Duke Power Want Deposit for Unsatisfactory Payments Charlotte, North Carolina

I have been sent a letter stating I am going to be charged a security deposit for Duke Power. I am being charged this because they say I have made unsatisfactory payments.       I am disabled and my heat pump went out. I could not afford to replace it. My payments have gone up because of trying to heat with space heaters and cool with window units.     I have called them and made payment arraingements with them which I have always paid and that they agreed to. Now they want me to pay this security deposit of $550 which is more than half of what I draw a month.    I not only think this is not right but seems to me somewhat illegal. If anyone can help me I would greatly appreciate it.