WO2015123549A1

WO2015123549A1 - Renewable energy generation plant and procedures for controlling a renewable energy generation plant

Info

Publication number: WO2015123549A1
Application number: PCT/US2015/015866
Authority: WO
Inventors: Stephan Galler; Kai SAEGEBARTH
Original assignee: First Solar, Inc.
Priority date: 2014-02-13
Filing date: 2015-02-13
Publication date: 2015-08-20
Also published as: DE102014101809A1; DE102014101809B4

Abstract

A procedure for controlling a renewable energy generation plant (1 ), where said plant (1 ) features multiple primary energy converters (2) connected with controllable energy converters (3) and a controller (10) to control the energy converters (3), consists of the following steps: - Assign at least one energy converter (3) to a primary group (7) of energy converters (3) set to maximum power output by the controller (10). - Assign energy converters (3) to at least one other group (8, 9) of energy converters (3) set to reduced power output by the controller (10), so that the desired total output of the renewable energy generation plant (1 ) is set by assigning the energy converters (3) to the groups (7, 8, 9). - Collect power data on the first group (7) of energy converters (3). - Determine the total potential output power available from the renewable energy generation plant (1 ) based on the power data collected from the first group (7) of energy converters (3).

Description

Renewable Energy Generation Plant and Procedures for Controlling a Renewable Energy Generation Plant

The invention concerns a renewable energy generation plant, especially photovoltaic plants, and procedures for controlling a renewable energy generation plant. Integrating renewable power stations such as wind-power and photovoltaic stations into power grids increasingly requires that a positive operating reserve be maintained during operation.

Different national grid codes occasionally stipulate requirements calling for a power reserve that can be accessed at various times. In Romania, for example, connecting large PV plants to the high-voltage network entails participation in primary control for frequency stabilisation. Another example can be found in the current version of the South African network regulations for renewable-energy power stations. Operating procedures for regulating the feed-in of active power are mandated there, and these too call for a power reserve.

Another possible requirement involves limiting power loss from renewable- energy power stations that have fluctuating primary energy sources

(photovoltaic, wind). If negative power gradients are permitted only up to a certain steepness, then the plant's technical control equipment can offset the difference between steeper drops in primary energy and the permissible power gradient.

Especially in so-called microgrids with a high concentration of feed-ins from renewable energy there is a need for network-stabilising system services, for example, to provide reactive power or, more importantly, to maintain primary balancing power for frequency stabilisation. Regenerative power stations that make system services like these available on demand to a superordinate energy management system can contribute directly to stabilising the microgrid.

What these and similar requirements have in common is that the plant has to deliver positive control energy in the short term. In PV plants, however, the strongly and stochastically fluctuating primary power source makes it extremely difficult to maintain an appropriate power reserve at all times, because this reserve has to orient itself on a primary energy supply that is sometimes highly variable.

By installing additional generators or energy storage devices, additional, controllable feed-in power for the required operating reserve can be made available to a renewable energy plant independently of its primary power source. In practice this involves, for example, the use of diesel generators that feed into the network parallel to a PV plant. This approach is

supplemented or substituted by installing energy storage devices such as batteries with suitable inverters.

These methods require additional investments in additional equipment. Furthermore, co-generation using non-renewable energy sources requires that additional costs connected with supplying the primary energy source (fuel) be taken into account. This is aggravated by the fact that the additional power producers must be able to offset the fluctuations of the PV plant's feed-in rapidly enough. In systems based on rotating masses (for example, diesel generators) this can lead to increased wear and tear. In any case, a rapid control system is needed to monitor feed-ins and regulate the co- generators accordingly. Another obvious approach is to directly determine the primary energy supply, for example, by measuring the amount of solar radiation at the module level of a PV plant. This makes it possible to estimate the amount of active power that can be fed into the power grid, taking into account the plant's variable efficiency and availability when generating electrical power. In detail, this procedure presents several obstacles to overcome, resulting in substantial metrological outlays in the field and considerable outlays for converting the measured values into available electrical power, so that adequately precise power estimates can be obtained.

As described in WO 2013/041534 A3, a power reserve can also be maintained by deliberately and controllably decreasing the efficiency of power conversion from the PV generator. In practice, this is accomplished by modifying the Maximum-Power-Point-Tracking (M PPT) at the DC input of the inverter. Rather than running the power adjustment of the inverter input on the MPP, it is run at an underlying, adjustable level.

Because this procedure can only be implemented by modifying the internal inverter control, not all existing plants can be retrofitted for it. For plant operators using inverters that do not support the procedure, this solution can only be implemented by replacing inverter technology at considerable cost.

The precision with which the percentage of power fed into the network can be determined using this method decreases drastically when larger deviations from instantaneous MPP are desired, because a power estimate for PV modules connected to arrays can only be analytically determined using a complex l-V curve to a very low degree of accuracy. This depends largely on radiation conditions and on the homogeneity of the power characteristics of the individual modules, although an inhomogeneity can also be characterised as a mismatch.

The invention addresses the problem of improving the control of a regenerative energy generation plant.

This problem is solved as described in Claims 1 and 9, respectively.

Additional advantageous implementations of the invention are defined in the dependent claims.

In accordance with a primary aspect of the invention, a procedure for controlling a renewable energy generation plant, where said plant features several primary energy converters connected with controllable energy converters and a controller to control the energy converters, consists of the following steps: Assign at least one energy converter to a primary group of energy converters set to maximum power output by the controller.

Assign energy converters to at least one other group of energy converters set to reduced power output by the controller, so that the desired total output of the renewable energy generation plant is set by assigning the energy converters to groups.

Collect power data on the first group of energy converters.

Determine the total possible output power available from the renewable energy generation plant based on the power data collected from the first group of energy converters.

For the purpose of the invention, an energy generation plant may be a power station, a part of a power station or a network of power stations, that is to say, it may span multiple stations. Assignment to the groups takes place dynamically, in other words, variably. The groups can also be understood as classifications. The groups are not necessarily assigned or formed physically, for example, by using cables; instead they are divided logically, as it were, preferably by the controller. The controller can operate in either a controlling or regulatory manner. The energy converters from the first group function to some extent as reference units or measuring devices to measure the currently available primary energy supply. The advantage of this is that no additional sensors are necessary and the measurement results precisely reflect the actual status at the energy conversion site. On the basis of the collected data on the maximum potential outputs of the energy converters from the first group, the determination step can make projections for all of the plant's energy converters, including those operating at reduced capacity in the other groups.

Even with distributed generation, the amount of primary power available can be determined without additional sensors. Thus, the total amount of power available can be determined for distributed energy converter systems even when total feed-in is reduced. The procedure behind the invention facilitates simple and highly precise determination of the total possible output power available, in other words, the power output of the plant when all controllable energy converters, such as rectifiers, are able to convert the available primary energy freely and without limit.

The invention allows distributed energy generation plants or units to be controlled in such a nuanced way as to facilitate both limiting the total output power and determining how much unlimited total output power is currently possible. Nor is this procedure limited to renewable, fluctuating energy sources with distributed primary energy converters, such as wind or solar radiation. In principle, the procedure can be applied to any primary energy source that fluctuates locally and temporally, yet occurs in a coupled or correlative form, insofar as the source can be gathered by distributed and controllable primary energy converters. Examples include distributed thermal converters at natural or artificial thermal-energy sources and bioelectric energy sources.

It is preferable that at least two additional groups be designated, with the energy converters in each group being operated at a different,

predetermined, reduced power output level. For example, four groups could be designated, the first group operating unregulated at 100% of its potential power output, and the three remaining groups operating at a reduced power output of 60%, 30% and 0%, respectively, of their potential power output. Because of the increased number of groups, the desired power output of the plant can be quantified more accurately by properly assigning or grouping the energy converters. Assignment to a group occurs when a control unit gives an energy converter a control command or a setpoint value, for example, to set power output at 30%. This energy converter then belongs to the group of energy converters that the control unit operates at a reduced power output of 30%. Ideally, the power outputs from energy converters in at least one of the other groups should be continuously controlled between a minimum and maximum power output level. This permits greater flexibility, in that it gives more leeway for adapting the plant's output without changing the groups or group assignments, while also enabling the plant's power output to be accurately adjusted between the group-specified quantisation steps by regulating or controlling the power outputs of the energy converters. The concepts minimum and maximum power output of the energy converter may refer, on the one hand, to the respective groups and, on the other, to plant-wide limits.

The number of energy converters in the first group may be maximised. This increases the number of energy converters operating as measuring devices, thus increasing the accuracy of the procedure. In implementing the invention, it is advantageous for energy converters to be assigned to the first group based on their spatial arrangement in the energy generation plant, their availability and/or the degree of correlation to the energy generation plant's mean total output power. When determining the degree of correlation, it is sometimes possible, without any down-regulating, i.e. with the energy converters set at maximum feed-in, to automatically examine the correlation between each energy converter's feed-in and the feed-in of the entire plant. This will reveal that certain energy converters represent the mean output power of the plant better than others. This is dependent on, for example, whether the unit is positioned at the edge or in the centre of the plant as a whole. Units that better represent the total output under various weather conditions should always be preferred when assigning converters to the primary group of reference units. Other units that are not as highly correlated can then be shifted to one of the other groups when down- regulating. Energy converters not assigned to the primary group may also be temporarily set to operate at maximum power output. This makes it possible to gradually employ each energy converter as a measuring instrument, which improves accuracy in determining the total output power. The energy converters can be exchanged cyclically on a rotational basis. The energy generation plant's radiation monitoring devices can be used to calibrate the calculated total possible output power. This makes it possible to review and/or improve the correlation between actual and calculated output power.

Another advantageous implementation of the invention calls for the actual feed-in power of the energy generation plant to be set to below its total possible output power in order to make an operating reserve available.

Because the invention facilitates an exact quantification of the potential feed- in power of the energy generation plant on the basis of the current primary energy supply, an operating reserve can be maintained with precision.

In calculating both the total feed-in that the renewable energy plant is currently capable of providing from the available generation units and the potential feed-in power of those units, the result can be affected by a value that is either fixed or adjustable to the plant's operating point and that offsets AC-losses on the transmission path to the grid connection point.

When setting the actual feed-in power, a predetermined feed-in setpoint can be taken into account. In so doing, it can be decided whether, for example, the priority is to ensure feed-in power, that is, the required feed-in limit, or to maintain an operating reserve.

Another implementation of the invention involves a renewable energy generation plant with multiple primary energy converters connected to controllable energy converters and a control unit for controlling energy converters, where at least one energy converter is assigned to a primary group set to maximum power output, the remaining energy converters are assigned to at least one other group of energy converters set to down- regulated, reduced power output, and the control unit is configured to adjust the total output power of the renewable energy generation plant to the desired level by assigning the energy converters to groups. The same advantages and modifications mentioned above apply.

In a particularly favourable version of this implementation, the controllable energy converters are individual inverters or inverter clusters, for example string inverters. However, individual controllable MPP trackers or DC-DC converters at the PV-module, PV-string or PV-array level can also be understood as controllable energy converters. Furthermore, any controllable, distributed energy converter equipped to measure power can be used, for example wind power stations with multiple wind turbines.

The energy generation plant should preferably be a photovoltaic power station whose primary energy converters are photovoltaic modules and whose energy converters are inverters. A primary power supply subject to powerful and rapid temporal and positional fluctuations, for example, due to cloud drift, is ideally suited for flexible use by renewable energy generation plants controlled in accordance with the invention. In what follows, the invention will be described in greater depth based on the diagrams, which depict:

Figure 1 . A block diagram of a renewable energy generation plant as per the invention.

Figure 2. A schematic of the layout of a PV power station. The diagrams serve only to illustrate the invention, and do not limit its scope. The diagrams and the individual parts are not necessarily to scale. Identical numbers indicate identical or similar parts.

Figure 1 presents a schematic diagram of a regenerative or renewable energy power station. This could be, for example, a PV or wind power plant. A number of distributed primary energy converters (2) captures primary energy (sun, wind) from a place and time dependent source and converts it into secondary energy (electrical energy).

Each primary energy converter (2) or group of primary energy converters (2) is connected to a controllable energy converter (3). The controllable energy converters (3) in turn supply energy over a standardised energy transport system (4) - while also permitting additional energy conversion or the conversion of energy form parameters (5) - to a superordinate energy distribution system (6), a power network or grid, as the case may be. In a specific example of a PV installation, the primary energy converters (2) are operated as PV modules and the energy converters (3) are implemented as inverters, 4-quadrant converters, negators, matrix inverters or the like.

This enables the primary energy converters (2), the controllable energy converters (3) and the other energy converters (5) to function as a unified group. They are oriented on the control unit, yet are repeatedly available in parallel.

By way of example, the controllable energy converters (3) are assigned to a first group (7) a second group (8) and a third group (9). The energy converters (3) in the first group operate freely, meaning that they are run at maximum power output. The energy converters (3) in the second group (8) are controlled or regulated to operate between a minimum and maximum power output level. The energy converters (3) in the third group (9) are operated at minimum power output. Distribution into groups, changes between groups, and operations within the groups will be described in detail later.

A superordinate control unit (10) receives target values from a superordinate control facility (1 1 ) for the power that is to be fed into the superordinate energy distribution system (6) or it calculates target values autonomously. The regulation or control unity (10) can be implemented as a centralised or distributed system that can function independently or be integrated into the architecture of the energy converter (3).

The control unit (10) issues control parameters or setpoints to the energy converters (3); to this end, they are either tethered or connected together wirelessly. These communication links can either be dedicated cables (12) or a bus system. It is preferable that the connection via the cables (12) be bi- direction, so that the power data, measured values, operating points, workloads and/or availability can be transferred from the energy converters (3) to the control unit (10). A wireless or radio-based communication link can be accomplished via WLAN or Bluetooth, for example. The procedure can be implemented in both a regulated and controlled manner in the superordinate energy distribution system (6) based on the actual feed-in. When regulating, the regulation and control unit (10) must be connected with a measuring device (13) that determines the actual feed-in at a feed-in point or grid connection point connected to the superordinate energy distribution system (6), and current feed-in data must be made available to it over this connection.

The control unit (10), the cables (12) and the measuring device (13) are elements of a monitoring and control system (14) that enables the energy generation plants (3) to monitor and control important operating parameters with high temporal resolution and high availability. There are multiple parallel- operating generation units (3) available that can be controlled independently of each other.

To facilitate regulation of the active power feed-in of the renewable power station (1 ), a superordinate control system (10) is connected to the generation units (3). This system groups the generation units (3) into several groups (7, 8 and 9), at least one of which (7) operates the generation units (3) without limiting active power, while at least one additional group (9) operates the generation units (3) at the lowest value within the adjustment range. By controlling the ratio of freely in-feeding generation units (3) from group 7 to minimally in-feeding generation units (3) from group 9, the power output of the power station (1 ) can be modulated in discrete steps between a minimum feed-in level and the maximum power output possible at a given moment (based on the current primary power supply). With at least one other group (8) of generation units (3) whose feed-in power can be regulated between the minimum output and the maximum output possible at a given moment, the feed-in power of the power station (1 ) can be precisely regulated between the discrete steps of groups 7 and 9. This makes it possible to precisely control all the power levels of the generation units (3) between maximum feed-in and the lower feed-in limit and, if necessary, to maintain them at a desired level within a closed-loop control system. In order to determine the reference capacity of the power station (1 ) independently of operating points, in each operating point of the power station (1 ) an attempt will be made to maximise the number of generation units (3) in group 7 that feed in unrestrictedly, On the basis of these reference units (3), which feed in freely depending on the extent to which the primary energy supply (for example, solar energy) is locally available at the plant (1 ), the potential total output power of the PV plant can be calculated, taking into consideration the availability of all generation units. The setpoint of an active power regulator or an active power controller for the plant as a whole can now be continually maintained at an arbitrary amount below the calculated potential current capacity to provide for an operating reserve. This makes the desired operating reserve available for retrieval, which, depending on the specific application, can be accomplished by either manipulating the setpoint of the active power regulator or overriding the method by which the operating reserve margin is calculated.

In order to maintain a required positive operating reserve at a predetermined feed-in limit or at the level of the maximum available primary supply, the available controllable energy converters (3) are dynamically assigned to the three groups (7, 8 and 9). The assignments are made by the superordinate control unit (10) by communicating over the cables (12) with the individual controllable energy converters (3).

The first group (7) consists of units (3) that deploy the maximum amount of overall power offered by the primary energy converters (2). The power data in this group (7) is used by the superordinate control unit (10) to calculate the total output power actually available from all controllable energy converters (3) or the power station (1 ).

The desired feed-in level is set by the appropriate groupings of controllable energy converters (3) based on calculations of the actually available total output power, with consideration given to maintaining the desired operating reserve. In the process, the amount of power to be fed in can be roughly adjusted based on the ratio of energy converters (3) from the first group (7), which are feeding in fully, and energy converters from the third group (9), which are feeding in minimally or not at all. The remaining fine-tuned adjustments are made by regulating the energy converters (3) in the second group (8) such that the output of these units (3) is appropriately modulated. Which of the three groups (7, 8 and 9) an individual controllable energy converter (3) belongs to is thus determined by the respective setpoint value assigned by the superordinate control unit (10).

In Figure 2, the layout of a photovoltaic power station (1 ) is depicted as an example. The layout depicts a plan view of a PV power station (1 ) with an installed capacity of roughly 27 MWp over an area of around 1200 m by 550 m. The PV modules (2) mounted on module tables in the ground-mounted PV power plant are organized into blocks (15). All of the modules (2) in one block (15) feed in via DC subdistributors located in the inverter station (3), which is centrally positioned for each block (15) and equipped with a medium-voltage transformer. These inverter stations (3) are connected along medium-voltage power lines to the substation (5), where measurement of the actual feed-in value is also conducted. The substation (5) feeds the power generated by the PV power station (1 ) into the high-voltage network and thus constitutes the grid connection point. PV-park regulation, or park control (10), is likewise conducted in the substation (5). A communication link runs from there to the inverter stations (3) along a secure fibre-optic Ethernet network. The plant depicted here as an example is organised into 15 blocks, each with a connected load of 1600 kW. Figure 2 depicts the current distribution of the individual blocks (15) into groups by way of example. Eleven blocks (A.1 to A.1 1 ) feed in at maximum available power; these blocks, and hence the controllable energy converters or inverter stations (3) belonging to them, are assigned to the first group (7).

Two blocks (C.1 to C.2) are set to the minimum adjustable output. These blocks and the controllable energy converters or inverter stations (3) that belong to them are assigned to the third group (9).

Based on the relationship between these two groups (7 and 9), the desired feed-in level can be set to below the potential total feed-in in a highly quantised manner. Two additional blocks (B.1 - B.2) are set to a setpoint between the minimum and maximum potential block capacity, so that the desired output of the power station (1 ) can be adjusted with greater precision. These blocks and the controllable energy converters or inverter stations (3) that belong to them are assigned to the second group (8).

It may be noted that the reference stations, or rather the inverter stations (3), of the first group (7) are distributed throughout the plant (1 ) as evenly as possible, so that the radiation conditions of the entire plant (1 ) can be determined,

The following table indicates the power values as an example. For the sake of simplicity, all losses are ignored and a uniform radiation distribution is assumed for the power station (1 )

It should be noted that based on the maximum power station output determined in the first group (7) by deducting the stipulated balancing power reserve, a desired feed-in power of 16,800 kW is calculated. This desired feed-in power is now initially approximated by a maximum number of freely running inverter stations (3) from the first group (7). Next, the minimally in- feeding inverter stations (3) from the third group (9) are set. Finally, the remaining inverter stations (3), which are assigned to the second group (8), fine-tune the desired output of the power station (1 ). Obviously, this procedure can also function without setting aside an operating reserve, in which case the groups would be assigned based on a currently available output of 21 ,600 kW.

Claims

Patent claims

1 . A procedure for controlling a renewable energy generation plant (1 ), where said plant (1 ) features multiple primary energy converters (2) connected with controllable energy converters (3) and a controller (10) to control the energy converters (3), consists of the following steps:

- Assign at least one energy converter (3) to a primary group (7) of energy converters (3) set to maximum power output by the controller (10).

- Assign energy converters (3) to at least one other group (8, 9) of energy converters (3) set to reduced power output by the controller (10), so that the desired total power output of the renewable energy generation plant (1 ) is set by assigning the energy converters (3) to the groups (7, 8, 9).

- Collect power data on the first group (7) of energy converters (3).

- Determine the total potential power output available from the

renewable energy generation plant (1 ) based on the power data collected from the first group (7) of energy converters (3).

2. Procedure based on Claim 1 , whereby at least two additional groups (8, 9) are designated, and the energy converters (3) in each group (8, 9) are operated at a different, predetermined, reduced power output level.

3. Procedure based on Claims 1 and 2, whereby the power outputs from energy converters (3) in at least one of the other groups (8, 9) are continuously controlled between a minimum and maximum power output level.

4. Procedure based on one of Claims 1 to 3, whereby the number of

energy converters (3) in the first group (7) is maximised.

5. Procedure based on one of Claims 1 to 4, whereby the energy

converters (3) are assigned to the first group (7) based on their spatial arrangement in the energy generation plant (1 ), their availability and/or the degree of correlation to the average total output of the energy generation plant (1 ).

6. Procedure based on one of Claims 1 to 5, whereby energy converters (3) not assigned to the first group (7) are temporarily set to operate at maximum power output.

7. Procedure based on one of Claims 1 to 6, whereby the radiation

monitoring devices of the energy generation plant (1 ) can be used to calibrate the calculated potential total output power.

8. Procedure based on one of Claims 1 to 7, whereby the actual feed-in power of the energy generation plant (1 ) is set to below its potential total output in order to make an operating reserve available.

9. Procedure based on Claim 8, whereby a predetermined feed-in setpoint is taken into account when setting the actual feed-in power.

10. A renewable energy generation plant (1 ) with multiple primary energy converters (2) connected to controllable energy converters (3) and a control unit (10) for controlling energy converters (3), where at least one energy converter (3) is assigned to a primary group (7) set to maximum power output, the remaining energy converters (3) are assigned to at least one other group (8, 9) of energy converters (3) set to down- regulated, reduced power output, and the control unit (10) is configured to adjust the total output of the renewable energy generation plant (1 ) to the desired level by assigning the energy converters (3) to groups (7, 8, 9).

1 1 . A renewable energy generation plant based on Claim 10, whereby the energy generation plant (1 ) is a photovoltaic power station whose primary energy converters (2) are photovoltaic modules and whose energy converters (3) are inverters.