Introduction
Le générateur 2# (125MW) de la centrale énergétique de Douhe de Datang Group opère généralement avec une charge de 70%-80%. Le flux des ventilateurs de tirage est ajusté par un aubage directeur d’entrée. Pour assurer des opérations sûres et stables du moteur d’entraînement des ventilateurs, la puissance nominale des moteurs est plus large que dans des conditions normales. Lorsque le générateur opère à pleine charge, l’aubage directeur d’entrée du ventilateur est ouvert à environ 60%. Lorsque le générateur compense la charge de pointe, l’aubage directeur d’entrée du ventilateur est ouvert à environ 40%. Aussi l’efficience du ventilateur est basse et une grande quantité d’énergie est gâchée dans l’aubage directeur d’entrée du ventilateur.

The company decided to adopt medium voltage VFDs for some fans and pumps in Douhe Thermal Power plant, Xiahuayuan Thermal Power Plant and Zhangjiakou Thermal Power Plant to realize saving energy, protecting environment, simplifying the operation and reducing the wearing of rotary equipment, etc. The company has made procurement of medium voltage VFDs by the way of international competitive bidding. Beijing Leader & Harvest Electric Technologies Co., Ltd. won the contract for 8 units medium voltage VFDs. Two units medium voltage VFDs of 6kV/1000kW produced by Beijing Leader & Harvest Electric Technologies Co., Ltd. was used in Douhe Thermal Power plant.
1.Principle of medium voltage VFDs
With multi-level cell-connection-in-series technology, medium voltage VFD is composed of phase-shifting transformer, power cell and controller (For the configuration of the 6kV system, see Fig 1). The 6 kV series VFD has 21 power cells and every 7 cells make up a phase in series.
All cells are identical in the structure and electrical parameters of and may be replaced each other. The circuit is basic AC-DC-AC single-phase VFD circuit (See Fig 2). The rectifier is a three-phase diode bridge. Controlling the IGBT inverter by sinusoidal PWM way, the output voltage waveform is shown in fig 3.

Each secondary of the phase shifting transformer supplies power to a power cell. The transformer's secondaries are divided into three groups. For 6000V series, 42 pulses rectifier mode is formed. The multi-pulse overlap rectifier mode greatly improves the quality of the line current waveform, and the input power factor can be improved up to near 1.
In addition, the independence of the transformer's secondaries makes the power cells' main circuit relatively separate. The main circuit of the power cell is similar to the circuit of the low voltage VFD.
At the output side, each cell's U terminal and the neighbor cell’s V terminal are connected in series to form a three phase star (wye) connection to supply power for the motor. By overlapping each cell's PWM waveform, we can get the multi-level SPWM waveform shown in Fig. 4. It is a excellent sine-shape, small dv/dt, no cable and motor insulation destruction, no output filter for long output cable, no motor derating, and it can be used in driving the old motor directly. Meanwhile the motor harmonics are reduced greatly, the mechanical vibration is eliminated, and the mechanical stress of shaft and vanes is reduced.
When one cell fails, this cell may be bypassed off the system through making the soft switch node K in Fig. 2 short and the operation of other cells will not be affected. In this cases, medium voltage VFD operates with derating, the loss resulted from VFD’s stop in many cases may be avoided.

2. A Brief Introduction to the Scheme of Medium voltage VFD Reform
The induced draft fans of #2 boiler are configured at both sides. The flux of the induced draft fans is adjusted manually by the inlet guide vanes. Since the designed redundant power of the motor is rather high and the inlet guide vanes used for controlling the air flux cause vast energy loss, the power used by the plant is high and the economic operation of the generator is affected.

In order to fully ensure the system reliability, a bypass device is configured with the medium voltage VFD. When the medium voltage VFD operates abnormally, it will stop and the motor may be manually switched to be driven directly by the power. The bypass circuit is composed of 3 medium voltage disconnect switches QS1, QS2 and QS3 (see the figure, in which QF is the original medium voltage switch of Party A). It is required that QS2 and QS3 may not be closed simultaneously and they are interlocked mechanically. When the motor is driven by medium voltage VFD, QS1 and QS2 are closed and QS3 is open; and when the motor is driven by the power, QS3 is closed and QS1 and QS2 are open.
In order to protect from medium voltage VFD failure, medium voltage VFD is interlocked with the 6kV switch QF. Once medium voltage VFD fails, medium voltage VFD will make QF switched off. It is required that the user makes suitable reform to the switching circuit. When the bypass circuit is acted, medium voltage VFD will allow QF switch-on and cancel the switch-off signal to enable the motor to start with the power through QF switching on.
3. Contrast of medium voltage VFD test data
The test results for the related parameters by China Electric Science Research Institute are as follows:
Table 1 Contrast of comprehensive input power of 2 induced draft fan systems for inlet guide vanes adjustment and medium voltage VFD adjustment

The test results show that the rates of the power saving are respectively 59% with the 72% load and 46% with full load. At the same time, when the motor starts with medium voltage VFD, the starting current rises smoothly and the motor starts very smoothly.
4. Computation of benefits after reform with medium voltage VFD
1) If operating with full load all the year and 2 medium voltage VFDs are used, the estimated annual energy savings will be: 520kW*5,500h=2,860,000 kWh
The electricity cost savings will be at least: 2860000 kWhRMB0.326/kWh=RMB932,000
2) If operating with 72% load all the year and 2 medium voltage VFDs are used, the estimated annual energy savings will be: 571kWRMB5,500h=RMB3,140,500 kWh
The electricity cost savings will be at least: 3,140,500 kWhRMB0.326 /kWh=RMB1,024,000
It is obvious that in the case of operating with full load all the year or operating with 72% load all the year and 2 medium voltage VFDs produced by Beijing Leader & Harvest Electric Technologies Co., Ltd. are used, the electricity cost savings for the company will be aroundRMB 1,000,000 for both cases. In addition, since the power factor for medium voltage VFD series of Beijing Leader & Harvest Electric Technologies Co., Ltd. may be more than 0.95, which is greater than the motor power factor 0.85, it will reduce the reactive power considerably. The soft starting of motors may be performed, thus affection to the motor insulation from high starting surge current may be avoided, maintenance of motors may be easier, service costs may be lower and motor life may be extended greatly.
5. Conclusion
Medium voltage VFDs have obvious energy savings effect. By using Medium voltage VFD, soft starting for motors may be performed and the motor life may be extended. The inlet guide vanes of the induced draft fans will fully open, vibration and wear of the air pipes will be reduced. In brief, the reliable operating performance and good power saving effects of medium voltage VFDs, taking the products of Beijing Leader & Harvest Electric Technologies Co., Ltd. as the representative, have created tremendous economic and social benefits for the company, so it is worth to be recommended and applied.