Measurement of the utility frequency
When generating capacitys or large consumers get offgrig unexpectedly, then deviations from 800 mHz are allowed for a short (49,200 Hz to 50.800 Hz). At higher deviations a massive failure in the network is likely. In this case a defined gradual plan for load-shedding comes into force, to avoid problematic network conditions. When dropping of loads at lower frequency or dropping of generation a hight frequencies do not help to stabilize the frequency, then the network is switched off (black out) and then rebuilt without the failures.
The definition of load shedding in Germany is described in the Technical requirements for automatic frequency relieve (Technische Anforderungen an die automatische Frequenz- entlastung) and is subject to the requirements of the Operation Handbook Policy 5 of the ENTSO-E. In the following, the exemplary implementation plan is represented with four stages:
| Frequency | Action | Sum of load | Typ of activation |
|---|---|---|---|
| 49,8 Hz | Activation of power reserves immediately and shedding of pump accumulators (t<10s) | manually / automatic | |
| 49,2 Hz | Immediate shedding of pump accumulators | automatic | |
| 49,0 Hz | Load Shedding Step 1, ca. 12.5 % | ca. 12.5 % | automatic |
| 48,8 Hz | Load Shedding Step 2, ca. 12.5 % | ca. 25.0 % | automatic |
| 48,6 Hz | Load Shedding Step 3, ca. 12.5 % | ca. 37.5 % | automatic |
| 48,4 Hz | Load Shedding Step 4, ca. 12.5 % | ca. 50.0 % | automatic |
| 47,5 Hz | Separation of power plants from the grid | automatic |
After the separation of the power plants from the network, the network is gradually built up again, which allows the identification of the disturbance and thus the workaround.