There are always small fluctuations in both frequency and voltage in the electrical grid. The size of these fluctuations depends on the power of the generators in the grid as well as the load currently present and the load being added.
Under normal circumstances 5 MW is not a big load compared to the load normally present in the grid. For example a train leaving a train station may easily exceed this. For comparison: In 2019 the power generation capacity in Germany was about 200GW with Germany being part of a huge synchronous grid covering almost the whole of Europe with a generation capacity of moren than 600GW (more information in the wikipedia: https://en.wikipedia.org/wiki/Synchronous_grid_of_Continental_Europe) - in this context 5MW is trivial and the change in frequency or voltage from that load will not be noticable.
The voltage and - actually more important - the frequency are monitored constantly and when they exceed certain thresholds generators will be added or removed from the grid or controllable loads will be added or removed.
Grid operators have many ways to adjust the load and generation in the grid. For example pumped-storage plants can provide or consume a huge power within seconds to keep the frequency stable.
Once the frequency drops below a certain value rolling blackouts will occur and cities and wherever possible industrial plants will be shut down to prevent a total blackout.
Big loads of several dozen or hundreds of Megawatts are mostly controllable by the grid operators so they can stabilize the grid. Aluminium huts e.g. have a load of several hundred Megawatts and have to be scheduled with the grid operator to be turned on or even off.
You can actually see periodic drops in frequency of the Swiss train grid every hour at minute 00 since that is when a lot of trains are leaving the stations.