation remains dry and free from contamination.

· For metering-type (LR-type) current transformers, if instrument protection readings (i.e., the FS value) are required, a shunt reactor with a toroidal core shall be connected in parallel across the secondary terminals of the current transformer.

The product features an inverted configuration and consists of three main components: the housing, the bushing, and the base. The housing is located at the top of the product, with both the primary and secondary windings housed inside. The primary winding passes through the center of the secondary winding, and the two windings are insulated from each other by SF6 gas. The housing is at the same electric potential as the primary circuit and is electrically insulated from the base via the bushing.

The secondary windings are secured within the enclosure by insulating components, and their leads are connected via metallic shielding tubes inside bushings to the secondary terminal blocks in the base, enabling users to connect measurement or protection loads. The base is equipped with desiccant, a secondary wiring panel, and an SF6 pointer-type density relay, among other components. An explosion-proof device is installed on the top of the enclosure.

Compared with other types of instrument transformers, epoxy-encapsulated insulated voltage transformers have the following key advantages:

(1) Minimal maintenance: Once the product is put into grid operation, it requires very little ongoing maintenance.

(2) Lightweight: The proportion of metallic components in the entire instrument transformer is relatively small, making it lighter than other types of instrument transformers with the same rating.

(3) Oil- and gas-free, environmentally friendly and clean: No oil or gas leakage due to poor sealing, thereby preventing environmental pollution.

(4) Convenient installation and transportation: As long as the equipment is securely fastened within the transport container, it can be loaded and transported in multiple orientations.

However, epoxy resin is prone to cracking, and its external insulation surface is susceptible to surface tracking, which can lead to irreversible discharge paths.

Configuration of Current Transformers

Current transformers shall be installed on all branch circuits in accordance with measurement and relay protection requirements. (This applies to circuits such as generators, transformers, outgoing lines, bus section switches, bus tie switches, and bypass circuits.) In generator circuits, main transformer circuits, large plant-service transformer circuits, and circuits in 110 kV and higher systems with high-ground-fault current, current transformers are generally configured on a three-phase basis; in low-ground-fault-current systems, they are generally configured on a two-phase basis.

The installation location of protective current transformers shall be determined so as to minimize the dead zone of the main protection scheme.

To prevent busbar faults caused by flashover of instrument transformer bushings, such transformers are typically installed on the circuit breaker’s outgoing side or on the transformer side.

To mitigate damage to the generator caused by internal faults, the excitation regulator current transformer should be installed on the generator stator outlet side. To facilitate fault analysis and enable the detection of internal faults before the generator is connected to the power system, the measuring current transformer should be installed on the generator neutral point side.

Typically, current transformers with dual cores or multiple cores are used to provide separate power supplies for measurement and protection.

For some circuit breakers and transformer bushings rated at 35 kV and above that are equipped with current transformers, no additional installation is required.