Objectif
To develop a new technology in power electronics and prediction control in order to accelerate and control A.C. motors by means of current feedback soft starters, operating 6 kV and in the power range up to 10 MW, in order to produce important energy savings during production dead times. Actually, direct line or conventional starting systems damage the motors very frequently and hence in many occasions these machines cannot be stopped, wasting large amount of energy.
Develop an industrial product to permit start/stop sequences every hour, to extend motor life without damages in the estator and/or rotor.
Reduction of the connection inrush current from 18 IN to 21 IN, average starting current from 7IN to 21IN, torsional torques during both connection and supply reconnection in a 15/2 times ratio.
Innovative aspects: operation at 6kV supply voltage by using thyristors in series, with adequate voltage sharing, leakage current compensation and dynamic supply voltage transients suppression.
- The proposed family of soft starters operating at 6 kV and high power levels will be both a new product and associated technology involved in it.
- I.E.S.S.A. has a positive experience in three feasibility operating models at 3 kV with a power range between 1 MW - 3,5 MW and these achievements are the departure to reach the 6kV operational level.
- Nevertheless, there are not existing thyristors rated at higher levels than 5,5 kV. For 3 kV it is possible to use only one single device per phase in forward and one in opposition, leading to a back to back thyristor pair per phase.
- But, the 6 kV line voltage requires to use three thyristors in series and three in opposition per phase, with fundamental problems to sorted-out.
PROBLEM DEFINITION AND POTENTIAL IN THE POWER ENGINEERING AREA
I. Acceleration. Different values of leakage currents in the thyristors, produce very bad voltage sharing and devices destruction as a consequence.
Experiences in 3kV : bad sharings in 5:1 ratio, causing devices destruction for TI > 70 deg. C
Potential research solutions : equal cooling for all thyristors, high power dissipation sharing resistors and large time constant heatsinks.
II. Very bad voltage sharing during aceleration can be generated by control sequence when the thyristor is in reverse bias cathode-anode voltage and the control system applies firing pulses.
Experiences : during acceleration interval, the thyristor limits the motor current by chopping the line voltage at variable firing angle A.C. motor power factor varies between 0,3-0,83 and hence the thyristor conduction varies widely.
RESULTS: POTENTIAL SOLUTIONS:
- predict the lagging current, avoid thyristors being fired under reverse voltage conditions.
- guarantee for all devices in series simultaneous firing, dv/dt and no overvoltages at turn-off (layout design).
III. Protections against transients in the supply voltage due to circuit breakers switch-off operation
Experiences : very fast transients up to60 kV
Solutions : High voltage/energy varistors, develop a circuit to empass very fast the anode-cathode junction with low voltage i.e. 2 volts, hence avoiding destruction.
IV. Insulation degradation with time
Solution : careful study of increasing corona and/or dust effects with time.
RESULTS AND PROBLEMS IN THE CONTROL AREA
- The variable current demand is a new prediction control system to adjust both the load and motor torques in parallel curves by means of current feedback prediction control. The nett result is a substantial current and torque pulsat minimization and also switching inrush current elimination.
- Following table shows preliminary comparative analysis with other conventional soft-starters.
TYPE OF SOFT STARTER AVERAGE CURRENT (p.u.) AVERAGE CURRENT (%)
Voltage control 4.1 100
Constant current 2.65 64
Project 1.89 46
The first application area is related to energy savings in actual installations, operating at 3/6kV with large motors driving pumps and fans with throttling and also with long dead time intervals. The main reason is due to a quick potential damage of these motors, if were started in a frequent manner, trying to obtain energy savings. The second area is mainly referred to the high repairs cost of the above motors and also to higher production costs due to plant shutdown. A particular emphasis in these applications are the high inertia fans, where the damages could be more severe, in particular in the rotor.
The third area concentrates in the peak energy contract between the user and the electricity generating board. Finally, there is a fourth area in the project orientated to additional shutdown due spurious relays trips; being produced by means of high connecting currents. The net effect on the production flan can lead to a very high penalty cost.
ENERGY SAVINGS
The actual application consists of a fumes cleaning plant in Esteban Orbegozo SA. with four A.C. squirrel cage motors rate at 6 kV, 1.100 kW, each, running with long dead times. Economic savings are up to 5,2 MW per year or 450 TOE/year.
SPECIFIC CHARACTERISTICS. TECHNICAL SPECIFiCATION
A. CONTROL SYSTEM
- Type of control during acceleration : microporcessor voltage rising ramp, to minimize the average current by means of automatic adjustment among the load and motor torque.
- Typical current during starting : 150%-250% of the nominal value.
B. POWER ELECTRONICS RATING
- Configuration : three phase, back to back thyristor connection. (3) in series per half phase for 6 kV.
- Nominal voltage : 6 kV
- Maximum continuous voltage : 18 kV
- Maximum starting current : 4 in for 60'
C. Operator adjustments
- Acceleration ramp : automatic adjustment depending on permitted current level.
- Current : during starting load torque curves can be programmed in the E2ROM memory of microprocessor.
D. INHERENT SOFT-STARTER PROTECTIONS
1. Motor stalled and shortcircuit : programmable maximum motor current (2-6 times nominal)
2. Large current fluctuations and motor unbalancements : by means of closed loop current feedback.
3. Overvoltages : large energy varistors.
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28925 ALCORCON (MADRID)
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