A pure insulator when is connected across line and earth, it behaves as a capacitor. In an ideal insulator, as the insulating material which acts as dielectric too, is 100 % pure, the electric current passing through the insulator, only have capacitive component. There is no resistive component of the current, flowing from line to earth through insulator as in ideal insulating material, there is zero percent impurity.
In practice, the insulator cannot be made 100% pure. Also due to ageing of insulator the impurities like, dirt and moisture enter into it. These impurities provide conductive path to the current. Consequently, leakage electric currentflowing from line earth through insulator has also resistive component.
Hence, it is needless to say that, for good insulator, this resistive component of leakage electric current is quite low. In other way the healthiness of an electrical insulator can be determined by ratio of resistive component to capacitive component. For good insulator this ratio would be quite low. This ratio is commonly known as tanδ or tan delta. Sometimes it is also referred as dissipation factor.
In the vector diagram above, the system voltageis drawn along x-axis. Conductiveelectric current i.e. resistive component of leakage current, IRwill also be along x-axis.
As the capacitive component of leakage electric current IC leads system voltage by 90°, it will be drawn along y-axis.
Now, total leakage electric current IL(Ic + IR) makes an angle δ (say) with y-axis.
Now, from the diagram above, it is cleared, the ratio, IR to IC is nothing but tanδ or tan delta.
NB: This δ angle is known as loss angle.
The cable, winding, current transformer, potential transformer, transformer bushing, on which tan delta test or dissipation factor test to be conducted, is first isolated from the system. A very low frequency test voltage is applied across the equipment whose insulation to be tested. First the normal voltage is applied. If the value of tan delta appears good enough, the applied voltage is raised to 1.5 to 2 times of normal voltage, of the equipment. The tan delta controller unit takes measurement of tan delta values. A loss angle analyser is connected with tan delta measuring unit to compare the tan delta values at normal voltage and higher voltages, and analyse the results.
During test it is essential to apply test voltage at very low frequency.
If frequency of applied voltage is high, then capacitive reactance of the insulator becomes low, hence capacitive component of electric current is high. The resistive component is nearly fixed, it depends upon applied voltage and conductivity of the insulator. At high frequency as capacitive current, is large, hence, the amplitude of vector sum of capacitive and resistive components ofelectric current becomes large too.
Therefore, required apparent power for tan delta test would become high enough which is not practical. So to keep the power requirement for thisdissipation factor test, very low frequency test voltage is required. The frequency range for tan delta test is generally from 0.1 to 0.01 Hz depending upon size and nature of insulation.
There is another reason for which it is essential to keep the input frequency of the test as low as possible.
That means, dissipation factor tanδ ∝ 1 / f.
Hence, at low frequency, the tan delta number is high, the measurement becomes easier.
Second one is, determining the condition of insulation from the value of tanδ, directly. No requirement of comparing previous results of tan delta test.
If the insulation is perfect, the loss factor will be approximately same for all range of test voltages. But if the insulation is not good enough, the value of tan delta increases in higher range of test voltage.
From the graph it is clear that, the tan&delta number non linearly increases with increasing test very low frequency voltage. The increasing tan&delta, means, high resistive electric current component, in the insulation. These results can be compared with the results of previously tested insulators, to take proper decision whether the equipment would be replaced or not.
A pure insulator when is connected across line and earth, it behaves as a capacitor. In an ideal insulator, as the insulating material which acts as dielectric too, is 100 % pure, the electric current passing through the insulator, only have capacitive component. There is no resistive component of the current, flowing from line to earth through insulator as in ideal insulating material, there is zero percent impurity.
In practice, the insulator cannot be made 100% pure. Also due to ageing of insulator the impurities like, dirt and moisture enter into it. These impurities provide conductive path to the current. Consequently, leakage electric currentflowing from line earth through insulator has also resistive component.
Hence, it is needless to say that, for good insulator, this resistive component of leakage electric current is quite low. In other way the healthiness of an electrical insulator can be determined by ratio of resistive component to capacitive component. For good insulator this ratio would be quite low. This ratio is commonly known as tanδ or tan delta. Sometimes it is also referred as dissipation factor.
In the vector diagram above, the system voltageis drawn along x-axis. Conductiveelectric current i.e. resistive component of leakage current, IRwill also be along x-axis.
As the capacitive component of leakage electric current IC leads system voltage by 90°, it will be drawn along y-axis.
Now, total leakage electric current IL(Ic + IR) makes an angle δ (say) with y-axis.
Now, from the diagram above, it is cleared, the ratio, IR to IC is nothing but tanδ or tan delta.
NB: This δ angle is known as loss angle.
The cable, winding, current transformer, potential transformer, transformer bushing, on which tan delta test or dissipation factor test to be conducted, is first isolated from the system. A very low frequency test voltage is applied across the equipment whose insulation to be tested. First the normal voltage is applied. If the value of tan delta appears good enough, the applied voltage is raised to 1.5 to 2 times of normal voltage, of the equipment. The tan delta controller unit takes measurement of tan delta values. A loss angle analyser is connected with tan delta measuring unit to compare the tan delta values at normal voltage and higher voltages, and analyse the results.
During test it is essential to apply test voltage at very low frequency.
If frequency of applied voltage is high, then capacitive reactance of the insulator becomes low, hence capacitive component of electric current is high. The resistive component is nearly fixed, it depends upon applied voltage and conductivity of the insulator. At high frequency as capacitive current, is large, hence, the amplitude of vector sum of capacitive and resistive components ofelectric current becomes large too.
Therefore, required apparent power for tan delta test would become high enough which is not practical. So to keep the power requirement for thisdissipation factor test, very low frequency test voltage is required. The frequency range for tan delta test is generally from 0.1 to 0.01 Hz depending upon size and nature of insulation.
There is another reason for which it is essential to keep the input frequency of the test as low as possible.
That means, dissipation factor tanδ ∝ 1 / f.
Hence, at low frequency, the tan delta number is high, the measurement becomes easier.
Second one is, determining the condition of insulation from the value of tanδ, directly. No requirement of comparing previous results of tan delta test.
If the insulation is perfect, the loss factor will be approximately same for all range of test voltages. But if the insulation is not good enough, the value of tan delta increases in higher range of test voltage.
From the graph it is clear that, the tan&delta number non linearly increases with increasing test very low frequency voltage. The increasing tan&delta, means, high resistive electric current component, in the insulation. These results can be compared with the results of previously tested insulators, to take proper decision whether the equipment would be replaced or not.
In Sweep Frequency Response Analysis a sinusoidal voltage Vi is applied to one end of a winding and output voltage Vo is measured at the other end of the winding. Other windings are kept open.
As the winding is itself an distributed RLC circuit it will behave like RLC filter and gives different output voltages at different frequencies. That means if we go on increasing the frequency of the input signal without changing its voltagelevel we will get different output voltages at different frequencies depending upon the RLC nature of the winding. If we plot these output voltages against the corresponding frequencies we will get a particular patter for a particular winding.
It also compares different transformers of the same design.
2) At low frequency winding inductances are determined by the magnetic circuitof the transformer core.
4) Winding exhibits many resonant points
5) Frequency responses are more sensitive to winding movement.
SIGNAL APPLIED ACROSS TRANSFORMER TERMINALS | CONDITIONS |
---|---|
HV Red phase to Neutral | LV Red Yellow Blue phases are open |
HV Yellow phase to Neutral | LV Red Yellow Blue phases are open |
HV Blue phase to Neutral | LV Red Yellow Blue phases are open |
HV Red phase to Neutral | LV Red Yellow Blue phases are shorted |
HV Yellow phase to Neutral | LV Red Yellow Blue phases are shorted |
HV Blue phase to Neutral | LV Red Yellow Blue phases are shorted |
LV Red to Yellow phase | HV Red Yellow Blue phases and LV Blue phase are open |
LV Yellow to Blue phase | HV Red Yellow Blue phases and LV Red phase are open |
LV Blue to Red phase | HV Red Yellow Blue phases and LV Yellow phase are open |
The purpose of the impulse voltage test is to secure that the transformer insulation withstand the lightning overvoltage which may occur in service.
The impulse generator design is based on the Marx circuit. The basic circuit diagram is shown on Figure above. The impulse capacitors Cs (12 capacitorsof 750 η F) are charged in parallel through the charging resistors Rc (28 kΩ) (highest permissible charging voltage 200 kV). When the charging voltage has reached the required value, breakdown of the sparkgap F1 is initiated by an external triggering pulse. When F1 breaks down, the potential of the following stage (point B and C) rises. Because the series resistors Rs is of low-ohmic value compared with the discharging resistors Rb (4,5 kΩ) and the chargingresistor Rc, and since the low-ohmic discharging resistor Ra is separated from the circuit by the auxiliary spark-gap Fal, the potential difference across the spark-gap F2 rises considerably and the breakdown of F2 is initiated.
Thus the spark-gaps are caused to break down in sequence. Consequently thecapacitors are discharged in series-connection. The high-ohmic dischargeresistors Rb are dimensioned for switching impulses and the low-ohmicresistors Ra for lightning impulses. The resistors Ra are connected in parallel with the resistors Rb, when the auxiliary spark-gaps break down, with a time delay of a few hundred nano-seconds.
The required impulse shape is obtained by suitable selection of the series and discharge resistors of the generator.
The front time can be calculated approximately from the equation:
For R1 >> R2 and Cg >> C (15.1)
The test is performed with standard lightning impulses of negative polarity. The front time (T1) and the time to half-value (T2) are defined in accordance with the standard.
In practice the impulse shape may deviate from the standard impulse when testing low-voltage windings of high rated power and windings of high input capacitance. The impulse test is performed with negative polarity voltages to avoid erratic flashovers in the external insulation and test circuit. Waveform adjustments are necessary for most test objects. Experience gained from results of tests on similar units or eventual precalculation can give guidance for selecting components for the wave shaping circuit.
The test sequence consists of one reference impulse (RW) at 75% of full amplitude followed by the specified number of voltage applications at full amplitude (FW) (according to IEC 60076-3 three full impulses). The equipment for voltage and current signal recording consists of digital transient recorder, monitor, computer, plotter and printer. The recordings at the two levels can be compared directly for failure indication. For regulating transformers one phase is tested with the on-load tap changer set for the rated voltage and the two other phases are tested in each of the extreme positions.
All the dielectric tests check the insulation level of the job. Impulse generator is used to produce the specified voltage impulse wave of 1.2/50 micro seconds wave. One impulse of a reduced voltage between 50 to 75% of the full testvoltage and subsequent three impulses at full voltage.
For a three phase transformer, impulse is carried out on all three phases in succession.
Temperature Rise Test for Top Oil of Transformer
During temperature rise test for top oil of transformer we make the LV winding short circuited and apply voltage to the HV winding. So for full load rated currentflows in the transformer, the supply voltagerequired will much less than rated transformer voltage. We know that core loss of a transformer depends upon voltage. So there will not be any considerable core loss occurs in the transformer during test. But for getting actual temperature rise of the oil in a transformer, we have to compensate the lack of core losses by additional copper loss in the transformer. For supplying this total losses, transformer draws currentfrom the source much more than its rated value for transformer.
TEMPERATURE RISE LIMIT FOR AIR AS COOLING MEDIUM | TEMPERATURE RISE LIMIT FOR WATER AS COOLING MEDIUM | CONDITION | |
---|---|---|---|
WINDING | 55oC | 60oC | When oil circulation is natural |
60oC | 65oC | When oil circulation is forced | |
TOP OIL | 50oC | 55oC | When transformer is sealed & equipped with conservator tank |
45oC | 50oC | When transformer is neither sealed nor equipped with conservator tank |
NB: These temperature rises limits mentioned in the above table are the temperature rise above the temperature of cooling medium. That means these are the difference between winding or oil temperature and temperature of cooling air or water.
The several secondary connections are available in respect of various primary three phase connection in a the three phase transformer. So for same primary applied three phase voltage there may be different three phase secondary voltages with various magnitudes and phases for different internal connection of the transformer.
Let’s have a discussion in detail by example for better understanding.
We know that, the primary and secondary coils on any one limb have induced emfs that are in time-phase. Let’s consider two transformers of same number primary turns and the primary windings are connected in star. The secondary number of turns per phase in both transformers are also same. But the first transformer has star connected secondary and other transformer has delta connected secondary. If same voltages are applied in primary of both transformers, the secondary induced emf in each phase will be in same time-phase with that of respective primary phase, as because the the primary and secondary coils of same phase are wound on the same limb in the core of transformer. In first transformer, as the secondary is star connected, the secondary line voltage is √3 times of induced voltage per secondary phase coil. But in case of second transformer, where secondary is delta connected, the line voltage is equal to induced voltage per secondary phase coil. If we go through the vector diagram of secondary line voltages of both transformer, we will easily find that there will be a clear 30o angular difference between the line voltages of these transformers. Now, if we try to run these transformers in parallel then there will be a circulating current flows between the transformers as because there is a phase angle difference between their secondary line voltages. This phase difference can not be compensated. Thus two sets of connections giving secondary voltages with a phase displacement can not be intended for parallel operation of transformers.
GROUP | CONNECTION | CONNECTION | ||||||||||||
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0 (0O) |
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1 ( 30O) |
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6 ( 180O) |
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11 ( – 30O) |
|
|
For YNd11 transformer, we will find,
2U-1N > 2V-1N > 2W-1N
2V-1W > 2V-1V or 2W-1W .
Actually the no load voltage ratio of transformer is equal to the turn ratio. Soratio test of transformer.
The above transformer ratio test can also be performed by portable transformer turns ratio (TTR) meter. They have an in built power supply, with the voltages commonly used being very low, such as 8-10 V and 50 Hz. The HV and LV windings of one phase of a transformer are connected to the instrument, and the internal bridge elements are varied to produce a null indication on the detector.
Let’s have a discussion on transformer turns ratio (TTR) meter method of turn ratio test of transformer.
در این قسمت بیشتر به تستهایی می پردازیم که در سایت ، پس از نصب ترانس و در مرحله پیش راه اندازی صورت می گیرند.
قبل از انجام تست ظاهر ترانس مورد بازبینی قرار خواهد گرفت و موارد زیر چک خواهد شد:
· سفت بودن پیچها و سایر اتصالات
· بررسی ترمینال ها
· بررسی قسمت های چینی ترانس
· عایق بندی بین سیم پیچی ها و تانک
· عایق بندی بین تجهیزات و سیستم Earth
· اتصال و سیستم Earth برقگیرها
· اتصال زمین تانک
· اتصال زمین مربوط به نوترال اولیه یا ثانویه
· بررسی تجهیزات خنک کنندگی مانند:
• جهت چرخش موتورها
• بازرسی مکانیکی فن ها
• جفت بودن و صحت اتصال رادیاتورها
همچنین بررسی عملکرد:
· تجهیزات ایمنی
· مرحله اول رله بوخهلتس : آلارم توسط تزریق هوا
· مرحله دوم رله بوخهلتس: تریپ
· آلارم ترموستات + فرمان قطع (تریپ)
· تپ چنجر(tap changer)
· Limit switch
· سنسورهای حرارتی سیستم خنک کنندگی ، راه انداز فن و سیستم گردش روغن و همچنین اندازه گیری مقاومت سیم پیچها در تپ های مختلف و تصفیه و تست دی الکتریک روغن ترانسفورماتورها تستها و چگونگی آنها
وجود چاه ارت برای حفاظ از کاربران و تخلیه بار الکتریکی روشی است که سال هاست اجرا می شود.اندازه گیری مقاومت این چاه ها از اهمیت ویژه ای برخوردار است با توجه به نیاز کاربر ارت سنج ه به سه دسته تقسیم بندی می شوند ادامه مطلب ...
رله یک سوئیچ الکترونیکی که تحت کنترل سایر مدارات الکترونیکی باز و بسته می شود. در اصل سوئیچ با یک آهنربای مغناطیسی برای باز و بسته کردن یک یا چند اتصال عمل می کند. این وسیله توسط “جوزف هنری” (Joseph Henry) در سال ۱۸۳۵ اختراع شد. چون رله می تواند مدار خروجی پر قدرتی را نسبت به مدار ورودی کنترل کند می توان آنرا به عنوان نوعی تقویت کننده در نظر گرفت.
وقتی جریان از سیم پیچ عبور می کند، میدان مغناطیسی حاصله یک میله فلزی را که به طور مکانیکی به یک اتصال متصل شده است، را جذب می کند. این حرکت موجب اتصال یا قطع یک اتصال با یک اتصال ثابت می شود. وقتی جریان قطع می شود، میله فلزی با نیروی تقریبی نصف قدرت میدان مغناطیسی به محل اولیه خود بر می گردد. معمولا این نیرو توسط یک فنر (spring) تامین می شود، البته از نیروی گرانش (gravity) نیز در موتورهای استارتر صنعتی ممکن است استفاده شود. اغلب رله ها برای عملیات سریع ساخته می شوند. در کاربردهای ولتاژ پائین، کاهش نویز دارای اولویت بیشتری است و در کاردهای ولتاژ بالا کاهش قوس الکتریکی اولویت بیشتری دارد.
اگر انرژی سیم پیچ توسط DC تامین شود، خیلی اوقات یک دیود به دوسر سیم پیچ متصل می شود تا انرژی حاصل از میدان مغناطیسی را به هنگام قطع مصرف و یا به عبارتی پراکنده کند، که می تواند یک ضربه ولتاژ باشد و به سایر قسمتهای مدار ضربه بزند. اگر سیم پیچ برای کار با AC طراحی شده باشد، یک حلقه مسی در انتهای سیم پیچ، تابیده می شود. این حلقه(Shading ring) یک جریان غیر هم فاز تولید می کند که کشش میله فلزی را در سیکلهای AC افزایش می دهد. یعنی هنگامی که جریان AC مقدار مینیمم خود را دارد این سیم با یک اختلاف فاز نسبت به آن دارای مقداری جریان است که می تواند میله را به سمت سیم پیچ نگه دارد و در غیر این صورت میله در هر سیکل از سیم پیچ جدا و دوباره متصل می شود و موجب ضربه زدن به سایر قسمتهای مدار می شود.
به تشابه با عملیات کارکرد رله مغناطیسی، خواهید دید که در رله های حالت جامد از تریستور یا سایر سوئیچهای حالت جامد استفاده می شود. برای رسیدن به ایزولاسیون الکتریکی از(light-emitting diode) یعنی LED با یک ترانزیستور نوری استفاده می شود.
ادامه مطلب ...
هرجا که صحبت از سیستم های جدید مخابراتی، سیستم های تلویزیون کابلی و اینترنت باشد، در مورد فیبر نوری هم چیزهایی میشنوید.
فیبرهای نوری از شیشه شفاف و خالص ساخته میشوند و با ضخامتی به نازکی یک تار موی انسان، میتوانند اطلاعات دیجیتال را در فواصل دور انتقال دهند. از آنها همچنین برای عکسبرداری پزشکی و معاینه های فنی در مهندسی مکانیک استفاده میشود.
یک رشته فیبر نوری
در این مقاله میخوانیم که این فیبرهای نوری چگونه نور را منتقل میکنند و نیز درمورد روش عجیب ساخت آنها !
فیبرنوری چیست؟
ادامه مطلب ...
پیشگفتار
ترانسفورماتورها یکی از مهمترین تجهیزات، در سیستم های الکتریکی است. درجه اهمیت آن از اینجا مشخص می شود که این دستگاه در هر سه قسمت تولید انتقال و توزیع کاربرد اساسی دارد، به جرات می توان گفت که ایجاد خطا در این دستگاه و خارج شدن از سیستم باعث می شود ، بقیه سیستم نیز از کار بیافتند. وقابلیت اطمینان را پایین می آورد.
شناخت قسمتهای مختلف این دستگاه و هماهنگی با سایر تجهیزات با عث می شود از این دستگاه به طرز صحیح استفاده شود و طول عمر آن افزایش می یابد که به تبع آن قابلیت اطمینان سیستم نیز افزایش یابد.
یکی از مواد حیاتی که در ترانسفورماتورها مورد استفاده قرار می گیرد روغن است، پس لازم است تا ما از خواص و انواع این ماده شناخت داشته باشیم.
هیدروکربن ها که قسمت اعظم روغن ها را تشکیل می دهند به سه بخش مهم تقسیم می شوند:
الف – پارافین ها
ب – نفتالین ها
ج – ترکیبات آروماتیک
ترانسفورماتورهای قدرت غوطه ور در روغن با توجه به حجم وبزرگی ترانسفورماتور ها مقدار زیادی روغن در تانک آنها وجود دارد که برای ترانسهای با قدرت بالا بیش از 500 بشکه روغن مصرف میگردد به دلایل مختلف روغن ترانسفورماتور احتیاج به تصفیه فیزیکی دارد که در بخش های قبلی توضیحات آن آورده شده بنا بر این در اینجا فقط نحوه فیلتر اسیون ورعایت استانداردها گفته شده است ادامه مطلب ...