ENHANCED ELECTRIC POWER CONVERTER DYNAMIC SWING EQUATION PARAMETER MODIFICATION FOR RATE-OF-CHANGE-OF-FREQUENCY RIDE THROUGH

Systems and methods for adjusting a controller for a power converter during a rate-of-change-of-frequency (RoCoF) event may include determining, based on first frequency information, a first RoCoF and a second RoCoF for the power converter; determining that the first RoCoF and the second RoCoF are both positive numbers or are both negative numbers; detecting a possible RoCoF event; comparing the first RoCoF to a positive RoCoF activation threshold when the first RoCoF is positive or to a negative RoCoF activation threshold when the first RoCoF is negative, and a power of the power converter to an upper or lower active power activation threshold; detecting, based on the threshold comparisons, a RoCoF event; and applying, during the RoCoF event, a RoCoF ride through.

TECHNICAL FIELD

This disclosure generally relates to control of a power converter connected to an electric power grid.

BACKGROUND

Electrical grid-forming converters connected to a network may replicate behavior of a synchronous machine. A swing equation often regulates the angle and rotational speed of a voltage vector of the internal voltage source, but sometimes does not allow for an acceptable response to both grid voltage phase jumps and Rate-of-Change-of Frequency (RoCoF) events.

SUMMARY

A method for tuning a swing equation for a power converter during a rate-of-change-of-frequency (RoCoF) event may include applying, by processing circuitry of a power converter control system, a filter to first frequency information of a power converter to obtain second frequency information of the source converter; determining, by the processing circuitry, based on the first frequency information, a first RoCoF for the power converter; determining, by the processing circuitry, based on the second frequency information, a second RoCoF for the power converter; determining, by the processing circuitry, that the first RoCoF and the second RoCoF are both positive numbers or are both negative numbers; detecting, by the processing circuitry, based on the first RoCoF and the second RoCoF both being positive numbers or both being negative numbers, a possible RoCoF event; comparing, by the processing circuitry, based on the detection of the RoCoF event, the first RoCoF to a RoCoF activation threshold; determining, by the processing circuitry, that the first RoCoF is greater than or equal to the RoCoF activation threshold for a first period of time; comparing, by the processing circuitry, based on the detection of the RoCoF event, a power of the power converter to an active power activation threshold; determining, by the processing circuitry, that the power is greater than or equal to the active power activation threshold for a second period of time; determining, by the processing circuitry, based on the first RoCoF being greater than or equal to the RoCoF activation threshold for the first period of time and the power being greater than or equal to the active power activation threshold for the second period of time, that the possible RoCoF event is a RoCoF event; and applying, by the processing circuitry, during the RoCoF event, a RoCoF ride through by adjusting at least one parameter of a swing equation used by the power converter.

A computer-readable storage medium comprising instructions to cause processing circuitry of a power converter control system for tuning a swing equation during a rate-of-change-of-frequency (RoCoF) event, upon execution of the instructions by the processing circuitry, to: apply a filter to first frequency information of a power converter to obtain second frequency information of the power converter; determine, based on the first frequency information, a first RoCoF for the power converter; determine, based on the second frequency information, a second RoCoF for the power converter; determine, that the first RoCoF and the second RoCoF are both positive numbers or are both negative numbers; detect, based on the first RoCoF and the second RoCoF both being positive numbers or both being negative numbers, a possible RoCoF event; compare, based on the detection of the RoCoF event, the first RoCoF to a RoCoF activation threshold; determine that the first RoCoF is greater than or equal to the RoCoF activation threshold for a first period of time; compare, based on the detection of the RoCoF event, a power of the power converter to an active power activation threshold; determine that the power is greater than or equal to the active power activation threshold for a second period of time; determine, based on the first RoCoF being greater than or equal to the RoCoF activation threshold for the first period of time and the power being greater than or equal to the active power activation threshold for the second period of time, that the possible RoCoF event is a RoCoF event; and apply, during the RoCoF event, a RoCoF ride through by adjusting at least one parameter of a swing equation used by the power converter.

A system for adjusting a swing equation for a power converter during a rate-of-change-of-frequency (RoCoF) event, the system including: a power converter; and a control system comprising memory coupled to processing circuitry, wherein the processing circuitry is configured to: apply a filter to first frequency information of the power converter to obtain second frequency information of the power converter; determine, based on the first frequency information, a first RoCoF for the power converter; determine, based on the second frequency information, a second RoCoF for the power converter; determine that the first RoCoF and the second RoCoF are both positive numbers or are both negative numbers; detect, based on the first RoCoF and the second RoCoF both being positive numbers or both being negative numbers, a possible RoCoF event; compare, based on the detection of the RoCoF event, the first RoCoF to a RoCoF activation threshold; determine that the first RoCoF is greater than or equal to the RoCoF activation threshold for a first period of time; compare, based on the detection of the RoCoF event, a power of the power converter to an active power activation threshold; determine that the power is greater than or equal to the active power activation threshold for a second period of time; determine, based on the first RoCoF being greater than or equal to the RoCoF activation threshold for the first period of time and the power being greater than or equal to the active power activation threshold for the second period of time, that the possible RoCoF event is a RoCoF event; and apply, during the RoCoF event, a RoCoF ride through by adjusting at least one parameter of a swing equation used by the power converter.

Certain implementations will now be described more fully below with reference to the accompanying drawings, in which various implementations and/or aspects are shown. However, various aspects may be implemented in many different forms and should not be construed as limited to the implementations set forth herein; rather, these implementations are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers in the figures refer to like elements throughout. Hence, if a feature is used across several drawings, the number used to identify the feature in the drawing where the feature first appeared will be used in later drawings.

DETAILED DESCRIPTION

Power plants that serve to generate electricity often include a synchronous generator that is directly connected to the electric grid. Power supporting for power grids is often based on voltage source converters. Synchronous grid-forming power converters (e.g., voltage controlled static synchronous compensators) connected to an electrical grid are commonly required to replicate behavior of a voltage source behind an impedance. The angle and the rotational speed of the internal voltage vector of the internal voltage source is regulated typically by a swing equation. The operation of a voltage source that synchronizes to a power grid may be based on the swing equation, which may define a change in angular velocity of the converter over time.

The swing equation tuning determines the response shape to grid-voltage phase jumps and Rate-of-Change-of-Frequency events (RoCoF). The RoCoF of a power system is the time derivative of the system's frequency (df/dt). The RoCoF can be noisy, so filtering may be applied in a RoCoF calculation. Sometimes an acceptable response to both grid-voltage phase jumps and RoCoF events (e.g., not blocking current and allowing a RoCoF event ride through in which the converter may continue to operate without needing to shut down or block current) cannot be achieved with a single tuning of a swing tuning. Therefore, it could be useful to detect a RoCoF event and change the swing equation parameters for a RoCoF event.

The parameters of the swing equation (e.g., start-up time, damping, and filter cutoff frequency) may be dynamic so that a response to a phase jump or RoCoF event is acceptable. However, existing techniques for tuning the swing equation may result in over-current on a converter, and the converter current blocking. In general, it can be difficult to find a set of swing equations parameters that have a good performance for both phase jumps and RoCoF events. A current limiter may solve the blocking issue, but such solutions where the current limit is reached are undesirable, and such solutions do not allow for riding through RoCoF events without limiting the current.

In one or more embodiments, when RoCoF events are detected, the swing equation parameters for synchronous grid-forming converters may be changed during a RoCoF event. After the RoCoF event concludes, the previous swing equation parameters may be restored.

In one or more embodiments, a frequency detector (e.g., a phased-lock loop (PLL) or other type of measurement) may be used to obtain first frequency information from a three-phase voltage Vabc. The first frequency information may be derived (e.g., calculated), such as from measured parameters, of a power system, and may or may not include a swing equation frequency.

In one or more embodiments, the frequency information (e.g., first frequency information) may be filtered to obtain second frequency information. Both the first and second frequency information may be differentiated to obtain two preliminary RoCoF information sets. Both the RoCoF information sets may be filtered similarly to obtain RoCoF1 and RoCoF2. When the signs (e.g., positive/negative) of RoCoF1 and RoCoF2 are the same as one another, an event is detected as a possible RoCoF event. When the signs of RoCoF1 and RoCoF2 are different from one another, the event is not detected as a possible RoCoF event. RoCoF1 may be compared to a RoCoF activation threshold. The RoCoF for the converter may be detected continuously for a predetermined amount of time to continue processing the signal. In addition to the detected RoCoF, an active power exchange may occur between the converter and the grid, and there may be an active power threshold to activate the RoCoF ride through. There may be separate active power and RoCoF deactivation thresholds. As a result of the enhanced techniques, a RoCoF event may be detected and a predetermined RoCoF ride through tuning may be applied to the swing equation. The response to a phase jump may remain unchanged, and the RoCoF ride through may not be activated. Existing techniques do not change swing parameters in a grid-forming converter during a detected RoCoF event, and the RoCoF detection logic herein is also different than any existing techniques.

FIG. 1 is an example process 100 for rate-of-change-of-frequency (RoCoF) event detection and swing equation tuning for grid-forming converters in accordance with one embodiment of the present disclosure.

Referring to FIG. 1, three-phase voltage Vabc for a converter at point of connection of a power grid (e.g., see FIG. 2) may pass through a measurement 102 (e.g., PLL or another measurement) to obtain first frequency information. The first frequency information may pass through a filter 104 (e.g., low-pass filter) to obtain second frequency information. The unfiltered (e.g., first) frequency information and the second frequency information may be differentiated (e.g., using derivative 106, derivative 108) to obtain preliminary RoCoF values, which may be filtered (e.g., using filters 110, filters 112 to address how noisy the preliminary RoCoF values may be) to obtain RoCoF1 and RoCoF2. When the signs of RoCoF1 and RoCoF2 are the same, an event is detected as a possible RoCoF event. When the signs of RoCoF1 and RoCoF2 are not the same, no RoCoF event is detected.

Still referring to FIG. 1, the absolution (abs) value of RoCoF1 may be compared to a RoCoF activation threshold 116. When RoCoF1≥ the RoCoF activation threshold 116, the RoCoF may be continuously detected for the power grid for a predetermined time period (e.g., ON delay 118) so that the signal is further processed. In addition to the detected RoCoF, there may be an active power exchange between the converter and the power grid. The absolute value of the active power P from the grid may be compared to an active power activation threshold 120. P being≥the active power activation threshold 120 may be a condition for activating the RoCoF ride through 121. Even when P≥ the active power activation threshold 120, ON delay 122 may be used to continuously monitor the P. For deactivation, a RoCoF deactivation threshold 124 and an active power deactivation threshold 126 may be used. The absolute value of P may be compared to the active power deactivation threshold 126 to detect when the absolute value of P is ≤the active power deactivation threshold 126. Even when the absolute value of P is ≤the active power deactivation threshold 126, P may be continuously monitored for ON delay 128 time. The absolute value of RoCoF1 may be compared to the RoCoF deactivation threshold 124, and when the absolute value of RoCoF1 is ≤the RoCoF deactivation threshold 124, the ON delay 130 time may allow for continued RoCoF monitoring. When the absolute value of RoCoF1≥ the RoCoF activation threshold 116 after the ON delay 118 and the absolute value of P is ≥ the active power activation threshold 120 after the ON delay 122, such corresponds to a detected RoCoF event, and the swing equation 132 parameters may be fine-tuned during the detected RoCoF event (e.g., while the RoCoF is still≥the RoCoF activation threshold and the power is still≥ the active power activation threshold) to activate the RoCoF ride through 121. After the RoCoF event concludes (e.g., when RoCoF1 is ≤the RoCoF deactivation threshold 124 for the ON delay 130 and P is ≤the active power deactivation threshold 126 for the ON delay 128), the swing equation 132 parameters may return to their previous values from before the detected RoCoF event. Activating the RoCoF ride through 121 may be optional even when the above conditions are satisfied, as there may be additional conditions or control signals that may prevent the RoCoF ride through. The ON delays for the RoCoF activation/deactivation thresholds and for the active power activation/deactivation thresholds may be the same or different.

In one or more embodiments, rather than using an absolute value of RoCoF1 in comparison with the RoCoF activation threshold 116, the RoCoF activation threshold 116 may be replaced with separate RoCoF activation thresholds-one for positive values of RoCoF1, and one for negative values of RoCoF1. The separate activation thresholds may be useful because there may be a steady active power flow that should not trigger the logic, and the active power thresholds may be adjusted as a function of the steady active power flow. Similarly, the active power activation threshold 120 may be replaced with separate active power activation thresholds—an upper threshold (e.g., a positive RoCoF threshold) for positive values of RoCoF1, and a lower threshold (e.g., a negative RoCoF threshold) for negative values of RoCoF1. Similarly, the RoCoF deactivation threshold 124 may be replaced with separate RoCoF deactivation thresholds-one for positive values of RoCoF1, and one for negative values of RoCoF1. Similarly, the active power deactivation threshold 126 may be replaced with separate active power deactivation thresholds—an upper threshold for positive values of RoCoF1, and a lower threshold for negative values of RoCoF1.

In one or more embodiments, the upper active power activation threshold and the lower active power activation threshold may be based on the RoCoF signal sign (e.g., during a negative RoCoF, the upper active power activation threshold may be triggered while the lower active power activation threshold may not be triggered, and during a positive RoCoF, the lower active power activation threshold may be triggered while the upper power activation threshold may not be triggered.

In one or more embodiments, the rate of change of active power may be used instead of the active power to trigger the active power threshold(s).

In one or more embodiments, an amount of energy may be used instead of the combination of active power and the on-delay (e.g., integral of power with respect to time=energy).

FIG. 2 is an example system 200 using a grid-connected grid-forming converter 202 in accordance with one embodiment of the present disclosure.

Referring to FIG. 2, the system 200 may include the grid-forming converter 202 (e.g., voltage source converter) for an electrical grid 204 via a point of common coupling (PCC 206). A three-phase voltage Vabc and a three-phase current Iabc from the PCC 206 may be input to signal processing 208 circuitry (e.g., including the process 100 of FIG. 1). Based on the signal processing 208, the parameters of the swing equation 132 of FIG. 1 may be set (e.g., applying or not applying the RoCoF ride through 121 of FIG. 1), and optional inner controls 210 (e.g., voltage and/or current controls, such as blocking/limiting, etc.). For example, the inner controls 210 may generate voltage source converter (VSC) pulses for the grid-forming converter 202. In one or more embodiments, the parameters of the swing equation 132 may be adjusted according the process 100 of FIG. 1.

A control system 212 may include the signal processing 208, the swing equation 132, and the inner controls 210. The control system 212 may operate on logic or a number of components, modules, or mechanisms. Modules are tangible entities (e.g., hardware) capable of performing specified operations when operating. A module includes hardware. In an example, the hardware may be specifically configured to carry out a specific operation (e.g., hardwired). In another example, the hardware may include configurable execution units (e.g., transistors, circuits, etc.) and a computer readable medium containing instructions where the instructions configure the execution units to carry out a specific operation when in operation. The configuring may occur under the direction of the executions units or a loading mechanism. Accordingly, the execution units are communicatively coupled to the computer-readable medium when the device is operating. In this example, the execution units may be a member of more than one module. For example, under operation, the execution units may be configured by a first set of instructions to implement a first module at one point in time and reconfigured by a second set of instructions to implement a second module at a second point in time. The control system 212 may include one or more processors and storage devices with a machine-readable medium on which may e stored one or more sets of data structures or software instructions embodying or utilized by any one or more of the techniques or functions described herein. The instructions may also reside, completely or at least partially, within a main memory, within a static memory, or within a hardware processor during execution thereof by the control system 212. In an example, one or any combination of a hardware processor, a main memory, a static memory, or a storage device may constitute machine-readable media.

FIG. 3 is an example process 300 for power synchronization of the swing equation 132 of FIG. 1 in accordance with one embodiment of the present disclosure.

Referring to FIG. 3, the swing equation 132 may include three parameters (e.g., Par. 1, Par. 2, and Par. 3 as shown in FIG. 3). Examples of the parameters include, but are not limited to: Par. 1=1/startup time (e.g., inertia of the system), Par. 2=damping, and Par.

where ωhp is a high-pass filter cutoff frequency (e.g., where s is from the Laplace transform, and an integrator 302 may be represented by

As shown, the swing equation 132 depends on a reference power Pref (e.g., a desired power level) for the system of FIG. 2, and a measured power Pmeas for the system of FIG. 2. A per-unit frequency fpu may be set using the swing equation 132 as shown. In one or more embodiments, the swing equation 132 parameters may be adjusted dynamically upon detection of a RoCoF event, using the process 100 of FIG. 1.

In one or more embodiments, the swing equation 132 may define a change in angular velocity of the converter 202 of FIG. 2 over time based on the angular velocity of the converter 202, a damping term, the startup time for the converter 202, a reference angular velocity (e.g., desired angular velocity), and a high-pass filter cutoff frequency used to high-pass filter the frequency. Any of the parameters, such as the damping term, the startup time, the high-pass filter cutoff frequency, or other parameters used in the swing equation 132 may be adjusted based on detection of the RoCoF event as in process 100.

FIG. 4A is an example response 400 to a 10 degree phase jump using the fine-tuned swing equation 132 of FIG. 1 in accordance with one embodiment of the present disclosure.

Referring to FIG. 4A, when the swing equation 132 is tuned using the process 100 of FIG. 1, the response 400 (e.g., a three-phase response) may be desirable. Rather than the current over time reaching a current limit (e.g., as in FIG. 4B), the response 400 avoids reaching the current limit due to the tuning of the swing equation 132 parameters based on detection of a RoCoF event.

FIG. 4B is an example response 405 to a RoCoF event of 2 Hz/s for 0.5 seconds with an undesirable response.

In contrast with FIG. 4A, when the swing equation 132 parameters are not tuned, over-current occurs in the three-phase response 450 of FIG. 4B, with a current limitation being reached. While the current limitation may prevent blocking, it is more desirable to avoid reaching the current limit as in FIG. 4A.

FIG. 5A is a comparison of responses to a RoCoF event of 2 Hz/s for 0.5 seconds with swing equation fine-tuning and without swing equation fine tuning in accordance with one embodiment of the present disclosure.

Referring to FIG. 5A, a three-phase response 502 without the swing equation 132 parameter fine tuning of the process 100 of FIG. 1, and a three-phase response 504 using the swing equation 132 parameter fine tuning of the process 100 of FIG. 1 are shown in comparison to one another. When a RoCoF event is detected and the RoCoF ride through 121 of FIG. 1 is applied, the result is the more desirable response 504.

FIG. 5B is an example of filtered RoCoF values using the process of FIG. 1 in accordance with one embodiment of the present disclosure.

Referring to FIG. 5B, the filtered RoCoF1 and RoCoF2 values of FIG. 1 (e.g., after the filters 110 and 112 are applied) are represented by filtered RoCoF 522 and filtered RoCoF 524, and may be evaluated for whether they have the same sign. In FIG. 5B, from time 0 to about 1.5 seconds, they both have a negative sign. The filtered RoCoF 522 briefly turns positive after 1.5 seconds, but not long enough for the ON delay. The result is the RoCoF ride through 121 state represented by FIG. 5C.

FIG. 5C is an example RoCoF ride through state 560 based on the filtered RoCoF values of FIG. 5C in accordance with one embodiment of the present disclosure.

Referring back to FIG. 5B, once the filtered RoCoF 502 and the filtered RoCoF 504 have had the same negative sign for some amount of time after time 0, a RoCoF event may be detected, and the RoCoF ride through 121 may be ON. While the filtered RoCoF 502 briefly exhibits a positive sign after 1.5 seconds, it is not long enough to deactivate the RoCoF ride through 121, which remains ON until about 2.7 seconds based on the process 100 of FIG. 1.

FIG. 6A is an example response 600 to a 10 degree phase jump using the fine-tuned swing equation 132 of FIG. 1 in accordance with one embodiment of the present disclosure.

Referring to FIG. 6A, the response 600 using the enhanced process 100 of FIG. 1 does not alter the response to a 10 degree phase jump when the swing equation 132 parameter tuning is not used. This is because high instantaneous RoCoF values may be measured, but mostly with different signs (e.g., no RoCoF event), so the RoCoF ride through 121 may not be triggered, as shown in FIG. 6B and FIG. 6C.

FIG. 6B is an example of filtered RoCoF values using the process of FIG. 1 in accordance with one embodiment of the present disclosure.

Referring to FIG. 6B, the filtered RoCoF1 and RoCoF2 values of FIG. 1 (e.g., after the filters 110 and 112 are applied) are represented by filtered RoCoF 622 and filtered RoCoF 624, and may be evaluated for whether they have the same sign. Because they mostly have different signs, no RoCoF event is detected, so the RoCoF ride through 121 remains off as shown in FIG. 6C.

FIG. 6C is an example RoCoF ride through state 660 based on the filtered RoCoF values of FIG. 6C in accordance with one embodiment of the present disclosure.

Referring to FIG. 6C, because the filtered RoCoF 622 and filtered RoCoF 624 do not lead to a RoCoF event being detected, the RoCoF ride through 121 remains OFF.