Unified-Power-Flow-Controller

A unified power flow controller UPFC abbreviated ( to German: Unified power flow controller) referred to in the electric power industry a component that is used to selectively control in national and meshed power networks, the transmitted electric power in individual lines as overhead lines and influence can. With UPFC target specific, contracted power through lines can thus be met in meshed networks.

The application is geared towards the powered with three- phase alternating-current high -voltage level of electrical interconnections, such as are customary in Europe voltage levels of 380 kV and 220 kV and employs to control a power electronics, which of the high voltage direct current transmission is similarly implemented (HVDC). In contrast to the HVDC power electronics is not directly the energy transfer by means of direct current, but the influence and modulate the transported on the AC side benefits. The Unified Power Flow Controller counts in the field of power electronics to the flexible AC transmission systems (English Flexible AC Transmission System ( FACTS) ).

Function

A UPFC is similar to a phase shifting transformer and as shown in the adjacent diagram schematically for an outer conductor of a parallel-connected power transformer T1 and a series transformer T2, which is implemented similar to a current transformer. The two transformers are connected via two inverters and an intermediate circuit connected. The high voltage line is connected in series with the UPFC, it being immaterial whether the UPFC is attached to the beginning or end of the line. For three-phase systems a corresponding expansion with three-phase alternating current transformers is necessary, the DC circuit is uniform.

By the timing control, in particular the current flow angle and the phase angle of the two inverters can be controlled between U1 and U2 with the UPFC, the power flow between T1 and T2 and the complex difference voltage, on the DC link generally only active power can be transmitted. It can use the following controls are made ​​independently of each other:

• With T1 in conjunction with the first inverter can be made ​​available to the longitudinal reactive power compensation of both inductive and capacitive reactive power in the grid. This circuit portion provides a Static Synchronous Compensator ( STATCOM ) dar. It can replace the UPFC otherwise necessary additional facilities for static reactive power compensation ( SVC).
• T2 associated with the second inverter a pitching compensation of the reactive power can be effected. This can be used for influencing the natural performance or to adapt the line impedance of the series-connected line.
• By an active power transfer between the two transformers can be forced in the conduit as in the phase shifting transformer a certain targeted active power flow. The phase angle can thereby be more accurate, faster and set over larger areas than in phase -shifting transformers.
• For three-phase application an unbalanced load can be achieved by asymmetric activations of the officers of the various phases inverter using UPFC, to a certain degree in order to compensate for unbalanced loads in the network. The power balance between the phases takes place via the common DC link.

The disadvantage is the comparatively complicated and costly HVDC power electronics of the inverter and the necessary control electronics including computer-aided control levels. The output of the inverter is in the order of 20% of the controlling power flows, which results for the dimensioning apparent power in the range of several 100 MVA. The DC voltage in the intermediate circuit are in the range of some 10 kV. Specific values ​​are largely determined by the particular application.