Current sensors

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General information and description

The development of microprocessor technology has led to changes in the requirements for the transformers used for measurement and protection in substations, due to self-powered relays based on microprocessors.

FMT has developed a series of low-power current sensors, compatible with modern microprocessor-controlled relays and secondary equipment with high input impedance. The sensors meet the requirements of the IEC 61869-10 standard.

Installation and mounting

Low-power current sensors are mounted inside the cable compartment of the main unit, designed for installation over the T-connector bushing-type insulator or directly around the power cable itself.

No subsequent field adjustment of the sensor is required, as they are factory-calibrated to deliver an accurate signal in accordance with the applicable IEC standard.

Key advantages of FMT sensors
Passive technology
Completely free of active components inside, with no need for any additional power supply.
Accuracy classes
High precision and accuracy classes fully tailored to all customer needs.
Temperature range
Stable operation and safe storage under extreme conditions from -40°C to +80°C.
Compact design
Reduced dimensions and minimal weight make installation in switchgear cells easier.
Maximum cost-effectiveness
Low device cost combined with maximum installation flexibility in the field.
Increased safety
High level of protection for personnel working in substations and minimisation of human error.
Current sensors

Low-power current sensors (SSO and SSD series)

Current: 40 - 1000 A | Type: Toroidal / Split core | Output: 225 mV | Standard: IEC 61869-10

Passive devices designed for medium-voltage measurement and protection systems.

Operating principle of current sensors

Current sensors are passive devices whose operation is based on the principle of a toroidal-core transformer or a split-core transformer with an integrated precision shunt. They are designed for medium-voltage measurement and protection systems in accordance with the IEC 61869-10 standard. Current sensors provide a voltage output proportional to the primary current. All current sensors are type- and routine-tested in accordance with the IEC 61869-10 standard.

Current sensor operating principle
Electrical wiring diagram (S1 red / S2 black)
Non-split current sensor
SSOxxx-xx
SSOxxx-xx

Mounted over the T-connector bushing-type insulator. The internal holders are elastic so they can adapt to any outer diameter of the T-connector.

Split-core current sensor
SSDxxx-xx
SSDxxx-xx

Mounted around the cable. The flexible clamp system enables simple and reliable installation on any cable up to 55mm in diameter.

Technical data for SSOxxx-xx and SSDxxx-xx:
TECHNICAL DATAVALUES AND CHARACTERISTICS
Isolation level0,72 / 3kV / -
Nominal primary current40-1000A; Extension 200%; or customer defined
Secondary output225mV acc. IEC; or customer defined
Nominal frequency50Hz
Thermic short current25kA 3s
Accuracy classes SSOxxx-xx0,2S / 0,2 / 0,5S / 0,5 / 1 / 3 & max. 5P20
Accuracy classes SSDxxx-xx0,5S / 0,5 / 1 / 3 & max. 5P20
StandardIEC60044-8 / IEC61869-11 (future)
Input impedances of measurement devices20kΩ¹; capacitance not relevant
Connection2pole, open wires inclusive screen
Temperature rangeOperating and storage: -40 to +80°C

¹ The input impedance of the measurement device influences the accuracy of the entire system. The higher the impedance - the better. With 20kΩ an additional error of 0.02% in amplitude error will be generated.