Thread breakage or shredding in an embroidery machine can be caused by several factors, including incorrect threading, incorrect needle type or size, tension issues, and dirty or damaged machine parts. Here are some steps you can take to fix thread breakage or shredding in your Brother PE550D Embroidery Machine:

1. Check the threading: Ensure that the thread is correctly threaded through the machine, following the path indicated in the machine manual. Make sure that the thread is properly seated in the tension discs and the take-up lever.
2. Check the needle: Ensure that you are using the correct type and size of needle for the fabric and thread you are using. Bent or damaged needles can also cause thread breakage or shredding, so replace the needle if necessary.
3. Check the tension: Adjust the tension to ensure that it is appropriate for the fabric and thread being used. Be sure to test the tension by sewing on a scrap piece of fabric before embroidering.
4. Check the machine parts: Check for any dirt, lint, or debris that may have accumulated in the machine, particularly around the bobbin case and hook area. Clean the machine as necessary, and replace any damaged parts.
5. Check the design: The design itself can also cause thread breakage or shredding if it has too many stitches in one area, or if it has very small or dense areas. Try reducing the density of the design in the embroidery software or selecting a different design.

By following these steps, you should be able to fix thread breakage or shredding in your Brother PE550D Embroidery Machine. If none of the above steps work, it may be best to take the machine to a qualified technician for further inspection and repair.