Patent Application
20120147506
1. Field
The disclosed concept pertains generally to utility grids and, more particularly, to protective apparatus for distributed resources, such as, for example, engine-generators. The disclosed concept also pertains to methods of detecting an unintentional island condition of a distributed resource of a utility grid. The disclosed concept further pertains to controllers for such distributed resources.
2. Background Information
Currently, industrial countries generate most of their electricity in relatively large centralized facilities, such as fossil fuel (e.g., without limitation, coal; gas powered), nuclear or hydropower plants. These power plants have excellent economies of scale, but usually transmit electricity relatively long distances, and can negatively affect the environment.
Distributed generation, also called on-site generation, dispersed generation, embedded generation, decentralized generation, decentralized energy or distributed energy, generates electricity from many relatively small energy sources. With the recent development of the U.S. “Smart Grid”, “MicroGrids” and other methods of supplementing the output of utility power resources, distributed resources, such as for example and without limitation, bio-gas power generation, have become more common These distributed resources generate electric power and export it to the utility grid.
Protection for life and safety is provided in a distributed resource system. An “unintentional island” condition can exist when: (1) a distributed resource is exporting power to the utility grid; (2) the power exported by the distributed resource exactly equals the power consumed by a number of connected grid loads; and (3) the sub-station switch at the utility sub-station is opened.
IEEE 1547™—IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems provides in Section 4.4.1 (Unintentional islanding) that for an unintentional island in which a distributed resource (DR) energizes a portion of an area electric power system (EPS) through a point of common coupling (PCC), the DR interconnection system shall detect the island and cease to energize the area EPS within two seconds of the formation of an island.
IEEE 1547™ also provides that the “DR contains other non-islanding means, such as a) forced frequency or voltage shifting, b) transfer trip, or c) governor and excitation controls that maintain constant power and constant power factor.”
Referring to
There is room for improvement in protective apparatus for distributed resources, such as, for example, engine-generators.
There is also room for improvement in methods of detecting an unintentional island condition of a distributed resource of a utility grid.
There is further room for improvement in controllers for distributed resources.
These needs and others are met by embodiments of the disclosed concept, which actively bias engine-generator speed of a distributed resource, and detect a predetermined change in frequency of the distributed resource and responsively indicate the unintentional island condition.
In accordance with one aspect of the disclosed concept, a method of detecting an unintentional island condition of a distributed resource of a utility grid comprises: actively biasing engine-generator speed of the distributed resource; and detecting a predetermined change in frequency of the distributed resource and responsively indicating the unintentional island condition.
The method may further comprise disconnecting the distributed resource from the utility grid responsive to the detecting.
The method may further comprise providing the detecting and the disconnecting within two seconds of formation of the unintentional island condition.
The method may further comprise employing an engine-generator as the distributed resource; and providing no impact to the engine-generator during normal operation without the unintentional island condition.
The method may further comprise providing no undue wear to a number of components of the engine-generator during normal operation without the unintentional island condition.
The method may further comprise performing the detecting by sensing frequency deviation of the engine-generator above or below predetermined limits.
The method may further comprise employing as the actively biasing engine-generator speed periodically adjusting the speed bias by a predetermined percentage thereof.
As another aspect of the disclosed concept, a protective apparatus is for a distributed resource of a utility grid. The protective apparatus comprises: a processor comprising: an input structured to input frequency of the distributed resource, a first output structured to control speed of the distributed resource, a second output, and a routine structured to actively bias engine-generator speed of the distributed resource through the first output, and detect a predetermined change in the frequency of the distributed resource from the input and responsively indicate the unintentional island condition through the second output; and a circuit interrupter.
As another aspect of the disclosed concept, a controller is for a distributed resource of a utility grid. The controller comprises: a processor; an input structured to input frequency of the distributed resource; a first output structured to control speed of the distributed resource; a second output; and a routine executed by the processor and structured to actively bias engine-generator speed of the distributed resource through the first output, and detect a predetermined change in the frequency of the distributed resource from the input and responsively indicate the unintentional island condition through the second output.
A full understanding of the disclosed concept can be gained from the following description of the preferred embodiments when read in conjunction with the accompanying drawings in which:
As employed herein, the term “number” shall mean one or an integer greater than one (i. e., a plurality).
As employed herein, the term “processor” shall mean a programmable analog and/or digital device that can store, retrieve, and process data; a computer; a controller; a workstation; a personal computer; a microprocessor; a microcontroller; a microcomputer; a central processing unit; a mainframe computer; a mini-computer; a server; a networked processor; or any suitable processing device or apparatus.
As employed herein, the statement that two or more parts are “connected” or “coupled” together shall mean that the parts are joined together either directly or joined through one or more intermediate parts. Further, as employed herein, the statement that two or more parts are “attached” shall mean that the parts are joined together directly.
As employed herein, the term “distributed resource” shall mean an engine-generator; a distributed engine-generator; a MicroGrid engine-generator; or a Smart Grid engine-generator.
An engine-generator is the combination of an electrical generator and an engine (or prime mover) mounted together to form a single piece of equipment. This combination is also called an engine-generator set or a gen-set. In many contexts, the engine is taken for granted and the combined unit is simply called a generator.
Engine-generators are available in a wide range of power ratings. Engine-generators can include, for example, relatively small, hand-portable units that can supply several hundred watts of power, hand-cart mounted units that can supply several thousand watts, and stationary or trailer-mounted units that can supply over a million watts. Regardless of the size, generators can run on gasoline, diesel, natural gas, propane, bio-diesel, sewage gas, hydrogen or any suitable fuel. Most of the relatively smaller units are usually built to use gasoline as a fuel, and the relatively larger ones have various fuel types, including diesel, natural gas and propane (liquid or gas). Some engines may also operate on diesel and gas simultaneously (bi-fuel operation). Some engine-generators use a turbine as the engine, such as industrial gas turbines used in peaking power plants and micro-turbines used in some hybrid electric buses.
The disclosed concept is described in association with a particular engine-generator, although the disclosed concept is applicable to a wide range of distributed resources.
The disclosed concept senses an unintentional island condition, and causes a distributed resource to be removed from a utility grid.
The disclosed concept considers that a connected utility is an “infinite source”. Since a distributed resource represents a relatively small fractional percentage of utility power capacity, the distributed resource cannot have any significant impact on utility frequency.
Under normal operating conditions, the connected distributed resource frequency is governed by utility frequency. Therefore, increasing or decreasing engine speed causes an increase or decrease, respectively, of the electrical power delivered by the engine-generator, with no appreciable impact on system frequency. For example, by suitably alternately increasing and decreasing the speed bias to an engine-generator, sometimes referred to as “push-pull” herein, an unintentional island formation can be detected.
When an engine-generator is in parallel with a utility, it is locked into the dominant utility frequency, regardless of changes in speed bias. When the engine-generator is islanded, changes in speed bias will cause a change in frequency, which can be captured and used to trigger a protective trip of an engine-generator circuit breaker.
There is a fundamental difference in electrical power generation between a generator that is running alone and a generator that is connected in parallel with a utility grid. This fundamental difference makes the disclosed protective function possible.
When an engine-generator is in parallel with an “infinite” utility source, electro-magnetic forces keep the engine-generator frequency in perfect synchronization with the utility frequency. Therefore, any change in engine-generator throttle position will only change the power output of the generator, and never the frequency at which the generator is running
On the other hand, if an engine-generator is operating alone, it will carry whatever suitable load (e.g., in kW) is connected to its output. No more, no less. In this case, a change in engine-generator throttle position will cause a change in output frequency.
Thus, by suitably adjusting the engine-generator throttle open and closed, and looking for a change in frequency rather than a change in generator power output, the disclosed concept can relatively very quickly determine whether the utility source is present (normal operation) or is not present (unintentional island).
One non-limiting example embodiment of the disclosed protective function is an example switchgear-mounted engine-generator controller 400 as is shown in
One example protective function is a known Delta Hz/Delta Time protective function that evaluates change in frequency per unit time. The Delta Hz/Delta Time function cannot reliably, in and of itself, detect all possible unintentional island conditions. Specifically, when the utility switch is opened, because the grid loads exactly equal distributed resource output, there would be no change in distributed resource frequency. Under these conditions, the Delta Hz/Delta Time function would not detect the island condition. This scenario, however, is detected by employing the disclosed active speed bias function. If there is some significant amount of power flow, in either direction, through the utility switch, then opening the switch will cause a step change to the load that the distributed resource is carrying and an attendant step change in distributed resource frequency, which is readily detected. The disclosed “push-pull” function ensures that when there is no power transfer across the utility switch, and the utility switch is open, the frequency deviation is artificially driven to a detectable level. The disclosed system can use, in part, the Delta Hz/Delta Time function for detection, but if the “push-pull” function was not there, then the Delta Hz/Delta Time function would not detect the island condition, under the above scenario.
Another known example protective function uses minimum/maximum setpoints of the expected utility frequency range. Over/Under Frequency protection is not associated with detecting an island condition. It is used to detect frequency deviation to a value that could cause equipment damage. For U.S. systems, Under Frequency is usually set to about 57.00 Hz and Over Frequency to about 63.00 Hz. The detection in the disclosed protective function assumes that the distributed resource is in parallel with a stable utility, and the setpoints are approximately 59.95 Hz and 60.05 Hz, respectively.
The disclosed concept alternately applies a positive and a negative speed bias to the engine 410, with the protective function looking to detect a change in frequency. If the utility (not shown) is present on the area EPS 412, no change in frequency will be detected. On the other hand, if the utility is not present on the area EPS 412 and an unintentional island has been formed, then a change in frequency will be detected by the disclosed protective function 414, and the point of common coupling (PCC) 416 will be immediately opened, thereby eliminating the unintentional island condition.
When the utility switch (not shown, but see the sub-station switch 10 of
Referring to
The inputs to the routine 600 include Enable Function=Enable (Enable is usually connected to the Point of Common Coupling (PCC) circuit breaker 416, which is the circuit breaker that connects the generator 420 (
The outputs from the routine 600 include Speed Bias Output to Engine Generator=BiasOut (this is the speed bias signal 402 of
First, at 602 of
On the other hand, if Ramping is false at 608, then, at 610, it is determined if GenHZ exceeds MaxHZ. If so, then the generator circuit breaker 416 is tripped at 613. Otherwise, if GenHZ is not greater than MaxHZ, but is less than MinHZ at 612, then the generator circuit breaker 416 is tripped at 613 and execution resumes at 622 of
Run PushMS Timer 615 and Run PullMS Timer 634 continue running the corresponding timers. These define the period of the Push-Pull function. While the PushMS Timer is running, the analog speed bias input (BiasIn) is passed unchanged to the analog speed bias output (BiasOut). When the PushMS Timer reaches its preset (PushMS), the pull cycle begins. There, the PullMS Timer is started at 616, and BiasIn is decremented by the pull increment (either Pull1 or Pull2), and the result is sent to BiasOut. When this timer reaches its preset (PullMS), the push cycle begins again, and so forth.
Next, at 618, it is determined if PullMS is complete. If so, then PushMS Timer is started at 620. Next, at 622, BiasOut is set equal to BiasIn. Then, at 624, it is determined if KWsetpt is different than CurrentSP. If so, then Ramping is set on at 626. Otherwise, Ramping is set off at 632. After 626, it is determined if GenkW is equal to KWsetpt at 628. If not, then execution resumes at 602 of
On the other hand, if PullMS was not complete at 618, then, at 634, PullMS Timer is run at 634. Then, at 636, it is determined if GenkW is greater than Xfer12. If not, then BiasOut is set equal to BiasIn less Pull1 at 638. Otherwise, if GenkW is greater than Xfer12, then BiasOut is set equal to BiasIn less Pull2 at 640. The Desired Speed Bias (BiasIn) is the basic desired speed command, before it is modified by the routine 600. Because of different engine dynamics at different loads, the routine 600 allows for a change in the way that the engine throttle 411 is cycled. When Current Generator kW (GenkW) is below Transition Point from 1 to 2 (Xfer12) in kW, Push-Pull Increment 1 (Pull1) is subtracted from Desired Speed Bias (BiasIn) and the resulting speed bias is sent to Speed Bias Output to Engine Generator (BiasOut). When Current Generator kW (GenkW) is above Transition Point from 1 to 2 in kW (Xfer12), Push-Pull Increment 2 (Pull2) is used. After either 638 or 640, execution resumes at 624.
The final result of the protective function routine 600 is a speed bias output (BiasOut) to the engine throttle 411 that cycles up and down, for example and without limitation, at about 12% of its range about every 1.5 seconds. This bias is enough to cause a change in generator frequency beyond the circuit breaker trip settings when the engine-generator 418 is islanded, but is not enough to cause a major disturbance in power output by the engine-generator 418 when it is in parallel with the area EPS 412. It will be appreciated that the non-limiting example 12% value and example 1.5 second time can vary for other types of engine-generators and other types of PCCs.
The disclosed concept provides IEEE 1547 compliance for unintentional islanding since the “push-pull” of the engine throttle 411 by the protective function routine 600 takes place about every 1.5 seconds, in order to be able to detect separation from the utility source (not shown) and open the generator breaker (PCC) 416 within 2 seconds.
The disclosed protective function routine 600 does not impact normal engine-generator 418 operation.
Distributed resource sites that are connected on a long-term basis to the utility grid usually are in the business of selling power to the local utility company. Hence, maximizing engine-generator power output is paramount. If the protective function causes a reduction in overall power output, then the protective function would be less desirable to its prospective users. At the same time, the throttle push-pull function is sufficient to the point that if the engine-generator is islanded from the utility, then the protective function will push or pull generator frequency beyond the corresponding trip settings. The disclosed push-pull adjustment and duration accomplish both of these goals.
The disclosed protective function causes no undue wear under normal operating conditions of a number of components of the engine-generator.
IEEE 1547 is based on UL 1741, which specifies that the protection must be active. Hence, in order to obtain UL certification for compliance, it must be demonstrated that the protection is not a passive function, such as monitoring for a change in frequency over time (DF/DT). Other standards, such a California Rule 21, specify that such protection must be an active function.
“Pumping” the engine throttle about every 1.5 seconds does not lead to an early throttle failure. The disclosed concept ensures that the protective function is strong enough to cause the generator breaker to trip when an unintentional island exists, but gentle enough not to cause early throttle failure under normal operations.
In the United States, utility grid frequency is relatively very constant. It is unusual to see variances of more than a few hundredths of a Hertz. Typical minimum and maximum settings for this protective function are from about 59.95 Hz (Minimum allowed Generator Frequency=MinHz) to about 60.05 Hz (Maximum allowed Generator Frequency=MaxHz). However, there is some variance in what is considered a normal variance from region to region in the United States. As such, in deploying the disclosed protective function, minimum and maximum settings can depend on what is considered to be a normal utility frequency variance for a given region. The goal of this setting is to be as sensitive as possible to an unintentional island condition without causing unnecessary interruption to the operation of the engine-generator.
While specific embodiments of the disclosed concept have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the disclosed concept which is to be given the full breadth of the claims appended and any and all equivalents thereof
