1 Copyright 2003 Kilowatt Classroom, LLC. CT1 Current Transformers Ratio / Polarity / Types Application Current Transformers (CT’s) are instrument transformers that are used to supply a reduced value of current to me- ters, protective relays, and other instruments. CT’s provide isolation from the high voltage primary, permit - grounding of the secondary for safety, and step down the magnitude of the measured current to a value that can be Transformers safely handled by the instruments. Ratio scale current - The most common CT secondary full - load current is 5 amps which matches the standard 5 amp full rating of switchboard indicating devices, power metering equipment, and protective relays. CT’s with a 1 amp load value and matching instruments with a 1 amp full - full range value are also available. Many new protective - relays are programmable for either value. CT ratios are expressed as a ratio of the rated primary current to the rated secondary current. For example, a 300:5 CT will produce 5 amps of secondary current when 300 amps flows through the primary. As the primary current changes the secondary current will vary accordingly. With 150 amps through the 300 amp rated primary, the secondary current will be 2.5 amps ( 150 : 300 = 2.5 : 5 ). When the rated primary amps is ex- ceeded, which is usually the case when a fault occurs on the system, the amount of secondary current will increase but, depending on the magnetic saturation in the CT, the output may not be exactly proportional. Polarity All current transformers are subtractive polarity. Polarity refers to the instantaneous direction of the primary cur- rent with respect to the secondary current and is determined by the way the transformer leads are brought out of same the case. On subtractive polarity transformers the H1 primary lead and the X1 secondary lead will be on the - side bushings). See the article Understanding Trans- side of the transformer (the left side when facing the low former Polarity in the Archive Catalog of the Kilowatt Classroom Web Site for more information on polarity. On the Window or Donut - type CT’s, such as pictured on - Type CT Donut or Window the left, the conductor, bus bar, or bushing which passes through the center of the transformer constitutes one primary type units with low primary current rat- turn. On Window - ings, where the primary conductor size is small, the ratio of White Lead is the transformer can be changed by taking multiple wraps of Secondary Polarity the primary conductor through the window. If, for example, a window CT has a ratio of 100:5, placing two primary con- ductor wraps (two primary turns) through the window will Primary Polarity change the ratio to 50:5. Some types of equipment employ Mark this method to calibrate the equipment or to permit a single ratio CT to be utilized for several different ampacities of Startco Engineering Ltd Photo equipment. Bar - Type CT - Type CT’s have primary connections that Bar Primary - up directly to the substation bus bars. bolt Polarity Mark rated versions of this equipment are Outdoor - used in pole - mounted primary metering H1 Terminal installations. Sheet 1 This type of CT often has compensating X1 Terminal windings which improve the accuracy across H2 Terminal - load range of the transformer. X1 Polarity Mark X2 Terminal the full Kilowatt Classroom Photo
2 CT2 Copyright 2003 Kilowatt Classroom, LLC. Current Transformers Symbols Current Flow Analysis In analyzing the current flow in a system utilizing CT’s the following observation can be made: polarity H2 lead, current will - ) to the non + When current flows in the CT primary from the H1 lead (polarity Transformers ) lead, through the burden (load), and return to the secondary X2 + be forced out the secondary X1 (polarity - polarity lead. The next half cycle the current will reverse, but for the purpose of analysis and for con- - non structing phasor diagrams, only the above indicated one half cycle is analyzed. - Electrical Drawing Conventions The polarity marking on electrical drawings may be made in several different ways. The three most common out schematic conventions are shown below. The drawing symbol for meters and relays installed in a draw - case that automatically short the CT secondary is shown in the drawing at the lower right. CT One Line Diagram Symbol - Secondary Winding - One Turn Primary Secondary Conductors to Relays or Instruments Polarity Marks Shown as Dots Polarity Marks Shown as Squares Source Source Current Elements in Meters or Relays Current Elements in Meters or Relays H1 X1 X2 H2 Secondary Safety Ground Secondary Safety Ground Load Load Out Meter or Relay Case Polarity Marks Shown with Slash - Draw out case with CT Shorting Source Source Symbol for draw - Current Elements in Meters or Relays Secondary Safety Ground Secondary Safety Ground Load Load Sheet 2
3 Copyright 2003 Kilowatt Classroom, LLC. CT3 Current Transformers Shorting Methods Caution: The secondary of a Current Transformer must always have a burden (load) connected; an open - circuited secondary can result in the development of a dangerously - high secondary voltage. Energized but circuited. unused CT’s must be kept short - Transformers Startco MPU 16 - Out Instrument Cases - Draw Motor Protective Relay - Meters and protective relays are available in draw out cases that auto- Retrofit installation in - matically short circuit the CT when the instrument is removed for test- out case. - draw - circuit contacts will be opened. ing and calibration. Voltage and trip - out case on Sheet 2. See symbol for draw Startco Engineering Ltd Photo CT Shorting Terminal Strips The illustration below shows the termination of a multi - ratio CT on a special shorting terminal strip. Insertion of shorting screw through shorting bar ties isolated terminal strip points together. Any shorted winding effectively shorts the entire CT. Shorting Bar X1 X2 Shorting screw in any other locations shorts CT. X3 Relay connected to CT tap which Ratio CT Multi - provides the desired ratio. X4 Lead X3 becomes polarity. X5 Shorting screw ties X5 CT lead to ground. Safety Ground Shorting screw ties shorting bar to ground. Spare Shorting Screw Stored for future shorting requirement. Terminal Strip Mounting Hole Auxiliary Current Transformers Startco Engineering Ltd has developed a system utilizing an Auxiliary CT wire protective relays from - (pictured at left) which permits safe removal of hard the system. The current transformers are permanently wired to the input of the Auxiliary CT and the output of the Auxiliary unit is wired to the protective relay current inputs. This arrangements keeps a burden on the CT secondary circuits and permits the protective relays to be removed for repair, calibration, or replace- ment. Sheet 3 The Auxiliary CT is installed as close as possible to the current transformers. Startco Engineering Ltd Photo This reduces the CT burden by reducing the length of the CT secondary current conductors.
4 Copyright 2003 Kilowatt Classroom, LLC. CT4 Current Transformers CT Accuracy Classes ANSI Accuracy Classes Current Transformers are defined by Accuracy Classes depending on the application. • are used where a high degree of accuracy is required from low - load values up to Metering Accuracy CT’s Transformers - load of a system. An example of this application would be the current transformers utilized by utility full companies for large capacity revenue billing. Relaying Accuracy CT’s are used for supplying current to protective relays. In this application, the relays do • not normally operate in the normal load range, but they must perform with a reasonable degree of accuracy at - current levels which may reach twenty times the full very high overload and fault load amplitude. - Notes: - 1978. 1) Instrument Transformers (PT’s & CT’s) are defined in ANSI C57.13 2) The load on an instrument transformer (PT or CT) is referred to as the “burden”. Metering Accuracy Classifications Available in Maximum Ratio Error Classes of: 0.3% , + 0.6% , + 1.2%, + 2.4%. + For Burdens (Loads) of: 0.1, 0.2, 0.5, 0.9, 1.8 ohms. Which equals 2.5, 5.0, 12, 22 - 1/2, 45 volt - amperes ( va ). 2 xR, use 5 amp secondary for I, and burden value for R. Since Power = I Typical Number B 0.3 0.2 + % Burden Ohms (Burden) Max Ratio Error Relaying Accuracy CT’s - typical of donut units Class C (C for Calculated) is low leakage reactance type Formerly Class L ( L for Low - Leakage). Class T (T for Tested) is high leakage reactance type - - type units - Formerly Class H ( H for High typical of bar Leakage). Typical Number 10 C 800 10% Max Ratio Error at 20 times Rated Current Low Leakage Unit Max secondary voltage developed at 20 times rated current without exceeding the +10% ratio error. Sheet 4 Available secondary voltages: 10, 50, 100, 200, 400, 800. Will support burdens of: 0.1, 0.2, 1.0, 2.0, 4.0, 8.0 ohms.
5 Copyright 2003 Kilowatt Classroom, LLC. CT5 Current Transformers Multi Ratio CT’s - - Ratio Bushing - Type CT Westinghouse Multi Installation Considerations For External Installation This bushing CT is designed for use on existing circuit breakers and power transformers and is Transformers installed externally (See Sheet 11). It is housed in an aluminum case which provides electrostatic shielding. Care must be taken with the installation to insure that the mounting clamp bolts do not contact the case resulting in a one - turn primary short circuit. Also because the case is metal and is installed externally it can decrease the bushing strike dis- tance. The circuit breaker or transformer manu- facturer should be consulted to verify acceptabil- ity of the installation. Secondary Turns Diagram 600/5 CT Diagram at the left shows the number of turns for each winding on a 600/5 multi - ratio CT. The full - number of 120 turns, from X1 X5, is used to obtain the 600/5 ratio. (Since there is one primary Polarity Mark turn, 120:1= 600:5). Another example: X1 - X2 has 20 turns, so 20:1= 100:5. Any combination of adjacent turns can be utilized. The lowest lead number of the combination will be the polarity lead. See the CT shorting strip diagram on Sheet 7 for a typical termination arrangement. Selection Guide Shows ANSI Accuracy Classes, Dimensions, and Mfg Style Number Sheet 5
6 CT6 Copyright 2003 Kilowatt Classroom, LLC. Current Transformers Typical Excitation Characteristics Excitation Curves The family of curves below describe the excitation characteristics for the 600/5 multi - ratio bushing current transformer shown on the previous sheet. This is a plot of the CT secondary current against secondary voltage. Transformers These curves illustrate how high the secondary voltage will in rise in order to force the rated secondary current through the burden. The effect of magnetic saturation is also illustrated by the knee of the curve. Next month’s article will show how to perform a CT Saturation Curve Test. Sheet 6
7 Copyright 2003 Kilowatt Classroom, LLC. CT7 Current Transformers Typical Installations - Side CT’s Line - Side CT’s Load Bushing CT’s Bushing CT’s may be mounted externally or internally Transformers on circuit breakers and transformers. - Multi ratio units are often used. Where single - ratio CT’s are employed, the CT primary rating may match either the full load ampacity of the - circuit breaker or of the feeder. In the latter case, up- grading the feeder ampacity will require replacement of the CT’s. in termination strip The CT secondary leads land on a the breaker or transformer control cubicle. Siemens SF6 Circuit Breaker With externally mounted bushing CT’s Kilowatt Classroom Photo External Portion of Bushing Note “Petticoats” which shed moisture and increase creepage distance. - Multi Ratio CT Bushing Internal Porcelain This section is submerged in oil. Westinghouse Oil - Filled Vacuum Recloser With tank dropped showing internally mounted CT’s on line - side bushings. In this configu- ration the protective relays fed by the CT’s are said to “look through” the breaker. Kilowatt Classroom Photo Control Circuit Contact Block Mates with breaker control block. Bus Bars Breaker Bus - Side Stabs Phase - With single donut CT on B Side Stabs - Breaker Machine With donut CT on A and C Phases Sheet 7 Clad Switchgear - General Electric 480 Volt Metal Cubicle with generator breaker racked out. Kilowatt Classroom Photo
8 CT8 Copyright 2003 Kilowatt Classroom, LLC. Current Transformers Types Hall Effect CT’s Hall - Effect CT’s are not current transformers in the conventional sense, rather they which can be applied in the measurement of either are electronic circuit transducers TPI AC or DC circuit currents. These devices have many applications; they are com- AC/DC Current Probe Transformers monly used in Variable Frequency Drives (VFD’s) to measure the DC link current and are also employed in AC/DC instrument probes such as the TPI - A254 Current Adapter shown at the right. balance type amplifier circuit. The magnetic - Effect devices contain an null Hall - flux (field) produced by the current flow through the primary (usually one turn) - which is balanced by an equal and opposite output from results in an output voltage the control or measuring circuit. Because the circuit is an amplifier, it requires ex- ternal operating power which is supplied by the control circuit power supply, or in the case of a portable instrument probe, batteries are used. Probe employs the Hall - As with conventional current transformers, Hall Effect devices provide isolation - Effect principle and produces from the high voltage circuit and reduce the measured current to a proportional a millivoltage output that is value which can be safely measured by the control or instrument circuit. applied to a TPI Digital Multimeter. The meter inter- Effect devices do not pose the same danger as conventional bus - Hall - bar or donut - prets the probe voltage as a type CT’s with regard to an open circuited secondary. (Note: some instrument cur- current value. Batteries are rent probes are conventional CT’s; these usually have a burden resistor within the used in the probe to power probe or may be protected from an open circuit with back - to - back zener diodes.) the amplifier circuitry. be However, good practice dictates that instrument current probes should not disconnected from the meter while current is passing through the device primary. Test Products International Photo Effect CT Hall - Used on the DC Link of a 5000 H.P. VFD. Control Circuit Connections Heat Sink Tubular High Voltage Bus Bar - Effect CT Passes through Hall Power Transistors Kilowatt Classroom Photo Typical VFD Block Diagram Showing Hall Effect CT (HCT) Connections - Phase AC Motor 3 - DC Link Single or Three - Phase Rectifier Inverter AC Input HCT HCT Output Signal Sheet 8 HCT Power Speed Frequency Control Signal Regulator Reference Motor Voltage Feedback