Unified power flow controller

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A unified power flow controller abbreviated UPFC : (to German Unified Power Flow Controller ) referred to in the electrical power engineering a component that is used in national and meshed power systems , the transmitted electric power in individual lines as overhead lines to be able to selectively control and influence . With UPFC, specific, contractually agreed service transfers can be met in meshed networks.

The area of ​​application is geared towards the high- voltage level of electrical interconnected networks operated with three-phase alternating current , such as the voltage levels of 380 kV and 220 kV common in Europe  , and uses power electronics to control power electronics that are implemented in a similar way to high-voltage direct current transmission (HVDC). In contrast to HVDC, the power electronics are not used directly to transmit energy by means of direct current, but to influence and modulate the power transported on the alternating current side. In the field of power electronics, the Unified Power Flow Controller is one of the flexible three-phase transmission systems ( Flexible AC Transmission System (FACTS)).

function

Basic circuit of a unified power flow controller (UPFC)

A UPFC consists of a parallel-connected power transformer T 1 and a series transformer T 2 , which is implemented in a similar way to a current transformer , similar to a phase-shifting transformer and as shown schematically for an outer conductor in the adjacent figure . The two transformers are connected via two converters and a DC link. The high-voltage line is connected in series with the UPFC, regardless of whether the UPFC is attached at the beginning or at the end of the line. For three-phase networks, a corresponding extension with three-phase alternating current transformers is necessary, whereby the direct current circuit is uniform.

Due to the timing, in particular via the current flow angle and the phase position of the two converters, the power flow between T 1 and T 2 and the complex differential voltage between U 1 and U 2 can be controlled with the UPFC , with only active power being transmitted via the direct current intermediate circuit can be. The following controls can be carried out independently of one another:

  • With T 1 in connection with the first inverter, both inductive and capacitive reactive power can be made available in the network for cross-compensation of the reactive power . This part of the circuit represents a Static Synchronous Compensator (STATCOM). The UPFC can replace any additional equipment required for static reactive power compensation (SVC).
  • With T 2 in connection with the second inverter, a longitudinal compensation of the reactive power can take place. This can be used to influence the natural power or to adjust the line impedance of the line connected in series.
  • By transferring active power between the two transformers, a specific active power flow in the line can be specifically enforced, as with the phase-shifting transformer. The phase angles can be set more precisely, faster and in larger areas than with phase shifting transformers.
  • In the case of three-phase applications, UPFC can be used to achieve an unbalanced load to a certain extent through asymmetrical control of the inverters responsible for the individual phases in order to compensate for unbalanced loads in the grid. The power equalization between the phases takes place via the common DC voltage intermediate circuit.

The disadvantage is the comparatively complex and costly HVDC power electronics of the inverters and the necessary control electronics, including the computer-aided control levels. The power of the inverters is in the order of magnitude of 20% of the power flows to be controlled, which results in apparent power in the range of several 100  MVA for the dimensioning . The DC voltages in the intermediate circuit are in the range of a few 10 kV. Specific values ​​are largely determined by the respective application.

Individual evidence

  1. HV Hitzeroth, D. Braisch, G. Herold, D. Povh: Stability of networks with UPFCs for feedthroughs and shielding by means of electronic fences . International Energy Industry Conference (IWET), conference proceedings, Vienna 1999 ( uni-erlangen.de [PDF]).
  2. ^ Nijaz Dizdarević: Unified Power Flow Controller in Alleviation of Voltage Stability Problem . Dissertation, University of Zagreb, 2001 ( eihp.hr [PDF]).

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