SE1750063A1 - Black start of nested microgrids - Google Patents

Abstract

The present disclosure relates to an electrical power network () comprising a first microgrid (A), a second microgrid (B), and a network controller (6). Each of the first and second microgrids are delimited by paired circuit breakers (). The controller is configured to black start each of the first and second microgrids when islanded, by means of a black start source (A;B) in the microgrid. The controller is also configured to connect the two microgrids to each other by opening a circuit breaker (AB) there between. The controller is also configured to pick up a load () in the first microgrid (A) by means of the black start source (B) in the second microgrid (B).(Fig 6a)

Description

KALLSTART AV KOPPLADE MIKRONÄTVERKBLACK START OF NESTED MICROGRIDS TECHNICAL FIELD The present disclosure relates to an electrical power network comprising a first microgrid, a second microgrid, and a network controller.

BACKGROUND Black start of an electrical power microgrid refers to the start-up of anelectrical network from de-energized state. In a microgrid, with multipledistributed generators (DG) and loads, the black start is initiated by one ofthe black start capable sources in the microgrid. The black start processcontinues with connecting other DGs and loads while maintaining systemvoltage and frequency within bandwidths specified for black start. The systemcontrol plays a significant role for a stable and quick black start in a microgrid.

One way of accelerating the process is to start up multiple DGs first with black start and then synchronize them, followed by a faster load pick up.

An other alternative is to black start different segments of the microgrid simultaneously and then connect them together.

Both the above mentioned alternatives are based on power balance of theenergized electrical network and system stability is ensured with appropriate voltage/ frequency control during the process.

A single microgrid controls a limited number of distributed generators, loadsand with no or one connection to another grid, e.g. a power distribution gridat a Point of Common Coupling (PCC). In a nested microgrid scenario, withmultiple interconnected microgrids and controllers, the black start strategycan have various alternatives. The selection of the most suitable strategy willhave significant impact in system start up time, load pick-up and load sharing, system stability and oscillations in the power flows.

SUMMARY In a nested microgrid scenarios, some different methods may be combined to improve the performance: 1. Parallel start-up of different microgrid segments to achieve faster black start of the whole microgrid. 2. A quicker load pick up and load sharing may be achieved by faster generation ramping. 3. System stability may be ensured during black start with voltage and frequency control. 4. The microgrid system oscillations may be minimized during black start by appropriate resynchronization of the segments.

By a combination of the methods mentioned above the black start process innested microgrids can be improved in accordance with the present invention.However, the methods may need to be coordinated e.g. regarding cranking paths and switching sequences.

A new method for coordinating the black start process in nested microgrid scenario is hereby proposed, using a decentralized control structure.

According to an aspect of the present invention, there is provided anelectrical power network comprising a first microgrid, a second microgrid,and a network controller. Each of the first and second microgrids aredelimited by paired circuit breakers. The controller is configured to blackstart each of the first and second microgrids when islanded, by means of ablack start source in the microgrid. The controller is also configured toconnect the two microgrids to each other by opening a circuit breaker therebetween. The controller is also configured to pick up a load in the first microgrid by means of the black start source in the second microgrid.

It is to be noted that any feature of any of the aspects may be applied to any other aspect, wherever appropriate. Likewise, any advantage of any of the aspects may apply to any of the other aspects. Other objectives, features andadvantages of the enclosed embodiments will be apparent from the followingdetailed disclosure, from the attached dependent claims as well as from the drawings.

Generally, all terms used in the claims are to be interpreted according to theirordinary meaning in the technical field, unless explicitly defined otherwiseherein. All references to "a/ an /the element, apparatus, component, means,step, etc." are to be interpreted openly as referring to at least one instance ofthe element, apparatus, component, means, step, etc., unless explicitly statedotherwise. The steps of any method disclosed herein do not have to beperformed in the exact order disclosed, unless explicitly stated. The use of“first”, “second” etc. for different features/ components of the presentdisclosure are only intended to distinguish the features/ components fromother similar features / components and not to impart any order or hierarchy to the features / components.

BRIEF DESCRIPTION OF THE DRAWINGSEmbodiments will be described, by way of example, with reference to the accompanying drawings, in which: Fig 1 is a schematic block diagram of an embodiment of a microgrid in accordance with the present invention.

Fig 2 is a schematic functional block diagram of an embodiment of a microgrid control system in accordance with the present invention.

Fig 3 is a schematic illustration of an embodiment of segmented microgrids in accordance with the present invention.

Fig 4 is an example sequence of black start, in accordance with the present invention.

Fig 5 is another example sequence of black start, in accordance with the present invention.

Fig 6a is a schematic illustration of an example of load pick-up, in accordance with the present invention.

Fig 6b is a schematic illustration of another example of load pick-up, in accordance with the present invention.

Fig 6c is a schematic illustration of another example of load pick-up, in accordance with the present invention.

Fig 7a is a schematic illustration of an example of coordination of load pick-up via interface between microgrids, in accordance with the present invention.

Fig 7b is a schematic illustration of another example of coordination of loadpick-up via interface between microgrids, in accordance with the present invention.Fig 8 is a schematic flow chart of an embodiment control of black start.

Fig 9 is a schematic illustration of an embodiment of information gathering of a microgrid network controller, in accordance with the present invention.

Fig 10 is a schematic illustration of an embodiment of a microgrid networkcontroller controlling assets in a microgrid during black start, in accordance with the present invention.

DETAILED DESCRIPTION Embodiments will now be described more fully hereinafter with reference tothe accompanying drawings, in which certain embodiments are shown.However, other embodiments in many different forms are possible within thescope of the present disclosure. Rather, the following embodiments areprovided by way of example so that this disclosure will be thorough andcomplete, and will fully convey the scope of the disclosure to those skilled in the art. Like numbers refer to like elements throughout the description.

Although the proposed method is discussed with an example approach here, the present invention is not limited thereto.

An example of an electrical power network 1 is shown in figure 1. The source(DG) 3 with black start ability can start the network energization. Loads 5 canbe connected to the network when energized and other grid parallel sources (DGs) 4 are connected when a stable microgrid is formed.

These different assets (DGs and loads) are used in a coordinated manner toaccelerate the black start process. One point is the selection of the blackstarting segments based on available breakers 2 which may divide thenetwork 1 into segments (herein also called microgrids or nested microgrids) between open breakers.Embodiments of the present invention include: - Island identification: Pairing of breakers 2, from the interface of thenetwork 1, based on available assets, to define black start islands (segments,nested microgrids within the network 1) which are sorted based on network energization time and sequenced to start and join together.

- Load pickup: Load 5 pickup during black start in nested microgrids issupported across the microgrid interface with identifying islands across the nested microgrids.

The microgrid 1 comprises an interface to a microgrid (network, or N) controller 6 which may in some embodiments: - have the ability to receive signals from microgrid assets 3, 4 and/ or 5 and breakers 2 to define black start islands between paired breakers. - be able to calculated the start-up time of the islands and sequence the islands an order to start and join the energized network. - support cold load pickup across the nested microgrids and communicate the power requirements at the PCC.

Thus, the network controller 6 may pair breakers 2 for formation of anisland/ segment for black start by microgrid interfacing controller as well assupporting cold load pick-up across microgrid connections /breakers byjoining two black starting microgrids. In embodiments, two connected(nested) microgrids are each black started and then connected to each other by closing a breaker 2 there between.

Figure 2 shows an embodiment of the interfacing controller 6 in controllingcommunication with the first microgrid/ segment 1A, the second microgrid/ segment 1B and the breaker 2 there between.

One example shown in figure 3 where two microgrids 1A and 1B areconnected via the breaker (brk) 2AB. Islands 7 (7Ab, 7Aa, 7Ba and 7Bb) areformed in each microgrid as shown in figure 3. The island forming startsfrom the interfacing N controller 6 at brk 2AB. The island is determinedbased on power balance and load pick-up cranking paths. Once it is pairedwith brk 2Aa to form island 7Aa, the next step is to find the pairing for brk 2Aa. This is continued till all the black start islands 7 are formed.

Figure 4 illustrates an example of a black start sequence for the network 1 offigure 3, and figure 5 illustrates an example of corresponding pairing sequence of the breakers 2.

Load pick up during black start may be in accordance with different scenariosas shown in figures 6a, 6b and 6c, respectively. These are the cases whereassets from one microgrid 1A/1B supports load pick up in the other microgrid(s), as indicated by the dotter arrows.

Cross microgrid cold load pick-up may go through the microgrid interfacingcontroller or the network controller 6. The microgrid control of assets andbreakers in the microgrids can be coordinated by the interfacing networkcontroller. The load pick up from other microgrid can be coordinated in twodifferent ways, scenarios 1 and 2 (below) as shown in figures 7a and 7b, respectively.

Scenario 1: Microgrid 1A requires load pick-up from microgrid 1B. The assets3-5 in microgrid 1A communicate the power generation and loads to bepicked up during black start to the network controller 6. The networkcontroller calculates the load pick-up requirement at the PCC. This is communicated to all the black starting assets in microgrid 1B.

Scenario 2: Microgrid 1A can provide load pick-up from microgrid 1B. Theassets 2-5 in microgrid 1A communicate the generation and loads to bepicked up during black start to the network controller 6. The networkcontroller calculates the load pick-up which can be offered to microgrid 1B.This is communicated to all the assets in microgrid 1B to decide the cranking paths.

An example of control methods for the proposed black start scheme in thenested microgrid network is shown in figure 8. The microgrid information onblack start sources, load location and breaker status may be communicated tothe network controller 6 as shown in figure 9. Once the black start strategy isdecided by network controller, including identifying islands 7 and load pickup coordination, the controller 6 may send control signals to the assets of the microgrid 1A to be black started.Advantages of embodiments of the present invention include: - A microgrid solution for faster and stable black start with connected microgrids 1.

- A network controller 6 for microgrid interfacing which can coordinate the black start and provide cold load pick-up across the microgrids.

- A microgrid solution to black start connected microgrid without black start ability through energization and load pick up.Below follow some embodiments of the present invention. 1. The black start or energization of at least two connected microgrids 1A and 1B is achieved by the interfacing network controller 6 - identifying black start islands 7 with pairing circuit breakers 2 from the microgrid interfacing point - support cold load pick-up across the microgrids by connecting two de- energized microgrids - arrange the islands for start-up and resynchronization based on location and required time - calculate load pick-up requirement at PCC and communicate to microgrid assets 3-5. 2. The circuit breaker 2 pairing is done based on location of black start sources 3 and loads 5 in the microgrids 1A and 1B. 3. Cold load pick-up is supported from sources 3 in other microgrid andthe cold load support requirement is exchanged from first microgrid 1A to the second microgrid 1B through the network controller 6 at PCC. 4. For cold load support, two de-energized microgrids are connected together by the interfacing network controller 6.

. The black start segments 7 are arranged in combination of series and parallel to minimize the system start-up time.

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

Claims (1)

1. CLAIM1. An electrical power network (1) comprising:a first microgrid (1A),a second microgrid (1B), anda network controller (6); wherein each of the first and second microgrids are delimited by paired circuit breakers (2),wherein the controller (6) is configured to: black start each of the first and second microgrids when islanded, by means of a black start source (3A; 3B) in the microgrid, connecting the two microgrids to each other by opening a circuit breaker(2AB) there between, and picking up a load (5) in the first microgrid (1A) by means of the black start source (3B) in the second microgrid (1B).