◊ This is part of the ‘Electricity Generation’ series of articles ◊
Electricity generation has many aspects which require consideration by energy professionals in the utility business. Here are just a few of the concepts related to generators which are managed by specialists to ensure the reliability of the grid.
Islanding
Islanding refers to the separation of the main grid into smaller autonomous geographic areas which are capable of operating with their own generation. Islands can be formed by planned switching operations or inadvertently as a result of electrical disturbances. Islands are usually formed on a short term basis. They may be strategically formed to mitigate the impact of cascading outages or to shift load between adjacent grids operating with different voltage or frequency limits.
Microgrids are capable of operating as an island to improve supply reliability and have the capability of reconnecting to the grid when operating conditions permit.
Inadvertent islanding poses a number of challenges for grid operation, equipment protection and worker safety. Grid operations require complex restoration procedures to re-establish the grid.
Restoration of an island requires synchronizing capability across an isolating device. Connecting two different ac systems with the same nominal frequency requires them to be in phase and the same voltage. Even though systems have the same nominal frequency rating, they inevitably operate at slightly different frequencies within their band of tolerance. The difference in frequency between systems is called ‘slip’. The slip between two different ac systems creates higher voltage stresses for switching equipment as the phases will experience opposite polarity on a cyclic basis. The isolating point will see twice the peak voltage when the polarity of the two system voltages are opposite.
An islanded system must be restored by reconnecting to the grid at the precise time when the systems are at the same point in the alternating cycle – preferably at the point when the voltage is zero. If two ac systems are connected when they are not in sync, it can be similar to switching into a short circuit. Synchronizing minimizes the jolt to both systems.
Anti-islanding
Operationally it is easier to manage inadvertent grid isolation by avoiding islanding. Anti-islanding protection schemes trip islanded generation off line during disturbances to minimize customer interruption and permit automatic reclosing of the interrupting device. Automatic reclosing can only be performed after the islanded generation has been taken off-line.
Anti-islanding protection schemes utilize high speed telecommunications to send trip and other command signals to generators during fault conditions to minimize outages and facilitate restoration. The signals must execute in a matter of milliseconds in bulk power system applications.
Reclosing is a common utility practice to improve reliability as the most common causes of outages self-clear. Outages caused by lighting, tree or animal contact are usually non-permanent and the system can be restored automatically by reclosing. Reclosing may be single or multi-shot depending on jurisdictional operating practices. The first reclose is typically around 150 milliseconds after the disturbance with various timing for multiple shots after that. Islanded generators need to be off line in under 150 milliseconds or the reclosing sequence will be blocked. Reclosing is not utilized where underground cabling is used as faults are usually permanent and require replacement or repair.
Black start
Following a major grid event– such as blackout – generators must systematically be brought back on line and pick up load. In the worst case scenario where the generator was forced off line and shut down, without any grid power available, it requires a black-start to resume operation. The black start must be performed without power from the grid.
Generating facilities require auxiliary power to operate services that support the generator and generating process. That includes governors, excitation systems, switchgear, cooling pumps etc. based on the type of primary fuel and generator type. The power required may come from battery supply, or auxiliary generator.
Not all generators have black start capability. Some generators require an energized grid to synchronize to, or power from it to operate.
Once a generator has been started it must have sufficient capability to restore the loads to which it is connected. Without sufficient capacity, the generator will trip offline from voltage or frequency deviations. Load restoration must be performed in stages to prevent overloading generators.
A regional operating authority will have an emergency power restoration plan which includes generators with black start capability along with strict operating procedures to ensure successful grid restoration.
Distributed Energy Resources (DERs)
In the context of electricity generation, distributed energy resources (DERs) are relatively small generators that supply loads in close proximity. DERs are relatively new to the grid and have proliferated through various incentive programs.
The grid architecture developed over the last century using large generating facilities connected to the transmission system. The transmission system can efficiently deliver large amounts of energy over long distances. In the last two decades, small renewable generation has been increasingly available for connection to the distribution grid. The distribution system is a low voltage network (less than 50kV) fed by the transmission system. The distribution infrastructure is what connects most electricity consumers to the grid. The grid was not designed to accommodate distributed energy resources on the distribution system and is in the process of transitioning to accommodate the new architecture. Most jurisdictions have developed DER connection standards that require specific technical features of distributed generators to ensure safety and power quality.
DERs are not part of the bulk power system and are embedded in the distribution system.
For additional reading, see the IESO – Distributed Energy Resources and the OEB ‘Charting the course for regulatory clarity and consumer value in Ontario’s energy transition’.
Reactive power compensation
Reactive power is required to supply the inductive and capacitive characteristics of certain types of generators, delivery infrastructure and load. It reduces the efficiency of the grid. Reactive power production by generators is one compensation option available to maximize power transfer to loads. The compensation may be:
- static – provided by capacitive or inductive elements grouped together in banks which may be switched on or off the grid – Static Var Compensators
- dynamic – provided by synchronous generators through their excitation or rapidly switched reactive elements using power electronics – Dynamic Var Compensator
Harmonics
In an ideal ac system, generators would produce power at 60 Hz with a perfect sinusoidal output. In practice however, all generators do not produce an ideal 60 Hz sinusoidal output. A distorted sine wave can be modelled by adding other sinusoidal functions at frequencies which are integer multiples of the fundamental. These additional components are called harmonics.
The presence of harmonics on the power system are important considerations for transformers and noise-sensitive equipment. Generators that use power semiconductors to produce outputs generate harmonics which may negatively impact grid infrastructure and customer power quality. Harmonics are limited by most jurisdictions to maintain power quality. Standards define limits to the magnitude of individual harmonics and the Total Harmonic Distortion (THD). Examples of Ontario standards are published by Hydro One here (see Table 3).
Additional information is available in the ‘Some physics’ article.
Generator transient response
The electrical grid is a dynamic machine with constantly changing loads and frequent transient disturbances. A transient disturbance is a temporary deviation from normal operation which may occur randomly or result from an intentional action. Transient disturbances may be caused by switching operations, faults, sudden load changes, equipment failure or lightning strikes. Disturbances may be as short as a few microseconds or last for minutes.
Generators respond to transient events to help maintain voltage and frequency within prescribed limits to maintain power quality. In the case of short circuits, generators provide the high currents used for protection coordination. Short circuits are sensed by protection devices which then trip circuit breakers to isolate faults.
A qualified planning authority ensures that the mix of generation on the grid has the capability of withstanding transient disturbances to ensure system resilience.
Generators and short circuit fault contribution
Generators provide fault current for short circuits which occur on the grid. The short circuit level at any point on the grid is determined by many different factors including the generators internal impedance (transient and subtransient reactance), short circuit path impedance and the amount of generation available. Short circuit levels are usually expressed in Mega-Volt-Amps (MVA) or current magnitude in kilo-amps (kA).
The short circuit current must be sufficient for protection equipment to detect without exceeding the fault and interrupt rating of the infrastructure in its path. Without proper consideration, generation could pose a problem for conductors, circuit breakers, transformers, switches, surge arrestors and ground grids. Bulk power grids may have short circuit levels between 50 and 100 thousand amps on the transmission system. A residential 120/240 volt service is typically short-circuit rated for 10 thousand amps.
The fault capability of a generator depends on its design, type and capacity. Synchronous generators will have the highest fault capacity while non-synchronous or asynchronous generators (wind and solar PV) can provide a minimal amount. Asynchronous generators are coupled to the grid through power semiconductor devices which limit the magnitude and duration of fault current contribution to the grid. Asynchronous generators using power semiconductor devices to produce their output may also be referred to as inverter-based resources or IBRs
Short circuit ratio (SCR)
The Short-Circuit Ratio (SCR) is an important indicator of grid resiliency and recognized by reliability authorities. SCR is the ratio of short circuit MVA available to the MVA rating of the generation. The higher the SCR, the more resilient the grid is considered to be. SCRs will be different throughout the grid depending on its configuration at any point in time. It is usually referenced at the point of interconnection (POI) of a resource. Reliability organizations have concerns about the increasing percentage of asynchronous generation and the impact it may have on grid resilience.
Additional reading – NERC, Short-Circuit Modeling and System Strength, February 2018
Additional considerations for inverter-based resources (IBRs)
Inverter-based resources use high frequency semiconductor switching to approximate a sinusoidal voltage at the grid fundamental frequency. The IBR output is never a perfect sinusoid and the distortion creates harmonics. Power quality standards set by reliability authorities limit the total harmonic distortion created by IBRs.
Reliability authorities also consider the possibility of harmful interaction between IBRs and existing grid infrastructure. The technical terms for the interactions are sub-synchronous control interactions (SSCI) and sub-synchronous resonance (SSR). The reliability authority must assess the risk associated with the interactions as part of the connection impact assessment process for generators.
Reference:
- The IESO Evaluation Stage Deliverability Test Methodology for the Long-Term 2 Energy Supply (Window 1) Request for Proposals, December 2024
- NERC An Introduction to Inverter-Based Resources on the Bulk Power System, June 2023
Grid forming and grid following
The terms grid forming and grid following are associated with inverter-based resources (IBRs) that connect to the grid.
IBRs with advanced control features are capable of managing their own voltage and frequency control independent of a grid reference. These IBRs can operate autonomously and are called grid forming inverters. Grid forming inverters are considered next generation and are being developed with dispatch and voltage support capabilities (see NREL). They are useful in microgrids and islanding situations where a grid connection may be interrupted or otherwise unavailable.
Grid following IBRs require an existing grid to operate. The existing grid provides voltage and frequency references for the IBR to follow. Without the grid reference, the IBR output shuts down.
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Derek
