METHOD FOR CONTROLLING PARALLEL-CONNECTED AC GENERATORS FOR DISTRIBUTING A REACTIVE LOAD

Description

[0001] The present invention relates to the management of a fleet of alternators supplying in parallel to a load.

[0002] When alternators in a bank of alternators are coupled in parallel and deliver power to the terminals of a load, the correct distribution of reactive load between all the alternators is an important factor in the proper operation of the bank. Indeed, reactive power impacts the value of the excitation current of the alternator, thus directly influencing the load on the electrical machine and its aging. A good distribution of the reactive load ensures a balanced load for the alternators coupled in parallel.

[0003] A method is known to ensure this load distribution, called statism, or " reactive droop » in English.

[0004] Application EP 2 722 955 describes a voltage regulator acting by the so-called statism method on a system of parallel alternators

[0005] EP 1 508 951 A1 discloses a real reactive power control system for wind turbine generators. This closed-loop voltage control system comprises a dynamic controller that provides a desired power to a load connected to the PCC.

[0006] US 2010 / 134076 A1 describes a method of reactive power control for renewable energy plants. An output signal of a reactive power controller is generated based at least in part on a reactive power control signal received from a decision unit.

[0007] US 2015 / 357819 A1 relates to a method for controlling the reactive power of a power generator by switching it from an initial reactive power state to a desired state. These states are determined by a central power controller.

[0008] CN 101 924 512 A discloses an automatic electrical generator control system for a ship. Several electrical devices are connected via a CAN bus to a remote control center ensuring reactive power balance.

[0009] The static method, illustrated in the figure 1 , is based on the measurement of the reactive power delivered by the alternator, expressed as a percentage of nominal kilovolt-amperes reactive (kVAr). The greater the reactive power delivered, the more the voltage is lowered proportionally according to the slope coefficient of the droop line. Thus, when coupling an alternator A to other alternators that deliver power to a load, the voltage of alternator A is equal to the no-load voltage for which no load is present at the alternator terminals during coupling, and the average voltage of the other alternators is lower than the no-load voltage because they are subject to the presence of the load. As the reactive load naturally moves towards the highest voltage, the voltage of alternator A will gradually decrease as it is taken over, and the voltage of the other alternators will increase, until the assembly reaches a point of equilibrium.

[0010] Droop allows the power measurement to be maintained on the regulator and an additional alternator to be easily added since it is sufficient to have identical settings for no-load voltage and droop coefficient. However, for ideal operation, all alternators must be set with the same no-load voltage and the same leading coefficient. If one of these two parameters is misadjusted, the sharing is no longer carried out correctly. Commissioning is therefore delicate. In addition, since the voltage varies according to the leading droop coefficient, the voltage setpoint is not maintained regardless of the load.

[0011] Another known method is the so-called reactive differential method, or « cross current compensation » in English.

[0012] US patent 7,053,503 describes a power control system using the reactive differential method, consisting of distributing the reactive loads on the different electrical machines in parallel from a loop physically created between the current measurements of the alternators.

[0013] As illustrated in the figure 2 , load sharing is achieved by a current loop created between stator current measuring transformers. At load balance or no-load, the currents entering each regulator are zero. Thus, power measurement by the regulator is not possible. The voltage setpoint is always maintained at the no-load voltage value, regardless of the reactive load.

[0014] The reactive differential method allows for a stable voltage setpoint and correct reactive load distribution between alternators, even if the no-load voltage of each alternator is out of adjustment, and to compensate for any misadjustments of the steering coefficient which is only used to return more or less quickly to the setpoint voltage.

[0015] However, this method requires complex wiring. Careful creation of a current loop between multiple alternators can lead to insulation problems. Adding an additional alternator therefore requires a complete modification of the loop. In addition, resistors must be determined based on the regulator, the rated reactive power of the electrical machine, and the stator current measuring transformer ratio, and switches must be used to short-circuit an alternator not coupled to the load. Part of the so-called droop method must be used in the regulator to allow the voltage setpoint to be raised and lowered. Power measurements on the regulators must be performed using another current measurement input.

[0016] In addition to droop and the reactive differential method, there is another method in the state of the art which consists of sharing reactive loads between regulators using an RS485 standard link. This makes it possible to compensate for the voltage drop of the droop. This method amounts to sharing by a field bus the total load present at the terminals of alternators connected to each other. The load value to be supplied by each alternator is then determined by a simple proportionality, the voltage setpoint being corrected accordingly.

[0017] There is a need to further improve the methods of reactive load distribution between alternators operating in parallel, in particular to ensure balanced use of the latter.

[0018] The invention aims to meet this need, and it achieves this, according to one of its aspects, by means of a method for regulating a bank of alternators comprising at least two alternators delivering power in parallel to a load, each equipped with a regulator configured to deliver an output signal representative of the reactive power rate of the corresponding alternator in relation to its nominal reactive power, and a regulation law making it possible to modify the reactive power rate of the alternator as a function of an input signal, a method in which a weighted signal sent as an input signal to these regulators is generated from the output signals representative of the reactive power rate of each of the alternators, delivered by the corresponding regulators, so as to cause each of the alternators to converge towards a predefined reactive power rate.

[0019] Thanks to the invention, it is possible to achieve a reactive load sharing function between different alternators operating in parallel, with simplified wiring and power measurements available without requiring the addition of additional transformers, and while keeping the delivered voltage permanently equal to the voltage setpoint, regardless of the value of the applied load.

[0020] Thanks to the invention, all the alternators in parallel deliver the same predefined reactive power rate to the load, and the exchange of reactive current between alternators is also avoided. An additional alternator can be easily added to the alternators already in place, without having to modify the regulation wiring and the configuration of the other alternators. Preferably, the weighted signal is the arithmetic mean of the output signals representative of the reactive power rate of each of the alternators. The invention makes it possible to compare the reactive power delivered by an alternator with the average reactive power of all the alternators operating in parallel.

[0021] The reactive power rate of an alternator is preferably expressed as a percentage of its nominal reactive power.

[0022] The predefined reactive power rate advantageously corresponds to a balance point, in particular calculated by dividing the value of the load by the sum of the nominal reactive powers of all the alternators and expressed as a percentage.

[0023] The predefined reactive power rate may correspond to the average of the reactive power rates of all the alternators in parallel. When executing the regulation method according to the invention, each of the alternators advantageously converges towards the same reactive power rate.

[0024] The regulation law makes advantageous use of the fact that, when alternators are coupled in parallel, an alternator with a voltage lower than the average of the voltages will deliver less reactive power, while an alternator with a voltage higher than the average will deliver more reactive power.

[0025] According to an advantageous characteristic, a new voltage setpoint is calculated by each regulator based on a difference between the current reactive power rate and the rate corresponding to the input signal, this new voltage setpoint making it possible to move the reactive power rate of the alternator to bring it closer to the predefined power rate.

[0026] The regulation law makes it possible to advantageously calculate at any time the difference between the current reactive power rate of an alternator and the weighted signal. Depending on this difference, the voltage setpoint of the alternator concerned is preferably modified according to a configurable steering coefficient line, in particular in order to bring the reactive power rate of the alternator closer to the predefined power rate.

[0027] According to the invention, the weighting of the output signals can be carried out analogically. In a variant, the weighting of the output signals is carried out digitally.

[0028] The invention also relates, according to another of its aspects, to a bank of alternators comprising at least two alternators delivering in parallel to a load, each equipped with a regulator configured to deliver an output signal representative of the reactive power rate of the corresponding alternator in relation to its nominal reactive power, and a regulation law making it possible to modify the reactive power rate of the alternator as a function of an input signal, the regulators receiving as input a weighted signal generated from the output signals representative of the reactive power rate of each of the alternators so as to make each of the alternators converge towards a predefined reactive power rate.

[0029] Each alternator can deliver its reactive power rate via an analog output configured as voltage.

[0030] In this case, the input of each regulator can be configured as voltage. The weighting of the output signals is preferably carried out by an analog centralization circuit.

[0031] The centralization circuit can advantageously connect the various analog outputs from the regulators to a common point connected to the analog inputs using resistors, in particular of identical values.

[0032] In one variant, the weighting of the output signals is carried out by a digital centralization circuit.

[0033] Preferably, the centralization circuit is an external system of the programmable logic controller type.

[0034] Each regulator advantageously delivers the reactive power rate of its alternator on a digital communication bus, called a “field bus”.

[0035] Preferably, the centralization circuit reads the reactive power rate of each regulator, determines a weighted signal and sends it to each regulator via said digital communication bus.

[0036] The invention may be better understood by reading the following description, non-limiting examples of its implementation, and by examining the attached drawing, in which: there figure 1 , described previously, represents a voltage regulation law curve, called “statism”, according to the prior art, the figure 2 , described previously, schematically illustrates a set of alternators connected in parallel using the reactive differential method according to the prior art, the figures 3 et 4 are diagrams of a park of alternators according to the invention, with respectively an analog and digital centralization circuit, the figure 5 illustrates an example of reactive load distribution according to the invention, and the figure 6 represents an example of a regulatory law according to the invention.

[0037] We have represented schematically in the figure 3 a park of N alternators supplying in parallel to a load C via a bus 11. Each alternator has a regulator 21, 22, 23, 24.

[0038] In this example, each alternator delivers via an analog output 12, 13, 14, 15 configured in voltage its reactive power rate in relation to its nominal reactive power.

[0039] An analog centralization circuit 20, which consists of weighting the output signals representative of the reactive power rate of each of the alternators, is produced by connecting to a common point the different analog outputs 11, 12, 13, 14 coming from each regulator 21, 22, 23, 24 by means of resistors RA, R1, R2 ... Rn.

[0040] These resistors RA, R1, R2...Rn are advantageously of identical value if the weighted signal corresponds to the arithmetic mean of the output signals representative of the reactive power rate of each of the alternators. In a variant, the resistors RA, R1, R2...Rn have different values.

[0041] The common point of the centralization circuit 20 is then connected to an analog input 16, 17, 18, 19 of each regulator 21, 22, 23, 24, configured in voltage, in order to measure a weighted rate of reactive power of all the alternators in parallel.

[0042] Preferably, the weighted rate corresponds to an average rate.

[0043] As explained previously, a regulation law is then applied by each of the regulators 21, 22, 23, 24 so as to make each of the alternators converge towards a predefined reactive power rate.

[0044] Preferably, the predefined reactive power rate corresponds to the average of the reactive powers of all the alternators in parallel.

[0045] There figure 4 schematically represents the case where the N alternators are connected to a digital centralization circuit 30 via a digital communication bus 31 called a “field bus”. In the example described, the digital centralization circuit 30 is an external system of the programmable controller type.

[0046] Each regulator 21, 22, 23, 24 delivers the reactive power rate of the corresponding alternator on the field bus 31.

[0047] The centralization circuit 30 reads the reactive power rate of each alternator, calculates a weighted reactive power rate and sends it to each regulator 21, 22, 23, 24 via the field bus 31.

[0048] Preferably, the weighted reactive power rate corresponds to the average of the reactive power rates of all the alternators in parallel.

[0049] As explained previously, a regulation law is then applied by each of the regulators 21, 22, 23, 24 so as to make each of the alternators converge towards a predefined reactive power rate. Preferably, the predefined reactive power rate corresponds to the average of the reactive powers of all the alternators in parallel.

[0050] There figure 5 illustrates an example of reactive load distribution according to the invention.

[0051] Two alternators 1 and 2 with different nominal reactive powers, equal to 100 kVAr and 200 kVAr respectively, supply a reactive load C of 125 kVAr via a bus 11. The reactive powers measured on the two alternators at time t are respectively 75% and 25% of their nominal powers. The average value of the reactive power rate at this time is 50%. The predefined reactive power rate T rp is, in this example, 41.6%, corresponding to a balance point calculated by dividing the value of the load C by the sum of the nominal reactive powers of the two alternators 1 and 2: T rp = 125 / (100 + 200) x 100 = 41.6%.

[0052] The regulation law of regulator 1 will decrease the reactive power of alternator 1 and the regulation law of regulator 2 will increase the reactive power of alternator 2, so that the two alternators converge towards 41.6% of their nominal reactive power.

[0053] There figure 6 represents an example of a voltage regulation law according to the invention. The abscissa axis corresponds to the difference between the reactive power rate of the alternator concerned and the weighted rate of the reactive powers of all the alternators in parallel.

[0054] As explained previously, the regulation law advantageously calculates at each instant the difference between the current reactive power rate of an alternator, in particular expressed as a percentage of its nominal reactive power, and the weighted rate of the reactive powers of all the alternators in parallel. Depending on this difference, the voltage setpoint of the alternator concerned is preferably modified according to a configurable directing coefficient line, as shown in figure 6 , in order to bring the alternator's reactive power rate closer to the predefined power rate.

[0055] As an example, we have represented at the figure 6 the voltage regulation of the two alternators of the figure 5 The difference in power rate on the abscissa, corresponding to the difference between the reactive power rate of each alternator and the predefined reactive power rate, as described previously, is equal to T r1 = 75 - 41.6 = 33.6% for the first alternator 1 and T r2 = 25 - 41.6 = -16.6% for the second alternator 2. The projection on the ordinate according to the voltage setpoint line gives the respective voltage values ​​u 1 and u 2 with which it is necessary to modify the setpoint voltages of the two alternators 1 and 2 at the input of the regulators 12 and 13 in order to have a balanced load sharing according to the invention.

[0056] The invention is not limited to the example just described. For example, other regulatory measures can be combined with the invention.

[0057] The alternators in the alternator pool according to the invention may be identical, or vary in size, nominal power and / or model. The invention relates in particular to all alternator regulators comprising one or more microcontrollers.

Claims

1. Method for regulating a bank of alternators comprising at least two alternators (1, 2) that deliver their output in parallel to a load (C), said alternators each being provided with a regulator (12, 13) that is configured to deliver an output signal representative of the reactive power level of the corresponding alternator divided by its nominal reactive power, and a control law allowing the reactive power level of the alternator to be modified depending on an input signal, a step wherein a weighted signal sent as the input signal of these regulators is generated from the output signals representative of the reactive power level of each of the alternators, i.e. the signals delivered by the corresponding regulators, so as to make each of the alternators converge to a predefined reactive power level (Trp) .

2. Method according to the preceding claim, the weighted signal being the arithmetic mean of the output signals representative of the reactive power level of each of the alternators (1, 2).

3. Method according to either of the two preceding claims, a new voltage setpoint (u1, u2) being calculated by each regulator (12, 13) depending on a discrepancy between the current reactive power level and the level corresponding to the input signal, this new voltage setpoint allowing the reactive power level of the alternator to be shifted in order to bring it closer to the predefined power level (Trp).

4. Method according to any one of the preceding claims, the output signals being weighted in an analogue manner.

5. Method according to any of Claims 1 to 3, the output signals being weighted in a digital manner.

6. Bank of alternators comprising at least two alternators (1, 2) that deliver their output in parallel to a load (C), said alternators each being provided with a regulator (12, 13) that is configured to deliver an output signal representative of the reactive power level of the corresponding alternator divided by its nominal reactive power, and a control law allowing the reactive power level of the alternator to be modified depending on an input signal, the regulators receiving as input a weighted signal generated from output signals representative of the reactive power level of each of the alternators so as to make each of the alternators converge to a predefined reactive power level (Trp).

7. Bank according to the preceding claim, each alternator (1, 2) delivering its reactive power level via an analogue output configured to deliver a voltage.

8. Bank according to Claim 7, the input of each regulator (12, 13) being configured to receive a voltage.

9. Bank according to either of Claims 7 and 8, the output signals being weighted by an analogue centralization circuit.

10. Bank according to the preceding claim, the centralization circuit joining at a common point connected to the analogue inputs the various analogue outputs coming from the regulators using resistors (R), in particular of identical resistances.

11. Bank according to Claim 6, the output signals being weighted by a digital centralization circuit.

12. Bank according to the preceding claim, the centralization circuit being an external system of programmable-logic-controller type.

13. Bank according to Claim 11 or 12, each regulator delivering the reactive power level of its alternator over a digital communication bus.

14. Bank according to Claim 13, the centralization circuit reading the reactive power level of each regulator, determining a weighted signal and sending it to each regulator via said digital communication bus.