How Do I Sign Connecticut Banking PPT

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[Music] hey Santino could you go get me a CT from the workroom yeah sure do you want a big one or a little one well we have to monitor the main breaker current in the switchboard and it has to be highly accurate for metering and protection then go around for different 500 MCM cables so so a big one as long as it fits around the wires it'll work right well sometimes that's true and there's actually a lot of different things to know about CTS because you have different ratings different classes different high frequency ones for measuring different transients hmm I didn't know that now I know you've been working on some new demos with CTS hey is there anything else you can tell me about current transformers yeah first let's back up a little bit and define what a CT or current transformer is a CT is a current measuring device that is used to safely reproduce a low-level current that accurately represents a higher current level for the purpose of metering and protection let's start with a simple explanation that describes the most common ways of calculating and measuring current using current transformers by applying Maxwell's equations specifically amperes law we can say that if a magnetic field is integrated around a closed loop of wire the value of that integral is equal to the net current enclosed by the loop CTS our closed loop instruments consisting of a magnetic core and a secondary winding around the core the primary winding of the CT the main loop has the wire with the current we wish to measure pass through the center of the core the primary winding that carries the main current is said to have a single loop for winding the wire produces the magnetic field that drives the current on the secondary winding which is used as the output of the CT the current on the secondary winding is proportional to the current flowing through the center of the core typically the secondary rating is 5 amps or 1 amp for example with a 1000 v rating or a turns ratio of 201 1 1,000 amps flows on the primary circuit 5 amps would flow on the secondary winding current transformers are primarily used for metering and protection applications and come in various sizes shapes and ratings CTS can be solid cord split cork or clamp on styles for both low voltage and medium voltage applications solid core CTS are generally more permanent are most commonly found for metering and protection in switchboards panel boards and switch gear split core and clamp on CTS are generally used in more temporary applications such as power quality instrumentation for permanent applications of protection and metering CTS can be found anywhere from generators to transformers to connected loads or anywhere we want to monitor current flowing in the system for example utilities use cts at their customers incoming service to monitor the current and power usage for billing purposes these CTS must be extremely accurate and are considered revenue grade since they are used for billing permanent CTS are also used to monitor power and power factor so that real and reactive power can be optimized on the protection side CTS can be used in combination with trip units for low-voltage circuit breakers and relays for medium voltage breakers to trip the breaker when there are overloads or faults on the system many circuit breakers have built-in CTS used to monitor current as a point of clarification a lot of people call CT sensors when they are mounted inside switch gear or circuit breakers but the functionality and purpose is the same to get current measurements to a useful level a single CT is generally needed for each phase and neutral to monitor current but for ground fault protection a special type of CT is used all of the phases and neutral conductors go through the ground fault CT and if any residual current exists in other words current comes in on one of the phases but doesn't return on the other phases or neutral then there is a ground phone in your house this is triggered by a five milliamp level and industrial plants this may be 30 milliamps or even a couple hundred amps ground fault protection is generally for personnel safety homes and equipment protection industrial applications in medium voltage applications a common misconception is that the insulation of the CT needs to be rated for the line voltage that the CT is measured for example thirteen point eight kV however this is not the case because the CT is installed around an already insulated and typically shielded conductor most CTS and secondary winding insulation is rated at 600 volts in medium voltage switchgear the CTS are physically mounted around an insulating material or use physical separation to insulate the CT from the energized bus with an air gap for all protection applications CTS must be rated to accurately measure the high currents that are possible during fault conditions usually 20 times full load amps so that the breakers can trip in the proper sequence without saturating which yields an incorrect result CTS come in many different ratios such as a hundred to five three hundred to five five thousand to five sixty to one and some have multiple taps where the ratio can be selected in the field or for a specific application let's take for example three hundred to five amps meaning that a wire carrying three hundred amps AC flowing through the core will produce five amps on the secondary winding most CTS have a 5 amp output but others have one amp so in interfacing with a meter or circuit breaker the correct multiplier must be used that will convert the five amp or one amp to the actual measured value but even though the current steps down 300 amps to five amps the voltage will step up on the secondary winding an open circuit on the CT secondary winding can have dangerously high voltage of thousands of volts when CTS aren't used they should always have their secondary winding shorted for safety by using a shorting block or a temporary jumper all cts output a current that is then used for metering but temporary cts like those used in Flook and other power quality instruments add a terminating resistor on the secondary winding so that the output a voltage and can be open circuited when connecting meter with this resistor these CTS typically have a rating of 10 millivolts per amp or 100 millivolts per in the lower range ratio is usually used for secondary measurements in other words clamping on to the 5 amp secondary of another CT sometimes the current value is too small a CT may not be accurate at less than 10 percent of its full load ratings so one way to increase the amount of current flowing through the core especially for temporary metering is to loop the primary wire several times through the core for example for a 500 to 5 amp CT or 100 to one ratio looping the wire 5 times through the core will make the CT ratio a hundred to five amps so if 20 amps of current is flowing on the primary circuit instead of 0.2 amps 1 amp of current will flow on the secondary making the measurement more accurate see T's come in a variety of accuracy classes for example the ratio of primary to secondary current error of a class 1 CT is 1 percent at rated current 0.5 rated CT is 0.5 percent or less the accuracy of these devices is very important for protective relaying and metering they have specific requirements for overload currents and excess of the normal rating to ensure accurate performance of the relays during system faults a current transformer can have a range of ratings from C 10 to C 800 which is related to the secondary winding output voltage capabilities and VA rating of the CT in simple terms this means that a CT with a higher rating can sustain more burden on the secondary without saturating for example a c 400 current transformer can have a burden that would drive a maximum of 400 volts based on these tables a c 400 would have an error of no larger than 10% at 20 times normal secondary current with a secondary burden of 4 ohms manufacturers will often offer a graph of the excitation performance of a particular CT the graph allows you to determine the performance of the CT over the entire range of secondary current and ensure that the CT will function as required this is an example of the typical CT saturation curve that shows the linear relationship and saturation point of a CT using a special tester like this one current transformers can be tested to verify CT ratio and saturation curves generally speaking CTS are fairly linear within 10 to 90 percent of their ratings but may change significantly outside of that range when operating outside the ratings of a CT the magnitude of the secondary voltage above which the output current ceases the linearly follow the input current is called the knee point voltage at this point the CT has been saturated and it's measurements can no longer be relied upon for metering for protection CTS this is especially important because these devices can see 20 to 30 times rated current during faults in addition to CT saturation the maximum frequency range that a CT can measure is typically 3 to 5 kilohertz for most applications this is OK since the harmonics and the power system are typically within this range however for high frequency applications like transients and the hundreds of kilohertz or megahertz range special highly accurate CTS like these Pearson CTS can be used cts have polarity dots arrows or markings indicating current flow direction the primary wire enters the CT on the side indicated by h1 or p1 and the load side is indicated by h2 or p2 when a CT has an arrow indicating direction the arrow points towards the load the current on the secondary comes out of x1 and returns on x2 a common problem when using cts is after installation a negative power is measured this can happen if the primary orientation or the secondary wiring is backwards for a setting is incorrect in the meter additionally CTS can also experience a phase shift between primary and secondary windings which can range between a few tenths of a degree up to 6 degrees that phase difference can make power measurements incorrect even to the point that power may look like it is incorrectly flowing out of the load like all transformers CTS have volt ampere or capacity ratings CTS have limits the number of devices and lengths of wire that can be connected to its secondary side which is called the burden items that contribute to burden are relays meters and wiring the best way to reduce burden is to keep the conductors between the CT and the metering devices as short as possible or increase the wire size to reduce resistance the burden for today's modern electronic relaying and metering is very low compared to older equipment but care must be taken to ensure CTS are not overloaded or inaccurate current measurements or protection will result in addition the CTS there are a few other ways of measuring current including current shunts Hall effect sensors and Rogowski coils one of the easiest ways of measuring current is with the current shunt or shut resistor by using Ohm's law unknown value sense resistor in combination with a differential voltmeter can be used to measure the current passing through the resistor because the resistor is small at value the voltage drop across it is also small but the value of current can actually be quite large this method can be used to measure both AC and DC current however as the current that is being measured increases implementing this method requires the use of large high current rated and low ohmic value resistors that requires making a break in the circuit and adds an additional electrical component to the system just the shut resistors can measure both AC and DC current Hall effect sensors can also measure AC or DC current Hall effect sensors require an additional DC supply to produce a constant current flow in a semiconductor creating a constant magnetic field when another magnetic field interacts with the Hall effect field it produces a change in current of the semiconductor and the sensor detects it the strength of the detected field is proportional to the amount of current Hall effect sensors are used extensively in UPS solar and micro grid applications for monitoring DC currents a disadvantage of Hall effect sensors is that they require a DC voltage source in the form of a battery or fixed power supply drift can also be a potential problem requiring to be compensated periodically through a degaussing or calibration process many multimeters utilize a combination of CTS and Hall effect sensors in order to get an accurate current measurement finally Rogowski coils are an open-loop current transformer using an open-air core with a very low inductance that is capable of measuring very fast transient currents and AC currents due to the low inductance the signals are a gauss ki coil generate are very small and require amplification and a DC power supply like a Hall effect sensor Rogowski coils are used for current monitoring and precision welding systems arc furnaces and other electronic equipment where high frequency measurements are required this thing is heavy man I didn't realize there's so many different ways to measure current yep and here at the Power Systems Experience Center you can see all these ways and more contact us or your local Eaton representative to schedule a visit to the Power Systems Experience Center today you [Music]