HVDC Transformers


Today, we will continue our online course EP-3 for Transformers, and still explaining different types of Power Transformers as follows:


HVDC (High Voltage Direct Current) transmission advantages over than AC transmission:
  • When bulks of power are transmitted over long distances by overhead lines or under the sea by means of submarine cable, HVDC transmission will be economical (see fig.1) and technically advantageous alternative to AC transmission. 

fig (1): Comparing Costs between High
 Voltage DC and AC Transmission
  • In DC transmission, the stability aspects do not limit the power transmission capability of the line. 
  • For a given quantity of conductor material in the line the losses in a dc line are lower than the losses in an ac line. 
  • Compared to AC transmission line, DC lines can operate with narrow corridors. Large areas of farm land or forests may then be saved. 
  • The conversion of alternating current into direct current allows for transmission with comparatively low losses and for the interconnection of grids of different frequencies. 

HVDC Transmission components:(see fig.2)

  • HV Overhead lines or Submarine cables 
  • Two convertor station; at each end of the line or cable 

fig (2): HVDC Transmission components



Now, we only interested in the second components; Converter stations which will be explained in the following paragraphs.

Convertor station:
The main components of the converter stations are as follows:

  1. HVDC transformers 
  2. Convertors 
Convertor Receiving Station

First: HVDC transformers
Rating:

HVDC transformers can operate in Transmission systems with dc voltage up to ± 800 KV.
HVDC Transformer 283.7 MVA 
HVDC Transformers uses:  
  • HVDC transformers make it possible to transport electrical energy over long distances with comparatively low losses and cost. 
  • HVDC transformers can easily combine two different AC networks together, even with different frequencies. 
  • HVDC transformers act as galvanic separation between the AC and DC systems and this prevents the direct current from entering the ac systems. 
  • HVDC transformers used to ensure an appropriate tapping range for part of the DC voltage and for reactive power control. 

Design Options of HVDC Transformers
Depending on voltage and transferred power rating, The HVDC converter transformer can be built as three-phase units or as single-phase units


fig (3): Two Valve Windings Transformer 
The converter transformer is generally built with two valve windings (see fig.3) of equal power and voltage ratings. One of the windings is connected in star and the other in delta. With this arrangement the dominant harmonics from the converter will be cancelled out.

Single-phase HVDC transformers: (see fig.4)

HVDC transformers are normally single-phase transformers, whereby the valve windings for the star and delta connection are configured either for one core with at least two main limbs or separately for two cores with at least one main limb, depending on the rated power and the system voltage.

fig (4): HVDC Transformer Configurations

Three-phase HVDC transformers:  (see fig.4) 

When HVDC transformers built as three-phase transformers, there is generally one unit with the valve winding arranged for star connection and the other unit for delta connection.

Tapping:
To control the service voltage and provide reactive power control, HVDC transformers are equipped with a fairly large tapping range on the line side which permits optimum operation and making a large variation in load without loss of efficiency.

Note: IEC 61378-2 specifies that the impedance variation at the principal tapping and the variation of the impedance over the tapping range for transformers of duplicate or similar design for the purpose of identical in service or interchangeable ability shall not exceed ± 3% of the mean tests values.

The reason for this requirement is to great extent the need for efficient cancelling of harmonics.


HVDC transformers insulation:
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. 
  • The valve windings which are connected to the rectifier and the converter circuit are subject to the combined load stress of AC and DC voltage. 
  • Transient voltages from outside caused by lightning strikes or switching operations. 

Consequently, HVDC transformers contain a much higher share of solid insulation than in ac transformers around three times as much and Special tests have to be performed to check these windings.


Testing:
To determine the ability to withstand dc voltage, the valve windings of HVDC transformers are subject to the following tests:

  1. Dc separate source voltage withstand test (IEC 61378-2 sections 8.2.3 and 10.4.3) 
  2. Polarity reversal test (IEC 61378-2 sections 8.2.4 and 10.4.4) 

You can watch a video showing HVDC Transformer Repair process by following the link.

Second: Convertors
1- Convertor Rectifier:
At the sending end of the line or cable there is a convertor station which operated as a rectifier. Firstly, the alternating voltage of supply system is transformed to voltage level suiting the convertor rectifier to transmit the intended power. It is then rectified with a controlled vales consisting of thyristors semi conductors. (see fig.5)


fig (5): Valve Module with Direct Light Triggered Thyristor

2- Convertor inverter:
At the receiving end of the line or cable there is another converter station. This is operated as an inverter that converts the direct current back to alternating current, which then is transformed to the voltage of the network being supplied.

Actually, the inverter can convert the current by any frequency. The frequency can be given by the ac system itself or by a separate synchronous machine. So an HVDC-link is suitable for connecting together ac systems with different frequencies, for example 50 and 60 HZ.


Note: The direction of power flow can easily be changed without interruption of the operation.



In the next topic, I will show another type of power transformers which will be " Industrial Transformers". please, keep following.

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