Black Entertainment Television, the District-based cable programming company, has announced that Group W Satellite Communications will market and promote the programming service.

Group W, a subsidiary of Westinghouse Electric Co., was formed last year to move the parent company into a series of ventures including an all-news cable service scheduled to begin operations next month in competition with Ted Turner's Cable News Network.

Robert L. Johnson, president of the black-oriented programming firm, said that Group W will market his service along with Group W's news service, the Disney Channel--another new cable service specializing in family entertainment--and a new country music network.

"We have had three people in affiliate relations," Johnson said. "Now we'll have access to hundreds of people they'll have in the market, including national representatives in every major city in the country."

The announcement came as the National Cable Television Association opened its annual convention here. NCTA expects more than 16,000 participants, including cable system operators, programmers and manufacturers.

In other developments here today, American Broadcasting Co. Inc. and Cox Cable Communications Inc. announced plans to form a joint cable venture specializing in cable equipment.

Cox is the fourth-largest cable system operator, and ABC, in addition to operating its television and radio networks, is involved in a series of cable joint ventures, including the Westinghouse All News Channel.

"We believe our strength lies in programming, marketing-promotion and technical talents," said Frederick S. Pierce. "We feel we can join forces to produce new forms of entertainment for a broad distribution base."

The venture appears to be designed to join ABC's programming experience with Cox's technical efforts to move into two-way cable services. Cable programmers are eager to use these technologies to develop "pay-per-view" systems, which will allow cable operators to charge for individual showings of particular programs.

Meanwhile, BET is beginning a major expansion. Black Entertainment Television's service is now three hours a week on Friday evenings and will shift in August to a six-hour-a-day, seven-day-a-week format. Johnson said he expects the service to be carried in 850 markets, including every major urban cable system.

In addition to the marketing clout that comes with ties to a national company, BET also expects to benefit by having an obvious inside track to distribution on cable systems owned by Group W, now the third-largest cable operator with more than 1.6 million subscribers.

Group W is the corporate unit that recently absorbed Teleprompter, which Westinghouse purchased last year.

"We expect to be on each of Group W's systems in markets with significant black populations," Johnson said.

Johnson is the majority stockholder in BET, although Tele-Communications Inc. of Denver and Taft Broadcasting Co. are equity partners in the firm.