MicroGrid Designer: user-friendly design,operation and control assist tools for resilient microgrid and autonomous community

0 Introduction

Stable and reliable power grid operations can be conducted securely if faults can be mitigated without threatening the integrity of the grid operations,preserving the continuity of supply to customers.However,the impact of recent events on power grids caused by extreme meteorological changes,such as typhoons,floods,and earthquakes,goes far beyond the conventional reliability standards for power utilities,and clarifies the need for restructuring current planning and designing of power grids [1].

In Japan,11 sites of nuclear plants were shut down and Liquefied Natural Gas (LNG) gasification plants were damaged by the Great East Japan Earthquake and the subsequent tsunami,which occurred on March 11,2011.It is estimated that it will take over 30 years for the decommissioning of the damaged nuclear plants.In August 2018,a magnitude 6.7 earthquake struck and knocked out power and train services across Hokkaido; 5.4 million people were affected by this disaster,and nearly 3 million households lost power,according to the Hokkaido Electric Power Company.It took two days to restore electricity for most households,and over one month in remote areas.In Chiba prefecture,hundreds of thousands of households near Tokyo were without power and water after the powerful typhoon on September 11,2019.Tokyo Electric Power Company Holdings Inc.stated that the blackout continued to affect 386,000 households in Chiba Prefecture as of 10 p.m.,and services would not be fully restored within a week at the earliest,although it aimed to fully restore them within a week.It marked the largest blackout on record for the company caused by a typhoon in terms of the number of households affected and the length of downtime [2].

These catastrophic extreme events have brought the topic of resilience to the discussions among power grid designers,regulators,and policy makers.The report“Microgrids for Energy Resilience: A Guide to Conceptual Design and Lessons from Defense Projects,” published by the National Renewable Energy Laboratory (NREL),US,in January 2020 includes the following statement [3].“Microgrids can enhance energy resiliency by providing energy surety (i.e.,loads have certain access to energy)and survivability (i.e.,energy is resilient and durable in the face of potential damage).Microgrids typically comprise distributed energy resources that can provide independent power to designated critical loads upon loss of their primary source of energy.However,a microgrid is distinct from an emergency back-up system (i.e.,UPS ) in that a microgrid can interact with the utility grid and operate in either gridconnected or islanded mode.” Thus,the microgrid has become a mainstream factor for future resilient energy and autonomous social service supply.

To create such resilient and autonomous future grids,user-friendly design,operation and control tools for resilient microgrids and autonomous communities are being developed under the support of the New Energy and Industrial Technology Development Organization(NEDO) in Japan.The proposed and developed tools consist of analysis and simulation software,including unit commitment,economic load dispatch (ELD),automatic generation control (AGC),load frequency control (LFC),dynamic power flow,line fault identification,black start procedure,and energy management,specifically for the compact design and operation of small- and medium-scale microgrids [4].The developed tools are applied to a typical microgrid to verify its effectiveness and performance.These tools are also applied to microgrids in Caribbean countries in the project Analysis and Stabilization of Micro Grids in the Caribbean Community and Future Developments under the support of Nippon Koei Co.Ltd.and the Japan International Cooperation Agency (JICA) [5].

1 Objectives and functions of MicroGrid Designer

There is no concrete definition of microgrid.The United States Department of Energy (DOE) provides the following definition of a microgrid [5]: “A microgrid is a group of interconnected loads and distributed energy resources within clearly defined electrical boundaries and acts as a single controllable entity with respect to the grid.A microgrid can connect and disconnect from the grid to enable it to operate in both grid-connected or island-mode.A remote microgrid is a variation of a microgrid that operates in islanded conditions.”

Similarly,a definition of microgrid is given by the International Council on Large Electric Systems (CIGRE)[6]: “Microgrids are electricity distribution systems containing loads and distributed energy resources (such as distributed generators,storage devices,or controllable loads)that can be operated in a coordinated way while connected to the main network or disconnected from the main network(islanded).”

As demonstrated in these definitions,a microgrid is a localized group of interconnected loads and distributed energy resources within clearly defined electrical boundaries (e.g.,cities,communities,campuses) that act as a single controllable entity with respect to the main regional or national electric grid.A microgrid can connect or disconnect from the main grid as physical and/or economic conditions dictate.It can operate in both grid-connected mode (synchronous with the grid) and island mode (operated autonomously and disconnected from the main electric grid)[6].The structure and components of a typical microgrid are shown in Fig.1.

The general procedure for planning and designing microgrids is illustrated in Fig.2.Firstly,grid information such as daily,weekly,monthly,or annual electric and heat demands,available generation resources,and meteorological conditions of the region are prepared as inputs for analysis tools.By taking the types,capacity,and fuel cost of generators,system operation conditions,and output characteristics of sustainable energy into consideration,appropriate generation resources can be selected to meet the required electric and heat demands.Based on the electric/heat demand and the selected generation resources(i.e.,diesel generators,small-scale hydro generation,and renewable energy generation),the minimum cost operation is determined using nonlinear mathematical methods.When excess electric power from renewable energy generation occurs,batteries must be installed in the grid.Grid engineers must determine the proper capacities (kW and kWh) of batteries to absorb the excess power.After preparation of this microgrid-related information,design,operation,and control of the grid are conducted using various types of analysis tools,as discussed below.

2 Components and algorithms of MicroGrid Designer

2.1 Components of MicroGrid Designer

MicroGrid Designer is comprised of a comprehensive suite of software for investigations and studies of electric power grids,transmission/distribution networks,and generation performance in both steady-state (single-stage)and dynamic (multi-stage) conditions.

MicroGrid Designer has the following functions(modules) available in the current version,as shown in Fig.3.

• Single-Stage ELD Module: Determines the optimal output of a number of electricity generation facilities to meet the system load at the lowest possible cost subject to transmission and operational constraints.

• Multi-Stage ELD and LFC Module: Chronologically determines the optimal output of a number of electricity generation facilities to meet time-varying system loads at the lowest possible cost and LFC commands to maintain the system frequency within the permissible range.

• Frequency Simulation Module under ELD and LFC:Detailed simulation software for frequency deviation to verify the performance of coordinated operation of the dynamic ELD and the LFC.

• Single-Stage Power Flow Analysis Module: Steadystate analysis tool to determine the voltages,currents,and real and reactive power flows in a grid under given load conditions,and planning ahead for various hypothetical situations.

• Multi-Stage Power Flow Analysis Module:Chronological power flow analysis for time-varying loads to determine the transitions of voltages,currents,and real and reactive power flows in a grid over a time horizon.By using the yearly load duration curve (LDC),quantitative evaluations of the annual fuel consumption and CO2 emissions of generators can be determined.

Additional functions can be supplied by customization based on the requests of grid engineers and users,such as:

• Load forecasting and assumption of future load

• Line fault analysis

• Fault identification (types and locations of faults)

• Setting of protection systems

• Black start procedure

• Determination of battery capacities (kW and kWh)

• Electricity and heat supply by cogenerations

• Energy Management System (EMS) design

The functions required to implement these key components are described in detail in the following sections.The key components consist of unit commitment of generators,single- and multi-stage ELDs,frequency simulation under LFC and single- and multi-stage power flows that are mandatory for microgrid analysis.

In Sections 3.2–3.6,detailed explanations and basic equations are provided for the key components of this tool.In particular,in Sections 3.4 and 3.5,fundamental equations and the optimization method are described for economic load dissipation and frequency control.In Section 3.6,conventional widely-used equations and the solution method for dynamic power flow analysis for microgrids are given.All the computations are carried out sequentially using the same input data.This means that all the computational modules are closely related,resulting in computational consistency.

2.2 Forecasting and assumption of future loads

Load forecasting is essential for grid engineers to predict the electric power and other energy needed to maintain the balance of supply and demand at the targeted time periods.Forecasting can be classified into terms of time periods—short-term (a few hours or a day),medium-term (a few weeks,a month,or up to a year),and long-term (over a year).Generally,in designing and planning microgrids for which MicroGrid Designer is applied,the amount of electric and heat demand is assumed or estimated by taking the composition and the area (m2) of buildings and facilities into consideration.In particular,for new microgrids in green fields,all the demand must be estimated by empirical knowledge.Therefore,load forecasting tools are not included in the current version.In MicroGrid Designer,short- and medium-term forecasting methods of electric and/or heat demands are facilitated for designing the EMS.For short-term forecasting,various approaches can be customized,such as regression models,time series,neural networks,fuzzy logic,and the ensemble method.

2.3 Determination of priorities of generators to be operated

To determine the priority of generators to be operated under a certain load band,the unit price at the maximum output of the generator is used,which is defined as follows:

The detailed calculation procedure is as follows.

Step 1: Obtain the fuel characteristic coefficients,which is necessary for calculating the fuel consumption (if the fuel consumption function is described as a second-order function,then obtain ai,bi,ci.[7]),and the output upper limit pi,max of generators (i=1,2,3,...n).

Step 2: Calculate the unit price at the maximum output of each generator μi,max (for simplicity,it is expressed as μi).

Step 3: Sort the calculated unit prices of generators μi in ascending order for determining the priority of generators μk,k(i),where,k is the number of the ascending order (k=1,2,3,...n),and i is the original number of the generator corresponding to k (i).

The obtained ascending order becomes the priority of generation to be operated at different load bands.

Step 4: When the ascending order is k=1,the load band is as follows.

The generator with the lowest unit price (the 1st generator in ascending order of the unit prices) is operated to meet the above load band.

When k=2～n,the load band is as follows:

The 1st,2nd,…,kth generators in ascending order of the unit prices of generators are operated to meet the above load band.

The above analysis is based on the following assumption:the total capacity of the thermal power generator can cover the net load (actual load minus the power generation output other than the thermal power generator).

2.4 Dynamic economic load dispatch

The ELD problem can be formulated mathematically as a constrained optimization problem with an objective function and constraints,as expressed by the following equations:

where f (pR) is the total generation cost,and K is a set of generators to operate based on the decision of unit commitment (see Table 1).From the point of view of practicality and convenience of mathematical processing,the fuel cost function can be expressed by the following formula [7]:

where fi (pi) is the power generation cost function of the i-th unit; pi is the power of the generating unit; and ai,bi,ci are the fuel cost coefficients of the generating unit.

All the constraints include the load balance constraint and the generator upper/lower limit constraint.If we need to treat a multi-stage decision problem,that is,to solve the dynamic ELD problem,then it is necessary to add a constraint of generator start/stop dynamic constraint,as follows:

where i is for all generators,and t is for all time sections.

2.5 Automatic generation control and load frequency control

Maintaining the power balance is commonly referred to as regulation and can be sensed by changes in the system frequency.If load (including losses) is exceeded,then necessary energy will be drawn from the kinetic energy stored in the rotating masses of the generators.Hence,the generators will begin to rotate more slowly and the system frequency will decrease.Conversely,if generation input exceeds the load,then the frequency will increase.The governor on a generator senses these speed deviations and adjusts the power input (e.g.,through the opening or closing of valves on a steam unit) as appropriate.AGC and LFC are functions to achieve these objectives (see Section 4.4 for details).

2.6 Dynamic power flow analysis for microgrid

The power flow problem consists of a given transmission network in which all lines are represented by a π-equivalent circuit and transformers by an ideal voltage transformer in series with an impedance.Mathematically,the power flow requires a solution of a system of simultaneous nonlinear equations as follows:

To solve such large-scale nonlinear equations,there are methodologies such as the Gauss–Seidel method and the Newton–Raphson method.In MicroGrid Designer,the Newton–Raphson method incorporating a high-speed sparse LU decomposition is adopted.

2.7 Quantitative evaluation of annual fuel consumption and CO2 emissions

In MicroGrid Designer,the CO2 emissions coefficients of generators are used to obtain CO2 emissions.The amount of CO2 emissions is calculated from the following equation:

CO2 emissions (tC)=fuel consumption (GJ) × carbon coefficient (tC/GJ).

The carbon coefficients are published by the related international organizations or the government of each country.

3 Application of the tools to a typical microgrid

3.1 Structure and components of the standard microgrid

In this section,the developed design tools are applied to a microgrid with standard structure and components,as shown in Fig.4.This microgrid is composed of 10 nodes and 9 distribution lines,and the capacity is assumed to be 3.2 GW.A gas turbine,a diesel generator,and a battery have been installed at Node G1,which is the center of consumption of this grid.Photovoltaic (PV) generation or wind generation are planned to be installed at Node G2 in the near future.There are five diesel generators at Node G3 and six diesel generators at Node G4.A small hydro generator (constant output) is also planned to be installed at Node G5.Nodes L1–L5 are load nodes that supply electricity to demands.According to the regulations,diesel generators must have mandatory maintenance once per year;therefore,it is assumed that two diesel generators at Nodes G1,G4 are in maintenance during this period.

3.2 Decision of priorities of diesel generators

The unit commitment is to determine the priority of generators to be used for the microgrid operation.Generation units to be committed and outputs to be supplied by the generation units to meet the electricity demand must be determined for each time period.In MicroGrid Designer,the schedule of power generating units and the generation output of each unit are determined by comparing the efficiency of generators (using the priorities of generation units),as mentioned in Section 3.3.

In the standard microgrid,one gas thermal generator and 12 diesel generators (two generators now in maintenance)are equipped.The largest gas thermal generator plays the role of the slack generator to absorb active and reactive power losses and to regulate the fluctuation of power from renewable energy generation; then,this generator is out of the unit commitment.Table 1 shows the list of priorities of generators at each demand level obtained by comparison of the efficiency of diesel generators at the maximum output limits.This priority can be revised by grid operators to reflect their intention.

In the table,0 means the generator is out of operation,and 1 means it is in operation.For example,when the load is 2500 kW,generators 1,2,3,4,5 should be operated and the others should be stopped,as shown in the table.

3.3 Dynamic economic load dispatch of the microgrid

The ELD software in MicroGrid Designer has a function to determine the production level of each diesel generator,so as to minimize the total cost of generation for a prescribed load schedule.The method of ELD for generating units at different loads must make the total fuel cost at the minimum value over the predetermined period.This approach is called dynamic (multi-stage) ELD.

Fig.5 shows the obtained results for a time series of generator outputs over 24 h without considering the priorities of generators (the unit commitment); therefore,all the diesel generators join the operation.This means that the unnecessary generators with lower efficiency are in operation,and dispatching does not attain the minimum cost operation.

On the contrary,Fig.6 shows the obtained results of a time series of generator outputs over 24 h considering the priorities of generators (the unit commitment).As shown,all the generators are not always in operation,and only the generators with higher efficiency run at the low demand level.Even at the highest demand level (at 12:00),only seven generators with higher efficiency out of 10 generators are used to attain the minimum fuel cost operation.

3.4 Detailed simulation of frequency deviation under LFC

The load of a microgrid varies instantaneously.Coping with the instantaneous variations of load requires a continuous change in the generation.When the load suddenly increases in the grid,initially the kinetic energy stored in the rotating masses of the generators will be utilized to meet the load change [7].Consequently,the speed and frequency drop.The governor mechanism then acts to increase the fuel input to the system to meet the increased load.The primary governor control alone cannot bring the frequency to the scheduled value,and the LFC is required to attain it.

The tasks of multi-stage ELD and LFC analysis are to determine outputs of available generators at the minimum cost and to regulate frequency deviations over a certain period.Conventionally,the economic dispatch problem of a power system is solved after determination of the unit commitment of generators by assuming that each of the online units is able to dispatch.The outputs of generation units are regulated continuously between the minimum generation limit Pmin and the maximum generation limit Pmax.At the same time,LFC is conducted to maintain the grid frequency within a permissible range [9].

In MicroGrid Designer,the detailed simulation software for frequency deviation is prepared to verify the performance of coordinated operation of the dynamic ELD and LFC,as shown in Fig.7.Precise descriptions of the frequency simulation and the related mathematical equations are not explained here due to space limitations.Depending on the prediction of demand every 5 min,commands Pgi of outputs by the ELD are sent to generators.The frequency deviation is detected by a monitoring device and the LFC produces the control signals to generators to regulate the frequency deviation every 5 min.

Fig.8 shows an example of the frequency deviation after regulating LFC generators over 1 h,as obtained by MicroGrid Designer.

3.5 Quantitative evaluation of annual fuel consumption and CO2 emissions

By conducting the dynamic load dispatching as shown in Fig.9,quantitative evaluation of fuel consumption as well as CO2 emissions is available.

From the result of optimizations and simulations from dynamic load dispatching,we can calculate the total summation of outputs of all generators to be operated.By using the fuel consumption functions,the total fuel consumption of all generators over 24 h is obtained and,at the same time,total CO2 emissions can be calculated by using coefficients of CO2 emissions for different types and capacities of generators.

Using MicroGrid Designer,reduction of fuel consumption and CO2 emissions by installation of renewable energy can be calculated quantitatively.

Fig.10 illustrates the reduction of generation outputs after installing PV generation.In the standard microgrid used for these applications and verifications,it is planned that 1–3 MW PV generation will be installed in the near future.

Fig.10 shows that PV generation produces electric power from 7:00 to 18:00,and that generation outputs of diesel generators are reduced remarkably during the daytime.

The LDC is used to evaluate the fuel consumption and CO2 emission of generators in the microgrid.If the curve is plotted over a time period of 24 h,it is known as a daily load curve.If it is plotted for a week,month,or year,then it is referred to as a weekly,monthly,or annual load curve,respectively.The LDC reflects the activity of society and industry quite accurately with respect to electrical power consumption over a given period of time.

In MicroGrid Designer,all types of LDCs can be utilized depending on the targeted time horizon,but for quantitative evaluation of annual fuel consumption and CO2 emission,the annual LDCs are used as in Fig.11.The dynamic ELD is conducted over 8760 h.By summing up the outputs of all generators,the annual total generation in this microgrid is calculated,and the annual total CO2 emission is also obtained.When renewal energy generation is installed,the reduction of fuel consumption and CO2 emission of the existing generators is obtained quantitatively.

MicroGrid Designer is an effective tool to evaluate the annual generation cost and environmental benefits of renewable energy for microgrids by using the LDC,which can be worked out for a given region (or for any collection of loads) by measuring or estimating the hourly load for 8760 h in a year.The loads for 8760 h are sorted and plotted to construct the annual LDC.

Figs.11 and 12 show the effects of installing renewable energy and conducting unit commitment.Fig.11 illustrates the results for the base case in which renewable energy generation is not installed,and unit commitment is also not conducted in this microgrid.Fig.12 shows the comparison of the base case and the second case for which renewable energy generation is installed and unit commitment is conducted.

By using MicroGrid Designer,we are able to evaluate the effect of installation of renewal energy and utilization of unit commitment.Finally,it is shown that the fuel consumption as well as CO2 emissions in the second case are reduced by 35.15% compared with the base case.This clarifies that installation of renewable energy and utilization of unit commitment (effective start and stop of generators depending on the priority of generator costs prepared by MicroGrid Designer) can greatly contribute to reduction of fuel consumption and CO2 emissions for the standard microgrids.

3.6 Dynamic power flow of the microgrid

In MicroGrid Designer,two types of power flow analysis tools are prepared.One is the Static (single-stage)Power Flow Analysis Module,which is a steady-state analysis tool whose target is to determine the voltages,currents,and real and reactive power flows in a grid under given load conditions and various hypothetical situations.The other is the Dynamic (multi-stage) Power Flow Analysis Module,which is a chronological power flow analyzer for time-varying loads to determine the transitions of voltages,currents,and real and reactive power flows in a grid over a time horizon.

Fig.13 shows a graphical interface of power flow and voltage profile conducted for the standard microgrid using the dynamic power flow analysis tool.As there is no installation of renewable energy generation here,most of the electric power is generated at Nodes G1,G3,and G4,and the power is transferred to Nodes L1 and L5,which do not have generation sources.The maximum power flow occurs at Branch B9,and the minimum voltage occurs at Node L1,which is a terminal end of the microgrid.It helps grid engineers and users of MicroGrid Designer to understand and recognize the situations and occurrences of the targeted microgrid effectively.

4 Conclusion

MicroGrid Designer can be used extensively by grid engineers for power flow and transmission loss analysis,ELD and cost analysis,evaluation of renewable energy sources,and overload diagnosis of distribution and transmission networks.

This article serves to show grid engineers how MicroGrid Designer is used for design,operation,control and economical evaluation of microgrids.

As mentioned in Section 1,microgrids can connect or disconnect from the main grid as physical and/or economic conditions dictate.It can operate in both grid-connected mode (synchronous with the grid) and island mode(operation autonomously and disconnected from the main electric grid).

In natural disasters and blackouts of conventional utility grids,the designed microgrid has to be operated in island mode,which runs autonomously and is disconnected from the main electric grid.By using the developed tool,we are able to verify whether the designed microgrid can operate in both grid-connected and island modes.In case the microgrid does not fulfill the resiliency,we change the combination and capacity of energy facilities and redesign the microgrid to attain the desirable resilience.

MicroGrid Designer provides compact but high applicability for the power flow profiles and economic power system operation states,such as active power,reactive power,voltage,phase angle,and frequency,and is applicable not only to conventional power systems,but to various kinds of new and advanced grids,such as micro grids,smart cities and autonomous communities.

MicroGrid Designer is also characterized by high computational performance.By proposing a simple algorithm for effective priority of generators and a highspeed power flow calculation methodology,the dynamic ELD calculation is very fast.For a system of 50 generators,the annual ELD simulation (8750 h) can be completed in less than 1 s,and dynamic power flow for 24-h calculation can also be completed in less than 10 s.

MicroGrid Designer is applicable for the following:

• Conventional electric power systems

• Distribution grids with sustainable energy generation

• MicroGrid and MiniGrid

• Autonomous regional grid

• Smart and ecocity

• Smart grid (including smart houses,smart buildings,smart industrial/science parks,smart packing,etc.)

• Smart community

• Grids in islands and remote areas

• Future grid planning for non-electrified areas

In this tool,all necessary data are input by wellknown Excel files.It is convenient for users to arrange the input data because all data input formats follow the IEEE Standard Input format in the US.All outputs are shown by tables that also follow the IEEE Standard Input format,and most of the simulation results are illustrated automatically by figures and graphs,as shown in Figs.5–13.

Declaration of Competing Interest

We declare that we have no conflict of interest.