The MAPP project will be using state-of-the-art Direct Current (DC) Voltage Sourced Converter (VSC) technology to transmit large quantities of power over long distances using underground cables, submarine cables and overhead transmission lines. This advanced VSC technology is different from the technology used in classical DC installations and offers many environmental and technical benefits that are useful for today's transmission systems.

Typical DC cable

When is DC transmission an option?

Due to the ease of converting AC voltage to the levels used by household appliances and other equipment, AC is the primary method of electricity transmission and distribution. DC transmission becomes an option when there is a need to transmit large quantities of power over long distances. Direct current (DC) transmission has less power loss than alternating current (AC) transmission over the long distances and design parameters necessary for MAPP.

More information on AC vs. DC  

The major difference with DC transmission is the cost of the converter stations that are necessary to convert between AC and DC. However, the benefits of reduced power loss and a flexible operating capability can justify the installation of a DC system. DC transmission is also used extensively with power transmission via submarine cables. The maximum transmission distance for AC cable systems is limited by the high reactive power flow due to the large cable capacitance of AC transmission cables. In many of these situations, DC transmission becomes the only practical technical alternative.

What does Direct Current mean?

Electricity is transmitted as current. Current flows from generating stations over transmission lines to substations and finally through distribution lines before it reaches our homes and offices. Transmission lines can carry either alternating current or direct current.

• Alternating current (AC) changes direction 60 times per second, which is referred to as a frequency of 60 Hertz (Hz).
• Direct current (DC) flows constantly in only one direction (0 Hz).

For more information on DC Technology.

What are the components of a DC system?

A typical DC system consists of two DC converter stations connected via a transmission line or cable. A converter is composed of high speed switches and filter banks and is used to convert between AC and DC. The converter station on the sending end will work as a rectifier converting AC power to DC power. The converter station on the receiving end will work as an inverter, converting DC power back to AC power. A single DC converter station normally contains the following electrical components:

• Transformers - Three single phase transformer units to step up or step down the voltage level.
• AC Harmonic Filters - Reactors and capacitor banks used to "smooth" the AC waveform.
• Converter Valves - High speed switches used to convert between AC and DC.
• DC Filters - Capacitors used to "smooth" any ripple in the DC waveform

The MAPP project will have two complete DC systems connected by a combination of underground, submarine and aerial transmission lines.

What is a Voltage Sourced Converter?

There are two types of converters that can be used in DC systems; a Line Commutated Converter (LCC) and a Voltage Sourced Converter (VSC). A DC system with VSC technology uses computer controlled switches which are operated in a controlled sequence at a very high speed to convert between AC and DC. When compared to a conventional LCC station, a VSC station typically takes up less space and needs very little filtering. This technology has been developed and implemented successfully over the last decade and provides many advantages to the transmission system.

What are the benefits of using a Voltage Sourced Converter DC system?

A DC system with VSC technology is an environmentally friendly solution for transmitting power that fits the needs of today's electric industry. The modern transmission system needs to be robust and have the ability to adapt to a constantly changing generation and load profile.

Some benefits of a DC system with VSC technology include:

• Utilization of polyethylene-based non-oil filled cables with little environmental impact.
• Ability to connect renewable energy sources to the transmission system (i.e. wind power).
• Magnetic and electric fields are static and are at very low levels.
• Assist with power quality control by reducing flicker and voltage excursions caused by system disturbances.
• No reactive power compensation necessary aside from small filter banks.
• Ability to control active and reactive power independently.
• Quick and smooth power reversal.
• "Black start" capability - Converter can act as a static generator waiting to supply power to an area after a blackout.
• Ability to supply electrical networks with little or no generation.

Many of the benefits stated above will allow for better control of the power flow across the Mid-Atlantic region, which reduce congestion costs and increase reliability.