POWER TRANSFORMERS FOR HIGH-VOLTAGE DIRECT-CURRENT (HVDC) TRANSMISSION SYSTEMS

Tailored-to-fit and delivered fully-assembled, Siemens HVDC Power transformers enable reliable and highly-efficient HVDC power transmission systems

Siemens expertise and experience produces superior transformers for the HVDC schemes, HVDC Classic and HVDC Plus, in bulk power transmission, offshore and onshore grid connections, back-to-back systems and other HVDC use cases. Portfolio: transformers, upgrades, replacement, repair, refurbishment, and the supply of selected components HVDC ratings: up to 12 GW and up to 1,100 kV DC AC grid connections: up to 1,050 kV AC Features: Compact designs, high reliability, and proven technology from Siemens’ global manufacturing network

Deregulation, privatization, growing demand, and environmental considerations have grid operators facing a new challenge: building secure and efficient power grids that delivers clean, reliable, and affordable power over long distances to increasingly urbanized societies. Innovative HVDC transformers play a key role in shaping environmentally sustainable power grids while offering economic efficiency.

Smart grids are emerging as a necessary response to the environmental, social and political demands placed on energy supply.

With their capacity to increase transmission efficiency, innovative HVDC solutions will play a key role in making sustainable low-loss AC/DC hybrid grids a reality. With the transmission power capability of HVDC systems steadily on the rise, requirements for converter transformer technology and design have greatly increased. Customers rely on Siemens’ experienced design teams to help them master transmission and grid challenges around the globe.

Bringing stability and security to grid and transmission

HVDC transformers are key components of HVDC stations for long-distance DC transmission lines or DC sea cables. As coupling elements between AC grids and high-power rectifiers, they adapt the voltage, insulate the rectifier itself from the AC grid, and generate a phase shift.

Back-to-back station

Back-to-back links are an easy solution for connecting two AC networks with deviating frequencies. Grid operators not only strengthen and optimize the performance of their grids with back-to-back links. They also allow both network users to balance varying loads and generation caused by different time zones or usage patterns.

Long-distance overhead and submarine transmission

HVDC transformers are used as coupling elements between AC grids and high-power rectifiers for very long overhead lines or submarine cables. Converting AC lines to DC can help increase the power transfer volume through existing rights of way and typically cuts line losses by 30-50%. HVDC is most efficient for overhead lines above 1,000 Mega Watt (MW) and 600 kilometers.

Replacement of HVDC transformer originals

With about one third of HVDC lines worldwide past the 25-year mark, operators need to decide – invest in repairs that will give them an as-good-as-new transformer from the 1980s, or benefit from the latest technology? Siemens engineers excel in replacing HVDC transformer originals, producing units that replicate performance, capacities, and dimensions of the original unit but offer the latest in transformer innovation.

HVDC technologies for long distances and low losses

Siemens offers two HVDC technologies: HVDC Classic and HVDC Plus. Siemens transformers meet all requirements for power, voltage, mode of operation, low-noise, connection types, types of cooling, transport, and installation. They also comply with specific national design requirements.

HVDC converter transformers for remote energy transmission

HVDC transformers are subject to operating conditions that set them apart from conventional system or power transformers. These conditions include:

·       combined voltage stresses with both AC and DC

·       high harmonics content of the operating current

·       DC pre-magnetization of the core

·       ideally, equal performance parameter, when installing multiple units.

In addition, the valve windings connected to the rectifier and the converter circuit are subject to the combined load stress of DC and AC voltage.

HVDC Classic

HVDC Classic transformers are line-commutated current-sourced converters. Siemens delivers HVDC Classic Converter transformers for transmission lines up to 12 GW, up to 1,100 kV DC with connections to AC grids up to 1,050 kV.

The high harmonics content of the operating current results from the virtually quadratic current blocks of the power converter. The odd-numbered harmonics with the ordinal numbers of 5, 7, 11, 13, 17 cause additional losses in the windings and other structural parts. Furthermore, additional DC currents on the valve and/or neutral terminals need to be considered and managed. The high bulk power ratings also require a high rating of HVDC classic converter transformers which must share specific performance parameters, e.g. short circuit impedance.

HVDC Plus

HVDC transformers are used for self-commutated voltage-sourced converters. Siemens delivers HVDC Plus Converter transformers for transmission lines up to 1,200 MW, up to 500 kV DC with connections to AC grids up to 525 kV.

In HVDC Plus mono-pole applications, no DC voltage stresses occurs at the transformer valve terminals. In HVDC Plus, bi-pole applications the valve windings which are connected to the HVDC Plus rectifier/converter circuit are subject to the combined load stress of DC and AC voltage. The harmonics content of the operating current is much lower than in HVDC Classic applications. It usually meets grid code requirements. Additional DC currents on the valve and/or neutral terminals have to be considered. They are much lower than in HVDC Classic applications.

Siemens HVDC Power transformers: Siemens Transmission Products

Transmission products play a vital role in the energy value chain. Siemens Transmission Products provides all key elements including power transformers and distribution transformers as well as gas-insulated and air-insulated switchgear and components. Products are offered individually, bundled or with related engineering upgrade to systems. Our products offer low life cycle costs with maximum availability, highest efficiency and short commissioning times.