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Contract No. DE-AC36-08GO28308

Design of a Microgrid Transition 

Controller II: System Recovery 

Under Abnormal Conditions

Preprint 

Jing Wang, Annabelle Pratt, and Murali Baggu

National Renewable Energy Laboratory 

Presented at the IEEE Power and Energy Society General Meeting

Portland, Oregon

August 5–9, 2018 

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Design of a Microgrid Transition Controller II: 

System Recovery Under Abnormal Conditions

Jing Wang, Annabelle Pratt, Murali Baggu

Power Systems Engineering Center, National Renewable Energy Laboratory Golden, CO 80401, USA

jing.wang@nrel.gov, annabelle.pratt@nrel.gov, murali.baggu@nrel.gov

Abstract—This paper presents a microgrid transition 

controller for managing emergency operation when the

microgrid experiences a system blackout caused by an internal

or external fault. The developed transition controller consists of 

various application function blocks, including normal 

operation, emergency operation, and coordination between 

them. The developed microgrid transition controller is 

validated by a sample microgrid, and two test cases are 

investigated: islanded black start and grid-connected black 

start. The simulation results demonstrate the feasibility and

effectiveness of the proposed microgrid transition controller for 

handling emergency operation and transitioning to normal 

operation.

Index Terms—Microgrid; Emergency Operation; Application 

Function Block; Black Start.

I. INTRODUCTION 

A microgrid’s ability to black start has attracted a lot of 

attention in recent years because it can rapidly restore the 

microgrid so that it continues providing uninterrupted power 

for critical loads under power outages and restore the 

distribution system if enough cranking power is available 

from distributed generators [1]. Also, the resiliency of electric 

grids is significantly improved by optimally managing the 

operation of a microgrid’s assets, such as dispatchable 

distributed energy resources (DERs) and controllable loads 

[2].

Therefore, extensive research regarding microgrid black 

start for distribution system restoration and to maintain 

critical loads has been performed. Regarding system 

restoration, a self-healing, resilient distribution system based 

on sectionalizing into microgrids is developed in [3] to 

provide continuous, reliable power to the maximum loads. A 

novel, networked, microgrid-aided approach for system 

restoration using the spanning tree search approach is 

presented in [4] to maximize the restored loads. An evaluation 

of a microgrid’s black-start capability and schemes is 

presented in [5] considering possible benefits and associated 

technical challenges. For a microgrid black start to support 

critical loads, an optimal black-start functionality procedure 

based on mathematical optimization is developed in [6] to 

achieve reliable black-start schemes. Generalized rules for 

microgrid black-start operation are proposed in [7] to restore 

critical loads as quickly as possible.

Significant progress has been made in these particular areas 

of research for microgrid black-start capabilities, but the 

capability to support critical loads needs more attention. Most 

of the existing literature treats a microgrid’s black start as a 

single operation, and great efforts are made to develop 

sophisticated optimization algorithms; however, from the

point of view of microgrid operation, black-start operation is 

also an operation mode—i.e., an emergency mode—similar to 

other operation modes, such as grid-connected, islanded, and 

transitioning between them. A complete microgrid control 

system should include the control and management of all 

different modes and the coordination among them. For 

instance, black-start functionality should override all other 

operation functionalities when system blackout and collapse 

occur. When the microgrid’s black start (self-start) is 

successfully performed, the islanded operation functionality 

should take control of the microgrid, then the black-start 

functionality will be running in the background. Reference [8] 

proposes conceptual coordination schematics for all of the 

microgrid’s operation modes. A microgrid controller that 

manages the operation and control of the microgrid under 

normal conditions (grid-connected, islanded, and the

transition between them) is developed in [9]. This paper aims 

to continue the research in [9] by developing a microgrid 

transition controller that performs a black start under 

abnormal conditions (e.g., natural disaster, system collapse)

with the objective of powering critical loads. As an extension 

of [8] and [9], the developed microgrid transition control 

system includes the application function blocks (AFBs) for 

black start, and we focus on the black start operation and 

coordination of black-start functionality with other operation 

functionalities. In consideration of the computation burden on

the microgrid central controller, a predefined sequence of 

actions for black start is inserted in the AFB for black-start 

operation mode rather than embedding a mathematical 

optimization algorithm. The developed microgrid transition 

controller will be tested via simulation of a sample microgrid 

that targets providing continuous power supply for critical 

loads.

II. OVERVIEW OF MICROGRID OPERATON

Fig. 1 presents a flowchart of microgrid operation under 

different conditions. In normal conditions, the microgrid 

system operates in any mode inside the inner loop, which is 

comprised of the operation modes in the green boxes, purple

diamonds, and purple boxes. The focus of this paper is on 

abnormal conditions, so the details of the operation process 

for normal conditions will not be discussed here but can be 

found in [8]. In this paper, abnormal operation is called 

emergency operation mode, and it is in the outer loop of the 

flowchart. The emergency operation mode is complex and 

might be caused by main grid or microgrid faults, 

unscheduled outages, or voltage and/or frequency transients. 

Some microgrid control systems can detect these unexpected 

This work was supported by the U.S. Department of Energy under Contract 

No. DE-AC36-08GO28308 with Alliance for Sustainable Energy, LLC, the 

Manager and Operator of the National Renewable Energy Laboratory.
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conditions and transition to a stable islanded operation mode. 

In most cases, however, the microgrid transitions to an

unstable system or black-out collapse.

When a microgrid black-out occurs, the microgrid system 

enters emergency operation mode. In this mode, two functions 

are included: (1) the decision on black-start option−gridconnected or islanded, based on the conditions of the

microgrid assets and availability of the main grid; and (2) the

sequence of actions. If the microgrid decides on an islanded

black start option, the green box for islanded operation mode 

will be selected, and the microgrid system will restart the 

master asset first, reenergize the transformer and lines, and 

then connect critical loads, etc. Once the microgrid system is 

stabilized in islanded mode and the main grid is ready, a 

reconnection signal might be sent from the distribution

network operator (DNO) to the microgrid central controller. 

Then the microgrid is on the path to normal grid-connected 

operation and will transition from reconnection control and 

check (marked by the purple diamond), to grid-connected 

operation (marked by the purple box), and to grid-connected 

mode (marked by the green box). Alternatively, the microgrid 

can choose to black start via connecting to the main grid 

first−once the main grid conditions are restored to normal, 

reenergize the transformer and line, and reconnect the critical 

loads. This work considers only the microgrid’s black start 

(either islanded or grid-connected). The situation in which the 

microgrid black starts and restores the distribution system will 

be included in future work.

Normal operation:

Grid-connected mode

- Economic dispatch

- Optimization

- Grid services

Normal operation:

Islanded mode

- Power balance

- Voltage and frequency control

- Load shedding

Reconnection check:

Voltage, frequency and angle

Reconnection 

request

Reconnection Disconnection 

check: active and 

reactive power, voltage and 

frequencyDisconnection

Disconnection 

request

Stable

Emergency operation:

- Decision on black start option

- Sequence of actions

Unstable system/blackout 

collapse

Fault/unintentio

nal islanding

Start

Transition to gridconnected operation

Transition 

to islading 

operation

N

N Y

Y

Fig. 1. Overview of microgrid operation under different conditions.

III. DESIGN OF THE MICROGRID TRANSITION 

CONTROLLER FOR BLACK START

Fig. 2 presents the schematic diagram of a microgrid 

transition controller. As shown, the microgrid transition 

receives a command from the DNO control room and feeds 

back the status of the microgrid’s operation. Based on the 

command from the DNO, network conditions, and 

measurements, the microgrid operates in any of the operation 

modes. The output signals from the microgrid transition 

controller are control commands and control references, 

which are sent to the device-control level for execution. At 

the field level, the microgrid assets receive the final 

command from the device-level controller and execute the 

command.

The microgrid transition controller includes nine AFBs 

that are located in the supervisory level (microgrid central 

controller). The AFBs in steady-state and transition-state 

operations are enabled-based and run only if the enable

conditions are satisfied. But the sub-AFB for detection of 

black out conditions within the emergency operation mode 

(AFB#8) will operate all the time. Based on the voltage and 

current measurements at the point of common coupling 

(PCC), the blackout/system collapse status will be detected. 

If these conditions are not detected, this sub-AFB will not 

trigger the other sub-AFBs (the decision on black start

option, sequence of actions, and end of black start) inside 

AFB#8, and it will send the status signal to AFB#1 

(operation-mode management) to inform it of the “normal” 

status of the system. Otherwise, once the detection of a black 

start is “true,” the sub-AFBs inside AFB#8 will be enabled to 

decide the black start option and then generate the sequence 

of actions. At the same time, the operation mode 

management (AFB#1) will get notified immediately, and it 

will disable the AFBs for steady state and transition state 

accordingly. The operation mode selection signal will change 

to the value that chooses AFB#8, and AFB#9 (control signal 

generation) will output the control command and references 

generated by AFB#8 to black start the microgrid. If the black 

start is successfully achieved, the end of black start sub-AFB 

will send a status signal to the detection of black start subAFB, then the other three sub-AFBs inside AFB#8 will be 

disabled. Also, AFB#1 will be informed of the success of the 

black start, and the microgrid operation mode will be updated 

to a normal condition – either grid-connected or islanded.

Utility operator 

control room

Supervisory 

level 

(Microgrid 

Central 

Controller)

#1 Operation 

mode 

management

#3 Optimal 

power dispatch

(Grid-connected)

#4 Planned 

disconnection

#5 Transition to 

islanding

#6 Reconnection 

control

#7 Transition to 

grid-connected

#9 Control signal 

generation

Detection of 

black start

Decision of 

black start 

Sequence of 

actions

End of black 

start

# 8 Emergency operation

Steady-state

Transition-state

DNO control room

PCC CB 

controller

DER 

application 

control

Device 

control level

DER

Load

...

Microgrid

Grid

Protection 

relay

DER

DER 

application 

control

Field level

PCC CB

Enable 

signal

Enable signal

Acknowledgement signal

Acknowledgement signal

Informing

signal

Status

Operation mode 

selection

PCC voltage and current

Measurements Control commands 

and references

#2 Robust stability 

control

(Islanded)

Status and request

Command

Load 

controller

Fig. 2. Schematic diagram of the microgrid transition controller.

IV. IMPLEMENTATION OF THE AFBS FOR EMERGENCY 

OPERATION

This section presents the detailed implementation of 

AFB#8. This paper is an extension of [9], and that paper

presents details on AFB#2 – AFB#7 and AFB#1 for normal 

operation modes. Since AFB#1 includes additional signals to 

implement emergency operation, we present it here again to 

give a good understanding of how emergency operation 

mode coordinates with other modes.
3

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#1 Operating mode management

DNO signal 

processing

PCC 

disconnection 

enable signal 1

PCC 

disconnection 

enable signal 2

PCC 

disconnection 

signal 3

PCC 

reconnection 

enable signal 

generation

Operating mode

decision 

DNO 

operating 

signal

PCC grid 

voltage

Steady-state mode

Transition mode

Enable signal 2

Enable signal 

3

Enable signal 

4

Enable signal 

5

Operating mode 

signal

Acknowledgment 

signal

PCC microgrid 

voltage

Alarm

PCC 

frequency

Acknowledgment 

signal

Coordination 

Status of completion signal

of islanded black start

Status of completion of 

grid-connected black start

Status of islanded 

black start

Status of gridconnected black start

Or

Request for confirmation

Request for reconnection

Enable signal 1

Fig. 3. Schematic diagram of AFB#1, operation-mode management.

Fig. 3 shows the schematic diagram of AFB#1 with the 

additional signals required to support emergency operation 

marked in red. The inputs from AFB#8 are the status of gridconnected black start, status of islanded black start, status of 

completion of grid-connected black start and status of 

completion of islanded black start. If either the status of gridconnected black start or the status of islanded black start is 

“true”, the steady-state operation and transition-state 

operation modes will be disabled and stop running their 

algorithms and the operation mode decision will switch to 

emergency operation and the output of the AFB#8 will be 

sent through AFB #9 to generate control references to the 

device control layer. If the black start is grid-connected, 

AFB#1 sends a request to the DNO for confirmation and the 

confirmation from the DNO is sent to AFB #8, then AFB #8 

manages the black start. Once the black start is completed, 

the sub-function block “operating mode decision” will 

change the operation mode of the microgrid according to the 

network conditions. For example, if the microgrid performed 

grid-connected black start successfully and operates stably 

for predefined time, then the microgrid will change to the 

grid-connected mode and the algorithm for this mode 

(optimal power dispatch, AFB #1) will be enabled. If the 

microgrid performed islanded black start successfully and 

operates stably for predefined time, then AFB#1 will send a 

request to the DNO for reconnection, and the reconnection 

control (AFB #6) operation will be performed if the DNO 

agrees.

Fig. 4 shows the schematic diagram of AFB#8, emergency 

operation. It starts from the first sub-AFB#8.1, detection of 

blackout, which uses the information from the PCC grid-side 

voltage, PCC microgrid-side voltage, and value of steadystate mode (from AFB#1). Each variable has two states: “0”

and “1.” If the voltage is normal, the state value is “1”, so 

there are eight combinations: two conditions resulting in 

microgrid islanded black start, one condition resulting in 

grid-connected black start, and five resulting in no black 

start. Based on this derivation, we have the implementation

of sub-AFB#8.1.

If the black start option is islanded, the sub-AFB#8.2 

(decision of black start option) will trigger islanded black 

start in sub-AFB#8.3 immediately. Otherwise, if it is gridconnected, we need to wait for confirmation from the DNO 

and only then launch the grid-connected black start. If no 

confirmation is received, the islanded black start will be 

launched. The outputs of sub-AFB #8.3 (control commands 

and references) will be sent to AFB#9, which will function 

as control signal generation to send all control signals to the 

device level and/or field level for execution. Sub-AFB#8.4 

will check whether the black start is completed based on the 

measurements. If black start is completed successfully, a 

status signal indicating the successful completion of the black 

start will be sent to sub-AFB#8.2 and from there to AFB#1. 

Once sub-AFB #8.2 receives this signal, it will stop enabling 

sub-AFB #8.3 and sub-AFB#8.4. At the same time, AFB#1 

will modify the operation mode of the microgrid. In Fig. 4, 

the red arrows indicate the input and output signals.

PCC grid

side voltage Grid voltage

check 

PCC microgrid

side voltage Grid voltage

check 

Steady-state

mode Check 

operation 

mode

Control 

logic

3

DNO 

confirmation

signal

Control 

logic

4

Grid-connected 

black start Check if 

black start 

finished?

Islanded black 

start Check if black start 

finished?

Measurements

Status of 

islanded black 

start

Status of gridconnected black 

start

Status of completion of gridconnected black start

Status of completion 

of islanded black start

Control commands 

and references

#8.1 Detection of blackout

#8.2 Decision of black 

start option

#8.3 Sequence of 

actions #8.4 Check completion of 

black start

Control 

logic

2

Control 

logic

1

Control commands 

and references

Fig. 4. Schematic diagram of AFB#8, emergency operation.

A more detailed implementation of sub-AFB#8.3 is 

presented here. For the sequence of actions in islanded black 

start, refer to [1]. It lists the sequence as: disconnection of all 

loads, building the low-voltage network, synchronizing small 

islands, connecting controllable loads to the low-voltage 

network, connecting non-controllable DERs or DERs 

without black-start capability, increasing the load, and

changing the control scheme of the inverters and microgrid 

synchronization with the medium-voltage network. The 

sequence of actions for grid-connected black start is slightly 

different from that for islanded black start because there is no 

need to build up the low-voltage network by using a DER 

with black-start capability. Critical loads are usually 

connected first after PCC circuit breaker is closed.

V. SIMULATION RESULTS FOR BLACK START USING A 

SAMPLE MICROGRID

The proposed microgrid transition controller offers an 

automatic solution to manage the normal and emergency

operation of a microgrid. The validation of the controller 

under normal operation is presented in [9], so this section 

presents only the validation of the microgrid controller under 

emergency operation in a sample microgrid to demonstrate 

its feasibility and effectiveness. This microgrid has 2 

Batteries, 2 PV, 3 critical loads and some noncritical loads.

One battery with 300 kW rated power is the black start 

source. The configuration of the sample microgrid and 

parameters can be found in [9].

Two scenarios are investigated in this section. Scenario 1: 

the microgrid experiences a blackout caused by an external 

fault, and islanded black start is launched. Scenario 2: The 
4

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microgrid experiences a blackout in islanded operation 

caused by an internal fault, and grid-connected black start is 

initialized. This section presents results on the coordination 

signals of the microgrid transition controller and 

measurements of the microgrid (such as PCC voltage and 

frequency, and critical loads circuit break status) to show the 

microgrid’s emergency operation. The performance of the 

proposed microgrid transition controller for emergency 

operation is evaluated assuming that the communications 

among different control layers are working well.

A. Scenario1: Islanded Black Start

In this scenario, the microgrid system starts from operating 

in grid-connected mode. Then an external fault is triggered 

and removed after a certain amount of time. The microgrid 

controller detects the blackout and black starts the microgrid

in islanded mode. Once islanded black start is completed, the 

controller sends a request for reconnection and after 

confirmation is received from the DNO, it performs

resynchronization and then operates in grid-connected mode.

Fig. 5 shows the operation of the microgrid and 

coordination signals inside the microgrid transition 

controller. As shown, the external fault is applied at 3 s and 

cleared at 4 s, and the relay at PCC circuit breaker detects the 

microgrid’s dead bus within 40 ms. The microgrid transition 

controller detects the blackout at 3.06 s and informs the 

DNO. The signal from DNO (the 3rd signal in Fig. 5) is 

informed about the microgrid’s blackout status, so the 

operation command changes to islanded operation at 3.1 s 

after the notice. The microgrid transition controller decides 

to launch islanded black start at 3.06 s, and it finishes the selfblack-start process at approximately 12.7 s. Once the black 

start is completed, the output of the black-start completion 

block is “high” to indicate that the black start is finished, and 

it holds the “high” status for a few seconds. Then a request 

for reconnection to the main grid is sent to the DNO at 15.2 

s, and this request is confirmed by the DNO at 17.5 s 

(indicated when the DNO signal changes from “0” to “2” and 

the microgrid needs to complete resynchronization before 

20.5 s).

Fig. 5. Coordination signals inside the microgrid transition controller for 

islanded black start.

The DNO signal “0” means islanded mode, “1” means 

grid-connected mode and “2” means transition from islanded 

to grid-connected mode. The microgrid is requested by the 

DNO to operate in grid-connected mode after 20.5 s. The 

translated steady-state signal and transition signal also 

validate the microgrid’s operation: before the blackout is 

detected, the microgrid is requested by DNO to operate in 

grid-connected mode (the “steady state” is “1” before 3.1 s); 

grid-connected mode is allowed only after synchronization is 

completed (“steady-state” is “1” again after 20.5 s); the 

microgrid operates in black-start mode between 3.06 s and 

12.7 s (black start is “1” during this time, and it disables all 

other operation functionalities) and switches to islanded 

mode between 12.7 s and 17.5 s (“steady-state” is “0” during 

this time); resynchronization is launched during 17.5 s and 

20.5 s (“transition signal” is “1” during this time). All the 

results shown in Fig. 5 demonstrate the correct functioning 

of the microgrid transition controller and the correct 

coordination and timing among different operation 

functionalities.

Fig. 6 shows the measurements at the PCC to further 

validate the performance of the proposed microgrid transition 

controller for emergency operation. The PCC circuit breaker 

status shows that the microgrid is connected to the grid before 

3.04 s, disconnects from the main grid at 3.04 s, and 

reconnects back to the main grid at 18.6 s. The PCC 

microgrid voltage and frequency show that the microgrid 

blackout occurs from 3.02 s to 4.68 s, and it recovered rapidly 

to normal operation after the black start is launched. Note that 

20 ms after the fault is applied due to the time delay 

introduced by the low pass filter used to calculate the voltage 

and frequency. The magnified view of voltage and frequency 

shows that the microgrid controller is able to successfully 

reestablish voltage and frequency within acceptable limits. 

The PCC active and reactive powers show the correct 

operation of the microgrid.

Fig. 6. Measurements at the PCC: PCC circuit breaker status, voltage, 

frequency, active and reactive power.

Fig. 7 shows the active and reactive power of the blackstart source – the battery – and the status of the circuit 

breakers for the critical loads. The BESS is shut off when the 

blackout is detected, and it is the first asset to be connected 

back to energize the low-voltage microgrid. Once the 
5

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microgrid is re-energized, the most critical load (Load 1) is 

reconnected, the second critical load (Load 2) is reconnected 

after a certain amount of time, and the third critical load 

(Load 3) is reconnected as well. Fig. 7 demonstrates that all 

critical loads are served in a very short time.

Fig. 7. Results of the assests: active and reactive power output of the blackstart source and the circuit breaker status of the critical loads.

B. Scenario 2: Grid-Connected Black Start

In this scenario, the microgrid system starts from operating 

in islanded mode. Then an internal fault is triggered and 

removed after a certain amount of time. The microgrid 

controller detects the blackout and black starts the microgrid 

in grid-connected mode (a request signal is sent to DNO, wait 

for DNO confirmation and black start is lunched). Once the 

grid-connected black start is completed, the microgrid 

controller transfers the operation mode to grid-connected 

operation. 

The results in this scenario are similar to the results shown 

in Scenario 1. The coordination signals demonstrate the 

correct function of the microgrid transition controller to 

manage the grid-connected black start and also the correct 

coordination and timing among different operation 

functionalities. Results of the assets (main DER source and 

critical loads) shows that critical loads and the main DER 

source are powered one by one based on the defined timing 

and operation procedures.

Fig. 8. Measurements at the PCC: PCC circuit breaker status, voltage, 

frequency, active and reactive power.

Fig. 8 shows the measurements at the PCC to illustrate the

grid-connected black start. The PCC circuit breaker status 

shows that the microgrid is connected to the grid at 4.56 s. 

The PCC microgrid voltage and frequency show that the 

microgrid is in blackout from 3.02 s to 4.58 s, and it recovers 

rapidly to normal operation after black start is launched. The 

magnified view of voltage and frequency shows that the 

microgrid controller is able to successfully reestablish 

voltage and frequency within acceptable limits during the 

black-start process. The PCC active and reactive powers 

show the correct operation of the microgrid.

The results here are in line with the designed operation 

procedures of grid-connected black start.

VI. CONCLUSIONS

This paper presents a microgrid transition controller for 

managing emergency operation when the microgrid 

experiences a system blackout caused by an internal or 

external fault. The proposed transition controller consists of 

various AFBs that include the operation functionalities for 

normal operation, emergency operation, and coordination 

between them. The focus is on the AFBs for operation mode 

coordination and emergency operation. The proposed 

microgrid transition controller for emergency operation is 

validated by using a sample microgrid with two test cases. 

The contributions of the paper are summarized as: 1) The 

proposed microgrid transition controller provides a design 

framework and microgrid automation solution for 

researchers and engineers working in the microgrid 

controller design area. 2) The simulation results demonstrate 

the feasibility and effectiveness of the proposed microgrid 

transition controller for handling emergency operation and 

transitioning to normal operation.

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0 5 10 15 20 24 0

10

20 x 104

Battery active

 power (W)

0 5 10 15 20 24 0

5

10 x 104

Battery reactive 

power (Var)

0 5 10 15 20 24 0

0.5

1

Critical

 load1 

0 5 10 15 20 24 0

0.5

1

Critical

 load2

0 5 10 15 20 24 0

0.5

1

Time (s)

Critical 

load3