Power system control apparatus

Abstract: A power system control apparatus has a power system impedance estimating circuit (105) for estimating a power system impedance from an internal power of a generator, determined by an internal current thereof, and from a terminal voltage of the generator, and having a power system impedance estimating table, a power system stabilizer (109) for suppressing a power fluctuation in power system in accordance with the estimated power system impedance, and an auto voltage regulator (103) for keeping an output voltage of the generator at a constant value during a normal operation.

Description: The present invention relates to a power system control apparatus for controlling a power system and, more particularly, to a power system control apparatus having an auto voltage regulator (to be referred to as an AVR hereinafter) and a power system stabilizer (to be referred to as a PSS hereinafter) connected to the AVR (see US-A-4080559). It also relates to a power system condition estimating unit and to an angular velocity detecting unit. When the state of a power system is changed due to a variation in load, connection/disconnection of the system, and the like, the generator terminal voltage is changed to cause a power fluctuation. In this case an AVR for controlling the excitation voltage is used as a power system control apparatus for stabilizing the generator terminal voltage. With only the AVR, however, the power fluctuation caused by the variation in load cannot be sufficiently suppressed. For this reason, a PSS has been put into practical use together with the AVR since the late 1960s to stabilize the power system. The PSS is a unit for increasing an effect for suppressing the power fluctuation by excitation control of a power generator. One example of PSS is disclosed in US-A-4 080 559, for damping shaft torsional frequency vibrations by including a torsional oscillation protection device comprising a composite band-reject filter. The system responds to a detected angular velocity signal. It is a known fact that stability of a power system is greatly changed by a power system impedance and a phase difference angle between a generator terminal voltage and an infinite bus voltage. In order to maintain the power system stable under any operating condition, it is preferable to change control gains of the AVR and the PSS in accordance with a state of the power system, particularly the power system impedance and the phase difference angle. Conventionally, in order to change the control gain, the power system impedance is estimated, thereby adjusting the control gain of the PSS. For example, when a model of a power system (single-generator-coupled infinite bus system) 5 as shown in Fig. 1, in which an infinite bus 1 is coupled to a single generator 3, is supposed, the amplitude and phase of its voltage e b are constant. An effective (active) power Pe and a reactive power Q can be expressed as follows: Pe = (eq+Xq·id) |et| · sinδ / (Xe + Xq) Q = (eq+Xq·id) id - Xq · i 2 where e q : internal quadrature-axis reactance voltage id: load current of direct axis Xq: quadrature-axis synchronous reactance δ: phase difference angle i: load current Note that the generator terminal voltage e t can be expressed by the following equation and assume that Xq =0. et 2 = eq 2 + ed 2 The active power Pe and the reactive voltage Q can be transformed as follows: Pe = et · eb / Xe · sinδ Q = et 2 / Xe - et · eb/ Xe · cosδ where e d is the internal synchronous reactance voltage (= Xq·id). When the voltage e b of the infinite bus which is difficult to measure is deleted from equations (4) and (5), the phase difference angle δ can be obtained from the following equation including amounts (the generator terminal voltage e t , the active power Pe, and the reactive voltage Q) that can be easily measured and Xe: δ = arctan {Pe / (et 2 /xe-Q)} Conventionally, when several presumed Xe values are to be obtained, the corresponding active power Pe, reactive power Q, and generator terminal voltage e t are substituted in equation (6) to obtain phase difference angles δ 1 , δ 2 , ..., and δ n . These calculated different phase difference angles δ are compared with corresponding internal phase difference angles δ gen measured at the generator terminal. An Xe value that yields a phase difference angle δ i providing the minimum change over time is determined as the power system impedance at this time. A conventional unit for estimating the power system impedance Xe comprises parameter measuring devices 9a to 9d, phase deference angle arithmetic units 11a to 11c, difference circuits 13a to 13c, correlation arithmetic units 15a to 15c, and a comparator 17, as shown in Fig. 2. According to the conventional impedance estimating unit, the comparison between a phase difference angle δ i (l ≤ i ≤ n) and an internal phase difference angle δ gen is performed by obtaining their correlation. An Xe value that yields the maximum correlation is estimated as,the power system impedance Xe. With this conventional method, the phase difference angle δ is obtained by equation (6) to estimate the power system impedance Xe under an assumption that Xq = 0. In practice, however, the quadrature-axis synchronous reactance Xq is as large as 1.5 pu (per unit) whereas the power system impedance Xe is as small as about 0.4 pu. Hence, equation (6) includes a significant error. This conventional method is disclosed in EP-A-17676 In the conventional method, the power system impedance Xe is estimated by obtaining the correlation between change in phase difference angle δ, calculated in accordance with above equation, and that in internal phase difference angle δ gen . Hence, the estimated power system impedance Xe includes a significant error. Furthermore, when several different power system impedances Xe are to be estimated in a real time manner, a plurality of phase difference angles δ are calculated by equation (6), and the correlation between the phase difference angle δ and the internal phase difference angle δ gen is obtained. Therefore, a large amount of calculation is needed, and many power system impedances Xe cannot be estimated from equation (δ) in a real time manner. For this reason, in practice, about three different power system impedances Xe are estimated. As a result, the estimated power system impedance Xe is not so accurate. Since the correlation between the phase difference angle δ and the internal phase difference angle δ gen is obtained as a function of time, the power system impedance Xe can only be estimated when a certain period of time elapses after the correlation is obtained. As a result, it is impossible to instantaneously estimate an abrupt change in power system impedance Xe to suppress fluctuation in the power system upon system disconnection and the like. The power system stabilizer PSS for suppressing the fluctuation of the system is to be described with reference to Fig. 3. Fig. 3 shows a typical example of a conventional power system control apparatus 19. The power system control apparatus 19 comprises an AVR 21 and a PSS 23. Excitation is performed only by a thyristor. The basic operation of the AVR 21 is to control the excitation voltage by using the thyristor such that a difference between a terminal voltage e t , measured by passing through a noise eliminating filter 25a, and a target voltage value e tref is reduced. An actual AVR 21 also includes a damping circuit and the like. In Fig. 3, the damping circuit is indicated as a gain/leading-delaying circuit 27. At this time, the PSS 23 corrects the target voltage value e tref to indirectly control the excitation voltage. As shown in Fig. 3, the PSS 23 generally has a reset circuit 29, a phase compensating circuit 31, a limiter 33a, and a noise eliminating filter 25b. Fig. 4 shows the response of the active voltage Pe when the target voltage value etref is changed in a stepwise manner to control the cross-compound type thermal power generator having a rated power of 600 MW by using the PSS 23. As is apparent from Fig. 4, when only the AVR 21 is used, fluctuations of 2.6 MW and 7.1 MW, at the first and second peaks, respectively, are excited. In contrast to this, when the PSS 23 is coupled to the AVR 21, the fluctuation of 7.4 MW at the first peak is converged. In this manner, the PSS 23 has a good performance for suppressing a fluctuation in power system near the rated point. The PSS 23 is a linear control circuit, as described above. In contrast to this, however, the power system does not have linear characteristics at all. These characteristics will be explained by way of the infinite bus system 5 coupled to a single generator as shown in Fig. 1. In the infinite bus system 5 shown in Fig. 1, the dynamic characteristic of the phase difference angle δ that governs the power and voltage at the generator terminal is expressed by the following equation: where M: unit inertia constant x 2 ω 0 : alternate frequency Tm: mechanical torque Te: electrical torque D: intrinsic braking torque coefficient (braking coefficient) The braking characteristic based on the phase difference angle δ is determined by the intrinsic braking torque coefficient D. When the coefficient D is large, the phase difference angle δ becomes stable against the disturbance, whereas when it is small, the angle δ fluctuates. When the angle δ fluctuates, the active power Pe fluctuates accordingly. The intrinsic braking torque coefficient D largely depends on the power system impedance Xe and the phase difference angle δ. D = eb 2 · ω0 [ (Xd'-Xd")Tdo" (Xd'+Xe) 2 sin 2 δ + (Xq-Xq")Tqo" (Xq+Xe) 2 cos 2 δ] (pu) where e b : infinite bus voltage Xd': direct-axis transient reactance Xd": direct-axis initial transient reactance Tdo": open-circuit initial time constant Xq: quadrature-axis synchronous reactance Xq": quadrature-axis initial transient reactance Tqo": short-circuit initial time constant Fig. 5 shows the characteristics of the braking coefficient D associated with the parameter (power system impedance) Xe and the phase difference angle δ, both of which change during operation of the generator. The larger the power system impedance Xe and the closer the phase difference angle δ to 90°, the smaller the braking coefficient D. Namely, the phase difference angle and the power system impedance of the single-generator-coupled infinite bus system 5 vary. Figs. 6A and 6B show the response state of the single-generator-coupled infinite bus system 5 when the power system impedance Xe is changed in a stepwise manner. when the power system impedance Xe is changed from 0.2 pu to 0.3 pu, both voltage and power are well controlled by the effect of the PSS 23. Namely, the fluctuation in voltage and power is suppressed within a short period of time. When the power system impedance Xe is changed from 0.2 pu to 1.0 pu, the conventional PSS 23 does not have a sufficiently large braking force for that and it takes time to suppress the fluctuation in power. This is because in the conventional PSS 23 the power system impedance Xe becomes large and the phase difference angle δ becomes 70° or more, and thus the braking coefficient D becomes small. In this manner, in the conventional PSS 23, when the power system impedance is changed significantly, due to partial disconnection of the power system, and the like, the braking characteristic is degraded. When the power system impedance is changed greatly, due to a fluctuation in load, the braking characteristic of the PSS against the fluctuation in power is degraded. The invention provides a power system condition estimating unit as defined in Claim 1 ; and an angular velocity detecting unit as defined in Claim 10. With the benefit of the invention, a power system control apparatus can correctly estimate power system impedance in real time and can sufficiently suppress power fluctuations due to large changes in power system impedance, by using the estimated impedance. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 shows the arrangement of a single-generator-coupled infinite bus system; Fig. 2 is a block diagram showing the arrangement of a conventional auto voltage regulator; Fig. 3 is a block diagram showing the arrangement of a conventional power system stabilizer (PSS); Fig. 4 is a graph showing the transient response of a power transmission active power when a target generator terminal voltage is changed in a stepwise manner by a conventional power system control apparatus; Fig. 5 is a graph of an intrinsic braking coefficient of a generator; Figs. 6A and 6B are graphs of power system response when the power system impedance of the conventional power system control apparatus is changed; Fig. 7 is a circuit diagram of a power system control apparatus according to the invention; Fig. 8 is a graph showing power system impedances and phase difference angles estimated by the power system control apparatus shown in Fig. 7 ; Fig. 9 is a circuit diagram of another power system control apparatus according to the invention; Fig. 10 is a circuit diagram of an angle detecting unit used in a power system control apparatus; Fig. 11 is a block circuit diagram of a power stabilizer controlled by a control signal from the angle detecting unit shown in Fig. 10 ; and Figs. 12 A and 12 B are graphs respectively showing a power stabilizing characteristic obtained by the power stabilizer of Fig. 11 and that obtained by the conventional power stabilizer. A first embodiment of the invention will be described with reference to Fig. 7. Primary delaying filters 131 to 134 receive the generator terminal voltage signal et, the active power signal P, the reactive power signal Q, and the rotational number signal ω, respectively, and remove noise components therefrom. The output signal e t from the primary delaying filter 131 is input to arithmetic circuits 135 to 137, and the output signal P from the primary delaying filter 132 is input to the arithmetic circuits 135 and 136. The output signal Q from the primary delaying filter 133 is input to the arithmetic circuit 135. The arithmetic circuits 135 and 136 calculate armature currents Iq and Id of the q-axis and d-axis, respectively, of the power generator in accordance with following equations (26) and (27): Iq = Pe t (PXq) 2 + (et 2 +QXq) 2 Id = P - Iq et 2 - (XqIq) 2 XqIq The arithmetic circuit 137 and an arithmetic circuit 138 calculate the generator internal voltages Eq and Eq' in accordance with following equations (28) and (29): Eq' = et 2 - (XqIq) 2 + Xd'Id Eq = Eq' + (Xq-Xd')Id The output terminals of the arithmetic circuits 135, 136, and 138 are connected to the input terminals of arithmetic circuits 139 and 140. The arithmetic circuit 139 calculates VL in accordance with the following equation (30) and inputs VL to the arithmetic circuit 140. The arithmetic circuit 140 calculates an approximated value Xe' of the power system impedance in accordance with following equation (31): VL = (EqId) 2 - (Eq 2 -1) (Iq 2 +Id 2 ) Xe' = EqId - VL Iq 2 + Id 2 - Xq The estimated power system impedance Xe is obtained from the approximated value xe' of the power system impedance output from the arithmetic circuit 140 by the primary delaying filter 141. The value Xe' corresponds to X ande shown in Fig. 8 , but the estimated value X ande is slightly delayed from the value Xe' due to the filter. The power system impedance changes in a substantially stepwise manner whereas the phase difference angle consecutively varies about every second. The primary delaying filter 141 utilizes the difference in response waveforms of the power system impedance and the phase difference angle and removes a change in phase difference angle included in the approximated value of the power system impedance. when the power system impedance Xe obtained by a primary delaying filter 141 is input to arithmetic circuits 142 and 143, VS and vC are obtained in accordance with following equations (32) and (33): VS = Iq (Xe+Xq) VC = Eq - Id (Xe+Xq) An arithmetic circuit 144 calculates the phase difference angle δ based on VS and VC obtained by the arithmetic circuits 142 and 143. Namely, the arithmetic circuits 142 to 144 constitute a non-linear arithmetic unit for calculating an estimated phase difference angle from the estimated power system impedance, the active power Pe, the reactive power Q, and the generator terminal voltage e t . Fig. 8 shows the phase difference angle δ and the power system impedance Xe that are estimated in the embodiment shown in Fig. 7. Note that in the drawing Xe is changed in accordance with 0.2+0.9 |sin(t-2)| (where t is a time in units of seconds). It is thus obvious that the power system impedance can be estimated considerably correctly. In this embodiment, the power system impedance and the phase difference angle can be estimated in a real time manner. Thus, the state of the power system can be obtained in a real time manner when a great change occurs in the power system due to partial disconnection of the system or the like, thereby estimating the stability of the power system in a real time manner. The control gains for the AVR and the PSS can be switched to optimize the power system from the estimated power system impedance and the estimated phase difference angle. As a result, when a great change occurs in power system, the generator terminal voltage Pe that tends to be unstable conventionally can be controlled considerably stably. Still another embodiment will be described with reference to Fig. 9. This embodiment has substantially the same circuit configuration as that of the embodiment of Fig. 7. The same reference numerals are used to denote the same parts and a detailed description thereof is omitted. In this embodiment, an arithmetic circuit 146 is provided to receive the rotational number signal ω and the active power signal Pe of the power generator. The arithmetic circuit 146 adds the rotational number signal ω and the active power signal Pe and filters the sum by a primary filter. An acceleration force Pv calculated by the arithmetic circuit 146 and the active power Pe are input to a subtracter 147, and a difference between the acceleration force Pv and the active power Pe is input to an integrator 148. The integrator 148 integrates the difference to output an integral of the phase difference angle δ. Namely, the arithmetic unit constituted by the arithmetic circuit 146, the subtracter 147, and the integrator 148 filters the sum of the active power signal and the rotational number signal of the power generator, calculates the mechanical power of the generator, and integrates the difference between the mechanical power and the active power, thereby estimating the rotational number of the generator. As a result, the noise included in the rotational number signal of the generator is removed. This embodiment has a similar effect as that of the embodiment of Fig. 7. An angular velocity detecting unit used for a PSS will be described. A turbine generator has a shaft type structure in which a generator rotor and inertia members of, e.g., a high-, medium-, or low-pressure turbine, are coupled by a shaft. The mechanical movement of the shaft in the rotating direction can be expressed in the form of a shaft torsion vibration model assuming that each of the inertia members is a mass point and the coupled shaft is a spring having a restoring force in the rotating direction. When the angular velocity of the turbine generator having such a shaft type structure is to be detected, assume that the state of the gear arranged at the end of the shaft of the generator rotor is detected by an electromagnetic pickup. Then, the detected signal includes a shaft torsion vibration component of the respective inertia members. In order to obtain the angular velocity (referred to as "average angular velocity") of the turbine generator as a rigid body, the shaft torsion vibration component must be removed by some method. For this purpose, an angular velocity detecting unit 150, as shown in Fig. 10 is used in the present invention. The angular velocity detecting unit 150 shown in Fig. 10 has a reset filter 151 and arithmetic circuits 152 and 153. The reset filter 151 receives a transmission active power signal ΔPe. The arithmetic circuits 152 and 153 receive a generator rotational number signal Δωg and a generator phase angle signal Δδg. The reset filter 151 is an incomplete differentiating filter having a time constant of 5 seconds, and cuts high- and low-frequency components from the active power ΔPe and passes only a signal of a frequency band concerning power stabilization. The signal ΔPe passed the reset filter 151 is input to the arithmetic circuit 153. The arithmetic circuit 152 calculates a change in phase angle from the signals Δωg and Δδg in accordance with the following equation (34), and the arithmetic circuit 153 calculates a change in angular velocity from the signals Δωg, Δδg, and ΔPe in accordance with the following equation (35): K 11 δ + K 12 ω = K 13 Δδg + K 14 Δωg K 21 δ + K 22 ω = K 23 ΔPe+ K 24 Δδg + K 25 Δωg Note that parameters K 11 , K 12 , ... are set at optimum values in accordance with the characteristics of the generator. The output terminals of the'arithmetic circuits 152 and 153 are connected to the input terminals of integrators 154 and 155, respectively. The integrator 154 integrates changes in phase angle to calculate an estimated phase angle δ and. The integrator 155 integrates changes in angular velocity to calculate estimated angular velocity ω and. when the parameters K 11 to K 25 are set to appropriate values, the arithmetic circuits 152 and 153 and the integrators 154 and 155 constitute a Kalman filter. A Kalman filter efficiently removes disturbance included in an input signal. In this embodiment, the Kalman filter removes a shaft torsion vibration of the turbine shaft. The arithmetic circuits 152 and 153 and the integrators 154 and 155 constitute a simulator that simulates power fluctuation of the generator. Therefore, the estimated angular velocity ω is output without substantially any response delay which poses a problem in a low-pass filter. The design of the arithmetic circuits 152 and 153 will be described. The fluctuation equation of a phase angle of a generator is expressed by the following equation (36): where ω0: standard angular velocity (60 Hz) D: attenuation coefficient (5 pu) Pe: active power Te = electric torque M: generator inertia (6 sec.) Tm: mechanical torque Pm = mechanical power The figures in parentheses are parameters of a standard thermal generator having an output of about 500 thousand kW. Substitutions of Pe - Pm = ΔPe and the various values into equation (36) yield the following equation (37): Namely, where X is a state vector (two-dimensional) and Y is an observation amount and is a vector consisting of a phase angle and an angular velocity. A matrix L is a Kalman gain to be set at an appropriate value: X = (A-L·C) X + Bu + LY where X is an estimated state amount. In this embodiment, L is determined such that the eigenvalue of (A - LC) becomes as follows: Eigenvalue = (-1.22 ± 0.38i) At this time, the gain becomes as follows: The coefficients K 11 ,..., K 24 are as follows: K 11 = -1.16, K 12 = 0.813, K 13 = 1.16, K 14 = 0.187 K 21 = -0.187, K 22 = -1.29, K 23 = -62.8, K 24 = 0.187, K 25 = 0.456 Fig. 11 shows a PSS of a generator using the angular velocity detecting unit described above. This PSS 156 comprises the angular velocity detecting unit 150 described above and a power stabilizing filter 157. An angular velocity (ω) output signal output from the angular velocity detecting unit 150 is filtered by the power stabilizing filter 157, and an output signal from the filter 157 is input to an AVR 159 as the output signal of the PSS 156. The excitation circuit of the generator is controlled by an output signal ΔEtd of the AVR 159. Fig. 12A shows the response of the PSS when a vibration component of 10 Hz corresponding to the shaft torsion vibration is superposed on the phase, the angular velocity, and the active power of the generator. Fig. 12B shows the response of the PSS when the noise in angular velocity is removed by a conventional delaying filter having a constant of 1/(1+0.025s). Note that these response characteristics are those when the power system impedance is changed from 0.2 pu to 0.3 pu in a stepwise manner. As is apparent from Figs. 12A and 12B, ΔEfd does not greatly fluctuate in the PSS of the present invention using the Kalman filter, whereas ΔEfd greatly fluctuates for 10 Hz in the conventional PSS. In this embodiment, divergence does not occur since a constant width vibration 10 Hz is applied to the generator. In practice, however, since the turbine shaft vibration is amplified by the PSS to resonate, a considerably large vibration is observed. In this embodiment, such a destructive phenomenon is prevented, and the generator turbine is safely maintained. It is known that a Kalman filter is effective for removing white noise. It is effective also for removing a high-frequency mode, as in this invention. As has been described above, according to the present invention, the power system impedance can be correctly estimated in a real time manner, and a braking force against a power fluctuation sufficient for a great change in power system impedance can be ensured.

Claims: A power system condition estimating unit comprising: input means (131 to 133) for receiving signals representing a terminal voltage (et), an active power (P), and a reactive power (Q) of a generator; and first arithmetic circuit means (135-138) for calculating an internal voltage of the generator and an armature current from the terminal voltage, the active power and the reactive power; second arithmetic circuit means (139, 140) for calculating an approximated value (Xe') of a power system impedance (Xe) from the internal voltage and the armature current; a filter (141) for filtering the approximate value (Xe') of the power system impedance (Xe) in order to obtain an estimated value of the power system impedance (Xe), the filter removing an error due to phase difference angle changes contained in the approximated value (Xe') of the power system impedance (Xe) by utilizing the response waveform difference between the waveform representing the power system impedance that changes in a stepwise manner, and the waveform representing the phase difference angle that changes continuously in a vibration-like manner; third arithmetic circuit means (142-144) for non-linearly calculating a phase difference angle (δ) from the estimated value of the power system impedance (Xe), the terminal voltage (et), the active power (P), and reactive power (Q), and outputting signals representing an estimated power system impedance (Xe) and an estimated phase difference angle (δ). A power system condition estimating unit according to claim 1, in which said input means comprises means (134) for receiving a signal (ω) representing a rotational number of said generator. A power system condition estimating unit according to claim 1, in which said input means comprises filter means (131 to 133) for receiving at least one of the signals representing the terminal voltage (et), the active power (P), and the reactive power (Q), and removing a noise component from the received signals. A power system condition estimating unit according to claim 3, in which said filter means (131 to 133) comprises a filter including a delaying element. A power system condition estimating unit according to claim 1, in which said filter (141) comprises a delaying filter including a delaying element. A power system condition estimating unit according to claim 1, which includes a means (146) for receiving the signal representing the rotational number (ω) of said generator, and means (147, 148) for calculating the active power, and outputting an estimated rotational number of said generator. A power system condition estimating unit according to claim 1, in which said first arithmetic circuit means (135 to 138) calculates d- and q-axes armature currents and an internal voltage of said generator from the terminal voltage, the active power, and the reactive power, said second arithmetic circuit means (140,141) calculates the power system impedance from the d- and q-axes armature currents and the internal voltage of said generator, and said third arithmetic circuit means (142 to 144) calculates the phase difference angle from the d- and q-axes armature currents, the internal voltage of said generator, and the power system impedance. A power system condition estimating unit according to claim 6, in which said estimating means (135-144) comprises means (146) for filtering a sum of the rotational number and the active power of said generator and outputting a sum signal, and means (148) for integrating or filtering a difference between the sum signal and the active power to obtain the estimated generator rotational number. A power state estimating unit according to claim 6 , further comprising filtering means (131 to 133) for filtering at least one of the signals representing the terminal voltage (et), the active power (P), and the reactive power (Q). An angular velocity detecting unit (150, Fig. 16) comprising: a first arithmetic means (152) for calculating a phase angle change from a rotational number signal (Δ ωg) and a phase angle signal (Δδg); a second arithmetic means (153) for calculating an angular velocity change from the rotational number signal (Δωg) and the phase angle signal (Δδg) and an active power (ΔPe); a first integrator (154) for integrating the phase angle change to obtain an estimated phase angle (δ); a second integrator (155) for integrating the angular velocity change to obtain an estimated angular velocity (ω); a filter (157) for filtering the estimated angular velocity signal; and an input means, coupled to a turbine generator, for receiving a signal representing said rotational number of said turbine generator (Δωg), a signal representing said active power (ΔPe); and said phase angle signal (Δδg).

Inventors: TAKAGI YASUO, TAKAGI, YASUO, TAKAGI, YASUO, C/O INTELLECTUAL PROPERTY DIVISION
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: H02P9
IPC codes: H02P9