Parameters of Voltage Transformers - Weikewei-GJB9001 Certification
Voltage TransformerThe parameter representation is used to specify the parameters of the device. The parameters of the voltage transformer are as follows:
Rated secondary voltage
Rated output or VA rated value
Accuracy level
Rated voltage coefficient
Insulation level
Rated secondary voltage
According to the standard, the rated voltage of a voltage transformer is 110V. Although we specify this voltage as the line voltage, the actual secondary voltage is 110/√ 3 volts. This is because usually one terminal of the transformer primary is grounded.
Rated output
In theory, we can design a transformer with any VA output. However, according to standards, only certain specific VA ratings are available on the market. For each single-phase unit, these rated values are 10, 25, 50, 75, 100, 150, 200, and 500 VA. Sometimes, transformers may have two or more secondary iron cores. So these values are the total output of all secondary windings of a single transformer.
Accuracy level
This is one of the important specifications of transformers. As is well known, there are always some errors in the performance of transformers. This is because no one can idealize it. These transformers have ratio errors and phase angle errors. Although we can minimize these errors to the greatest extent possible. However, as a result, the cost of the equipment has increased. In fact, for different purposes, we can allow errors to expand to a limited scope. Therefore, we intentionally compromise on errors to optimize the cost of potential transformers.
This is because voltage transformers have different purposes. In some cases, the erroneous performance of voltage transformers will not affect actual requirements. Similarly, in some cases, in order to obtain the desired results, the error must be minimized. The accuracy level or simple accuracy level represents the error limit of the voltage transformer.
Rated voltage coefficient
It is determined by the maximum operating voltage and the grounding condition of the primary winding. The following table lists the recommended voltage factors under different grounding conditions.
| Rated voltage coefficient | Rated time | The method of connecting the primary winding to the system grounding condition. |
| one point one | continuity | Between lines and between transformer star points and ground. |
| one point one | continuity | Between the line and ground in an effectively grounded neutral wire system. |
| 1.5 | 30 seconds | Between the line and ground in an effectively grounded neutral wire system. |
| one point one | continuity | In a neutral wire system with ineffective grounding (automatic grounding fault tripping) between the line and ground. |
| one point nine | 30 seconds | In a neutral wire system with ineffective grounding (automatic grounding fault tripping) between the line and ground. |
| one point one | continuity | In isolated neutral wire systems or resonant grounding systems, between the wire and ground. |
| one point nine | 8 hours | In isolated neutral wire systems or resonant grounding systems, between the wire and ground. |
Rated insulation level
The rated insulation level refers to the rated voltage of a voltage transformer with standard capacity. The rated insulation level includes the rated system voltage rating, the highest system voltage rating, the highest system voltage rating, and the power frequency voltage.
Nominal voltage
The nominal voltage is the voltage at which the voltage transformer operates. That is the normal system voltage. For example, 11KV for 11KV system, 33KV for 33KV system, 132KV for 132KV system, etc.
Maximum system voltage
The highest system voltage refers to the maximum possible system voltage that may last for any period of time. This is the maximum system voltage that may occur under no-load or low load conditions. Therefore, PT must be able to withstand high voltage for any duration. For example, the maximum system voltage for an 11KV system is 12KV. Similarly, the maximum system voltage for a 33KV system is 36KV. The similarities of the 132KV system are 145 KV, etc.
Power frequency withstand voltage
There may be certain situations in the power system due to sudden withdrawal of large loads; The system voltage will increase. This high voltage may have a power frequency voltage, but it does remain constant during each nominal cycle. Therefore, the insulation level of the voltage transformer must be able to withstand at least a certain amount of time. Usually, for this type of power frequency high voltage, we consider the standby capacity of PT within 30 seconds. For example, the power frequency withstand voltage of 11KV PT is 28KV.
Voltage for impact resistance test
PT also has another rated voltage or insulation level. We refer to it as the impulse withstand test voltage with standard full wave positive and negative polarity KV peak values. Due to lightning and switch pulses, high voltage may be applied to the insulation layer of PT. But we are fortunate that these pulse high voltages will last for a very short period of time. The duration of these pulse high voltages ranges from a few microseconds to a few milliseconds. Therefore, we can design an insulation layer for a voltage transformer that can withstand standard applied pulse voltages.
The interference of these pulse voltages is very uneven under different waveforms and frequencies. Therefore, it is impossible to determine which type of pulse PT insulation will impact during its service life. Therefore, we standardized some pulse voltages for different nominal system voltages. We design insulation for standard pulse voltage. This means that during the testing period, the voltage transformer should withstand these standard pulse voltages. For example, 75KV is the pulse voltage level of an 11KV voltage transformer.