证明ACS1000输出谐波文件

附件2

New Variable Speed Drive with Proven Motor Friendly Performance for Medium

Voltage Motors

Juergen K. Steinke and Risto Vuolle

ABB Industrie AG Dept. Drive Products CH-5300 Turgi

Herbert Prenner

ABB Industrie AG Dept. Large AC Machines

CH-5242 Birr

Jukka J?rvinen

ABB Industry Oy

Induction Machines Division

FIN-00381 Helsinki

Abstract – A medium voltage VSD is described, where existing standard motors can be connected. The sinusoidal quality of its output voltage is proven by ac machine manu-facturer tests. No additional motor losses exist compared to operation direct on line. For quadratic loads like pumps and fans, self-ventilated motors can be operated at variable speed without cooling problems.

I. INTRODUCTION

input transformer can be placed where it is most convenient with respect to available space.

II. NEW CONVERTER

Energy savings are today a major reason for considering to replace an existing fixed speed drive by a variable speed drive. Two important points may stand against it: The total costs of the new installation are too high, i.e. pay back time is too long, and second a too long time for replacing the old drive by the new one, which results in a too long standstill of production. Taking a converter that allows leaving the existing motor within the installation and which only takes a small amount of floor space can solve both problems. This paper describes a new developed Voltage Source Inverter (VSI) based on the principle of the Neutral-Point Clamped (NPC) Inverter. The new converter is equipped with a LC output filter in order to feed the motor with a sinusoidal output voltage. Because of the sinusoidal motor voltage, existing motors can be fed without any derating, without reinforcing the insulation and without substantial additional motor noise. These advantages were proven by measurements, which were done by machine manufacturers. The described new inverter is very compact concerning its footprint because it uses the newly developed power semiconductor called IGCT [1] and because the necessary Figure 1 shows the basic circuit diagram of the converter. A. Rectifier

The 12-pulse diode bridge is an optimized solution concerning cost, efficiency, reliability and harmonics as long as no regeneration of energy to the line is necessary. Diodes are the lowest cost semiconductors and have lower losses than actively controlled devices. The failure probability of a diode bridge is low because no control circuitry is needed. The harmonics on the line side can be kept below the IEEE 519-1992 limits with a properly designed transformer, as long as the short circuit power of the feeding line is at least about 30 times higher than the rated drive power. In case of a weak feeding network or more strict harmonic limits, a 24-pulse diode rectifier can fulfil the requirements. B. Inverter

An inverter for 4 kV motor voltage has to operate at a dc-link voltage of about 6000 V. None of the today’s commercial available power semiconductors is able to block as a single device this high level of dc voltage. The new IGCT (Integrated Gate Commutated Thyristor) is available for a peak blocking capability of 6 kV with a dc blocking capability of 3.3 kV. Taking the NPC topology [2] a minimization of the number of parts is achieved because RectifierIsolationTransformerDC-Linkdi/dt-chokeInverterFilterMotor3NInd.MotorFig. 1 Basic circuit diagram of the new converter series connection would need for each semiconductor switch an individual voltage-sharing network. The IGCT allows to work without a turn-off snubber network, which reduces also the number of auxiliary components dramatically compared to the GTO-thyristor. The IGCT allows working with an average switching frequency of 500 Hz at acceptable losses, which together with the advantage of the three levels of output voltage [3] leads to an acceptable size of the LC output filter. The resulting switching frequency at the output is 1000 Hz. Fig. 2 shows for a 4.0 kV converter a measured curve of the inverter voltage before the LC filter is applied. Also shown is a harmonic analysis of this voltage, which gives a total harmonic voltage distortion of 12.8 %.

The IGCT as a thyristor device has less on state losses than the transistor device IGBT. Also its turn-off switching losses and the turn-on switching losses are lower than those of the IGBT are. The later is compensated by the drawback, that the IGCT cannot be turned on smoothly like the IGBT. In order to protect the freewheeling diode against excessive di/dt at turn-off, an IGCT inverter has to be equipped in each commutation path with a di/dt limiting passive device, a di/dt-choke. Demagnetizing this choke gives losses, which are in the same order than the turn-on losses of the IGBT at smooth turn-on. C. LC-Filter

The LC filter is tuned to operate at a resonance frequency of about 360 Hz. Together with an active filter resonance damping control which is an integrated part of the drive control, a sinusoidal output voltage is achieved [4]. Fig. 2 shows a measured curve of the motor voltage and a harmonic analysis of it. The harmonic content of this voltage (0.82 % THD) is lower than the limits that are given by IEEE 519-1992 for the line voltage. Therefore each existing or new standard motor can be operated at this voltage without extra losses i.e. without derating. D. Common Mode Circuit

The output voltage of each type of inverter is because of its switched operation not a pure positive or a pure negative phase sequence system. Always a zero phase sequence system is present. In a symmetrical three-phase load like the LC filter or the motor a zero phase sequence voltage system (also often called common mode voltage) does not result in a current. But the real system is a 4-wire system. The 4th wire is the ground connection. The input and output of the converter is connected to it by parasitic capacitors, which are mainly resulting from the capacitance of the cables to ground. By grounding the starpoint of the output capacitor bank, the motor can be protected from common mode voltages. This avoids the risk of bearing currents resulting from the inverter zero phase sequence voltage. An optional common mode choke reduces the amplitude of the charging and discharging current of the parasitic capacitors to ground, which may be a problem if the cable to the input transformer is long. The common mode choke is basically a transformer. The dc-link charging current is only affected

by its stray inductance, while the common mode current is affected by its main inductance. A measurement of the ground current at 300 meters of cable between transformer and rectifier showed that the common mode choke reduces the peak instantaneous earth current by a factor of 5. V_uvV_Inv_uvkV76543210-1-2-3-4-5-6-7180185190195200205210msV_Inv_uv_spec10^-3 130120110THD: 12.8% 10090807060504030201000.00.51.01.52.02.53.0kHzV_uv_spec10^-3 130120110THD: 0.82% 10090807060504030201000.00.51.01.52.02.53.0kHzFig. 2 Phase to phase voltage on inverter and on motor side of the filter (top); spectrum of the inverter side voltage (middle) and spectrum of motor voltage (bottom); 10 units are equal to 1% of fundamental

E. Protection

Each extra part like a fuse reduces the reliability of a converter and adds costs to the equipment. DC fuses for several thousand volts at low rated currents are very expensive devices. The new converter is instead of dc fuses equipped with two semiconductor switches called “Protection IGCT” which immediately separate the rectifier from the dc link in case of an inverter failure. The reaction is so fast that the line current does not rise more than some percent above its normal level. Only the energy of the dc-link capacitor is dissipated into the failed inverter, no additional energy from the line goes to the failed device. Therefore any mechanical destruction is avoided. On the line side of the rectifier no fuses are needed as long as the transformer has high enough impedance. In case of a diode failure, which has a low probability, the transformer protection switch is fast enough to protect also the rectifier from mechanical destruction. F. Motor Control

Motor Control is based on the stator field oriented Direct Torque Control (DTC) [5], which works without speed sensor and gives the physically fastest possible dynamic response as well as an accurate speed control. The static speed control error is only about 10 % of the motors slip i.e. for a standard ac machine it is in the range of 0.1 %. DTC is a proven method which works today in thousands of low voltage drives in all kinds of industrial applications. The control software is implemented into a fast signal processor. The inner control loop is calculated each 25 μs, which allows to keep the torque within a narrow band around the reference value. For easy commissioning, the control software has an identification run (ID-run) for identifying the motor parameters before the first start-up of the motor. This standard ID-run is done at standstill. If a very high performance is needed and autotuning of the speed controller is desired, an extended ID-run is available, which utilizes a defined operating test sequence with turning motor.

G. User Interface

The drive can be controlled either locally or remote. The local control panel is that of the low voltage drive family, which allows the plant operator to handle the medium voltage converter in the same way as all the low voltage ones. In remote control, either an analog signal transmission is possible or different types of industrial field busses. This gives a maximum flexibility to connect the drive to existing process control equipment. For diagnostics or comfortable parameter selection, a PC can be connected to the drive control. The SW-tool is the same as for low voltage drives, therefore once the operator is familiar with the low voltage SW-tool he is also familiar with the medium voltage one. The same is valid for the service SW-tool. This gives the advantage, that the service people trained to service low voltage drives and medium voltage drives keep to be familiar with this tool even if they only have to service a medium voltage drive once a year.

III. TEST RESULTS

The negative impact of VSDs without output filter to squirrel cage motors has been addressed in many papers [6,7]. The most common and detrimental effects of medium voltage VSDs to motors are in summary the following: ?

Winding isolation stresses due to high dv/dt output voltages resulting from dv/dt up to 10 to 20kV/?s in IGBT or IGCT medium voltage switches

? High temperature rises resulting from a high harmonic current content in the motor, which reduces loadability ? Bearing currents caused by steep common mode voltage steps at the motor terminals

?

Increased audible noise due to non-sinusoidal magnetic flux in the iron core

The goal of the tests was to prove the motor friendly performance of the new VSD topology. Further motor manufacturer’s interest was to define new dimensioning rules for VSD duty motors in order reduce motor costs. Until now all VSD duty motors have been oversized, i.e. de–rated.

All the tests were made with the same motor. The motor is specified according to IEC standards and is a closed circuit cooled TEWAC machine. See table 1 for the motor nameplate data.

TABLE 1

TEST MOTOR DATA

U1 4160 V I1 356 A P1 2000 kW n 715 rpm f1 60 Hz cos? 0.81 Connection Y A. Motor Temperature Rise Tests

The temperature rise tests were performed with the same machine with two different supplies, direct on- line and VSD supply, at rated load. The measurement results are in table 2. The resistance method was used for temperature measurement.

TABLE 2

TEMPERATURE RISE AT FULL LOAD

DOL New VSD fed U in V 4000 4020 I in A 357 356 f in Hz 60 60 Temperature rise in °C, measured 45.5 44.8 by resistance method According to this measurement the temperature rise is a little lower in VSD fed motor than in DOL motor. This can be explained by measurement inaccuracies in the system. As a conclusion, it is proven for the new VSD, that the temperature rise in a VSD fed motor is not higher than it is

in a DOL motor. This is a clear improvement compared to other medium voltage VSD concepts, which require always at least 5 % to 15 % of derating.

B. Voltage Rise Time and their Effects on Motors

The short voltage rise time generated by medium voltage VSDs has caused many user concerns in terms of winding damages and failures. Figure 2 shows a typical output voltage without output filter. These kinds of voltages are the root cause of following major effects in the motor:

? Higher harmonics in the motor current

?

Very high voltages at the motor terminals due to voltage reflections

The higher harmonic content causes higher losses and thus the temperature rise is higher than it is for DOL operation of the motor.

High voltage rise times can cause voltage reflections, which make the terminal instantaneous voltage higher than the incident one. This can lead to accelerated aging of the motor stator winding insulation.

These effects are practically eliminated by introducing the output filter in this VSD topology. Voltage rise times don’t deviate at all from the network ones (see the Fig. 2). C. Noise

Noise plays a major role in variable speed drive applications due to the negative environmental impact. Table 3 shows the measured values for three different cases: ? VSD (NPC inverter) without output filter ? Direct on line motor

?

New Topology VSD with output filter

All measurements were done in no load conditions at nominal speed.

TABLE 3

AUDIBLE NOISE MEASUREMENTS

System type Sound Pressure Sound Pressure Level Level in dB(A) (linear, related to DOL operation) VSD without output filter 74.5 145 % Direct on –line 71.3 100 % New VSD Topology with 72.8 119 % output filter The sound pressure level for a motor fed by a VSD with NPC inverter without output filter is 45 % higher than it is for the motor in direct online operation, although the NPC inverter has already the best output performance of all of the conventional VSI drives. In certain installations this can be a criteria for not selecting VSD control for a motor. Due to the output filter included in the new converter the VSD control related increase of the noise level is reduced by more than a factor of 2. D. Loadability

If a standard IEC or NEMA motor is driven by a VSD without output filter the loadability is decreased. Until now

motor manufacturers had to de-rate the self-ventilated motors for VSD duty due to the higher harmonics – that results in larger machines than necessary for industrial application.

120100Loadability? euqroT60r otoSquared LoadM40200020406080100Speed in %Figure 3. Loadability of a motor, which is driven by the new AC drive topology. Measured at variable torque and speed

The loadability curve in figure 3 of the motor with closed circuit cooling (e.g. TEWAC) depicts the actual situation with the new converter topology. The measurements were performed at variable torque and speed. Also a typical squared load curve for fans and pumps is shown in the same figure. The loadability for machine types with frame surface cooling (e.g. TEFC) is even better in lower speeds like 0 to 30%.

Motor Losses in the New Topology

Due to the output filter the motor losses are very close to the direct on-line operation (Fig.4).

Thus the dimensioning of a motor can be made in terms of losses also according to DOL dimensioning rules.

Motor Efficiency in Constant Torque Applications98.0DOL96.094.0New VSD Topology with Output Filter%][ ycne92.0iciffE90.088.086.015.00%.005.00E.00U.00e.00u.00?.00?.00%Speed [%]Figure 4. Efficiency of a squirrel cage induction motor in operation at a pure sinusoidal voltage source (“DOL”) and driven by the new VSD topology converter with output filter. (Operation with 100 % constant torque) IV. CONCLUSION

This paper describes a new converter topology for medium voltage AC drives. Focus is on VSD’s impact on standard medium voltage motors.

Based on the measurements performed by a motor manufacturer the results are very satisfactory.

Due to practically sinusoidal output waveforms the following results are achieved: ? ? ?

No de-rating of motor required

No additional isolation in windings required

Retrofitting of old motor with VSD control is safe and cost efficient

? Noise level increase is more than a factor of 2 lower

than in motor driven by a VSD without output filter For retrofit applications this new drive concept has tremendous advantages. Thus old motors, now VSD controlled, can be driven with the same nameplate data as earlier. With conventional medium voltage drives they had to be derated by 5 to 15 %.

The grounding of the starpoint of the output filter avoids any common mode voltage on the motor terminals. Therefore no problems with bearing currents resulting from inverter operation are expected. Measurements on bearing currents are under way and will be reported soonest.

REFERENCES

[1] [2]

H.M.Stillman, “IGCT – Megawatt Power Switches for Medium Voltage Applications,” ABB Review no. 3/97, pp. 12-17, 1997 A.Nabae, T.Takahashi, H.Akagi, “A New Neutral-Point-Clamped PWM Inverter,” IEEE Transactions on Industry Applications, vol. IA-17, no.5, pp. 518 – 532, 1981

M.K.Buschmann and J.K.Steinke, “Robust and Reliable Medium Voltage PWM Inverter with Motor Friendly Output,” Proceedings of the European Conference on Power Electronics (EPE’97), held in Trondheim (Norway), Sept. 1997, pp. 3.502 – 3.507

J.K.Steinke, P.A.Pohjalainen and Ch.A.Stulz; “Use of a LC Filter to Achieve a Motor-friendly Performance of the PWM Voltage Source Inverter,” Proceedings of the International Electric Machines and Drives Conference (IEEE-IEMDC’97), held in Milwaukee (WI, USA), May 18-21, 1997, pp. TA2-4.1 – TA2-4.3 P.Pohjalainen, P.Tiitinen and J.Lalu; “The Next Generation Motor Control Method – Direct Torque Control (DTC),” Proceedings of the EPE Chapter Symposium on Electrical Drive Design and Application, held in Lausanne (Switzerland), 1994, pp. 115 – 120

A. Binder, \Insulation Stress of Low Voltage A.C. Motors due to Inverter Supply,\Proceedings of the International Conference on Electrical Machines ICEM 94, held in Paris, France, September 5 - 8, 1994, pp. 431-436

A.H. Bonnett, \Inverter Voltage Waveforms on AC Induction Motors,\IEEE Transactions on Industry Applications, vol. 32, no. 2, pp. 386-392, March/April 1996

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