Congratulations is used to express joy and fulfillment at something someone has done.
"Dave congratulated his friend on the completion of his new office complex."
Congradulations is simply a misspelling of congratulations and is commonly misused and should be avoided in official writings.
Congratulations means to express one's sympathetic feeling or emotion, pleasure and joy to the person it is felt for.
"Congratulations on your reelection to office."
Congradulations is an inappropriate spelling of the original word, congratulations which should be avoided in official writings due to it being regarded as incorrect.