WO2011150247A1

WO2011150247A1 - Power grid compensation system

Info

Publication number: WO2011150247A1
Application number: PCT/US2011/038198
Authority: WO
Inventors: Charles W. Botsford
Original assignee: Aerovironment, Inc.
Priority date: 2010-05-28
Filing date: 2011-05-26
Publication date: 2011-12-01

Abstract

Embodiments of the power grid control system (310) compensate for power usage spikes from EV fast charging that could otherwise negatively impact grid stability and capacity, especially during peak grid (301) usage.

Description

TITLE: Power Grid Compensation System

DESCRIPTION

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to and benefit of Provisional Patent Application Nos.

61/349,699 filed May 28, 2010 and 61/390,983 filed October 7, 2010 and are hereby incorporated by reference for all purposes.

TECHNICAL FIELD

Power grid compensation system for accommodating electric vehicle charging, particularly fast charging units drawing from the power grid.

BACKGROUND

Synchrophasors are measurements of the phase angles of voltage, current and frequency; measurements that may be derived from locations on the electricity grid according to the disposition of phasor measurement units (PMUs). Synchrophasor measurements may be taken from grid monitoring devices e.g., PMUs at speeds of thirty observations per second.

DISCLOSURE

Some embodiments of the power grid control system compensate for power spikes from EV fast charging that could otherwise negatively impact grid stability and capacity, e.g., during peak grid usage. A power grid control system may comprise a control processing unit that has a processor and addressable memory, where the control processing unit may be configured to receive feedback signals from phasor measurement units (PMUs) of active transformers of a power grid; and feedback signals from PMUs of power grid substations. The control processing unit may be further configured to provide control signals to grid-level energy stores, each energy store may be configured, responsive to a control signal from the control processing unit, to draw power from, and provide power to, the power grid. The control processing unit may be further configured to provide control signals to electric vehicle chargers configured, responsive to control signals from the control processing unit, to draw power from, and provide power to, the power grid. The control processing unit may be further configured to provide control signals to electric vehicle chargers having an energy store where the electric vehicle chargers may each be configured, responsive to control signals from the control processing unit, to interrupt drawing power from the power grid and to resume drawing power from the power grid. One or more of the PMU s may be configured to output synchrophasors. The control signals of the power grid control system may be based on a difference between at least one PMU feedback signal indicative of power grid cycle frequency and a reference frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example and not limitation in the figures of the accompanying drawings, in which:

FIG. 1 depicts a top-level functional block diagram of an exemplary grid control system;

FIG. 2 depicts an exemplary active transformer;

FIG. 3 depicts an exemplary top-level functional system block diagram;

FIG. 4 depicts an exemplary bank of EV chargers; and

FIG. 5 depicts an exemplary top-level functional system block diagram.

BEST MODES

PMUs may be integrated with a Global Position System (GPS) antenna and a receiver, and accordingly, PMU measurements may be associated with a time and a location.

Synchrophasor measurement rates are more than two orders of magnitude faster than

Supervisory Control and Data Acquisition (SCAD A) techniques, which measure once every four seconds. Because synchrophasors are measured at discrete points in time and space, via GPS, the measurements may be correlated and synchronized against grid activity. This also enables grid control systems to combine synchrophasors to provide an instantaneous, all- encompassing view of the grid, thereby providing an assessment that may include systemic changes and grid stresses. Accordingly, the application of synchrophasors, via PMUs, may be feedback elements in a grid control system attempting grid attribute compensation in the face of the addition and operation of multiple EV fast chargers. As depicted in FIG. 1, an exemplary grid control system 100 may comprise a control processing unit 110 that comprises a processor 111 and addressable memory 112 that may be configured, e.g., by processor-readable instructions, to receive a feedback signal 113, 114 from at least one of: (a) a first phasor measurement unit (PMU) 122 of an active transformer of a power grid 130; and a second PMU 121of power grid substation 131; where the control processing unit 110 may be further configured to provide a control signal 1 15 to at least one of a grid-level energy store 140 configured. The energy store 140 is configured to be responsive to a control signal 115 from the control processing unit 110, to draw power, via a path 132, from, and provide power, via a path 132, to the power grid 130. The control processing unit 110 may be further configured to provide a control signal 1 16 to a first electric vehicle charger 150 configured to be responsive to the control signal 116 from the control processing unit 110, to draw power, via a path 133 from, and provide power, via a path 133, to the power grid 130. The control processing unit 110 may be configured to provide a control signal 1 17 to a second electric vehicle charger 160 having an energy store, where the second electric vehicle charger 160 may be configured to be responsive to the control signal 117 from the control processing unit 110, to interrupt drawing power via a path 134 from the power grid 130, and to resume drawing power via 134 from the power grid 130.

Grid-level energy storage may include, in any combination: (a) compressed air energy storage (CAES); (b) pumped hydroelectric; (c) flywheels; and (d) batteries. For example, grid-level battery systems for grid frequency regulation may be categorized by levels such as: one-half megawatt-hr (0.5MW-hr) of energy storage, and a megawatt of power (MW).

Battery systems typically require a bi-directional inverter and transformer for grid

connection. An active transformer may combine these functions into a robust and efficient grid interface.

Conventional transformers that tie into medium voltage grid feeder lines have high efficiencies, but create potential power factor issues. FIG. 2 depicts a general arrangement of components of an exemplary active transformer 200. For example, active transformer elements may form a portion of an integrated housing, where elements of an active transformer may include: a plurality of radiators 210; cooling fans 221, 222; a plurality of interphase transformers 240; a control element 250; and an output filter 230. Active transformers, or power converters for medium voltage, e.g., 13kV, may employ fast switching transistors and high frequency transformers that can provide a unity power factor. Applications for this type of power converter include: (a) unidirectional power flow from the grid to 240/480V 3-phase commercial/industrial equipment; (b) bi-directional power flow between the grid and electric vehicle charging equipment; and (c) bi-directional power flow between the grid and energy storage devices. Operationally, the active transformer accepts power from a 13kV, or similar, grid feeder line allow for bidirectional flow, and provides power to the commercial/industrial equipment, EV direct current (DC) charger, or grid energy storage device and allow for bi-directional flow. Bidirectional flow enables the energy storage devices to provide grid ancillary services such as frequency regulation, and, to be applicable for EVs providing vehicle-to-grid, V2G, services. A leading power factor can be provided to achieve VAR Compensation, i.e., to provide fast acting reactive power on a high- voltage electricity transmission network. Accordingly, losses within the grid distribution system can be reduced by the application of active transformers.

A DC fast charging station, e.g., a Level III fast charger, comprises an AC/DC converter and access to an AC line from which DC voltages may be generated, via the charger, to support direct EV charging. Fast charger products may range, for example, from 30kW to 500kW, and provide DC power at low voltage (48 to 96V) and medium voltage (250V and above). Fast charger products may allow EV charging in ten to thirty minutes, depending on the battery state of charge, battery type, and other factors. Fast chargers may include communication modules that provide, in addition to grid communication, operational data to a charger operator and other stakeholders via an Internet back office service.

EV chargers, both on-board the EV and off-board the vehicle, may be electronic- based charging systems that convert AC utility power into controlled DC power that then may be used to charge the EV battery pack. An EV charger typically comprises an external utility isolation transformer and a charging module. The charging module may be capable of operating from 3 -phase utility input voltage ranging from 400-600 VAC (50or 60Hz). In addition, the charging module may be capable of delivering up to 600Amps DC to charge battery packs up to 500VDC for passenger vehicles and higher voltages for heavy duty vehicles such as buses. For example, a 250kW charger may be capable of charging a 35kWh battery pack (0-1 00% SOC) in less than ten minutes. To meet future V2G infrastructure requirements, EV chargers may be configured to provide bi-directional power flow.

FIG. 3 depicts a top-level system block diagram of a power grid 301 where one or more of the phasor measurement units (PMUs) may be configured to output synchrophasors via, for example, land lines and or wireless links, having optional peaker turbines 303, and where a power grid control system 300 comprises a control processing unit 310 that has a processor and addressable memory (FIG. 1), where the control processing unit 310 may be configured, e.g., by loaded computer-readable instructions and/or data of the addressable memory, to receive feedback signals 31, via a wired 371 and/or one or more wireless channels 372,373 from phasor measurement units (PMUs) 321 of active transformers 320 of a power grid 301, e.g., at the substation grid level; and feedback signals from PMUs 331of power grid substations 330. The control processing unit 310 may be further configured to generate commands 360 for or more elements of the grid 301, and provide control signals, via a wired link 312 or a wireless link 313, to grid-level energy stores 302, each being configured, responsive to a control signal 312,313 from the control processing unit 310, to draw power from, and provide power to, the power grid 301. The control processing unit 310 may be further configured to provide control signals 312,313 to electric vehicle chargers 341 configured, responsive to control signals 312,313 from the control processing unit, e.g., of one or more generated commands, to draw power from, and provide power to, the power grid 301. Energy stores 350 may be disposed at the substation grid level. The control processing unit 310 may be further configured to provide control signals 312, 313 from the control processing unit, e.g., of one or more generated commands, to electric vehicle chargers 342 having an energy store where the electric vehicle chargers may each be configured, responsive to control signals from the control processing unit, to interrupt drawing power from the power grid and to resume drawing power from the power grid. The generated command signals, and/or the control signals, of the power grid control system may be based on a difference between at least one PMU feedback signal indicative of power grid cycle frequency and a reference frequency, that, for example, may be ascertained from the addressable memory or from a remote source. The nodes of the exemplary grid network may communicate via any one, or a combination of, the following: wired, e.g., plain old telephone service (POTS), wireless; cellular; Ethernet, and/or power line communications. Simultaneous EV fast charging may negatively impact the grid. For example, by placing multiple, eight to ten, 250kW loads, or DC fast chargers, on a remote distribution feeder line, those loads could induce voltage sag and enhance grid congestion.

The effect on the grid of an array of EV fast charging stations will be dependent on the utility system site and that compensation techniques may be necessary. A 24.9kV line location may demonstrate voltage sag with more than three EVs charging simultaneously, and a 13.2kV line location may demonstrate only a single EV may be charged with negligible grid impact. FIG. 4 depicts a grid system 400 where a local grid 403 is drawn, via a local transformer 402, from the grid 401, and where, at the local grid 403, a charging station 420 may be configured or converted to a bank 420 of eight 250kW fast chargers 412-428.

Without assistance from compensation techniques such as ramp up, an onsite energy storage facility, or interlock strategies, eight EVs, such as the EV depicted as 430, could be charged simultaneously with minimal voltage sag impact.

A 13.2kV distribution line, without the use of compensation techniques, may readily handle multiple CHAdeMO 50kW EV fast chargers at a single location, while simultaneously charging vehicles at full power. EV fast chargers typically ramp up power levels rather than supply instantaneous power. This alleviates short time scale grid shocks. They are also designed with the capability to continuously communicate with the grid. If a problem occurs with the distribution line the utility could command the EV charger(s) to ramp down in power level, or shut down completely as a form of demand response.

For weak grid distribution lines, charging schemes that include battery storage between the grid and the charger bank, may provide a buffer, and thereby further reduce the potential for adverse grid impacts like voltage sag. Utility control, coupled with a high peak use rate structure, works to modify consumer behavior, and may lower or minimize potential adverse grid impacts from fast charging. For grid services, including compensation for distribution lines, the energy storage technology should have a high round trip efficiency, a high power to energy ratio, and a system life of at least ten years. Flow batteries are often mentioned and are in use for many demonstration projects, but have relatively low round trip energy conversion efficiency and a low power-to-energy ratio. They may be better suited to peak shifting applications. Lithium batteries, of which several chemistries are on the market, are also the subject of many demonstration projects. They exhibit extremely high round trip efficiency. Two specific lithium chemistries, iron phosphate and lithium titanate (titanium oxide), have high power-to- energy ratios, high cycle life and long calendar life. Lithium titanate, if proven cost-effective, appears ideally suited to distribution line compensation.

Electric utilities monitor grid activities with supervisory control and data acquisition (SCAD A) techniques, which provide data not normally useful in fast responses for controlling grid problems because the sample rates are low and the measurements are not synchronized. SCADA techniques are used to monitor local areas.

While SCADA techniques are used for local monitoring, synchrophasor

measurements allow grid operators to apply static VAR (reactive power, volt-ampere reactive) compensation (SVC), protective relay control and other system asset control. This also allows identification of assets that can respond in real-time to local grid problems. For existing grid assets with phasor measurement technology, synchrophasor functionality may be added to provide increased control. One may include synchrophasor functionality via, e.g., exemplary instrumentality and functionality such as GPS (for location), time synchronicity (standard IRIG-B format time code), and voltage and current inputs/filtering to an EV fast charger/transformer systems, e.g., multiple charger systems, have the potential to turn a traditional grid load into a valuable compensation asset. DC fast chargers may be configured to continuously or continually communicate with the grid. FIG. 5 depicts a local feeder line control 510 in communication with a node 520 collecting synchrophasors of a power grid 530 from PMUs 570,751. Power is depicted as proved via active transformers 541, 542 to one or more electric vehicle charging station 550, and optionally to a grid energy storage unit 560. Given the ability of future Smart Grids to monitor health via synchrophasors, if a feeder line were to exhibit congestion or other issue as may be signaled via a PMU 573 and/or a grid level PMU 572, and processed by the local feeder line controller 510; the grid controller 510 may signal a fast charger 581 to reduce charge power level or cease operation altogether as depicted in FIG. 5. Electric utilities typically use Demand Response as a more active technique than

Time-of-Use (TOU) to actually curtail loads, when necessary, to provide grid stability. Time- of-Use pricing allows electric utilities to price electricity according to the time of day and value of grid electricity at the time. Electric utilities have, and are in the process of applying the TOU pricing technique to modify the behavior of residential, commercial and industrial customers. This technique could also apply to EV drivers, where the electric utility could price a fast charge event at several times the dollar per kilowatt hour rate during a late summer afternoon peak use period versus six in the morning. It is contemplated that various combinations and/or sub-combinations of the specific features and aspects of the above embodiments may be made and still fall within the scope of the invention. Accordingly, it should be understood that various features and aspects of the disclosed embodiments may be combined with or substituted for one another in order to form varying modes of the disclosed invention. Further it is intended that the scope of the present invention is herein disclosed by way of examples and should not be limited by the particular disclosed embodiments described above.

Claims

CLAIMS: What is claimed is:

1. A power grid control system comprising:

a control processing unit comprising a processor and addressable memory; the control processing unit configured to receive a feedback signal from at least one of:

(a) a first phasor measurement unit (PMU) of an active transformer of the power grid; and

(b) a second PMU of power grid substation;

wherein the control processing unit is further configured to generate a command, based on the feedback signal, for at least one of: (i) a grid-level energy store configured to draw power from the power grid and to provide power to the power grid; (ii) a first electric vehicle charger configured, to draw power from the power grid, and provide power to the power grid; and (iii) a second electric vehicle charger, having an energy store, the second electric vehicle charger configured to interrupt drawing power from the power grid and to resume drawing power from the power grid.

2. The power grid control system of claim 1 wherein the first PMU may be configured to output one or more synchrophasor measurements.

3. The power grid control system of claim 1 wherein the second PMU may be configured to output one or more synchrophasor measurements.

4. The power grid control system of claim 1 wherein the control processing unit is further configured to generate the command based on a difference between at least one PMU feedback signal indicative of a power grid cycle frequency and a reference frequency.

5. A power grid control system comprising:

(a) a first phasor measurement unit (PMU) of an active transformer of a power grid; and

(b) a second PMU of power grid substation; wherein the control processing unit is further configured, based on the feedback signal, to provide a control signal to at least one of: (i) a grid-level energy store configured, responsive to a control signal from the control processing unit, to draw power from, and provide power to, the power grid; (ii) a first electric vehicle charger configured, responsive to a control signal from the control processing unit, to draw power from, and provide power to, the power grid; and (iii) a second electric vehicle charger having an energy store, the second electric vehicle charger configured, responsive to a control signal from the control processing unit, to interrupt drawing power from the power grid and to resume drawing power from the power grid.

6. The power grid control system of claim 5 wherein the first PMU may be configured to output one or more synchrophasor measurements.

7. The power grid control system of claim 5 wherein the second PMU may be configured to output one or more synchrophasor measurements.

8. The power grid control system of claim 5 wherein the control signal is based on a difference between at least one PMU feedback signal indicative of a power grid cycle frequency and a reference frequency.