1 Answer
The CT (Current Transformer) ratio for a 33 kV system depends on the specific current rating of the equipment you are measuring, such as the transformer or circuit breaker, and the load current.
In general, the CT ratio is given as:
$$
CT \, \text{Ratio} = \frac{\text{Primary Current}}{\text{Secondary Current}}
$$
For a 33 kV system, the primary current depends on the power being transmitted and the load.
**Example Calculation:**
If you have a 33 kV system and need to measure the current on a line that carries 10 MVA of power, the primary current can be calculated using the formula:
$$
I_{\text{Primary}} = \frac{\text{Power}}{\sqrt{3} \times \text{Voltage} \times \text{Power Factor}}
$$
Assuming a power factor of 0.8 (common in many cases):
$$
I_{\text{Primary}} = \frac{10 \, \text{MVA}}{\sqrt{3} \times 33 \, \text{kV} \times 0.8} \approx 174.9 \, \text{Amps}
$$
For the secondary current, CTs usually have a standard secondary rating of 1 or 5 Amps.
So, for a CT with a 5A secondary current:
$$
CT \, \text{Ratio} = \frac{174.9 \, \text{Amps}}{5 \, \text{Amps}} \approx 35:1
$$
In this example, a CT ratio of **35:1** would be used.
### Common CT Ratios
* For 33 kV systems, CT ratios typically range from **50:1** to **200:1** or higher, depending on the specific load and application.
It's important to note that the CT ratio is selected based on the primary current, so you'll need to know your system's load current to determine the exact ratio for your application.
In general, the CT ratio is given as:
$$
CT \, \text{Ratio} = \frac{\text{Primary Current}}{\text{Secondary Current}}
$$
For a 33 kV system, the primary current depends on the power being transmitted and the load.
**Example Calculation:**
If you have a 33 kV system and need to measure the current on a line that carries 10 MVA of power, the primary current can be calculated using the formula:
$$
I_{\text{Primary}} = \frac{\text{Power}}{\sqrt{3} \times \text{Voltage} \times \text{Power Factor}}
$$
Assuming a power factor of 0.8 (common in many cases):
$$
I_{\text{Primary}} = \frac{10 \, \text{MVA}}{\sqrt{3} \times 33 \, \text{kV} \times 0.8} \approx 174.9 \, \text{Amps}
$$
For the secondary current, CTs usually have a standard secondary rating of 1 or 5 Amps.
So, for a CT with a 5A secondary current:
$$
CT \, \text{Ratio} = \frac{174.9 \, \text{Amps}}{5 \, \text{Amps}} \approx 35:1
$$
In this example, a CT ratio of **35:1** would be used.
### Common CT Ratios
* For 33 kV systems, CT ratios typically range from **50:1** to **200:1** or higher, depending on the specific load and application.
It's important to note that the CT ratio is selected based on the primary current, so you'll need to know your system's load current to determine the exact ratio for your application.