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Zhuhai Sinopak Electric Ltd

Add:  Office 801,  No. 1316 Caixia Street, Hengqin, Zhuhai City, Guangdong Province, China

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Medium Voltage Variable Frequency Drives

Medium Voltage Variable Frequency Drives

Sinopak Multi-level voltage source VFD with the technology of cascaded-connected power cells can directly output medium voltage, ie. 3kV, 3.3kV, 4.16kV, 6kV, 6.6kV, 10kV and 11kV.

Medium voltage variable frequency drives



3~11 KV 250kW-20MW/315-25000kVA 


1. Composition

Transformer cabinet

Isolated transformer: supplies phase shift supply for power cells and reduces the harmonic interference to grid.


Power Cell Cabinet

Controller: The voltage vector PWM control, signals measurement and controlling with complete electric isolation by the opticfiber communication between controller and power cells.

Power Cell: The modular designs, easy to production, maintenance and replacement with each other. I/O Board: The interface of digital and analog signals, adaptable for the various applications on site. HMI: LCD display in multiple-language, easy to operation, parameter setting, status display, fault recording, etc. 


Manual / Automatic Bypass Cabinet (Optional)

To ensure the continuous production in case that the VFD is fault or stopped, the bypass cabinet allows the motor to be driven by the grid system.


Master-Slave Drive Control System (Optional)

It is used for multi-drive system in master-slave mode to keep the speed and torque balance between all drives (motors), which are synchronously controlled by the opticfiber communication.


Grid-Connected Synchronous Switching System (Optional)

It is composed of the switch cabinets, a reactor cabinet and a sync-controller cabinet. The whole switching system is controlled synchronously by a sync-controller to ensure that when the VFD’s output voltage keep the phase-sequence and voltage value same as the grid system, the motor can be driven at the industrial frequency by switching into the grid system from the VFD.


2. Principle

Topology of System

 3kV, 4.16kV, 6kV, 6.6kV, 10kV, 11kV




Voltage Overlapping Method

Sinopak HVF has a number of power cells connected in series, the independent phase-shift power is supplied to each power cell by the isolated transformer. Changing   the power cells number in each phase you can get different output voltage conveniently without the limitation for the components’ withstand voltage. For example, the 6kV VFD has 5 cells in each phase, the rated voltage of each power cell is 690V, the phase voltage is 3460V (the line voltage is 6kV); 



Power Cell Topology

The power cell adopts AC-DC-AC mode, it is equal to a low voltage voltage-source inverter with three-phase input and single phase output. All power cells are same electrical and mechanical features, thus very easy to maintenance and replacement.


The internal structure of the power unit is shown in the figure above. The power input ends of R, S, and T are connected to the three-phase low voltage output ends of the secondary coil of a transformer. The three-phase diode full wave rectifier charges the DC link capacitor. The capacitor voltage is supplied to the single-phase H-bridge inverter circuit composed by IGBT.

Each inverter unit outputs the PWM sine wave. The multiple technologies are adopted in units connected in series. That is to say, all amplitudes and frequencies of the output voltage of  units on each phase are identical, but there exists a phase difference of a certain angle (with a difference of 1/n switching period). Therefore,the phase voltage waveform generated by units connected in series has (2n + 1) levels. As a result, the harmonic content in the waveform is significantly reduced and the waveform is almost similar to a perfect sine wave. The figure below shows the overlap waveform generated by 3 units connected in series and the load voltage and current waveform actually measured.


3. Features

By adopting the improved and perfected flux optimization and control with space vector PWM technology of the latest generation and in combination of state-of-the-artcontrol theory, the control software is developed by a Germany R & D team on its own. The control precision of the dynamic rotational speed is lower than ± 2% of the rated speed. The torque step response time < 10 ms. The output frequency resolution is up to 0.01 Hz.

High reliability design

High reliability and long service life

Reliability and stability shall be taken as the principle when selecting the models of core components. It is recommended that IGBT, rectifier bridge, driver module, capacitor, and optical fiber of internationally famous brands should be adopted. Sufficient safety surplus should be left when selecting the capacity to enhance overload capacity of the products and guarantee their reliability and long service life.

In terms of structure design, we absorb the leading design idea from the internationally famous brand companies like ABB, Siemens, and Robicon and adopt the mature and stable structure with multiple units connected in series. Also, we use a module design and make the standard unit have the auto bypass function to enhance the operation reliability of the products and thus significantly reduce the possibility of failure shutdown The designed service life is 20 years, designed MTBF is longer than 100 thousand hours, and designed MTTR is shorter than 10 minutes. Good heat dissipation and ventilation effect can be guaranteed as internationally famous brand cooling fans with long service life are installed on the top of the transformer cabinet, the inverter unit cabinet, and the control cabinet. The MTBF of the air cooling system is longer than that of the devices.

Infallible power supplied by the control power supply
The power for the system is supplied by a power supply controlled by double loops with standard configuration. (1-way internal loop of 220 VAC and 1-way external loop of 220 VAC). The internal control power supply is taken from an auxiliary secondary line winding of the input side isolation transformer. The control power supply will not lose power as long as the high voltage main power supply is active. When the high voltage main power supply lose power, the external power supply (220VAC) will maintain a supply.

The optional external control power supply (220 VDC or 110 VDC) input mode is specially designed for the direct power supply measuring and controlling system in a high-voltage power distribution room to obtain a stable control power supply from the DC system in a convenient way. 

An optional UPS is available to obtain a more stable redundant power supply.

Optical Fiber Communication

Optical fiber communication is adopted inside the VFD to realize the complete optical isolation and a good anti-interference performance.

Automatic current-limiting operation function

Sinopak HVF MVVFD adopts the space vector and PWM control technology of the latest generation to realize the automatic torque limitation function.

When the situation of abnormal load such as short-time high overload or short-time mechanical jamming occurs abruptly, Sinopak HVF MVVFD can automatically reduce the operating frequency, limit the output power, torque, and current, and maintain its operation to avoid over-current trip.

After this phenomenon of abnormal load disappears in a short time, the VFD will return to normal operating frequency.




Power unit bypass operation

The entire Sinopak HVF series feature hardware bypass operation for power units. When a power unit is faulty, this faulty power unit can be automatically switched to the bypass through the automatic bypass technology and the output three-phase voltage is automatically balanced for continuous operation of the VFD.

At most 3-stage bypass function is offered. After the unit is switched to the bypass mode, the output can be adaptively regulated according to bypass stages and the current load to maintain a higher output power.

     ▲ A contactor is taken as the hardware bypass component for excellent anti-interference performance and greater security.

     ▲ The bypass and the inverter circuit work independently and thus have higher stability.

Perfect and reliable structure design

Electromagnetic shielding has been performed on the cabinet and the electromagnetic compatibility (EMC) meets stringent IEC1000-4 and IEC1800-3 requirements.

The integrated design includes the dry-type transformer (H class insulation) with high reliability. The temperature of the iron core and windings of a phase-shifting transformer can be monitored.

A threshold switch is designed on the transformer cabinet to ensure that the system will generate an alarm if the cabinet door is opened when the transformer is in operation.

The heating equipment preventing condensation due to low temperature can be adopted to ensure that the transformer can operates in a cold and humid environment for a long time.

Surge absorption capacity for power protection

The entire Sinopak HVF serial products have the surge absorption capacity for power protection which enables them to fully absorb the peak current for successful poweron at once. The surge absorption capacity for power protection of Sinopak HVF includes the following two parts:


Anti-surge measure for a unit

The input voltage of a power unit, through a fuse, enters a three-phase bridge rectifier for rectification, and then it is filtered by an

electrolytic capacitor to become a direct current. To prevent the surge current upon poweron, a contactor and a resistor should be put in parallel,

and then they should be connected in the middle of a three-phase bridge rectifier and a filter electrolytic capacitor in series. As a result,

the electrolytic capacitor will be charged through a power resistor upon poweron.

After the charging is finished, the contactor is sucked.



Anti-surge measure for a system

The main circuit of a phase-shifting transformer is equipped with anti-surge resistors upon poweron and a vacuum contactor, which can effectively reduce the excitation inrush current and the electrolytic capacitor charging current at the moment of switching on, for a successful switch-on operation at once.


High voltage surge arrester for surge protection

The input terminal of a high voltage power supply is equipped with a surge arrester, which can absorb a lightning surge and the surge formed by the switching overvoltage in the power grid.








Start with full torque at low speed

Benefiting from the leading "dynamic PWM" software, Sinopak HVF MVVFD can operate stably at an extremely low frequency (approximately 0.1 Hz) with the widest range of speed regulation.

The starting torque of the VFD is adjustable. When starting a overloaded device, like a belt conveyor, a roller mill, or a draught fan suffering from jamming due to a seriously corroded bearing shell, the VFD can output an extremely large starting torque at an extremely low operating frequency (approximately 0.1 Hz) to ensure a normal start of a load.

Sinopak HVF MVVFD is a high-performance VFD which is not only suitable for variable torque loads, but also suitable for constant torque loads.

Starting a motor inversely rotating at a low speed

Benefiting from its excellent features, Sinopak HVF MVVFD has the function of "starting a motor inversely rotating at a low speed". It adopts a technology equivalent to the DC braking (DC brake) when starting a motor inversely rotating at a low speed. First, it slows the speed of a motor inversely rotating at a low speed down to zero, and then make the motor run from zero speed. The function of "starting a motor rotating inversely at a low speed" allows the MVVFD to start a motor rotating inversely in a safety mode without current trip.

Restart with speed tracking (Flying start)

Sinopak HVF MVVFD adopts the unique "slip current control algorithm" to automatically search and recognize the motor rotating speed. It starts the rotary electric motor at current speed rather than the zero speed for a safe startup at a low current. Therefore, it reduces the impact on the power grid and weakens the influence on productio-n when an immediate outage occurs. 



Excellent input side features















For 6 kV devices: 30/36 impulse input.

For 10 kV devices: 48/54 impulse input.

The grid-side harmonic is low. No need to install the noise filter. The input harmonic content can meet the requirements of I-EEE 519-1992 and GB/T14549-2002.

The power factor exceeds 0.97 at rated load in the range of normal speed regulation.

The blue curve and yellow curve in the diagram refers to the input current curve and the input voltage curve respectively. Bo-th waveforms are close to the sine wave, with negligible harmonic content.

Excellent output features 
The Sinopak HVF MVVFD can output excellent multi-level PWM sine waves. A 6 kV MVVFD can output an 11- or 13-level phase voltage, and a 21- or 25-level line volta-ge; a 10 kV MVVFD can output a 17- or 19-level phase voltage, and 33- or 37-level line voltage. It has a small dt/dv value, and the output voltage and current waves are n-ear perfect sine waves. It has no special requirements to the driven motor and can drive motors made in China.
Common power cables can be adopted as input and output cables. The output cable stretches as long as 1,500 m (inform Cumark in advance if you need a longer cable). The Sinopak HVF MVVFD can operate stably with small output voltage and current harmonics, small motor torque ripple, and low noises. 





Complete fault handling functions

With its complete fault diagnosis, location, and handling functions, the Sinopak HVF MVVFD categorizes and handles faults based on severities, outputs the fault and content in real time, and record the fault in the log.


The Sinopak HVF MVVFD also has a series of complete protection functions for MVVFD overheat, input overvoltage, input undervoltage, output overcurrent, motor overload, output grounding, output short-circuiting, equipment overload, power unit fault, cooling fan fault, and interlocking of doors to the high voltage switchgear. Some fault can intertrip the high voltage switchgear on the input side.




Powerful adaptative feature to the voltage fluctuation of power grid 

Sinopak HVF MVVFD can operate at full load when the power grid voltage ranges from -20% to 15%

Sinopak HVF MVVFD can continue to operate at a derated load with decrease of the power grid voltage by 35%

Power loss ride-through and power failure recovery features
Sinopak HVF MVVFD can maintain a normal operation within 10 cycles after immediate power outage. A longer power outage duration is allowed when Sinopak HVF MVVFD operates under light load.
When VFD loses power for a period longer than 10 cycles, it will automatically operate at a derated load. The VFD will shut down when the power outage duration exceeds 10 seconds.
When the power is transmitted again by the power grid, it is recommended that the Sinopak HVF MVVFD should be re-started after the automatic speed tracking is set based on the requirements. 

Reliable redundancy control feature

The core control part of Sinopak HVF MVVFD is designed by two control systems adopting PLC and touchscreen based on the DSP control board, which is a dual redundancy control mode. The PLC system is taken as the external interface, realizing a transition from the external control port to the internal DSP control board. The external port is completely isolated from the internal control board. This not only significantly enhances the reliability and anti-interference ability of the system to avoid interference on the control part of the VFD from an external system, but also is conducive to the upgrade and maintenance of the system and addition of the monitoring function. The design is more reliable when compared with the design of adopting only a single chip control board adopted by most similar manufactures in China.

Safe resonance avoidance

Sinopak HVF MVVFD allows users to configure three sets of resonance frequency hopping points to effectively avoid mechanical resonance of the electromechanical system and thus to ensure the safe and reliable operation of the drive system.

Automatic flux optimization

Sinopak HVF MVVFD adopts the exclusive flux optimization technology which enables real-time monitoring and flux optimization on the entire speed regulation system including the motors, and thus increases the system efficiency by 1% to 10% and save more energy than a similar VFD does.

The automatic flux optimization also facilitates reduced operating current of the motor and improved stability of the device. Current motors are made based on the power frequency design patterns. When some motors are running at a variable frequency, the best magnetic intensity rather than rated magnetic intensity may occur at a particular frequency (especially within the medium or low frequency range), which may cause large operating current and unstable operation if the traditional vector control strategy is adopted. In this case, Sinopak HVF MVVFD will enable automatic flux optimization in combination of the space vector PWM control algorithm to adjust the motor flux to the specification value of the motor, reduce the operating current of the motor and improve the stability of the system.

Large capacity design

The designed power of 10 kV is 20,000 kW, 6 kV 12,000 kW respectively. Tens of synchronous/asynchronous motors with high power above 5,000 kW have enjoyed success in many applications.

4. Technical Parameters

Medium voltage input

Voltage range

3kV, 4.16kV, 6kV, 6.6kV, 10kV, 11kV  (-20%~+15%)

Frequency range

50/60 Hz±10%

Medium voltage output

Output voltage

0~Rated voltage


Output frequency and accuracy

0~50/60 Hz, 0.01 Hz

Output waveform

Multilevel PWM sin wave, total harmonic distortion (THD) < 4%



Inversion efficiency at an rated load > 98.5%, overall efficiency (including the transformer) > 97%

Power factor

Power factor (within the range of speed regulation) > 0.95, power factor (at a rated load) > 0.97

Overload capacity

110%: long-term stable operation, 120%: 1 minute, 150%: 2 seconds, and 160%: immediate protection

Allowable outage duration

200 ms (and longer under light load)

Mean time between failures

> 100,000 hours


Control mode

Flux optimization and control with space vector PWM

Acceleration/deceleration time

1 to 3,600 seconds

Switching quantity input/output


Analog input

4-way (optional: 0 to 5 V, 4 to 20 mA)

Analog output

3-way (optional: 0 to 10 V, 4 to 20 mA )


RS485 interface, ModBus RTU, Profibus DP (optional), Industry Ethernet Protocol (optional)

Control power supply

Single-phase 220 V AC, 5kVA

Excitation control

(dedicated for the synchronous motor)

Control mode

Automatic excitation control, manual excitation control, multi-stage speed excitation control, and external excitation control for the MVVFD


Operating environment

Operating in indoor environment without explosive or corrosive gases, conductive dust, or oil fog

Operating ambient temperature


Storage and transportation temperature

-40℃ ~+70℃

Ambient humidity

< 90% (No condensation)

Operating altitude

Derating required for an altitude greater than 1000 meters


Protection function

Overcurrent, overload, short circuit, three phase current unbalance, immediate electric outage, input/output phase loss, overvoltage, undervoltage, body overheat, transformer overheat, shutdown due to external fault, and power unit auto bypass

Cooling mode

Forced-air cooling

User interface

An 10-inch Human Machine Interface

Protection grade: IP31



5. Application