Beijing Leader & Harvest Electric Technologies Co., Ltd.
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Medium voltage starter aplication in site
I. Project introduction
Qinghai Zhongxinguoan Technology Developing Co., Ltd is a manufacturer of fertilizer and chemical material. It locates in center of ChaidamuBasin. Producing/living power is provided by itself power plant. The power grid capacity is limited and can not stand big impact. Principle of spray drying tower is like this: air passes filter and heater, then enters in dividing device of dry tower, and then enters in dry room evenly in form of helix. Passing filter via liquid channel, material liquid is delivered to centrifugal atomizer of dry tower. Material liquid is sprayed into very fine mist-state drops. It contacts with hot air. Water is evaporated quickly and material liquid is dryed into product. The product is discharged from dry tower underneath and cyclone separator. Waste gas is discharged by fan. Spray dry tower is equipped with four sets of high voltage suction fan. If start fan in full voltage, big start current and big impact torque accelerate ageing of cable and motor, as well as wear of impeller and bearing. Start-up impact may cause stop of whole power grid. Whereas above state, soft starter for high voltage fans is indispensable.
Motor parameters

Model YKK500-4 Power 630kW
Rated voltage 10kV Speed 1489r/min
Rated current 45.2A Power factor 0.86

Fan parameters

Flow 180000m3/h Speed 1450r/min
Power 630kW
Manufacturer Sichan Fan Plant

II. Scheme and equipment
After comparing kinds of schemes, we decide to use Leader&Harvest scheme at last.
In order to reduce start current, big power motor needs starter. Common soft starter adopts mode of reducing voltage but not reducing frequency. It reduces start current, but torque is also reduced a lot, so its application is restricted. In addition, there is also big harmonic to power grid. Leader&Harvest product is soft starter of variable voltage variable frequency. Voltage and frequency rise from zero. Rate of voltage and frequency(V/f=C) keeps constant. Motor current is decided by load. Start torque can reach max torque of motor, so it is not restricted. Start time and start torque can be adjusted, so it is very convenient. The product can prolong lifetime of motor and mechanism, without impact on grid and without affection on other electrical equipments.
This project adopts scheme of "one drive four", namely one VFD drives four motors. In plateau of 2800 altitude, soft starter connects to 10kV cable and makes motor soft start. Once motor frequency reaches line frequency, soft starter disaffiliates motor and stop running. Motor is switched to direct on line simultaneously.
Variable frequency soft starter is controlled by operation box. It implements soft start for each motor according to operation state. When motor speed accelerates from zero to rated speed, soft starter sends signal to operation box and operation box makes motor finish switch.
Soft starter adopts technology of modern SPWM. It starts up in the state of keeping constant magnetic flux. Soft starter adjusts motor voltage and frequency, makes motor start from zero to rated value according to expected curve. Motor starts up smoothly. That can lower requirement of power source and reduce affection on grid and motor. Soft starter has protection of overload, short circuit, overcurrent, low voltage, lack phase for motor

Main circuit:

This is bypass scheme. It is composed of 8 contactors KM1~KM8 and 4 isloators QS1~QS4. KM1 and KM2 can not close together, KM3 and KM4 can not close together, KM5 and KM6 can not close together, KM7 and KM8 can not close together. KM2, KM4, KM6, KM8 can not close together and they interlock electrically.
Soft starter is HARSVERT-A10/050, rated capacity is 400kVA, max capacity is 790kVA.

III. Characteristic of soft starter
1. High-high structure, cell-cascade PWM mode, 10kV output directly.
2. 8 grade series connecting, 48 pulse input without harmonic.
3. Soft starter can run during voltage +10%~-10% and derate to run during voltage -10%~-35%. That avoids damage on motor resulted from grid voltage down.
4. Big power components contactless control in start-up process, motor accelerates smoothly, which protects motor and its connecting mechanism farthes.

IV. Operation effect

In order to implement ligh-load start, close air gate of fan in start-up process. In actual start process (0~50Hz), motor current does not change suddenly. It runs stably and current is less than rated current(45.2A) all the time. Soft starter starts each motor successfully, protects motor and fan effectively and ensures grid stability.
In contrast to common soft starter, variable frequency soft starter has following advantages:
1. It does not need full load operation for long time. It only drives motor to line frequency. So we can take advantage of strong overload ability of dry transformer to reduce transformer capacity. This can save cost.
2. Variable frequency soft start is more smooth, without current sudden change, no impact on motor and grid. Start-up time is in wide range.
3. Completed protection, such as over voltage, low voltage, overload, over current, lack phase, over heat.
4. High reliability and stability of components.
5. Optical fiber connecting between high voltage and low voltage, with good separation and reliable triggering.
6. Small volume and compact structure make it nice figure.
7. HMI makes operation visual and easier.

V. Concluding remarks
Application of variable frequency soft starter exploits new field of Leader&Harvest product, makes wide variety of product.

Variable Frequency Drive
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