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پرسش ترانسفورماتور شماره 15- تلفات در ترانسفورماتور
اگه ميشه کمي درباره حالت no load losses /load losses برام بگيد يا درباره short- circuit impedance.
نویسنده : احسان
 
#1
ساعت: 18:25 - تاریخ: 21 بهمن 1388
اصطلاحات مذکور به بخشی از تستهای ترانس که جهت تعیین بعضی مشخصات مهم آن طرح ریزی می شود مربوط  هستند. تلفات بی باری به تلفات ناشی از ولتاژ دار کردن ترانس و آثار مغناطیسی ناشی از آن و تلفات بار داری به تلفات ناشی از بار گیری ترانس و تلفات موسوم به تلفات اهمی بر می گردد. امپدانس اتصال کوتاه ترانس نیز معرف امپدانس معادل ترانس که عمدتاً فلوهای پراکندگی مسبب آن هستند اطلاق می شود. وجه تسمیه اتصال کوتاه نیز به روش تست مرتبط با آن مربوط می شود. در این روش ثانویه ترانس اتصال کوتاه شده و درصدی از ولتاژ نامی را به اولیه ترانس چنان تزریق می کنند تا از ثانویه جریان نامی عبور نماید. در صد مذکور که به Uk% نیز معروف است معادل امپدانس پریونیت ترانس در مبنای ظرفیت نامی آن می باشد. در زیر برخی اطلاعات مفید در مورد اصطلاحات مطرح شده را مشاهده می فرمائید.

1-Losses

The no-load losses are essentially the power required to keep the core energized. These are commonly referred to as “core losses,” and they exist whenever the unit is energized. No-load losses depend primarily upon the voltage and frequency, so under operational conditions they vary only slightly with system variations. Load losses, as the terminology might suggest, result from load currents flowing through the transformer. The two components of the load losses are the I2R losses and the stray losses. I2R losses are based on the measured dc (direct current) resistance, the bulk of which is due to the winding conductors and the current at a given load. The stray losses are a term given to the accumulation of the additional losses experienced by the transformer, which includes winding eddy losses and losses due to the effects of leakage flux entering internal metallic structures. Auxiliary losses refer to the power required to run auxiliary cooling equipment, such as fans and pumps, and are not typically included in the total losses as defined above.
Transformer losses represent power that cannot be delivered to customers and therefore have an associated economic cost to the transformer user/owner. A reduction in transformer losses generally results in an increase in the transformer’s cost. Depending on the application, there may be an economic benefit to a transformer with reduced losses and high price (initial cost), and vice versa. This process is typically dealt with through the use of “loss evaluations,” which place a dollar value on the transformer losses to calculate a total owning cost that is a combination of the purchase price and the losses. Typically, each of the transformer’s individual loss parameters — no-load losses, load losses, and auxiliary losses — are assigned a dollar value per kW ($/kW). Information obtained from such an analysis can be used to compare prices from different manufacturers or to decide on the optimum time to replace existing transformers. There are guides available, through standards organizations, for estimating the cost associated with transformers losses. Loss-evaluation values can range from about $500/kW to upwards of
$12,000/kW for the no-load losses and from a few hundred dollars per kW to about $6,000 to $8,000/ kW for load losses and auxiliary losses. Specific values depend upon the application.

2- No-load losses

When alternating voltage is applied to a transformer winding, an alternating magnetic flux is induced in the core. The alternating flux produces hysteresis and eddy currents within the electrical steel, causing heat to be generated in the core. Heating of the core due to applied voltage is called no-load loss. Other names are iron loss or core loss. The term “no-load” is descriptive because the core is heated regardless of the amount of load on the transformer. If the applied voltage is varied, the no-load loss is very roughly proportional to the square of the peak voltage, as long as the core is not taken into saturation. The current that flows when a winding is energized is called the “exciting current” or “magnetizing current,” consisting of a real component and a reactive component. The real component delivers power for no-load losses in the core. The reactive current delivers no power but represents energy momentarily stored in the winding inductance. Typically, the exciting current of a distribution transformer is less than 0.5% of the rated current of the winding that is being energized.

3-Load losses

A transformer supplying load has current flowing in both the primary and secondary windings that will produce heat in those windings. Load loss is divided into two parts, I2R loss and stray losses.

I2R loss:
Each transformer winding has an electrical resistance that produces heat when load current flows.
Resistance of a winding is measured by passing dc current through the winding to eliminate inductive effects.

Stray Losses:

When alternating current is used to measure the losses in a winding, the result is always greater than the
I2R measured with dc current. The difference between dc and ac losses in a winding is called “stray loss.”
One portion of stray loss is called “eddy loss” and is created by eddy currents circulating in the winding conductors. The other portion is generated outside of the windings, in frame members, tank walls, bushing flanges, etc. Although these are due to eddy currents also, they are often referred to as “other strays.” The generation of stray losses is sometimes called “skin effect” because induced eddy currents tend to flow close to the surfaces of the conductors. Stray losses are proportionally greater in larger transformers because their higher currents require larger conductors. Stray losses tend to be proportional to current frequency, so they can increase dramatically when loads with high-harmonic currents are served. The effects can be reduced by subdividing large conductors and by using stainless steel or other nonferrous materials for frame parts and bushing plates.

3- Short circuit impedance

The short circuit impedance is obtained from short circuit test can be performed by one of the two techniques, viz. pre-set short circuit and post-set short circuit. In the pre-set short circuit test, a previously short-circuited transformer (i.e., with a short-circuited secondary winding) is energized from its primary side. If the secondary winding is the inner winding, the limb flux is quite low as explained in Section resulting in an insignificant transient inrush current. Hence, the method will work quite well. If the primary is the inner winding, there is substantial flux density in the limb and hence the inrush current gets superimposed on the short circuit current. Since the inrush current flows through the primary winding only, it creates a significant ampere-turn unbalance between the primary and secondary windings resulting in high short circuit forces. Depending upon the instant of closing and the core residual flux, the magnitude of the inrush current varies. In order to reduce inrush current and its effects during the test, the core can be deliberately pre-magnetized with the opposite polarity.
In the post-set short circuit test method, in which the transformer is in the energized condition, the secondary winding is short-circuited. Naturally, this method is preferred as there are no inrush currents and the related problems, and also due to the fact that it represents the actual fault conditions at site. However, the disadvantage of this method is that the short circuit capacity of test stations has to be much higher than the first method to maintain the rated voltage across the transformer terminals (by overcoming the voltage drop across the series impedance between the source and transformer) and establish the required value of short circuit current. If the source impedance is not negligible as compared to the transformer impedance, a higher voltage needs to be applied, subject to a limit of 1.15 p.u. (on no-load source voltage) as per IEC standard 60076–5 (second edition: 2000–07). Thus, the required short circuit capacity of the test stations increases. The capacity of the test stations should be at least 9 times the short circuit power of the transformer for a 15% over-excitation condition. The test stations may not have such capability, and hence short circuit tests on large transformers are usually carried out by the pre-set method.

 
نویسنده : حمید - از: فیروزآباد فارس
 
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