A comma and a coordinate conjunction are often used to separate the clauses of a compound sentence. Sometimes the ideas combined in a compound sentence are so closely related that it is not necessary to use a conjunction. In that case, a semicolon is used to separate the two clauses. The following are two important uses of the semicolon in the compound sentence:
1. A semicolon should be used between the independent clauses of a compound sentence when they are not joined by a coordinate conjunction. In the following sentence, there is no conjunction between the two independent clauses; therefore, a semicolon is used.
The doctor came in late; he did not stop to read the telegram.
2. When the independent clauses of a compound sentence are very long, or have internal punctuation, a semicolon is generally used before the coordinate conjunction (underlined). Internal punctuation means that there are commas within one or both of the independent clauses.
Shakespear, a great dramatist, wrote a great many plays; and he also wrote a number of sonnets.
Temperamental and lazy, John manages to get along without working; but he was never contented or happy.
Both of these sentences have one or more commas in the first independent clause; that is, the first clause has internal punctionation. A semicolon is used between the two independent clauses even though a coordinate conjunction is used.
-- The New Webster's Grammar Guide