HVDC transformers are key components of HVDC stations.
HVDC converter and inverter stations terminate long-distance DC transmission lines or DC sea cables. This type of transformer provides the interface between AC grids and high power rectifiers and are used to control the load flow over the DC transmission lines.
The design concept of HVDC transformers is mainly influenced by the rated voltage, rated power and transportation requirements like dimensions, weight and mode of transportation. Many large power HVDC converter station are located in rural areas of low infrastructure.
Frequently, special geometrical profiles have to be fulfilled in order to move such transformers by railway.
Typically, HVDC transformers are single phase units containing 2 winding limbs. This concept can include either 2 parallel valve windings (two for delta or two for wye system, figure 1) or two different valve windings (one for delta and one for wye, figure 2).
In order to reduce the total transportation height frequently the core assembly includes 2 return limbs.
Due to redundancy requirements in HVDC stations 3 phase units are quite uncommon.
The valve windings are exposed to AC and DC dielectric stress and therefore a special insulation assembly is necessary. Furthermore, special lead systems connecting the turrets and windings have to be installed in order to withstand the DC voltage of rectifier.
Additionally, the load current contains harmonic components of considerable energy resulting in higher losses and increased noise.
For approving the proper design and quality of manufacturing special applied DC and DC polarity reversal tests have to be carried out. The test bay has to be equipped with DC test apparatus accordingly and needs to provide adequate geometry to withstand the DC test voltage.
In addition to the standard parameters of power transformers, special performance requirements have to be known for the design of HVDC transformers.
These parameters are jointly defined by designers of the HVDC station and transformer design engineers in order to reach a cost-effective design for the entire equipment. Special parameters are:
- Test levels: DC applied, DC polarity reversal and long-time AC defines the insulation assembly of the transformer
- Harmonic spectrum of the load current and phase relation generate additional losses, which have to compensated by the cooling circuit
- Voltage impedance impacting the dimensions of windings and the total height of the transformer
- DC bias in load and current and transformer-neutral have to be considered for no-load noise and no-load losses
- Derivative of the load current (di/dt) is a key parameter for the on-load tap changer
- Overload requirements have to be considered for cooling circuit and capacity of coolers
- Regulation range and number of steps influence the voltage per turn which is a key design parameter
- Seismic requirements have to be considered for mechanical strength of turrets, outlets and bushings.
HVDC – Connecting remote power generation to the market
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Reference: Siemens Energy Sector – Power Engineering Guide Edition 7.0