Patent Application
20200259332
This disclosure relates generally to a system and method for selectively connecting or disconnecting loads in a fragile power system network, such as a micro-grid.
An electrical power distribution network, often referred to as an electrical grid, typically includes a number of power generation plants each having a number of power generators, such as gas turbines, nuclear reactors, coal-fired generators, hydro-electric dams, etc. The power plants provide power at a variety of medium voltages that are then stepped up by transformers to a high voltage AC signal to be connected to high voltage transmission lines that deliver electrical power to a number of substations typically located within a community, where the voltage is stepped down to a medium voltage for distribution. The substations provide the medium voltage power to a number of three-phase feeder lines. The feeder lines are coupled to a number of lateral lines that provide the medium voltage to various distribution transformers, where the voltage is stepped down to a low voltage and is provided to a number of loads, such as homes, businesses, etc.
Some electrical grids may include one or more micro-grids, where each micro-grid includes one or more power sources, such as photovoltaic cells, generators, battery modules, wind turbines, etc., and where the power sources may be distributed throughout the micro-grid. The power sources in the micro-grid may be generating power during normal operation when the micro-grid is connected to the electrical grid, where the micro-grid power sources may be reducing the amount of power that the loads in the micro-grid are drawing from the electrical grid or may be placing power onto the electrical grid.
When a micro-grid is disconnected from the electrical grid as a result of power loss from a fault in the utility grid or otherwise, circuit breakers in the micro-grid are opened at the interconnection point of the micro-grid and the main utility grid prior to the micro-grid power sources being switched on to allow the power sources to start. Once the power sources are providing electrical energy, then the circuit breakers or other switching devices are closed in a certain sequence to add load to the sources. This operation typically occurs very quickly where significant load is coupled to the power sources in a relatively short amount of time. The power sources in a micro-grid are often low-inertia power sources, such as combustion based rotating generators, that only generate a small amount of power relative to the power providing capability of the utility grid. The low inertia is a consequence of a relatively small rotating mass in the generator or generators, where coupling or decoupling of loads in the micro-grid has an effect on the frequency of the system voltage. Thus, when a large load is coupled to the generator, the rotating speed of the generator significantly decreases. In other words, when the kinetic energy of the rotating mass in the generator is converted to electrical energy to meet the power demand, the rotational speed of the rotating mass decreases, causing the reduction in electrical system frequency. Thus, the mechanical power input to the generator needs time to ramp up to meet the increased electrical demand output of the generator in order to restore system frequency. Hence, the frequency will decrease until the generator prime mover increases its active power output to match the load and recover when the generator prime mover output exceeds the load as the governor and voltage regulators bring the generation output (watts and vars) back in balance with the load. The same phenomenon occurs when load is removed from the generator in that the reduced conversion of mechanical energy to electrical energy causes the rotating generator to speed up and increase the frequency. These types of frequency and voltage deviations can limit the generator's ability to accept or reject loads and cause power quality issues at the loads that are undesirable.
It is possible for the utility controlling the electrical grid to provide switches and other devices at various locations throughout the electrical grid and the micro-grid and provide the necessary communications therebetween to control how loads are being added and removed from the grid based on some predetermined priority so as to prevent the type of frequency and voltage changes of the power sources described. Further, the power sources employed in the micro-grid could be made larger to more easily support larger load changes. It is also possible to provide an energy storage system, such as a battery bank, that can be discharged when large loads are added to the grid to provide the additional power necessary to meet the demand without adversely affecting the power source. However, because of the necessary controls, communications, hardware, etc. these are expensive solutions, and a lower cost solution is desired.
The present disclosure describes a system and method for selectively connecting loads in a fragile power system network, such as a micro-grid, in a predetermined autonomous manner, using only locally measured quantities having no interaction with any remotely located control system devices, where the loads are coupled to a number of lateral lines and the lateral lines are coupled to a feeder line, when the feeder line is disconnected from an electrical grid as a result of a power loss in the electrical grid, or at other times when the feeder line is connected to the electrical grid when low short circuit level and/or low inertia conditions occur. Each lateral line includes a switching device positioned where the lateral line connects to the feeder line, where each switching device is provided with a timing control to switch on or off the switching device after a certain delay timer expires. As an example, when the power system network is disconnected from the electrical grid, the switching devices detect the voltage is not within a predetermined range and then open. Power sources in the power system network are then turned on. The switching devices detect that system voltage is within the predetermined range indicating that power is now available and are closed based on a predetermined delay. The delay can be randomly selected according to a predetermined statistical distribution or can be based on a priority of the loads. If the utility grid is operating in the low short circuit and/or low inertia condition with multiple sources of electrical power disabled, it may be desirable to restore power to the loads in smaller increments in order to minimize voltage and frequency disturbances to customers that are already on line.
Additional features of the present disclosure will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.
The following discussion of the embodiments of the disclosure directed to a system and method for selectively connecting loads in a distribution feeder or a micro-grid in a predetermined autonomous manner, where the loads are coupled to a number of lateral lines and the lateral lines are coupled to a feeder line, when the feeder line is disconnected from an electrical grid as a result of a power loss in the electrical grid is merely exemplary in nature, and is in no way intended to limit the disclosure or its applications or uses.
A feeder line 20 is connected to the electrical line 18 on the micro-grid side of the switch 16 in the micro-grid 14, where the feeder line 20 includes a power source disconnect switch 22, and where the feeder line 20 is typically a three-phase line. A number of lateral lines 28 are coupled to the feeder line 20, where each lateral line 28 can be a single-phase line, a two-phase line or a three-phase line. A number of loads 30 are coupled to each of the lateral lines 28 and can be any suitable load, such as a home, a building, a business, etc. A switch 32, discussed in detail below, is provided in each lateral line 28 and is operable to connect or disconnect the lateral line 28, and all of the loads 30 coupled thereto, from the feeder line 20 under a certain control scheme as discussed herein. A power source 34, such as photovoltaic cells, generators, battery modules, wind turbines, etc., is connected to the feeder line 20 and is intended to represent one or all of the power sources in the micro-grid 14, where the power sources 34 may be distributed throughout the micro-grid 14, that can power the micro-grid 14 and put excess power onto the utility grid 12 when the micro-grid connection switch 16 is closed. A controller 36 includes the necessary hardware, such as grid forming devices, ground source devices, balancing devices, etc., and software for controlling the power source 34.
The present disclosure proposes an autonomous control scheme for selectively opening and closing the switches 32 in response to the voltage applied at the switch location, such as the micro-grid 14 being disconnected from the utility grid 12, where power is provided to the micro-grid 14 by the power source 34, and being connected to the utility grid 12, where power is provided to the micro-grid 14 by the utility grid 12.
During normal operation, the switches 16, 22 and 32 are all closed, the power source 34 is off and the loads 30 are being powered by the utility grid 12. In response to a fault, or otherwise, in the utility grid 12, where power is not being provided to the micro-grid 14, the switches 16 and 22 are opened to disconnect the micro-grid 14 from the utility grid 12 and disconnect the loads 30 from the power source 34. The switches 32 detect the loss of voltage, such as a reduction in voltage below a predetermined threshold, and automatically open in response thereto. The power source 34 is then started. Once voltage is restored indicating that power is available from the power source 34, the switch 22 is closed. Each switch 32 detects that voltage has now returned, such as a rise in voltage above the predetermined threshold, and then closes based on a predetermined time delay so that power is provided to the lateral lines 28 from the power source 34 in a sequential manner, and thus not all at once. Thus, the power being drawn from the source 34 during this start-up sequence is provided in small segments and is ramped up to a maximum power draw. This prevents a large power draw on the power source 34, and thus allows the power source 34 to meet the power demand without significant frequency or voltage changes. The timing control of the switches 32 can be configured so that they are switched on in a random delayed manner according to a predetermined statistical distribution or high priority loads, such as police, hospital, etc., can be turned on first using predetermined time delays configured into the switches 32.
The switches 32 can be any switch suitable for the purposes discussed herein. One suitable example includes modifying a known single phase recloser including a vacuum interrupter switch so that it is able to detect the presence or absence of voltage, and a delaying opening and closing based on an assigned delay time in response thereto.
In order for electrical power to be generated and delivered to the loads 30 in a stable, reliable and cost-effective manner, it is necessary that the frequency of the AC signal be maintained as close as possible to a desired frequency. Therefore, power systems employ various control schemes so that the frequency of the AC signal provided on the grid 12 is maintained at the desired frequency. Generally, systems increase or decrease power generation as needed as load is added to and removed from the grid 12. However, some electrical power distribution networks are limited in their ability to respond quickly enough to an event that results in an under-frequency condition, such as a loss of a major generator or an abrupt increase in loads being applied to the grid 12. For example, certain power distribution networks, such as island grids, have a limited number of power plants that provide the power, and thus the system has a reduced ability to respond to changes in the frequency of the AC signal as needed. With an increase in distributed generation that is not controlled by the utility, or if the generation has low or zero inertia, the ability to control frequency is reduced further.
For those infrequent under-frequency transient events when the frequency of the AC signal on the electrical grid decays beyond the ability of the system to control it, the system must immediately reduce the load on the grid 12 within milliseconds so as to avoid potential system failure, referred to in the art as under-frequency load shedding. However, known under-frequency and under-voltage load shedding schemes typically shed large blocks of loads, such as a block serviced by a single substation, and do not provide any discrimination for higher priority loads.
According to another embodiment, the switches 32 can be configured to detect an under-frequency or under-voltage event at their location and then be selectively opened in a certain sequence to remove the loads 30 on the particular lateral line 28 so that lower priority loads are shed first, and higher priority loads 30 are removed last if system conditions require more load removal.
The discussion above talks about selectively turning on the loads 30 in the micro-grid 14 because the power that is able to be provided by the power source 34 may not be enough to allow energization of the loads 30 at the same time. However, this general control scheme can be used through the utility grid 12 during a black start where the entire grid 12 is in the process of recovering from an out of service condition. Specifically, the switches 32 can also be strategically provided throughout the grid 12 so that during a black start when bringing the grid 12 back on line, separate portions of the grid 12 can be started before other portions of the grid 12 so as to bring the grid 12 back on line more slowly to prevent overloading of the sources providing energy to the grid 12.
The foregoing discussion discloses and describes merely exemplary embodiments of the present disclosure. One skilled in the art will readily recognize from such discussion and from the accompanying drawings and claims that various changes, modifications and variations can be made therein without departing from the spirit and scope of the disclosure as defined in the following claims.
This application claims the benefit of priority from the U.S. Provisional Application No. 62/804,865, filed on Feb. 13, 2019, the disclosure of which is hereby expressly incorporated herein by reference for all purposes.
| Number | Date | Country | |
|---|---|---|---|
| 62804865 | Feb 2019 | US |
