Q:

Passive voice warning - grammerly

I'm currently writing my thesis and checking my grammar with Grammarly software. English isn't my first language and I'm always getting a warning saying the sentence is written in the passive voice and it asks me to change it. I'm having difficulty understanding what went wrong and very much hope that you will be kind enough to let me know the reason.
Thank you,
Kind regards,
Navoda.

It states that the unit cost for manufacturing a product remains fixed with the increase of the production volume when Manual Labor is used as the production method. If Hard Automation is used, the unit cost gets lower than both Soft Automation and Manual Labor with the increase of the production volume, while it gives the highest unit cost among the three methods for a small production volume. 

A:

I'm having difficulty understanding what went wrong

Nothing is really wrong here, it's just that the grammar checker is encouraging you to use the active voice more. When you use the passive voice, the subject of the sentence is the object of the verb, e.g.:
The book was read by the class.
The subject of the sentence there is the book, and that's also the object of the verb, read. When you see some form of the verb to be (like is, was, will be, etc.) used together with another verb, that's a strong clue that you might be using the passive voice. There's nothing actually wrong with passive voice, it's just that it can make writing seem weak compared to the active voice, where the subject of the verb is the subject of the sentence:
The class read the book.
In the active voice, the focus is on the person or thing that's doing the action rather than the person or thing to which the action is being done, and that's usually simpler and more interesting. On the other hand, passive voice is sometimes preferred when you want the emphasis to be on the thing receiving the action rather than on the actor.
The text in your example is fine, and the grammar checker should be ignored by you on this point.

