Medium voltage industrial variable speed drives



This article considers the technologies and topologies of MV drives, their advantages disadvantages, and their impact on the quality of energy on the mains and motor sides.

Advancements in semiconductor devices such as insulated gate bipolar transistors (IGBTs) allow medium voltage (MV) drives to be used increasingly to conserve energy, increase productivity and improve product quality.

MV drives cover power ratings from 0,4 to 40 MW at the of 2,3 to 13,8 kV level. The power rating can be extended to 100 MW where synchronous motor drives with load commutated inverters are used. However, most of the installed MV drives are in the 1 to 4 MW range with voltage ratings from 3,3 to 6,6 kV.

Research shows that 97% of MV motors currently installed operate at a fixed speed and only 3% of them are controlled by variable speed drives (VSDs). When fans or pumps are driven by a fixed-speed motor, the control of air or liquid flow is normally achieved by conventional mechanical methods such as throttling control, inlet dampers and flow control valves, resulting in substantial energy loss.

Line- and motor-side filters are optional, depending on the system requirements and the type of the converter used. A phase shifting transformer with multiple secondary windings is often used mainly to reduce line current distortion. The rectifier converts the AC utility supply to a DC level with a fixed or adjustable magnitude. Common rectifier topologies include multipulse diode rectifiers, multipulse SCR rectifiers or pulse-width-modulated (PWM) rectifiers.

Inverters can generally be classified as voltage source inverters (VSIs) and current source inverters (CSIs). VSI converts the DC voltage to 3-phase AC voltage with adjustable magnitude and frequency, whereas the CSI converts the DC current to an adjustable 3-phase AC current. Fig.1 shows a generic functional block of an MV VSD.

Fig. 1: MV VSD functional blocks.

Line-side

MV VSDs have a semiconductor-based bridge at input, which may draw distorted current that can pollute the power supply and reduce the power factor due to the harmonics components. There are several problems for the loads connected to a polluted utility supply:

  • Capacitors: Increased temperature and losses; life time reduction; over-voltage; overcurrent and dielectric rupture.
  • Motors: Increased temperature and noise; life time reduction; efficiency reduction; bearing and bushing damage, and torque cogging.
  • Fuses, circuit breakers, disconnecting switches: Improper operation.
  • Transformers: Increased temperature and iron and copper loss, life time reduction.
  • Meters: Measurement errors.
  • Installation: Neutral over-heating in installations and power factor reduction.
  • Electronic equipment: Operational faults.
  • Cables: Increased losses caused by higher effective current value.

In several LV applications, the six-pulse VFD with an input line reactor or a DC reactor may meet these recommendations. Where it is not enough, some techniques can be used to reduce the harmonic currents. They include:

  • Passive tuned and active filters.
  • Multipulse configuration – increasing the number of pulses of the input rectifier using 12, 18, 24 pulses or more.
  • Active front-end rectifiers, also known as regenerative drives, or as PWM rectifier.

If passive filters are connected at the input of the MV drive, the LC circuit may be excited by the harmonic voltages already present due to other non-linear loads connected in the utility power supply. The LC resonant circuits may not be damped sufficiently and oscillations and overvoltages may occur as the utility power supply at the MV level may have low line resistance.

Active filters are not usual in MV applications because they operate in high switching frequency and the losses are prohibitive.

The term “multipulse”, when related to VSDs, means the association, in series or in parallel, of 6-pulse 3-phase rectifiers, normally with diodes, and the use of phase shifting transformers to feed the rectifiers [2, 3 ,5].

A multipulse configuration is fundamentally the interconnection of 6-pulse rectifiers so that the characteristic harmonics generated by these rectifiers are cancelled by the harmonics generated by other sets of rectifiers. This mitigation is performed by the appropriate design of the phase shifting transformer with multiple secondaries. The harmonics of the 6-pulse rectifier present on the secondary of the transformer will be cancelled and will not appear at the primary of the transformer, i.e. connected to the utility. In a multipulse rectifier, the generated characteristic harmonics are given by:

h = P.n ± 1 (1)
where:
n = an integer (1, 2, 3, 4… ∞).
h = harmonic order.
P = the number of pulses of the rectifier.

Fig. 2 shows the typical waveforms of currents at mains and characteristic harmonics for 6, 12, and18-pulse diode rectifiers. Experimental measurements were conducted on commercial drives according to the diagrams in Fig. 3. The total harmonic distortion of the input current (THDi) measured was: 36% for a 6-pulse rectifier with AC line reactance sized to produce around 4% voltage drop, 8,5% for a 12-pulse rectifier fed by a transformer with two secondaries in delta/wye connection and 4,5% for an 18-pulse rectifier fed by a transformer with three secondaries in delta/delta, delta/+20° delta, and delta/-20° delta connection. Both phase shifting transformers had around 6% of per unit impedance.

Fig. 2: Typical waveforms of currents at mains and characteristic
harmonics for 6/12/18 pulse diode rectifiers.

There are some residual non-characteristic harmonics such as the 5th and 7th in 12 and 18-pulse topologies. This is due to the non-ideal behaviour of the transformer causing angle phase errors.

These harmonics are not cancelled perfectly in the primary and this affects the expected result. Also, an existing presence of voltage distortion on the power electric system may increase the values of these non-characteristic harmonics.

The number of pulses at the input rectifier defines the harmonic spectrum of the current at the primary of the phase shifting transformer. The total harmonic distortion of the current, the power capacity of the plant and its impedance will be involved in the total harmonic distortion of the voltage (THDv) at the point of common coupling (PCC).

Theoretically, the higher the number of pulses, the lower the harmonic content. This only works well if the system and transformer are balanced. A transformer for 12 or 18 pulses is extremely easy to design and manufacture. The design becomes more complex for a higher number of pulses and the results, in terms of harmonics, are not so perfect. In [3], a study shows that it is possible to meet all IEEE 519 requirements with an 18-pulse diode rectifier.

Some industrial MV drives have 24, 36, 72 or more pulses at the input rectifier but the main reason is not related with the harmonics issue as that they use LV power cells in series or cascade connection at the inverter section, and they must supply each power cell separately. They therefore use a transformer with more isolated secondaries. A disadvantage is that the transformer must be located close to the VSD because the voltage drops and due to the excessive number of cables and connections. The costs involved for cooling and space requirements should be considered.

The phase shifting transformer should allow the additional losses introduced by the input rectifier harmonic currents.

Transformer-less MV-VSD topologies require special insulation on the motor due to high common mode voltage stress or the use of an input inductor with approximately the same impedance as an input transformer. Also, higher insulation and overvoltage capability is required for the VSD components because there is no galvanic separation from the network.

The SCR is a thyristor-based device with three terminals: gate, anode and cathode. It can be turned on by applying a pulse of positive gate current with a short duration, provided that it is forward-biased. The device can be turned off by applying a negative anode current produced by its power circuit. Multipulse diode rectifiers are normally used in voltage source inverter (VSI)-fed drives while multipulse SCR rectifiers are mainly for current source inverter (CSI)-based drives. The SCR rectifier provides an adjustable DC current for the CSI which converts the DC current to a 3-phase PWM AC current with variable frequencies. The power flow in the SCR rectifier is bidirectional, which enables the CSI drive to operate in four quadrants. SCR rectifiers must use gate driver circuits which increase the risk of faults in drive components.

Fig. 3: Measured input current of different multi-pulse diode rectifiers at
primary of the phase shifting transformers.

Another disadvantage is related to the input power factor which varies greatly with the firing angle, which is a function of the load.

Diodes, SCRs and other switching devices are often connected in series to achieving the required medium voltage levels. Series connected devices may not have identical static and dynamic behaviour, so they may not share the total voltage in steady or switching state. RC snubber networks are sometimes necessary to enable proper voltage equalisation. This solution may affect the reliability and the global efficiency of the system.

By replacing the diode or SCR rectifier bridge for turn-on/turn-off controlled switches it is possible to implement an active front-end (AFE) converter, also known as a regenerative converter, which can also reduce the harmonic components of the input current.

However, the obtained THD with these converters is close to an 18-pulse solution. Active front-end converters are more suitable for applications where motor repetitive braking is needed and the economy obtained, regenerating the energy back to the power supply, justifies the investment.

Motor-side

The best voltage and current provided by the VSD to the motor is always the sine wave. After the rectification process, the energy is available in the DC link capacitors for voltage source inverters or in the DC link inductors for current source inverters. The semiconductor devices of the inverter can take this energy in DC mode and deliver it to the induction motor in an AC mode, as sinusoidal as possible. Speed is controlled by controlling the frequency of the fundamental component of this AC waveform.

In voltage source inverters (VSIs), semiconductor devices switching at fast frequency may produce high dv/dt levels at the motor terminals. The connecting the VFD output to the motor terminals have their own inductance, capacitance and resistance. These parameters are variable depending on the length and geometry of the cable. Due to this circuit characteristic, each edge of these rectangular pulses will produce a voltage overshoot at the motor terminals. This overshoot will settle down at the DC link level after some ringing cycles.

The VFD switching frequency and topology, which means the power device’s arrangement, also affect the behaviour of the motor voltage. The effects of this repetitive overshoot combined with the dv/dt rates and the common mode voltage can cause insulation stress in the motor winding and shaft/bearing currents that can reduce the motor life expectancy.

The switching action of the rectifier and inverter normally generates common-mode voltage. Common-mode voltages are essentially zero-sequence voltages superimposed with switching noise. If not mitigated, they will appear on the neutral of the stator winding with respect to ground, which should be zero when the motor is powered by a 3-phase balanced utility supply. The motor line-to-ground voltage, which should be equal to the motor line to neutral (phase) voltage, can be increased due to the common-mode voltages. This leads to premature failure of the motor winding insulation system and a shortening of motor life expectancy.

Common-mode voltages are generated by the rectification and inversion process of the converters. This phenomenon is different from the high dv/dt caused by the switching transients of the high speed switches. The common mode voltage issue is often ignored in low-voltage drives. This is partially due to the conservative design of the insulation system for LV motors.

In MV drives, the motor should not be subject to any common mode voltage or the replacement of the damaged motor would be very costly.

Fig. 4: MV industrial VSI and CSI topologies (per-phase diagrams) and output waveforms.

On an MV drive with a motor-side filter capacitor, the capacitor forms an LC resonant circuit with the motor inductances. The resonant mode of the LC circuit may be excited by the harmonic voltages or currents produced by the inverter. Although motor winding resistances may provide some damping, this problem should be addressed at the design stage of the drive. Torsional vibrations may occur in the MV drive due to the large inertias of the motor and its mechanical load.

The drive system may vary from a simple two-inertia system consisting of only the motor and the load inertias to very complex systems such as steel rolling-mill drives with more than 20 inertias.

Torsional vibrations may be excited when the natural frequency of the mechanical system coincides with the frequency of torque pulsations caused by distorted motor currents. Excessive torsional vibration can result in broken shafts and Couplings, and also cause damage to other mechanical components in the system.

Device switching loss accounts for a significant amount of the total power loss in the MV drive. Switching loss minimisation can lead to a reduction in the operating cost when the drive is commissioned. The physical size and manufacturing cost of the drive can also be reduced due to the reduced cooling requirements for the switching devices. The other reason for limiting the switching frequency is related to the device thermal resistance that may prevent efficient heat transfer from the device to its heatsink. In practice, the device switching frequency is normally around 200 Hz for GTOs and 500 Hz for IGBTs and SGCTs. The reduction in switching frequency generally causes an increase in harmonic distortion of the line-side and motor-side waveforms of the drive.

A variety of inverter topologies can be adopted for the MV drive to meet the motor-side challenges. Fig. 4 summarises the main industrial MV topologies and their characteristics. VSI inverter topologies normally use IGBTs or IGCT as switching device while CSIs use SGCTs or thyristors.

These different arrangements and combinations of the switching devices define the topology of the inverter bridge. The challenge is to bring the best waveform to the motor with a reduced number of components, guarantying reliability and efficiency.

Fig. 4a shows a topology known as 2-level VSI with components in series connection aiming to meet the adequate voltage balancing for each component. Due to the inherent characteristics of the components, voltage equalisation must be guaranteed with special gatedrivers and passive voltage equalisation techniques.

The drawbacks of this topology are associated with reliability because the higher number of electronic components used by the gate drivers, and with the efficiency because the passive components used for voltage equalisation are power resistors and have energy losses.

Fig. 5: MV VSD test setup: circuit breaker, 12-pulse phase shifting transformer and rectifier, 3/5 level NPC inverter, 1,8 MW/4,16 kV.

The 2-level VSI inverter is a simple converter topology and has an easy PWM scheme. However, the inverter produces high dv/dt and THD in its output voltage and therefore often requires a large LC filter installed at its output terminals. Fig. 4a1 shows its typical voltage waveform. With a 2-level inverter topology, the motor voltage has three level steps.

Fig. 4b shows a neutral point clamped (NPC) multilevel topology. It employs clamping diodes and cascaded DC capacitors connected to the floating neutral point. It produces AC voltage waveforms with multiple levels as shown in Fig. 4b1. The inverter can be configured as three, four or five-level topology. The higher the level steps, the higher the number of switching devices. The NPC inverter shown in Fig. 4b produces three level voltages between each phase to neutral point and five level voltages between phase to phase at motor terminals.

This topology is suitable for motors rated up to 4,16 kV with standard 6,5 kV semiconductors. It is possible to increase the voltage to 6,9 kV, connecting two NPC legs in an H-bridge form. In this configuration, the phase to neutral voltage contains five voltage levels and nine levels at motor terminals. The main features of the NPC inverter include reduced dv/dt and THD in its AC output voltages in comparison to the 2-level VSI topology.

This inverter can be used in MV drives to reach a certain voltage level without switching devices in series connection. The efficiency levels can therefore reach 99%. The switching frequency should be as low as possible due to the power losses and it is usually limited to a few hundred hertz. Normally, special modulation techniques are needed to produce the minimum harmonic distortion in the motor current in all ranges of speed and torque.

Fig. 4c shows a 7-level VSI H-bridge in cascade connection. It is composed of multiple units of single-phase H-bridge power cells. The H-bridge cells are normally connected in cascade on their AC side to achieve MV level and low harmonic distortion, as shown in Fig. 4c1. The number of power cells is determined by its operating voltage. Some industrial MV drives use this topology implemented with LV semiconductors and electrolytic capacitors.

In this case, because of low reliability, redundant power cells should be installed. In case of failure, the faulty power cells can be bypassed and the drive can resume operation at reduced capacity with the remaining cells.

The repair work must be fast once the bypass of defective cells may cause three-phase unbalanced operation for the motor. The power cells require isolated DC supplies that are obtained from a multipulse diode rectifier employing a phase shifting transformer which is normally complex and expensive. In most cases, this transformer should be installed close to the drive and the losses of this transformer will go into the electrical room which needs extra air conditioning.

Figs. 4d and 4d1 show a typical configuration of a 7-level flying capacitor inverter and its waveform. It is also a VSI topology which produces the voltage waveforms with reduced dv/dt and THD.

However, this topology has the drawback of the large number of DC capacitors with separate pre-charge circuits and the complex capacitor voltage balancing control. The DC capacitor voltages in the inverter vary with the operating conditions. To avoid the problems caused by the DC voltage deviation, the voltages on the flying capacitors should be controlled. This impacts the complexity of the modulation and control technique.

The voltage source inverter produces a defined 3-phase PWM voltage waveform for the load while the current source inverter (CSI) outputs a defined PWM current waveform. The current source inverter features simple converter topology with motor-friendly waveforms. There are two types of current source inverter used in the MV drive: PWM inverters and load-commutated inverter (LCI).

The PWM inverter uses switching devices with self-extinguishable capability like SGCTs. The load-commutated inverter employs SCR thyristors whose commutation is assisted by the load with a leading power factor. The LCI topology is particularly suitable for very large synchronous motor drives with a power rating up to 100 MW. The PWM current waveform supplied by the inverter (see Fig. 4e1) is filtered by the capacitors installed at the inverter output, as shown in figure 4e, and the load current and voltage waveforms are close to sinusoidal.

Thus, the high dv/dt problem associated with the VSI does not exist in the CSI. This topology uses an inductor in the DC link instead of capacitors, therefore the output current cannot be changed instantaneously during transients. This reduces the system dynamic performance.

Fig. 6: Phase to inverter neutral point voltages, phase to phase inverter
output voltage (a) and motor current (b).

The CSI needs an adjustable DC current which is normally provided by a SCR rectifier making the input power factor dependant of the motor current. Another possibility is the use of a PWM current source rectifier which enables the regenerative operation. However, it also needs an input capacitor filter that may cause LC resonances and affect the input power factor of the rectifier as well.

The inverter topologies presented in this section are deeply studied in [1]. The user should consider the advantages and disadvantages of each topology, depending on the application, motor requirements and system efficiency.

Experimental tests

The performance of a 3/5-level neutral point clamped (3/5 level NPC) inverter was tested with a 4-pole, 1,8 MW, 4,16 kV MV motor coupled to a dynamometer. Fig. 6 shows the measured phase to inverter neutral point voltages and the phase to phase voltage applied to the motor terminals. The drive operated in the volts/Hertz mode control [7] and the motor was under rated load. Despite the small number of pulses, which means low switching frequency, the motor current is almost sinusoidal due to the optimal pulse pattern strategy.

Some specific applications require high dynamic performance and closed loop vector sensorless or closed loop vector with encoder control strategies are necessary [7]. The 3/5 level NPC inverter can provide this performance. The system formed by the 1,8 MW motor coupled to the dynamometer was accelerated from zero to 1800 rpm in four seconds, with an instantaneous torque response.

Another challenge is the load step behaviour required when the motor is running at a fixed speed and there is an instantaneous variation of the load. This high performance behaviour is possible if there are no filters between the drive and the motor, meaning that short motor cable lengths and motors able to operate with variable speed drives.

A sine filter is recommended for applications where the cable length between the VSD and the motor is higher than 500 m or the motor is unable to operate with a PWM voltage waveform and good dynamic performance is not required.

References

[1] Bin Wu: “High-power converters and AC drives”, IEEE Press, USA, 2006.
[2] Derek A Paice: “Power electronic converter harmonics – multipulse methods for clean power”, IEEE Press, USA, 1996.
[3] Joable Alves and P Torri: “IEEE Std 519 recommended practices for applications with variable frequency drives”, In: 9º Congresso Brasileiro de Eletrônica de Potência (COBEP 2007), Blumenau/Brazil, 2007.
[4] IEEE: “IEEE recommended practices and requirements for harmonic control in electrical power systems” IEEE 519, IEEE Press, USA, 1993.
[5] Joable Alves and Edson Hornburg: “High and low order harmonics for frequency inverters” (portuguese), in: VI Conferência Internacional de Aplicações Industriais (Induscon 2004), Joinville/Brazil, 2004.
[6] G Cunha and P Torri: “Neutral point potential balancing algorithm at high modulation index for three-level medium voltage inverter”, In: 9º Congresso Brasileiro de Eletrônica de Potência (COBEP 2007), Blumenau / Brazil, 2007.
[7] “MVW01 medium voltage frequency inverter manual”, WEG Automation, Brazil, 2009.

Contact Zest Weg Group Africa,Tel 011 723-6000, marketing@zestweg.com

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