ABB 15.04.2005 Instrument Transformer

ABB 15.04.2005 Instrument Transformer

Current transformer

Basic functions and principles

Function: Convert high current in power transmission and distribution systems into low current for use in low-voltage measurement and protection equipment. The secondary current is usually 1A or 5A, connected in series, and the secondary winding is mainly connected to an ammeter and protection relay.

Principle: Through electromagnetic induction, the secondary current is proportional to the primary current, and the conversion ratio K=I ₚᵣ/I ₛᵣ=N ₂/N ₁, where I ₚᵣ is the primary current, I ₛᵣ is the secondary current, N ₁ and N ₂ are the turns of the primary and secondary windings, respectively. The secondary current I ₛᵣ=I ₚᵣ × N ₁/N ₂. It can have one or more secondary windings, each with an independent magnetic circuit, used for different purposes such as metering or protection.

Key parameter

Rated primary current (I ₚₑ): The primary current value based on the performance of the transformer, which needs to be equal to or higher than the highest primary current of the system. The standard value is in accordance with IEC regulations, such as 10, 12.5, and their decimal multiples. It can also be produced in any proportion according to the protocol, usually with a 120% extended current range to avoid high temperature rise during installation.

Frequency: It can be set as 50Hz or 60Hz, or a combination of 50/60Hz, and 16 ⅔ Hz is also possible, depending on the network of each country.

Equipment maximum voltage (U ₘ): The insulation level should be set to be equal to or higher than the system maximum voltage U ₛᵧₛ (except for cable current transformers, whose insulation is provided by the application), with standardized values, such as 3.6kV corresponding to system voltage up to 3.3kV, etc.

Insulation level: Based on the U ₘ value, such as IEC insulation level 12/28/75kV, it represents the highest voltage, power frequency withstand voltage (effective value), and lightning impulse withstand voltage (peak value) of the equipment, respectively.

Accuracy level: divided into measurement class (0.2, 0.2S, 0.5, 0.5S, 1, 3) and protection class (5P, 10P, PX, TPX, TPY, TPZ), different levels are suitable for different purposes such as tariff measurement, laboratory measurement, protection, etc. The chart shows the current ratio error limit of commonly used measurement classes.

Secondary current and number of windings: The secondary current is 1A or 5A, and the number of windings depends on the size of the body. For example, the maximum 36kV TPU 7x.6x can have up to 8 iron cores.

System primary current (I ₛᵧₛ): The system current for installing transformers needs to be equal to I ₚᵣ, and different applications have different calculation methods. For example, the transformer feeder I ₛᵧₛ=S/(√ 3 × U ₛᵧₛ), the capacitor feeder I ₛᵧ=1.3 × Q/(√ 3 × U ₛᵧₛ), where S is the apparent power (kV). A), U ₛᵧₛ is the system voltage (kV), and Q is the reactive power of the capacitor (kVar).

Rated short-time thermal current (I ₜₕₙ): The maximum primary current that a transformer can withstand for a specified short period of time without causing harmful effects. The standard values are 2, 4kA, etc., and the duration standard is 1 second, or possibly 3 seconds. The rated dynamic current I dyn is 2.5 times I ₜₕ at 50Hz.

Extended current rating (EXT): The range of current increase that a current transformer can measure at an accuracy level, such as 120%.

Maximum continuous thermal current (I ₜₕ): The maximum continuous thermal current that the current transformer can still operate but is not at the accuracy level.

Accuracy Limiting Factor (ALF): Used for protection purposes, it defines the multiple of primary current that the error must meet. For example, 5P20 represents a composite error of ≤ 5%, and the error limit from 100% primary current to 20 times primary current must be met. ALF is related to the load, and unexpected increases in load can cause ALF to decrease. The actual load required for the secondary iron core, secondary circuit, and connecting equipment needs to be calculated. Different instrument loads vary, such as 0.5-4VA for ammeters, and cable losses need to be compensated.

Safety factor (FS): Used for measurement purposes, it protects all devices connected on the secondary side by setting iron core saturation to avoid secondary high current transmission during faults. For example, FS5 indicates an error of ≥ 10% under rated load at 5 times the rated primary current.

Environmental temperature and insulation level: Environmental temperature is divided into three categories according to IEC: -5/40 ° C, -25/40 ° C, and -40/40 ° C. The insulation level of ABB products corresponds to Class E, which means the maximum temperature rise of all active components is about 75K.

Other features

Capacitive voltage divider: integrated into the current transformer body, making voltage indication more convenient.

Reconnect: It can be achieved on the secondary or primary side. The left side features a dual core design with a secondary reconnection tap, while the right side has a primary reconnection that requires manual operation.