HomeApplicationVFD in Thermoelectricity Plant

VFD in Thermoelectricity Plant

The so-called Variable Frequency Drive (VFD) parallel connection technology indicates that two or more sets of small power VFDs are connected in parallel to drive a high power motor. However, since it is inverting frequency output, several problems as follows need be solved firstly:

1. When two or more sets of small power VFDs are connected in parallel to drive a high power motor, it is required that their frequency, phase and limit must keep the same. Any hairlike bias will lead the system unstable, or trigger circumfluence among the inverting units.

2. The output must be the same, as accumulated bias in the frequency will cause regular fluctuate in the system's output limit, and difference in phase will result in limit fluctuating.

3. High requirement to equalizing current. Equalizing current can be classified as active power equalizing current and reactive power equalizing current, namely the power equalization is classified as that of active power and reactive power.

4. On the aspect of protection, there must be a new function that can switch off the faulty VFD in time besides protection to the single VFD, which can guarantee the stability of the VFD.

Among the problems listed above, the most important is current equalization. So adopt the control mode of active power and reactive power parallel connection. Now, China's current equalization technology is very mature, with unequalizing content within ±1% of the rated current. Gozuk has successfully launched low voltage high power VFDs based on the parallel connection structure, and advanced SVPWM technology, photoelectrical isolation/transmission technology.

A large amount of pumps and fans in the power plants and the boiler industry have been reconstructed with the advanced frequency conversion speed control technology, with distinctive energy saving efficiency, majority of which, however, are low voltage motors with power lower than 315KW. Yet Gozuk has successfully reconstructed a high power AC motor (690V, 560KW) in a Shenzhen-located thermoelectricity plant supported on its self-developed parallel connection technology.

Main advantages:
  • No need to design the structure respectively
  • Delivery period can be steeply shortened
  • More easy to control the quality
  • High performance but low cost
Prominent features of the parallel connection technology.
1. The common digital operator and control board can guarantee the balance of the system's frequency, phase and limit
2. The common input and output compound reactor can ensure the balance of the output frequency.
3. The common DC bus realizes the parallel of active and reactive power, moving the current un-equalization problem.
4. All the parallel-connected VFD adopt digital control, making equalizing current control and communication more convenient, and realizing complex operation inside the module of the variable frequency drive, so as to enhance reliability of the system
1. ±20% voltage fluctuating width is quite applicable to Chinese grid status
2. The SVPWM technology, in addition to professional wave processing circuit, can reduce 1/3 heat loss and accordingly prolong the using life of the VFD.
3. The photoelectrical isolation technology is adopted in the VFD's control circuit and drive circuit, to realize accurate signal receiving and control. Fault alarm and protection are pretty sensitive and accurate
4. various protection functions: over current, over load, over voltage, undervoltage, phase loss and shortcircuit
5. Flexible control mode: RS-485 control, PI regulating, multi-step speed, procedure control, proportion linkage, programming control terminals and analog signal terminal, several control mode.

Gozuk VFD features:
  • Control mode: vector control, V/F control, vector control with PG, V/F control with PG.
  • Owns motor parameter autotuning function, applicable for broad range of load.
  • In the mode of open-loop, can realize rated torque output at low speed, speed control range hits 100:1
  • In the mode of closed-loop, can realize full torque output at zero speed, with speed control range up to 1000:1
  • Energy saving control function enables the VFD run with high efficiency. It can realize automatic operation with PID.
  • Slip compensation function makes the system more stable
  • Built-in automatic torque compensation realizes high starting torque.
  • Torque control function enables the motor be able to run with a constant torque.
  • Friendly user interface, large screen LCD. Can realize parameter online monitoring and online regulation, in addition to parameter copy function, making operation more convenient.
  • Less pollution to the grid.
1) air valve + motor: total investment cost: RMB 200,000
Cost (RMB)
Motor (560kw)
Low voltage controller

Advantages: low investment cost, and simple system
Disadvantages: a) heavy starting current, low power saving efficiency
                     b) low control accuracy, and inconvenient maintenance

2) hydrodynamic coupling + motor:
Cost (RMB)
Motor (560kw)
Low voltage controller
Hydrodynamic coupling

total investment cost: RMB 260,000
Advantages: small current, less impact on the grid, stable speed control,
Disadvantages: high investment cost, and high maintenance cost, and unsatisfying energy saving efficiency

3) VFD + motor: total investment cost: RMB 360,000
Cost (RMB)
Motor (560kw)
Low voltage controller
Variable frequency drive

Advantages: prominent energy saving efficiency (30% or above), realize soft start and no impact on the grid, high reliability,
Disadvantages: harmonic current exists, bringing interference to the grid and peripheral devices.

From the three tables above, it is easy for us to draw a conclusion that the third method can help the customer save 500KW×5000h×30%×0.4=336000 RMB more per year (providing the VFD works 5000 hours per year). And it can realize economic benefits in half a year.

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