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 busway via the safety and making switches, where they are then connected in parallel.

The main technical data can be found in Table 1. A 3 MW asynchronous machine serves as the drive motor. Contactor-controlled starting resistors are used to start up the generator and the starting time is roughly 15 minutes. A DC injection circuit attends to braking, with a braking time of approximately 30 minutes to standstill.

Each generator has 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 (Generators 3 and 4 plus Generator 2, which has a lower output), 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.

Since the breaking capacities of today's high-voltage circuit-breakers can in the majority of cases no longer be tested directly in the test circuit, 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 lay down 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 heavy-current circuit can contain at least six interrupter units. This is high enough for all synthetic test circuits in use today, including the three-circuit connection.

To take into account the high mechanical loads that arise, the transformers have six legs, each of the four central legs being equipped with a 35 kV coil on the High Voltage (HV) side. The two outer legs are not wound and they act as the magnetic return path. The four coils can be connected in any sequence in series, in parallel or combined. The transformers are tap-changed simply and reliably on the HV side by turning tube straps. Metal-oxide arresters are used to protect the windings against switching surges that can arise between phase and ground and phase-to-phase.

The high voltage and heavy current are distributed to the three test bays by an aluminum-tube, duplicate busbar system - rated for 150 kV and a short-circuit current of 170 kA - and located between the machine room and the test bays.

Located next to one another in the tension section bay are two busbar systems, each of which is assigned a specific impulse transformer set and consequently one of the two independently usable short-circuit generators. Both busbar systems are connected to the three-phase test-bay feeds located on the lower level via motorized linear-travel disconnectors. These allow variable connection to the individual test bays and therefore the necessary degree of flexibility in operation. If the two busbar systems are connected in parallel, the total current of the two systems is available in the test bays. In addition, the overhead line system to the old, smaller test bay can be coupled up to one of the busbar systems by means of center-break disconnectors in order, when required, to further increase the testing power available or to obtain other combined testing possibilities.

The disconnectors of the tension section bay 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 new test bay to the old one - and yet still being able to operate both separately - was the chief criterion behind the control concept.

The bays 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:

Each bay 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 heavy-current path are, except during parallel operation, only to be assigned to their respective control desk, whereas all the subsequent zones on the heavy-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 test 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 U act matches the preset charging voltage U set. In the process, all analog inputs and outputs are transferred to the databus by means of an optocoupler.

ll 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 synthetic circuit for short-line faults is available for simulating short circuits remote from the generator terminals.

In the near future, a new synthetic circuit for closing operations will be put into service, which will make it possible to switch on the appropriate nominal voltage for a synthetic test too.

Furthermore, a new TRV reactor is being installed in order to upgrade the synthetic 4-parameter test circuits.

The data is transferred from the test bays and the oscillograph board in the control room to the transient recorder via a fiber-optic system with (in part) up to 120m long fibers. After being digitized, the data is transferred to the UNIX workstation connected by means of a GPIB bus insulator. The data is then edited there and displayed on a monitor in a graphical user interface for analysis and further processing. The measured data is then archived on magneto-optical discs.

 

April 1, 1928 in the Schaltwerk Berlin of the Siemens Schuckertwerk

April 1, 1928 in the Schaltwerk Berlin of the Siemens Schuckertwerk

Excerpt from the book entitled "Deutsche Grossbetriebe Bd.11", Dominik H., 1938 (Large-scale German Plants, Vol. 11):

In close cooperation with the design engineer, they create - through scheduled examinations - the bases for electrical and mechanical loading of individual design elements and for expedient designing of the devices. The foremost task of the testing and experimental bays is that of research; this forms the basis for all work performed in the plant.

It would be inefficient to begin testing only once the finished units had been built. One would then probably discover faults whose cause has nothing to do with the design or construction of the unit, but with fundamental factors, such as the materials used.

In these test bays, the devices are tested not only under the same electrical and mechanical conditions as during normal operation, but also very often under far more rigorous stresses. The testing and experimental bays are an important part of the overall organization of the Schaltwerk.

In a system of this kind, however, the consequences of short circuits, insulation faults and other similar operational disturbances that cannot always be avoided are, due to the very size and nature of the system, far more serious than in the older, smaller power stations. The moment a short circuit occurs, all the energy from the power sources supplying the network will attempt to balance itself out by means of the easy route offered by the short circuit and cause massive destruction at the point of the short circuit or on its way there. Molten cables and burst insulators would mark its path and long-term operational outages would be the result if heavy-duty circuit-breakers did not intervene immediately and shut down the faulted section of the network with total reliability. This means the switchgear must be able to reliably withstand not only the normal currents and voltages, but also the huge short-circuit power levels that ensue when a short circuit occurs. This indicates the sheer magnitude of the task the power supply industry presents the switchgear constructor.

It is a proven principle of Siemens' factories to test each and every machine and device at least up to its operating capacity - what is termed its nominal capacity. But how is one to follow this principle when testing heavy-duty circuit-breakers if such testing almost always necessitates short-circuiting gigantic machine units?

It was for these reasons that a separate building was built close by the manufacturing plant, which houses a high-power testing station with an output corresponding to that of a 45,000 kVA power station. When a short circuit is initiated, such a system offers more than ten times the switching capacity, so that circuit- breakers can be tested for up to half a million kVA.

The photograph shows the machine room of the high-power test bay with the first generator to be installed there. A three-phase AC motor with a nominal output of 3000 kilowatts drives the three-phase generator coupled to it that provides the short-circuit power required to test the switchgear. The three-phase generator is excited by the three-phase dynamo, which is driven by the three-phase motor.

The voltage of the three-phase generator can be controlled within wide limits and ranges up to 12,000 V. For tests involving high voltages, the generator operates with two transformers that enable tests to be conducted with voltages of up to 70,000 or 140,000 V respectively. The level of the impulse currents can be restricted precisely by activating various reactors.

The great benefits of such a powerful test bay were revealed just a year after it was commissioned, when development of the expansion circuit-breaker began. Without this test bay, it would have been absolutely impossible to create such circuit-breakers. It is a fact that the entire field of heavy-duty circuit-breaker construction has experienced an undreamt-of impetus since test bays of this kind came into existence.

At the time the high-power testing station at the Siemens-Schuckertwerke plant was built, it was the largest of its kind in Germany. But development has raced on at such a pace that there are already plans to expand it. Thanks to progress in the construction of electrical machines, the new test generator, which in terms of weight is twice the size of the first one, is capable of outputting more than three times the short-circuit power. Three new single-phase transformers will make it possible to test the largest circuit-breakers available - for a nominal voltage of 400 kV - under normal operating conditions. The preparatory work is already underway. Testing and experimental bays with similar power ratings are unheard of in other sectors of the power engineering industry. They only exist in the switchgear construction sector; it is in their very necessity that the nature of all switchgear is actually revealed. Whereas all other switchgear and machines, including simple disconnectors, only have to withstand the normal operating power levels, automatic circuit-breakers embody so to speak the 'security police' of the power supply system.

 

Wherever faults arise in the extensive branches of the networks, these breakers must be a match for the short-circuit power levels that ensue. In 1938 Generator 1, with a design rating of 40 MVA (and a short-circuit breaking capacity of 400 MVA), was supplemented by Generator 2, which had a design rating of 64 MVA (and a short-circuit breaking capacity of 1200 MVA). After World War II, parts of the larger generator fell into the hands of the allies. A new Generator 2, with a design rating of 64 MVA (and a short-circuit breaking capacity of 2460 MVA) was built. This generator is still in use today, mainly for tests conducted on medium-voltage systems.

An overhead power transmission line installed in the plant enables this generator to be coupled up to Generators 3 and/or 4 that were commissioned back in 1961 and 1985 respectively. These generators form today's high-power testing and experimental bay, which is located in another part of the plant site. And the same principles as were laid down all those years ago still apply to development and testing at Siemens.