رابطه کلی زیر بین ولتاژ زانویی، جریان نامی ثانویه، ولت آمپر نامی، مقاومت داخلی و ALF یک ترانس جریان وجود دارد.
برای مثال ولتاژ زانویی یک ترانس جریان 5 آمپری با ولت آمپر 10 و مقاومت داخلی 4 اهم و فاکتور محدوده دقت معادل 5 ( 0.5M5)از رابطه زیر بدست می آید.
اگر منظور بدست آوردن ولتاژ زانویی از طریق آزمایش است که به هیچیک از این روابط نیاز نیست. شما با اعمال یک جریان مستقیم قابل کنترل به ثانویه ترانس، می توانید منحنی مغناطیسی آنرا برحسب ولتاژ به جریان ترسیم کنید. با افزایش جریان عبوری ولتاژ خروجی افزایش می یابد تا حدی که یک افزایش 50 درصدی در مقدار جریان تنها منجر به افزایش 10 درصدی ولتاژ شود. ولتاژ متناظر با نقطه مزبور مطابق تعریف ولتاژ شروع اشباع یا ولتاژ زانویی است. روشهای دیگری نیز با اعمال ولتاژ یا جریان ac وجود دارد که همراه با استانداردهای مربوطه ذکر شده است.
Current transformer standards are designed to simplify CT purchasing by eliminating the need to provide a complicated description of the requirements by simply selecting from a range of available classes and rating. There are two major standard organizations that issue CT standards for relaying:
• ANSI/IEEE: the dominant standard for North American manufacturers
• IEC: the dominant European standard
ANSI/IEEE C57.13 recognizes three classes of CT for relaying purposes, but only the low leakage type "C" is used in large numbers by the industry. The class was named “C” because its excitation curves can be prepared by calculation. The standard details many physical and electrical characteristics of this CT type, but it is vague in terms of characteristics discussed in this report. The "C" classification only guaranties that the CT can deliver, with less than 10% accuracy at 20 times rated current, into one of 7 standard burden values: 0.1, 0.2, 0.5, 1, 2, 4, and 8 ohms. The first three are metering burdens with 0.9 Power Factor, the others are relaying burdens with a Power Factor of 0.5. The code for this accuracy classification is the letter C followed by the voltage across the burden at the specified burden. For example if a 5 A CT is purchased for 0.2 burden, then the accuracy classification would be C20 (100 A develops 20 V across 0.2 ). The standard also specifies that the manufacturer must supply typical excitation curves and give the secondary winding resistance.
The weakness of this classification is that the tolerances for the excitation curves are very wide, and the remanence factor is not part of the specification. Therefore, matching CTs manufactured to the same standard by different manufacturers is difficult. An attempt was made to tighten the tolerances by introducing the "K" classification, which places a limit on the knee-point voltage with respect to secondary terminal voltage rating, but not many manufacturers offer such CTs as standard product.
The IEC 60044-6 standard has five classes for relaying CTs and a much wider range of options for specifying accuracy requirements. The important difference between the ANSI and IEC standards is that IEC has two classes of CTs where the residual magnetism is limited. The TPY transformers have a limit of 10% on remanence, while in the TPZ class transformers the remanent flux is practically negligible.
Before any test on a CT, first the CT should be demagnetized. ANSI C57.13.1 Standard lists several methods. All involves driving the CT into saturation one way or another and
than slowly reducing the magnetizing force to zero.
Excitation curves are usually measured by connecting a voltage source to the secondary winding while the primary is open-circuited. The applied voltage is gradually increased until the excitation current reaches the accuracy class rated value (10 A for 5 A CT). The voltage waveform during the test should remain sinusoidal, while the current becomes increasingly distorted. This is only possible if the voltage source is capable to supply the very high peak power is required to measure the saturated part of the excitation curve.
For example, a 200-5 A CT with ANSI accuracy class of C20 is expected to draw 10 A rms during this test at 40 V. At this level, the current is heavily distorted and its peak to rms ratio (crest factor) will be in the order of 2.5. This means the test source should be capable of supplying a peak power of:
Power = 1.4 ×40 V ×2.5 ×10 A = 1400 VA
An alternative to this method is to use a current source. In this case the current is applied to a test winding which gives a convenient range of currents for the tests. For example, to measure the excitation curve of a small CT, rated 1 to 0.001 A, it is better to apply the current to the primary winding and measure the voltage at the secondary. This way both the currents and voltages are in a convenient range. The results should be plotted in diagram with logarithmic scale of equal cycle length on the horizontal and vertical scales.
The measured current should be scaled to the secondary winding by dividing the measured value with the turn ratio. The measurement points should define two straight lines as shown above. The intersection of these lines defines the Saturation Voltage, one of the key CT parameters, which 20 V in this particular case. The other key CT item, the secondary winding resistance, should also be shown with the excitation curve. The secondary resistance is measured with a DC source.