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High-Power Testing Laboratory

At the high-power testing laboratory, the switching capacity of high and medium-voltage equipment is tested in terms of thermal stress and dynamic short-circuit performance, opening, breaking, and insulation capacity after short-circuit breaking, and operational behavior. Short-circuit tests with surge arresters in conformance with standards including pre-stress, can also be performed.

Testing portfolio

The synthetic test circuits are arranged in a flexible manner so that the switching capacity for capacitive tests can be tested at 50 and 60 Hz according to the standards. Moreover, the high-power test facilities comprise a laboratory for basic physical studies, where short-circuit currents up to 63 kA at 50/60 Hz can be generated through capacitor banks and synthetic voltages up to 120 kV are available.

Extract from the testing portfolio of the high-power testing laboratory

Short-circuit generator

  • short-circuit tests for opening and breaking capacity
  • test of the capacitive switching capacity
  • test of bus transfer currents
  • short-time and peak-withstand current test for determination of dynamic and thermal stress
  • laboratory for basic physical studies

 

 

Specifications

Performance data of the high-power testing laboratory

Maximum generator output6,400 MVA
Maximum short-circuit current (single and three phase [peak/rms])270/100 kA
Maximum short-time withstand current, 3 s80 kA
Maximum voltage for synthetic tests1,150 kV

Characteristics of the laboratory for basic physical studies

Maximum short-circuit current (one phase)63 kA
Frequency50/60 Hz
Maximum voltage for synthetic tests120 kV

Overview of the technical equipment

Siemens Energy's high-power testing laboratory at the Schaltwerk Berlin that went into service in 1961 consists of several system components.

The heart of the high-power testing laboratory is formed by two almost identical generators supplied by Siemens Energy's AC/DC generator plant. Furthermore, it is possible to connect another third generator, which is located on the medium-voltage laboratory. Each of the two generators have a three-phase short-circuit power of 2800 MVA for a short-circuit on the secondary side of the transformers. When the three generators are connected in parallel, a direct testing power of 6400 MVA can be obtained. This corresponds to a maximum single-pole short-circuit current of 120 kA with a source voltage of 35 kV.

Both generators have been built with two parallel three-phase stator winding systems so that the forces that arise from the impulse current can be more easily controlled. The two systems are run to the primary windings of the transformers and to the bus bar via the safety and making switches, where they are connected in parallel.
A 3 MW asynchronous machine operates as the drive engine. Starting resistors are used to start up the generator and the starting time is roughly 15 minutes. Braking to complete stop takes about 30 minutes and is achieved by means of a DC braking circuit.

Technical data of short-circuit generator

  • Nominal voltage (star/delta) 19/11 kV
  • Maximum operating voltage 21 kV
  • Reference power 200 MVA
  • Nominal speed 750 U/min
  • Operating frequency 50 Hz / 16 2/3 Hz
  • Maximum short-circuit current 100 kA bei 19 kV
  • Length of generator shaft 9,990 mm
  • Outside diameter of laminated stator core 4,250 mm
  • Weight of stator 390 t
  • Weight of rotor 225 t
  • Overall weight 680 t
  • Flywheel effect 13,000 kNm 2

Since the switching capacities of today's high-voltage circuit-breakers in the majority of cases no longer can be tested directly in the test circuit, in the high-power testing laboratory short-circuit transformers step up the voltage of the generators to the values required for testing in the synthetic test circuit.
IEC and DIN/VDE guidelines define limits for the permissible current displacement of the final half-wave during the testing of high-voltage switchgear. These limits specify either the level of the source voltage or the maximum permissible arc voltage and therefore the number of contact gaps in the test circuit. On this basis and with a voltage of 35 kV, the high-current circuit can contain at least six interrupter units. This is high enough for all synthetic test circuits in use today.

Electrical characteristics of the transformers

  • Manufacturer Transformatoren-Union
  • No. of single-phase units 3
  • Nominal design rating of one unit 250 MVA
  • Nominal voltages ---
  • Low voltage max. 19 kV
  • High voltage, single-phase max. 420 kV
  • High voltage, three-phase max. 140 kV
  • Nominal frequency 50 Hz
  • Short-circuit voltage of one unit 92 %
  • Max. permissible primary short-circuit current 65 kA
  • Max. permissible peak short-circuit current 182 kA
  • Weight per unit (filled with oil) 170 t

To take into account the high mechanical loads that arise, the transformers have six-leg type, each of the four central legs being equipped with a 35 kV coil on the high-voltage (HV) side. The two outer legs are unwound and form the return path for the magnetic flux. The four coils can be connected in series, in parallel or any combination. Metal-oxide arresters are used to protect the windings against switching overvoltages that can arise between phase and ground and phase-to-phase. 

Star-delta tap-changing (SLG)

The generator is connected to the transformers and the power supply fed directly to the high-power testing laboratory by means of a 20 kV bus bar run. Starting from the star-delta tap-changing operation at the generator terminals, each of the two winding systems is run into the safety-switch compartments. The bus bar run consists of an insulated aluminum tubular system. In each phase of the two winding branches, a combination of two circuit-breakers assumes the role of safety switch. A modified SF6 circuit-breaker 3 AS1 (up to 80 kA at 19 kV) has been used as the primary-current breaker (safety switches), while a 3 AF vacuum circuit-breaker serves as the residual-current breaker.

Safety switch

In the subsequent transformer undervoltage switchover, the two generator systems can be connected in parallel and then run along the underground bus bar directly into the test laboratories for testing the switching capacity without the transformers being interposed. As of the transformer switchover, the bus bars are run uninsulated on supports. The two generators can also feed in in parallel. If the circuit contains three single-phase transformers, each phase of a generator system is connected to the appropriate terminals of the low-voltage windings. Switchover takes place by means of straps.

Making switch

The making switches are tripped with the generator voltage. The system can be switched on simultaneously or time-graded, whereby exact connection is specified for each desired instant. Located behind the safety switches are the making switches. These are single-pole, oil-insulated devices developed specifically for this purpose in the high-power testing laboratory at the Siemens Energy Schaltwerk. The operating of this type of making switch is 5 ± 0.1 ms and the maximum permissible peak short-current circuit 230 kA.

Current-limiting reactor

The high-current bus bar runs from the making switches (which are located in separate compartments) to the reactor compartments of the two generator windings, in which there are two tappable coils for each phase. Each of these coils has twelve taps that enable very finely stepped current grading by connecting various coils in parallel and series.

The high voltage and high current are distributed to the three high-power testing laboratories by an aluminum-tube, duplicate bus bar system – rated for 150 kV and a short-circuit current of 170 kA - and located between the machine room and the test laboratory.
Located next to one another in the tension section laboratories are two bus bar systems, each of which is assigned a specific impulse transformer set and consequently one of the two independently usable short-circuit generators. Both bus bar systems are connected to the three-phase test-laboratory feeds located on the lower level via motorized linear-travel disconnectors. These allow variable connection to the individual test laboratories and therefore the necessary degree of flexibility in operation. If the two bus bar systems are connected in parallel, the total current of the two systems is available in the test laboratories. When required, the generator of the medium voltage test laboratory can be coupled to the bus bar systems via the overhead-line system to increase the testing power or to obtain other combined testing possibilities regarding switching capacity.
The disconnectors of the tension section laboratory can be remotely controlled by means of a mimic diagram with position indicator located in the control room. This ensures quick switching and a clear visual overview of the momentary operating state of the high-voltage circuit.

Connecting the generators and to operate separately was the chief criterion behind the control concept. The laboratories are selected by means of the higher-level control system that combines the various parts of the station to create a controllable and operable unit. Based on the structural and electrical configuration of the installation as a whole, the test laboratory has been constructed with 13 independent control zones that can be divided up into the following four main groups:

  • Source (high-current generator)
  • Path (high-current route)
  • Destination (testing laboratories) and
  • Synthetic testing circuits

Each laboratory is assigned its own operator keyboard that is used to assign control of the hardware installed in the selected zone to a position on the control desk.

The generator and the switchgear (safety and making switches) immediately downstream in the high-current path are, except during parallel operation, only to be assigned to their respective control desk, whereas all the subsequent zones on the high-current side can also be assigned by the selecting unit (i.e. the higher-level control system) to any of the control desks. A logic circuit contained in the selecting unit prevents illogical assignments and combinations from being executed.

In the context of expansion of the high-power testing laboratory, the synthetic test circuits were also provided with voltage and current injection and the re-ignition circuits automated. A microprocessor-controlled control system continues running up the motor actuator of the control transformer, taking into account the charging current and the operating state of the station, until the charging voltage Uact matches the preset charging voltage Uset. In the process, all analog inputs and outputs are transferred to the data bus with the aid of an optocoupler.
All synthetic test circuits can be assigned to each of the generators and transformers for the voltage and current-injection circuits, including the re-ignition circuits. A line is available to copy short-circuits far from the generator. By means of a one switching synthetic it has possibly to be switched on in a synthetic test on the corresponding nominal voltage.
Three-phase synthetic tests are possible up to a tension of 170 kV and 80 kA.

A digital transient measuring system is used to record the measured values of the switching capacity. This comprises two systems of identical design, which can be operated independently of one another. In turn, each system is equipped with 20 channels with a sampling rate of 100 MSamples and a 14 bit resolution and switchable time-bases.
The data of the switching capacity tests are transferred from the test to the transient recorder in the control room via a fiber-optic links with (in part) up to 120m long glasfiber cables.

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