Vacon replaced aging current source inverters with new voltage source inverters using their DriveSynch technology to control special purpose motors that compress gas for underground storage. The retrofit was challenging due to the motor's unique features and explosive environment. Vacon's solution used parallel connected standard inverters and a step-up transformer to meet power needs while reusing the existing motor. This innovative concept increased reliability and grid power factor over the old design.

1. Reference Story
drive retrofit for
synchronous motor
Forward-thinking concept for replacing current source inverters
In a joint project with RWE Gasspeicher GmbH, Vacon has re- The shaft power required by the special-purpose motor used
placed a conventional current source inverter (CSI) with volt- is 3.7 megawatts, and piston compressors are needed to
age source inverters (VSI), using an advanced drive concept achieve the required injection pressure. Because these com-
based on Vacon’s DriveSynch technology. The motor used pressors are located in a potentially explosive atmosphere
in the underground gas storage incorporates many special- (Ex-area), they must comply with regulations regarding pro-
ized features, which made the task particularly challenging. tection against explosion (ATEX).
The installation has a large number of aging CSI drives, for
which spares are becoming hard to find or are unavailable.
Technical requirements
The technology introduced now can be used in retrofitting the
old drives, changing the electronics and retaining the existing The RWE specifications required a 12-pulse current source
motor. inverter for the large motor in the underground reservoir
in Epe, with an active voltage conditioner (AVC) to reduce
A new drive concept was needed for the drive control of the harmonic distortion in the medium voltage grid. They also
special-purpose motors that are used to drive the compres- stipulated that a 12-pulse synchronous motor with 30 degree
sors injecting gas at the Gronau-Epe storage facility of RWE phase-shifted windings and a rated voltage of 990 volts was to
Gasspeicher GmbH in Germany. The purpose of the gas stor- be used. This motor would not be changed during the retrofit,
age facilities is to compensate for seasonal fluctuations in which was a challenge. Replacement parts for the old spe-
consumption, thus ensuring a reliable and continuous sup- cial-purpose AC drive were no longer available. This meant a
ply of natural gas. In summer, when consumption is low, the high risk of system failure. Since system availability is a high
natural gas is stored underground and it is withdrawn during priority, the risk of system standstill had to be minimized with
winter when consumption is higher. a new drive concept.
RWE Gasspeicher GmbH accounts for the safe operation of Vacon had already completed several successful joint proj-
fi ve gas storage facilities in total. The company operates four ects with various divisions of RWE Power, so an internal sur-
caverns and one high-capacity aquifer reservoir. The under- vey at RWE led to a request for a quotation for this project
ground gas storages are approximately 1,000 metres under- from Vacon. “We achieved the objective using a configuration
ground, so the gas has to be compressed at a high injection that was completely different from what was requested in the
pressure of up to 240 bar. This means that the compressors request for quotation,” explains Friedhelm Harf from Vacon
used for injection require substantial power. GmbH.

2. Alternative, innovative concept from Vacon chronous motor in the gas reservoir uses brushless external
excitation because it is located in an Ex-area. After the retro-
To meet the required cost-efficiency goals, Vacon submitted fit the excitation is done by a Vacon NXP drive.
an innovative, alternative concept based on its extensive ex-
pertise with active front-end (AFE) AC drives. The configu- The motors each have a voltage of 990 volts, but the drives
ration of the Vacon solution completely eliminates the need used only have a voltage of 690 volts. This is because the
for an active filter system because with the AFE solution, the drives are ‘ready-made’, mass-produced products. This of-
voltage source inverter reduces harmonic distortion to less fers the customer the advantage that replacement parts and
than 5% current harmonic distortion (THD-I). A medium volt- service personnel are always readily available.
age active voltage conditioner (AVC) is then not needed be-
cause the power factor cos φ is better than 0.99. This meets Vacon solved the problem of the difference in voltage by em-
the customer’s requirements without having to install addi- ploying a step-up transformer that converts 690 volts to 990
tional filter and conditioner components, which would have volts. A sinusoidal filter is located upstream of the transform-
cost around EUR 150,000 per drive. This saved a total of EUR er, which has the added effect of reducing the reflected volt-
450,000, and also saved considerable space. age at the motor. This is important, since the motor is located
in an Ex-area.
In the Vacon concept, four AC drives are connected in parallel
to control the motor. This redundancy ensures reliability and The DriveSynch concept made this whole solution possible,
system availability, and the parallel configuration also allows but the redundantly connected Vacon NXP drives with AFE
the required power to be achieved using standard compo- technology have also dramatically increased the grid power
nents. factor: at full load cos φ is equal to 0.99; in the partial load
range it is equal to 0.98. In contrast, current source inverters
DriveSynch – The innovative concept behind the idea fall to cos φ = 0.25 in the partial load range. “This is a quan-
tum leap compared to the previous solution,” says Mr Harf.
The Vacon DriveSynch is an innovative concept for achieving “The benefit for us,” he continues, “is that we do not have to
high drive powers by connecting standard components in par- build one large drive, but can work with standard modules
allel. This has made it possible to retain old motors in retrofit instead.”
projects, as long as they are still in good working order. The
power electronics are the only components to be replaced by
more modern technology. These old systems have often been The impossible made possible
discontinued, and it is either not economical or not possible Another special feature of this retrofit project for RWE Gass-
to repair them. peicher GmbH was that the piston compressor, whose me-
chanical parts are subject to natural wear, is subjected to
By connecting standard drive components (frequency in- enormous loads because of the extremely high pressure and,
verter modules or complete converters) in parallel, the Va- at the same time, is required to maintain a tight seal, because
con DriveSynch concept can achieve output powers of up to 6 the gas is highly explosive. The piston rings, which provide
megawatts in the low-voltage range of up to AC 690 volts − in the necessary seal, are inspected and installed by positioning
the present case up to 990 volts, through a special solution the pistons in jog-mode − this is a manual operation. This had
using a step-up transformer. to be possible after the retrofit as well. “We have a set point
of only 0.15 Hertz for the jog mode − which is usually impos-
Fibre optics are used to synchronize the modules, and out- sible for step-up transformers. But we achieve this without
put filters to ensure load balance. Motors with normal three- any other auxiliary equipment,” stresses Mr Harf.
phase windings and motors with multiple windings and phase
shifted windings can be controlled. One or more drive mod- Vacon’s software designers have created their own software
ules are employed per winding and the phase shift is set via for controlling both the excitation of the synchronous motor
parameters. and the four main drive inverters.
Because the retrofit at RWE Gasspeicher GmbH involved con-
trolling a synchronous motor, several challenges had to be
overcome. Synchronous motors either have permanent mag-
nets in the rotor, or the motor is excited externally. The syn-
Vacon Group
Runsorintie 7, 65380 Vaasa, Finland
RS000146A
Tel. +358 (0)201 2121
Fax +358 (0)201 212 205
www.vacon.com
info@vacon.com