Authors: Heiko Kraut, TEN Thueringer Energienetze and Walter Schossig, VDE Thuringen, Germany
In the 20- or 10/0.4-kV-low voltage stations HH (=high-voltage-high breaking-capacity, h..v.h.b.c. fuse) or NH- fuses(low-voltage high breaking-capacity fuse, l.v.h.b.c. fuse) are used with transformers 250 up to 630 kVA (Dyn5).
In the 20 kV load-disconnecting switches are used on the lines, every 10 stations circuit breakers equipped with protection for the lines are available. Two power station infeeds with 310 MW are connected via unit transformers to the 110 kV grid. Additional infeeds from power stations are in the medium voltage.
The typical setup of a line feeder, a transformer feeder and a 110 kV coupling for double busbar stations is shown in Figures 2 and 3. In case of looping-in of lines a “compact switchgear” is used consisting of two line feeders and two transformer feeders. On the medium voltage a switchable busbar is realized in BB1 and BB2 with circuit breaker. So in normal state one transformer per section A and section B powers the busbar.
The 110-kV-grid as well as the medium voltage are operated compensated. This means that the star points of the transformers are connected with Petersen coils to the station ground for compensation of the capacitive residual current. In some medium voltage grids a mechanism KNOSPE is used.
This German abbreviation means short-time-low-resistive-star-point-grounding.Power station’s supply as well as some municipality infeed is realized with permanent low-resistive-star-point-grounding.
For the protection of lines, transformers, generators and switchyards 3780 digital relays and 790 electromechanical relays are in operation.
Fundamentals of Protection Design
For new substations as well as retrofit the following fundamentals apply:
Table 1 provides an overview.
The detailed functions should be explained later in this article covering the different protection objects.
In the medium voltage integrated protection and control is used: