Remnant and Existent DC Systems
Some cities continued to use DC well into the 20th century. In central Helsinki, there was a DC network in existence up until the late 1940s, and in the 1960s, Stockholm's dwindling DC network was eliminated. A mercury arc valve rectifier station could convert AC to DC where networks were still used. In 1942, the Greenwich Village neighborhood in New York City used DC. Parts of Boston, Massachusetts along Beacon Street and Commonwealth Avenue still used 110 volts DC in the 1960s, causing the destruction of many small appliances (typically hair dryers and phonographs) used by Boston University students, who ignored warnings about the electricity supply. New York City's electric utility company, Consolidated Edison, continued to supply direct current to customers who had adopted it early in the twentieth century, mainly for elevators. The New Yorker Hotel, constructed in 1929, had a large direct-current power plant and did not convert fully to alternating-current service until well into the 1960s. This was the building in which AC pioneer Nikola Tesla spent his last years, and where he died in 1943. In January 1998, Consolidated Edison started to eliminate DC service. At that time there were 4,600 DC customers. By 2006, there were only 60 customers using DC service, and on November 14, 2007, the last direct-current distribution by Con Edison was shut down. Customers still using DC were provided with on-site AC-to-DC rectifiers. The city of San Francisco, California featured a DC power grid to supply power for pre-1940s winding-drum elevators. Around the end of 2010, the DC grid was divided into 171 separate islands with each island supplying 7 to 10 customers.
The Central Electricity Generating Board in the UK continued to maintain a 200 volt DC generating station at Bankside Power Station on the River Thames in London as late as 1981. It exclusively powered DC printing machinery in Fleet Street, then the heart of the UK's newspaper industry. It was decommissioned later in 1981 when the newspaper industry moved into the developing docklands area further down the river (using modern AC-powered equipment). The building was converted into an art gallery, the Tate Modern.
Electric railways that use a third-rail system generally employ DC power between 500 and 750 volts; railways with overhead catenary lines use a number of power schemes including both high-voltage AC and high-current DC.
High-voltage direct current (HVDC) systems are used for bulk transmission of energy from distant generating stations, or for interconnection of separate alternating current systems. These HVDC systems use electronic devices like mercury arc valves, thyristors, or IGBTs that were unavailable during the War of Currents era. Power is converted to and from alternating current at each side of the HVDC link. An HVDC system can transmit more power over a given right-of-way than an AC system, which is an advantage in overall cost. HVDC systems allow better control of power flows in transient and emergency conditions, which helps prevent blackouts. HVDC is an alternative to AC systems for long-distance, high-load transmission, see List of HVDC projects for example projects.
DC power is still common when distances are small, and especially when energy storage or conversion uses batteries or fuel cells. These applications include:
- Electronics, including integrated circuits, high-power transmitters and computers
- Vehicle starting, lighting, and ignition systems
- Hybrid and all-electric vehicle propulsion with internal power-supply
- Telecommunication plant power (wired and cellular mobile)
- Uninterruptible power for computer systems
- Utility-scale battery systems
- "Off-grid" isolated power installations using wind or solar power
In these applications, direct current may be used directly or converted to alternating current using power electronic devices. In the future, this may provide a way to supply energy to a grid from distributed sources. For example, hybrid vehicle owners may rent the capacity of their vehicle's batteries for load-levelling purposes by the local electrical utility company.
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