WO2015139061A1

WO2015139061A1 - Ramp rate control system and methods using energy storage devices

Info

Publication number: WO2015139061A1
Application number: PCT/US2015/020842
Authority: WO
Inventors: Vahid SALEHI POUR MEHR; Kevin Meagher; Brian RADIBRATOVIC
Original assignee: Power Analytics Corp
Current assignee: Power Analytics Corp
Priority date: 2014-03-14
Filing date: 2015-03-16
Publication date: 2015-09-17
Anticipated expiration: 2016-09-14

Abstract

A power generation system includes at least one photo voltaic (PV) unit for generating power, a first data unit receiving and storing irradiance data measured by the PV unit, a second data unit having irradiance forecast data, a controller receiving the data from both data units and an energy storage device receiving instructions from the controller, wherein the instructions from the controller include ramp up or ramp down messages.

Description

RAMP RATE CONTROL SYSTEM AND METHODS USING ENERGY STORAGE DEVICES

RELATED APPLICATION DATA

[0001] This application claims priority from U.S. Provisional Application No.

61/953,708 filed on 14 March 2014, the entire subject matter of which is incorporated herein.

FIELD OF DISCLOSURE

[0002] The present disclosure relates generally to power systems and more particularly to methods and devices for limiting power ramps in real time utilizing an energy storage device.

BACKGROUND

[0003] Distributed power generation and alternative power generation benefit significantly from the inclusion of energy storage. This is especially true of non-traditional forms of generation including photovoltaic, wind generation and fuel cells. The introduction of distributed power generation and variable renewable energy resources into the electrical networks, however, raises additional issues and cost penalties due to adverse effects on the network.

[0004] These effects include intermittency of a fuel source (sunlight in particular) for both the planning and operation of the generation. The effects could also include network congestion, instability, environmental impacts and security of the supply. In mixed generation, it is frequently possible to use the more traditional resources to mitigate some of these issues. In many cases, however, it is not possible.

[0005] With photovoltaic (PV) power generation, concerns about intermittency are not only based on sunrise and sunset but cloud and other atmospheric conditions also limit or reduce the availability of incident solar irradiance. Since PV does not have rotating machinery, traditional solutions to electric fault clearing for example must be dealt with in unique ways. [0006] PV inverter technology provides advanced capability for voltage and frequency support but it still requires measures to be taken to ensure network stability and predictability of response. These measures can include such alternatives as providing significant levels of energy storage that dramatically increase the overall cost of the solution.

[0007] The output power variation of photovoltaic (PV) plants that are connected to the power grids affect the stability of the grids (i.e. destabilizing the grids). This is especially true for weak grids which are grids having a high penetration with respect to local generation. These problems originate from simple voltage and frequency variations and can extend to protection malfunction, system inertia limitations for following fast frequency changes, power uncertainty and constrains on available spinning reserve. Therefore, energy storage system (ESS) is introduced to mitigate adverse impacts of rapid variation of renewable resources on the local grid and maintain voltage and frequency for islanded area.

[0008] The intermittent nature of PV generation causes considerable changes of magnitude and rate of change in output power. On some days, clouds passing over a PV plant can cause the PV system to go from full output to nearly zero output and back to full output in a matter of seconds. At a high level of PV penetration, this intermittency adversely affects utility operation and load side equipment due to fluctuations in grid voltage and frequency.

[0009] Utility companies have developed minimal technical requirements for interconnection of photovoltaic (PV) facilities for ramp rate control. According to some requirements, the PV facility should have the ability control the rate of change of power output during some circumstances such as rate of increase/decrease of power when a curtailment of power output is engaged. In addition, for limiting ramp rates, there are some constrains such as ten percent (10%) per minute rate (based on PV installed capacity or nameplate rating) which should be considered. [0010] The use of storage systems in grid-connected solar installations is a relatively new phenomenon for applications such as peak shaving, load shifting, demand response, outage protection, economic optimization of energy flows and grid power quality control. In addition, PV storage inverters include the settings for limiting the rate of power delivery to the grid in response to intermittent PV generation. It is desirable for storage systems to manage their charging/discharging modes such that the rate of change in power delivery to and from the grid remains below this limit even with intermittent PV generation.

[0011] One method for dealing with performance of photovoltaic is to de-rate the output financial benefit of PV. This reduction in both operational and financial performance reduces the use and expansion of this very clean alternative energy source and in some cases is the barrier to implementation. While solar forecasting and weather forecasting have been invaluable in predicting the general performance of PV, the lack of an integrated local control that is both real-time and based on a detailed power system model limits the availability of photovoltaic to specific areas and applications.

SUMMARY

[0012] According to an exemplary embodiment, a power generation system is disclosed. The system comprises: at least one photo voltaic (PV) unit for generating power; a first data unit receiving and storing irradiance data measured by the PV unit; a second data unit having irradiance forecast data; a controller receiving the data from both data units; and an energy storage device receiving instructions from the controller, wherein the instructions from the controller include ramp up or ramp down messages.

[0013] According to another exemplary embodiment, a power generation method is disclosed. The method comprises: receiving irradiance forecast data; evaluating the received data; and selectively charging/discharging an energy storage device based on the evaluation. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] The several features, objects, and advantages of exemplary embodiments will be understood by reading this description in conjunction with the drawings. The same reference numbers in different drawings identify the same or similar elements. In the drawings:

[0015] FIG. 1 illustrates an exemplary output power of a photo-voltaic plant for different irradiance profiles;

[0016] FIG. 2 illustrates ramp rate occurrence frequency for two different irradiance profiles;

[0017] FIG. 3 illustrates dynamic model of a Lithium- ion battery;

[0018] FIG. 4 illustrates a ramp-rate limit control logic circuit in accordance with exemplary embodiments;

[0019] FIGs. 5(a) - 5(c) illustrate ramped power, PV plant output power and battery power for a day and for an hour of the day and energy changes of the battery for the day;

[0020] FIGs. 6(a) and 6(b) illustrate ramped power, PV plant output power and battery power based direct measurement and based on forecast data for a time period;

[0021] FIGs. 7(a) and 7(b) illustrate ramped power, PV plant output power and battery power for an hour and battery energy change based on forecast data for a day;

[0022] FIG. 8 illustrates energy changes of the battery for a day based on forecast data and on utilizing the control circuit of FIG. 4;

[0023] FIGs. 9(a) and (b) illustrate a PV plant and a PV dynamic model in accordance with exemplary embodiments;

[0024] FIG. 10 illustrates a PV plant output power with and without battery storage system;

[0025] FIG. 11 illustrates a PB system in accordance with exemplary embodiments; [0026] FIG. 12 illustrates a system in accordance with exemplary embodiments; and

[0027] FIG. 13 illustrates a method in accordance with exemplary embodiments.

DETAILED DESCRIPTION

[0028] In the following description, numerous specific details are given to provide a thorough understanding of embodiments. The embodiments can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of the exemplary embodiments.

[0029] Reference throughout this specification to an "exemplary embodiment" or

"exemplary embodiments" means that a particular feature, structure, or characteristic as described is included in at least one embodiment. Thus, the appearances of these terms and similar phrases in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. The headings provided herein are for convenience only and do not interpret the scope or meaning of the embodiments described herein.

[0030] Exemplary embodiments disclose methods for controlling the charging and/or discharging modes of storage devices in order to mitigate adverse effects of high ramp rates in point of common coupling (PCC) of PV plant connection to the grid. Such a method can be implemented in real-time systems. It may be based on utilizing instantaneous (real-time) irradiance data and forecast irradiance data for better ramp rate control in order to minimize the battery size and increase battery life.

[0031] Irradiance data for a proposed PV plant location can be taken into account in determining the size of an energy storage device that is to be implemented within the proposed PV plant. A dynamic model of a PV plant, energy storage devices and realistic sun irradiance data may be implemented to verify the performance of the exemplary methods.

[0032] Irradiance and insolation are two major measures of solar resource. Irradiance is a measure of solar power on a given plane expressed in W/m (watts per square meter). The output power of a PV plant is generally proportional to the irradiance across the area of the plant.

[0033] An exemplary output power of a fifty (50) MW PV plant for three different days with different irradiance profiles is illustrated in FIG. 1. The profiles illustrated include a profile for a clear sunny day, an overcast day and a partly cloudy day. The PV output variability is measured based on computing AC power output changes over multiple time intervals.

[0034] A ten (10) minute magnified data of this exemplary data for a partly cloudy day is also illustrated in FIG. 1. The three arrows illustrate how the ramp rates are calculated for different time scales. For a typical PV plant, a number of change-in-power (ΔΡ) observations for four ramp rate intervals may be measured. These rate intervals may include ten (10) seconds, one (1) minute, ten (10) minutes and one (1) hour. The observation data indicates the frequency and extent with which output ramping occurs during daytime hours.

[0035] For utilities, ramping events are a concern for PV interconnection even if they rarely occur. Exemplary embodiments disclose novel methods for reducing and mitigating the power ramping issues in real-time by limiting power ramps utilizing an energy storage device.

[0036] According to exemplary embodiments, solar irradiance forecast data may be utilized to predict or estimate output power ramps. Therefore, based on available measured irradiance data at five (5) second intervals, the output power ramp rate increase or decrease for different time scales can be estimated or computed. In an exemplary embodiment, ramp rate occurrence frequency for two different irradiance profiles representing a partly cloudy day and an overcast day can be considered. These profiles are illustrated in FIG. 2.

[0037] A ten percent (10%) power change of five (5) MW (based on 50 MW nominal power) may be applied with various time scales such as one (1) hour, ten (10) minutes, one (1) minute and ten (10) seconds for example. The power change, ΔΡ (in MW), may be multiplied by the time step (i.e. 5 second measurement data resolution) and then divided by each of the different time scales to find proper ramp rate to compare with the irradiance data.

[0038] The ramp rate may be computed by: RampRate = (ΙΔ PI x T_s)/(time scale) where ΙΔ PI is expressed as MW and T_s and time scale are expressed as seconds. The computed ramp rate for each of the time scales may be 0.006944 MW (for 1 hour), 0.041666 MW (for 10 minutes), 0.41666 (for 1 minute) and 2.5 MW (for 10 seconds).

[0039] The frequency of ramp occurrences in the output power for the partly cloudy day and for the overcast day for the time scales specified above are also illustrated in FIG. 2. In this example, for the partly cloudy day, the ramp occurrence frequency of five (5) MW per one (1) hour ramps is 83.93% of the total daytime from 6 AM to 6 PM. For the five (5) MW per ten (10) minute ramps, the occurrence frequency is 52.13% of daytime. For the five (5) MW per one (1) minute ramps, the occurrence frequency is 15.28% of daytime. There is no ramp with five (5) MW per ten (10) second rating.

[0040] For the overcast day, the ramp occurrence frequency of five (5) MW per one

(1) hour ramps is 82.97% of the total daytime from 6 AM to 6 PM. For the five (5) MW per ten (10) minute ramps, the occurrence frequency is 42.77% of daytime. For the five (5) MW per one (1) minute ramps, the occurrence frequency is 4.9% of daytime. There is no ramp with five (5) MW per ten (10) second rating.

[0041] Exemplary embodiments as described herein focus on the ten percent (10%) of nominal power per one (1) minute ramp ratings. This rating needs faster control from AC grid based on the system inertia and available spinning reserve to reduce power ramp effects of PV plant output. The focus on the ten percent (10%) of nominal power for one (1) minute results from typical existing requirements. The selection of this particular ramp rating is exemplary and not limiting. A battery energy storage device may be utilized for mitigating the occurrence of the ramp rate in the five (5) MW per one (1) minute case.

[0042] An energy storage device may be implemented to meet specific output variability limits at the point of common coupling (PCC). Decreasing ramp rate limits at PCC may be applicable to specific circumstances based on effects on local voltage and frequency, islanded and grid connected operations and cost preference of the energy storage devices.

[0043] Batteries having a lower cost and lower power requirement may not store large amounts of power. Other batteries that can store large amount of power may have a higher cost. Existing PV systems utilize lead-acid battery products which are inexpensive but not efficient in weight and size.

[0044] Lithium-ion batteries are utilized in specialized applications such as large PV installation plants. These batteries deliver better performance in a small size without the need for maintenance. A Lithium-ion battery provides greater deep discharge which means more energy can be stored in these systems. A Lithium-ion battery also has a longer life which makes it appropriate for remote locations.

[0045] For ramp rate application of a PV plant, a battery should have fast response in charging and discharging performance which helps in absorbing fluctuation of PV output power. Factors that are considered may include battery storage capacity, rate of charge and discharge power, state of charge, maximum depth of discharge for longer battery life and transient time response of a storage device. [0046] ynamic model of a Lithium-ion battery that is implemented for studying the transient behavior of energy storage devices is illustrated in FIG. 3. In order to improve power quality and to smooth the PV output power, a real-time ramp-rate controller may be designed for monitoring the sun irradiance or the PV output power.

[0047] The controller can also take action to absorb or inject active power to the PCC.

The controller may send charge or discharge command to the battery based on desired power level. A bidirectional converter control may be used to interlink the DC storage device to the AC utility. This controller can be integrated to the PV inverter control or can be independent from PV control circuit.

[0048] A ramp-rate limit control logic circuit in accordance with exemplary embodiments is illustrated in FIG. 4. The PV power output or sun irradiance real-time data may provide the input to the logic circuit and ramped up/down data may be output by the logic circuit. Rising or falling ramp rates can be the parameters for this circuit. The output can be subtracted from the input in order to achieve required power signal for a battery controller.

[0049] As described, the PV output power signal [MW] or sun irradiance data [W/m²] can be used as input for ramp rate control. Real-time data can be used to calculate the required power of a battery bank.

[0050] An expected PV output (dashed curve) and smoothed PV output (dotted curve) are illustrated in FIG. 5A for example. The rate of change of the smoothed curve may be limited to five (5) MW per one (1) minute for example. In order to smooth the PV plant output power, the dotted curve has to be generated in the PCC. The solid line illustrates the power that needs to be injected to the PCC by the available energy storage system. The positive and negative power signs correspond to charge and discharge modes for battery controller. [0051] The expected PV output, the smoothed PV output (limited to 5 MW/min) and the power needed for injection to the PCC for a one hour interval between 10AM -11AM in an exemplary setting are illustrated in FIG. 5B. This particular time range represents the period during which the maximum and minimum requested battery power occurs in this example. As shown, the energy storage system should be capable of producing 10.78 MW active power instantly and be capable of consuming 11.03 MW active power instantly. This amount of power is approximately twenty percent (20%) of the PV plant nominal power (of 50 MW).

[0052] Daily energy changes of stored energy in the battery for this example are illustrated in FIG. 5C. If the battery is fully charged at the beginning of the day (i.e. at 6 AM), its state of charge decreases by ΔΡ = 0.759 MWh by the end of the day (i.e. at 6 PM). The battery has to be charged by this amount by or at the beginning of the next day.

Therefore, the capacity of the energy storage system also has to correspond to this value (i.e. 0.759 MWh).

[0053] According to this exemplary embodiment, the size of the battery is approximately 0.8 MWh for full discharge depth but it should be capable of instantly providing approximately 11 MW as discharge/charge power.

[0054] Typical batteries allow less charge/discharge amount of power but are designed in high capacity range. In order to extend the battery life, they allow limited depth of discharge.

[0055] Irradiance data may be used to predict PV power generation. It can also be utilized to ramp down (and ramp up) PV output power. The ramp rise event and the battery power request without having any knowledge prior to ramp occurrence is illustrated in FIG. 6A. In this setting, the real-time system should immediately take over this large power production duty with the storage device. The dashed line indicates the PV output and the dotted line represents a ramped power value. The ramped value trails the actual PV output during changes toward an increase or toward a decrease.

[0056] An upcoming ramp event that is predicted a minute ahead of the actual ramp event and the beginning of the ramping up according to exemplary embodiments is illustrated in FIG. 6B. In this illustration, the ramped (dotted line) value leads the actual PV output (dashed line) before the actual output value increases or decreases. Both discharging and/or charging modes may be used to decrease requested power amount during this event. The decrease in the battery power exchange results in a reduced need in its size and increases its life.

[0057] The application of forecasted data for controlling battery power to start charge or discharge the ramping event fifty (50) seconds prior to the actual ramping event is illustrated in FIG. 7. This time frame is chosen in a way to achieve the smallest power exchange between the battery and the PCC point. The battery output power can be limited to 6.618 MW in discharge mode and 6.85 in charging mode. The battery state of charge (SOC) decreases by 0.759 MWh during the daytime.

[0058] Control based on forecasted data may be enhanced in a way so as not to use the battery power when there is no predicted ramp event. In existing designs, the controller keeps charging the battery until the maximum power output time of the day (i.e. noon) and then discharges the battery for the rest of the duration of daytime. This keeps the battery in service all day unnecessarily. According to exemplary embodiments, a real-time controller may be equipped with a bypass controller for the time periods during which the forecasted data does not include (or, predict) an upcoming ramp event. The bypass controller can keep the battery converter disconnected from the circuit.

[0059] Results from applying this controller for the same day profile are illustrated in

FIG. 8. Keeping the battery power at zero when there is no ramp event will cause less battery discharge at the end of the day. Such reduced discharge can facilitate faster charging back in the morning and increasing the battery life expectancy. This controller will also turn off the battery controller for all sunny days.

[0060] The results from using the ramp rate control methodology are presented for a typical PV plant. To achieve this, the PV plant is designed with appropriate PV/battery modeling in Paladin DesignBase® software developed and sold by the assignee of this disclosure. The single line diagram and the dynamic PV model used for transient study is illustrated in FIG. 9A. Thirty six (36) PV blocks are distributed in 6 branches with a battery unit for each branch. A detailed structure of a PV is illustrated in FIG. 9B. Each battery includes the dynamic model illustrated in FIG. 3. A time series power flow is used for measuring sun irradiance data every 5 seconds.

[0061] The results of load flow study for days with different sun irradiance profile and application of this real-time controller to mitigate 5MW per one (1) minute ramps at PCC is shown in Table I. In addition to the daily energy production, losses and availability are also shown. The performance of the battery for three different sun irradiance profiles are also shown for better comparison and proper storage device selection.

[0062] In addition, a transient simulation may be performed to show the effectiveness of usage of battery storages to smooth the power ramp rates for the total PV generation. In order to achieve this, the output power control signal is calculated for each battery to decrease the ramping rate of the generation of the PVs. The PV plant output power [MW] with (dashed line) and without (solid line) battery storage system is illustrated in FIG. 10 for comparison.

[0063] The use of storage devices for reducing PV plant output power intermittency and a real-time control strategy based on forecasted sun irradiance data enhances a PV system operation integrated with a power system by considering all power quality constrains. [0064] By combining the advanced power modeling in both planning and real time operation in combination with solar forecasting and PV inverter technology, an accurate modeling of the actual performance may be achieved. Solar irradiance forecast can further enhance the prediction of the performance of a photovoltaic array. This combination of base line performance calibrated with the real time information and solar forecasting provides accurate information to adequately size energy storage requirements, and optimize the charging and discharging of connected energy storage. In addition, the real time model incorporates the optimization and control of the PV to maximize electrical performance and minimize or eliminate sudden swings in generation (ramp up and ramp down) associated with the intermittency of photovoltaic in an electric grid.

[0065] The Power Analytics Real-Time model integrates the solar forecasting data with the detailed electrical model of specific photovoltaic inverters and the control of those inverters as well as the simulated and actual charging and discharging of the battery storage. This accurate model also eliminates the variability energy storage so that the use of energy storage is minimized depending on the specific operation requirements of the power network owner or operator.

[0066] In existing systems, utility operators, virtual power plants and power generation provides lack the knowledge of the range of performance and that risk required the inclusion of energy storage frequently at multiple times over photovoltaic system rendering the solution financially and operationally impractical or even impossible.

[0067] The present disclosure develops a real-time power ramp-rate limiter feature for photovoltaic plants subjected to intense daily power variations. A method in accordance with exemplary embodiments facilitates the smoothing of PV output power at the point of common coupling (PCC) below a requested ramp rate, i.e. 10% Power_nom/lmin by using energy storage devices which are controlled by a real-time model based system. Using forecasted sun irradiance/power data enhances the design and helps in selecting smaller storage device.

[0068] An exemplary PV system is illustrated in FIG. 11. A plurality of PVs arranged in banks with each bank connecting to the power network via an inverter. A battery energy storage is connected to the PVs and the power system via a bi-directional inverter. The bi-directional inverter facilitates charging and discharging of thee battery energy storage.

[0069] A system in accordance with exemplary embodiments is illustrated in FIG.

12. A PV unit provides actual (or real-time) irradiance information to an irradiance data source. The information provided by the PV is actual data measured by the PV. A forecast irradiance data is also provided by a forecasting service or system that may utilize sensors for example. The forecast data may be gathered by a sensor (such as a camera) that is located upstream of a PV generation area. In this context, upstream may indicate that wind and clouds may be flowing from the sensor location toward the PB generation area.

[0070] In operation, the PV provides generated power to a grid for example. The actual and forecast irradiance data may be provided to a ramp rate controller. The ramp rate controller issues charge or discharge commands to the energy (e.g. battery) storage device in the form of ramp up or ramp down messages. The communication may be via wired or wireless means. If the message is a ramp up event, the battery is discharged and if the message is a ramp down event, the battery is charged. Power for charging the device may be from the power generated by the PV and/or from the grid. Discharge from the battery may be provided to the grid.

[0071] A method in accordance with exemplary embodiments is illustrated in FIG.

13. Irradiance forecast data is received. If the forecast indicates a ramp down event (i.e. power generation from the PV is decreasing), the energy storage device is charged before the ramping occurs. As the forecast indicates a leveling or ramping up event, the device is discharged during the ramping. If the forecast indicates a ramp up event, the energy storage device discharged before the ramping event. The device is charged during the ramping. In both cases, the process returns to evaluating received irradiance forecast data.

[0072] Exemplary systems and methods described herein can be specially constructed for the required purpose such a general purpose computer that is selectively activated or configured by a computer program stored in the computer. The embodiments as described herein may also be embodied as computer readable code on a computer readable medium. The computer readable medium can be any data storage device that can store data that can thereafter be read (and executed) by a computer or computer system. The irradiance data can be stored in network locations remote from the PVs and can be accessed by a network (public, private, etc.) connection that is wired or wireless.

[0073] Although exemplary embodiments have been disclosed, it will be apparent to those skilled in the art that various changes and modifications can be made which will achieve some of the advantages of embodiments without departing from the spirit and scope of the disclosure. Such modifications are intended to be covered by the appended claims in which the reference signs shall not be construed as limiting the scope.

[0074] In the description and the appended claims the meaning of "comprising" is not to be understood as excluding other elements or steps. Further, "a" or "an" does not exclude a plurality, and a single unit may fulfill the functions of several means recited in the claims.

[0075] The above description of illustrated embodiments, including what is described in the Abstract, is not intended to be exhaustive or to limit the embodiments to the precise forms disclosed. Although specific embodiments and examples are described herein for illustrative purposes, various equivalent modifications can be made without departing from the spirit and scope of the disclosure, as will be recognized by those skilled in relevant art. The various embodiments described above can be combined to provide further embodiments. Aspects of the embodiments can be modified, if necessary to employ concepts of the various patents, applications and publications to provide yet further embodiments.

[0076] These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.

Claims

What is claimed is:

1. A power generation system comprising:

at least one photo voltaic (PV) unit for generating power;

a first data unit receiving and storing irradiance data measured by the PV unit;

a second data unit having irradiance forecast data;

a controller receiving the data from both data units; and

an energy storage device receiving instructions from the controller, wherein the instructions from the controller include ramp up or ramp down messages.

2. The system of claim 1, wherein a ramp up message indicates a discharge of the energy storage device.

3. The system of claim 1, wherein a ramp down message indicates a charge of the energy storage device.

4. The system of claim 3, wherein a charge for the energy device is provided at least by the PV unit.

5. A power generation method comprising:

receiving irradiance forecast data;

evaluating the received data; and

selectively charging or discharging an energy storage device based on the evaluation.