WO2016008544A1

WO2016008544A1 - System and method for peak current control of a power converter exchanging power with the grid

Info

Publication number: WO2016008544A1
Application number: PCT/EP2014/065525
Authority: WO
Inventors: Jesús MUÑOZ-CRUZADO ALBA; Javier VILLEGAS NUÑEZ; José Alberto VITE FRÍAS; Ezequiel RUBÉN HERNÁNDEZ
Original assignee: Green Power Tecnologies, S.L.
Priority date: 2014-07-18
Filing date: 2014-07-18
Publication date: 2016-01-21

Abstract

System and method for peak current control of a power converter exchanging power with the grid. The method comprises: - saturating the duty cycle control signal (D) of the converter (6) to limit the grid current (I_grid) to a maximum current limit (I_lim) when a fault condition (8) in the grid (1) is detected; - updating the duty cycle control signal (D) with a new duty cycle control signal value (D1) either during the same upslope (50) or downslope (51) cycle of the modulation signal when the new duty cycle control signal value (D1) is calculated (T1) or in the following peak or valley (T2, T0) depending on the comparison of the previous duty cycle control signal value (D0) and/or the new duty cycle control signal value (D1) with the modulation signal value when the new duty cycle control signal value (D1) is calculated (T1), such that the overall switching frequency remains constant. The method achieves faster signal update.

Description

System and method for peak current control of a power converter exchanging power with the grid

Field of the Invention

The subject matter disclosed herein relates to a system and method for peak current control of a power converter which exchanges power with the grid. The power converter may be used, for instance, with a distributed generator, a static synchronous compensator (STATCOM) or a battery charger. Background of the Invention

The amount of distributed generation power installed in the power network is currently increasing; therefore, these systems have a high influence over the grid behaviour during low voltage and phase jump event. For this reason, large capacity system generation is desirable in view of the improved performance during voltage dips, which is similar to a conventional plant.

Initially, for smaller power sources, it has been acceptable and desirable for the power source to go offline when a voltage reduction of a certain characteristics occurs. This operational mode has been acceptable since the total amount of power being provided by these power sources has been relatively small in comparison with the total amount of power provided by the rest of power sources on the grid. Hence, disconnection of that power sources has little, if any, impact on the recovery ability of the power network after a fault occurred. Given the importance of voltage sags and their effect on the network stability, together with the increment of the amount of power coming from these smaller power sources, requirements regarding low voltage in photovoltaic and wind installation have been introduced by the grid codes in order to limit their operation during the mentioned events. Thus, renewable generation system, especially small independent networks, need to keep the synchronization with the power system and must provide voltage and frequency support to the power network with purpose to overcome any fault or dip voltage, preventing the spread of any event to the rest of the network.

One of the most used proposals in order to overcome low voltage events and their consequences is the saturation of any of their references in order to protect the correct operation. Patent US 7558093 B1 , entitled "Power converter with emulated peak current mode control", discloses a method including a voltage threshold and its dependence on the on-time T_on of the switch in a switching cycle. Together with the operation mode and the imposed limits, response time is crucial in this kind of events whose lengths are in the order of milliseconds. Digital modulators are helping to achieve shorter time response with the chance to update duty signals immediately; however they present certain restrictions regarding duty cycle update and constant switching frequency.

Description of the Invention

In view of the exposed problems and the solutions given recently, it would be desirable to have other options of peak current control with an improvement of the parameter saturation and a faster signal update. The present invention consists of a method and system for peak current control of a power converter exchanging power with a grid.

The power converter comprises an AC connection through which it is connected to the AC grid. It may also comprise a DC or AC connection through which it may be connected to distributed generator. Any number of power supply systems, such as wind or solar generator systems, can be connected to the modular power converter through the DC or AC connection. These supply systems transfer power to the power converter; therefore that power can be injected into the AC grid. In addition, the power converter, through a proper controller, can detect if low voltage or a phase jump is happening.

The controller obtains different signals from the AC line connection. With those signals, the proposed method will be able to detect if a transient fault is ongoing because the grid voltage would fall suddenly. In this situation an overcurrent is expected. Therefore, the controller will reduce and control the peak current in the power converter. The reduction of these peak currents removes current errors in drivers. Moreover, peak current reduction decrease IGBTs degradation along with unexpected grid disconnections.

In addition to normal operation control, the present invention proposes an innovative method for peak current control. The method is based in two main actions: - On the one hand, the duty cycle control signals are saturated with the objective to limit a theoretical peak current.

- A second characteristic is proposed through a faster duty cycle update without increasing PWM switching frequency.

As it was mentioned, together with current limitation, overcurrent peak could be significantly fast. Therefore, signal delay is very important in order to guaranty synchronization with the network as well as equipment protection. Typically, pulse width modulation (PWM) control signals are updated in valleys and peaks of the triangular carrier to guaranty non-desirable firing, maintaining switching frequency and avoiding power lost and unwanted behaviour. This triangular carrier is periodic and together with the control signals, is used to regulate semiconductor switching. Each controlled semiconductor is closed and opened according to whether the triangular carrier is under or over the corresponding control signal. Updating the control signal in the peaks and valleys of the triangular carrier ensures that each controlled semiconductor will complete one switch cycle (i.e. it will be switched once on and once off) in every cycle of the triangular carrier. Thus the switching frequency is known in every moment. Switching frequency value is important because part of the power lost in the power converter is proportional to that frequency; in addition, switching the frequency has high influence over the semiconductor temperature. However, it could be possible to update the control signal in the middle of the slope of the triangular carrier with some restrictions such as small delays (DSP delays) due to the mathematical operations and the controller. The objective is to use the chance to update the control signal in the middle of the slope in order to update the reference signal in the shortest time possible during any drop in voltage or other similar event. Therefore, during the upslope, if previous control signal is greater than triangular carrier value in the update instant, no extra transition is guaranteed and control signal could be updated without switching frequency change.

On the other hand, if previous control signal is lower than the triangular carrier in the update instant, at least three transitions may occur: the first one belongs to the previous cycle; a second transition is due to control signal is update in the defined instant; and a third transition in order to achieve the desired level. For this reason, in order to have a stable switching frequency, the control signal may be updated in the middle of the slope depending on whether its previous or its new value are higher or lower than the triangular carrier and on whether the triangular carrier is rising or lowering. For example, during upslope cycle of the triangular carrier, if previous control signal value is greater than the triangular carrier in the update instant; otherwise, signal will be updated in the next valley or peak.

Following the same steps, during the downslope cycle of the modulation signal, if previous control signal is lower than the modulation value in the update instant, control signal could be updated without any change in the switching frequency (no extra transition is guaranteed).

Brief Description of the Drawings

A series of drawings which aid in better understanding the invention and which are expressly related with an embodiment of said invention, presented as a non-limiting example thereof, are very briefly described below.

Figure 1 shows a scheme of the peak power control system according to the present invention.

Figure 2 depicts a detailed control block diagram of the peak current controller depicted in Figure 1 .

Figure 3 depicts a method operation flow diagram which includes the control characteristics of Figure 2.

Figure 4 shows the strategies used to update the control signal (D); the one update in valleys and peaks: D0_A -D1 _A, D0_A -D1 _B; and the chance to update immediately taking into account controller delay: D0_B -D1 _A; D0_B -D1 _B.

Figures 5A and Fig 5B show, according to the present invention, the immediately update (in instant T1 ) of the control signal (D) when the previously calculated control signal (D0_B) is greater than the triangular carrier during its upslope cycle. Figure 5A shows the transition D0_B - D1 _A, whereas Figure 5B shows the transition D0_B - D1 _B. Figures 6A and 6B show the immediately update (in instant T1 ) of the control signal (D) when the previously calculated control signal (D0_A) is lower than the triangular carrier during upslope cycle of the triangular carrier. Figure 6A shows the transition D0_A - D1 _B, whereas Figure 6B shows the transition D0_A - D1 _A.

Figure 7 shows, according to the present invention, the update of the control signal (D) in the peak of the triangular carrier (instant T2), when in instant T1 (when the new control signal value D1 _A is calculated) the previously calculated control signal (D0_A) is lower than the triangular carrier during upslope cycle of the triangular carrier and the new control signal value (D1 _A) is greater.

Description of a Preferred Embodiment of the Invention

The invention is further detailed with an example, which is not intended to limit its scope. The example illustrates a possible embodiment of the invention in connection with the aforementioned figures.

Figure 1 shows a scheme of the peak current control system 3 according to the present invention, including a power converter 6 connected to the grid 1 through the power converter breaker 2 and inductor 9 (which is not an essential element). A controller 4 and a pulse-width modulator (PWM) 5 are also shown. The power converter 6 is used to transfer power coming from a supply system (in this case, a distributed generation system or distributed generator 7) to the grid 1 . For this example, the distributed generator 7 is a photovoltaic generator. The presence of a power converter breaker 2 allows the power converter 6 to be disconnected from the rest of the grid 1 in case it is required by the controller 4, for instance due to island conditions or for maintenance operations.

Like most of power converters for photovoltaic generation, the power converter 6 operation uses a maximum power point tracking (MPPT) algorithm based on an external control loop of voltage. Active power P and reactive power Q references are generated and transmitted to an inner control loop.

A possible fault condition 8 which may cause voltage sag in the power converter and the consequent peak current is represented by a surge symbol. If such a fault 8 takes place and the controller 4 detects it, voltage dip operation will be activated. As a consequence, current reference will be changed and a different control signal D will be transmitted to the pulse-width modulator (PWM) 5.

Therefore, if a transient network fault occurs (mainly, voltage sags in the network V_grid), sudden changes in the voltage (V_L) applied to interconnected elements (such as an inductor) between the network and the power converter are observed. As a consequence, a fast current increase could occur in the output of the inverter due to the fast transition in V_grid if the inverter control signal (D) does not respond quickly enough to change.

Whenever a fault condition is detected, or even on every control cycle if desired, a theoretical current value that the grid current would reach, according to the measured current (l_grid) and the value of the control signal (D), is calculated and compared with a maximum current limit (l|_im). If the theoretical current is greater than the maximum current limit (l|_im), a limit control signal (D_max) is calculated such that the grid current is limited to the maximum current limit (l|_im), i.e. the control signal (D) is saturated. Thus, limit control signal (D_max) is achieved as a function of dc-link voltage (V_dc) and the sampling frequency (F_s) or sample time (At_sampie)- Next, the mathematical methodology to obtain the parameters described and the desired saturation is exposed, when using the inductor 9:

Δ/ ^llim ^'

V_L = I— = L

At sample J

¾ - D · Y_dc, - V_ffri

D

dc

Active power is obtained through the d axis current, being the current as a function of the maximum power point tracking (MPPT), the voltage of the DC bus and the power saturation. On the other hand, reactive power is given by the q axis current which is achieved from the reference given, the perturbation current obtained in any anti- islanding method as well as the maximum saturation allowed to the reactive power. If system is operating in normal mode, those currents reference will be sent to the controller in order to obtain the required procedure in the power converter. Otherwise, in case a drop in network voltage or any other event with the chance to produce a current peak is given, the controller will change the operation mode. Therefore, reference currents in d and q axis will be limited by the maximum current limit (liim) by applying the limit control signal (D_max) saturation. In order to change the operation mode, switching signal will be updated instantaneously without increasing switching frequency as it was previously mentioned.

Figure 2 shows the control block diagram used by the controller 4. The controller 4 is able to obtain the current reference to be injected (for instance, using the dq axis representation). In addition to peak current control capabilities, the controller 4 is able to perform other tasks which are specific of the converter application. For example, since the converter 6 is used for a photovoltaic application, the controller 4 is also responsible of tracking the maximum power point as well as to decide tripping conditions of the power converter breaker 2 in case of islanding or other emergency conditions. In any case, peak current control is the objective in this innovation.

Voltage V_grid frequency F_grid and current l_grid signals are measured to perform the control algorithm, as it can be observed in Figure 1 and it is also shown in step 23 of Figure 3, which depicts a flow diagram of the control method.

The first step is to check 24 if the operation is according to the limits, if the voltage V_grid frequency F_grid and current l_grid signals of the grid are in the allowed range. In case of normal operation 25 (voltage is within a limit, current is under a threshold and/or phase jump is lower enough), maximum power point tracking (MPPT), DC bus voltage (V_dc), active power saturation limit (P_ref_sat), reactive reference (Q_ref), reactive power saturation limit (QreLsat) and anti-islanding perturbation reference (Q_ref__Ai) are calculated 26 as they would be in a typical converter used for the same application. With those values and parameters, current references in dq axis are obtained and transferred 27 to the duty controller 21 . Once the modulation signal is achieved from current reference, the control signal D is update 28 in the peaks and/or valleys of the triangular carrier.

Otherwise, if the operation parameters are out of limits, current peak and phase jump have an important effect over the normal operation; the operation mode is changed to voltage dips operation governed by the voltage dips controller 19 in Figure 2. Upon this situation, as shown in Figure 3, the voltage dips operation 29 is started. Following the succession of steps, the limit control signal (D_max) is obtained from theoretical maximum current limit (l_Nm) 30. Once the limit control signal (D_max) is calculated, the instant (T1 ) when it is transfered 31 to the duty controller 21 plays an important role in order to obtain a fast actuation and a quick protection of the system. At this point, during the upslope 50 of the triangular carrier, two possible cases can be presented and should be considered as the comparison step 32 is showing: if previous control signal (D0_B) value is greater than the triangular carrier, as can be observed in Figures 5A and 5B, then an immediate update of the duty control 33 can be applied; otherwise, previous control signal (D0_A) value may be lower than the triangular carrier, an example is presented in Figure 7, in that case, the duty control would be updated in the next peak of the modulation signal 34. The opposite situation occurs during the downslope cycle 51 of the triangular carrier. Due to how similar these cases are, only the upslope situation is described in detail.

Figure 4 shows the two possibilities at the moment of update (two possible previous control signals, D0_A and D0_B) as well as the two possible modes to update the signal: immediately (D0_B - D1 _A; D0_B - D1 _B) and peak-valleys (D0_A - D1 _A, D0_A - D1 _B). Together with this explanation, Figures 5A, 5B, 6A, 6B and 7 explain in detail both situations for the case of upslope and their consequences.

Figures 5A and 5B present the process started in step 32 and ended in the step 33, in which previous calculated control signal is D0_B. Previous control signal D0_B is greater than the triangular carrier, during its upslope cycle 50, in the instant the control signal would be updated (T1 ). As a result, any immediate change in control (increment or decrement of the control signal D will just need a switching process; hence, switching frequency will not change. Figure 5A shows the transition D0_B ->D1 _A, whereas Figure 5B shows the transition D0_B ->D1 _B. In both cases there is only one switch during the upslope cycle 50.

However, if the process 32 finally ends in the procedure 34, the situation shown in Figures 6A and 6B (in which previous control signal is D0_A may occur; meaning that previous control signal D0_A is lower than the triangular carrier in the instant the control signal would be updated (T1 ). In that case, any immediate change does not ensure a stable switching frequency.

In case of the new control signal (D1 _B) is also lower than the triangular carrier (D0_A->D1 _B) switching process will not change, as shown in Figure 6A. In this case there is only one switch during the upslope cycle 50.

However, if the new control signal D1 _A is higher than the modulation signal (D0_A ->D1 _A), immediate switching process will be quicker as shown in Figure 6B. In this case there are three switches during the upslope cycle 50. Hence, switching frequency would be higher, needing three changes (first switch previous to instant T1 , when triangular carrier exceeds D0_A; second switch in T1 instant; and third switch when triangular carrier exceeds D1 _A). Therefore, in this case immediate update would mean uncertainty and higher power loses. For this reason in this situation (when the previously calculated control signal D0_A is lower, in instant T1 , than the triangular carrier during upslope cycle 50 of the triangular carrier), the control signal is actually updated in the following peak as shown in Figure 7, to ensure a stable switching frequency. Equivalently, it is possible to decide if the update should occur when the new control signal value (D1 ) is calculated (in T1 ) or in the next peak or valley (T2, TO) depending on whether the new value is greater or lower than the triangular carrier (instead of the previous value). In this case, during the upslope cycle 50 of the triangular carrier, the control signal D will be updated in instant T1 if its new value is lower than the triangular carrier (Fig. 5B and 6A), while during the downslope cycle 51 it will be updated in T1 if it is higher than the triangular carrier. Otherwise, it will be updated on the next peak or valley (T2, Fig. 7) of the triangular carrier. Finally, providing the comparisons can be done fast enough, it is possible to decide when to update the control signal (D) depending on both values: the previous one and the new one.

According to the two pillars on which the proposed invention is based on, the control scheme may contain two control loops for the active and reactive power together with the option to activate the peak current control method in case of undesirable event. This structure is shown in Figure 2.

Claims

1 . Method for peak current control of a power converter exchanging power with the grid, the method comprising saturating the duty cycle control signal (D) of the converter (6) to limit the grid current (lg_rid) to a maximum current limit (l|_im) when a fault condition (8) in the grid (1 ) is detected, characterized in that the method further comprises, when such fault condition (8) is detected, updating the duty cycle control signal (D) with a new duty cycle control signal value (D1 ) either during the same upslope (50) or downslope (51 ) cycle of the modulation signal when the new duty cycle control signal value (D1 ) is calculated (T1 ) or in the following peak or valley (T2, TO) depending on the comparison of the previous duty cycle control signal value (DO) and/or the new duty cycle control signal value (D1 ) with the modulation signal value when the new duty cycle control signal value (D1 ) is calculated (T1 ), such that the overall switching frequency remains constant.

2. Method according to claim 1 , wherein the updating of the duty cycle control signal (D) is made when the new duty cycle control signal value (D1 ) is calculated (T1 ), for those cases occurring:

during the upslope cycle (50) of the modulation signal if previous duty cycle control signal value (DO) is greater than the modulation signal value; or

during the downslope cycle (51 ) of the modulation signal if previous duty cycle control signal value (DO) is lower than the modulation signal value.

3. Method according to claim 2, wherein the updating of the duty cycle control signal (D) is made:

in the following peak (T2), if previous duty cycle control signal value (DO) is lower than the modulation signal calculated during the upslope cycle (50); or

in the following valley (TO), if previous duty cycle control signal value (DO) is greater than the modulation signal value calculated during the downslope cycle (51 ).

4. Method according to claim 1 , wherein the updating of the duty cycle control signal (D) is made when the new duty cycle control signal value (D1 ) is calculated (T1 ), for those cases occurring:

during the upslope cycle (50) of the modulation signal if the new duty cycle control signal value (D1 ) is lower than the modulation signal value; or during the downslope cycle (51 ) of the modulation signal if the new duty cycle control signal value (D1 ) is greater than the modulation signal value.

5. Method according to claim 4, wherein the updating of the duty cycle control signal (D) is made:

in the following peak (T2), if the new duty cycle control signal value (D1 ) is greater than the modulation signal calculated during the upslope cycle (50); or

in the following valley (TO), if the new duty cycle control signal value (D1 ) is lower than the modulation signal value calculated during the downslope cycle (51 ).

6. Method according to claim 1 , wherein the updating of the duty cycle control signal (D) is made:

in the following peak (T2), if previous duty cycle control signal value (DO) is lower and the new duty cycle control signal value (D1 ) is greater than the modulation signal calculated during the upslope cycle (50);

in the following valley (TO), if previous duty cycle control signal value (DO) is greater and the new duty cycle control signal value (D1 ) is lower than the modulation signal value calculated during the downslope cycle (51 );

in any other case, when the new duty cycle control signal value (D1 ) is calculated (T1 ).

7. Method according to any of previous claims, comprising measuring (23) voltage (Vgrid), frequency (F_grid) and current (l_grid) signals of the grid (1 ), and checking (24) if any of said signals are out of an allowed range to determine a fault condition (8) in the grid (1 ).

8. System for peak current control of a power converter exchanging power with the grid, the system comprising a controller (4) configured to saturate the duty cycle control signal (D) of the converter (6) to limit the grid current ( lg_rid) to a maximum current limit ( liim) when a fault condition (8) in the grid (1 ) is detected, characterized in that the controller (4) is further configured to, when such fault condition (8) is detected, update the duty cycle control signal (D) with a new duty cycle control signal value (D1 ) either during the same upslope (50) or downslope (51 ) cycle of the modulation signal when the new duty cycle control signal value (D1 ) is calculated (T1 ) or in the following peak or valley (T2, TO) depending on the comparison of the previous duty cycle control signal value (DO) and/or the new duty cycle control signal value (D1 ) with the modulation signal value when the new duty cycle control signal value (D1 ) is calculated (T1 ), such that the overall switching frequency remains constant.

9. System according to claim 8, wherein the controller (4) is configured to update the duty cycle control signal (D) when the new duty cycle control signal value (D1 ) is calculated (T1 ), for those cases occurring:

10. System according to claim 9, wherein the controller (4) is configured to update the duty cycle control signal (D) in the following peak (T2), if previous duty cycle control signal value (DO) is lower than the modulation signal calculated during the upslope cycle (50), or in the following valley (TO), if previous duty cycle control signal value (DO) is greater than the modulation signal value calculated during the downslope cycle (51 ).

1 1 . System according to claim 8, wherein the controller (4) is configured to update the duty cycle control signal (D) when the new duty cycle control signal value (D1 ) is calculated (T1 ), for those cases occurring:

during the upslope cycle (50) of the modulation signal if the new duty cycle control signal value (D1 ) is lower than the modulation signal value; or

during the downslope cycle (51 ) of the modulation signal if the new duty cycle control signal value (D1 ) is greater than the modulation signal value.

12. System according to claim 1 1 , wherein the controller (4) is configured to update the duty cycle control signal (D) in the following peak (T2), if the new duty cycle control signal value (D1 ) is greater than the modulation signal calculated during the upslope cycle (50), or in the following valley (TO), if the new duty cycle control signal value (D1 ) is lower than the modulation signal value calculated during the downslope cycle (51 ).

13. System according to claim 8, wherein the controller (4) is configured to update the duty cycle control signal (D):

14. System according to any of claims 8 to 13, comprising means for measuring voltage (Vg_rid)_, frequency (F_grid) and current (lg_rid) signals of the grid (1 ), wherein the controller (4) is configure to determine a fault condition (8) in the grid (1 ) by checking (24) if any of said signals are out of an allowed range.