Introduction:
Phase modifiers are installed in transmission lines to improve the performance and stability of the system. They are used to compensate for the reactive power in long length transmission lines.

Explanation:
1. Short transmission lines:
Short transmission lines are typically less than 80 km in length. These lines have a low voltage drop and low reactance. Since the length is short, the power factor correction can be achieved using other devices such as capacitors and reactors. Therefore, phase modifiers are not normally installed in the case of short transmission lines.

2. Medium length lines:
Medium length transmission lines are typically between 80 km and 250 km in length. These lines have a moderate voltage drop and moderate reactance. In some cases, phase modifiers may be required to compensate for the reactive power and improve power factor correction. However, it is not a common practice to install phase modifiers in medium length lines.

3. Long length lines:
Long length transmission lines are typically greater than 250 km in length. These lines have a significant voltage drop and high reactance. The high reactance causes a large phase angle difference between the voltage and current, resulting in a poor power factor. Phase modifiers are installed in long length lines to compensate for this reactive power and improve power factor correction. They help in maintaining voltage stability, reducing losses, and improving the overall performance of the transmission system.

4. For all length lines:
Although phase modifiers are not normally installed in short and medium length transmission lines, there may be exceptional cases where phase modifiers are required based on specific system requirements. However, as a general rule, phase modifiers are commonly installed in long length transmission lines to address the reactive power issues and improve power factor correction.

Conclusion:
Phase modifiers are normally installed in the case of long length transmission lines to compensate for the reactive power and improve power factor correction. While short and medium length lines may not require phase modifiers, there may be exceptional cases where they are needed based on specific system requirements.