JP2018196190A - Power conversion device - Google Patents

Abstract

To provide a power conversion device that lessens overcurrent as much as possible by further improving the responsiveness to delayed following of secondary side voltage (output voltage of the power conversion device) caused when a system accident point is generated on the primary side of an interconnection transformer.SOLUTION: The power conversion device comprises an inverter connected to a system via an LC filter, and an inverter control unit that controls the inverter. In control which is performed by the inverter control unit, a transient variation current suppression voltage estimation value which is a voltage deviation at the time of a system accident, is estimated on the basis of inverter output current sampled multiple times during a calculation cycle and an interconnection point voltage at an inverter interconnection point, the transient variation current suppression voltage estimation value is added, as a feedforward, to a signal for controlling current flowing through the inverter, so that a voltage command is created, and the voltage command is used as a PWM voltage command for the inverter.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a power converter.

  2. Description of the Related Art There is known a power conversion device that can detect a voltage fluctuation of a system voltage and prevent an overcurrent. Patent document 1 is mentioned as an example of such a power converter device.

JP 2004-153957 A

  Control responsiveness can be improved by selecting an LPF with a shorter time constant than usual for the detection value of the connection point voltage used for voltage feedforward of the inverter ACR when the system voltage fluctuates. For higher requirements such as continuing operation without stopping the gate, further control responsiveness is required. When a system fault occurs on the primary side of the interconnection transformer, a follow-up delay of the secondary side voltage (output voltage of the power converter) occurs, so it is necessary to further improve the response and suppress the overcurrent as much as possible. is there.

  An inverter connected to the system via an LC filter and an inverter control unit for controlling the inverter. The control performed by the inverter control unit includes an inverter output current sampled a plurality of times within the calculation cycle and an inverter connection. Based on the interconnection voltage of the system point, the estimated voltage value for suppressing the transient fluctuation current, which is the voltage deviation at the time of the system fault, is estimated, and the estimated voltage value for suppressing the transient fluctuation current is fed to the signal that controls the current flowing through the inverter. Provided is a power converter having a control in which a voltage command is created by adding as a forward and the voltage command is used as a PWM voltage command for an inverter.

  It is possible to provide a power conversion device that can respond to an output voltage at a high speed in response to a sudden change in system voltage and suppress the generated overcurrent value to be small.

The block diagram which shows the power converter device of a present Example. FIG. 6 is a process flow diagram of a voltage estimation calculation unit for suppressing transient fluctuation current. The connection point voltage fluctuation detection processing flowchart. The comparison figure of the system voltage and output current at the time of the system voltage sudden change of a conventional system and a present Example.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the text, symbols enclosed in [] represent vector quantities, symbols not enclosed in [] represent scalar quantities, and in the drawing, vector quantities are represented by adding arrows on the symbols. Moreover, || indicates the absolute value of the vector quantity enclosed thereby.

  FIG. 1 shows an outline of a power conversion device 2 of this embodiment. In this figure, a DC voltage generated by natural energy is set as an input 1, and an output 16 of the power conversion device 2 shown as a large frame of a one-dot long chain line is connected to a system 18 via a connection transformer 17. In addition, as for natural energy, solar power generation, wind power generation, etc. can be considered, for example.

  A DC voltage input 1 using natural energy as an energy source is input to the input circuit 5 of the interconnection inverter 6. The AC output circuit 11 of the interconnection inverter 6 is connected to the secondary circuit 16 of the interconnection transformer 17 via an LC filter 14 for the purpose of reducing harmonics.

Next, the inverter control unit 21 will be described. The inverter control unit 21 that controls the interconnection inverter 6 includes an ACR 3 that is a current controller. The ACR 3 includes a current command I _ref that is an output of the DC voltage controller DC- _{AVR 4} that _receives the DC voltage command V _dcref and the DC voltage detection value V _dc of the power conversion device 2, and an interconnection point current detection value [ I] and the phase signal [V _2ph ] calculated by the phase detector 19 are input. The phase detection unit 19 receives the connection point voltage detection value [V ₂ ] as an input, and outputs the phase signal [V _2ph ] of the _connection point voltage [V ₂ ] calculated by the phase detection unit 19 to the current regulator (ACR 3). The current regulator (ACR3) outputs [V _2ph ] as an input and performs vector control to output an AC voltage command value [V _ref ].

Next, voltage feed forward [V _FF ] to be added to the AC voltage command [V _ref ] of ACR 3 will be described. The voltage feed forward [V _FF ] is calculated from the connection point voltage detection value [V ₂ ].

A low-pass filter pass value [V _LPF ] obtained by removing harmonics from the connection point voltage detection value [V ₂ ] by the low-pass filter (LPF) 13 is connected to the input terminal 9 of the switch 10. In addition, the output signal [V _est ] of the transient fluctuation current suppression voltage estimation calculation unit 12 having the connection point voltage detection value [V ₂ ] and the connection point current detection value [I] as inputs is input to the input terminal 8 of the switch 10. Connect to. The voltage feed forward [V _FF ] uses [V _LPF ] or [V _est ] which is a signal selected by the switch 10. The above-described inverter control unit 21 performs the calculation at 1/2 of the inverter pulse period. When the low pass filter pass value [V _LPF ] is selected by the switch 10 for voltage feed forward. Equivalent to conventional inverter control.

Next, the transient fluctuation current suppression voltage estimation calculation unit 12 will be described. The transient fluctuation current suppression voltage estimation calculation unit 12 includes the connection point current [I] and the connection point voltage [V ₂ ] obtained by sampling N times (N ≧ 2) within the calculation cycle detected by the current sensor 15. As an input, and the voltage change estimated value [V _est ] for suppressing the transient fluctuation current obtained by subtracting the voltage change estimated value Δ [V ₁₂ ] applied between the primary and secondary of the interconnection transformer 17 due to the current transient fluctuation from the detected voltage [V ₂ ]. It has a function of outputting to the terminal 8 of the switch 10. Here, the calculation cycle in this embodiment is a time corresponding to the calculation cycle of the PCS output waveform control calculation.

Here, the calculation method of the transient fluctuation current suppression voltage estimated value [V _est ] will be described using Equations 1 to 3, and the calculation processing flow will be described with reference to FIG.

Equations (1) to (3) are calculation formulas for determining the transient fluctuation current suppression voltage estimated value [V _est ], and the impedance of the interconnection transformer 17 due to the interconnection point voltage [V ₂ ] and the current transient fluctuation and the fault point. Obtained from the difference from the estimated voltage change Δ [V ₁₂ ] applied to the system impedance.

The voltage change estimated value Δ [V ₁₂ ] is a voltage drop caused by the current flowing through the grid impedance and the system impedance up to the fault point when the power transformer on the primary side of the grid transformer 17 is instantaneously reduced. It corresponds to the deviation voltage between the voltage and the transformer secondary voltage (interconnection point voltage [V ₂ ] (inverter output voltage)). Since an AC overcurrent occurs due to this deviation voltage, in order to compensate for this, as shown in Equation 1, a voltage change estimated value Δ [] which is a deviation voltage from the connection point voltage [V ₂ ] (inverter output voltage). The voltage estimated value [V _est ] for suppressing transient fluctuation current is calculated except for V ₁₂ ] and used as voltage feed forward [V _FF ]. The estimation calculation unit 12 is operated at a cycle of 1 / N of the calculation cycle of inverter control.

A voltage change estimated value Δ [V ₁₂ ], which is a voltage drop up to the point of the accident at the time of a system voltage sudden change, is calculated by Equation 2. The resistance R component and inductance L component in Equation 2 correspond to R and L from the inverter to the infinite bus voltage. The line impedance [Z] = R + jωL from the voltage detection point of the inverter to the infinite bus voltage is the impedance of the interconnection transformer 17 ([Z _Tr ] = R _Tr + jωL _Tr ) and the primary side of the interconnection transformer 17 Corresponds to the total impedance [Z] = R _Tr + R _g + jω (L _Tr + L _g ), which is the sum of the estimated system impedance [Z _g ] (= R _g + jωL _g) from the terminal to the infinite bus . Since it is difficult to actually determine the system impedance [Z _g ], an estimated value from the short-circuit capacity is used. Here, since the R component is sufficiently smaller than the L component, it is ignored and the right side of Equation 2 is obtained. The differential term di / dt is obtained from the average value Δ [I _ave ] of the time variation Δ [I _N-1 ] of the interconnection point current in the calculation cycle Ts and the sampling cycle Δt (= T _s / N). Here, Δ [I _ave ] is detected at the previous sampling every sampling based on the premise that the interconnection point current is sampled N times (N ≧ 2) within the calculation cycle T _s as shown in _Equation 3. Time difference Δ [I _m-1 ] / (T _s / N) (= ([I _m ]-[I _m-1 ]) / (T _s / N)), and the sum is divided by N. It should be noted that by calculating the average value Δ [I _ave ] of the time variation of the connection point current, it is possible to suppress the influence on instantaneous noise mixed in the connection point current detection value.

FIG. 2 is a flowchart of the voltage estimation calculation unit for suppressing transient fluctuation current. Input the connection point current [I ₁ ] of the _first sampling and the connection point current [I ₀ ] of the _0th sampling (the last value sampled in the previous calculation cycle) as input, and the amount of time change of the connection point current Δ [I ₁ ] (= [I ₁ ] − [I ₀ ]) is calculated (process 1), and then the second connection point current [I ₂ ] and the first sampling point current [I ₁ ] is input, and the time change Δ [I ₂ ] (= [I ₂ ] − [I ₁ ]) of the interconnection point current is calculated (processing 2). This is repeated until the Nth sampling (N ≧ 2), and each current change amount is obtained from Δ [I ₁ ] to Δ [I _N ]. The average value Δ [I _ave ] of the current change amount is calculated using these current change amounts (processing N). Next, using the average value Δ [I _ave ] of the current change amount and the total impedance estimated value [Z], the voltage change estimated value Δ [V ₁₂ ] due to the interconnection transformer 17 and the system impedance when the system voltage drops is calculated. . A transient estimated current suppression voltage estimated value [V _est ] is calculated from the difference between this and the value of the interconnection point voltage [V ₂ ] (processing N + 2). The above is the calculation flow of the transient estimation current suppression voltage estimation calculation unit.

Next, the interconnection point voltage fluctuation detection unit 7 will explain the command SWREF for selecting the output of the switch 10. FIG. 3 shows a processing flow of the interconnection point voltage fluctuation detection unit 7. The fluctuation detecting unit 7 receives the interconnection point voltage [V ₂ ] as input, and calculates the amplitude | [V ₂ ] | of the interconnection point voltage (processing 1). In this amplitude calculation, the UVW phase of the interconnection point voltage [V ₂ ] is αβ converted using Equation 4, and the amplitude is calculated using Equation 5 from the voltage [V _α ] and [V _β ] after the αβ conversion. To do. A command to select switch terminal 8 (process 3) when | [V ₂ ] | is smaller than a predetermined value | [V ₀ ] | (process 2), and to select switch terminal 9 (process 4) when larger | SWREF is output to the switch 10.

  FIG. 4 is a simulation waveform of the system voltage and output current when the system voltage suddenly changes in the power conversion apparatus to which the present embodiment is applied and the power conversion apparatus using the conventional method. These waveforms are waveforms in which the system voltage is suddenly changed from 100% to 0% at time 0 second in both the present embodiment and the conventional system, and the output current of the conventional system at time 0 second jumps up to 150%. The conventional waveform in FIG. 4 stops the operation of the PCS when the current exceeds 150%, and rises to 6 pu when the PCS continues to operate. On the other hand, in this embodiment, the output current is suppressed to jump to about 140% at time 0 second, and the current value can be reduced.

When the system voltage fluctuates rapidly, the voltage feed forward [V _FF ] of ACR3 for inverter current control is switched from the low-pass filter output [V _LPF ] to the transient estimated current suppression voltage estimated value [V _est ]. By using _est ], the follow-up to the system voltage sudden change is accelerated, and it becomes possible to prevent the overcurrent and continue the operation. Furthermore, by sampling the current a plurality of times and calculating Δ [I _ave ] based on the difference from the previous value, it is possible to suppress the influence of the detection noise of the connection point current. This is because, for example, when the connection point current is changing at a constant rate, instantaneous noise is mixed into the connection point current at the sampling time of the 0th time, and the difference between the 0th time and the first time becomes large. In addition, since the average value of the current change is calculated using the difference between the first time and the second time and the difference between the (N-1) th time and the Nth time thereafter, the influence of noise mixed in the current change detection value is reduced. It becomes possible to suppress.

  In this embodiment, an example is shown using an AC base (phasor) formula, but the same effect can be obtained by using a detection value after conversion to a DC base after DQ conversion.

DESCRIPTION OF SYMBOLS 1 Input of power converter 2 Power converter 3 Current controller (ACR)
4 DC voltage controller (DC-AVR)
5 Linkage inverter input 6 Linkage inverter 7 Linkage point voltage fluctuation detector 8 [V _est ] selection switch terminal 9 [V _LPF ] selection switch terminal 10 Switch 11 Linkage inverter output 12 Voltage estimation for transient fluctuation current suppression Arithmetic unit 13 Low-pass filter 14 LC filter 15 Current sensor 16 Power converter output 17 Interconnection transformer 18 System 19 Phase detector 20 Power converter 21 Inverter control

Claims (4)

An inverter connected to the system via an LC filter;
An inverter control unit for controlling the inverter;
For the control performed by the inverter control unit,
Based on the inverter output current sampled multiple times within the calculation cycle and the connection point voltage of the inverter connection point, estimate the voltage estimation value for transient fluctuation current suppression which is the voltage deviation at the time of system fault,
A voltage command is created by adding the voltage estimation value for suppressing transient fluctuation current as a feed forward to a signal for controlling a current flowing through the inverter,
There is control using the voltage command as a PWM voltage command for the inverter,
Power conversion device. The inverter control unit includes a switch capable of switching two inputs according to the magnitude relationship between the inverter connection point voltage value and a predetermined value,
One input to the switch is the estimated voltage value for suppressing the transient current fluctuation, and the other input is the LPF passing value of the inverter connection point voltage,
The power converter according to claim 1, wherein the output of the switch is used as a voltage feedforward added to an output of inverter current control.   The said inverter control part is a power converter device of Claim 2 which uses the said estimated voltage value for transient fluctuation current suppression as voltage feedforward, when the said inverter connection point voltage value is smaller than predetermined value.   The power conversion device according to claim 1, wherein the calculation cycle is a half value of a cycle determined by a carrier frequency of the inverter.

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