Merge pull request #979 from PowerGridModel/feature/update-docs

update voltage sensor angle required

Author: nitbharambe

docs/user_manual/components.md

@@ -731,10 +731,14 @@ voltage is a line-to-line voltage. In a `asym_voltage_sensor` the measured volta
 
 ##### Input
 
-| name               | data type        | unit     | description                                                          |              required              |  update  | valid values |
-| ------------------ | ---------------- | -------- | -------------------------------------------------------------------- | :--------------------------------: | :------: | :----------: |
-| `u_measured`       | `RealValueInput` | volt (V) | measured voltage magnitude                                           | ✨ only for state estimation | ✔ |    `> 0`     |
-| `u_angle_measured` | `RealValueInput` | rad      | measured voltage angle (only possible with phasor measurement units) |              ❌              | ✔ |              |
+| name               | data type        | unit     | description                                                          |              required                                                                                      |  update  | valid values |
+| ------------------ | ---------------- | -------- | -------------------------------------------------------------------- | :--------------------------------------------------------------------------------------------------------: | :------: | :----------: |
+| `u_measured`       | `RealValueInput` | volt (V) | measured voltage magnitude                                           | ✨ only for state estimation                                                                         | ✔ |    `> 0`     |
+| `u_angle_measured` | `RealValueInput` | rad      | measured voltage angle (only possible with phasor measurement units) | ✨ only for state estimation when a current sensor with `global_angle` `angle_measurement_type` is present | ✔ |              |
+
+```{note}
+When a current sensor with `global_angle` `angle_measurement_type` is present there needs to be a voltage sensor with `u_angle_measured` in the grid as a reference angle (when performing a state estimation). 
+```
 
 ##### Steady state output