The principle of current transformer is based on the principle of electromagnetic induction. Current transformer is composed of closed core and winding. Its primary winding has few turns and is connected in series in the line that needs to measure the current. Therefore, it often flows all the current of the line. The secondary winding has many turns and is connected in series in the measuring instrument and protection circuit.
We should first determine the magnification of the transformer according to the load, and then wind the primary wire from the center of the transformer as required. Note that the number of turns wound in the outer ring should not be regarded as the number of turns wound, but the number of turns penetrated into the current transformer should prevail. If the current transformer with the maximum conversion ratio of 150 / 5 has a maximum rated current of 150A at a time, if it needs to be used as a 50 / 5 transformer, the conductor should be wound 150 / 50 = 3 turns, that is, the inner ring should be wound for 3 turns, and the outer ring is only 2 turns (that is, no matter how many turns of the inner ring, as long as you pass through it from inside to outside, the number of turns of the outer ring is always 1 turn less than that of the inner ring. Of course, if the conductor passes through it from outside to inside, the opposite is true.) At this time, if the number of turns of the outer ring is counted and the outer ring is 3 turns, the actual number of core threading turns of the inner ring is 4 turns, and the transformed primary current is 150 / 4 = 37.5a, which becomes a 37.5/5 current transformer with a magnification of 7.5. In meter reading, the staff calculates the electricity degree with a 50 / 5 current transformer with a magnification of 10, and the error is: (10-7.5) / 7.5 = 0.33, that is, multiple electricity degree 33.
The nameplate of some current transformers is lost during use. When the user's load changes and the transformation ratio of the current transformer needs to be changed, the transformer shall be verified first to determine the maximum primary rated current of the transformer, and then the transformation ratio and the number of turns shall be converted as required. For example, a current transformer with the maximum primary rated current of 150A shall be used as a 50 / 5 transformer, and the conversion formula is one The number of primary core threading turns = the maximum primary rated current of the existing current transformer / the primary current of the transformer to be transformed = 150 / 5 = 3 turns, that is, the current transformer transformed into 50 / 5, and the number of primary core threading turns is 3 turns. The maximum primary rated current can be calculated from this. For example, the transformation ratio of the original current transformer is 50 / 5 and the number of core threading turns is 3 turns. When it is used to change it into a 75 / 5 transformer, we first calculate the maximum current High primary rated current: maximum primary rated current = primary current in original use Original core threading turns = 50 3 = 150A, the number of core turns after conversion to 75 / 5 is 150 / 75 = 2 turns, that is, when the original 50 / 5 current transformer with 3 turns is converted to 75 / 5 current transformer, the number of core turns shall be 2 turns.
Another example is that the original 50 / 5 current transformer with 4 turns of core threading needs to be changed to 75 / 5 current transformer. We first calculate that the maximum primary rated current is 50 4 = 200A, the number of core threading turns after conversion should be 200 / 75 2.66 turns. In actual core threading, the number of winding turns can only be an integer, either 2 turns or 3 turns. When we wear 2 turns, the primary current has changed to 200 / 2 = 100A, forming a 100 / 5 transformer, which produces an error of (original transformation ratio - current transformation ratio) / current transformation ratio = (15-20) /20 = --0.25, i.e. - 25, that is, if we still calculate the electricity degree according to the transformation ratio of 75 / 5, we will count 25 less electricity. When we wear 3 turns, we will count more electricity of the user. Because the primary current becomes 200 / 3 = 66.66a, a 66.6/5 transformer is formed, and the error is (15-13.33) /13.33 = 0.125, that is, 12.5 more kwh is calculated when calculating the kWh according to the transformation ratio of 75 / 5. Therefore, when we don't know the maximum primary rated current of the current transformer, we can't replace the transformation ratio at will, otherwise it is likely to cause measurement error.
Wrong winding wiring of current transformer for substation protection
In a 220kV substation, the current transformer circuit for differential protection of main transformer is wrongly connected to the measuring winding of bushing current transformer, resulting in serious saturation of current transformer in case of fault in the area (differential protection) The differential current reached the set value of transformer differential protection action, resulting in differential protection action. This substation has been put into operation for many years, and many technical supervision and regular inspection tests have been carried out in the process, but hidden dangers have not been found in time. Looking back on this event, it should be said that "the problem of secondary circuit of relay protection" As electricians, we all know that because the secondary circuit is too complex, cumbersome and wired in a variety of ways, few people will carefully check the correctness of the wiring when there is no problem ("maybe there is no achievement in the secondary work, so it is not necessary to invest too much energy and time in drilling").
In view of the reflection on the wrong wiring of current transformer winding for substation protection, two basic works should be done:(1) During the capital construction or technical transformation period, the review, test and acceptance of relevant protection current circuits (whether the current transformer windings are mixed and misunderstood; whether the configuration and use of protection windings are reasonable; whether there is protection action dead zone, etc.) shall be done well, and the key nameplate parameters and circuit requirements shall be photographed and archived.(2) During the operation and maintenance period, it shall focus on the sorting and compilation of basic data and archives, technical supervision test and regular inspection.
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