1P. (see Single phase)

3P. (see Three phase)

AC. Alternating current. Electric current that periodically reverses direction, typically twice per second (see Frequency, Hertz). Direct current (DC) flows in only one direction. Alternating current runs household appliances, whereas flashlight batteries provide direct current. AC power can be transformed (by voltage transformers) into high voltage with lower current and distributed more efficiently over long distances, then reduced to low voltage for consumption.

ATS. Automatic Transfer Switch. A “gatekeeper” switch that can automatically connect or disconnect the MicroGrid from the utility grid. If the grid fails and the MicroGrid is producing electricity, the ATS can route power onto the grid.

Base load. The minimal level of demand on an electrical grid over 24 hours.

Blockchain. A growing list of records, or blocks, linked using cryptograph; a distributed ledger without central authority that votes by consensus algorithms and is resistant to data modification by design.

BMS. Battery management system. A BMS electronically monitors and manages a rechargeable battery, maintaining safe operating parameters, controlling its environment, balancing it, and reporting its data, such as voltage, temperature, state and depth of discharge. 

CA ISO. California Independent System Operator. The independent, non-profit transmission authority that oversees movement of bulk electric power long distances and also operates a marketplace for commercial generators to sell power to utilities; a new rule allows small private generation owners to aggregate and sell power.

CCHP. Combined cooling, heating, and power system; a trigeneration system.

Controllable assets. Devices responsive to control signals, ranging from simple (on/off) to more complex (adjusting speed or output incrementally).  Assets that can curtail power are important in balancing small power systems with greater load variability than utility-scale grids.

Converter. A device that (usually) converts AC into DC (see Inverter).

CPUC. California Public Utilities Commision. The agency that regulates privately owned public utilities in the state, including electric power.

Charge controller. Device connected between a solar array and battery to control the rate of charge.

Critical Grid. A backup system of lower-voltage, solar-charged “buffer” batteries that provide an uninterruptible power supply (UPS) to protect essential equipment.

DC. Direct current.  Electric current that flows in just one direction. Alternating current (AC) periodically reverses direction. Direct current is generated by solar (photovoltaic) cells and is used to charge batteries. DC can be converted to AC with an inverter.

DER. Distributed energy resource. Typically, DERs are small, decentralized, renewable energy generation or storage devices. Microgrids are considered DER systems because they connect to the periphery of the macrogrid.

Disconnect. A “smart” safety device that can interrupt a circuit.

EMS. Energy management system. A computerized network that monitors, controls, and optimizes the performance of an energy generation and distribution system, including a microgrid. For multiple assets with different technologies controlled by an EMS, a hierarchical master/slaves architecture works well, especially in island mode.

Frequency. The number of occurrences of a repeating event (or cycle) per unit of time (see Hertz).

Fuel cell.  A device that converts chemical energy into electricity from the reaction of hydrogen with oxygen to form water. Fuel cells reverse the electrolysis process.

Hertz (Hz). A unit of electrical frequency, defined as one cycle per second. AC typically operates at 60Hz, or 60 cycles per second. A cycle, or oscillation, is represented by a sine wave.

Inverter. A device that (usually) converts DC into AC.

Kilowatt (kW). A measure of 1,000 watts of electrical power; the rate at which electric power is generated or consumed.

Kilowatt-hour (kWh). A measure of 1,000 watt-hours of energy; the generation or consumption of 1,000 watts of electrical power for one hour.

Load. A part or component of a circuit that consumes electrical power.

Low Carbon Fuel Standard. A program of the California Air Resources Board that gives a credit of $2.17 per kilogram of hydrogen gas produced.

MAC switch. Microgrid Actuation Circuit. A specially developed ATS from Emerson ASCO that measures power quality from the utility grid. If quality is normal, the connection is maintained. If not, the switch looks for voltage and frequency on the emergency (MicroGrid) side and, finding it, first synchronizes them before switching to island mode.

Management system. The communication and control components of the MicroGrid.

Megawatt (MW). A measure of 1 million watts of power; the rate at which electric power is generated or consumed.

Megawatt-hour (MWh). A measure of one million watt-hours of energy; the consumption of 1 million watts of power for one hour.

Microgrid. (U.S. Dept. of Energy definition) “A group of interconnected loads and distributed energy resources (DERs) with clearly defined electrical boundaries that acts as a single controllable entity with respect to the grid (and can) connect and disconnect from the grid to enable it to operate in both grid-connected or island mode.” Read more

(CPUC definition) “’Microgrid’ means an interconnected system of loads and energy resources including, but not limited to, distributed energy resources, energy storage, demand response tools, or other management, forecasting, and analytical tools, appropriately sized to meet customer needs, within a clearly defined electrical boundary that can act as a single, controllable entity, and can connect to, disconnect from, or run in parallel with, larger portions of the electrical grid, or can be managed and isolated to withstand larger disturbances and maintain electrical supply to connected critical infrastructure.”

Microinverter. A small inverter placed on each solar panel to produce AC more cost-effectively. A “smart” inverter, like the Enphase Energy S280, enables curtailment of solar output.

Optimization mode. A state in which the aggregate net of power generation and use balances at zero. All systems act as one entity, and fluctuations in voltage, frequency, real power and reactive power are minimized with controller algorithms. In effect, assets bid to provide the most economical power.

PCC. Point of common coupling. The point where the customer connects to the utility grid, typically considered at the meeting point, service entrance or service transformer. The connection point between the MicroGrid and the utility grid.

Peak. Period(s) of relatively high system demand for electricity.

Power. The rate of flow of energy past a given point of an electric circuit.

PV. Photovoltaic. Production of electric current at the junction of two substances exposed to light; converting sunlight into electricity.

Reactive power. The portion of power due to stored energy, which returns to the source in a complete AC cycle.

Real power. The portion of power In a complete AC cycle that results in net transfer of energy in one direction. Also called “active power.”

Relay. An electrically operated switch to implement a safety function.

Round-trip efficiency. The ratio of energy put into a storage device to energy taken from it, expressed as a percentage. The lesser the percentage, the greater the loss of energy.

Rule 21. A CPUC document, the law for all utilities, that addresses safety concerns, particularly regarding the PCC, and elements to accomplish compliance. Currently, there is no export provision in Rule 21.

Single phase (1P). An AC service with two energized wires that typically serves smaller needs of 120/240V, such as residences, and requires less equipment and is less expensive to install and maintain.

Switches. Devices that open and close electrical circuits. Smart switches can be programmed to perform tasks on their own, like controlling a water pump, eliminating the need to put communication in the pump control.

Thermal runaway. A situation in which a rise in temperature alters conditions to cause further increases in temperature that can produce failure or even fire with certain types of batteries.

Three phase (3P). An AC service with three energized wires that typically serves power needs larger than 120/240V, such as industrial or agricultural uses.  The 480V/3P system is also called 480V 3 Phase 4 Wire, 480V 3 Phase Wye, and 480Y/277V, all of which refer to the fourth (neutral) wire that connects with the center of a “Y” intersection (common star point) of the three power wires. A smaller ground wire normally carries no current. The four-wire wye is used to accommodate both single-phase and three-phase (e.g., electric motors) loads. Neutral provides three separate single-phase supplies at constant voltage. Read more

Conductors between the voltage source and load are called lines. Voltage between two lines is called line voltage. Voltage between any line and neutral is called phase voltage. In a symmetrical 3-phase, each line carries AC at the same frequency and voltage amplitude relative to neutral, but a phase difference of one-third of a cycle (120°). This difference gives constant power transfer to a balanced linear load (much as each cylinder in a three-cylinder engine fires offset from the others). The line voltage will be 480V line-to-line and phase voltage 277V line-to-neutral.  

The amplitude of the voltage difference between two phases is the square root of 3 (or 1.732) times the voltage of the individual phases (277V x 1.732 = 480V). The sum of the instantaneous currents of the three conductors with a balanced linear load is zero; the current in each is equal in magnitude to the sum of the currents in the other two, but with the opposite sign. The return path for the current in any phase conductor is the other two conductors. (end Read more)

Trigeneration. A system of combined cooling, heating and power (CCHP) production.

UPS.Uninterruptible power supply. An electrical device that provides emergency backup power when the input power source fails. See Critical Grid.