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How can a CT generate a lethal high voltage when not connected to burden resistance?
Wiki User
∙ 15y ago
A CT is a current transformer. The key word is transformer. Any transformer has a turns ratio. Take a 480V to 120V transformer for instance. The ratio is 4:1, that is for every turn of wire on the secondary winding, there are four windings on the primary. So, if there are 200 turns on the secondary, there are 800 on the primary. We can reverse that same transformer to make it a step-up transformer. Now we would say it has a turns ratio of 1:4. Lets take that transformer and modify it (in our imagination) to see what will happen. Every time, we will apply 120V to the primary, and measure the output voltage on the 800 turn secondary. We will change the number of turns on the primary each time. First, the original ratio: 200T (turns) : 800T (turns) = 1:4 (ratio) = 120V (input) : 480V (output) OK, that seems pretty obvious. But now lets start removing turns from the primary. Watch the trend of secondary voltage: 100T : 800T = 1:8 = 120 : 960 50T : 800T = 1:16 = 120 : 1920 25T : 800T = 1:32 = 120 : 3840 5T : 800T = 1:160 = 120 : 19200 2T : 800T = 1: 400 = 120 : 48000 Wow, the output voltage for our transformer with a two-turn primary is a whopping 48000 volts! Is it possible to have a transformer with a one-turn primary? That is exactly what a current transformer is! the bus wire that passes through the CT forms a one-turn primary, and the many-turn CT coil forms the secondary. If the secondary is shorted, the secondary reflects the primary current divided by the number of secondary turns. But if the CT is open, the current ratio is now 1:zero (no load), the voltage ratio is the inverse - 1:infinity. So, if the CT is not shorted with a very low resistance, it tries to be a voltage transformer with an extremely high secondary voltage. In practice, the voltage is limited by coronal discharge and/or insulation breakdown of the windings. But it can produce some pretty good fireworks and smoke while it is happening! I once observed a facility electrician at one of my customers industrial plants accidentally cut through CT wiring while it was energized. He thought the conduit was abandoned and he was removing it. That CT wiring easily produced an impressive 2-inch continuous arc. This is why CT's are wired through special terminal blocks that short out the CT when the disconnect is pulled. You should never try to work CT wiring hot, and not under any circumstances if you are not trained and qualified!
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∙ 15y ago
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What is CT burden in electrical engineering?
CT (current transformer) Burden is the maximum load that the CT secondary can drive (e.g. meters or protection relays) Why do you care about burden when ordering current transformers? The burden is the termination impedance of the measuring instrument. The measuring instrument can be an analog or digital energy meter, a data logger or a recorder. All instruments that use a current transformer to measure line current must terminate the CT with a resistance (impedance in same case which means there is some inductance involved). A CT supplier should be given the burden when ordering. Technically, the total burden for a CT is the internal winding resistance, the connecting lead resistance and the measuring instrument input resistance. Some Ct's are installed a long distance from the measuring instrument and the connecting leads can have a resistance as much as 0.002 ohms per foot (both ways because the current makes a complete round trip). This resistance will cause an error if not taken into account.
Why PT Burden is always higher than CT Burden?
PTs are high voltage, low current transformers and so need a relatively high impedance (or burden) load to function accurately. CTs are high current, low voltage (if not open circuited!) transformers and so need a very low impedance load to function accurately.
What is a c200 current transformer?
This is a class of current transformers, and is a fairly low class. This has to do with what kind of burden can be placed on the secondary. A general rule is a C200 current transformer can supply ~200 volts at full ratio to its' secondary. If the burden (the CT resistance + cable resistance + relay or instrument resistance) times the maximum expected current is higher than 200 volts, the CT is likely to saturate. During multiple fault events, a CT may keep some magnetizing current causing CT saturation to be higher on a reclose event. Typically CT's are sized and their ratios are chosen to minimize saturation when feasible.
What are the three basic parts of transformer?
core - air, iron, ferrite, etc.windings - one or more coils wound around the core.bobbin - insulating structure used to hold the windings. optional, not needed in every type of transformer. in some types of transformer the windings are prewound on the bobbin then the core is inserted into the bobbin.
Why current transformers secondary is always short circuited?
Unless a burden (i.e. meters, relays, etc.) is connected to the CT, current transformers should always be shorted across the secondary terminals. The reason is very high voltages will be induced at the terminals. Think of the CT as a transformer, with a 1 turn primary and many turns on the secondary. When current is flowing through the primary, the resulting voltage induced in the secondary can be quite high, on the order of kilovolts. When a CT fails under open circuit conditions, the cause of failure is insulation breakdown, either at the shorting terminal strip, or at the feedthrough (in the case of oil filled apparatus), because the distances between terminals are not sufficient for the voltages present.
Why metrosil is provided in differential protection?
under internal fault conditions , CT primary current produces steep insrease in voltage across the relay. This will exceed the burden 3kv normally designed as per. To get protected from the voltage, metrosil is connected across the relay , to drop the voltage.
What is mean by burden of the current transformers?
The burden on a CT is the sum total impedance connected to the secondary - including the CT secondary winding coil resistance, lead resistance, and any loads attached (relays, meters, etc.).If this is for power calculations and CT saturation in three phase systems, remember you must multiply the lead resistance by two to get the total path resistance.Another AnswerWe use the word, 'burden', to describe the load supplied by the instrument transformer's secondary. This is normally a measuring instrument or protection relay. We use the word 'burden' to distinguish it from the 'load' supplied by the circuit to which the primary winding is connected.
How do you calculate burden resistor fo current transformer?
The method I have seen employed is to estimate the maximum current that will flow in the secondary side of the CT due to fault currents on the primary side, and calculate the total load (resistance) on the secondary side, including the CT resistance, cable resistance (2 way for ground faults, 1 way for three phase and L-L), resistance of any meters/relays connected to the CT. Once this is known, the voltage rise in the CT can be determined to see if the CT is likely to saturate.Alternately, you can determine the VA burden by the above, VA = V*I = (I*R)*I, using Ohm's law.The above answers the question of how to calculate the VA burdon on a current transformer. The VA burden of a CT is determined by the resistivity of the CT, multiplied by the current squared through the secondary. In general, this will be provided by the manufacturer in the form of a CT burden characteristic, developed through testing of the CT in question, not through calculations.
What is VA burden in electrical engineering?
The term 'burden' is used to describe the secondary load applied to an instrument transformer; the reason for this is so that it is not confused with the 'load' supplied by the high-voltage circuit to which the transformer's primary is connected. A burden is either a measuring instruments (ammeter,voltmeter, wattmeter, etc.) or protective relay, depending on the function of the instrument transformer. 'V.A' is the symbol for 'volt ampere', which defines the 'power' rating of the instrument transformer.
What is CT burden in electrical engineering?
CT (current transformer) Burden is the maximum load that the CT secondary can drive (e.g. meters or protection relays) Why do you care about burden when ordering current transformers? The burden is the termination impedance of the measuring instrument. The measuring instrument can be an analog or digital energy meter, a data logger or a recorder. All instruments that use a current transformer to measure line current must terminate the CT with a resistance (impedance in same case which means there is some inductance involved). A CT supplier should be given the burden when ordering. Technically, the total burden for a CT is the internal winding resistance, the connecting lead resistance and the measuring instrument input resistance. Some Ct's are installed a long distance from the measuring instrument and the connecting leads can have a resistance as much as 0.002 ohms per foot (both ways because the current makes a complete round trip). This resistance will cause an error if not taken into account.
What was the white mans burden during the Spanish American War?
A lack of resistance to tropical diseases.
Why PT Burden is always higher than CT Burden?
PTs are high voltage, low current transformers and so need a relatively high impedance (or burden) load to function accurately. CTs are high current, low voltage (if not open circuited!) transformers and so need a very low impedance load to function accurately.
Why secondary windings of current transformers are shorted?
Current transformers (CTs) -together with voltage (or potential) transformers- are 'instrument transformers'. CTs are used to enable large currents flowing in high-voltage systems to be measured, while electrically-isolating the ammeter (termed a 'burden') from the h.v. system. They are also used to monitor h.v. currents for high-voltage protection systems. Before disconnecting its burden, the secondary of a CT must be short-circuited, and remain short-circuited until after the burden has been replaced. This is because a dangerously-high voltage will appear across the open terminals of any energised CT.
Will superconductivity make ohms law obsolete?
Most certainly not, resistance plays important role in electronic circuits, it is not just a burden.
What is the function of burden resistors?
To load a circuit to a specific resistance value. This is useful in impedance matching networks, power transmission, telecommunications, etc.
How do you make current transformers to saturate?
Over voltage, severe under frequency, or add too large of a secondary burden (the last is what typically happens).
What happend if secondry is kept open in Current transformer?
A CT attempts to push a specific ratio of primary current through the secondary. If the secondary burden is a very high resistance (as in an open circuit), the voltage developped across the secondary will build up until it reaches the theoretical limit of the primary voltage. Before it gets to this voltage, it will arc across the air gap in the secondary open (often between two terminal blocks). As an old tech once told me, you only open the secondary of a CT once...the arcs can be fairly spectacular.
