Patent Description:
There have been developed a number of solutions for power source control in dual and multi-source power systems. In the solar tracker scenario there have been developed certain control systems. One of these control systems determines the source of the power to be applied to a load such as a power grid. While one source is the power generated by a solar power array the second is often a battery system.

There are times when the energy produced by the solar power plant is insufficient to supply to the grid. This may occur when the sun is obscured by clouds or demand is such that the solar power plant cannot generate sufficient power to supply an energy grid. In these types of instances, a DC storage plant is employed to provide additional power. As will be appreciated, the ability to switch or combine the energy from the two power providing systems (i.e., the solar power plant and the DC storage plant) is an important feature of any such system. Though there have been developed systems enabling this transition, there is always a need for improved and more efficient systems.

<FIG> depicts a prior art multi input port inverter solar tracker system <NUM>. This architecture for DC coupling solar and storage utilizes a multi input port inverter <NUM>. The multi input port inverter <NUM> manages solar array <NUM>, following the maximum power point tracking (MPPT) algorithm, and battery <NUM> charging and discharging separately for distributed and large applications. The solar array <NUM> to battery <NUM> ratio is determined by the specific situation for every installation. Typically the multi input port inverter <NUM> will be oversized to meet large power requests from the grid and the responses from the solar array <NUM> or battery <NUM>. Correlation between the central multi input port inverter <NUM> and battery charger <NUM> can be complicated and communication delay also limits this configuration for fast response micro-grid applications.

<CIT> discloses a method for regulating DC voltage of DC distribution network, which involves dividing the degree of DC bus voltage deviation from rated value into three levels such as mild deviation, moderate deviation and severe deviation. The voltage of DC bus is adjusted by main voltage mode, backup voltage regulation mode and emergency voltage regulation mode. The main voltage mode and standby voltage regulation mode are provided for regulating DC bus voltage of network converter and energy storage device. The emergency voltage regulation mode regulates the DC link voltage by reducing power consumption and load shedding of distributed power supply.

Accordingly, in view of these short comings, improved solutions with greater efficiency and higher speed are desired.

Embodiments of the present disclosure are described in detail with reference to the drawing figures wherein like reference numerals identify similar or identical elements.

The invention is defined by a method for implementing a multi-power distributed power storage and generation system with the steps in independent claim <NUM> and by a multi-power distributed storage system with the technical features of independent claim <NUM>.

Various aspects of the present disclosure are described herein below with reference to the drawings, which are incorporated in and constitute a part of this specification, wherein:.

The present disclosure is directed to systems and methods for implementing a multi-power and distributed power storage system whereby a single load may be driven by two separate power sources, both individually and together. Though described generally herein in the context of a solar tracking system that utilizes both a photovoltaic (solar) power plant and a DC storage plant to provide energy to drive an energy grid, the systems, schematics, and algorithms described herein in any situation where there is are two power sources. In particular the systems and algorithms of the present disclosure are useful where there is one power source that is the preferred power source to be utilized but the system should experience little to no lag in transitioning to the other power source.

Typically a photovoltaic power plant is connected to a large power grid, and may be associated with large battery banks that can be used to provide power to the grid when the solar panels are unable to meet demand. Commonly owned <CIT> and teaches a battery container and <CIT> entitled "Direct Current Battery String Aggregator for Standard Energy Storage Enclosure Platform," teaches a controller and system for connecting a battery and photovoltaic system to an energy grid. Both references are incorporated herein by reference. Other dual power source energy systems requiring monitoring and switching between energy supply systems are also contemplated within the scope of the present disclosure.

<FIG> depicts a distributed solar and storage system <NUM> which may be deployed as part of a larger array. It is contemplated that this application could be utilized for solar tracker systems, fixed tilt solar systems, roof top solar, and any type of solar array. Each distributed solar and storage system <NUM> includes a single input port inverter <NUM> which is fed an output voltage from a first power source such as a solar power plant <NUM>, and from a second power source such as a DC storage plant <NUM>, by way of a common bus <NUM>. Other types of power sources are contemplated such as steam, nuclear, geo-thermal, hydroelectric, wind, etc. It is contemplated that more than two power sources can be utilized. The single input port inverter <NUM> can be sized for the AC output requirements of the grid to which it is connected. The common bus <NUM> allows for near instantaneous response to changes in the system power requirements.

The DC storage plant <NUM> typically includes a plurality of battery banks <NUM>, bi-directional DC/DC converters <NUM>, and a controller <NUM>. The controller <NUM> can govern the charge and discharge rate. The bi-directional DC/DC converters <NUM> can be configured to charge the battery banks <NUM>. The bi-directional DC/DC converters <NUM> can be sized to the battery output or input. Every battery bank <NUM> and DC/DC converter <NUM> is separately managed. The bi-directional DC/DC converters <NUM> employ a power droop algorithm which maintains constant power output in the normal MPPT region. When the voltage is higher or lower than the MPPT region, a bi-directional DC/DC converter <NUM> could ramp up power or ramp down power output to the bus <NUM>. The power droop algorithm enables local control of power output from the DC power plant <NUM> based on the external load. Additionally internal resistance of the parallel connected bi-directional DC/DC converters <NUM> maintains relatively equal current sharing between the battery banks <NUM>. The power droop algorithm, in combination with the battery banks <NUM> and the photovoltaic panel arrays <NUM> sharing a common bus <NUM> eliminates the need for additional communication and allows for a fast response to micro-grid applications (e.g., changes in load on the inverter <NUM>).

The single input port inverter <NUM> receives power from the common bus <NUM> and converts it to an AC voltage. The single input port inverter <NUM> can be sized for the AC output requirements of the distributed solar and storage system <NUM>. The single input port inverter <NUM> maintains the output power at the Maximum Power Point (MPP) by using, for example, the voltage tracking method. It is contemplated that other methods known in the art can be used. Maximum Power Point Tracking (MPPT), which is the process of finding the keeping the load characteristic at the point where the system is optimized to give the highest power transfer, is run at the input port of the single input port inverter <NUM>. The output power from the solar plant is sampled and the proper load characteristic (resistance) is applied so as to obtain maximum power. When a grid curtailment command arrives at the solar power plant <NUM> and the DC storage plant <NUM>, the single input port inverter <NUM> experiences a rise in system voltage at the bus <NUM>, and both the solar power plant <NUM> and the DC storage plant <NUM> reduce power output without active control. Similarly, in an increasing load scenario, the single input port inverter <NUM> experiences a drop in system voltage at the bus <NUM>, and both the solar power plant <NUM> and the DC storage plant <NUM> increases power output without an active control. These changes in voltage are near instantaneous at the single input port inverter <NUM>.

As can be seen in <FIG>, as the system voltage increases beyond MPPT, the power and current output drop dramatically with the rise in voltage. In parallel with this drop in power and current, the controller <NUM> can communicate with the individual bi-directional DC/DC converters to manage the output of each of the individual battery banks <NUM> to achieve the same power and current output in a more efficient manner and in accordance with the health and states of charge of each of the individual battery banks <NUM> as described in greater detail below. It is contemplated that the battery banks <NUM> and DC/DC converters <NUM> are separately managed. The single input port inverter <NUM> is thus able to maintain power output of the solar power plant <NUM> and the DC storage plant <NUM> in the MPPT region.

<FIG> depicts a graph <NUM> depicting the MPPT region of a distributed solar and storage system <NUM> in accordance with the present disclosure. MPPT can be implemented generally to sources with variable power. The graph <NUM> depicts a series of V/I and P/V curves. To achieve maximum solar energy, the photovoltaic panel arrays <NUM> of the solar power plant <NUM> are equipped with a system for calculating and maintaining MPPT. As demonstrated by the graph <NUM> as the amount of sunlight varies, the load characteristic which provides the maximum power transfer efficiency also changes. By changing the load and supply characteristic, the power transfer can be kept at a point of high efficiency. The distributed solar and storage system <NUM> depicted in <FIG> will operate such that the single input port inverter <NUM> can operate within the MPPT region, and simply add or shed supplied power in response to the demand.

<FIG> depicts a graph of bi-directional DC/DC converter output curve <NUM> in accordance with the present disclosure. The curve depicts output power vs voltage. The power droop algorithm employed by the bi-directional DC/DC converters <NUM> and inverter <NUM> maintains the output at a relatively constant power. As shown in the graph <NUM> a constant power region <NUM> depicts the power output of the DC/DC converters <NUM> at a given voltage. Though relatively constant, a droop in power is depicted as the voltage increases. The constant power region <NUM> of the graph <NUM> substantially overlaps the MPPT region <NUM> of <FIG>. When the common bus's <NUM> output voltage is higher than the MPPT region, the bi-directional DC/DC converter can automatically react and increase its output voltage, as depicted in <FIG>, to a point higher than the MPPT region depicted in <FIG>. When the common bus's <NUM> output voltage is lower than the MPPT region, the bi-directional DC/DC converter can automatically react and the output power can be ramped up. This allows the bi-directional DC/DC converters <NUM> to respond changes in load automatically as a result of changes in current drawn from the battery banks <NUM> to maintain the inverter <NUM> at or near MPPT at all times.

As the voltage increases, and the power delivered from the battery banks <NUM> through the bi-directional DC/DC converters <NUM> drops, the controller <NUM>, though lagging the near instantaneous response caused by the changes in bus <NUM> voltage caused by the change in load, is able to communicate with the bi-directional DC/DC converters to adjust how that reduced or increased power is being delivered to the common bus <NUM>. That is the controller can adjust which battery banks <NUM> are actually feeding the common bus <NUM>. In this way, controller can remove battery banks <NUM> from the common bus <NUM> to both adjust the delivered power, and to reduce the bus voltage, shifting the operating point of the inverter <NUM> to within the inverter nominal operating region, and to a more efficient point on the MPPT curves depicted in <FIG>. As will be appreciated, this also works in reverse as a load is added to the inverter <NUM>. As load increases on the common bus <NUM>, the voltage as observed in connection with the power droop curve of <FIG> will drop, and the bi-directional DC/DC converters will recognize that they are outside of the constant power region and additional battery banks <NUM> will connect to the common bus <NUM>. While some number of battery banks <NUM> will be able to automatically connect to the common bus <NUM> based on their state of charge, other battery banks <NUM> may have received communication from the controller preventing their further discharge until achieving some level of charge (e.g., <NUM> % of maximum capacity) before being permitted to discharge.

Generally, the response to load by the bi-directional DC /DC converters <NUM> both connecting to or separating from the common bus <NUM> is initially driven by the bus voltage. Secondarily it is driven by the controller <NUM> to manage more directly which battery banks <NUM> and bi-directional DC/DC converters <NUM> are supplying power to the common bus <NUM> and inverter <NUM> or are being charged by the solar power plant <NUM>.

<FIG> depicts a graph of grid demand response <NUM> in accordance with the present disclosure. In an embodiment, for example, an 80kW solar power plant and ten 10kW batteries sharing the common bus <NUM> could have all the battery banks <NUM> running full discharge as in the top trace in the graph <NUM>. As the voltage increases, so does the output power, as the system <NUM> maintains output power in the MPPT region as depicted in <FIG>. By turning off all of the batteries <NUM>, only the solar power plant <NUM> would remain providing power to the grid, as in the middle trace <NUM>. If for example, half of the battery banks <NUM> needed charging the curve would follow the lower trace <NUM>. In all of the above cases, the distributed solar and storage system <NUM> would attempt to remain operating in the MPPT region as depicted in <FIG>, such that the single input port inverter <NUM> will still be able to provide maximum power. If the single input port inverter <NUM> voltage rises past the MPPT region and into the power curtailment region of the graph <NUM> in any of the above traces <NUM>, <NUM>, <NUM>, the solar power plant <NUM> and the DC storage plant <NUM> will automatically respond by reducing power to the common bus <NUM>, without the need for a command.

In another embodiment, when the AC load demand changes, it creates a change in frequency, which will translate as a change in bus voltage on the input side of the single input port inverter <NUM>. When the grid frequency is low, and there is a need for frequency support, the single input port inverter <NUM> will reduce the system voltage on the common bus <NUM> to search for the system maximum power.

<FIG> depicts a logic flow for a control algorithm in accordance with the present disclosure. In one embodiment, the system controller <NUM> can monitor the power demand of a particular load at step <NUM>. The controller <NUM> can monitor the DC charging state of the DC storage plant <NUM> at step <NUM>, and monitor the output power and voltage of the solar power plant <NUM> at step <NUM>. In another embodiment, based on the monitoring, the controller <NUM> can detect whether the load has increased <NUM>, and after the load has stabilized, the controller <NUM> can adjust at step <NUM> the power available from the DC storage plant <NUM> in accordance with the power droop algorithm. In yet another embodiment, based on the monitoring steps <NUM>, <NUM>, and <NUM>, the controller <NUM> can detect or receive communications indicating at step <NUM> that there are one or more bad batteries in a battery bank <NUM>. In a DC storage plant <NUM>, a bad battery can cause the entire battery bank <NUM> to be bad and thus unavailable for meeting load demands safely. Thus, in step <NUM> the controller can switch the affected battery bank <NUM> off, preventing it from being employed to respond to changing load demands, and ensure that any battery bank <NUM> with a faulty battery is not permitted to be connected to the bus <NUM> and feed the inverter. It is contemplated that the controller <NUM> maintains the operating state of charging of the DC storage plant <NUM> at or above a minimum level of charge. In yet another embodiment, based on the monitoring steps <NUM>, <NUM>, and <NUM>, the controller <NUM> can detect whether demand has dropped at step <NUM>. Generally, the response to load by the bi-directional DC /DC converters <NUM> is initially driven by the bus voltage. However, when the load demand is stabilized, the controller <NUM> can adjust the amount of power being supplied from the DC storage plant <NUM>. It is contemplated that any of the detecting steps <NUM>, <NUM>, and <NUM> can happen in parallel with each other.

Claim 1:
A method for implementing a multi-power distributed power storage and generation system comprising:
feeding power generated by a plurality of solar arrays (<NUM>, <NUM>) of a solar power plant (<NUM>) directly to a common bus (<NUM>);
discharging, by a plurality of bi-directional DC/DC converters (<NUM>) of a DC storage plant (<NUM>), power from at least one of a plurality of battery banks (<NUM>), respectively, directly to the common bus (<NUM>);
feeding a voltage on the common bus (<NUM>) to a single input port invertor (<NUM>);
monitoring (<NUM>), by a system controller (<NUM>), a power demand of an external load;
maintaining constant power output from the DC storage plant (<NUM>) in a maximum power point tracking, MPPT, region (<NUM>) of the solar power plant (<NUM>) using the power droop method enabling local control of power output from the DC storage plant (<NUM>) based on the power demand of the external load;
ramping down an output power of the plurality of bi-directional DC/DC converters (<NUM>) when a voltage on the common bus (<NUM>) is higher than the MPPT region (<NUM>);
ramping up an output power of the plurality of bi-directional DC/DC converters (<NUM>) when a voltage on the common bus (<NUM>) is lower than the MPPT region (<NUM>); and
maintaining maximum power output of the solar power plant (<NUM>).