WO2012101318A1

WO2012101318A1 - An arrangement and a method for synchronizing a generator set to an electric network

Info

Publication number: WO2012101318A1
Application number: PCT/FI2012/050013
Authority: WO
Inventors: Sören NYMAN
Original assignee: Wärtsilä Finland Oy
Priority date: 2011-01-28
Filing date: 2012-01-09
Publication date: 2012-08-02
Also published as: FI20115086A0

Abstract

The objective of the invention is to improve synchronization of a generator set. The invention predicts phase angle difference at synchronous speed of the generator and adjusts a controller of the prime mover for accelerating the prime mover in such a way that the generator will be the in phase with the network at the synchronous speed as a response to the prediction. A generator breaker is commanded to be closed (at 43) when the phase angles, frequencies and voltages match.

Description

An arrangement and a method for synchronizing a generator set to an electric network

Field of technology

This invention relates to arrangements to synchronize a generator set to a electric network. The generator set comprises a prime mover (for example a gas turbine or a combustion engine, such as a diesel engine), and a generator. The generator is, for example, a synchronous electric machine. The electric grid is for transmitting electric power.

Prior art

The synchronization of the generator set requires that the generator must have substantially same voltage, frequency and phase angle with the electric network before the set is connected to the network. The prime mover is accelerated to a nominal synchronous speed and the field of the generator is energized by supplying a field current to the generator. The synchronizing controller adjusts the field current and the speed reference for a speed/load controller of the generator set in order to match the voltage of the generator (also frequency and phase angle of the voltage) with the voltage of the network.

Fig. 1 shows an example of a known embodiment 10 for synchronizing the generator set. The speed/load controller 1 performs control signals 3 to an actuator means of the prime mover, like to a fuel injection system. The speed/load controller has inputs to receive prime mover speed data 4 and speed reference data 5. The voltage regulator 2 performs the speed reference data 5 to the speed/load controller and con- trol 6 the field current of the generator. The voltage regulator 2 has inputs for receiving voltage information 9 of the generator and voltage information 11 of the electric network. When the generator's voltage (and it's frequency and phase angle) matches with the network's voltage, the voltage regulator can give commands 7 to a generator breaker 8 for connecting the generator set to the network. Another way for command- ing the generator breaker 8 is to use synchronizing relays that act when the voltages, frequencies and phase angles are equal. After the closing of the generator breaker the electric load affects to the speed of the generator. It can be seen as speed reduction of the generator set, which means that the frequency of the generator decreases. The speed/load controller and the voltage regulator drive the frequency towards the stable state 27 as illustrated in Fig. 2.

Fig. 2 illustrates the synchronization action. The curve 21 represents the frequency of the prime mover, i.e. it also represents speed. As can be seen, the speed of the prime mover is accelerated towards a nominal speed value 25 that also represents the frequency of the grid. The acceleration is reduced near the nominal value in order to not overshooting the speed. When the voltage, frequency and phase angle are inside acceptable tolerances 26, the generator breaker is closed 22 for connecting the generator set to the network. This moment of closing 22 ends the synchronization sequence 23 and starts the loading sequence 24. Fig. 3 illustrates an example of the actions during the synchronization sequence as a flow chart.

When a synchronization command has initiated 31 the voltages and the frequency and phase angle values of the voltages of the generator and the electric network are monitored 32. Next, there is checked whether the voltages and frequencies are within an allowable ranges or not 33. If the voltages and frequencies are not within their range, the speed/load controller is commanded 35 to adjust it's output for the prime mover's actuator means. The field current of the generator can also be adjusted at this method step. If they are in the range, the phase angle of the voltage is checked 34 whether it is within an allowable range of phase angle. If the phase angle is not within the range, the voltages and frequencies are monitored again 32. If the phase angle is in the range, the generator breaker 36 is commanded to be closed. The said ranges of voltage, frequency and phase angle determine allowed errors with respect of reference values. The reference values are the network's voltage, frequency and phase angle. So it can also be said that errors of voltage, frequency and phase angle of the prime mover are checked.

As can be noted, the voltages and their frequencies are checked first when the synchronization is arranged according to Fig. 3. When they match the phase angles are checked. The voltage and frequency values change with time (There is little difference between the actual rotation speed and the synchronous rotation speed). Due to this the phase angles match at some moment, and the generator breaker can be closed. This phase angle shift time when the phase angles match are illustrated as a period 28 in Fig. 2.

As can be further noted, the synchronization and loading sections 23, 24 took time. These time periods are required each time when the generator is connected to the electric network. It means that power recourses are needed for keeping the network in operation, for example, in malfunction situations of the network. For example, if a short-cut causes shutdown of the network or it's part, the generator/s drived down are tried to connect back to the network as soon as possible. Said sequences determine when the generator is capable of supplying power to the network.

Short Description of Invention

The objective of the invention is to improve the current situation of the synchronization of the generator set. The objective is achieved in a way that is described in the independent claims. The independent claims disclose the inventive method and arrangement. Dependent claims disclose different embodiments of the invention.

The method of the invention for synchronizing a generator set to a electric network comprises steps of monitoring 92 (Figure 9) voltage of the generator of the set, voltage of the electric network and frequencies and phase angles of the voltages; estimating 93 phase angle difference at target synchronous speed of the generator using at least some values achieved from the monitoring step, and adjusting 94 acceleration of the generator set as a response to the estimating step 93 and using at least some values achieved from the monitoring step in such a way that the generator will be the in phase with the network at the target synchronous speed. These steps are needed for obtaining the inventive progress in the synchronization action.

When the method is described in more detail (Figure 8) it further comprises conventional steps of checking 85, 86 whether the voltages, frequencies and phase angles match in such a way that they are within allowable ranges or not. Further, The check- ing step 85, 86 is arranged to direct the method back to the monitoring step 82 or steps of conventional synchronization 88 in case of the unmatched voltages and/or frequencies, and also arranged to direct the method to the phase angle checking if the checking step is arranged into two sub steps like illustrated in Figure 8. Finally the method comprises a step of commanding 87 a generator breaker to be closed if the phase angles match.

An embodiment of the invention comprises a controller 10 to drive the prime mover of the set and to control field current of the generator of the set, which embodiment is arranged to realize the actions of the inventive method.

Drawings

Next the invention is described in more detail with the figures of the attached drawings in which drawings:

Figure 1 illustrates

trailer,

Figure 2 illustrates

Figure 3 illustrates

Figure 4 illustrates

set,

FFiigguurree 55 iilllluussttrraatteess an example of an embodiment of the invention,

Figure 6 illustrates

tion,

Figure 7 illustrates

and

Figure 8 illustrates

Figure 9 illustrates

vention. Description

Figure 4 illustrates how the invention works. The prime mover is accelerated ac- cording to a slope 41 in the synchronization section 23. At a certain moment 42 voltage of the generator of the set, voltage of the electric network and frequencies and phase angles of the voltages are monitored. The monitored values are used for predicting (estimating) phase angle difference at synchronous speed 25 of the generator. So, the moment 44 of the synchronous speed of the generator is predicted as well by utilizing information of the acceleration slope. If the phase angle difference is predicted at this moment 44, the synchronizing controller can be arranged to adjust the acceleration slope for changing the moment 44 when the generator runs at synchronous speed to another moment 43. In the example of figure 4 the acceleration is increased. In this way the synchronous speed (the voltages, frequencies and phase angles match) is achieved faster than in prior art solution. In other words, the synchronization sequence 23 is shorter.

The acceleration power is greater at the synchronizing moment 43 than in known solutions. The generator transmits the acceleration power to the electric network at the beginning of the loading sequence 24. Due to this the loading sequence 24 is shorter as well. In other words the acceleration power carries a greater part of the load of the electric network at the synchronization moment 43. The mechanical power used for generator acceleration is shifted to electrical power delivered by the generator. The power is given by:

cU

Where J_genset is the total inertia of the generating set, and ω is the angular velocity expressed in radians per second. This affects as said above. Since the whole time (the synchronizing sequence and the loading sequence together) for connecting the generator set in its allocated power to the network is shorter than in known prior art solutions, power resources of the electric network can be smaller.

Figures 5 - 7 illustrate different embodiments of the invention that are arranged to process the steps of the inventive method. Figure 9 shows the steps that are needed for obtaining the inventive progress in the synchronization action. However, when looking figure 9 it should be understand that synchronization command 91 and continuation to checking are conventional steps. Figure 8 illustrates an example as a flow chart how the inventive method can be constructed in practice.

Referring to figure 9 voltage of the generator of the set, voltage of the electric network and frequencies and phase angles of the voltages are monitored 92. It is estimated 93 phase angle difference at target synchronous speed of the generator using at least some values achieved from the monitoring step. Acceleration of the generator set is also adjusted 94 as a response to the estimating step 93 and using at least some values achieved from the monitoring step in such a way that the generator will be the in phase with the network at the target synchronous speed.

Next there is referred to figures 5 - 8. It must be noted that the steps 91 , 92, 93 and 94 of figure 9 correspond with the steps 81 , 82, 83 and 84 of figure 8. When the synchronizing system receives a synchronization command 81 , it starts to monitor voltage, frequency and phase angle values of the network and the generator 82. The monitored values are used to predict 83 a phase angle error when the generator (also the prime mover) will run at the synchronous speed.

As said above, the generating set is typically accelerated according to a reference speed slope 41. At time 42 the controller monitors the frequency and phase angle of both the generator voltage and the grid voltage. The controller 10, 210 and method performs a calculation, where the time when the generator will reach the grid frequency with the present slope rate is predicted. This time is represented by moment 44 in figure 4. The calculation can be performed according to the following formula:

wherein t_pi is the predicted moment 44, t_ci is the monitoring moment 42, f_grici is the frequency of the network 25, f_gen the frequency of the generator i.e. the curve 41 and a1 _gen is the slope coefficient 41.

The time t_c1 can be chosen so that the generator and grid are in phase at the moment, otherwise the phase angle error i.e. difference can be described by a factor e_P (t_d), which indicates the angular lead of the grid (network) voltage compared to the generator voltage in terms of a decimal value between 0 and 1 , where 0 means no lead, and a value close to 1 means almost one period ahead.

As the time t_p1 is determined, the number of elapsed periods of the generator voltage and grid voltage (here denoted by Aph) between t_c1 and t_p1 with current grid frequency and generator frequency can be determined with the following respective formulas:

- l , --1

wherein Aphg_en is a number of elapsed periods of the generator's frequency and Aphgrid is a number of elapsed periods of the network's frequency. The difference in number of elapsed periods eA_P between the generator voltage and grid voltage at t_pi is obvious:

It should be noted that frequency is a parameter of voltage so it can be said that the elapsed period are either between the frequencies or the voltages. By adding the phase angle error e_ph(t_ci) at t_c1 to the expression above, we get a representative for the sum of the phase angle error at f_pi and the difference in number of elapsed frequency (voltage) periods. When the integer of this representative is subtracted from the representative itself (i.e. the fractional part remains), we obtain the predicted phase angle error at t_p1

wherein again e_Ph(t_pi) represents the lead of the grid voltage in relation to the generator voltage at time t_pi. As can be seen, the phase angle error at the monitoring moment can be taken into account.

In order to reach the grid frequency at the same moment as the generator is in phase with the grid the speed ramp shall be adjusted 84, so e_ph(t_pi) goes to zero i.e. there is no phase angle difference between the grid voltage and the generator voltage at synchronous speed.

This can, for example, be done by either increasing or decreasing the acceleration slope 41 of the generator from a1_gen to another value a2g_en. If the correction is done by accelerating the generator, the speed slope can be changed according to the formula:

and if the correction is done by decreasing the acceleration, the speed slope can be changed according to the formula:

As can be seen the above formulas concern a linear acceleration slope. However, the shape of the acceleration slope can be another form, for example an exponential or logarithmic curve, in which case the formulas are designed to that specific acceleration slope.

For obtaining the required adjustment, the controller of the prime mover of the set is adjusted in such a way that its output directs the generator (through the prime mover) to be in phase with the network at the synchronous speed. The adjustment may also comprise adjustment of field current of the generator.

The precision of the correction can be improved by iteratively repeating the process with sampling at time t_c2, predicting the time for synchronous speed t_p2, and calculating a third speed slope a3_gen, and so on. By increasing the number of repeti- tions to °°, the correction process becomes continuous. Figure 8 shows that the iterative repetitions are initiated when checking 85 whether the voltages, frequencies and phase angles match in such a way that they are within allowable ranges or not. If they are not, the steps are repeated according to the figure 8. If they are, the checking 86 whether phase angles of the voltages match within an allowable range or not is made. If the phase angles match, the generator breaker is commanded 87 to be closed.

If the phase angles do not match, the method and system may comprise the steps of the conventional synchronization 88 as backup steps. The conventional synchronization steps are showed in the figure 3. If the iterative repetitions are not used, the conventional synchronization can be as backup as well when checking 85 whether the voltages and frequencies match.

As said before the reference voltage (frequency and phase angle as well) is the network's voltage and the difference between the reference voltage and the monitored voltage of the prime mover can be expressed an error, which is going to be li- mited into the allowed range. In some case wherein the network is very stable, the reference voltage can be predetermined.

Arrangements for synchronizing a generator set to an electric network are illustrated in the figures 5 - 7. The arrangements comprise a controller 210, 10 to drive 3 the prime mover of the set and to control field current 6 of the generator of the set, which arrangement is arranged to monitor voltage of the generator 9, voltage of the electric network 11 , and frequencies and phase angles of the voltages to check whether the voltages and frequencies match in such a way that they are within allowable ranges or not, and to check whether phase angles of the voltages match within an allowable range or not. The measurements of the generator and network voltages are transmitted to the arrangement. The voltage regulator is arranged to monitor all required parameters (voltage, frequency, phase angle) from the measurements.

The controller is further arranged to predict phase angle difference at synchronous speed of the generator after the monitoring function, and to adjust the controller for driving the prime mover in such a way that the generator will be in phase with the network at the synchronous speed as a response to the prediction and before the checking of the voltages and frequencies.

The checking function is arranged to direct the arrangement back to the monitoring function or functions of the conventional synchronization in case of the unmatched voltages and/or frequencies, and also arranged to direct the arrangement to the phase angle checking if the voltages and frequencies match. The controller is also arranged to command 7 a generator breaker 8 to be closed, if the phase angles match.

It is practical that the controller comprises a module 71 , 51 that is arranged to perform at least the prediction and the adjustment functions. As showed in the examples of the figures 5 and 6 the controller can comprise a speed/load controller 1 and a voltage controller 2 in such a way that the module 61 , 51 is in the voltage controller. The voltage regulator 2 is arranged to provide speed reference commands 5 to the speed/load controller 1 . When the module 51 performs the adjustment functions, the signal 5 to be sent to the speed/load controller 1 carries control commands created by said adjustment functions for adjusting the output 3 of the speed/load controller.

The figure 6 shows an example wherein the speed/load controller 1 comprises a second module 62 that is arranged to perform the prediction and the adjustment functions for driving the prime mover. The module 61 in the voltage controller 2 is for controlling the field current of the generator in this example. The figure 7 shows an ex- ample wherein the speed/load controller and the voltage regulator have been inte- grated into one entity 210. It can be seen from the examples above that the speed/load controller 1 or the integrated controller 210 is arranged to receive speed data 4 from the prime mover.

The functions of the invention can be utilized in such a way that delays of the ar- rangement are taken into account. Delay may exists in the measurement, in the circuits of the voltage regulator and/or speed/load controller and in the circuit breakers, The delays may depend on voltage, which means that the voltage level should be taken into account when determining the delays.

This text describes only some embodiments of the invention. However, other poss- ible solutions exist. For example, the calculations of the phase angle differences described above can be replaced by monitoring moments when the phase angles are equal, which gives a positive phase angle matching signal to the other functions of the invention. A logical entity such as software component may perform this task. In general, the invention can, for example, be constructed from software components or a printed circuit, like ASIC circuit (Application Specific Integrated Circuit).

So, it is clear that the invention is not restricted to the embodiments described in this text, but the invention can be utilized in any suitable manner within the scope of the independent claims.

Claims

1. A method for synchronizing a generator set to an electric network, which method comprises steps of

- monitoring (92) voltage of the generator of the set, voltage of the electric network and frequencies and phase angles of the voltages

- estimating (93) phase angle difference at target synchronous speed of the generator using at least some values achieved from the monitoring step,

- adjusting (94) acceleration of the generator set as a response to the esti- mating step (93) and using at least some values achieved from the monitoring step in such a way that the generator will be the in phase with the network at the target synchronous speed

2. A method according to claim 1 characterized in that it further comprises a step of checking (85, 86) whether the voltages, frequencies and phase angles of the vol- tages match in such a way that they are within allowable ranges or not,

3. A method according to claim 2 characterized in that it the checking step (85) is arranged to direct the method back to the monitoring step (82) or steps of conventional synchronization (88) in case of the unmatched voltages, frequencies and/or phase angles.

4. A method according to claim 3 characterized in that it further comprises the steps of the conventional synchronization (88) as backup steps in case the phase angles fail to match.

5. A method according to claim 1 , 2, 3 or 4 characterized in that the step of adjusting (94) comprises also controlling of a controller of a prime mover of the genera- tor set and/or adjustment of field current of the generator.

6. A method according to any of claims 1 - 5 characterized in that the estimation step (93) is arranged to determine a prediction moment when the generator rotates at synchronous speed by utilizing an acceleration slope of the generator set and the monitored frequencies of the grid and the generator.

7. A method according to claim 6 characterized in that phase angle difference between the grid and the generator is achieved by calculating periods of the generator voltage and periods of the grid voltage between the prediction moment and the monitoring moment, calculating difference of these periods, and calculating a fractional part of said difference of the periods.

8. A method according to claim 7 characterized in that the calculation of the fractional part takes into account a phase angle error at the monitoring moment.

9. A method according to any of claims 6 - 8 characterized in that the step of adjusting (94) the controller of the prime mover is arranged to adjust the acceleration slope for increasing the acceleration.

10. A method according to any of claims 6 - 8 characterized in that the step of adjusting (94) the controller of the prime mover is arranged to adjust the acceleration slope for decreasing the acceleration.

1 1. A method according to any of claims 1 - 10 characterized in that the monitor- ing moment is arranged to be a moment when the phase angles of the grid and the generator are equal.

12. A method according to any of claims 1 -10 characterized in that at the monitoring moment a phase angle difference between the phase angles of the grid and the generator are arranged to be taken into account by a factor between zero and one.

13. An arrangement for synchronizing a generator set to an electric network, which arrangement comprises a controller (10) to drive the prime mover of the set and to control field current of the generator of the set, which arrangement is arranged to monitor voltage of the generator, voltage of the electric network and frequencies and phase angles of the voltages, to check whether the voltages, frequencies and phase angles match in such a way that they are within allowable ranges or not,

characterized in that the controller is further arranged to

- estimate phase angle difference at synchronous speed of the generator after the monitoring function, - adjust the controller for driving the prime mover in such a way that the generator will be the in phase with the network at the synchronous speed as a response to the prediction utilizing values from the monitoring

14. An arrangement according to claim 13 characterized in that the checking of the voltages and frequencies is arranged to direct the arrangement back to the monitoring function or functions of conventional synchronization in case of the unmatched voltages, frequencies and/or phase angles.

15. An arrangement according to claim 13 or 14 characterized in that the control- ler comprises a module (71 , 51 ) that is arranged to perform at least the prediction and the adjustment functions.

16. An arrangement according to claim 15 characterized in that the controller comprises a speed/load controller (1 ) and a voltage controller (2), the module (71 , 51 ) being in the voltage controller.

17. An arrangement according to claim 16 characterized in that the speed/load controller comprises a second module (62) that is arranged to perform the prediction and the adjustment functions for driving the prime mover, the module in the voltage controller being for controlling the field current of the generator.

18. An arrangement according to claim 16 characterized in that the speed/load controller (1 ) and the voltage regulator (2) have been integrated into one entity (210).