◊ This is part of the ‘Electricity Generation’ series of articles ◊
Connecting generation to the grid can be as simple as a routine switching operation for existing facilities or a complex formal process for constructing new ones.
Existing generators
Existing generation will be equipped with synchronizing controls which match generator output to grid voltage and frequency to permit connection to an energized grid. In special cases of grid restoration, some generators have black start capability for startup and can connect to a de-energized grid.
New generator connection
Connecting new generating capacity to the grid is the final step in a formal process which includes identification of needs, procurement, connection impact assessment, construction and commissioning. At the end of the process, a generator connects to the delivery infrastructure and becomes part of the supply.
Generator connection requirements
There are many requirements which must be met before a generator can be part of the grid. The entity to which the generator connects will stipulate the conditions of service to which they must comply. The conditions will include compliance to standards set by organizations with jurisdiction locally, nationally and internationally. The applicable connection criteria is most stringent for generators that supply the bulk power system and least for micro generators. The requirements are set to ensure safety and reliability of supply to all stakeholders.
Generators that are connected to the bulk power system generally require:
- motorized isolating device at the demarcation point between the generator and the delivery infrastructure suitable for being energized from both sides
- interrupting device(s) suitable for protection and synchronized switching
- meet the insulation requirements of the system being connected to
- voltage measuring devices for protection, regulating, statistical and revenue metering
- approved metering device to measure energy delivered to the grid for financial settlement
- transformation equipment with an approved winding configuration suitable for grid connection
- active and reactive power capability stipulated by the delivery entity and reliability authorities
- meet applicable electrical safety codes
- protection equipment for the facility in accordance with the delivery and reliability authority
- telecommunication facilities to support protection, control and annunciation
- local and remote control as required by the operating authority
- disturbance, sequence of event and power quality monitoring equipment
- grounding and isolating equipment to prevent fault impact outside of the generator facility
Generation connection point
The grid delivery infrastructure has two systems – transmission and distribution. Each system is distinguished by voltage level. The transmission system has the highest voltage levels for bulk power transfer. The distribution system has lower voltages more suitable for transferring power over shorter distances. Voltage demarcation between the two systems is region-specific.
A generator connects to either the transmission or distribution system depending on its capacity and proximity to delivery infrastructure. The connection requirements on the transmission system are more stringent than the distribution system due to reliability criteria.
In Ontario, the transmission system is a high voltage network (greater than 50 kV) that handles the bulk of power from generators. The high voltages are required to transfer energy efficiently over long distances. Typical transmission voltages in Ontario are 230 kV and 500 kV. The transmission system and the generators connected to it form the bulk power system (BPS) – subject to definition by the reliability authority.
Large load customers like major manufacturers and distribution companies connect directly to the transmission system.
Transmission voltages are stepped down to a lower level (50 kV or less) to create the distribution infrastructure to which most load customers connect.
The generator connection point may also be called the point of common coupling subject to criteria set by the delivery entity.
Large scale generation
Large scale generators will produce a three phase output and couple with the transmission system.
Three phase systems are characterized by three separate conductors supported by large insulators. Three phase systems have been the standard for grid infrastructure for over a century.
The output voltage of synchronous generators can range from 2kv for a small 1 MVA hydroelectric unit up to 22 kV for an 880 MW nuclear unit.
A generator does not produce sufficient voltage to couple directly to the transmission system and requires step-up transformers.
Embedded generation
Small generators connect to the distribution system and are referred to as embedded generators. Embedded generators may be three phase or single phase depending on the capacity and point of connection. Embedded generators do not typically need to meet all of the requirements a transmission connected generator does. In some jurisdictions, embedded generation is called distributed generation.
Embedded generators will have capacity limitations based on
- voltage of the feeder
- single phase or three phase
- short circuit limitations of the infrastructure
Residential roof-top solar PV generators produce single phase electricity because most residential service entrances are single (split) phase and the solar PV’s intended function is load displacement.
Examples of technical requirements for embedded generation are available from Hydro One:
That completes the series on electricity generation! At this point you know the high level concepts of generating electricity and how it connects to the grid.
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Derek
