Patent Application
20150168981
The present invention generally relates to synchronous condenser, and more specifically to high-inertia synchronous condenser.
The frequency response of power systems is decreasing due to the displacement of conventional power plants with alternate power sources such as wind plants (without inertia option) and solar plants. In addition, motor loads are increasingly connected to variable speed drives that de-couple the machine from the grid.
As conventional power plants are replaced by alternative power sources, such as wind plants and solar plants, and as more and more synchronous and induction motors are connected to the grid via motor drives, the inertia associated with these conventional power plants and motors are also removed from the electric power grid. The inertia is an energy stored as rotating energy in machines on a system. One way to restore the inertia to the grid is through use of synchronous condensers. Directly connected synchronous machines and induction motors contribute to inertia.
Synchronous condenser is a device commonly used to adjust conditions on the electric power transmission grid. Synchronous condensers have many characteristics which can benefit the power grid including: the ability to generate or absorb reactive power to adjust the grid's voltage, provide short circuit current, have reactive power overload capability, have excellent fault ride-through capability, as well as provide inertia to the power grid. Synchronous condenser has an inertia constant (H), and depending on the machine design, H varies from approximately 0.75 to 2.5 for horizontal shaft machines. This value compares to approximately 4 to 8 for power plants which obtain additional inertia from the turbine.
Normally, the synchronous condenser has an inertia constant H in the order of approximately 2. When a large amount of inertia is desired for a grid, multiple synchronous condensers are used or larger condensers are used, and this increases the cost for adding the inertia to the power grid.
Therefore, it is to an alternative system that provides inertia at a lower cost that the present invention is primarily directed.
Embodiments of the present invention overcome the aforementioned deficiencies noted in the conventional modeling methods.
In one embodiment, a hybrid high-inertia synchronous condenser facility is provided. The hybrid high-inertia synchronous condenser facility comprises at least one synchronous condenser, a voltage transformer connecting the at least one synchronous condenser to an electrical grid, at least one bank of capacitors switchably connected to the electrical grid, at least one bank of reactors switchably connected to the electrical grid, and a controller for controlling connection of the voltage transformer, the at least one bank of shunt capacitors, and the at least one bank of shunt reactors to the electrical grid.
In another embodiment, a method for delivering reactive power from a hybrid high inertia synchronous condenser facility to an electrical grid is provided. The method comprises monitoring the electrical grid, monitoring the hybrid high inertia synchronous condenser facility, computing an amount of reactive power to be delivered by the hybrid high inertia synchronous condenser facility, controlling operations of the hybrid high inertia synchronous condenser facility according to a computation result.
The foregoing and other objects, features, aspects and advantages of the present invention will become better understood from a careful reading of a detailed description provided herein below with appropriate reference to the accompanying drawings.
The present invention can be understood in more detail by reading the subsequent detailed description in conjunction with the examples and references made to the accompanying drawings, wherein:
While the present invention is described herein with illustrative embodiments for particular applications, it should be understood that the invention is not limited thereto. Those skilled in the art with access to the teachings provided herein will recognize additional modifications, applications, and embodiments within the scope thereof and additional fields in which the invention would be of significant utility.
The hybrid high-inertia synchronous condenser facility of the present invention provides a high inertia constant that reduces the number of conventional condensers or the total installed condenser MVAr nameplate capacity to achieve the desired amount of inertia. The synchronous condenser facility of the present invention uses a fly wheel to increase the inertia constant H and also uses banks of shunt capacitors and/or shunt reactors together with the reactive power rating of the condenser to provide the required level of leading and lagging MVArs to the grid.
The synchronous condenser facility of the present invention can deliver the inertial effect of a large synchronous condenser through a small synchronous condenser with a fly wheel while shunt capacitors and/or shunt reactors compliment the condenser reactive rating such that the desired level of compensation is provided to the grid.
The main technical advantage of the hybrid high-inertia synchronous condenser facility of the present invention is that a single machine can appear to the power grid as multiple machines (from an inertia perspective) and enables the option to reduce the total machine MVAr rating to reduce cost and make up the reduction in condenser nameplate MVArs with low cost shunt capacitors, shunt reactors, and a controller.
The inertia constant H is calculated according to the equation below:
In a power generation facility, under steady state, mechanical energy (M) and electrical energy (E) are balanced. When a power plant is lost or trips, electrical energy (load) is greater than mechanical energy, and system frequency drops. The rate of frequency drop (df/dt) is a function of initial power mismatch and the inertia constant H of the system. The machine speed in the power generation facilities decline until the mechanical power increases to match the electrical load when a new steady state is reached.
Each shunt reactive component and machine component is connected through a switch 208. Each reactive component 210 can absorb 60 mega volt-ampere reactive (MVAr) of reactive power and each machine can supply 120 MVArs or absorb 60 MVArs (+120 MVArs means 120 MVArs from the apparatus 200 to the grid 202 and −60 MVArs means 60 MVArs from the grid 202 to the apparatus 200, ignoring reactive losses in the transformer). The output from the components passes through a transformer 206 for adjusting the voltage of the output from the components to the voltage of the grid 202. The output from the transformer 206 is 240 MVA minus the reactive loss in the transformer. The transformer 206 may be equipped with a tap changer (not shown). The apparatus 200 provides an inertia rated as 2*120H or 480 MWs if H=2.
The same inertia from the apparatus 200 can be provided through a different configuration shown in
The configuration of
For some conditions, the components with higher capacity may not be available or may not be suitable in stations with limited available space.
There is yet another way to deliver inertia and MVArs to the grid as the configurations denoted in
By adding a flywheel to the synchronous condenser, the H of the synchronous condenser with an H of nominal value of 2 can be increased to 3 or more including 6 to 8 and beyond if required and the design made feasible. The addition of a flywheel to the synchronous condenser can reduce the required amount of machine capacity for a given application while the overload capacity of the machine plus shunt capacitors and/or shunt reactors, which are all controlled by a controller, can operate satisfactory and have the necessary MVArs for quasi and steady state voltage regulation and can replace an all-machine installation at a lower cost, smaller footprint, and lower operating losses while providing the same or more inertia to the system compared to the all-machine option.
The two high inertia synchronous condensers 408 provide a total of 2*60*4=480 MVAs for H=4 and the missing MVAr capacity of the facility 400 is provided by banks of shunt reactors 404 and shunt capacitors 406. Each bank of shunt reactors provides −90 MVArs and each bank of shunt capacitors provides 60 MVArs. So the total capacity of the facility 400 is +240/−240 MVArs (ignoring MVArs losses in the transformer). This hybrid facility 400 has banks of shunt reactors, banks of shunt capacitors, and multiple high-inertia synchronous condensers 408.
The operation of the synchronous condenser facility 400 is controlled by a controller 410. The controller 410 determines when shunt elements (banks of capacitors and/or banks of shunt reactors) are connected to the grid and how much MVArs are provided or absorbed by the synchronous condenser with fly wheel (by adjusting a field current for each condenser).
The controller 410 controls all the circuit breakers 412 and also the synchronous condenser 408, the tap changer in the transformer 402 (if provided), and banks of shunt reactors 404 and shunt capacitors 406. The controller 410 may be a computing device with a non-transitory storage unit (memory) and a set of instructions inside the storage unit. The controller 410 is programmed by a user and executes the programmed instructions to control delivery of dynamic reactive power by the facility 400. The controller 410 monitors the power grid and the operation of different components of the hybrid high-inertia synchronous condenser facility 400 through information received through the input 414. The amount of reactive power absorbed or delivered to the grid (bus) is controlled by the controller 410 according to the instructions and the information received.
The hybrid high-H synchronous condenser facility of the present application takes advantage of (1) the flywheel that can make a single condenser act on the grid, from an inertia perspective, as multiple condensers and (2) the low-cost shunt capacitors and reactors, which are up to an order of magnitude less cost per MVAr compared to traditional condenser MVArs, for reducing the nameplate of the condenser and lowering the cost of the overall installation compared to an all-machine installation.
The operation of the synchronous condenser and the switched shunt capacitors and/or reactors are all controlled by a controller that can be programmed or structured to operate according to an objective function that varies from installation to installation depending on the customer's requirements. For example, the objective function can include, but not be limited to: regulating a bus (or the machine terminals) to a voltage setpoint, regulating to a MVAr setpoint, i.e. providing a constant level of MVArs to the grid, operating the condenser facility at or near 0 MVArs (floating) to minimize losses, regulating a voltage at the bus only after leaving a deadband, and other control objectives as required by the customer.
The deadband is one of the control modes defined by the user. An example of the deadband is to provide no MVArs when the voltage is within a range and then operate, i.e. provide or absorb MVArs, when voltage is outside the range, e.g. do not provide or absorb MVArs when the voltage is between 95% and 105% but provide MVArs when voltage drops below 95% and absorb MVArs when voltage exceeds 105%.
The hybrid high-H synchronous condenser facility of the present application provides dynamic reactive power via operation of the automatic voltage regulator (AVR) during a fault and clear scenario, regulates the quasi and steady state voltage, provides inertia to the electrical grid system, and provides local short circuit current. The most common applications for synchronous condensers are near high-voltage direct current (HVDC) terminals and remote wind plants that use the condenser to provide short circuit current. AVR is an excitation system controlling the field current in the condenser via field voltage to maintain a voltage or MVAr setpoint.
Because a high inertia constant H can be achieved with addition of a relatively inexpensive fly wheel (compared to additional condenser(s)) and banks of shunt capacitors and shunt reactors are also utilized, the delivery of desired reactive power together and the desired level of inertia to the grid can be provided at a lower cost.
Although the present invention has been described with reference to the preferred embodiments, it will be understood that the invention is not limited to the details described thereof. Various substitutions and modifications have been suggested in the foregoing description, and others will occur to those of ordinary skill in the art. Therefore, all such substitutions and modifications are intended to be embraced within the scope of the invention as defined in the appended claims. It is understood that features shown in different figures can be easily combined within the scope of the invention.
