WO2016176628A1

WO2016176628A1 - Controller for micro-grid generator and renewable power and method of use

Info

Publication number: WO2016176628A1
Application number: PCT/US2016/030220
Authority: WO
Inventors: Sandeep Agarwal; Kartik Somashekar VASUDEV; Andras Boross; Rasool AGHATEHRANI
Original assignee: Sunedison, Inc.
Priority date: 2015-04-29
Filing date: 2016-04-29
Publication date: 2016-11-03

Abstract

An example micro-grid includes a load meter, a renewable-source power generation device, and a generator bank. The load meter is coupled between a load and a distribution bus and is configured to measure a load value representing total power delivered to the load. The renewable-source power generation device is coupled to the distribution bus and is configured to supply a first power to the load. The generator bank is coupled to the distribution bus and is controllable according to the load value to supply a second power to the load. The first power and the second power sum to the total power delivered to the load.

Description

CONTROLLER FOR MICRO-GRID GENERATOR

AND RENEWABLE POWER AND METHOD OF USE

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Patent Application Serial No. 62/154,370, filed April 29, 2015, which is hereby incorporated by reference in its entirety.

FIELD

[0002] This disclosure generally relates to micro-grid installations of generators and renewable power sources and, more specifically, to a controller for generators and renewable power sources in a micro-grid.

BACKGROUND

[0003] Facilities, sites, and buildings are typically served by a local utility's electric grid. These facilities, when described in terms of their electrical loads, often include components considered critical and others considered non-critical. Facility owners and operators generally utilize a back-up energy source to ensure service to at least the critical loads is maintained in the event power from the grid is lost. Back-up energy sources include generators, energy storage devices, and renewable energy sources, such as solar, hydroelectric, and wind systems. Generators are typically fossil fuel based and can include a single generator or a bank of generators with a controller. A distribution system for back-up energy sources is referred to as a micro-grid. More specifically, a micro-grid typically supplies a site that wants to continue normal operation when grid power is lost. A micro- grid can also serve a site that is rarely or never served by the grid, such as, for example, a marine vessel, ship, remote village, mine, island, or resort.

[0004] A micro-grid generally includes a load, a distribution bus, and at least one back-up energy source that connects to a load. If a utility grid is available, the energy source connects to the load through a transfer switch. During normal operation under grid power, the transfer switch generally disconnects the micro-grid from the load. The transfer switch connects the micro-grid to the load upon loss of grid power.

[0005] This Background section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present disclosure, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure.

Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art .

BRIEF DESCRIPTION

[0006] One aspect is directed to a micro-grid for serving a load. The micro-grid includes a load meter, a renewable-source power generation device, and a generator bank. The load meter is coupled between a load and a distribution bus and is configured to measure a load value representing total power delivered to the load. The renewable-source power generation device is coupled to the distribution bus and is configured to supply a first power to the load. The generator bank is coupled to the

distribution bus and is controllable according to the load value to supply a second power to the load. The first power and the second power sum to the total power delivered to the load.

[0007] Another aspect is directed to a method of controlling a micro-grid that serves a load . The method includes measuring a value of the load that represents total power delivered to the load. The method also includes serving a portion of the load by a renewable-source power generation device, thereby leaving a remaining portion unpowered. The method also includes commanding, according to the value, a generator bank to serve the remaining portion .

[0008] Various refinements exist of the features noted in relation to the above-mentioned aspects. Further features may also be incorporated in the above- mentioned aspects as well. These refinements and

additional features may exist individually or in any combination. For instance, various features discussed below in relation to any of the illustrated embodiments may be incorporated into any of the above -described aspects, alone or in any combination.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] Figure 1 is a block diagram of an electrical power system for a facility;

[0010] Figure 2 is a block diagram of one embodiment of a micro-grid for serving a load; and

[0011] Figure 3 is a flow diagram of one embodiment of a method of controlling a micro-grid. [0012] Like reference symbols in the various drawings indicate like elements.

DETAILED DESCRIPTION

[0013] The embodiments described herein generally relate to micro-grids. More particularly, embodiments described herein relate to apparatus, methods, and systems for controlling micro-grids including

photovoltaic and generator power sources.

[0014] FIG. 1 is a block diagram of an electrical power system 100 for a facility. The facility is associated with a load 110. Load 110 is serviced by a utility grid 120 and, for example, if utility grid 120 is lost, a micro-grid 130. Utility grid 120 and micro-grid 130 are coupled to load 110 by a transfer switch 140. During normal operation when utility grid 120 is available, transfer switch 140 couples utility grid 120 to load 110 and disconnects micro-grid 130. When utility grid 120 is lost, transfer switch 140 disconnects utility grid 120 from load 110 and couples micro-grid 130 to load 110, providing service to at least critical components of load 110, if not all of load 110, enabling normal operation while utility grid 120 is down. Transfer switch 140 is an automatic switch that detects whether utility grid 120 is available or down and makes and breaks the connections among load 110, utility grid 120, and micro-grid 130 accordingly. In an alternative embodiment, transfer switch 140 is a manual switch that requires an operator to recognize the loss of utility grid 120 and control the transfer from utility grid 120 to micro-grid 130, connecting micro-grid 130 to load 110. [0015] FIG. 2 is a block diagram of one embodiment of a micro-grid 200 for serving a load 202. Load 202 is an alternating current (AC) load that may include critical components and non-critical components. Load 202 is a variable load characterized by a maximum power draw, referred to as 100% load or simply 1. In alternative embodiments, load 202 is a direct current (DC) load.

[0016] Micro-grid 200 includes a generator bank 203 including generators 204, a photovoltaic (PV) cell array 206, a generator bank meter 208, a PV meter 210, and a load meter 212. PV cell array 206 couples to micro-grid 200 through an inverter 214 and a relay 216. Generators 204 are respectively controlled by controllers 218. Micro-grid 200 is controlled by a controller 220.

[0017] Power is delivered by micro-grid 200 to load 202 through a distribution bus 222. Distribution bus 222 is an AC bus. In alternative embodiments, distribution bus 222 is a DC bus and micro-grid 200 further includes suitable AC/DC and/or DC/AC converters.

[0018] Generators 204, as a group, are referred to as generator bank 203. The generator bank for a micro-grid is typically sized for a power output 25-30% higher than maximum load to ensure the load is not

compromised under any condition, including transient and steady state conditions. The generator bank including generators 204 is rated at a maximum power output 30% higher than the maximum load of load 202, or 1.3 times the maximum load of load 202. The maximum power output of generator bank 203 varies according to the actual

generators installed and the actual loads to be powered. The amount of extra capacity designed into the generator bank also varies per installation, depending on the acceptable level of risk of an outage.

[0019] Each of generators 204 also has a minimum power output level, below which operation may damage the generator or at least significantly reduce efficiency. Each of generators 204, when operating, has a minimum power output of 30% of its maximum rated power output. In alternative embodiments, generators 204 can each operate safely and economically below 30%. When all of generators 204 are running, generators 204, as a whole, have a minimum power output of roughly 40% of the maximum load of load 202 (1.3 x 0.3 = 0.39 times maximum load) . The generator bank including generators 204 may operate below 40% of the maximum load of load 202 if one or more of generators 204 are off. However, a consequence of powering off generators is that 10s of seconds may be needed to power on and spool up a given generator in the event or a sudden and dramatic load increase. The minimum power output of a generator can be reduced through the use of

improvements such as variable frequency drives .

[0020] PV cell array 206 converts solar energy into a DC current. The generated DC current is passed to inverter 214 to generate an AC current that can be provided to distribution bus 222 through relay 216. Generally, PV cell array 206 is sized such that maximum power output of PV cell array 206 does not exceed the difference between the maximum load, which is typically the daytime load, and the minimum power output of generator bank 203 for those conditions of load and PV power. PV penetration is defined as the percentage of load that is supplied by PV cell array 206. In the case of a single generator, the size of PV cell array 206 is limited to 60% of the peak daytime load, given that 40% of the load is supported by the single generator due to its minimum loading constraints. When there are multiple generators, some can be turned off during the day, permitting PV cell array 206 to exceed 60% depending on the acceptability of complete or partial loss of power in the event of a cloud.

[0021] PV penetration is theoretically limited to 100%, but that level of PV penetration carries the risk of compromising the load in the event of a cloud. PV penetration is further practically limited by safety precautions, such as protecting against power flow from PV cell array 206 into the generators in the event of a load drop. The limit of a particular PV cell array is determined through detailed simulations specific to the exact models of generators and PV inverters used.

[0022] In alternative embodiments, PV cell array 206 has a maximum power output greater than 60% of maximum load, which can be achieved through various improvements in the generator bank, such as variable frequency drives that allow generators to operate at low output with high efficiency.

[0023] The actual power outputs, during operation, of generators 204 and PV cell array 206 sum-up to the actual value of load 202. As load 202 and power ouput of PV cell array 206 vary, so does the power output of generators 204. In certain known micro-grids, generator bank meter 208 senses the value of load 202 and provides that measurement to controllers 218, which modify the operation of generators 204 to generate the appropriate amount of power. Generator bank meter 208 may include a meter for each of generators 204, the some of which is the total load sensed by at generator bank 203. When PV cell array 206 is generating power, generator bank meter 208 senses a value of load 202 that is less than the actual value of load 202. More specifically, the value of load 202 sensed by generator bank meter 208 is the actual value of load 202 minus the power generated by PV cell array 206. Generator bank meter 208 thereby senses both fluctuations in load 202 and fluctuations in the power output of PV cell array 206.

[0024] Here, load meter 212 senses the actual value of load 202. In contrast, generator bank meter 208 senses load 202 minus the output of PV-cell array 206. The load value sensed by generator bank meter 208 is referred to as a virtual load value. A virtual load signal 224, generated by generator bank meter 208, is provided to controller 220. Likewise, a load signal 228, generated by load meter 212, is passed to controller 220. Similarly, a PV load signal 226, generated by PV meter 210, is provided to controller 220. Controller also reads a relay state 230 of relay 216 and an inverter output 232 of inverter 214. Controller 220 controls relay 216 by a relay control line 234 and controls inverter 214 by an inverter control line 236. Controller 220 also controls controllers 218 and generators 204 by a generator control line 238.

[0025] Controllers 218 receive the value of load 202 as measured by load meter 212 directly via a load line 240, rather than the load value measured by generator bank meter 208. Controllers 218 use the value from load meter 212 in evaluating generator pickup/drop-off

thresholds . [0026] Controller 220 configures micro-grid 200 such that all of generators 204 that are online are operating at least at minimum power output. As the power output of PV cell array 206 fluctuates, as indicated by inverter output 232, controller 220 commands generators 204 to increase or decrease power output to meet the actual demand of load 202, as measured by load meter 212. By having all of generators 204 operating at least at minimum power output, controller 220 creates a spinning reserve, effectively eliminating the startup time of a powered off generator that must be brought online to meet demand.

Controller 220 avoids shutting down one or more of

generators 204 when generator bank meter 208 senses a low- load condition due to a high power output of PV cell array 206, although load 202 may actually be large. Controller 220 achieves this without use of an energy storage device, such as a battery, to provide interim power. Micro-grid 200 is configured to operate without an energy storage device to, for example, reduce the cost of the micro-grid 200, reduce the space needed for the micro-grid 200, and/or to reduce the amount of work needed to maintain the micro-grid 200. For example, systems that use batteries as an energy storage device are more expensive due to the need to purchase the batteries, require additional space to house the batteries, and require additional effort to properly maintain the batteries. Use of energy storage does, however, facilitate PV penetration beyond 100%.

[0027] For example, assume load 202 is at 1 and power output of PV cell array 206 is at 0.7. Generator bank meter 208 would sense a load of 0.3, or the difference between load 202 and the output of PV cell array 206.

Conventionally, one or more of generators 204 may be shut down to avoid damage or inefficiencies incurred by operating below the minimum power output. If the output of PV cell array 206 were to unexpectedly drop, for example, when a cloud moves through, generators 204 must increase output to meet the demand of load 202. In this example, assume power output of PV cell array 206 drops from 0.7 to 0.1. Given a load of 1, generators 204 must increase output from 0.3 to 0.9. To meet the demand, generators that were shut down may have to be brought online and running generators are spun up. The time needed to power on and spin up generators 204 can lead to brown-outs and possibly black-outs for load 202.

[0028] In the example above, controller 220 receives load signal 228 from load meter 212 and ensures enough of generators 204 are spun up to meet the 1 demand level of load 202. Controller 220 commands all of

generators 204 to operate at least at minimum output, which generates an output of 0.4. The remaining portion of load 202, which is another 0.6, is powered by PV cell array 206 or generators 204, depending on the power output by PV cell array 206. Thus, even if the power output by PV cell array 206 drops to zero, or near zero, generators 204 would be capable of increasing output from 0.4 to 1.

[0029] Improvements to PV penetration can be made using forecasting techniques whereby a controller ascertains, for example, the PV array will not drop more than some amount of power output over the next 10 seconds. PV penetration and cloud resiliency can also be improved through use of flywheels, ultra-capacitors, high DC-AC ratios, and trackers with backtracking.

[0030] In certain embodiments, based on load signal 228 measured by load meter 212, controller 220 commands one or more generators 204 to power down when the value of load 202 is sufficiently low. For example, if load 202 is 0.1, or 10%, one or more of generators 204 may be shut down, because the remaining online generators can generate sufficient power to meet any spike in demand from load 202. While the number of generators 204 running is controlled based on load signal 228 generated by load meter 212, the output of generators 204 is controlled based on load signal 228 and the output of PV cell array 206, or virtual load signal 224.

[0031] FIG. 3 is a flow diagram of one embodiment of a method of controlling a micro-grid. The method begins with step 310. At a measuring step 320, a value of a load is measured. The load can be measured by a load meter coupled between the load and a distribution bus of the micro-grid. The load varies over time.

[0032] At 330, a portion of the load is served by a renewable-source power generation device. The

renewable-source power generation device may include a photovoltaic cell array, a wind generator, hydroelectric generator, or other suitable renewable power generation source. The remaining portion of the load is not powered by the renewable-source power generation device. The power generated by the renewable-source power generation device can vary over time.

[0033] At 340, a generator bank is commanded to generate a certain amount of power for serving the remaining portion. As the power generated by the renewable- source power generation device declines, the generator bank must increase production to meet the demand of the load. Also, as the load varies, the power generated by both the generator bank and the renewable -source power generation device increase and decrease such that the sum of the generated power equals the power demand of the load.

[0034] The commanded output level is determined according to the value of the load. The demand of the load, as sensed by the generator bank, is reduced by the power generated by the renewable-source power

generation device. However, controlling the generator according to the load sensed at the generator results in an under-capacity in the event the power generated by the renewable-source power generation device reduces sharply.

[0035] For example, if power generated by the renewable-source power generation device is high and the load is at 75% of maximum, the generator bank may only detect a fraction of the 75% load and command a portion of the generator bank to shut down. If the power generated by the renewable-source power generation device reduces sharply, the generator bank would lack the reserve capacity in its running generators to meet the 75% load, resulting in a brown-out and possibly a black-out.

[0036] Determining the output level of the generator bank according to the measured load results in sufficient reserve capacity in the running generators. The generator bank is commanded to run at least at minimum capacity, facilitating an increase in power production in the event the renewable -source power generation device is lost or its output is reduced. The method then returns to measuring step 320 at a repeat step 350.

[0037] An exemplary technical effect of the methods, systems, and apparatus described herein includes at least one of: (a) ability to operate generators in optimum generation range; (b) improved running-reserve generator capacity; (c) increased portion of load served by renewable energy sources, such as solar or wind; (d) improved transition time between renewable energy source and generator bank when power output of renewable reduces sharply; and (e) improved reliability of micro-grid.

[0038] This written description uses examples to disclose various embodiments, which include the best mode, to enable any person skilled in the art to practice those embodiments, including making and using any devices or systems and performing any incorporated methods . The patentable scope is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial

differences from the literal languages of the claims.

[0039] When introducing elements of the present invention or the embodiment (s ) thereof, the articles "a", "an", "the" and "said" are intended to mean that there are one or more of the elements. The terms "comprising", "including" and "having" are intended to be inclusive and mean that there may be additional elements other than the listed elements.

[0040] As various changes could be made in the above without departing from the scope of the invention, it is intended that all matter contained in the above

description and shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.

Claims

WHAT IS CLAIMED IS:

1. A micro-grid for serving a load, comprising:

a load meter coupled between the load and a distribution bus, and configured to measure a load value representing a total power delivered to the load;

a renewable-source power generation device coupled to the distribution bus and configured to supply a first power to the load; and

a generator bank coupled to the distribution bus and controllable according to the load value to supply a second power to the load, wherein the first power and the second power sum to the total power delivered to the load.

2. The micro-grid recited in Claim 1 further comprising a controller coupled to the load meter and the generator bank, wherein the controller is configured to:

receive the load value from the load meter; and control the generator bank according to the load value .

3. The micro-grid recited in Claim 1 wherein the controller is further configured to:

control a number of running generators in the generator bank according to the load value; and

control an output power of the running generators according to the total power minus the first power.

4. The micro-grid recited in Claim 1, wherein the renewable-source power generation device comprises a photovoltaic cell array.

5. The micro-grid recited in Claim 4 further comprising an inverter and a relay coupled between the photovoltaic cell array and the distribution bus.

6. The micro-grid recited in Claim 5 further comprising a controller coupled to the inverter and the relay, wherein the controller is configured to:

monitor respective states of the inverter and the relay; and

respectively control the inverter and the relay.

7. The micro-grid recited in Claim 1, wherein the generator bank comprises a plurality of generators

respectively controlled by a plurality of controllers according to the load value.

8. The micro-grid recited in Claim 7, wherein each of the plurality of controllers is coupled to the load meter and configured to receive the load value.

9. The micro-grid recited in Claim 7, wherein each of the plurality of generators is configured to operate at least at a minimum power output .

10. The micro-grid recited in Claim 9, wherein the minimum power output of the plurality of generators is 30% of each generator's maximum rated power output.

11. The micro-grid recited in Claim 1, wherein a maximum rated power output of the renewable-source power generation device is at least 60% of a maximum value of the load .

12. The micro-grid recited in Claim 1, wherein the load is not served by an energy storage device.

13. A method of controlling a micro-grid that serves a load, comprising:

measuring a value of the load representing a total power delivered to the load;

serving a portion of the load by a renewable- source power generation device, thereby leaving a remaining portion unpowered; and

commanding, according to the value, a generator bank to serve the remaining portion.

14. The method recited in Claim 13, wherein

commanding the generator bank to serve the remaining portion comprises commanding generators of the generator bank to each operate at least at a minimum power output .

15. The method recited in Claim 13, wherein serving a portion of the load comprises converting solar energy into electrical energy for consumption by the portion of the load .

16. A micro-grid for serving a load, comprising:

at least one load meter coupled between the load and a distribution bus, and configured to measure a load value representing a total power delivered to the load;

a renewable-source power generation device coupled to the distribution bus and configured to supply power to the load; and

a generator bank coupled to the distribution bus and configured to supply power to the load.

17. The micro-grid recited in Claim 16 further comprising a controller coupled to the at least one load meter and the generator bank, wherein the controller is configured to:

receive the load value from the load meter; and control the renewable-source power generation device according to the load value.

18. The micro-grid recited in Claim 16 wherein the generator bank comprises a diesel generator.

19. The micro-grid recited in Claim 16 further comprising a relay coupled between the renewable-source power generation device and the distribution bus.

20. The micro-grid recited in Claim 19 further comprising a controller coupled to the renewable-source power generation device and the relay, wherein the

controller is configured to:

monitor respective states of the generator bank, the renewable-source power generation device, the load, and the relay; and

respectively control the renewable-source power generation device and the relay.

21. The micro-grid recited in Claim 16 wherein the renewable-source power generation device is sized to operate at a maximum capacity for a load value of at least the most commonly occurring day time load.

22. A method of controlling a micro-grid that serves a load, comprising:

measuring a value of the load representing a total power delivered to the load; commanding a renewable source to supply a maximum power without violating a minimum loading requirement of a generator bank; and

supplying remaining load requirements from the generator bank.

23. The method recited in Claim 22 further comprising commanding the renewable source to track power output to a varying load.