While the use of high-voltage DC (HVDC) results in lower transmission losses compared with high-voltage AC (HVAC), 5-6% losses for HVDC versus 8-10% for HVAC, the overall cost-justification for HVDC is more complicated. HVAC terminals are much less expensive than the power electronics-intensive HVDC terminals. But HVDC cabling can be less costly than an HVAC transmission system carrying the same power level.
As seen in the image below, the breakeven distance is generally considered to be about 500km; however, with recent advancements in power semiconductors, the breakeven has moved as low as 50km for some use cases. Various use cases for HVDC and HVDC system architectures will be the subject of the next installment in this FAQ series, “HVDC Transmission System Architectures and Use Cases,” this FAQ will delve into HVDC converter topologies, components, and control systems.
HVDC power converters
Classic HVDC using thyristors (typically SCRs) employs a line commutated converter (LCC) topology. The name is derived from the fact that the ac line is used to turn off (commutate) the thyristor current. This requires a short period of ‘reverse’ voltage (turn-off time) to switch the thyristor off. Modern LCC converters use 12 valves in a 12-pulse bridge. The input is split into two three-phase supplies, one configured as a star (wye) configuration and the other as a delta. That results in a 30° phase difference between the two sets of three phases. This arrangement significantly reduces harmonics and system costs.
Performance limitations of LCC converters are addressed by inserting series capacitors into the ac line connections resulting in the capacitor-commutated converter (CCC) topology. The series capacitors partially offset the commutating inductance of the converter and reduce fault currents. It also allows a smaller extinction angle to switch off the thyristors, reducing the need for reactive power support.
The introduction of high-voltage and high-current GTO thyristors and IGBT modules for HVDC converters has enabled developing the self-commuted voltage-source converter (VSC) topology has relegated the CCC topology to near extinction and replaced much of the LCC applications. In addition, multilevel topologies have the advantage that harmonic filtering needs can be significantly reduced or eliminated. The AC harmonic filtering equipment of LCC HVDC stations can cover nearly half of the overall station area. For the highest power installations, thyristors continue to be used due to their lower cost and higher efficiencies.
Power Semiconductors for HVDC
Since modern thyristors can switch power on a megawatts’ scale, thyristor valves have become the heart of high-voltage direct current (HVDC) conversion either to or from alternating current. Both electrically triggered (ETT) and light-triggered (LTT) thyristors are available. The optical triggering of LTTs simplifies circuit design by eliminating the requirement for an external electrical trigger circuit. And modern thyristors can include integrated safety features such as overvoltage protection, DV/DT protection, and protection against forwarding voltage transients during recovery time, further reducing component count and design complexity. Thyristors are arranged into a diode bridge circuit, and to reduce harmonics, they are connected in series to form a 12-pulse converter. Typical HVDC systems have about 100 thyristors in series connection in a single valve.
Originally, thyristors relied only on a current reversal to turn them off, making them difficult to apply for direct current; newer device types can be turned on and off through the control gate signal. The latter is known as a gate turn-off thyristor or GTO thyristor. More recently, Insulated gate-controlled thyristors (IGCTs) have been developed. The IGCT’s much faster turn-off times than the GTO’s allow it to operate at higher frequencies — up to several kHz for very short periods of time. However, because of high switching losses, the typical operating frequency is up to 500 Hz.
High power IGBT modules have become available rated for over 1MW. IGBT modules are available in various industry-standard housings such as the 190 x 140 mm, 130 x 140 mm, and 140 x 70 mm footprints. Current ratings range from 300A to 3,600A. IGBT modules rated up to 10.2kVRMS are designed for use in electricity transmission and distribution systems such as HVDC installations and large-scale renewable energy installations.
In the future, silicon carbide may enter into HVDC converter designs. An experimental 6-kV, 1-kA SiC GTO thyristor has been fabricated. In another development, a 22 kV SiC emitter turn-off thyristor (ETO) has been demonstrated. The device is based on a 2cm2 22kV p-type gate turn off thyristor (p-GTO) structure. The developers believe that the technology can be scaled up for ultra-high voltage (over 800kV) applications in the future.
HVDC Converter Transformers
The converter transformers step up the voltage of the AC supply network. Using a star-to-delta or “wye-delta” connection of the transformer windings, the converter can operate with 12 pulses for each cycle in the AC supply, eliminating numerous harmonic current components. The insulation of the transformer windings must be specially designed to withstand a large DC potential to earth.
Converter transformers can be built as large as 300 MVA as a single unit. It is impractical to transport larger transformers, so several individual transformers are connected when larger ratings are required. Either two three-phase units or three single-phase units can be used. With the latter variant, only one type of transformer is used, making the supply of a spare transformer more economical.
Cabling for HVDC versus HVAC
There is a growing variety of cables designed for use in HVDC transmission systems. A conventional high-voltage cable for HVDC transmission has the same construction as a cable used for HVAC transmission. In terms of performance, however, HVAC’s use necessitates taking the skin effect into consideration, which limits the power handling capabilities of cables used in AC systems. There is no skin effect for DC systems, and cable utilization is, therefore, more efficient. Many HVDC cables are used for DC submarine connections because, at distances over approximately 100 km, AC can no longer be used. Most of these long deep-sea cables are made in older construction, using oil-impregnated paper as an insulator.
Newer technology is mass impregnated (MI) cables. These cables are currently suitable for voltages of up to 500 kV DC. The insulation consists of high-density paper tapes impregnated with a high-viscosity compound, which does not require fluid pressure feeding.
Cross-linked polyethylene (XLPE) cable is another common technology found in HVDC transmission systems. XLPE insulation performs at both high and low temperatures. XLPE is extremely resistant to abrasion and other wear and tear. It also boasts resistance to high voltage electricity, chemicals, and other hazardous materials.
NKT offers a 640kV XLPE DC extruded cable system for use with HVDC transmission and renewable energy resources. It has been optimized specifically for use in underground installations.
Sumitomo Electric offers its HVDC XLPE cable that enables polarity reversal operations and significantly higher operational conductor temperature (90°C) than that of MI cables. These features allow system designers to achieve various benefits, including reduced CAPEX and increased operating margins with a conventional LCC converter.
There are already a number of superconducting cables operating in AC networks. However, the EU-funded ‘Best Paths’ project has focused on investigating HVDC solutions for bulk power transmission with a modular design that is easily adaptable so that the rated current and voltage can be matched to any power grid specification. Nexans recently completed successful qualification testing of a ‘Best Paths’ superconductor cable for HVDC power links. The Nexans qualified the 320 kV direct current superconducting cable for currents up to 10 kA with a 3.2 GW power transmission capability.
Control and protection
Controllability is an important aspect of maximizing the performance of modern HVDC systems. The type of hardware and system software used for a VSC-HVDC control system are the same as in an LCC-HVDC control system; only the application software and the valve controls differ. In a typical control and protection system, data transmission is via fiber optics based on an IEC standard protocol and communication with SCADA through high-speed Ethernet. Customer requirements and specifications drive the design of the control and protection systems for individual HVDC installations. There is no overall performance benchmark. That is going to change.
CIGRE has established a working group (WG) titled, “Wide Area Monitoring Protection and Control Systems – Decision Support for System Operation.” The current state of monitoring, diagnosis for HVDC systems will be studied in this WG. The pertinent health indicators will be identified for the different HVDC components and subsystems. The different condition monitoring approaches which lead to the diagnostics will be compared. Furthermore, in regard to safety, reliability, and cost-effectiveness of HVDC systems, traditional, preventive, and predictive maintenance strategies will be benchmarked.
The WG will work toward identifying and qualifying the indicators to help the decision making of facilities owners to develop/implement optimized station maintenance strategies (i.e., in response to the degradation of the detected components to decide on the follow-up such as refurbishment, replacement, possible life extension measures in comparison to the maintenance cost and the operation needs). The working group started in July 2020, and the final report is due September 2022.
You might also enjoy reading part 1 of this HVDC FAQ series, “Over a Century of High Voltage DC Power Transmission,” and part 3, “HVDC Transmission System Architectures and Use Cases.”
Resources:
22 kV SiC Emitter turn-off (ETO) thyristor and its dynamic performance including SOA, ResearchGate
An Innovative Silicon Carbide (SiC) 6-KV, 1-KA Gate Turn Off (GTO) Thyristor, SBIR announcement
High-voltage direct current, Wikipedia
Introduction to HVDC Architecture and Solutions for Control and Protection, Texas Instruments
Nexans completes successful qualification testing of ‘Best Paths’ superconductor cable for HVDC power links, Nexans
Wide Area Monitoring Protection and Control Systems – Decision Support for System Operation, CIGRE
Good information for HVDC and HVAC