Temperatures rarely rise above freezing between November and April in Moscow. Reliable and efficient delivery of heating energy is essential to improving the quality of life of Russians living in the city.
Energy provider MosTeploEnergo generates and transfers steam heat to 56 separate district heating stations including Lublino District Heating Station (LDHS) in the new residential region of Moscow called Lublino. Boilers at the station regulate the temperature and 630-kilowatt (kW) pumps send the heated water through pipelines to provide heat and hot water for approximately 300,000 people in this region.
The Challenge
The new Lublino heating station was needed to meet customer’s increasing demands. Maintaining efficient energy consumption and low harmonic distortion values were also required to minimize its impact on the electrical distribution system.
Traditionally, water pumps in Moscow were run directly across-the-line, and the pressure in the system was maintained with a regulation valve arranged either in series with the pumps (throttle method) or in parallel with the pumps (shunt method). While the regulation valve solution is initially economical because it requires less equipment, its inherent inefficiencies and maintenance requirements add to costs in the long term. Additionally, low mechanical reliability and mechanical and electrical starting stresses limit this method.
To meet the demanding application needs, Rockwell Automation distributor SPEL Ltd. of the Czech Republic (now called Rockwell Automation Services, s.r.o), teamed with RETEMP management, a Moscow heating station control systems company.
The specific project objectives of the Lublino heating station were to:
- reduce additional losses from throttling valves
- increase operation stability of the heat station
- reduce thermal and mechanical stress on motors that are started frequently
- generate minimal harmonic distortion
- increase production
- and reduce energy consumption and related costs.
The Solution
Instead of using valves, the team decided to use variable frequency drives (VFDs) to control pump speed, flow and pressure. They chose the Rockwell Automation PowerFlex 7000 18-pulse medium-voltage drive with synchronous transfer as the best solution.
The PowerFlex 7000 drive is an ideal soft starting and speed control method for pump motors because it:
- draws minimal inrush currents while starting, minimizing voltage drop and system electrical-stress
- reduces mechanical shock (starting torque is controlled at nominal levels)
- enhances process control through VFD speed response for pressure and flow control
- provides cost savings and system flexibility by using one VFD with synchronous transfer capability, and several input/output/ bypass contactors for multi-motor operation
- optimizes energy savings through higher system efficiencies, with much greater savings seen at lower speeds
- and increases overall reliability and maintainability.
The PowerFlex 7000 uses a current source inverter drive technology with a pulse width modulation switching pattern (CSI-PWM) on the inverter. The CSI-PWM topology allows the easy connection of power semi-conductors in series for use in the higher range of medium voltage (in this case, 6.0 kilovolts [kV]) without a step-up transformer. This technology decreases the amount of space required for drive mounting and reduces cost. It also provides near sinusoidal input and output waveforms for current and voltage.
The synchronous transfer solution provides a low-cost and space-efficient system to start and operate more than one motor with one drive system. It allows motor systems to have reduced starting current and be transferred between the drive and a fixed frequency line supply without stopping. Compared to a simple non-synchronous transfer in which power to the motor is interrupted for a significant length of time, the transient drop in motor speed is much less with synchronous transfer.
In order to perform an in-phase synchronous transfer, both a drive output contactor and a bypass contactor are required for each motor. The "bypass" is a contactor that connects the motor directly to the fixed frequency supply, bypassing the drive.
The basic functionality of the building blocks of the heating station’s synchronous transfer drive system is standard, but the actual components are tailored to the customer’s needs.
In the Lublino system, the first component is the 10kV input power cell containing an isolation switch, power fuses and a vacuum contactor. The second component is the 18-pulse isolation transformer, which provides phase shifting of the secondary voltages to enable harmonic cancellation of the principal harmonics.
The main component is the PowerFlex 7000 6.3 kV 18-Pulse variable frequency drive. It provides the conversion from the fixed frequency input to a variable voltage and frequency output in order to vary the speed of the controlled motor. Following the drive is the output contactor for the drive, which includes an isolation switch and contactor.
The final component before each motor is the Output/Bypass Starter Unit. This unit allows the motor to be connected to a fixed frequency supply bus or the variable frequency bus connected to the VFD output.
A motor management relay protects the motor and both isolation switches are mechanically interlocked to isolate the power cells from the electrical system.
The entire system is controlled by the drive via an Allen-Bradley programmable logic controller (PLC) installed in the drive control section. It manages all of the components in the synchronous transfer operation. In particular, when transferring from drive to bypass, the time interval between the drive shutting off and the bypass contactor closing must be accurately controlled.
Typically, the PLC gives control of the bypass contactor to the drive before performing the transfer, and takes back control after the transfer is completed. The PowerFlex 7000 drive is the perfect solution in terms of motor waveforms when attempting in-phase synchronous transfer.
The Results
The successful commissioning of the entire system was completed in less than 10 days, in part due to the testing procedure developed by the experienced team of one Rockwell Automation engineer, one engineer from SPEL Ltd., and one RETEMP service engineer.
M. Korshunov, technical manager of LDHS, was impressed with the quality of the product and the knowledge of the engineering team.
"The new equipment has been used to start and run the pumps and no problems have occurred during this initial period of operation," he said. "This satisfactory result demonstrates the good quality of the work done by Rockwell Automation, SPEL and RETEMP. Thanks to the exhaustive tests that were carried out in Cambridge, it was possible to commission the installation in short time. The commissioning engineers – John Lyle, Vladimir Kraft and Michael Sypchenko – were first class. We are very satisfied with the way the work was carried out and with the result."
RETEMP performed the harmonic analysis of the drive system at full speed and full load for one motor. The system met or exceeded the project criteria. The total harmonic disturbance (THD) during motor running was 2.1 percent. That is much less than the published limits for total harmonic current distortion for medium voltage systems, which is generally specified as no greater than 5 percent.
Following implementation, Korshunov wrote, "Owing to the 18-pulse technology, the line harmonics are a great deal below the limits defined by Russian standards. Rockwell Automation’s installation of the medium-voltage drive in the heating station has completely eliminated all potential problems."
