JP3602352B2 - Method for suppressing harmonic inflow of PWM current-type power converter - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for suppressing harmonic inflow of a PWM current type power converter, and in particular, when connected to an electric power system such as a power storage, the reactance on the power supply side of the capacitance of an LC filter attached for harmonic measures is reduced. The present invention relates to a harmonic inflow suppression method for suppressing the inflow of harmonics existing in the above.
[0002]
[Prior art]
FIG. 3 is a circuit diagram of a conventional PWM current type power converter. Such a PWM current-type power converter is used, for example, in a power storage or the like to obtain a sine wave AC power using a redox flow battery as a DC power supply.
[0003]
In FIG. 3, the PWM current type power converter is a PWM inverter, and the electric circuit works as a “voltage source” on the AC side ideally and the electric circuit works as the “current source” on the DC side. It is attached.
[0004]
AC power source AC is supplied to the switching circuit SW through the reactor _{L f.} The switching circuit SW switches according to the PWM pulse. The output side of the switching circuit SW is a reactor L _d is connected to a current source I _G the DC side. In order to use the AC side as a voltage source, a capacitor _Cf is connected to the AC side.
[0005]
4 and 5 are equivalent circuit diagrams of the PWM current type inverter shown in FIG. In FIG. 4A, in the PWM current type inverter, the AC side is represented as a voltage source and the DC side is represented as a current source. Then, as shown in FIG. 4 (b), when the upper element of the switching circuit SW is turned on, the current becomes equal to the alternating current (the direction is from right to left). When the lower element of the switching circuit SW is turned on, the current value of the DC current source is equal to AC (the direction is from left to right).
[0006]
However, when both the upper and lower elements of the switching circuit SW are turned off, there is no place for the current of the current source to go, the voltage becomes infinite, and the switching element is destroyed. When both the upper and lower elements of the switching circuit SW are turned on, commutation occurs due to the voltage on the AC side, the AC side is opened, and the DC side is short-circuited. Therefore, control is performed so that the upper and lower elements of the switching circuit SW are not turned off at the same time.
[0007]
Thus, in the PWM current type power converter, the state of the circuit is controlled by the PWM control so that the current to the alternating current becomes a sine wave on average. From this, the PWM current-type inverter can be viewed as a current source from the AC side. In other words, the fundamental wave has a sine wave current source as shown in FIG. 5 (a), and has an open (high impedance) as shown in FIG. As a result, it behaves as a current source as shown in FIG.
[0008]
FIG. 6 is a circuit diagram of the PWM voltage type power converter. 6, in the PWM voltage type power converter, the AC side works as a “current source” and the DC side works as a “voltage source”. In practice, in order to the AC side current source, reactor L _f is serially connected to the AC power source AC, capacitor C _d is connected in parallel to the DC power source to a voltage source DC side. Further, a freewheel diode FWD is connected in parallel to the switching circuit SW.
[0009]
7 and 8 are equivalent circuit diagrams of the PWM voltage type power converter shown in FIG. In FIG. 7A, in the PWM voltage type inverter, the AC side is represented as a current source, and the DC side is represented as a voltage source. Then, as shown in FIG. 7A, if both the upper and lower elements of the switching circuit SW are off, the AC voltage becomes zero. As shown in FIG. 7 (b), when the upper element is turned on, the voltage value of the DC voltage source is equal to the voltage of AC plus, and when the lower element is turned on, the voltage of the AC voltage is equal to minus the voltage of the DC voltage source. Value. When both the upper and lower elements are turned on, the voltage source is short-circuited, the current becomes infinite, and the switching circuit SW burns. When both the upper and lower elements are turned off, the current on the AC side commutates to one of the freewheeling diodes. For this reason, the state of the circuit is controlled by the PWM control so that the voltage to the alternating current becomes a sine wave on average.
[0010]
That is, from this, the PWM voltage-type inverter uses a sine wave voltage source as shown in FIG. 8A for the fundamental wave from the AC side, and shows a low-order harmonic wave as shown in FIG. 8B for the fundamental wave. As shown in FIG. 8C, it acts as a voltage source against short-circuit (low impedance) and noise at the carrier frequency.
[0011]
The circuit operation in the above description, the PWM current-type inverter shown in FIG. 3, in principle, a capacitor C _f connected in parallel to the AC side, or is only connected in series reactor L _d to the DC side, in order to suppress the outflow of noise in the vicinity PWM carrier frequency, connected in series the reactors L _f to the AC power source AC, constitute an AC LC filter and reactor L _d and a capacitor C _f.
[0012]
In the same manner the voltage-type inverter shown in FIG. 6, in principle, a reactor L _f are connected in series to the AC power source AC, may but only connected in parallel a capacitor C _d with respect to the DC power source , in order to suppress the outflow of noise, connected in parallel with capacitor C _f to the AC power source AC, constitute an AC LC filter by the reactor L _f and capacitor C _f.
[0013]
However, how to select the resonance frequency of the LC filter on the AC side of the PWM current-type power converter shown in FIG. 3 becomes a problem. In order to suppress outflow of noise at the carrier frequency of PWM, the resonance frequency f = 1 / (2π√ (LC)) is selected to be smaller than the carrier frequency f _{PWM of PWM} . That is,
f _PWM > 1 / (2π√ (L _f C _f )) (1)
Further, in order to avoid a series resonance with the AC power source AC, it is selected AC side frequency (e.g. 50Hz or _60Hz) greater than _{f AC.} That is,
f _PWM > 1 / (2π√ (L _f C _f ))> f _AC (2)
By the way, the carrier frequency of the PWM varies depending on the power conversion function and the power device. For example, if a GTO (gate turn-off thyristor) is used, it is selected to be several hundred Hz, and an insulated gated bipolar transistor (IGBT) is used. If the MOSFET is used, it is selected to be several hundred kHz. Since the AC side frequency is 50 Hz or 60 Hz, the resonance frequency of the AC LC filter of the current-type PWM power converter is selected between those as shown in FIG. FIG. 9 shows the frequency characteristics of the LC filter as viewed from the AC side, and | Z | is the combined impedance of the LC filter.
[0014]
[Problems to be solved by the invention]
By the way, with the spread of power electronics equipment in recent years, the voltage waveform distortion of a power system has increased, and the power system has often been separated from a sine wave. Specifically, the voltage waveform of the power system has odd harmonics such as the fifth harmonic, the seventh harmonic, the eleventh harmonic, the thirteenth harmonic, and so on. In the worst case, it is more than a few percent. This is particularly noticeable in the fifth and seventh harmonics.
[0015]
As shown in the operation equivalent circuit of the lower harmonics of the PWM current-type inverter, the PWM current-type inverter acts as “high impedance”, but in actual impedance, it suppresses the outflow of noise at the PWM carrier frequency. Since an LC filter is added to the AC side and its resonance frequency is selected between f _AC and f _PWM , the fifth, seventh,... Harmonics of the waveform distortion of the power system of the LC filter are eventually obtained. As shown in FIG. 10, the combined impedance becomes relatively low, and it is inevitable that harmonic current flows from the power system.
[0016]
Specifically, when the capacitor _{C f} to the rated output of the power converter and 1 power unit (PU), the need to sufficiently absorb the PWM _{noise, (f AC / f PWM ×} 1) (PU) sufficiently larger than the capacity It must be a capacitor with For example, if f _AC = 60 Hz and f _PWM = 3 kHz, a capacity sufficiently larger than 1 × 60/3000 = 0.02 PU is required. For example, a capacity of about 0.1 PU is selected.
[0017]
In addition, the reactor L _f, AC power (= power system) and the leakage reactance of the interconnection transformer component for connecting the power converter is always present without installing an AC reactor, the value of 5% to 15% PU Therefore, for example, a value of 10% PU is set.
[0018]
As described above, the combined impedance of the LC filter with respect to the n-th harmonic is
From J · (0.1n−10 / n), it can be understood by substituting the values of 5, 7, 11,... Into n. Compare the series resonance frequency of the LC filter with the frequency of the harmonic of the voltage distortion of the system. It will be close to the target.
[0019]
As described above, the PWM current-type inverter is likely to cause inflow of harmonic current in the power system. Depending on the voltage distortion of the power system, a higher harmonic current than the control value indicated in “Technical Guideline for Harmonic Suppression Measures” JEAG9702-1995 was caused.
[0020]
Therefore, a main object of the present invention is to provide a method for suppressing harmonic inflow of a PWM current type power converter that can suppress inflow of harmonics by inserting an LC filter on the AC side and appropriately selecting a resonance frequency. To provide.
[0021]
[Means for Solving the Problems]
The invention according to claim 1 is a PWM current type power converter in which the AC side functions as a voltage source and the DC side functions as a current source. In the PWM current type power converter, at least one series LC filter is inserted between the lines on the AC side, and the PWM current is supplied from the AC side. There are two or more series resonance frequencies of the filter when looking at the LC filter of the power converter, so that the frequency of the order of the harmonic having a large voltage distortion on the AC side exists between the two series resonance frequencies. To be elected.
[0022]
BEST MODE FOR CARRYING OUT THE INVENTION
FIG. 1 is a circuit diagram of one embodiment of the present invention. In Figure 1, a series LC filter consisting of reactor L ₁ and capacitor C ₁ Tokyo between lines of the AC power supply AC side current type PWM power converter is provided at least one. Other configurations are the same as those in FIG.
[0023]
FIG. 2 is a diagram showing a frequency characteristic with respect to a combined impedance in one embodiment of the present invention. 2, filter shown in FIG. 1 is the total impedance is changed as shown in FIG. As apparent from FIG. _{2, 1 / (2π√ (L} 1 C 1)) < In the region _{f <1 / (2 π√ (} L f C f)), it can be kept high combined impedance of the filter . This state is a series LC filter consisting of reactor L ₁ and capacitor C ₁ Tokyo, LC filter consisting of a reactor L _f and capacitor C _f is (are anti-resonance) is a parallel resonance state and the power conversion If the current is flowing out of the harmonic current, it must be avoided as a measure against high frequency. However, in the PWM current type power converter, the lower harmonic current flows in by the previous induction. since it could be shown that, although contrary to common sense, the fifth order by the anti-resonance (= parallel resonance), it is possible to increase the combined impedance of the filter against the low order harmonic voltage such seventh order, Inflow suppression can be achieved.
[0024]
The embodiments disclosed this time are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.
[0025]
【The invention's effect】
As described above, according to the present invention, at least one series LC filter is inserted between the lines on the AC side of the PWM current type power converter, and the series resonance frequency of the filter when the LC filter is viewed from the AC side is reduced. By providing two or more, and having a frequency of the order of a harmonic having a large voltage distortion on the AC side between the two series resonance frequencies, the combined impedance of the filter can be increased, and the inflow of the harmonic voltage can be reduced. Can be suppressed.
[Brief description of the drawings]
FIG. 1 is an electric circuit diagram showing an embodiment of the present invention.
FIG. 2 is a diagram illustrating a frequency characteristic of a combined impedance according to an embodiment of the present invention.
FIG. 3 is a circuit diagram of a conventional PWM current type power converter.
4 is an equivalent circuit diagram of the PWM current type inverter shown in FIG.
FIG. 5 is an equivalent circuit diagram of the PWM current-type power converter shown in FIG. 3 with respect to a fundamental wave, lower harmonics, and a carrier frequency.
FIG. 6 is a circuit diagram of a PWM voltage type power converter.
FIG. 7 is an equivalent circuit diagram of the PWM voltage type power converter shown in FIG.
FIG. 8 is an equivalent circuit diagram of a PWM voltage type inverter at a fundamental wave, a lower harmonic, and a carrier frequency.
FIG. 9 is a diagram illustrating impedance versus frequency characteristics of the LC filter as viewed from the AC side.
FIG. 10 is a diagram showing a combined impedance and frequency characteristics for fifth, seventh,...
[Explanation of symbols]
AC AC power supply SW Switching circuits L ₁ , L _f , L _d reactor C ₁ , C _f capacitor _IG current source

Claims (1)

In a PWM current-type power converter in which the AC side works as a voltage source and the DC side works as a current source,
At least one series LC filter is inserted between the lines on the AC side, and two or more series resonance frequencies of the filter when the LC filter of the PWM current type power converter is viewed from the AC side are present;
A harmonic inflow suppression method for a PWM current-type power converter, wherein a frequency of a harmonic having a large voltage distortion on the AC side is present between the two series resonance frequencies.

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