GB/T 12668.8-2017Adjustable speed electrical power drive systems -- Part 8: Specification of voltage on the power interface (English PDF)

3.1 System Overview

The electric drive system (PDS) consists of an electric motor and a complete set of drive modules (CDM), excluding equipment driven by electric motors.

The CDM includes a Basic Drive Module (BDM) and its periphery, such as an input section or some auxiliary equipment (such as a ventilator). BDM includes Power converter with control and self-protection. The boundaries between the PDS and other parts of the equipment and/or production process are shown in Figure 1. Such as the PDS uses a dedicated transformer that is included in the CDM.

In this section, the following conventions are used for all symbols.

--- Use the "^" symbol to indicate the peak value;

--- Use the "*" symbol to indicate the amount of bipolarity.

For a defined driver topology, the pattern of voltage waveforms between the various parts defined later is substantially identical in shape (including peaks) Values), and their magnitude depends on the appropriate operating voltage, which is the reference value in each section.

Whether the reference voltage between the parts uses the DC average or the AC fundamental RMS, depending on the interface and the detection voltage of the considered part the actual situation (differential mode or common mode).

In the differential mode and the common mode model, the actual voltage value between the various parts is considered as the peak value. the relevant given value can be multiplied by some the appropriate coefficient is obtained, and the coefficient includes the influence of the overvoltage phenomenon.

Figure 1 Definition of equipment and some equipment

3.2 Terms and definitions

The following terms and definitions apply to this document.

3.2.1

Power interface the connections required for PDS internal power distribution.

[GB/T 12668.3-2012, definition 3.3.11] 3.2.2 Dual port network two-portnetwork A two-port network (or four-terminal network, or quadrupole network) is a circuit or device with two pairs of terminals.

3.2.3

Converter reference point; NP the point in the converter where the potential is equal to (Vd Vd-)/2. The converter reference point can be used in different topologies. Electricity from NP to ground the voltage is usually the common mode voltage.

3.2.4

DC link DClink

A DC power circuit connecting an input converter and an output converter in an indirect converter, including a capacitor and/or a reactor to reduce DC Voltage and/or DC current ripple.

3.2.5

DC link voltage DClinkvoltage Ud;Vd ;Vd-DC link voltage of the converter section. Vd refers to a positive potential and Vd- refers to a negative potential.

3.2.6

Filter resonance frequency.

3.2.7

The fundamental frequency of the inverter output voltage.

3.2.8

Phase pulse frequency.

3.2.9

The fundamental frequency of the power supply voltage system.

3.2.10

Fsw

The switching frequency of each semiconductor active device.

3.2.11

Ideally idealground

The reference ground point of the equipment.

3.2.12

Magnification (peak) of the relevant part of the common mode model.

3.2.13

kDv

Magnification (peak) of the relevant part of the differential model.

3.2.14

Number numberoflevels the number of levels N is equal to the number of possible voltage steps between the output phase and the NP point.

3.2.15

Ndcmult

The number of DC links per phase in a multi-DC link inverter topology.

3.2.16

System star point; SP the reference point for the inverter output. Different points in the system can be used as system star points. Used to define the total between the three-phase system and the ideal ground Mode voltage.

3.2.17

Rise time

The time between 10% and 90% of the instantaneous peak voltage is equivalent to t90-t10 (see Figure 2).

Figure 2 Two-level inverter voltage pulse waveform parameters. Among them, the rise time tr=t90-t10

3.2.18

Transient overvoltage UB the voltage value exceeding the steady state value of the step voltage "Ustep" (see Figure 2).

3.2.19

Ground potential VGi the reference potential of the i-th part to ground. Sometimes use "ground potential" or "ground".

3.2.20

UPP

The voltage between the phase and the phase (line voltage).

3.2.21

UPNP

The voltage between the output phase of the inverter and the NP.

3.2.22

UPSP

The voltage between the inverter output phase and the star point.

3.2.23

UPG, motor

The voltage between phase and ground at the motor terminals.

3.2.24

UPP, motor

The voltage between the phase and phase at the motor terminals.

3.2.25

U^PP

The peak value of the voltage (line voltage) between the phase and phase. For the two-level case, there is U^PP=Ustep UB.

3.2.26

U^PP*

The peak value of the voltage between the two bipolar voltage peaks.

Figure 3 Typical voltage curve at the motor terminals and the relationship between parameters and time (line voltage) when the two-level inverter is powered

3.2.27

U^PP_fp*

The peak of the voltage between the phase and phase containing twice the overvoltage spike.

3.2.28

The voltage between the phase and phase of the converter power supply (feeder). In this section, this voltage is used to peak voltage and DC link the voltage is normalized to the standard value and contains all tolerances according to IEC 61000-2-4.

3.2.29

USN

The nominal voltage between the phase and phase of the converter power supply (feeder supply), ie the secondary voltage of the input transformer when the tolerance is not taken into account.

3.2.30

Ustep

The difference between the steady-state voltage values before and after switching (see Figure 2).

Figure 4 Typical voltage curve at the motor terminals and the relationship between time and time (line voltage) when the three-level inverter is powered

3.2.31

Ustep_PP

Voltage step between phase and phase voltage (line voltage) UPP.

3.2.32

Ustep_PNP

The voltage step of the voltage UPNP between phase and NP.

3.2.33

Ustep_PSP

The voltage step of the voltage UPSP between the phase and the star point.

3.2.34

Ustep_Gi

The voltage step of the common mode voltage UGi.

4.1 Overview

The voltage source drive system (see Figure 5) consists of the following sections. grid section, grid side filter (if required), grid side rectifier, DC reactance (if required), DC capacitor bank in DC link, self-commutated motor-side converter output filter (if required), converter and power the cable system between the motives and the motor.

Figure 5 Transmission system consisting of a voltage source inverter (VSI) and an electric motor

4.2 High frequency grounding performance and topology

Connecting a PE using a cable is a so-called low frequency grounding. In order to describe the dynamic voltage characteristics using the system method, we are more concerned about high Frequency grounding performance and topology.

In the actual equipment, the ground potentials VG0~VG4 of different parts are shown in Fig. 5. As long as the grounding impedance is different, the ground potential is different, and the ground potential should be based on high frequency (if the ground line performance is not good), although the ground potential based on low frequency may have the same value.

--- The high-frequency grounding performance of the single-point grounding topology is not good. High frequency based ground potentials VG0~VG4 may contain additional parasitics Voltage component.

--- High-frequency grounding performance of multi-point or mesh grounding topology is very good. The high frequency based ground potential VG0~VG4 does not contain additional Parasitic voltage component.

4.3 Dual Port Method

The two-port method is suitable for describing the voltage waveform between the motor terminals.

There are two main types of dual port components, which divide the system into two overlapping parts.

--- Amplifying components in the differential mode;

--- Additive components in the common mode model.

4.3.1 Amplifying components

The amplifying elements are shown in Figure 6. In this case, the output voltage of the dual port component is calculated as follows.

Uout=k×Uin (1)

Figure 6 Dual port amplification component

4.3.2 Adding components

The case of the two-port adder is shown in Figure 7. The output voltage is calculated as follows.

Uout=Uadd Uin (2)

Figure 7 Dual port adder

Consider the relationship between the output voltage Uout of the dual-port component and the input voltage Uin, taking into account the main parameters (such as peaks) in Chapter 4.

Voltage, rise time), will be the characteristics of the entire power supply network, converter input, converter output, filter, cable and motor input Research provides a method. Grounding conditions can affect or change the voltage relationship, and these effects will be discussed in conjunction with different ground potential conditions.

4.4 Differential mode and common mode system

4.4.1 Overview

In signal theory, it is a common method to divide a known system into a common mode and a differential mode system. In the differential mode system, including the conductor all signals appearing between. The common mode system includes all signals present in all conductors and all signals to ground.

In the PDS, an example in which the inverter output portion is divided by this method is shown in FIG.

The output voltage of the inverter (UU, UV, UW) can be divided into differential mode (also known as symmetrical) voltage system (UUD, UVD, UWD) and common mode (also Called the asymmetric voltage system (UG2).

The differential mode voltage represents the voltage between the three output phases. For each phase, the difference between the inverter output voltage and the common mode voltage can be calculated.

Come. For example U phase.

UUD=UU-VG2 (3)

PDS is usually a symmetrical system, that is to say the amplitude of all AC differential mode voltages in all phases (eg supply voltage, inverter) the output voltages are the same and the voltage vectors have a phase shift of 120° from each other (see Figure 9).

Figure 8 Differential mode and common mode voltage system

a) DC link voltage b) Rotary inverter output voltage

Figure 9 Voltage in a differential mode system

The reference point of the DC differential mode voltage is the neutral point of the DC link, and the voltage (Udc D, Udc-D) appears as an angle of 180°. therefore, the magnitude of the DC differential mode voltage is always 50% of the total voltage of the DC link from the positive supply terminal to the negative supply terminal.

The common mode voltage represents the voltage from the ideal star point of the three output phases to the ideal ground potential. Calculated as follows.

UG2=(UU UV UW)/3 (4) for differential mode and common mode systems, an equivalent circuit diagram can be drawn using the dual port components described above.

4.4.2 Differential mode system

The differential mode block diagram is shown in Figure 10.

Figure 10 Block diagram depicting the motor terminal voltage using a differential mode model consisting of two-port components

The highest line voltage at the motor input is calculated as follows.

U^PP, motor=US×prod

kDi (5)

Figure 11 shows an example of actual equipment.

Figure 11 is an equivalent circuit block diagram for calculating the differential mode voltage

In the step-by-step calculation, the differential mode voltage is calculated as follows.

Grid section. US (6) Input converter section. Ud=kD1×US (7) Inverter section. U^PP2=kS×Ud (8) Filter section. U^PP3=kD3×U^PP2 (9) Cable section. U^PP4=kD4×U^PP3=U^PP,motor (10)

4.4.3 Common mode system

The block diagram of the common mode system is shown in Figure 12.

Figure 12 Block diagram depicting the motor terminal voltage using a common-mode model of dual-port components

Figure 13 shows an example of actual equipment.

In the step-by-step calculation, the common mode voltage is derived as follows.

Power grid part. V^G0=kC0×US (11) Input converter section. V^G1=^VG0 kC1×US=(kC0 kC1)×US (12) Inverter part. V^G2=^VG1 kC2×kD1×US=(kC0 kC1 kC2×kD1)×US (13) Filter section. V^G3=kC3×^VG2=kC3×(kC0 kC1 kC2×kD1)×US (14) in Figure 12, a common mode filter is shown and connected to ground potential. In some applications, the common mode output filter is connected to the NP potential.

In this case, the filter only affects the common mode voltage of the inverter output. The formula (14) is amended as follows.

Figure 13 is an equivalent circuit block diagram for calculating the common mode voltage

V^G3=^VG1 kC3×kC2×kD1×US=(kC0 kC1 kC3×kC2×kD1)×US (15) Cable section. V^G4=kC4×^VG3 (16) the maximum voltage of the motor end relative to ground is calculated as follows.

U^PG, motor=

×U^PP,motor U^G4=

×US×prod

kDi US× sum

kCi( )×prod

kCi (17)

According to the topology of the PDS section, the amplification factors kD1 to kD4, kC3 to kC4, and the common mode factors kC0 to kC2 will be explained and determined hereinafter.

5.1 Overview

This chapter will explain the impact of the power supply system. Several of the most common power supply systems (TN, TT, and IT systems) are listed in Appendix A, including Grounding and its effects.

For the grid section and the input converter section in Chapter 6, since the TT power system has the same effect as the TN power system, the TT power supply system is not discussed separately.

5.2 TN power supply system

5.2.1 Overview

The TN power supply system has a point that is directly grounded, and the exposed conductive parts of the equipment are connected to this point through the protective conductor. According to neutral conductor There are three types of TN systems connected to the protective conductor.

---TN-S system. use a separate protective earth conductor throughout the system;

---TN-CS system. In a part of the system, the neutral and protection functions share a single conductor;

---TN-C system. Neutral and protection functions share a single conductor throughout the system.

5.2.2 Star grounding and corner grounding

Usually any point in the above power supply system can be used as a grounding point. The common mode voltage is different depending on the grounding point. root According to the two figures in Figure 14, the common mode voltage value ranges between the minimum and maximum values.

- When the star point is grounded, there is a minimum value kC0=0;

- When the corner is grounded, there is a maximum value of kC0=US/SQR3.

Figure 14 TN-S power supply system

5.3 IT power supply system

In an IT power supply system, all conductors are insulated from ground potential. This causes the value of VC0 to be indeterminate (see Figure 11). The actual situation is, the parasitic impedance is more or less symmetrical, which results in kC0=0.

If a ground fault occurs in the above equipment, a deviation will occur. In this case, this value will be close to kC0=1/SQR3.

5.4 Magnification of the differential mode model of the grid part

Table 1 Magnification of the differential mode model of the grid part

TN network IT network US/USN 1 1 Note. In the worst case, the tolerance of the grid voltage should be included in the US value.

5.5 Relevant conclusions of the common mode model of the grid part

Table 2 Parameters of the common mode model of the grid part

TN network IT network kC0 Midpoint grounding system potential associated with nominal supply voltage Grounding of the star point. 0 Angle grounding. 1/3 Uncertain, at least 1/3

6.1 Voltage Analysis

The low-frequency ground potential at the output of the inverter is determined by the DC link voltage (Ud) and the reference potential of the DC link voltage (VG1) (see Figure 5).

The ground potential at the output of the converter is.

VG1±Vd/2 (18)

When the upper switch of the inverter is turned on, the ground potential VG1 Vd/2 appears at the output of the inverter. If the switch at the bottom of the inverter is turned on, the ground potential at the output of the inverter is VG1-Vd/2.

6.1.1 DC link voltage (Ud) of the converter part

The DC link voltage is mainly caused by the type of rectifier and the impedance of the power supply and/or DC grid and the filtering effect of the large DC capacitor.

If you decide. DC voltage ripple is usually negligible.

The DC link voltage is usually affected by the following factors.

---Rectifier type (single phase diode, three phase diode, active converter);

---Inverter type (single phase/three phase and with/without DC braking);

--- Grid side commutation impedance;

---load.

6.1.2 Reference potential of NP ...