A small part of the primary current of a secondary transformer is used to excite the core. If the secondary of the transformer is open, the primary current remains the same because it is determined by the load on the power circuit and is all used to magnetize the transformer core. The core saturates, and the voltage induced on the secondary develops high voltage spikes due to the high rate of change of magnetic flux around the zero crossings of the primary current. The voltage peaks can be several kV high.
The secondary terminals should always be short-circuited before a meter or other load is removed from service. It is recommended that shorting blocks or knife switch shorting assemblies be used with current transformers.
I would just like to add a small comment to the previous excellent posting. DO NOT attempt to measure the secondary voltage of an open circuit CT with a standard voltmeter! As the previous posting indicated, the voltage waveform is highly spiked and will probably have a significantly lower RMS value than the peak value. It is the peak value that can damage the CT and possibly kill you, especially on large CTs. Unfortunately, most voltmeters measure RMS and not peak voltage.
Greg May
Two Sockets - Two Meters, Inc.
WEB: socket-two-me.com
You really don't need to know about saturation and magnetizing impedances to understand the concept. An ideal transformer model works nicely. An ideal transforemer changes voltage by the turns ratio, and current by the inverse of the turns ratio. An open secondary means no current in either the secondary or the primary. No current in the primary means an open circuit to the load, meaning full line to ground potential or higher is impressesed across the primary winding. The secondary voltage is the primary voltage stepped up by the CT ratio. High enough yet? The non-ideal aspects of the transformer keep the load connected and the voltage much lower. Still high enough to be dangerous.
The big difference between CTs and power transformers and VTs is in the connection. CT primaries are connected in series with the load, the others in parallel.
I'm not sure if I'm reading your post correctly or not, but to clarify, the open circuit voltage of a CT (or any secondary voltage of the CT) has abosolutely nothing to do with the line-to-ground voltage on the primary winding.
The open circuit voltage is a function of the primary current and the core characteristics (cross sectional area, number of secondary turns, core material).
A basic principle of any transformer is ampere-turn balance. Thus, primary amps x primary turns = secondary amps x secondary turns.
By opening the secondary with the primary energised, you are upsetting the ampere-turn balance relationship. In order to maintain ampere-turn balance, the secondary voltage rises until insulation breaks down allowing current to flow and restore balance, or until the core saturates. Saturation of the iron core keeps the voltage well below its theoretical limit, although a big CT will easily develop several kV open circuit, more than enough to kill you.
With the ideal transformer model I proposed the CT primary essentially interrupts the load current so this CT could see line to ground or higher voltage depending on the load connection.
This model is good enough to explain why the voltages are high enough to be dangerous, but is a long way from real world. This model lies at the core of any equivalent transformer model and must be understood first. The question is pretty elementary, so I believe an elementary model is the best place to start.
Primary voltage is certainly relevant. Zero voltage on the primary, for example, translates to zero on the secondary. If you want to get picky, ambient temperature, humidity, pressure and tidal forces all probably play a role. Everything is an approximation. Which model suits your needs?
I'm not sure I completely agree with your analogy.
The voltage developed across an open secondary is a function of the current that flows through the magnetizing branch of the CT. The magnetizing branch is in parallel with the secondary winding in an equivalent circuit. When the secondary winding is open, all of the primary current is passed through the magnetizing branch, thus creating a high voltage(there is a substantial impedance in the mag branch). The voltage is limited by the saturation of the core, and as is mentioned above, the RMS voltage measured is not that high. However, the peaks can be extremely high because the rate of change of the flux as it passes through zero current is not limited by the core saturation.
For a 2000:5A, C800 rated CT, it is not incommon to have an open-circuit voltage of in excess of 20 kVp.
The main thing I want to clarify is that the voltage developed across the secondary is a function of primary current and not an impressed voltage breaking down insulation...and then current flowing.
Just to clarify, the peak voltage that you are referring to has nothing to do with any electro-static voltage buildup that would be present as the result of an absence of a secondary ground reference. This electro-static voltage buildup could also be of significant magnitude.
Greg May
Two Sockets - Two Meters, Inc.
WEB: socket-two-me.com
The electro-static voltage induced really depends on the type of CT. Most MV and all HV (at least they should) have a grounded shield between the HV winding and the LV winding, which negates any electro-static charge. The only real charge would be a function of the winding to ground and the winding to winding capacitances, which, under steady state primary conditions, would result in little voltage on the secondary...10's of V or so. Electro-static charges being picked up in the wiring running in a s/s is a different story, but here we're only address terminal voltages at the CT.
Any CTs that don't have a shield (i.e. most 600 V type and some MV types) would get some charge transfer, but would still pale in comparison to the OC voltage of the CT.
Very interesting. As you may know, I was almost killed as the result of an electrostatic voltage buildup on a medium voltage CT. I was very careful to close the shorting link on the CT, but neglected to maintain a secondary ground reference. When attempting to rewire the secondary (why is a long story), the ensuing shock startled me to the point where my uninsulated arm missed a primary conductor by an inch or less! I didn't measure the electrostatic voltage present, but it was enough to knock the snot out of me!
Greg May
Two Sockets - Two Meters, Inc.
WEB: socket-two-me.com
These were Schlumberger outdoor CTs mounted in a ground line cabinet. There were "in and out" elbows for underground primary (13,800/7970 Y) on one side that were placed backwards, causing the watthour meter to reverse. The socket was then rewired, creating an inadvertent lagging "Q" meter that resulted in a significant overregistration.
I wanted to have the large consumer take an outage, unplug the "in and out" elbows, and rewire the socket to correct the error. Since this was found to be politically difficult, I decided to rewire the CT secondaries to accomplish a "double polarity reversal."
The entire story can be found on WEB site TheMeterGuy.com in the archived Mystery! series.
Greg May
Two Sockets - Two Meters, Inc.
WEB: socket-two-me.com
Your explanation is technically superior to mine - I was trying to keep things at a pretty simple level as an explanation to the original post, which didn't seem too familiar with CT's and their models. Might just be my interpretation.
Ahh...would you believe me if I said I was going to guess it was an underground application? I've heard tell of this happening on underground applications...if memory serves me correct I believe the higher then normal line-to-ground capacitances can work to "couple" the HV and LV windings, if the LV loses a ground reference.
The confusion about the value of voltage developed across an open secondary CT winding can perhaps be cleared up by this explanation.
You can use a simple transformer model to get the voltage value, but it is the voltage drop across the primary winding(same as any transformer) which is transformed to the open circuit secondary voltage, not the line-to-line or line-to-ground voltage, which has nothing to do with the concept.
Since there is only one primary winding in a CT, the maximum voltage drop across the primary is based on the burden that the CT can put on the primary circuit. A larger capacity CT can build up a larger open circuit voltage since it can impose a higher burden on the primary circuit.
And leaving a CT open circuited is not always the kiss of death(though I am certainly not recommending it). In one case, I personally have found three 400:5A CTs in this condition, and yes, they are small and the primary current was low. They were in this condition for years. They were buzzing at the time, but were not damaged. I shorted out the secondary winding(the buzzing immediately stopped) and used them for metering for a week with no problems.
In another location, two much larger CTs (around 3000:5A) were found open circuited by an IR scan from the heat they generated. The elctrician installing the metering circuit did not wire the ammeter switch correctly to short out the unused CTs.
A year or two ago I disturbed a bad connection (by the transformer manufacturer) on a CT secondary. The CT involved was an 18000/5 Class X type in a differential scheme. The arc was quite spectacular, although at the time I think 'terrifying' is the word I would have chosen. I have no recollection of vaulting over the platform guardrail eight feet above ground level to get away from it. My colleagues told me about that later. Big CT's are things to be treated with respect.
"You can use a simple transformer model to get the voltage value, but it is the voltage drop across the primary winding(same as any transformer) which is transformed to the open circuit secondary voltage, not the line-to-line or line-to-ground voltage, which has nothing to do with the concept."
I disagree. Put an open circuit secondary power or voltage transformer in series with a load, and you will nearly disconnect the load. The simple (ideal) transformer model has no shunt admittance. Zero current on the secondary transformed by the turns ratio translates to zero current on the primary. Ideal transformers like ideal mates do not actually exist in nature, though. And especially in the case of real CTs, shunt admittances are significant.
I have two problems with many of the other posts. The first is that that they answered a basic question with more advanced concepts than required. The second is that shunt admittance and saturation are given as the cause of the high open circuit voltage when they actually act to reduce it.
stevenal, I don't know what you mean by saying: "Put an open circuit secondary power or voltage transformer in series with a load, and you will nearly disconnect the load."
If I connect a load across the open secondary of a power or voltage transformer, I will cause current to flow through the load.
I believe my explanation is a simple way to explain what kind of voltage can be developed across an open-circuited CT secondary winding.
In fact, you were the person who first proposed the use of a simple transformer model to explain the concept; you were just incorrect about the voltage developed being based on the line-to-ground voltage of the system.
It is also unclear to me why you first propose a simple explanation, then chastise other posters for presenting what you consider explanations that are too complex, and then in the same paragraph bring up shunt admittance and saturation as necessary to understand the concept!
As a matter of fact, your original post had another incorrect statement. You said:
"The big difference between CTs and power transformers and VTs is in the connection. CT primaries are connected in series with the load, the others in parallel."
This is a misconception. The load of the CT is the relay and/or ammeter, and it is connected in the same way any other load is connected: across the power source, which is the secondary winding. Neither is the primary winding in a CT connected any differently than any other transformer, there is simply just one turn. The only difference is that a CT secondary circuit is (usually) low impedance, to allow the full current(based on the primary to secondary ratio and circuit impedances) to flow without an appreciable voltage drop across the 'load'. If you place a low enough impedance across the secondary of a power transformer, you will also get the full load current, and it will be based on the primary to secondary ratio and circuit impedances. The power will come from the same place as in a CT: the current flow through, and voltage drop across, the primary winding.
