Introduction

I.1 Electric Power System and the Need for Change

Electric power system (EPS) is the infrastructure to generate, transmit, and distribute electricity. The existing EPS is the largest and most complex man-made system. The bulk electricity is generated in large power plants in the form of alternating current (ac) using synchronous generators (or alternators). The ac voltage is then boosted using transformers and transmitted via long transmission lines. After transmission, the voltage levels are decreased and the electricity is used for various industrial, commercial, and residential applications. Electrification has been the greatest engineering achievement of the twentieth century [1].

I.1.1 Review of Operational Principles of EPS

The existing EPS has a hierarchical structure with central power plants generating the electricity and sending to users through the transmission system. The power flow is unidirectional from Generation to Distribution via Transmission.

• Generation
  1. A Power Plant is an industrial plant to generate bulk electric power.
  2. Dominant sources of bulk electric energy are fossil fuels (coal, natural gas, petroleum), nuclear reaction, and stored water. Corresponding turbine types are steam (for fossil fuels and nuclear) and hydro turbines.
  3. Popular generator is the three-phase synchronous generator (ac, 50 or 60 Hz).
  4. Unit Transformer steps up the voltage at the power plant.
• Transmission
  1. High-voltage overhead lines (ac voltage is boosted using transformers).
  2. Meshed network to increase reliability.
  3. Voltage is stepped down at transmission substations.
• Distribution
  1. Voltage is further lowered at distribution substations.
  2. Medium to low-voltage transmission lines; radial feeder topology; pole transformers (three-phase and single-phase low voltage end users).
  3. Industrial, commercial, and residential users.
  4. Residential applications: cooling; heating; lighting; refrigerating; washing; drying; entertainment; cooking; etc.

The EPS is highly interconnected: many generation and transmission systems are connected together to form a large pool of energy resulting in a highly reliable system. Maintenance of such a large interconnected ac system in terms of synchronized, stable operation, and protection of all components while preventing cascading failures is the everyday challenge of electric utility companies.

I.1.2 Problems with the Existing EPS

According to US Energy Information Administration (EIA) [2], about kWh of electricity was generated in United States in the year 2016.1 This caused a total emission of  kg.2 Every kWh of electricity caused about 0.44 kg of emission. An average house in United States consumes about 900 kWh of electricity per month which corresponds to release of about 13 kg of per day, about 400 kg per month and about 4800 kg per year. This is a major problem with the existing EPS among several others summarized as follows.3

1. Environmental impacts ( emission, impacts on nature, green-house effect)
2. Unsustainable (ever increasing energy demand versus limited resources; dependence on oil market)
3. Low generation efficiency (typical efficiency of coal, petroleum, and nuclear power plants being around 30% and that of natural gas plants around 40%)
4. Transmission losses (amount to 5-10% of the total transmitted power)
5. Maintenance (synchronization, stability, protection, and cascading failures in a large interconnected ac system in addition to the cost of infrastructure).

I.2 A Potential Solution: Renewable Integration

Renewable energy4 (from sources such as sun, wind, moving or stored water, etc.) can be converted to electricity. Only a small portion of the total energy in the solar rays reached to the earth is sufficient to supply our total energy demand [4, 5].

Dispersed or distributed generation systems are small generators interfaced with the distribution (low-voltage) or sub-transmission (medium-voltage) lines.

I.2.1 Examples of Distributed Generation Systems

1. Renewable: Solar photovoltaic (dc), solar thermal, wind, tidal, micro hydro5
2. Nonrenewable: Micro-turbines, diesel generators (DGs)6

Geothermal generators tap to the earth heat at locations that are susceptible for it.

Biomass generators burn biological materials from nature to generate steam.

Fuel Cell technology uses chemical reactions to generate electricity (dc).

I.2.2 Additional Benefits of Deploying Distributed Generation

1. Reduce transmission loss.
2. Reduce/defer transmission system expansions.
3. Recovering the heat loss (combined heat power, CHP, systems).
4. Participation of consumers in market (producing consumers: prosumers!).
5. Autonomous (or islanded) operation of a section of distribution system: increased reliability and resilience.
6. Offering ancillary services to the grid.
7. Keeping the oil and gas prices more stable and lower for longer time.
8. Lower dependence of power industry on oil and gas industry changes/uncertainties.

I.2.3 Technical Challenges with this Solution

Technical challenges arise when high level of distributed generation is integrated.

1. Bidirectional power flow can substantially change the voltage profile. This will cause malfunctioning of voltage regulating devices. The transformer-based voltage regulators, capacitor banks, and protection equipment need to be properly upgraded and/or modified.
2. The system's responses to faults change. This will require readjustment (of settings) and rearrangement (of locations) of relays and other protection devices.
3. Grid stability problems due to uncertain and variable nature of renewable sources.
4. Grid-scale battery energy storage (BES) systems may be required to address variable and uncertain nature of the renewable resources because the conventional generators have limited ramp up/down rates and cannot respond to those variations. The BES technology is not yet fully an economical and environmental-friendly technology at large.
5. Islanding prevention (when the local EPS is unavailable, the distributed generators should not energize it).
6. Coordination and control of high number of distributed generators.

There are also the regulatory and policy-related challenges which are not discussed here. For example, the regulation of possible ancillary services that the distributed generators can provide to the grid, given the wide range and variety of services that can be possible, is an ongoing challenge.

I.3 Microgrid

A microgrid (G) is a cluster of distributed energy resources (DERs) and loads that is connected to the grid at a single location [6]. The G can operate in grid-connected (GC) and in islanded mode. It may also operate in isolated mode without a grid connection. From our technical discussions, islanded and isolated conditions are often similar. Thus, the term standalone (SA) is used to describe this mode.

DER includes distributed generator, distributed storage, and distributed load [7].7 The G concept may be the key concept to address the aforementioned challenges.

I.3.1 Properties and Advantages of G

1. When connected to the grid, the G performs as a controllable entity with controlled interaction with the rest of the grid. This interaction may be characterized as follows.
   • Real and reactive power exchange
   • Harmonic filtering
   • Fault ride through and grid support (and ride through grid frequency swings)
   • Grid stability support and improvement, frequency response
   • Power quality aspects: harmonics, unbalance, flickers
2. In SA mode, the G supplies power to its own loads. Major aspects are as follows.
   • Voltage quality and stability
   • Power management and power sharing
3. The G makes seamless transition from GC to islanded and vice versa.
4. The G concept can help realizing the smart grid functionalities such as demand response (DR) management.

Low rotating inertia of distributed generators is a possible concern. This will reduce the total inertia of the EPS and makes it susceptible to larger frequency swings. Low over-current capability and low over-load limit of power electronic switches are other issues that must be respected. For instance, a conventional induction motor requires high level of current to start.

In order to smooth down the variable generation of renewable sources, certain amount of nonrenewable distributed generation (such as diesel and gas turbine generators) and distributed storage resources should be included in a G. Distributed storage technologies include battery, ultra-capacitor, flywheel, pumped hydro, stored hydrogen, etc. Electronically controlled loads (e.g. active rectifiers and motor drives) may be considered among DERs as they can actively...