Detailed:
Use of current transformer
Correct threading method
We must first determine the transformer ratio according to the load, and then thread the line from the center of the transformer as required. Be careful not to take the number of turns around the outer ring as the number of turns around, but to take the number of turns into the current transformer as the standard.
For example, a current transformer with a maximum transformation ratio of 150/5 has a primary rated current of 150A. If it is to be used as a 50/5 transformer, the conductor should be wound around 150/50=3 turns, that is, the inner ring is wound around 3 turns, and the outer ring has only 2 turns at this time (that is, no matter how many turns the inner ring is, as long as you thread from the inside out). Of course, if the conductor is threaded from the outside to the inside, the opposite is true. At this time, if the number of turns in the outer ring is counted, the actual number of turns in the inner ring is 4 turns, and the converted primary current is 150/4=37.5A, which becomes a current transformer of 37.5/5, with a magnification of 7.5, while the staff uses 50/5 when reading meters, with a magnification of 10.
Conversion between ratio and turns
Some current transformers have lost their nameplates in use. When the user's load changes, it is necessary to change the ratio of current transformer. First of all, the transformer should be verified to determine the highest rated current of the transformer, and then the transformation ratio and turns should be converted as needed.
For example, a current transformer with a maximum rated current of 150A should be used as a 50/5 transformer, and the conversion formula is
The number of core-threading turns at a time = the highest rated current of the existing current transformer/the primary current of the transformer to be replaced = 150/5=3 turns.
That is, the current transformer is converted into 50/5, and the primary iron core turns are 3.
On this basis, the highest primary rated current can be calculated. For example, the transformation ratio of the original current transformer is 50/5, and the number of turns through the core is 3 turns. When the transformer is used as 75/5, we first calculate the highest primary rated current: the highest primary rated current = the primary current in use × the number of turns through the core =50×3= 150A, which is converted into 75/5.
That is to say, the original current transformer with 50/5 core threading turns is converted into a current transformer with 75/5 core threading turns, and the strain of core threading turns is 2 turns.
For example, the original 4-turn core threading current transformer needs to be used as a 75/5 current transformer. First of all, we calculate that the highest rated current is 50×4=200A, and the number of turns of the wire core after conversion should be 200/75≈2.66 turns. When actually threading the core, the number of winding turns can only be an integer, which can be 2 turns or 3 turns. When we wear it twice, the primary current has become 200/2= 100A, forming a transformer of 100/5, resulting in an error, which is (original transformation ratio-current transformation ratio)/current transformation ratio = (15-20)/20 =-. Moreover, if we wear three turns, we will definitely overcharge the users. Because its primary current is 200/3=66.66A, a 66.6/5 transformer is formed, and the error is (15-13.33)/13.33 = 0.125, that is, according to 75/. So when we don't know the highest rated current of current transformer, we can't change the ratio at will, otherwise it is likely to cause measurement error.
Transformer polarity judgment
Voltage transformer (PT) and current transformer (Ct) are important electrical equipment in power system, which undertake the functions of isolation between high and low voltage systems and high and low voltage conversion. Whether its wiring is correct is of great significance to the normal work of system protection, measurement, monitoring and other equipment. When newly installed PT and CT are put into operation or the secondary cables of PT and CT are replaced, it has become an essential procedure for relay protection workers to check the correctness of PT and CT wiring through polarity test.
To avoid polarity inversion is to find the "homonymous terminal" of transformer input and output, and the specific method is "point polarity". Take current transformer as an example to explain how to point polarity.
The specific method is to connect the pointer multimeter to the secondary output winding of the transformer, and the multimeter is within the DC voltage range; Then the negative electrode of the dry battery is fixed on the primary output lead of the current transformer; Then use the positive pole of the dry battery to "point" the primary input conductor of the current transformer, so that a+(positive) pulse current will be generated in the primary circuit of the transformer; At the same time, observe the direction of pointer multimeter pointer deviation. If the pointer of the multimeter shifts from left to right, the pointer opens, indicating that the primary input terminal of the current transformer and the secondary output terminal of the current transformer connected with the positive terminal of pointer multimeter are the same terminal, and this connection is called "positive polarity" or "positive polarity". If the pointer of the multimeter shifts from right to left and the pointer of the multimeter is reversed, it means that the primary input end of the current transformer and the secondary output end of the current transformer connected with the positive terminal of pointer multimeter are not the same name, and this connection is called "reverse polarity" or "reverse polarity";
note:
1, the above method is not accurate, and it depends on whether the "dry battery" is pulled back.
2. If there is no response, check wiring. For CT with large equipment capacity, if the transformation ratio is large, 9V battery string polarity can be used.
Pointing should be a thing of the past. At present, most of the primary current CT testers used in the field are three-in-one (point polarity, secondary winding volt-ampere characteristics), which is too convenient.
4. The principle and method of voltage transformer are the same.
By the way, both "point" and "pull" are instantaneous, and the time cannot be too long. For CT, the primary winding is quite short-circuited, and it will discharge after a long time. Only communication can be reflected through the transformer. After a while, it will become DC.
6, must use pointer multimeter, digital type.
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