power systems loss

QUICK CURRENT TRANSFORMER FACTS Current Transformer Facts 1) Bushing CTs tend to be accurate more on high currents (due to large core and less saturation) than other types. 2) At low currents, BCT’s are less accurate due to their larger exciting currents. 3) Rarely, if ever, is it necessary to determine the phase-angle error. 4) Accuracy calculations need to be made only for three-phase and single-phase to ground faults. 5) CT burden decreases as secondary current increases, because of saturation in the magnetic circuits of relays and other devices. At high saturation, the impedance approaches the dc resistance. 6) It is usually sufficiently accurate to add series burden impedance arithmetically. 7) The reactance of a tapped coil varies as the square of the coil turns, and the resistance varies approximately as the turns. 8) Impedance varies as the square of the pickup current. 9) Burden impedance are always connected in wye. 10) "Ratio correction

IEEE GUIDE FOR FIELD TESTING OF RELAYING CURRENT TRANSFORMER C57.13.1 A guide for field testing of relaying current transformer In the application of protective relays, the most widely used input quantity is current. A multiplicity of different protective relays either utilizes current directly, combines it with other currents as in differential schemes, or combines it with voltage to make impedance or power measurements. The source of relay input current is from current transformers which may be located on the bushings of power circuit breakers and power transformers, on the bus bars of metal clad switchgear, or installed as separate items of equipment located as required. Relaying accuracy classes have been established in ANSI/IEEE C57.13-1978, Requirements for Instrument Transformers, to specify the performance of relaying current transformers. During faults on the electric power system, relaying current transformers must operate at high overcurrent levels. ANSI classiÞcations,

VOLTAGE TRANSFORMERS ROUTINE TEST PROCEDURE C57.13.5 Test procedures for voltage transformers. Applied voltage test on the neutral terminal With the ground link temporarily removed from the neutral terminal of the transformer, a voltage of 19 kV (r.m.s.) shall be applied between the neutral terminal and the ground for 60 s. The gas pressure of the gas-filled transformer may be at any setting equal to or less than the pressure given in Clause 5 for the test. The transformer shall be considered as having met the requirements if no external and internal disruptive discharge or collapse of voltage is observed. Accuracy test The test shall be performed in accordance with 8.1 of IEEE Std C57.13-1993. The calibration of the test system given in 4.3 shall apply. The gas pressure of the gas-filled transformer may be at any setting for the test. The transformer shall be considered as having met the requirements if the performance is within the limits of the accuracy class. Excitation ch

CURRENT TRANSFORMERS ROUTINE TEST PROCEDURE C57.13.5 Test procedures for current transformers. Inter-turn overvoltage test The test shall be performed in accordance with one of the following procedures. If there is no agreement between the manufacturer and user, the choice of the procedure shall be left to the manufacturer. If secondary protective devices are provided, these devices shall be installed for the inter-turn overvoltage test. Procedure A With the secondary windings open-circuited (or connected to a high impedance device which reads the peak voltage), a substantially sinusoidal current shall be applied to the primary winding. The test current shall be increased until one of the following conditions is reached:    a) The r.m.s. value of the applied current is equal to the maximum primary current corresponding to the continuous thermal current rating factor (RF), or    b) The peak value of the secondary induced voltage is equal to:       1) 280 V for metering rated cur

STANDARD REQUIREMENTS IN DEALING WITH VOLTAGE TRANSFORMERS What is a Voltage Transformer with respect to C57.13? Terms in which ratings shall be expressed The ratings of a voltage transformer shall include:      a) Basic impulse insulation level in terms of full-wave test voltage      b) Rated primary voltage and ratio      c) Frequency (in Hertz)      d) Accuracy ratings      e) Thermal burden rating Standard burdens Standard burdens for voltage transformers for rating purposes are shown in table below. Assignment of accuracy ratings A voltage transformer shall be assigned an accuracy rating for each of the standard burdens for which it is rated. For example, an accuracy rating might be 0.3W and X, 0.6Y, 1.2Z. Accuracy classification for voltage transformers with two secondary windings or tappedsecondary windings - The burden on any two secondary terminals affects the accuracy on all other terminals. The burden stated in the accuracy ratings is the total burden on the

STANDARD REQUIREMENTS IN DEALING WITH CURRENT TRANSFORMERS What is a Current Transformer with respect to C57.13? Terms in which ratings shall be expressed The ratings of a current transformer shall include:     a) Basic impulse insulation level in terms of full-wave test voltage.     b) Nominal system voltage, or maximum system voltage.     c) Frequency (in Hertz)     d) Rated primary and secondary currents.     e) Accuracy classes at standard burdens.     f) Continuous thermal current rating factor based on 30 °C average ambient air temperature.     g) Short-time mechanical current rating and short-time thermal current rating. Standard burdens Standard burdens for current transformers with 5 A rated secondary current shall have resistance and inductance according to table below. Accuracy ratings for metering A current transformer for metering shall be given an accuracy rating for each standard burden for which it is rated. For example, the accuracy ratings assigned to

INSTRUMENT TRANSFORMER THEORETICAL DISCUSSIONS Basic theoretical concept of instrument transformers guide Instrument transformer is the standardized term used for current and voltage transformer. Theoretical Reminder: Current Transformers Current transformers have a similar composition to "conventional" transformers. A magnetic circuit (generally made of an iron alloy) in the shape of a toroid is surrounded by N1 turns on the primary and N2 turns on the secondary. The primary can be reduced to a simple conductor passing through the toroid (n1 = 1) (see figure 6-2). General Application Rule Current transformers feed measuring, control and monitoring devices. Galvanic insulation electrically seperates the primary circuit from the secondary circuit. It allows earthing of the electrical measuring device and thus ensures safety of operating personnel. The current transformer is designed to give the secondary a current that is proportional to the primary current. The seondary

IMAGES OF VOLTAGE TRANSFORMERS INCLUDING TYPES AN DEFINITIONS Images and Illustration of a typical voltage transformer for medium and high voltage level. A voltage transformer or potential transformer is an instrument transformer intended to have its primary winding connected in shunt with a power supply circuit, the voltage of which is to be measured or controlled. A cascade-type voltage transformer is a single high-voltage line terminal voltage transformer with the primary winding distributed on several cores with the cores electromagnetically coupled by coupling windings and the secondary winding on the core at the neutral end of the highvoltage winding. Each core of this type of transformer is insulated from the other cores and is maintained at a fixed potential with respect to ground and the line-to-ground voltage. A double-secondary voltage transformer is one that has two secondary windings on the same magnetic circuit insulated from each other and the primary. Either or

IMAGES OF CURRENT TRANSFORMERS INCLUDING TYPES AN DEFINITIONS Images and Illustration of a typical current transformer for medium and high voltage level. A current transformer is an instrument transformer intended to have its primary winding connected in series with the conductor carrying the current to be measured or controlled. The ratio of primary to secondary current is roughly inversely proportional to the ratio of primary to secondary turns and is usually arranged to produce either five amperes or one ampere (IEC Standard) in the full tap of the secondary winding when rated current is flowing in the primary. Bar: A bar-type current transformer is one that has a fixed, insulated straight conductor in the form of a bar, rod, or tube that is a single primary turn passing through the magnetic circuit and that is assembled to the secondary, core, and winding. Bushing: A bushing-type current transformer is one that has a round core and a secondary winding insulated from an

INSTRUMENT TRANSFORMERS SERVICE CONDITIONS FOR OPERATION What are the conditions that are considered in choosing instrument transformers? The standard ratings of instrument transformers are based on operation at the thermal rating of the instrument transformer for defined ambient temperature conditions provided the altitude does not exceed 1000 meters (3300 feet). Instrument transformers may be used at higher ambient temperatures, at altitudes higher than 1000 meters, or for other unusual conditions if the effects on performance are considered. Consult the manufacturer for specific applications. Altitude: A higher standard BIL may be required at high altitudes in order to obtain the insulation required for the voltage used. The decreased air density at higher altitudes also affects heat dissipation and the permissible loading on instrument transformers. Current transformers may be operated at altitudes greater than 1000 meters if the current is reduced below rated current by 0.3 p

BASIC INSTRUMENT TRANSFORMER INFORMATIONS AND DISCUSSIONS TAKEN FROM STANDARD HANDBOOK FOR ELECTRICAL ENGINEERS How does instrument transformers being use in the power system? The discussion that follows is a short summary of information on instrument transformers as measurement elements. For more extensive information, consult American National Standard C57.13, Requirement for Instrument Transformers; American National Standards Institute; American National Standard C12, Code for Electricity Metering; Electrical Meterman’s Handbook, Edison Electric Institute; manufacturer’s literature; and textbooks on electrical measurements. AC range extension beyond the reasonable capability of indicating instruments is accomplished with instrument transformers, since the use of heavy-current shunts and high-voltage multipliers would be prohibitive both in cost and power consumption. Instrument transformers are also used to isolate instruments from power lines and to permit instrument circuits t

69KV CURRENT TRANSFORMER SAMPLE TECHNICAL SPECIFICATION Sample specification of a 69kv Current Transformer. Description: For Oil-Filled Type: The unit shall be hermetically sealed and of the minimum oil-filed type and compact design. All sealing shall be located below the oil level. The expansion room shall be of a gas cushion type filled with nitrogen. Oil level should be of the reflection type and without moving parts. Primary terminals shall be suitable for connection of copper or aluminum connectors. The external ferrous parts shall be of hair pin type insulation consisting of oil-impregnated paper and capacitor layers for voltage grading. It should be preferably provided with a capacitance voltage tap throughout thru an insulated, factory grounded, bushing for checking the condition of its primary insulation. It should have a high seismic withstand capability of 0.5G. The unit must be able to be tilted to 60 deg. C. For Gas Type: The primary and secondary winding of the SF6

69KV VOLTAGE TRANSFORMER SAMPLE TECHNICAL SPECIFICATION Sample specification of a 69kv Voltage Transformer. Description: The unit should be magnetic type and for connection between phase and earth in networks with isolated or earthed neutral. • Designed with low flux density in the core and can therefore be operated at 190% rated voltage for more than 8 hours • Fitted with a secondary measuring winding and a tertiary earth-fault winding • Hermetically sealed, which means no need for regular maintenance procedures • Primary terminals shall be suitable for connection of copper or aluminum connectors. • All external parts are hot-dip galvanized • High seismic withstand capability (0.5G) For Oil-Filled Type: The unit shall be hermetically sealed and of the minimum oil-filed type and compact design. All sealing shall be located below the oil level. The expansion room shall be of a gas cushion type filled with nitrogen. Oil level should be of the reflection type and without moving

Guide for instrument transformer standard referring to IEEE standards. Scope of the Standard This standard is intended for use as a basis for performance, interchangability, and safety of equipment covered, and to assist in the proper selection of such equipment. This standard covers certain electrical, dimensional, and mechanical characteristics, and takes into consideration certain safety features of current and inductively coupled voltage transformers of types generally used in the measurement of electricity and the control of equipment associated with the generation, transmission, and distribution of alternating current.

How Instrument transformer contributes to system losses? Like any other transformer in general, Instrument transformers are intended to convert current or voltage from the high level in the transmission and distribution systems to the low levels that can be used by low voltage metering devices. Three primary applications for which instrument transformers are usually known: metering (for energy billing and revenue purposes); protection control (for system protection and protective relaying function); and load survey (for economic management of industrial loads).