The high voltage DC power line transmission is a system in which the bulk power is transmitted over long distances using direct current at a much-elevated voltage level. For long-distance transmission, the conventional AC system incurs various power losses making it an expensive mode of transmission. However, high voltage DC power lines cut down on these losses to a greater extent.
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High voltage DC power lines do not require any heavy current for charging and discharging of its capacitance each cycle. High voltage DC power line transmission allows operation between unsynchronized AC transmission systems. As the flow of power is controlled independently of the phase angle between source and load, the disturbances in the network due to rapid changes in power can be stabilized. High-voltage DC power lines are capable of transmitting power between grids of different frequencies.
Components of high voltage DC Power line
Converter station:
The major component of the high voltage DC power line is the converter stations which convert the AC to DC also known as rectifier stations and from DC to AC commonly known as inverter stations. High voltage DC power line requires two numbers of these stations so that the DC power can be linked to AC grids on both sides. In the case of reversed power flow, the role of rectifier and inverter stations can be reversed by the use of converter control.
Converter Unit:
These units basically consist of two numbers of three-phase converter bridges, series connected which form a twelve pulse converter. The valves switch the segment of the AC voltage waveform. The total number of valves in such units is twelve and can be arranged in single-valve, double-valve, or quad-valve arrangements. The converter is powered by the converter transformers connected in star/star or star/delta arrangements.
The cooling of these valves can be air-cooled or liquid-cooled. Liquid cooling with deionized water results in efficient cooling and a reduction in station loss. The rating of these valves depends on the short circuit current capacity and the steady-state load requirements. The design of these valves depends on the number of series connected thyristor levels that can be contained in a module.
These valves regulate the power flow by adjusting the firing angle in the LCC or modulation index in the VSC. The firing signals are generated in earth potential, and transmitted to each thyristor in the valve through fiber optics. This signal at the thyristor level is converted to an electrical signal using gate drive amplifiers and pulse transformers.
Controlling the conduction period of the valve, the voltage at the AC and DC sides can be maintained. The quick shut-off operation of the valves during the fault condition helps protect the converter. The valves are also protected by snubber circuits and gapless surge arresters.
Converter Transformers:
These have different configurations
Three-phase two winding: which is ideal for large-scale systems for robust transformation and balancing,
Single phase three winding: These offer greater flexibility and voltage control suitable for single-phase system connections
Single-phase two-winding: This type is mostly used for simpler and small-scale projects with basic isolation and transformation.
The valve side windings are configured in star or delta with ungrounded neutral. The delta configuration cancels out the third harmonics and its multiples while ungrounded neutral reduces the excessive ground fault current to flow in the winding allowing better transient performance. On the AC side, the transformers are connected in parallel and neutral grounded. The leakage reactance of these transformers is chosen to limit the short circuit currents through the valves.
The converter transformers are designed to withstand high voltage DC stress and increased eddy current losses resulting from harmonics. Unsymmetric firing of the valves can cause DC magnetization of the core, which can become a big issue for these transformers.
Filters:
Three types of filters are used in the converter stations:
AC filters: With tuned and damped filter arrangements these act as passive circuits which provide low impedance and shunt path for AC harmonic currents.
DC filters: They are used to filter out the DC harmonics
High-frequency RF and PLC filters: They are connected between the converter transformer and AC bus to suppress the high-frequency currents. They are also connected between the DC filters and the high voltage DC power line.
Reactive power source:
A converter station requires a reactive power source depending on the active power loading of the high voltage DC power line which is about 50-60% of the active power. Part of the requirement is fulfilled by AC filters. Static VAR systems, shunt-switched capacitors, and synchronous condensers are used depending on the desired control as the quick response of reactive power sources helps stabilize the voltage. The presence of a reactive power source helps mitigate harmonics generated by the converter equipment.
Smoothing Reactor:
The series reactor is basically used on the DC side to smooth the DC current and also aid in the protection system. The reactor is designed as line reactor and is connected in the line side.
DC switchgear:
Modified AC equipment is used for interruption of the DC current example is a disconnecting switch or isolator. Metallic return transfer breakers are used for interruption of the rated load current which uses a metallic return or ground return path.
In addition, the AC switchgear and associated circuitry are also used in the AC side of the converter stations.
Types of high voltage DC power line Link
Three types of DC links are used in high-voltage DC power line applications
Monopolar Link:
A monopolar link basically has one power conductor through which DC current flows. This type uses the ground or sea as a return path. Metallic return paths can also be taken in use where harmonic interferences are high. As the effects of corona are less in the negative polarity of DC lines compared to positive polarity, hence monopolar links are essentially operated with negative polarity.
Bipolar Link:
A bipolar link as the name suggests, uses two conductors one positive and another negative. The terminal has two sets of converters of equal ratings connected in series on the DV side. The junction between the sets is grounded via a short electrode. During normal operating conditions, zero ground fault current flows through the electrode. In the first stage of development, monopolar operations can be done by one conductor. However, during faulty converter conditions, the metallic return can be made out of one conductor temporarily with suitable switching arrangements.
Homopolar Link:
In this type of link, two conductors with the same polarity mostly negative are operated with a ground or metallic return. Because of the disadvantages of operating an high voltage DC power line link with ground return, bipolar links are mostly used. The homopolar links require less insulation however it outweighs the disadvantage of using a ground return.
Advantages of high voltage DC power line transmission
Some advantages of DC transmissions which are lacking in AC are
- Full control over transmitted power
- It provides enhanced transient and dynamic stability in associated AC networks.
- Faster control in limiting the fault current in DC lines.
Disadvantages of HVDC power line transmission
- The cost of equipment is high
- Inability in direct use of transformer for changing voltage levels
- The generation of harmonics and the requirement of AC DC filters add to the cost of the conversion
- Complex control
Application of HVDC power line transmission
The following are the application areas:
- Used for long-distance and bulk power transmission
- Used for underwater or underground cable transmission
- Used to provide a synchronous interconnection between two AC grids working at different frequencies
- In an integrated power system, the control and stabilized power flow can be achieved by high voltage DC power line transmission.
