OF DISTRIBUTED GENERATION ASSETS

BACKGROUND

[001] Embodiments of tbs invent! ve subject matter relate to electric power systems and, more pariieufarly, to power grids utilizing distributed generators.

[002] During the last several years, there has been a fast-develpp g Rend to transform the grid energy production from central power stations to production using assets that are more distributed generation in nature. Fast growth of wind, solar, grid-tied energy storage and; flexible power generation plants is creating: a new grid interconnect dilemma.

[003] Traditional utilities used central power stations and were built as a generally unidirectional grid. In a unidirectional grid, protective devices (breakers, fuses, etc.) capable of handling and interrupting high currents were installed upsteeahi only, Moving downstream from the central power stations, available fault currents were reduced, and less robust protective: devices were required.

[004] In this traditional utility distribution: system:, only the load was downstream from the central power station. Therefore, the grid was built: such that, each step of the way with lower voltages and higher impedance closer to consumer. Therefore, the last leg ofpower distribution grid did not require expensive, high current capable infrasiructure, The capability of a protective device is: rated by its ability to safely interrupt fault currents and is expressed in Amperes Interapting Capacity (AIC).

[005] The addition of distributed generation (DG) close to fee las legs of the existing distribution grid ma change fault current dynamics of the grid. With the addition of a new generation Station, we can have higher than expected fault currents, which may compromise the safety of the system. The higher fault current may render the existing circuit protection devices ineffective In isolating a fault on the revised grid. Ά ith the increasing use of distributed generation, the grid may need to be more symmetrical, with high current capability interruption and protections at both ends of the system this may require the utility to evaluate fee grid and make upgrades for higher fault current capability close to the loads when adding distribute generation,

[006] Changes to fee existing gri can be costly an time consuming. Once fee changes have been identified, there may also be contention over who should fund the infrastructure upgrade. The utilities have tended to delay identify these changes, resulting in long approval delays. They also ten to assess high grid interconnect fees: to the distributed power generation developers to cover fire Infrastructure changes. The delays anc hgh fees can make a proposed distributed power generation project unprofitable and a non-starter.

[007] Some embodiments of the inventive subject ate provide a _: system incliid i ng a breale-before-make automatic: transfer switch (ATS) configured to selectively couple a utility grid and at least one distributed generator (e,g., a plurality of paralleled generators) to a load bus such that, in a state transition of the ATS, the utility grid arid the at least; one: distributed generator are both disconnected. from the load bus before the utility grid or the at least one distributed generator is connected to the load bus. The system thither includes at least one converter configured to be coupled to the load bus and configured to provide power ip the load bus during the state transition of the ATS . 1 n some embodiments, the at least one converter may be configured to provide power to the load bus: from the utility grid uring the state transition of the ATS. I some embodiments, the at least one converter may include a first converter having a first port configured to be coupled to the utility grid and a second converter having a first port coupled to a second port of the first converter by a DC bus and a second port configured to he coupled to the load bus. in some embodiments, the at least one _: converter may be bidirectional an support transfer of power from the at least one distributed generato to the utility grid.

[008] In some embodiments, the at least one converter may be configured to commence providing power to the load bus before the state transition when the utility grid is connected to the load bus via the ATS, to continue providing power id the loa bus through the state transition, and to gradually cease providing power to the load bus ate the state ttesition when the at least one distributed generator is connected to tile load bus via the ATS. The at least one converter may be further configured to provide _: power to the load responsive to an unavailability of file at least one distributed generator ate the: state transition.

[009 ] in further embodiments, the at least one converter may be configured to commence providing power to the loa bus before tire state transition while the at least one distribute generator is connected to the load bus via the ATS, to _: continue providing power to the load bus through the state transition, and to gradually cease providing power to the load bus ate the State: transition when the utility grid is connected to the load bus via the ATS.

[0010] Additional embodiments provide a system including at least one distributed generator, a switch configured to connect the at least one generator to a load bus, and a converter coupled between a utility grid and the; load bus and configured to provide power to the load bus from the utility grid while the at least one: distributed generator is connected to the load bus and providing power thereto.

[0011] Still further embodiments provide system including a first switch configured to couple a utility grid to a load bus via an inductor, _: a second switch configured to be coupled at least one distributed generator to the load bus, and at least one converter configured to be coupled _: to the load bus and configured to provide power thereto to support a first mode In which the first and second switches are closed, a second mode in: which the first switch Is Open and die second switch is Closed and a third mode in which the first switch is closed and the second switch is open. The at least one converter may be configured to support bidirectional power transfers between the utility grid and the load bus in the first mode. The: at least one converter may be configured: to provide voltage and transient load response support in the second and third modes.

BRIEF ESCRIPTION OF THE DRAWINGS

[0012] FKSl 1 is a schematic diagra illustrating a soft grid interconnection system according to some embodiments.

[0013] nC . 2-4 illastrate _: example operations of the system of FIS. 1.

[0014] FIG 5 is: schematic diagram illustrating a soft grid interconnection syste according to some embodiments.

[001:5] FIG 6 illustrates _: the system of FIG. 1 with additional energy storage.

[0010] FIG 7 is a schematic _^ diagram illustrating a soft grid interconnection system according to some embodiments.

[0017] FIG. 8 is a seheniatic diagram ilinstxaiing a soft grid interconnection system with.a current-limiting impedance according to some embodiments. DETAILED DESCRIPTION

[0018] Specific exemplary embodiments: of the inventive subject matter will be describedwith reference: to the acco panying drawings, This inventive subject matter may, however, be embodied in many different forms and should not be eonstmed ,as limited to the embodiments _: set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will folly convey the scope of the inventive subject matter to those shilled in tire art. In the drawings, like numbers refer to like items. It will be understood that when an item is referred to as; being "connected" or "coupled" to another item, it can be directly connected or coupled to the other item o Intervening items may be present. As used herein the term "and/or" incl udes any and all combinations of one of more of the associated listed items,

[0019 j The terminGl og used herein is for the purpose of describing particular embodiments only and is not Intende to be limiting of the inventive subj ect matter. As used herein* the singular forms; "a", "an" and "the" are intended to include the plum! forms as well, unless expressly stated otherwise:. It will be further understood that the terms "includes," "comprises," "including” and/or ''comprising," when used in this specification, specify ^" the presence of stated features, Integers, steps, operations, items, and/or components, but do not preclude the presence or addition of one or more other features, integers steps, operations, items, components _: , and/or groups thereof

[Q02Q| Unless: otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which tills inventive subject matter belongs. It will be further understood that terms, such as those defined in commonl used dictionaries, should be interpreted as laving a meaning that is consistent: with their meaning in foe context ofthe specification arid the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein.

[0021] Some embodiments of the inventive subject matter can provide a workable solution to the above-mentioned challenges by removing the need for grid side substation upgrades as a result of b iMing a new distributed power generation project. Some embodiments of the inventive: subjectmatter can achieve the following objectives to reduce or eliminate; negative impacts to the existing grid:; I ) BG power plant incoming power line short circuit current rating (AlCj does not significantly increase compared to the original installation capability before installing the (DG) device; 2 ) a symmetrical uninterrupted transfer capability is provided between grid mode and DG mode: (meet base line power quality, such as ITIC-GBEMA, if desired.}; 3) die cost of the soi¾” transfer described provi es an economically feasible business case for the distributee power generation project vs. paying grid infrastructure upgrades or high variable energy rates; and/or 4} the up time of proposed solution meets and exceeds the grid power availability.

ffi§22] As distributed generation is installed downstream In the existing: grid at a distance from the central power station generation, tliere is a concern that fault currents will exceed the interruption current levels of local protection devices:. As result, the utility companies commonly require prolonged evaluations of the grid in the vicinity of the proposed connection to the grid The result of the evaluation is most likely to include grid inifastructiite upgrades before the distributed generation can be connected to the grid. Once the grid infrastructure upgrades have been identified, the cost is typicall passed to the distributed generation developers in the form of high grid interconnect fees.

[0®23| Some embodiments of the inventive subject _: matter provide methods and systems for integrating distributed generation (DO) to an existing grid without the need for gri

infrasiructure upgrades and without unduly increasing the distribute generation power line short circuit current rating (AiC) compared to the original installation.

[0024] Referring to Figure 1, a grid 130 is connected to a load bus 10 through a first pole i lOa of a break-beiore-make Automatic Transfer Switch ( TS) 10 of a soft grid connection system. The soft gri connection: system further includes a solid state generator (SSG) power system ISO; having first and second bidirectional converters 152, 154 coupled via DC link 15:3. The input of the soft grid connection system is connected to the grid BO via the first: converter 152 and the output is connected to tire load bus 10 via the second converter 154. Distributed generation (DG) assets 140 a e _: coupled to a common Hue 120 and to the load bus 10 through a second pole 10fe of the break-hefbre-make ATS 1 10.

[0025] In a first mode where the first pole 1 :10a of the ATS 110 is open and the second pole I 10 b of the ATS is closed, the DG power plant 140 is operating and is supporting the load. To obtain a seamless transfer from the DG 140 to grid 130 _: with a break-before-make, the soft gri connection system takes power from fire grid 130 and supplies it to the load bus 10. Once the load is supported by tire grid 130 through the S5G system 150 of tire soft grid conneetion system, tlie ATS: 1 10 is commanded to transition to a state in which the second pole _: 110b ofithe ATS 110 is open and. then the first pole 110a of the ATS 110 is closed. During: the transition, the first and second poles 110a, IlOh are both open, thus preventing parallel connection of the grid I SO and the DO 140, while the $S<3 system 150 provides power to the load bus 10. After tire transfer, the SSG power system 150 ramps down its power output and allows the grid 130 to supply the load directly.

f0§2&] Wh fhe load bus IQ is being powered by the grid 130, the soft grid connection system starts delivering power to the load bus: [ft from the grid 130. The DG assets 140 are synchro ized with the load bus IQ but not connected. The ATS 1 10 is commanded to transfer .from the grid 130 to the DG assets 149 and responds by opening the first pole 110a of tlie ATS MO and then closing the second: pole 110b of the ATS 110. The : soft grid connection system will supply the load bus 10 from the grid 139 during the ATS transfer time. After the transfer is complete the soft grid connection system reduces the power supplied by the grid 130 and support of the l oad is transferred slowly to the DO assets G40 until the load bus 10 i s completely supported by the G assets 140. In this case, the DG assets 140 are never connected to the grid 130 in a directly coupled parallel configuration, in the event of a loss of pari of the DG assets 140, the soft grid connection system can supplement the DG assets 1 0 with power from the grid 130 without any irect connection between the grid 130 and DG assets common line 120,

[9027] Figure 6 illustrates a similar configuration except energy storage has been added at tile DC link 153 between the SSG converters 152, 154, either: directly or via a DC/DG converter (not shown). The energy storage may include ultra-capacitors, electro-chemical storage or a combination of both. This configuration can he used to buffer load transients: seen by the grid 130 and/or the DG assets 140, especially ii the load steps are asynchronous or repetitive and 5- 30% in magnitude.

[0028] Figure 7 illustrates a variation of the system of Figure 6, where an SSG 750 comprises a 4-quadreftt converter 752 configured to be connected to the load bus 10, In this configuration, electrochemical storage (e.g., a baftery) may be connected to a DC link 753 of the SSG 750, directly or via a DC/DC converter (not shown). This configuration may be desirable if the load steps are asynchronous or repetitive and 30- ί 00% in magnitude. This variation would enable provision: of a 100% island grid system with maximum power quality and energy

ft Figures 2, 3, and 4 illustrate the three modes of operation of the system of Figure L Figure 2 shows a transfer ofthe load &»i® the grid 130 to the DG assets 140. A command is received from the utility to shed load or conversely transfer the load to one or more: of the DG assets 140. The SSG 150 starts, and is placed in: standby mode, in a voltage/cnrrent (VI) support mod that provides voltage and transient load response support. The on or more DG assets 1 0 are started and are synchronized to the grid 130 The SSG 150 starts supplying the load using the grid 1 30 as a source. The ATS 1 10 is commanded to switch, resulting in the, disconnection of the grid 130 from the load: bus: 10. During this time, the SSG 1:50 supports the load, since, the second pole i 10b of the ATS 1 10 has not closed. The ATS 1 10 finally closes the second pole 110b, connecting the common line 120 to the output of the SSG 150. Once the connection is made, the SSG 150 ramps down its output power as the one or more DC assets 140 ramp up power supporting the load. After the SSG: 150 has transferred all the load support to the one, or more DG assets 140, the SSG 150 is placed in a standby, voltage and current (VI) support mode. |003b) FI gure 3 Hi ustraies transfer of the load fro one ox more of t he DG assets 140 to the grid 130. A command is received from the utility indicating that the load can be supporieci b the grid 130. The SSG 150 starts loading the gri 130 and supplying power to tire load. As the grid loading is ramped up, the loading of the one or more DG; assets 140 is ramped down until the load power comes from only from the grid 130. The ATS 110 is commanded to transfer the load from the one or more DG assets 1 0 to the grid 130, The ATS 110 first disconnects the common line 120 from the load bus 10 and the SSG 150 supports the load. Thereafter, the ATS 110 connects the grid 130 to the load and power output of the SSG ISO: is reduced as the grid 130 assumes the load directly. After the transfer is complete, the SSG 150 arid the ope or more DG assets 140 may be tamed off

0031] Figure 4 illustrates operations to support the load in the event one or more of the DG assets 140 fails or trips; off line when load is supported by the one or more DC assets 140. As described above, the _: SS 150 is operating in standby, VI suppotl mode. W:hen the one or more DC assets 140 becomes unavailable, the SSG 150 nearly Instantaneously provides power from the grid 130 to replace po wer that was lost by the loss of the one or more DC assets 140. The SSG 150 can continue supplying power to the load from the grid 130 or may retransfer the load back to the one or more DG assets 140 if restored or to % replacement one or more of the DC assets 140, The replacement DG asset(s) 140 may be ready and connected to the common line 120, so the SSG 150 can transfer the load to the replacement DG assetfs) by reducing the power taken from the grid 130. When power received from the grid reaches zero, the SS6 150 can resume operation in standby, ¥1 support mode,

}QQ32| Figure 5 illustrates a weak grid condition wherein the grid 130 cannot alone support the load. In this case, the BG assets 140 may be the primary power source for the load and the SSG 150 stabilizes the island grid frequency. Although the grid 130 cannot support tire entire load, it can provide redundancy for the DG assets 140 by providing power from the grid 130 via the soft grid connection system. The SSG 150 draws power from the grid. 130 and supplies it to iheToad bus 10, In this co figuration, the grid 130 is not parallele with the common line 120 (which Is coupled to the load bus 10 via a switeh 540), although _: the grid 130 can supply power on as needed basis up to the grid capability limit; This operational mode allows for redundancy if one or more of the DG assets 140 are removed, as the missing power to satisfy the loa requirements will be supplied by the grid 130 through the SSG 150 The SSG 150: provides a method to prevent DO export to the grid.

1003 J In foe above cases, there is no export of power to toe grid 130. However, in some modes, the SSG 150 could export power to the grid 130 an may not: exceed the original incoming power line short circuit rating (AIC) because of the current limiting capability of the SSG 150.

pG34| As discussed earlier, a concern of the utilities is the change in fault current dynamics with the addition of a DG asset close to the last legs of the existing _: distribution. Thi s concern can be mitigated if the avail able fault current remains at the same level before and after the addition of the DG installation. Figure 8 shows an arrangement similar to that of Figure I , but with a coupling impedance (inductor) 6 _. 10 placed between the gri 130 and load bus 10 when a: first switch 620 (K1 ) is closed, The impedance 610 is sized to limit the fault current to toe desired level (e.g , the impedance predDG installation}. The load bus 10: remains synchronized with the gri 130.

f0@35| Power flow between the grid: 130 and the load bus 10 can be controlled by control hng the voltage phase angle between them arid the ¥AR magnitude: between toe sources may be controlled by adjusting the amplitude difference, With two source of similar amplitudes and the first switch 620 ( l) and the second switch 640 K2) closed, _: power can flow from toe grid 130 and DG common line 630 can be controlled by adj usting the phase angle of the load bus 10 to lag the grid. 130, is., increasing the lag will increase the magnitude of power flowing from the grid .130 to the loa bus 10, Conversely, adjusting the phase angle of the load bus 10 to lead the grid 30 will cause power to Sow from: the load bus 10 to the grid ] 30, i.e.. increasing the lead will increase the amount of power flowing from the DG load bus 10 to the grid 130. When die phase an le _: is zero, there is no power flow between the two sources, The capability of the topology also extends to control of the volt-amperes reactive (VARs) that are exchanged between the two sources if the amplitude of voltage on the load bus 10 is increased in relation to the amplitude _: of the voltage o the grid 130, it will result in leading VARs and conversely if the amplitude of the voltage on the load bus 10 is lowered with respect to the amplitude of the voltage on tlie gri 130, the VARs will be lagging,

036] In the system illustrated in Figure 8, the gri 13.0 isconnected to a first terminal of coupling impedance 610 and the second terminal of the coupling impedance is connecte to the first switch 620 vl ). The soft grid connection system can include a SS system 650 having a bidirectional first and second converters coupled via DC link or just a bidirectional converter coupled lo a DC linh (e,g., similar to soft grid connection configurations in Figures 6 and 7, respectively) DG assets 140 are coupled to a common line 630 and a second switch 640 (K2).

In a fust mode, the second switch 640 (K2) is closed and the DG assets 140 support the load.

The SSG 650 is operated in V I support mode: wherein the SSG 650 controls the frequency of the load bus 10. The gri 130 can be coupled to the: load bu 10 by having the SSG 650 synchronize the load bus 10 to the grid 130 and then closing the first switch 620 (K 1). The SSG 650 can adjust the phase angle between the grid and the load bus to either export or import power fro the grid 130, As discussed above, the magrurude of power is controlled by the adjustment of the phase angle between the two sources in this mode, the qualit of the load bus 10 can be controlled by maintaining the- load bus 10 voltage independently of the grid voltage. The power is either delivered to or taken from the grid 130 independent of the magnitude of the grid voltage. The difference in voltage magnitude between the two sources results in VARs exchanged.

In a mode in which the DO assets 140 are disconnected, the SSG 650 can continue to support the lead bus 10. As long as the SSG 650 controls the voltage and phase angle with respect to the grid 130, power will be _: taken from: the grid 130 to support the load. The SSG 650 can also provide transient support for: step loads that may be encountered. It Is also possible to power the load directly by the grid 130 without the SSG 650 operating. This can be accomplished by shorting out the: coupling impedance 610 using a switch that bypasses the impedance 61:0 , which can avoid a drop in the voltage as a result of the line impedance 610. Similar to the variations illustrated in Figures 6 and 7, this configuration can include an SSG with t o converters and an energy store or an SSG with only one converter and an energy store, [0038| In this line interactive operating mode, the SSG operation will condition the load by providing voltage support and transient load response. With the impedance between the two sources, the fault current available at the grid connection will he controlled. It can he arranged that the first switch 620 (K1 ) he opened after number of fault cycles. When the first switch 620 (Kl ) is opened, the two sources will be isolated.