Concept of Protection, Automation and Control of a German Distribution System Operator

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:

  • The main protection for lines, transformers, generators, motors and couplings shall be digital protection
  • The protection relays have to be powered with 220 VDC, a nominal current of 1A, a nominal voltage of 100 V is mandatory as well as serial interface, switchboard or cubicle design

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:

  • The relays used have to be compliant to the rules in Germany (VDE/FNN), Austria (VEO OSTERREICHS ENERGIE) and Switzerland (VSE Verband Schweizerischer Elektrizitatsunternehmen)
  • The principle main protection and backup protection is guaranteed permanently
  • The fault clearance times that allowed detection of fault and interruption are presented in Table 2.
  • In the compensated grid earth faults are indicated only but switched off manually
  • Test plugs are mandatory for all relays. When plugging in the CTs are short-circuited automatically, tripping- and indicating connections are opened and the serial connection is set to test mode. Now the test set can be connected without any danger
  • In the substations all circuit breaker, disconnectors and earthing switches are operated by the central control center (“load dispatcher”) in Erfurt. All substations are equipped with local HMI
  • The VTs have an open earth fault detection winding (da(e)-dn(n)), the dn(n) of phase L1 is earthed. To damp relax-ation oscillations a resistance of 25 Ω, 6 A, is built in
  • On the secondary side the CTs are earthed into the direction of the assets to be protected
  • The designation of all devices is standardized. For instance distance relays -F301, fuse connected-F301F, trip-ping relay -K301A, terminal strip -X301, auto recloser ON/OFF -S301W, test socket -X301P
  • The documentation has to be realized according to the definitions of IG EVU – a working group of utilities and vendors

Power. Flexible. Easergy.
Let?s start with organization in protection testing