What is High Voltage Direct Current Transmission : Advantages & Disadvantages

An Uno Lamm is the father of the High Voltage Direct Current (HVDC) Power Transmission. He is a Swedish Electrical Engineer born on 22nd May 1904 in Sweden and died on 1st June 1989 in California. He completed his masters in “Stockholm at Royal Institute of Technology” in 1927. Some of the companies which provide High Voltage Direct Current (HVDC) products are GE Grid Solutions, ABB (ASEA Brown Boveri) Limited, Siemens AG, General Electric Company, etc. The transmissions are of different types such as overhead transmission, underground transmission, bulk power transmission, etc. The HVDC is one type of power transmission used to transmit power over long distances. This article discusses an overview of HVDC.

What is High Voltage Direct Current Transmission?

The High Voltage Direct Current (HVDC) Power Transmission is used for transmitting huge power over a long distance typically hundreds of miles. When the electricity or power is transported over a long distance, the high voltages are used in power distribution to decrease the ohmic losses. A brief explanation about high voltage direct current transmission is explained below.

HVDC System Configurations

There are five HVDC configuration systems namely Monopolar, Bipolar, Back-to-Back, Multiterminal & Tripolar HVDC Configuration. The explanation of these HVDC system configurations is briefly explained below.

Monopolar HVDC System Configuration

The Monopolar the HVDC system configuration contains DC transmission lines and two converter stations. It uses only one conductor and the return path is provided by the ground or water. The monopolar HVDC configuration figure is shown below.

Bipolar HVDC System Configuration

The bipolar configuration of the HVDC transmission system represents a parallel connection of the two monopolar HVDC transmission system. It uses two conductors one is positive and another one is negative. Each terminal in the monopolar has an equal rated voltage of two converters connected on the DC side in series and the junction between the converters is grounded. In the two poles, the current is equal and there is no ground current. The bipolar HVDC configuration figure is shown below.

Back-to-Back HVDC System Configuration

The back-to-back HVDC system configuration consists of two converter stations in the same location. In this configuration, both rectifier and the inverter are connected in the DC loop at the same place and there is no DC transmission in the back-to-back high voltage direct current transmission system configuration. The back-to-back HVDC system configuration figure is shown below.

Multiterminal HVDC System Configuration

The multiterminal HVDC system configuration consists transmission line and more than two converters connected in parallel or in sequential. In this multiterminal HVDC configuration, the power is transmitting between two or more AC substations and the frequency conversion is possible in this configuration. The Multiterminal HVDC system configuration figure is shown below.

Tripolar HVDC System Configuration

The tripolar HVDC system configuration used for transmission of electricity using Modular Multilevel Converter (MMC). The tripolar HVDC configuration figure is shown in the below.

The rectifier and inverter consist of three-phase six bridge arms MMC converters and two converter valves on the DC side within the structure of this configuration. This configuration is highly reliable and this is the main advantage of tripolar.

HVDC Transmission

The HVDC is an interconnection of AC and DC transmission. It employs positive points of both the AC and DC transmissions. The basic terminologies used in high voltage direct current transmissions are AC generating source, a step-up transformer, rectifier station, inverter station, step-down transformer, and AC load. The high voltage direct current transmission is shown in the below figure.

AC Generating Source and Step-up Transformer

In the AC generating source the power is supplied in the form of AC. Now in the AC generating source, the power is step-up or the voltage of the power is step-up by the step-up transformer. In the step-up transformer, input voltages are low and the output voltages are high.

Rectifier Station

There is an interconnection unit of HVDC in the rectifier station transmission. In the rectifier, we have an AC power supply as input and the DC power supply as output. These rectifiers are grounded and the output of the rectifier employed on overhead transmission lines of HVDC for long-distance transmission of this high DC output and this high DC output from rectifier transfers through over a DC line and supplied to inverters.

Inverters and Step-Down Transformer

An inverter converts the DC input power supply to the output and these AC outputs are supplied to the stepdown transformer. In the step-down transformer, input voltages are high and the output voltages are decreases by sufficient values. The DC step-down transformers are employed because at the consumer ends, if high voltages are provided or supplied then the devices of the consumers are may get damage. So we have to decrease the voltage levels by employing step-down transformers. Now this step-down AC voltage can be supplied to the AC loads. This whole high voltage dc system is very efficient, cost-effective and can supply bulk power over a very long distance.

Comparison of HVDC and HVAC Transmission Systems

The difference between HVDC and HVAC Transmission Systems are shown in the below table:

Advantages and Disadvantages of High Voltage Direct Current

The advantages of the high voltage direct current transmission are

• Current charging is absent
• No proximity and No skin effect
• No stability problem
• Due to reduced dielectric losses, the current carrying capacity of HVDC cable is large
• Compared to AC transmission the radio interference and the corona power loss are less
• Less insulating devices are required
• compared to AC the switching surges are lower in DC
• There are no Ferranti effects
• Voltage regulation

The disadvantages of the high voltage direct current transmission are

• Expensive
• Complex
• Power faults
• Causes radio noise
• Difficult grounding
• Installation cost is high

Applications of High Voltage Direct Current

The applications of the high voltage direct current transmission are

• Water crossings
• Asynchronous interconnections
• Long-distance bulk power transfers
• Underground cables

In this article, the High Voltage DC Transmission advantages, disadvantages, applications and the comparison of HVDC and HVAC Transmission Systems are discussed. Here is a question for you, how to identify the faults in High Voltage DC (HVDC) Transmission?

FAQs

1). What is considered high voltage DC?

The cables or wires considered high voltage over an operating voltage of 600 volts

2). High voltage power lines AC or DC?

The high voltage power lines are Alternating Current (AC) because the resistance losses are low in the cables or wires

3). Why is DC voltage transmitted at high voltage?

There are no stability problems and also no difficulties in synchronization in DC. Compared to Ac systems the DC systems are more efficient therefore the cost of the conductors, insulators and towers are low

4). Which is better AC or DC?

Compared to alternating current the direct current is better because it is more efficient and has lower line losses.

5). What is meant by High Voltage?

When more energy is used from the same amount of current, then it is said to be a high voltage and the range of high voltage is from 30 to 1000 VAC or 60 to 1500 VDC. Some of the high voltage products are power transformers, switch gears, etc