KR20080087107A - Variable frequency drive system apparatus and method for reduced ground leakage current and transistor protection - Google Patents

Abstract

As applications of variable frequency drives (VFD) (50) continue to grow so do challenges to provide VFD(50) systems meeting application specific requirements. For multiple reasons to include safety standards and electromagnetic interference, reduced ground leakage current is desireable. Building high ouput voltage VFDs (50) using transitors rated at voltages lower than the VFD ouput votlage is desireable for economic resons. The apparatus and method described herein meet these challenges and others, in part by placing an electrically insulating plaste (cpl76) having high thermal conductivity, a low dielectric constant, and high dielectric strenght between the heat sink plate of a VFD power semiconductor module and a grounded cooling plate (80 TE). The positive effects of this plate installation include reducing ground leakage current induced by system capacitances to ground upon high frequency voltage steps and increasing the effective dielectric strength of the VFD's(50) transistor modules engaging in high reliable VFD (50)votlage ouput for a given transistor rating.

Description

VARIABLE FREQUENCY DRIVE SYSTEM APPARATUS AND METHOD FOR REDUCED GROUND LEAKAGE CURRENT AND TRANSISTOR PROTECTION

Conventional sinusoidal AC power supplies only fixed motor speeds and cannot respond quickly to changes in load conditions. With the development of variable frequency drives (VFDs), better performance motors can be achieved at low energy costs. The VFD drive motor responds quickly to changes in load conditions, for example in response to shock loads. The VFD drive motors also provide precision torque output and continuous speed control. Because of these many advantages, the use of VFDs in industry is increasing.

A conventional medium voltage VFD drive motor system is described below in connection with FIG. The intermediate point N 26 of the DC bus 20 is grounded to protect the transistor switch from potential voltage spikes that cause insulation degradation and component abnormalities. The heat sink of the transistor of the inverter bridge is also grounded, the ground connection is not shown in FIG. 2A shows the ground of the inverter bridge transistor module through heat sink 126 and will be described in more detail below. Referring again to FIG. 1, three phase cables 30 are connected at one end to the output terminal 52 of the VFD 50. The cable 30 has an intrinsic capacity per unit length. Total cable capacity is shown as C _C 32. These cables are pervasive to motor M 40, which also has the capacity and motor impedance represented by Z _M 44 due to the winding represented by C _M 42.

FIG. 3A is a schematic circuit diagram 300 of a conventional VFD drive motor system, for example the drive system shown in FIG. 1. The switch S 60 displays the voltage change output from the VFD 50. Upon closing of switch S 60, a voltage change from the grounded middle to the positive potential 22 or negative potential 23 of the DC bus 20, output from the VFD 50, is applied to the circuit 300. Is given. The ground leakage current I _GND 200 flows freely at the ground connection with the voltage change due to the motor capacity C _M 42 and the cable capacity C _C 32. Inverter bridge capacitance C _IB 62 is connected to device ground PE 70 and earth ground TE 80 from midpoint N 26 in parallel with the short circuit of the midpoint N 26 connection with earth ground TE 80. Connected. In this conventional configuration, since the C _IB 62 is in parallel with the short circuit connection to ground, the C _IB 62 contributes to the ground leakage current to a negligible extent.

With high performance and low power consumption, VFDs are desired in a variety of application areas to cover fan and pump loads. However, the use of VFD in medium voltage applications can be complicated when low ground leakage current is required. Low ground leakage currents may be needed in potentially explosive environments or in environments requiring reduction of electromagnetic interference (EMI). High frequency ground leakage currents up to the MHz range can induce EMI, for example, in wireless receivers, computers, bar code systems, and vision systems.

An example of an application requiring low ground leakage currents is underground mining: this underground mining environment has its own conditions and safety standards. Underground mining motors preferably have an intermediate voltage range (between 690V and 15kV) and are typically driven at 4160V. A conventional intermediate voltage VFD that provides a 4160V output can produce a ground leakage current I _GND 200 in excess of 10 amperes, which flows from VFD 50 to motor M 40 at the ground wire. Using a medium voltage motor facilitates the use of smaller cables, but the maximum allowable drive motor ground line leakage current I _GND 200 may be 1 Amp or less.

Unlike conventional AC sine wave motor drives, the VFD outputs a voltage change in the order of microseconds. Thus, larger ground leakage currents are induced due to the capacitances C _M and C _C inherent in the VFD drive motor system even at relatively low voltages, for example 690 volts. With reference to FIG. 3A, separating the midpoint N 26 of the DC bus 20 from TE ground 80 is a practical means of reducing ground leakage current. A schematic description of separating intermediate point N 26 from the ground of conventional VFD system 302 is shown in FIG. 3B. As shown in FIG. 3B, separating the midpoint N 26 of the DC bus from TE ground 80 changes the circuit model for ground current leakage current I _GND 202. The inverter bridge capacity C _IB 62 is now in series with the parallel coupling of the cable capacity C _C 32 and the motor capacity C _M 42. This results in higher impedance to ground leakage current due to a reduction in total system capacity. However, the separation of intermediate point N 26 and TE ground 80 makes transistors S ₁ -S ₁₂ of the inverter bridge susceptible to voltage spikes.

Separating the midpoint N 26 of the DC bus from TE ground 80 causes the transistor to float with respect to the midpoint N 26 of the DC bus. Voltage spikes at the full DC bus potential can be applied across the transistors of the inverter bridge. Referring to FIG. 2A, these voltage spikes are transferred between the semiconductor substrate 122 of the transistor and the heat sink 126 of the transistor across the thin insulator 124.

For low drive voltages, an effective transistor rated above the difference between the positive and negative DC buses can be used in a VFD system having a midpoint of a floating DC bus separated from TE ground. This configuration has been successfully used, for example, on SMC's Microdrive 2,300V model. However, when a higher VFD voltage output is needed or desired, and when a transistor rated at full DC bus potential is not practical, protecting the transistor from full DC bus potential spikes can be used to prevent component life and reduction of anomalies. need. Many challenges exist for VFD drive applications. One challenge is to reduce leakage ground currents, for example, while protecting VFDs, especially inverter bridges. Another challenge is to reduce the ground leakage current as much as possible.

In other applications, the challenge is to provide a reliable VFD system for motors rated greater than 4160V. For example, for a motor rated above 4160V, a VFD that provides a 6.9kV output is desired. However, currently available transistors for manufacturing VFD at 6.9 kV have to compromise transistor isolation and are more prone to component problems. A DC bus rated at 11.5 kV is required to obtain a VFD output of 6.9 kV. Inverter bridge transistors are available with an insulation rating of 5100V. Even when the midpoint of the DC bus is grounded, the transistor module isolation 124 (FIG. 2A) breakdown voltage 5100V is less than half of the potential on the DC bus 11.5 kV. Another challenge of the VFD system is to protect the inverter bridge including the series-connected transistors to provide VFD output voltages greater than 4160V when the transistors are rated to less than half the DC bus voltage.

<Overview of invention>

The present invention provides a reduction in the ground leakage current of the VFD drive motor system while protecting the inverter bridge.

It is an object of the present invention to reduce the ground leakage current of a VFD drive motor system.

Another object of the present invention is to float the midpoint of the DC bus to reduce ground leakage current.

Another object of the present invention is to float the midpoint of the DC bus while protecting the VFD from component anomalies due to voltage spikes.

Another object of the present invention is to increase the impedance to ground leakage current.

Another object of the present invention is to further reduce the ground leakage current of the medium voltage VFD drive motor system while reducing the system capacity to ground.

Another object of the present invention is to reduce the total capacity to ground of the VFD motor drive system.

Another object of the present invention is to reduce the total capacity to ground of the VFD motor drive system by using a high dielectric strength and low dielectric constant plate disposed between the VFD transistor module heat sink and the grounded cold plate.

It is another object of the present invention to improve inverter bridge reliability and component life in VFD systems with transistors rated to less than half the full DC bus potential with additional effective isolation when the midpoint of the VFD's DC bus is grounded. It is to let.

Exemplary embodiments of the present invention can be used in low, medium, and high voltage drive applications.

According to the object of the present invention, the midpoint of the DC bus in the device according to an exemplary embodiment of the present invention is a floating state separated from ground.

In an apparatus according to another exemplary embodiment, an electrically insulating plate having high thermal conductivity, high dielectric strength, and low dielectric constant is thermally and electrically connected between the transistor semiconductor substrate and the cold plate.

In an apparatus according to another exemplary embodiment, the common mode filter is equipped at the output of the VFD.

The method according to an embodiment of the present invention includes floating the midpoint of the DC bus and increasing the impedance of the ground leakage path using a dielectric substrate.

Another method according to an embodiment of the present invention includes increasing the impedance of the ground leakage path using a dielectric substrate and increasing the dielectric strength of the transistor module of the VFD above the full DC bus voltage.

Another method according to another embodiment of the present invention is to float the midpoint of the DC bus, increase the impedance of the ground leakage path using a dielectric substrate, and apply a common mode filter across the three phase drive cable of the VFD system. Installing.

Other objects and advantages of the present invention will become apparent to those skilled in the art from the following description with reference to the accompanying drawings.

1 shows a conventional VFD drive motor system.

Figure 2a shows the grounding of the inverter bridge across the insulation plate of the transistor module and the resulting inverter bridge capacitance in accordance with a conventional VFD system.

2B illustrates a series connection between an inverter bridge and ground across an insulating plate of a transistor module, a heat sink of the module, and an electrically insulated, high dielectric strength, low dielectric constant and high thermal conductive plate in accordance with an embodiment of the present invention.

FIG. 3A shows a schematic representation of the ground current loop in a conventional VFD system to include the capacity of an inverter bridge, for example as shown in FIG. 1.

3B shows a schematic representation of a VFD system with floating midpoints on a DC bus in the absence of the present invention.

4 is a schematic representation of a ground current loop in a VFD system implementing an exemplary embodiment of the present invention, showing the capacitance of a transistor module and the capacitance of a low dielectric constant isolator.

5 shows another exemplary embodiment of the present invention including a common mode filter installed in a three phase drive cable.

6 shows an exemplary embodiment of mounting means for enabling electrical insulation and heat conduction between the VFD power semiconductor module and the grounded cold plate, securing the VFD power semiconductor module to the insulation plate and securing the insulation plate to the ground cooling plate.

The present invention reduces ground leakage current by allowing the midpoint N of the DC bus to float without exposing the transistors of the inverter bridge to excessive voltage spikes.

Referring first to FIG. 2A, a conventional ground of the inverter bridge of a transistor module is shown. The ground of the inverter bridge is through the heat sink 126 of the transistor module. The capacitor C _IB 62 is formed between the semiconductor substrate 122 and the heat sink 126 across the thin insulating plate 124. The semiconductor substrate 122, the thin insulating plate 124, and the heat sink 126 together form a conventional VFD power semiconductor module, which is also referred to as a VFD transistor module. The heat sink 126 is mounted on and electrically connected to the grounded cooling plate 130.

2B illustrates grounding of an inverter bridge in accordance with an exemplary embodiment of the present invention. The semiconductor substrate 122, insulating plate 124 and heat sink 126 of the transistor module are connected together as in the conventional grounding means of FIG. 2A. However, according to an exemplary embodiment, an electrical insulation plate P 175 having high dielectric strength, low dielectric constant, and high thermal conductivity is mounted between the heat sink 126 and the grounded cold plate 130. The heat sink is usually made of aluminum silicon carbide. However, the heat sink 126 need not necessarily be made of aluminum silicon carbide; Materials that provide good electrical and thermal conductivity can be replaced as appropriate.

The inverter bridge semiconductor substrate 122 is thermally coupled to and electrically insulated from the cooling plate 130 through the plate P 175. Since the dielectric constant of the insulating plate P is low, a small capacitance C _P 176 is formed between the heat sink 126 and the cooling plate 130 of the transistor module. This capacitance is less than and in series with the transistor internal capacitor C _IB 62 (FIGS. 2A, 2B and 4). Capacitor C _IB 62 is a byproduct of the capacitive coupling between semiconductor substrate surface 122 and transistor heat sink 126, which is insulated from the thin insulator 124, as shown in FIG. 2A.

Referring to FIG. 3A, in the conventional inverter bridge, when one end of the intermediate point N 26 and the C _IB 62 of the DC bus 20 is grounded to the same TE ground 80, a displacement current is formed at every pulse transition. To form a path to the DC bus. This ground prevents one transistor from affecting another transistor in the inverter bridge. More specifically, grounding off the midpoint N 26 of the DC bus 20 causes one side of the C _IB 62 to float relative to the midpoint N 26 of the DC bus, as shown in FIG. 3B. This allows displacement current crosstalk between the transistors of the inverter bridge via each transistor capacitor C _IB 62 interconnected through the cold plate 130. This crosstalk can cause excessive voltage spikes between the semiconductor substrate 122 of the transistor module and its heat sink 126, which can result in component failure.

Those skilled in the art will appreciate that the module or individual base high voltage IGBT transistors need to be grounded at the midpoint of the DC bus. This grounding ensures that the maximum voltage between the transistor terminals and the cold plate is only half the maximum DC bus voltage. When the intermediate point is separated from the ground, the capacitor C _IB formed between the semiconductor substrate of the transistor module and its heat sink is still grounded on the cooling plate 130 side. The inverter bridge capacitance is now floating relative to the DC bus positive and negative voltages 22/23 and electrically connected to the internal capacitance C _IB 62 of the other transistors of the inverter bridge via a grounded cold plate 130. do. When the transistor attached to the positive voltage of the DC bus is turned off, the semiconductor substrate is directly connected to the positive voltage of the DC bus. Next, when the transistor connected to the negative voltage is turned on, this causes the internal transistor capacitor C _IB 62 to be charged to the negative voltage potential of the DC bus. Since both sides of each internal capacitor C _IB 62 of all transistors are connected together, the semiconductor substrate of the transistor connected to the positive voltage is subjected to the full DC bus voltage. Typically, the full DC bus voltage is significantly greater than the insulation voltage rating of the transistor, resulting in damage to the transistor.

4 is a schematic representation 304 of an exemplary embodiment of a VFD system according to the present invention with an insulating plate P 175 coupled, for example, as shown in FIG. 2B. Insulator plate capacitance C _P 176 is currently in series with C _IB 62. Insulator plate capacitance PC 176 is connected to TE grounded cold plate 130. The series capacitance of C _IB and C _P provides a high impedance in the return path to voltage source V 140, reducing ground leakage current I _GND 204. Insulation plate P 175 (FIG. 2B) also increases the insulation strength between the cold plate and the power semiconductor. The total system capacity C _SYS is reduced as shown by equations 1 and 2 corresponding to the circuit shown in FIG. First, in Equation 1, we define the capacity of the VFD, C _VFD .

The reduction of the total system capacity C _SYS then reduces the ground leakage current IGND 204 from the voltage change at the output of VFD according to equation (3).

The following experimental data shown in Tables 1 and 2 are obtained in the presence and absence of exemplary embodiments of the present invention and confirm their validity. Table 1 summarizes the data obtained under control conditions. The VFD module is a 4160V output microdrive (SMC Electrical Products, US Pat. No. 6,822,866). The motor is a 500HP induction motor rated at 4000V or less. Ground current measurements are made for three-phase shielded cable lengths of 30, 250, and 1300 feet. Ground current is measured continuously at the output terminal of the VFD and at the motor. VFD switches at 1kHz and the motor speed is maintained at 30 percent. As shown in FIG. 3B, measurement control cannot be performed when the intermediate point N of the DC bus is separated from the ground in the absence of the insulating plate P 175 and is in a floating state. Separation of the midpoint N from ground allows the transistor of the inverter bridge to be protected only by the thin insulator 124, which leads to component anomalies due to crosstalk, as predicted and described above. The 1.5μF capacitor is connected in series from the midpoint N to ground and in parallel with C _IB , acting as a very high impedance path, providing more protection than open circuits. A 1.5μF capacitor provides a high impedance path from the midpoint to ground without sacrificing the inverter bridge. Table 1 summarizes the control data obtained without insulation plate P 175. The current value in Table 1-3 is RMS.

TABLE 1

Ground Current Measurements for Cables of 30, 250, and 1300 Feet

No boron nitride plate

30 '250' 1300 '

@drive 4.7 8.0 38.3 @motor 2.3 2.0 3.7

Table 2 summarizes the experimental data obtained using an exemplary embodiment of the present invention. The test condition is the same as that of the control condition in which the following test condition is modified. An insulating plate P 175 is provided between the heat sink 126 and the cooling plate 130 (shown in FIGS. 2B and 6). The middle point N of the DC bus is suspended from ground. In the test embodiment of this example, the insulating plate P is a ceramic made of boron nitride. 4 is a schematic representation of the test conditions summarized in Table 2. FIG.

TABLE 2

Ground Current Measurements for Cables of 30,250 and 1300 Feet

In boron nitride plate

30 '250' 1300 '

@drive 0.7 1.1 5.5 @motor 0.5 0.3 0.4

Additional experimental measurements are made on a system according to another example embodiment that includes a common mode filter (CMF 150) shown for the example of FIG. 5. The CMF 150, transformer and resistor ballasts are coupled across a three phase cable connecting the VFD output to the motor. The data in Table 3 below is obtained with a 500HP induction motor operating at 50 percent of full speed. All other test conditions are the same as those used to obtain the test data summarized in Table 2. VFD switches at a frequency of 1kHz. Ground current measurements are made at the output terminals of the VFD and at the motor. The current value shown is RMS as above.

TABLE 3

Ground current measurement for 250 feet of cable (Amps)

In boron nitride plate

No CMF CMF

@drive 5.2 0.05 @motor 0.8 0.05

As can be seen from the data in Table 3, the ground leakage current is reduced to a negligible amount of 50 mA in the installation of the boron nitride ceramic plate and the CMF according to another exemplary embodiment of the present invention.

While boron nitride is used in the exemplary embodiment for obtaining the above experimental data, other oxide and nitride materials or other insulating components having the desired dielectric and thermal properties described according to the present invention may be used. For example, synthetic diamond plates with the desired dielectric properties and good thermal conductivity can be used.

As shown in FIG. 2B, the provision of insulation plate 175 serves another object of the present invention, which is rated at less than half of the full DC bus potential when constructing a reliable VFD system having a midpoint of a grounded DC bus. Enable the use of transistors. For example, for a motor rated at 4160V or higher, a VFD providing an output voltage greater than 4160V, for example 6.9 kV, is preferred. Rated at 11.5kV, the DC bus is readily available for VFD and is suitable for providing 6.9kV of VFD output. Transistors that build inverter bridges are readily available at 5,100V ratings. Even when the midpoint is grounded, the transistor isolation (ie, element 124 of FIG. 2A) breakdown voltage 5100V is less than half of the full potential on the DC bus 11.5 kV. As shown in Figure 2b, the insulating plate insulates the P (175) having sufficient dielectric strength described above is increasing the effective isolation of C _IB bays and similar compared to the inverter bridge, and C _IB and C in series the effect of the _P transistor. Increased insulation improves component life and system reliability. Thus, the provision of insulation plate P 175 is such that a plurality of transistors are connected in series (eg, shown in FIGS. 1 and 5, S1-S12) to common cold plate 130 (eg, shown in FIG. 2B). By cooling, the high output voltage VFD system can be achieved at low cost using currently available transistors rated at less than half the total DC bus voltage, while retaining the advantages of a single grounded cold plate.

6 allows electrical insulation and thermal conduction between the VFD power semiconductor module 50 and the grounded cold plate 130 and secures the power semiconductor to the insulating plate 175 and the insulating plate to the grounded cold plate 130. An exemplary embodiment of mounting means is shown. The VFD power semiconductor module 50 (or shown in FIGS. 2A and 2B) is fixedly mounted to the insulating plate 175, which is fixed to the grounded cooling plate 130 using an L-shaped steel bracket 180. Is mounted. The bracket 180 is directly fixed to the cooling plate 130 at one end and secures the VFD power semiconductor module through the set screw 182 and the ceramic pellet 184 at the other end.

6 shows an exemplary embodiment of many possible means for securing the VFD power semiconductor module and insulation plate to the cold plate. The bracket may be made of a material having sufficient strength to support the clamping force required by the transistor module specification. Those skilled in the art will understand the various ways of physically securing the VFD power semiconductor module to the insulating plate and the insulating plate to the cold plate while allowing electrical insulation and thermal conduction of the insulating plate to be realized.

In summary, the reduction of ground leakage current in a VFD system is desirable for a variety of reasons in various application environments. With high dielectric strength, low dielectric constant, and high thermal conductivity, an electrical insulator mounted between the VFD power semiconductor module and the grounded cold plate is an efficient means of reducing system capacity, reducing ground leakage current induced by the high frequency voltage shift of the VFD. You can. The installation of the insulator plate described above increases the insulation between the VFD power semiconductor module and ground to protect the transistors of the inverter bridge from dielectric breakdown.

While specifically illustrated and described in accordance with exemplary embodiments of the invention, it will be understood by those skilled in the art that various modifications may be made to the type and details without departing from the spirit and scope of the invention as defined in the following claims. will be.

Claims (21)

As a method of reducing ground leakage current in a VFD system, Floating a neutral point on the DC bus of the VFD; And Disposing an electrical insulating plate having high thermal conductivity, low dielectric constant and high dielectric strength between the VFD power semiconductor module and the cooling plate Ground leakage current reduction method of the VFD system comprising a. The method of claim 1, further comprising connecting a first end of a cable to each output terminal of the VFD and connecting a second end of the cable to a device driven by the VFD. Way. 3. The method of claim 2, further comprising using a common mode filter on the cable connected to each output terminal of the VFD. 3. The method of claim 2 wherein the ground leakage current is reduced to less than 1.5 amps. 4. The method of claim 3 wherein the ground leakage current is reduced to less than 1 amp. 5. The ground leakage of the VFD system according to claim 4, wherein the ground leakage current is reduced to less than 1.5 amps, and the reduction to less than 1.5 amps is obtained despite the capacity for ground due to a cable connected to the output terminal of the VFD. Current reduction method. 5. The method of claim 4, further comprising driving a motor using the output voltage of the VFD, wherein the ground leakage current is reduced to less than 1.5 amps and the reduction to less than 1.5 amps is obtained despite the motor capacity. How to reduce ground leakage current in VFD systems. 6. The ground leakage of a VFD system according to claim 5, wherein the ground leakage current is reduced to less than 1 amp, and the reduction to less than 1 amp is obtained despite the capacity for ground due to a cable connected to the output terminal of the VFD. Current reduction method. 6. The method of claim 5, further comprising driving a motor using the output voltage of the VFD, wherein the ground leakage current is reduced to less than 1 amp, and the reduction to less than 1 amp is obtained in spite of the motor capacity. How to reduce ground leakage current in VFD systems. A method of reducing ground leakage current in a medium voltage VFD system, Floating a neutral point on the DC bus of the VFD; And Disposing an electrical insulating plate having high thermal conductivity, low dielectric constant and high dielectric strength between the VFD power semiconductor module and the cooling plate And wherein the ground leakage current is reduced to less than 1.5 amps. Medium voltage VFD system with low ground leakage current A DC bus with a floating neutral point; An inverter bridge of a VFD power semiconductor module electrically connected to the DC bus; Electrical insulating plates having high thermal conductivity, low dielectric constant and high dielectric strength; And Cold plate Wherein the VFD power semiconductor module is mounted on the electrically insulating plate having high thermal conductivity, low dielectric constant and high dielectric strength, the insulating plate is mounted to the cold plate, and the cold plate is grounded. Medium voltage VFD system with ground leakage current. The method of claim 11, A first end of a cable connected to each output terminal of the VFD, A common mode filter used for the cable to filter the output of the VFD, and And a second end of the cable coupled to the device driven by the VFD. 13. The medium voltage VFD system of claim 12 wherein the ground leakage current is reduced to less than 1 amp. 12. The medium voltage VFD system of claim 11 wherein the ground leakage current is reduced to less than 1.5 amps. The VFD system of claim 13, wherein the cable connected to the output terminal of the VFD is greater than 30 feet. 13. The intermediate voltage VFD system of claim 12 wherein the output voltage at the VFD output terminal is greater than 690 volts. 12. The medium voltage VFD system of claim 11 wherein the insulator plate is a ceramic material. The medium voltage VFD system of claim 11, wherein the insulation plate is made of boron nitride. 12. The capacitor of claim 11, wherein a capacitance is formed across the electrically insulating plate between the VFD power semiconductor module and the cooling plate, wherein the capacitance between the VFD power semiconductor module and the cooling plate is less than the internal capacity of the inverter bridge transistor for ground. And the total capacity to ground of the VFD system is reduced. 12. The apparatus of claim 11, wherein electrical insulation and thermal conduction between the VFD power semiconductor module and the grounded cold plate is enabled, the at least one fixing the VFD power semiconductor to the insulating plate and the insulating plate to the grounded cold plate. The medium voltage VFD system with low ground leakage current further comprising one mounting device. Medium voltage VFD system, A DC bus having a grounded midpoint; A VFD power semiconductor module, in which a transistor module of an inverter bridge is connected in series, the inverter bridge is electrically connected to the DC bus, and the insulation of the transistor module is rated to less than half of the full maximum DC bus voltage rating; Electrical insulating plates having high thermal conductivity, low dielectric constant and high dielectric strength; And Grounded Cooling Plate Wherein the VFD power semiconductor module is disposed on the electrically insulating plate having high thermal conductivity, low dielectric constant, and high dielectric strength, the insulating plate being mounted to the grounded cold plate, to the ground of the VFD system. Medium voltage VFD system with effective insulation capacity increased.

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