Coordinated frequency load shedding protection method using distributed electrical protection devices

A method for providing frequency load shedding in a power distribution network. The network includes a number of distributed switch-gear assemblies that control whether AC power is provided to groups of loads. The distributed switch-gear assemblies monitor the frequency of the AC signal to determine if a frequency event is occurring and also determine the direction of the power flow at the time of the event. The switch-gear assembly may open in an underfrequency event only if the loads are drawing power from the network, and the switch-gear assembly may open in an overfrequency event only if there is reverse power flow during the event. In addition, the order of operation of which switch-gear assemblies may open first in response to the frequency event is determined in advance by the location of the switch-gear assembly in the network and a corresponding time delay and coordinated frequency set-points.

BACKGROUND

Field

This disclosure relates generally to a frequency load shedding scheme for disconnecting load in an electrical power distribution network in response to an unusual frequency event and, more particularly, to a frequency load shedding scheme for removing load in an electrical power distribution network in response to a frequency event, where the scheme selectively opens certain switch-gear depending on the direction of the power flow at the switch-gear during the event, and where all of the switch-gear are calibrated and coordinated in advance with a control time-frequency delay so that those switch-gear at an outer edge of the network shed first and that the scheme does not require fast communications during the event.

Discussion of the Related Art

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 or steam turbines powered by thermal energy sources and hydraulic turbines powered by dams. The power plants provide an AC output that enables delivery of electrical power via high voltage transmission to a number of substations, where the voltage is stepped down from high voltage to a medium voltage for distribution. The substations provide the medium voltage electrical service to a number of feeder circuits that are typically delivering power using three phase AC power. The feeder lines are connected to a number of lateral lines that provide the medium voltage to distribution transformers, where the voltage is stepped down to a single or three phase low voltage and is provided to a number of loads, such as homes, businesses, etc. In addition, distributed generation sources may be interconnected with loads in distribution resulting in periods of negative load from time to time.

The high voltage AC provided by the power plant is also a signal in that the AC frequency can be measured at any point in the network and this frequency is controlled at either 50 or 60 Hz. Unlike AC voltage or current measurements, which are driven primarily by local conditions near the point of measurement, the system AC frequency signal can be measured at any point of the voltage step down processes where it is ultimately delivered at that frequency to the load. As the amount of loads and generation varies over time, the frequency of the AC signal provided on the entire electrical grid changes, where, generally the addition of a load causes the frequency to be reduced and the removal of a load causes the frequency to be increased. In order for the electrical power to be generated and delivered to the loads 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 50 or 60 Hz. Therefore, power generation plants and system operators employ various generation control schemes so that the frequency of the AC signal provided on the grid is maintained at the desired frequency. Generally, rather than shed loads, system operations increase or decrease generation as needed as load is added to and removed from the grid. However, some electrical power distribution networks are limited in their ability to respond quickly enough to an underfrequency event, such as a loss of a major generator or an abrupt increase in loads being applied to the grid. 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 increased in uncontrolled distributed generation, the ability to control frequency is reduced further.

For those rare underfrequency transient events when the frequency of the AC signal on the electrical grid decays beyond the ability of the system operator to arrest it with additional generation, the system must immediately reduce the load on the grid within milliseconds so as to avoid potential system failure, referred to in the art as underfrequency load shedding (UFLS). For example, known UFLS schemes typically cause one or more substations in the network to disconnect substation breakers so as to disconnect load. The UFLS schemes impact a relatively large service area in response to a low frequency event due to being deployed at a substation level rather than further down the network closer to loads. Specifically, a controller in pre-selected substations identifies a grid frequency below a certain value, such as 59.6 Hz, and will automatically and autonomously open a breaker within milliseconds to remove the loads that the substation services. The substations are selectively calibrated in advance so that depending on where the substation is located and to who the substation is providing power determines when that substation will remove its loads from the grid. In other words, the various substations are controlled so that they do not all disconnect their loads at the same predetermined low frequency, whereas when a particular substation remove its loads from the grid, the frequency of the AC signal on the grid will go up, which reduces the likelihood that other substations will need to remove their loads from the grid.

With the increase of distributed generation connected at low or medium voltage, sometimes the loads that are disconnected in an underfrequency load shedding scheme may be loads that are actually generating power. More specifically, distributed power generation is becoming more prevalent in electrical power grids where a certain load, such as a home, may actually be generating power through any of a number of power generation techniques, such as solar power, wind power, geo-thermal power, etc., and where at least for certain times, those loads may be injecting power on the grid instead of consuming power from the grid. Thus, since it is unknown in advance what loads may be producing power and at what time, removing those loads from the electrical grid during an underfrequency load shedding event may make an underfrequency transient worse and reduce the ability of the event to be quickly overcome since those loads were acting to support the frequency of the AC signal on the grid. Hence, when an underfrequency condition is detected and load shedding is necessary, the current method of providing underfrequency load shedding via substation-based load shed is not precise in that large areas may be unnecessarily affected and loads generating power may be removed.

Further, there is also less available governor response in smaller grids and grids with a higher proportion of distributed generation to help dampen frequency oscillations that occur during a loss of generation or loss of load events. What is meant by governor response is the ability for generation to increase or decrease power autonomously as needed to counteract frequency transients outside the desired value. Unlike central generation, distributed generation sources typically operate at maximum power and are not either able or not regulated via interconnection tariffs or controlled to provide governor response like central generation.

Moreover, if an underfrequency event occurs, and the underfrequency load shedding process removes large amounts of load to overcome the event, it is possible that removal of the loads could cause the frequency to go above 50 or 60 Hz, which could also cause similar stability control and have wide-area impact as in an underfrequency event.

SUMMARY

Disclosed and described herein are a system and method for providing frequency load shedding in an electrical power distribution network, where the network includes a power source, such as an electrical substation, providing an AC power signal on one or more electrical lines, such as feeder lines and lateral lines, to be distributed to a number of loads electrically coupled to the lateral lines. A number of switch-gear assemblies are provided along the electrical lines to provide electrical control and protection. These assemblies control whether the AC signal is provided to certain groups of the loads depending on where the switch-gear assembly is located in the network. The switch-gear assembly monitors the frequency of the AC signal to detect whether a frequency transient event is occurring, where the frequency of the AC signal falls outside a frequency control deadband. If the control detects that a frequency event is occurring, the switch-gear assembly then determines the direction of the power flow to determine whether the switch-gear assembly should open or remain closed. If the power flow is in the reverse direction indicating that those loads are combining to generate net power, the switch-gear assembly would not open during an underfrequency event. Furthermore, the order of operation of which switch-gear assembly opens first in response to the underfrequency event is determined by the location of the switch-gear assembly and a time delay, where those switch-gear assemblies that are farther from the power source are opened first to limit the number of loads that are affected by the underfrequency event. This scheme does not require any communication during the frequency event. Frequency events occur in milliseconds and require accurate frequency measurements and fast and autonomous switch-gear apparatus to be effective.

Additional features will become apparent from the following description and appended claims, taken in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following discussion of exemplary embodiments directed to a system and method for providing frequency load shedding in an electrical power distribution network is merely exemplary in nature, and is in no way intended to limit the embodiments of the disclosure or applications or uses.

As will be discussed in detail below, a frequency load shedding scheme is disclosed for an electrical power distribution network that controls switch-gear at the outer edges of the network so that the number of loads that are removed from the network during a frequency event is limited. More specifically, switch-gear is distributed within a distribution network that supplies power to a group of loads. Each switch-gear detects a frequency event and which direction the power is flowing, i.e., is the group of loads providing power to the network. Thus, if the switch-gear determines that power is being provided to the network because the group of loads is generating more power than it is consuming, it will not remove the group of loads in response to the frequency event, but if the group of loads is drawing power from the network, then the switch-gear will open to remove the group of loads and help arrest the frequency occurrence. Further, the switch-gear are coordinated and calibrated in advance relative to each other so that those switch-gear that are servicing a fewer number of loads are opened before other switch-gear so that only a relatively small number of the loads are affected during the underfrequency event.

FIG. 1is a general block diagram of a power distribution network10intended to represent any electrical power distribution system or network of any size and configuration that provides electrical power from any number or type of power plants (not shown) over any suitable distance on any type of transmission line (not shown) to electrical substations (not shown) to be distributed on feeder lines (not shown) to any suitable load (not shown).

The network10includes a grid12that is intended to represent all of the power distribution devices and systems in the network10that deliver power to the various loads in the network10. Some of those components will be switch-gear, such as switch-gear14that represents one of the many switching devices, breakers, reclosers, interrupters, etc. that may be provided in the network10that controls power flow from a certain substation on the grid12to a load/generation block16, which is intended to represent any suitable number or group of loads receiving power from the network10, such as homes, businesses, manufacturing facilities, etc.

Some of the loads that may be drawing power at one point in time may be generating power, such as by, for example, an array of solar panels, wind turbines, storage cells, etc., at another point in time and be putting power on the grid12, where the group of loads is consuming less power than it is producing. It would be unknown which particular location in the network10could be generating power at what particular point in time. As discussed above, any power generated on the network10helps reduce the magnitude and duration of an underfrequency event. It is noted that in this non-limiting illustration, power flows from the grid12through the switch-gear14to the load/generation block16when the load/generation block16acts as a load, and flows from the load/generation block16to the grid12when the load/generation block16acts as a generator. However, modern electrical systems are often configured such that power may be provided to the load/generation block16through a switch-gear (not shown) from the right of the load/generation block16if the power is coming from, for example, a different substation or feeder line in the grid12.

The switch-gear14includes a controller18that controls whether the switch-gear is open or closed in order to connect or disconnect the load/generation block16to and from the grid12so as to prevent the load/generation block16from drawing power from the grid12or placing power on the grid12. The controller18receives electrical measurement signals from the switch-gear14that allow the controller18to identify the direction of flow of current through the switch-gear14and to calculate the frequency of the AC power signal. The algorithms operating in the controller18will be coordinated with the algorithms operating in other switch-gear in the network10depending on their location so that as each controller18for each switch-gear14identifies a frequency event at the same time, those switch-gear that may be at a more remote location from the substation may be opened before other switch-gear upstream of the flow of power so that loads are removed from the grid12to stop the frequency event, but as few as possible load/generation locations will be affected. As load/generation locations are removed from the grid12eventually the frequency event will be removed.

FIG. 2is a flow chart diagram20illustrating a simplified control algorithm operating in the controller18for determining whether to open the switch-gear14in response to an underfrequency event. At box22, the algorithm identifies the frequency of the power signal traveling through the switch-gear14, the direction of the power flow through the switch-gear14and a time delay value for the switch-gear14that is calibrated relative to the other switch-gear in the network10. As the frequency of the power signal changes and is determined at each sample point, the algorithm may determine that the frequency of the AC signal has fallen below a predetermined minimum frequency set-point deadband specific for the switch-gear14and an underfrequency event is occurring. The algorithm then starts a timer, and after the time delay, the algorithm determines whether the frequency is still below the minimum frequency set-point at decision diamond24before taking any load shedding action. The switch-gear14that are farther downstream of the power flow in the grid12, and thus have fewer load/generation blocks16after them, will have shorter time delay values so that they are the first ones to be opened in response to an underfrequency event. Thus, if the switch-gear14has a longer time delay value because it is farther upstream towards the substation in the grid12, the frequency of the AC signal may go back up above the minimum frequency set-point because of loads being removed by other switch-gear before the time delay has passed.

If the frequency is not below the set-point at the decision diamond24, then the algorithm returns to the box22to identify the frequency of the power signal at the next sample time. If the frequency of the AC signal is below the minimum frequency set-point at the decision diamond24, the algorithm then determines whether the power flow through the switch-gear14is in the normal direction meaning that the load/generation block16is drawing power from the grid12and not putting power on the grid12. If the power flow is not in the normal direction at the decision diamond26meaning power is being placed on the grid12by the load/generation block16, then the switch-gear14is not opened because the load/generation block16is actually providing power generation of a reverse power flow. However, if the power flow is in the normal direction at the decision diamond26meaning the load/generation block16is drawing power from the grid12, then the algorithm opens the switch-gear14at box28and returns to the box22to again determine the frequency, set the time delay value, and determine the direction of power flow. Once the frequency of the AC signal subsequently goes above the minimum frequency set-point, then the algorithm will reclose the switch-gear14.

The methods discussed herein can also remove the load/generation block16from the grid12if there is an overfrequency event where the frequency of the power signal is above a certain value, which also could have detrimental effects on the stability of the grid12. In this embodiment, the same process is performed where the fewest number of the load/generation blocks16are removed from the grid12so as to reduce the number of locations where power is disrupted. The algorithm operating in the controller18identifies the frequency of the AC signal, the power flow direction of the AC signal, and the time delay value for the particular switch-gear14at box32in the same manner as the box22. As the frequency of the power signal changes and is determined at each sample point, the algorithm may determine that the frequency of the AC signal has risen above a predetermined maximum frequency set-point and an overfrequency event is occurring. The algorithm starts a timer, and after the time delay, the algorithm determines whether the frequency is still above the maximum frequency set-point at decision diamond34before taking any action, and if not, returns to the box32. The switch-gear14that are farther downstream of the power flow in the grid12, and thus have fewer load/generation blocks16after them, will have shorter time delay values so that they are the first ones to be opened in response to an overfrequency event.

If the frequency is above the set-point at the decision diamond34, then the algorithm determines whether the power flow direction is in a reverse direction at decision diamond36meaning that the load/generation block16is putting power onto the grid12. If the power flow is in the normal direction and not in the reverse direction at the decision diamond36, the load/generation block16is drawing power like a normal load and is helping to lower the frequency of the power signal, and the algorithm returns to the box32. However, if the load/generation block16is putting power on the grid12at the decision diamond36meaning the power flow is in the reverse direction, then the algorithm opens the switch-gear14at box38so that the load/generation block16is prevented from providing power onto the grid12. Once the frequency of the AC signal subsequently goes below the maximum frequency set-point, then the algorithm will reclose the switch-gear14.

FIG. 4is schematic block diagram of an electrical power distribution network40illustrating an example of the scheme for removing loads discussed above during an underfreqeuncy event. In this non-limiting example, the network40includes an electrical loop44that receives a power signal from an electrical substation42and provides the power signal on an upper segment46of the loop44and a lower segment48of the loop44that travel in opposite directions. The upper segment46of the loop44and the lower segment48of the loop44are electrically separated by a switch-gear assembly50that is in a normally open position to prevent power flow between the upper segment46and the lower segment48through a connector segment52. The upper segment46includes a number of series connected switch-gear assemblies60and62that each includes a switch-gear similar to the switch-gear14and a controller similar to the controller18. The switch-gear assemblies60and62are separated by load/generation blocks64and66similar to the load/generation block16. Likewise, the lower segment48includes a number of series connected switch-gear assemblies68and70separated by load/generation blocks72and74. The load/generation blocks66and74would be electrically coupled together if the switch-gear assembly50was closed.

Part of the method can be illustrated by the following example. Consider that the load/generation block66has a load of 100 kW and a distributive power generation of 50 kW providing a net 50 kW load and the load/generation block64has a load of 100 kW and a distributive power generation of 125 kW providing a net −25 kW load, i.e., the combination of loads from the blocks66and64is consuming 25 kW in power from the network40. Thus, the switch-gear assembly62sees the net 50 kW load from the block66and the switch-gear assembly60sees a net 25 kW load because of the combination of the net power values of the blocks64and66. The load/generation block74has a load of 100 kW and a distributive power generation of 150 kW providing a net −50 kW load and the load/generation block72has a 100 kW load and a 50 KW distributive power generation providing a net 50 kW load. Thus, the switch-gear assembly70sees the net −50 kW load and the switch-gear assembly68sees a net 0 kW load because of the combination of the net power values of the combined load the blocks72and74. This provides a combined upper and lower segment load on the substation42of net 25 kW composed of 400 kW in load and a distributive power generation of 375 kW.

In this example, the switch-gear assemblies62and70have the same time delay value, which is shorter than the time delay of the switch-gear assemblies60and68, which are also the same. Since the switch-gear assembly62has a net 50 kW load and the switch-gear assembly70has a net −50 kW load, the switch-gear assembly62is opened and the switch-gear assembly70is not opened, which would remove the 100 kW load and the 50 kW distributive generation from the load/generation block66. This will remove the net 50 kW load from the network40, where the substation42would then see the reduction of 50 kW load to a net −25 kW load, and thus help support the underfrequency event. This scheme therefore reduces 50 kW in net load by only dropping 100 kW in loads at the block66. This is more net load reduction and less gross load interruption in contrast to a substation based scheme that would have all the dropped loads and generation at the blocks64,66,72and74in which 400 kW load would be disconnected and only result in 25 kW in net load. Hence, only the load/generation block66is removed from the network40instead of all of the load/generation blocks64,66,72and74as would occur in the known load shedding schemes. If additional load is needed to be removed from the network40to overcome the underfrequency event, then the switch-gear assembly having the next shortest delay time and having a load would be opened, which is not the case in this example.

The normally-open switch-gear assembly50can be closed if a fault occurs and the circuit needs to be reconfigured. For example, if a fault occurs at or near the switch-gear assembly62and the switch-gear associated therewith is opened so that power is not delivered to the load/generation block66, then the switch-gear assembly50can be closed so that power can be delivered to the load/generation block66from the lower segment48. Therefore, the switch-gear assembly50would see the net 50 kW load of the block66. In the same manner as discussed above, if the underfrequency event occurs in this configuration of the network40, then the switch-gear assembly50would be the first to be opened to remove the load/generation block66, which would again cause the substation42to see the net −25 kW load. In this scheme, the switch-gear must maintain topology information as to which direction is the grid12. This topology direction is maintained in advance like the time-frequency set-points and changed from time-to-time as the circuit is reconfigured.

The foregoing discussion discloses and describes merely exemplary embodiments. 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.