Using variable speed drives for flexible and efficient pumping

Paul Boughton

The state water supply utility in Baden-Württemberg is one of the largest long distance water supply utilities in Germany. More than three million people are supplied with drinking water through a long distance piping system extending over 770km.

Up to 450000m3 of water can be pumped every day. This corresponds to 5200litres per second. In addition to Stuttgart, Aalen, Esslingen, Göppingen, Ludwigsburg, Schwäbisch Gmünd and Ulm are just some of the 250 cities and communities that are supplied.
Approximately 70percent of the state water supply for drinking water comes from ground water in the Schwäbischen Alb. Parts of the karst ground water of the Schwäbischen Alb flows in an extensive series of branched underground fissures and crevices of the Donauried between Langenau and Niederstotzingen. The water is pumped to the Langenau water plant from a total of six water storage facilities with 220 individual springs.
Thirty per cent of the total water pumped is taken from the Danube River after which it is treated to become top quality drinking water. More than 90millionm3 of drinking water, with a hardness of grade two, is provided each year.
In order to ensure that the drinking water supply is kept at a constantly high level, it is necessary to flexibly operate the ground water storage facilities.
This means that every storage facility must supply precisely the amount of water that is required for the optimum mixing ratio and therefore the optimum drinking water quality. The water pumping system must be able to continually adapt the volume of water pumped to the actual demand.

Precise and energy-saving

Variable-speed drive systems are the most practical solution -- both technically and economically -- in order to be able to quickly and precisely adjust the required amount of water being pumped.
When compared with mechanical control concepts such as slide valves, variable-speed drives allow the flow to be significantly more accurately controlled and at the same time with shorter response times.
The operating costs of the variable-speed drive solution are significantly lower too, due to their substantially lower energy requirements. For pumps, fans and compressors, the energy consumption represents approximately one third of the operating costs.
Contrary to fixed-speed motors that always operate at their full rated speed, variable-speed drive systems use drive converters to adapt to their operating requirements. Put another way, the motor only draws that amount of power that is precisely necessary at that particular operating point.
The result is a significantly lower energy requirement when compared with fixed-speed drives operating in the partial-load range. Depending on the plant characteristics, this energy saving can be between 30 and 50percent.
The reduced wear of the mechanical pump system components also plays an important role in cutting operating costs. When variable-speed drive converters are used, current spikes generated during acceleration and the associated torque surges are eliminated. This soft starting and stopping reduces stress on the complete drive mechanical transmission line and increases its operational lifetime.
As a result of the technical and economic benefits, the state water supply utility equipped the new pump systems with variable-speed drives when modernising water storage facility number six. In that facility, for a maximum possible flow rate of 350l/s that can be withdrawn using both pumps, it is now possible to continually set any value between 50l/s and 230l/s per pump. Previously, only three fixed flow rates were possible.
Siemens supplied the variable-speed drive systems for the new pumps used at the storage facility. Two rugged and reliable 1LG4132-kW low-voltage motors drive the pumps. SinamicsG150 drive converters -- with a 160kW power rating -- are used to control the motor speed.

Optimised for pump applications

SinamicsG150 drive converters have been specifically designed for pump, fan and compressor drives -- applications with a square-law load torque.
These applications use closed-loop vector control to optimise motor control and therefore minimise motor losses. However, they require neither speed feedback, high dynamic control response, nor high overload reserves. The SinamicsG150 has been designed to address this requirement profile and is correspondingly straightforward and rugged. Its functionality has also been optimised to address pump, fan and compressor drives. The result is reliable in operation, requires little maintenance, is simple to handle and has the appropriate price-performance ratio.

Only the necessary functions

Pumps that we use in our systems only run in the first operating quadrants explained Martin Krasser, planning engineer with the state water supply utility. Until nowwe had to use drive converters in our pumping systems that could handle multi-quadrant operation -- which is not necessary. They also had a whole raft of functions that we didn't need for pump drivesbut which we had to pay for. SinamicsG150 doesn't have such superfluous functions and the rice-performance ratio is significantly better."
"Furtherthe Sinamics G150 is far more user-friendly than the previous units. It is more transparent and has clearer operator prompting and navigation. The plain-text graphics display provides all of the operator control and commissioning functions. In operationdrive parameters -- currentspeed actual value and speed setpoint -- are displayed as a bar diagram. This type of display provides an overview at a glance he added.
SinamicsG150 is presently the quietest and most compact drive converter in a standard cabinet. It has a noise level of only 69dB when operating at its rated power which means that additional noise protection measurements are not required when planning and installing the drive system.
The footprint of the cabinets, including the components to connect to the line supply, is approximately 50percent of that of conventional drive converters. Without components to connect to the line supply -- which is possible when a central power supply is used -- then the footprint is 70percent less.
In addition to supplying the motors and SinamicsG150 drive converters, Siemens also installed and commissioned the variable-speed drive systems for this water utility. The company also integrated the drive systems into the higher-level supervisory control system.
The SinamicsG150 is connected to a SimaticS5PLC that in turn is linked to the central and area control rooms through fibre-optic cable. This means that the drive converter -- and therefore the pump speed and flow rate -- can be controlled at a local operator panel or from the main control rooms.

Stefan Rausch is with Siemens Automation and Drives in Nuremburg, Germany. Wolfgang Wagner is with Siemens Industrial Solutions & Services, Germany.

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