WO2015090375A1

WO2015090375A1 - Microgrid black-start

Info

Publication number: WO2015090375A1
Application number: PCT/EP2013/077025
Authority: WO
Inventors: Ritwik MAJUMDER
Original assignee: Abb Technology Ltd
Priority date: 2013-12-18
Filing date: 2013-12-18
Publication date: 2015-06-25

Abstract

The present invention relates to a method for black-start of a microgrid (1) comprising a plurality of distributed electrical generators (DG:s) and a plurality of loads, the method comprising: segmenting the microgrid to form a plurality of islanded network segments (4) which are electrically disconnected from each other, each segment comprising at least one of the DG:s and at least one of the loads; black-starting a first segment of the plurality of islanded network segments; connecting the first segment with a second segment, thereby electrically combining the first and second segments; power balancing the combined first and second segments with respect to power generated by the DG:s of the first and second segments and the loads connected to the combined first and second segments; and sequentially connecting and balancing any further of the network segment(s) to the combined first and second segments, thereby black-starting the whole microgrid. The invention also relates to a microgrid configured for performing the method.

Description

MICROGRID BLACK-START

TECHNICAL FIELD

The present invention relates to a method for black-start of a microgrid comprising a plurality of distributed electrical generators (DG:s) and a plurality of loads. The present invention also relates to a black-start capable microgrid comprising a plurality of distributed electrical generators (DG:s) and a plurality of loads.

BACKGROUND

A microgrid is a localized grouping of electricity generation, energy storage, and loads that normally operates connected to a traditional centralized grid via a point of common coupling. Microgrids are part of the structure for so called distributed generation (DG) aiming at producing electrical power locally from many small energy sources which may be called distributed generators (DG:s) or micro sources. In case of total voltage collapse, the microgrid needs a black start. Since all the micro sources do not have black-start capability and the voltage build up with load pick up is done in a step by step process, a well-planned black-start procedure must be in place for a remote or islanded microgrid.

SUMMARY

A microgrid may span over a moderate area and consist of different micro sources. Based on the controllability of the sources, the black start process may be initiated step by step with a smaller part of the microgrid. The black- start process may be coordinated through a microgrid central controller. In the absence of a central microgrid controller, each micro source or load may, in accordance with the present invention, participate in a segmented black- start process based on pre-defined rules and local measurements.

According to an aspect of the present invention, there is provided a method for black-start of a microgrid comprising a plurality of distributed electrical generators (DG:s) and a plurality of loads. The method comprises segmenting the microgrid to form a plurality of islanded network segments which are electrically disconnected from each other, each segment comprising at least one of the DG:s and/or at least one of the loads. The method also comprises black-starting a first segment of the plurality of islanded network segments, said first segment comprising at least one of the DG:s and at least one of the loads. The method also comprises connecting the first segment with a second segment, thereby electrically combining the first and second segments. The method also comprises power balancing the combined first and second segments with respect to power generated by the DG:s of the first and second segments and the loads connected to the combined first and second segments. The method also comprises sequentially connecting and balancing any further of the network segment(s) to the combined first and second segments, thereby black-starting the microgrid.

According to another aspect of the present invention, there is provided a microgrid comprising a plurality of DG:s and a plurality of loads. The microgrid is configured for being segmented to form a plurality of islanded network segments which are electrically disconnected from each other, each segment comprising at least one of the DG:s and/or at least one of the loads. The plurality of segments comprises a first segment which is capable of being black-started, and a second segment which is capable of being electrically combined with the first segment after the first segment having been black- started such that the combined first and second segments are capable of being power balanced with respect to power generated by the DG:s of the first and second segments and the loads connected to the combined first and second segments.

By segmenting the microgrid, the microgrid can be stepwise black-started, starting with the first segement and then combining it with the second and further segments in a suitable sequence for keeping a sufficient power balance throughout the black-start method until the whole microgrid is black-started. The first segment should be black-start capable, implying that it comprises at least one DG (also called micro source) capable of black-start, typically by being associated with an auxiliary power source, e.g. a battery, and a load which can be increased step-wise for balancing the DG till it reaches its full capacity (i.e. its full power generating capability). Typically, also the second segment comprises at least one DG and at least one load, but it is also contemplated that the second segment may comprise a DG and no load, or a load and no DG (e.g. be a fixed load). Since the power generating capacity of each DG in the microgrid may vary over time, especially for DG:s depending on wind or sunlight, the amount of power generated by a DG may vary over time why continuous measurements of the current power generation of the different segments may be needed for determining which segment should be chosen as the first segment and which segment should be chosen as the second segment in order to obtain sufficient power balance for enabling the black-start of the microgrid. The power balancing may not achieve perfect balance between the DGs and the loads, but should achieve sufficient balance for enabling the black-start of the microgrid to proceed as segments are sequentially connected to each other until all segments are reconnected and the whole microgrid is successfully black-started.

Predefined rules may be used for determining the sequence in which the segments should be connected to each other during the black-start. These rules may relate to power, e.g. voltage and/or frequency, measurements performed locally and in real-time on the different segments for compiling a priority list dictating the order in which the different segments should be sequentially black-started and/or connected to each other. Bearing in mind that not all segments may have black-start capability, some of the segments may need to be started by connecting them to other, already

energised/started, segment(s), e.g. in accordance with the priority list. Other considerations than the power balance may be relevant when determining the black-start sequence (the priority list), such as criticality of certain DGs or segments. Such critical segment(s) may be moved up the list to be started as early as possible during the black-start sequence.

Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to "a/an/the element, apparatus, component, means, step, etc." are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. The use of "first", "second" etc. for different features/components of the present disclosure are only intended to distinguish the features/components from other similar features/components and not to impart any order or hierarchy to the features/components. BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments will be described, by way of example, with reference to the accompanying drawings, in which:

Fig l is a schematic block diagram of an embodiment of a microgrid in accordance with the present invention. Fig 2 is a schematic block diagram of an embodiment of a microgrid segment with black-start capability, in accordance with the present invention.

Fig 3 is a schematic block diagram of another embodiment of a microgrid segment with black-start capability, in accordance with the present invention.

Fig 4 is a schematic flow-chart of an embodiment of black-starting a first segment with black-start capability, in accordance with the present invention.

Fig 5 is a schematic flow-chart of an embodiment of starting a second segment without black-start capability, in accordance with the present invention.

Fig 6 is a schematic flow-chart of an embodiment of combining a second segment without black-start capability with a black-started first segment, in accordance with the present invention. Fig 7 is a schematic block diagram of an embodiment of a black-started first segment being connected to a second segment without black-start capability, in accordance with the present invention.

Fig 8 is a schematic block diagram of an embodiment of a black-started first segment being connected to a second segment with black-start capability, in accordance with the present invention.

Fig 9 is a schematic flow-chart of an embodiment of the method of the present invention.

DETAILED DESCRIPTION

Embodiments will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments are shown.

However, other embodiments in many different forms are possible within the scope of the present disclosure. Rather, the following embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers refer to like elements throughout the description.

Figure 1 is a schematic block diagram of an example embodiment of a microgrid 1 in accordance with the present invention. The microgrid 1 is segmented into several network segments 4 by activating the black-start isolators 5 disconnecting the segments 4 from each other. A first segment 4a has black-start capability and comprises a black-start capable DG 2 and a step load 3. A second segment 4b does not have black-start capability and comprises a DG 2 without black-start capability and no load 3. A second segment 4c does have black-start capability and comprises a DG 2 with black- start capability and a step load 3. The figure 1 also shows a second segment 4d consisting of an islanded fixed load which does, of course, not have black- start capability.

The microgrid can thus be divided into multiple network segments 4 comprising DG(s) and load(s) by opening the dedicated isolators 5. Some segments have DG 2 with black-start capability (herein also called DGBS) and some may not have black-start capability (herein also called DCWBS). Based on power balance information about the segments 4 of the microgrid 1, a priority list of the segments 4 may be created, e.g. by means of one or several control units comprised in the microgrid 1. A segment 4 with high power balance (power generated by the DGs of the segment in relation to a load 3 which can be connected to the network segment) and which has black-start capability may preferably be chosen as the first segment for initiating the black-start of the microgrid 1.

Figure 2 is a schematic block diagram of an embodiment of a microgrid segment 4 with black-start capability, in accordance with the present invention. A segment 4 with black-start capability typically has some sort of back-up power 21 and/or power storage 22 allowing the segment to black- start (i.e. without available power from the grid). As is shown in figure 2, a DG 2 may be associated with an auxiliary power source 21 and/or a power storage 22 e.g. an uninterruptable power supply (UPS) such as a battery.

Figure 3 is a schematic block diagram of another embodiment of a microgrid segment 4 with black-start capability, in accordance with the present invention. Black-start of a segment 4 with black-start capability may be done with step-wise voltage and load build-up while the system frequency and terminal voltage is controlled by means of a control unit 31 of the segment 4.

Figure 4 is a schematic flow-chart of an embodiment of black-starting 92 a first segment with black-start capability, in accordance with the present invention. Figure 4 illustrates a black-start 92 of e.g. the black-start capable segment 4 in figure 3. The black-start capable DG 2 is associated with an auxiliary power source 21. A first step load 3 is connected to the segment, after which it is checked whether the voltage and frequency are stable. When the voltage and frequency are stable, the next step load 3 is connected to the segment and the process is repeated until the DG 2 is at full capacity, whereby the segment 4 is ready to be connected to be connected with a second segment. Figure 5 is a schematic flow-chart of an embodiment of starting a second segment without black-start capability, in accordance with the present invention. In contrast to the starting of a segment with black-start capability as set out with reference to figure 4, a segment without black-start capability may only be started with voltage and frequency from the network, e.g. from the already black-started first segment 4a. The second segment may e.g. be a fixed load 4d or a segment 4b comprising a DG 2 without black-start capability. The method illustrated in figure 5 may e.g. be performed as part of the power balancing 94 after the second segment has been connected to the first segment 4a. The power output of the DG 2 of the second segment is increased and the load 3 of the second segment is connected during continuous power balancing with the first segment, until the voltage and frequency are stable for the combined first and second segments when the DG 2 of the second segment is at full capacity. Figure 6 is a schematic flow-chart of an embodiment of combining a second segment 4b without black-start capability with a black-started first segment 4a, in accordance with the present invention. The black-start of the microgrid 1 starts with the first segment, as chosen based on available power balance. The choice of the second segment may be based on available power balance as well as on frequency and/or voltage stability. First, the first segment 4a is black-started 92 as in figure 4. Then, a second segment is connected 93 to the first segment. If it is determined, by a control unit, that it is possible to achieve power balance with a second segment 4b or 4d without black-start capability, then that second segment is connected 93 to the first segment and the power is balanced as in figure 5. If not, then a second segment 4c with black-start capability may be chosen to be connected 93 and power balanced 94 to the first segment 4a. Then, maybe the combined first and second segments are able to be connected to and balanced with the segment without black-start capability as a third segment. If not, a third segment with black- start capability may be connected instead etc.

Fig 7 is a schematic block diagram of an embodiment of a black-started first segment4a being connected to a second segment 4b without black-start capability, in accordance with the present invention. The operation sequence is shown with circled letters. It start with connecting the main blackstart breaker at A. Once the line is energised, the auxiliary switch (if present) is closed at B followed by the DG breaker at C. Once the DG 2 has started injecting power, the load switch is closed at D and ramped up step wise.

Fig 8 is a schematic block diagram of an embodiment of a black-started first segment 4a being connected to a second segment 4c with black-start capability, in accordance with the present invention The operation sequence is shown with circled letters. In the second segment 4c, the DG auxiliary power 21 is connected at A and when the DG 2 is started, the DG breaker is closed at B. Then, the loads are connected at C and step wise increased as illustrated by the arrow. The breaker to auxiliary power from the microgrid 1 is also connected at D. The voltage and frequency is matched to the measured value of the first segment 4a and finally the blackstart breaker connecting these two segments is closed at E.

Figure 9 is a more general flow-chart of an embodiment of the method of the present invention. The method is performed in a microgrid 1 comprising a plurality of DG:s 2 and a plurality of loads 3. The microgrid 1 is segmented 91 to form a plurality of islanded network segments 4 which are electrically disconnected from each other, each segment 4 comprising at least one of the DG:s 2 and/or at least one of the loads 3. The segmenting 91 may e.g. be in response to a grid failure which has left the microgrid shut down and in need of black-start due to a lack of connection to a macro grid or the like. Then, a first segment 4a of the plurality of islanded network segments is black- started, said first segment comprising at least one of the DG:s (2) and at least one of the loads 3. The first segment is black-start capable and may have been chosen based on predetermined rules, e.g. that it has the best available power balance of the segments 4 with black-start capability or that it is a more critical segment in the microgrid. Then, the black-started 92 first segment 4a is connected 93 with a second segment 4b, 4c or 4d, thereby electrically combining the first and second segments into a combined segment. The power is balanced 94 over the combined segment with respect to power generated by the DG(s) 2 of the first and second segments 4 and the load(s) 3 connected to the combined first and second segments. The process may then be repeated to add further segments 4 until the whole microgrid is black- started. In some embodiments of the present invention, the second segment does not have black-start capability. If a segment 4 of the microgrid 1 does not have black-start capability, it needs to be started with the help of other already black-started segment(s).

In some embodiments of the present invention, the black-starting 92 of the first segment 4a comprises power balancing the first segment up to the full power generation capability of the DG(s) 2 of the first segment, before the connecting of the first segment 4a with the second segment 4b, 4c or 4d. Thus, the DG of the first segment may be up and running fully before combining with further segments 4. In some embodiments of the present invention, the black-starting of the microgrid 1 further comprises, after the segmenting of the microgrid and before the black-starting of the first segment: obtaining, by a control unit 31, power balance information about the first segment; and determining, based on the obtained information, that the first segment can be black-started. In some embodiments, the determining comprises determining, based on the obtained information, that the first segment can be black-started with balancing of the first segment up to the full power generation capability of the DG(s) of the first segment.

In some embodiments of the present invention, the black-starting of the microgrid 1 further comprises, before the connecting of the first segment 4a with the second segment 4b, 4c or 4d: obtaining, by a control unit 31, power balance information about the second segment; and

determining, based on the obtained information, that the second segment 4b can be combined with the black-started first segment 4a and that the combined first and second segments can be power balanced.

The control unit 31 discussed herein may be a control unit comprised in each of the segments 4, e.g. associated with a specific DG 2 as shown in figure 3 and/or the control unit may be a central control unit of the microgrid 1. In some embodiments, the power balance measurements and setting up of the list may be done with a designated blackstart controller for each segment, but in decentralised embodiments the power output and power consumption of the DGs 2 and loads 3 may be broadcasted within the segment 4 and each DG 2 calculates the power balance for the segment 4a. The balance may then be broadcasted to the other segments 4b with blackstart ability for making the priority list. This priority list may then be used to control the blackstart isolators 5 in the microgrid 1. Below follow another aspect of the present invention.

According to an aspect of the present invention, there is provided a microgrid 1 comprising a plurality of distributed electrical generators (DG:s) 2 and a plurality of loads 3. The microgrid comprises means for segmenting the microgrid 1 to form a plurality of islanded network segments 4 which are electrically disconnected from each other, each segment 4 comprising at least one of the DG:s 2 and/or at least one of the loads 3. The microgrid also comprises means for black-starting a first segment 4 of the plurality of islanded network segments which first segment comprises at least one of the DG:s 2 and at least one of the loads 3. The microgrid also comprises means for connecting the first segment 4a with a second segment 4b, thereby electrically combining the first and second segments. The microgrid also comprises means for power balancing the combined first and second segments with respect to power generated by the DG:s 2 of the first and second segments 4 and the loads 3 connected to the combined first and second segments. The microgrid also comprises means for sequentially connecting and balancing any further of the network segment(s) to the combined first and second segments, thereby black-starting the microgrid l.

The present disclosure has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the present disclosure, as defined by the appended claims.

Claims

CLAIMS l. A method for black-start of a microgrid (l) comprising a plurality of distributed electrical generators, DG:s, (2) and a plurality of loads (3), the method comprising: segmenting (91) the microgrid (1) to form a plurality of islanded network segments (4) which are electrically disconnected from each other, each segment (4) comprising at least one of the DG:s (2) and/or at least one of the loads (3); black-starting (92) a first segment (4a) of the plurality of islanded network segments, said first segment comprising at least one of the DG:s (2) and at least one of the loads (3); connecting (93) the black-started (92) first segment (4a) with a second segment (4b; 4c), thereby electrically combining the first and second segments; power balancing (94) the combined first and second segments with respect to power generated by the I)G(s) (2) of the first and second segments (4) and the load(s) (3) connected to the combined first and second segments; and sequentially connecting and balancing any further of the network segment(s) to the combined first and second segments, thereby black-starting the microgrid (1).

2. The method of claim 1, wherein the second segment does not have black-start capability.

3. The method of claim 1 or 2, wherein the black-starting (92) of the first segment (4a) comprises power balancing the first segment up to the full power generation capability of the DG(s) of the first segment, before the connecting of the first segment (4a) with the second segment (4b; 4c; 4d).

4. The method of any claim 1-3, further comprising, after the segmenting of the microgrid and before the black-starting of the first segment: obtaining, by a control unit (31), power balance information about the first segment; and determining, based on the obtained information, that the first segment can be black-started.

5. The method of claim 4, wherein the determining comprises determining, based on the obtained information, that the first segment can be black-started with balancing of the first segment up to the full power generation capability of the DG(s) of the first segment.

6. The method of any claim 1-5, further comprising, before the connecting of the first segment (4a) with the second segment (4b): obtaining, by a control unit (31), power balance information about the second segment; and determining, based on the obtained information, that the second segment (4b; 4c; 4d) can be combined with the black-started first segment (4a) and that the combined first and second segments can be power balanced.

7. A microgrid (1) comprising: a plurality of distributed electrical generators, DG:s, (2); and a plurality of loads (3); wherein the microgrid is configured for being segmented to form a plurality of islanded network segments (4) which are electrically disconnected from each other, each segment (4) comprising at least one of the DG:s (2) and/or at least one of the loads (3); wherein the plurality of segments (4) comprises a first segment (4a) which is capable of being black-started, and a second segment (4b) which is capable of being electrically combined with the first segment after the first segment has been black-started such that the combined first and second segments are capable of being power balanced with respect to power generated by the DG:s (2) of the first and second segments (4) and the loads (3) connected to the combined first and second segments.

8. The microgrid of claim 6, wherein the microgrid (1) is configured for being segmented by means of comprising a plurality of black-start isolators, e.g. in the form of switches or circuit breakers, configured for electrically disconnecting the network segments (4) from each other.

9. The microgrid of claim 6 or 7, further comprising a control unit (31) configured for: obtaining power balance information about the first segment; and determining, based on the obtained information, that the first segment can be black-started; before black-starting the first segment (4a).

10. The microgrid of any claim 6-8, further comprising a control unit (31) configured for: obtaining power balance information about the second segment (4b); and determining, based on the obtained information, that the second segment (4b) can be combined with the black-started first segment (4a) and that the combined first and second segments can be power balanced; before electricaUy combining the second segment with the first segment.