Classifications

• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
  • H02P23/00—Arrangements or methods for the control of AC motors characterised by a control method other than vector control
  • H02P23/14—Estimation or adaptation of motor parameters, e.g. rotor time constant, flux, speed, current or voltage
• • H—ELECTRICITY
  • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
  • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
  • H02P2207/00—Indexing scheme relating to controlling arrangements characterised by the type of motor
  • H02P2207/05—Synchronous machines, e.g. with permanent magnets or DC excitation

Definitions

• the present invention relates generally to a brushless excited synchronous apparatus, such as a synchronous motor. More particularly, the invention relates to a method for estimating the field current in a brushless excited synchronous apparatus according to the preamble of claim 1 , and a field current estimator according to the preamble of claim 7. The invention also relates to a computer program according to claim 5, and a computer readable medium according to claim 8.
• the present invention is useful in high- and medium voltage dive systems.
• the invention is particularly useful for synchronous machine drive systems using HV (High Voltage) converters and HV machines, such as cable wounded machines, and for HV synchronous machine drive systems in conjunctions with transmission systems.
• HV High Voltage
• excitation has to be provided in order to build up a magnetic flux.
• excitation can be achieved by means of a dc voltage supplied to the rotor field winding via brushes and slip rings, mounted on the rotor shaft.
• brushes and slip rings are not accepted.
• the brushes are associated with periodic maintenance. This makes it attractive to supply excitation to the rotor machine part by other means.
• One way is via an exciter such as a rotating transformer.
• the synchronous exciter comprises a stator part and a rotor part com- prising a rectifier.
• the exciter can be of either synchronous type or asynchronous type.
• the synchronous exciter is fed by a dc-voltage on the stator part.
• the rotor part comprises three phase windings.
• the three phase windings in the rotor part supplies a three-phase rectifier connected to the field winding of the machine. It is obvious that the shaft must rotate to induce anything in the exciter rotor winding. Therefore, the synchronous exciter is not suitable if operation at zero or low speed is desired.
• the asynchronous exciter machine operates as an induction machine in plugging operation, i.e slip higher than 1 .
• the stator windings are connected to a three phase voltage source and the rotor windings are connected to a rectifier that supplies the field winding. Since voltage is induced in the rotor, even at standstill, the asynchronous exciter machine is suitable also at zero or low speed.
• the three phase voltage source, feeding the exciter stator is normally a simple thyristor inverter.
• the thyristor inverter supplies a fixed frequency and variable voltage to the exciter.
• the thyristor inverter is chosen because it is robust and low cost. Its disadvantage is the fixed frequency, large harmonic content and low control bandwidth.
• a variable speed drive using a synchronous motor with a rotor field winding requires the field current to be controlled and in particular measured precisely, to obtain a high performance dynamic and steady state control of motor torque, flux and power factor.
• a disadvantage of brushless excitation is that it does not allow for direct measurement of the field current. Thereby, de- tection and control of the field current becomes difficult.
• wireless current transducers has to be mounted to the rotor shaft or the field current have to be reconstructed from the exciter stator voltages and currents. Detection schemes using telemetry from rotor to stator exists, but with poor reliability. Thus, It is a desire to compute the field winding current from signals that are measurable on the stator side of the brushless excited motor. It I also desirable to obtain accurate information in steady state as well as during dynamic conditions.
• a model for calculating the field current in a synchronous appa- ratus denoted a current model
• the field current is calculated based on exciter current measurement.
• the exciter rotor phase currents are estimated and the field current is calculated as the sum of the rotor currents.
• the rotor current is calculated and transformed into rotor coordinates.
• the field current has to be estimated from the rotor current.
• a disadvantage with the current model is that there are still transient model deviations in the reconstructed currents.
• the field current estimate is based on a maximum over time of the rectified rotor currents and therefore the ripple will cause a too high value of the field current estimate.
• free-wheeling i. e. short circuit
• Freewheeling may also occur when the field current is forced towards zero, for example when the speed are reduced.
• a problem with the current model is that the field current can't be controlled during the freewheeling since the field current can't be estimated. The problem concerning short circuit is always present in the current model.
• the object of the present invention is to provide a method for estimating the field current in a brushless excited synchronous apparatus that solves the above-mentioned problems with the prior art.
• This object is achieved by means of a method characterized by: receiving measured values of the current and voltage of the ex- citer stator part, calculating the voltages of the exciter rotor part based on the values of the current and voltage of the exciter stator part by means of a mathematical model of the exciter, calculating the voltage of the field winding based on said calculated voltages of the exciter rotor part by means of a mathemati- cal model of the rectifier, and calculating the field current of the field winding based on said calculated voltage of the field winding by means of a mathematical model of the field winding.
• This method requires that the rotor position of the machine is known.
• the field current is estimated based on the field voltage.
• the rotor exciter voltages are estimated and the field voltage is calculated based on the exciter rotor voltages.
• the case when the rectifier is free-wheeling is simplified considerably since no special treatment is required. Since the field current is estimated from the field voltage, parameter sensitivity is transferred from the exciter to the machine. Also noise in the estimated field voltage will be effectively filtered by the field current estimations. Further advantages with the method according to the invention is that it is provides a faster and more precise estimation of the field current than known methods, in turn giving a better control of machine torques, flux and power factor. This allows lower power rating for the same dynamic and steady state performance.
• the method according to this embodiment of the invention makes it possible to estimate the field current during steady state conditions.
• the field current is calculated with regard to the flux ( ⁇ ac ⁇ ) in the machine. This embodiment makes it possible to estimate the field current not only during a steady state conditions, but also during a transient condition.
• said mathematical model of the exciter comprises a plurality of differential equations describing the exciter and that the voltages of the exciter rotor part is calculating by solving said differential equations.
• said rectifier is a diode rectifier
• the voltage of the field winding is calculated as the difference between the maximum and minimum voltage of the exciter rotor part.
• the field voltage is calculated as the difference between the maximum and mini- mum phase voltage of the exciter rotor part.
• a further object of the invention is to provide a field current estimator for estimating the field current in a brushless excited synchronous apparatus.
• This object is achieved by a field cur- rent estimator as defined in claim 7.
• the object is achieved by a computer program directly loadable into the internal memory of the computer or a processor, comprising software code portions for performing the steps of the method according to the invention, when said program is run on a computer.
• the computer program product is provided either on a computer readable medium or through a network, such as the Internet.
• the object is achieved by a computer readable medium having a program recorded thereon, when the program is to make a computer perform the steps of the method according to the invention, and said program is run on the computer.
• Fig. 1 shows an example of a synchronous apparatus comprising an exciter.
• Fig. 2 shows an exciter per-phase equivalent circuit as seen from the rotor.
• Fig. 3 shows a steady state ideal supply excitation system equivalent circuit.
• Fig. 4 shows a machine model for stator current reconstruction.
• Fig. 5 shows numerical construction of exciter currents at stand still and zero torque.
• Fig. 6 shows example of curve shapes when the exciter operates at half rated speed and half rated torque.
• Fig. 7 shows a reconstruction of rotor winding phase current and measured rotor phase current at field current corresponding to zero torque. No compensation for iron losses.
• Fig. 8 shows a reconstruction of rotor winding phase current and measured rotor phase current at field current corresponding to nominal torque. No compensation for iron losses.
• Fig. 9 shows a reconstruction of rotor winding phase current and measured rotor phase current at field current corresponding to zero torque. Compensation for iron losses.
• Fig. 10 shows a reconstruction of rotor winding phase current and measured rotor phase current at field current corresponding to nominal torque. Compensation for iron losses.
• Fig. 1 1 shows an equivalent circuit for field current estimation.
• Fig. 12 shows reconstructed rotor terminal voltage and measured voltage at zero speed and no load.
• Fig. 13 shows reconstructed rotor terminal voltage and measured voltage at half speed and half rated torque.
• Fig. 14 shows a flow diagram of a method for estimating the field current according to an embodiment of the invention.
• Figure 1 shows a synchronous apparatus comprising an asyn- chronous exciter 1 and a machine 2.
• the exciter 1 is adapted for brushless excitation of the machine 2.
• a thyristor converter 3 supplies a fixed frequency, for example 50 Hz, and a three phase voltage, for example 400V, to the exciter 1 .
• the exciter comprises a stator part 4 and a rotor part 5 having three rotor windings 6 and a diode rectifier 8.
• the exciter rotor part 5 is electrified from the exciter stator part 4 by means of induction.
• the machine 2 comprises a rotor part 10 and a stator part 12 having three phase windings 13,
• the machine rotor part 10 comprises a field winding 14.
• the machine stator part 12 is con- nected to a three phase voltage source.
• the exciter rotor windings 6 are connected to the diode rectifier 8 that supplies the field winding 14.
• stator frequency is ⁇ » ⁇ and the rotor frequency is w 2 and both sides experience the same flux level
• stator and rotor EMF is known as
• the rotor EMF can be calculated from the stator voltage and current as
• the magnetising current is a
• E ⁇ E r ⁇ m jX m jsX m (4)
• the rotor phase voltage can be calculated from the rotor EMF, assuming rotor current reference direction into the machine, as
• V r Er + ⁇ jaX tract + Rr)I r . (6)
• FIG. 2 shows an exciter machine per-phase equivalent circuit as seen from the rotor.
• the circuit in Figure 2 can be translated into a Thevenin- equivalent by calculating the open circuit rotor voltage and the reactance as seen from the rotor.
• the open circuit voltage becomes
• Figure 3 shows a steady state ideal supply excitat on system equivalent circuit.
• the field winding can initially be treated as a current source. If the resistive part Rt is neglected the analysis is simple and straightforward. However, if the resistive part is considered the analysis becomes compli- cated. Therefore, in the following, the resistive voltage drop is taken into account but its effect on current commutation is ne- glected. The commutation angle is assumed to be less than 3 , since this is the case for the experimental exciter. The mean value of the dc-voltage can be calculated as
• V d0 - ⁇ V ⁇ Eth- (13)
• a simple expression for the resistive voltage drop can be de- rived if the resistance Rth is moved from the ac-side of the rectifier to the dc-side. The voltage drop then becomes
• the commutation duration is also affected by the resistive part.
• the diode voltage drop is calculated as
• V d V d0 - ⁇ V d>1 - ⁇ V d>2 - ⁇ V d ⁇ 3 ( 1 7) or
• V d ⁇ L6E tk - -Xthh - 2RthI d - 2V f . ⁇ g
• the direct voltage can in steady state be calculated as
• Figure 4 shows a machine model for stator current reconstruc- tion. No proper analysis of stator current and power factor is carried out in this thesis. However, if the stator and rotor resistance are neglected, the rotor current can be reconstructed numerically. By neglecting the stator resistance and leakage reactance, the simple machine model according to Figure 4 can be used. The magnetizing current can then be calculated as
• FIG. 5 shows a numerical construction of exciter currents at standstill and zero torque. Then, the stator current becomes
• the RMS value of the stator current and power factor can be calculated.
• simulation can be used.
• the thyristor bridge introduce three main difficulties to the analysis.
• the analysis in the previous section was based on a fixed magnitude and frequency supply. If the harmonics cannot be neglected the concept of slip is of no use. Since the harmonics influence the commutation of the rectifier circuit, the analysis cannot be done for each harmonic separately.
• the two commutating circuits influence each other. Since the rotor rotates asynchronously to the stator supply there is no periodicity in the commutation pattern, not even in steady- state. It is therefore hard to define the stationary properties for the circuit. If at all possible, statistical methods should be considered.
• FIG. 6 shows example curve shapes when exciter is operating at half rated speed and half rated torque. The short circuits are clearly visible in the rotor line-to-line voltage.
• Top graph stator current (solid) and rotor current (dashed).
• Bottom graph Stator line-to line voltage (solid) and rotor line-to-line voltage (dashed).
• the stator of the exciter machine was connected to the supply grid via an auto transformer.
• the rotor was rotated by a speed controlled induction machine on the same shaft.
• the synchronous machine was at standstill with the stator circuit open.
• the stator voltage, stator current, field voltage, field current, active and reactive power was measured. From the measured quantities the Thevenin-equivalent was calculated according to (9), (1 1 ) and (12).
• the direct current was calculated according to (20). Also the individual voltage drops was calculated assuming a diode forward voltage drop of 1 Volt or 0.0215 p.u.
• the relative error in both calculated dc-voltage and dc-current was calculated according to
• the mean value of the field current is found to be approximately half the RMS value of the stator cur- rent.
• the estimation error in both dc-current and dc-voltage is approximately one percent except for very small stator voltages.
• stator flux Since the stator flux is not directly known it has to be estimated. One possible way is to use the voltage model for estimation of the flux. It is known that
• Simulating (25) directly results in an open integration and cannot be used. Instead the integration is replaced by a low pass filter.
• the flux estimate then becomes where P is the differentiation operator.
• the cut-off frequency * > 0 has to be chosen so the phase is close to -90 degrees at the fundamental frequency and that low frequency noise is suppressed sufficiently.
• the rotor current then has to be transformed from stator reference frame into rotor reference frame
• the two phase complex representation of the rotor current has to be transformed into three phase representation
• the rectified rotor current is
• the maximum decrease is set from the field winding time constant.
• the field current is estimated by letting it decrease as it would, if the synchronous machine d-axis current was constant. The field current can however be forced to decrease faster if the synchronous ma- chine stator current in the d-axis is increased.
• the current model above depends on a good relation between flux and stator and rotor current. If the iron losses are low and the exciter operates at a low magnetic utilization the linear relation (25) holds. If the machine operates at a high magnetic utilization, saturation has to be taken into account.
• the field current is then calculated in the same way as when the position is known.
• the following aspects of the current model performance are evaluated be experiments: The reconstruction of the rotor currents with the stator of the synchronous machine disconnected with and without compensation for iron losses and the reconstruction of the field current with the stator of the synchronous machine disconnected with and without compensation for iron losses.
• the rotor current reconstruction is verified at field currents corresponding to no torque and nominal torque.
• the reconstruction is performed with and without iron loss compensation.
• Figure 7 shows reconstruction of rotor winding phase current (solid) and measured rotor phase current (dashed) at field current corre- spending to zero torque. No compensation for iron losses.
• Figure 7 shows reconstruction of rotor winding phase current (solid) and measured rotor phase current (dashed) at field current corresponding to nominal torque. No compensation for iron losses. From Figure 7 and Figure 8 it is clear that a great improvement is gained when iron loss compensation is used. However, there are still large high frequency ripple in the reconstructed currents.
• the field current estimate is based on a maximum over time of the rectified rotor currents and therefore the ripple will cause a too high value of the field current estimate.
• the exciter will go into saturation at low speeds. The remaining error is belied to be caused by saturation.
• Figure 9 shows reconstruction of rotor winding phase current (solid) and measured rotor phase current (dashed) at field current corresponding to zero torque. Iron losses are compensation for.
• Figure 10 shows reconstruction of rotor winding phase current (solid) and measured rotor phase current (dashed) at field current corresponding to nominal torque. Iron losses are compensation for. Saturation cause large error in rotor current reconstruction as seen in Fig- ure 10.
• the current model is very sensitive to exciter machine parameters.
• the ripple in the phase currents cannot be filtered since the full bandwidth is needed for detection of short circuit. Saturation cause large errors in the experimental setup.
• the stator flux is believed to be well estimated.
• the error in rotor current estimation is mainly due to the flux to current relation (25).
• the flux to current relation is considerably affected by saturation and iron losses. Using compensation for iron losses and fine tuning of the machine parameters, the error in field current estimation is approximately 10%.
• the problems concerning short circuit is always present in the current model.
• the magnetic flux in the exciter stator can be estimated with good accuracy.
• the relation between the flux and currents is affected by saturation and iron losses. Since the effects of saturation and iron losses are of the same magnitude as the rotor current and effect the rotor current estimate directly, large errors occur.
• One way to avoid this is to estimate the field voltage instead of the field current. The errors in flux to current relation will then only cause estimation errors in the resistive and inductive voltage drops. Since the voltage drops should be magnitudes smaller than the main voltage, the total estimation error should decrease significantly.
• the case when the rectifier is short circuited is simplified considerably since no special treatment is required. Since the field current has to be estimated from the field voltage parameter sensitivity is transferred from the exciter machine to the main machine. Also noise in the estimated field voltage will be effectively filtered by the field current estimator.
• the rotor flux is given by
• stator voltage and flux should be dominant and therefore the voltage estimate should be influenced by saturation and iron losses to a notably lesser extent compared with the current model.
• the positive conducting phase is the phase with maximum phase volt- age.
• the negative conducting phase is the phase with minimum phase voltage.
• Vpn ma (Vrai V r b, Vra) — ⁇ in (v r a, V r b,V r c) (46 )
• v ra , v rb , v rc is the three phase voltage of the exciter rotor part.
• the third voltage should be somewhere in between the two conducting voltages or at the same voltage as one of the other during commutation. Since the dc voltage is calculated as the difference between a maximum and a minimum, estimation errors and noise is expected to raise the estimated voltage above the real value. Also, during short circuit, the dc-link is short circuited and therefore all phases are at zero potential. It is clear that during this condition, the voltage model is most sensitive for estimation errors in the three phase voltages.
• Vff Vpn - 2V f .
• Figure 1 1 shows an equivalent circuit for field current estimation. Any damper-windings are neglected. Using the synchronous machine equivalent circuit in Figure 1 1 , the field winding flux linkage can be expressed by
• the field winding flux linkage is the sum of the field winding leakage flux ⁇ j and the air gap flux ⁇ / .
• the field winding leakage flux is the product of field current and field winding leakage reactance.
• the air gap flux estimate must be provided by the main flux con- trol.
• the rotor terminal voltages are reconstructed by calculation of the rotor phase voltages according to (45) and transforming these into the rotor reference frame.
• the line-to-line voltages are calculated and compared to the measured values. The error is approximately constant at 10%.
• Figure 12 shows reconstructed rotor terminal voltage (solid) and measured voltage (dashed) at zero speed and no load.
• Figure 13 shows reconstructed rotor terminal voltage (solid) and measured voltage (dashed) at half rated speed and half rated torque.
• the instantaneous rotor voltage in Figure 12 and Figure 13 show a good agreement between reconstructed rotor voltage and measured rotor voltage.
• FIG 14 is a flow chart illustration of the method and the computer program product according to the present invention. It will be understood that each block of the flow chart can be implemented by computer program instructions.
• the position of the rotor is a necessary input parameter to the method according to the invention. Measured values of the current (i s ) and voltage (u s ) of the exciter stator part 4 is received. The voltages (v ra ,V ⁇ - b ,v rc ) f the exciter rotor part 5 is calculated based on the values of the current (i s ) and voltage (u s ) of the exciter stator part 4 by means of a mathematical model of the exciter, box 10.
• the voltage (v ff ) of the field winding 14 is calculated based on the calculated voltages (v ra v rb ,v rc ) of the exciter rotor part 5 by means of a mathematical model of the rectifier.
• the mathematical model of the rectifier is the previously discussed mathematical formula (46).
• the field current (i ff ) of the field winding 9 is calculated based on the calculated voltage (v f ) of the field winding by means of a mathematical model of the field winding.
• the mathematical model used for calculating field current (i ff ) is base on the exciter equivalent circuit shown in figure 4.
• the voltage model is sensitive to model parameters since the calculation of field voltage is based on two extreme functions, a maximum and a minimum.
• the field current estimator based on the voltage model is found to have about the same steady-state performance as the estimator based on the current model. However, the voltage model is found to be more stable and have a higher bandwidth. Since the properties of the main machine used for experimental verification are well known compared to the properties of the exciter machine, the voltage model is favorable over the current model. With the voltage model, the field current becomes possible to observe even during short circuit.
• the voltage model also takes actions from the stator of the main machine into account.
• Vff field voltage mean value

Landscapes

• Engineering & Computer Science (AREA)
• Power Engineering (AREA)
• Synchronous Machinery (AREA)
• Control Of Eletrric Generators (AREA)
• Control Of Ac Motors In General (AREA)

Applications Claiming Priority (3)

Publications (1)

Family

ID=32990893

Family Applications (1)

Country Status (4)

Families Citing this family (6)

Family Cites Families (1)

• 2004
  • 2004-03-15 WO PCT/SE2004/000374 patent/WO2004082105A1/en active Application Filing
  • 2004-03-15 BR BRPI0408343-1A patent/BRPI0408343A/pt not_active IP Right Cessation
  • 2004-03-15 EP EP04720782A patent/EP1609232A1/de not_active Withdrawn
  • 2004-03-15 CA CA002518953A patent/CA2518953A1/en not_active Abandoned

Non-Patent Citations (1)

Also Published As

Similar Documents

Legal Events