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

Line protection

In normal case for the 110 kV lines and cables distance protection (21) -F301 is the choice. Options used are R-X-startup, auto recloser, fault locator, disturbance recorder and earth fault direction. This feature set allows to protect 85% of line length with instantaneous tripping. The last 15% will be protected with 2nd stage in 300 ms. To achieve 100%-instantaneous tripping sometimes signal protection or line differential is used. The communication between the relays is realized in fiber in the overhead ground wire. To eliminate arc failures on lines a single reclose (0.4 s dead time) is realized.

In case of switch-onto-fault the line is tripped immediately (realization with binary inputs).
In the medium voltage also distance protection (21L) F301A with voltage controlled overcurrent startup is realized. The options are the same as in the 100 kV grid, but combined protection and control is chosen. The auto recloser used is working 2 times (first dead time again 0.4s, the second long time is 10s. Table 3 shows the relays used typically.
As in the 110 kV feeders three single phase isolated voltage transformers are used (built in between line disconnector and earthing switch), in the medium voltage only the transformer infeeds and busbar units are equipped with voltage transformers. The auxiliary contacts of the busbar disconnectors are used to transmit the voltage to the distance relays.

To block R-X-startup or unselective tripping in case of fuse failures the voltage is disconnected in that case and the backup overcurrent activated.
The backup protection is taken over by the upstream line protection (stage 2 or stage 3).The backup protection if line differential is overcurrent.

In all substations in medium voltage so called 3-way-switches are used. This is a combination of busbar disconnector -Q1 and earth switch -Q8. The earthing is short circuit proof realized with the circuit breaker -Q0. Electrically or mechanically the unwanted connection to the end of line in case of protection. These devices are used in the 110-kV in case of GIS substations only.

Transformer protection
Transformers are the most important assets. That’s why they are equipped with several protection devices to be on the safe side in case of protection failures. This will also guarantee in case of loss of auxiliary voltage (battery) or protection failure immediate trip. Figure 4 shows the concept of 10/20(10)/20-kV-Transformator, 40/20/20 MVA.

Failures in the transformer -T101 and/ or arc suppression coil -L21, as well as in the protection zone of differential will startup Buchholzschutzes CF050B or CF061 (63) and differential -F321 (87) and trip the transformer immediately.
Overcurrent (51) - F311 on the high voltage side realizes (depending from fault location) a none delayed or delayed backup protection.
Device-F321 contains zero sequence current differential (87REF). In case of single phase failures in the zero current zone (between the transformers -T1 and -T90) as well in case of isolation failures in star resistance–R21 the relay trips immediately all transformer circuit breakers. The application of REF allows the zero current elimination and avoids the decrease of sensitivity to 2/3. Table 4 contains the relays used.

Coupling protection
The 110-kV-couplings are equipped with distance protection as the lines. 4 setting groups are in use:

  • Setting group 1:   Coupled busbar
  • Setting group 2:  Standard characteristic for short lines
  • Setting group 3: Dtandard characteristic for long lines
  • Setting group 1:  Characteristic for transformer feeders

Depending on the operational conditions the corresponding parameter set is chosen with binary inputs. To guarantee the proper operation of the directional elements the scheme (e.g. -T1: P1(K) to –Q0) used as well as the operation (e.g. infeed of line via BB1 using –Q1, -Q0, -T1, -Q20 via BB2 to line = E02) has to be done right (see Figure 5).
In the medium voltage couplings there will be no protection in couplings. In case of a connection to customer’s substations the “handover-protection” is realized as overcurrent or distance protection.

Busbar protection and plant protection

In normal case the busbar protection or plant protection can be guaranteed by the line protection of upstream substation. In case of 110 kV GIS plants current differential is used.
In the medium voltage infeeds distance protection with R-X-startup and increased fast time are in operation. See Figure 4=J05-F301A (21B). This allows detection of busbar failures within 1 and backup protection for long lines. The integrated breaker failure guarantees even in case of failures between -T1 and circuit breaker - Q0 (= outside the protection area) a trip within 1 s.

Earth fault protection
The grid is connected with the low voltage side of the transformer infeeds with the zero voltage relay–K5 (59N) to the da(e)-dn(n)-winding und supervised.  For selective earth fault detection different methods are in use.
In the 110 kV grid the transient earth fault relays are in use (see Figure 6).
For the compensated medium voltage also the transient earth fault principle is used as well as the harmonics- relative- principle and the qu-method (Figure 7).

Cable-type current transformers are in use for zero current measurement (60/1 A). Since some of the municipalities use short time star point grounding the single phase short circuit current is limited to 2 kA maximum and the faulty line can be separated by zero current time protection. Power plant grids will be also operated with this short time star point grounding but limited to 90A.
In some special cases this application is used in the medium voltage grid as well (transmission lines). In that case the Petersen coil should clarify the arc fault. In case of metallic faults after 1s a resistance (0,3…2 kOhms) is connected in parallel and the line with the earth fault can be detected. Figure 8 shows the protection and control cubicle and Figure 9 the switchyard with combined protection and control in the 20 kV.

Automation
Automatic frequency load shedding (AFE)
In the substations in the medium voltage frequency load shedding is built in -A371 (81). In case of distributed generation connected the direction of power is checked in addition.  The AFE trips preselected line feeders.

Automatic resonance setting adjustment
Tap changers are mandatory for 110 kV/MV transformers (± 16 % in 19 steps).  The controller to be used for every transformer is–A211. The measured value is taken from the VT in the medium voltage transformer infeed.
It is a 2 point controller with 2 voltage bands. The nominal value as well as the operating time is supervised.
The Petersen coils are connected to the resonance setting adjustment-A211. The principle used is the neutral displaced voltage as the setpoint for the control of the Petersen coil.

Automatic Changeover
The AC station service system is powered in the 0.4-kV-side via unit auxiliary transformers, the switchover is handled automatically. Additional grid changeover is realized in special cases only. Since the SCADA system is available there is no need for special automation system for the grid transformers.

Auxiliary power supply
To achieve redundancy the DC system is doubled in the substation. The battery capacity is halved. The DC distributions are connected via diode-coupling. Figure 10 shows the doubled DC system as it is used by default. On the left and on the right hand side the DC infeed is visible. In the middle is the step-up chopper and the normally closed diode coupling.
The auxiliary power for the protection is separated in such a manner, that the backup protection is guaranteed (Table 5). If for instance is a failure in the transformer and at the same moment the DC system (=NK) would be out of service the differential protection does not operate anymore. But the trip would be possible by the Buchholz protection, powered by DC #2 (=NL). 
If there is a fault in the 20-kV and DC #2 (=NL) fails the backup protection is guaranteed by the distance protection in transformer infeed -F301A (21B), powered by DC#1 (=NK).

AC power
The 0.4 kV power supply is realized with 2 unit auxiliary transformers. They supply the rectifiers, fans on the transformer and the stepchanger on transformers and Petersen coils.
SCADA
All digital relays are connected serially via fiber (IEC 60870-5-103) to station control. For new substations IEC 61850 shall be the choice. The measurement is realized with the relay’s displays. No analog measurement needed, as well as transducers. Disturbance records (events and lists) are transmitted via communication system TechLAN to the protection engineers. 78.000 messages and 13.000 measurements are collected in central control center.

Summary
Digital technology has been introduced since 1990. The vendors used for protection and control are ABB, AEG (today Schneider Electric), SIEMENS, EAW (today Sprecher Automation) and A. Eberle. The devices work properly. The standardization and guidelines developed showed advantages in commissioning, operation and extension.

Biographies

Heiko Kraut is the leader of the special department protection and control at TEN Thuringer Energienetze GmbH in Erfurt.
He leads the VDE-AK „Protection“of VDE Bezirksverein Thueringen.

Walter Schossig is in the industry for 40 years. He worked as protection engineers in Thuringer Energie AG. He was a member of working groups as VDEW AA relay protection and standardization at DKE Normenausschuss K434 protection and control. He is still very active in the VDE AK protection. He is the author of the PACWorld series about history.

Relion advanced protection & control.
BeijingSifang June 2016