For example, one secondary may be used for branch overcurrent protection, while a metal oxide gapless lightning arrester may be used in a bus differential protective scheme, and a third winding used for power and current measurement. Current transformers, especially those intended for high voltage substation service, may have multiple taps on their secondary windings, providing several ratios in the same device.
Current transformers used to sense ground fault (zero sequence) currents, such as in a three-phase installation, may have three primary conductors passed through the ring. Bar-type current transformers have terminals for source and load connections of the primary circuit, and the body of the current transformer provides insulation between the primary circuit and ground. This problem can be reduced by using thicker cables and CTs with lower secondary currents (1 A), both of which will produce less voltage drop between the CT and its metering devices.
Items that contribute to the burden of a current measurement circuit are switch-blocks, meters and intermediate conductors The most common cause of excess burden impedance is the conductor between the meter and the CT. When substation meters are located far from the meter cabinets, the excessive length of cable creates a large resistance. A CT with a rating of 2.5L400 specifies with an output from its secondary winding of twenty times its rated secondary current (usually 5 A × 20 = 100 A) and 400 V (IZ drop) its output accuracy will be within 2.5 percent. Accuracy classes for various types of measurement and at standard loads in the secondary circuit (burdens) are defined in IEC 61869-1 as classes 0.1, 0.2s, 0.2, 0.5, 0.5s, 1 and 3. The class designation is an approximate measure of the CT’s accuracy.
The secondary of a current transformer should not be disconnected from its burden while current is in the primary, as the secondary will attempt to continue driving current into an effective infinite impedance up to its insulation break-down voltage and thus compromise operator safety. Split-core current transformers are typically used in low current measuring instruments, often portable, battery-operated, and hand-held (see illustration lower right). A current transformer has a primary winding, a core and a secondary winding, although some transformers, including current transformers, use an air core.
Continental Control Systems sells several different series of current transformers (CTs) in various window sizes, current ranges, and accuracy grades. Outdoor Oil Cooled Instrument Transformers (CT/PT) up to 500kV Voltage Class. High voltage circuit breakers-These breakers are rated for use at voltages greater than 2 kV. High voltage circuit breakers are further subdivided into transmission class breakers.
ZW7 outdoor hv 35kv vacuum circuit breaker TENPRO Dry-Type Transformer Manufacturers: $42.99. 10KV outdoor current transformer LJW-10 type high voltage current transformer. Founded in 2001 located at Pulandian District of Dalian City, is a professional manufacturer of 0.5kV-330kV current transformer and voltage transformer involved cast resin, oil immersed, inductive, capacitor, gas SF6, organic silicone rubber dry type and so on. The company covers an area of 150,000㎡ more than 400 employees, achieved more than USD 40 millions for annual sales.