Fault Analysis in Integrated Wind Generation Networks

Morris Brenna, Giuseppe Esposito, Federica Foiadelli

Politecnico di Milano – Department of Energy morris.brenna@polimi.it

Mariacristina RosciaUniversità degli Studi di Bergamo

cristina.roscia@unibg.it

Abstract- This paper deals with the dynamic behavior of the

wind farm and the risk to lose wind energy production, due to isolated faults on the HV net. Many simulations have been carried out in order to determine the stability condition for different wind farm and protection typologies. In particular, regarding the wind farms, fixed and partially variable rotation speed turbine has been considered. While, considering the protection system, different settings of the second stage has been introduced. Finally, the influence of different factors on the wind production lose due to the faults has been studied.

I. INTRODUCTION

Wind energy, as many renewable resource, is a variable energy both in the short time (seconds, minutes) and in longer time (years). The wind plants become competitive compared with the traditional ones, but they operate with very low marginal costs therefore it is important to catch all the energy available by the wind.

The long time variability affects the producibility of the power plant during its life and therefore the correct sizing of the wind generators.

Instead, the short time variability cause disturbances especially when the generators are connected to weak grids. Indeed, the typical network at which the wind farm are connected, are characterized by a low short circuit power as in the case of long medium voltage distribution lines in rural or remote areas. Therefore the wind farms are sensible to the perturbations incoming from the national high voltage transmission network that propagate themselves at the distribution level. These perturbations are often due to faults that occur in the high voltage grid and that cause the trip of the protection system and the out of services of the faulted line.

In this paper a study of the wind farm dynamic behavior and of the risk to lose wind energy production, due to isolated faults on the HV net, has been carried out. All the studied faults are three-phase short circuit ones on HV 400 kV and 225 kV lines. Each fault occurs at a distance equal to the 10% of the line length and it is isolated thank to the fast trip of the circuit breakers. The trip of the circuit breakers at the extremity of the line has been simulated in two different times (first and second stage) and the characteristic operating times of the line protection has been considered.

The influence of different factors on the wind production lose due to the faults has been studied.

II. WIND FARMS PRODUCTION

The energy given by the wind is essentially constituted by the kinetic energy of the moving air mass. The available power Pw in a fluid vein of section A (perpendicular to the wind direction) is the product between air flux and the kinetic energy for vein volume unit. Its expression is the following:

( )2

3 312 2wvP A v A v K vρ ρ⎛ ⎞⋅= ⋅ ⋅ = ⋅ = ⋅⎜ ⎟

⎝ ⎠

where ρ is the air density (standard value 1.225 kg/m3) and v the wind speed.

Only a part of the power owned by the wind can be theoretically converted by the turbine and it is equal, as demonstrated by A. Betz, to the 59,3%. Therefore, the maximum power Pmax that can be obtained by the wind in the ideal operating conditions of the wind farm is given by the following:

3 30.5930.593 0.29652w vP P A v A vρ ρ= ⋅ = ⋅ = ⋅ ⋅

where, in this case, A is the surface covered by the wind generator rotor blades.

The mechanical power given to the wind generator axel is always lower than the theoretical maximum power due to different factors such as different wind behavior depending on the soil high, friction phenomena dissipation and turbulence present in the air flux disturbed by the blade crossing. In order to consider these factors, a power coefficient Cp, that is the relationship between the mechanic power really given to the axel and the power Pv, is defined. This coefficient depends from the rotor type, the wind speed and other different factors, but it is never higher than the 90% of the ideal value 59.3%. Therefore, the power given to the wind generator axel can be expressed as the product between this factor and the available power in the wind:

312p v pP C P C A vρ= = ⋅

The aerodynamic behavior of each wind generator can be expressed by the power coefficient, that is function of λ that is the relationship between the peripheral speed at the blade extremity and the wind one before the rotor:

( ) , pp p

vC C

vλ λ= =

The wind generator performances are generally synthesized in a curve that represents the electric generated power in function of the wind speed. It is important to consider the cut-in speed, that is the wind speed at which the wind generator starts to give electric energy, the rated speed and the cut-out

speed, that is the maximum wind speed at which the wind generator can work. After this speed the generator has to be stopped to reduce the mechanical stress.

Considering that the wind speed is strongly variable, in order to maintain the power constant and equal to its rated value, it is important to control the Cp factor. This coefficient can be changed varying the blades pitch angle, that can be used to limit the absorbed mechanical power for high wind speeds.

III. THE STUDIED CASE

The studied case considers a wind farm of 1200 MW, that is really high value that can introduce dynamic stability problems to the net. The production level of the each wind farm has been determined using a deterministic approach. Two different cases has been studied:

o functioning of all the wind farms at a power equal to its own rated power;

o functioning of all the wind farms at a power equal to the 50% of the rated power.

Both the cases give high values of wind power available, but surely the first one is the heaviest situation for the dynamic stability of the net. The deterministic approach has been preferred to the probabilistic one, because it was wanted to study the worst case, that is when all the wind farms work at their rated power.

For a deep stability study, it is necessary to consider the differences among the different wind generators. For this reason, the two types of wind generator more used have been considered:

o wind generator at fix rotation speed; o wind generator at partially variable rotation speed.

In the carried out simulation, it has been supposed that the wind network was constituted by homogeneous farms, in order to show the different dynamic behaviour of the two wind generator types and the problems that come out following possible faults.

For each farm the rated power Sn has been calculated thank to the active rated power Pn, considering a power factor equal to 0.89. The parameters of all the generators are the same and defined in p.u. based on the network power.

The wind farms have been connected to transmission grid at 63 kV through a 20 kV underground medium voltage line as reported in Figure 1.

Figure 1: Electric scheme considered in this study

All the wind generation systems connected to the same 63 kV node of the net have been grouped and modelled in one generator. The length of the connecting cables is equal to the mean distance between the considered generation stations and the 63 kV nodes and it is equal to 10 km. The cable numbers have been calculated in function of the rated power of the connected farm, considering for each cable a rated power of 25 MVA. These two different protection types have been considered:

o a maximum and minimum speed protection. Its limit values have been regulated considering the analyzed wind farm type.

o a maximum and minimum voltage protection, regulated considering the actual standards.

IV. FAULTS SIMULATION

In order to evaluate the stability of the wind generators, many three phase faults on the 400 kV or 225 kV lines have been considered. Each fault is at a distance equal to the 10% of the line length and it is isolated thank to the fast trip of the circuit breakers. The system protection causes the trip of the circuit breakers at the extremity of the line. The trip time of the circuit breaker nearest the short circuit is call first stage time, while the one of the farther is call second stage time. If the fault occurs in the middle of the line, these two times are similar. Therefore, the worst condition is when the fault occurs at a distance between 0 and 20% of the line length, because their isolation is slower. For this reason, all the studied faults have been located at a distance equal to 10%, in order to analyze the worst condition for the dynamic stability of the net.

There is also the possibility to use a device able to accelerate the second stage, so to reduce the time needed to open the farther breaker from the fault. This solution lets to reduce at 100 ms the second stage time, considering that its normal value is around 500 – 900 ms. Instead, the first stage time is always equal to 80 ms.

The influence of different factors on the wind generation loss has been studied. A really influent factor is the employed technology and then the dynamic behaviour of two different wind systems has been considered:

o wind system with fix rotation speed; o wind system with partially variable rotation speed.

A. Wind system with fix rotation speed with accelerated second stage

In this case, the simulated faults, located at a distance equal to 10% of the length line, are isolated in first stage in 80 ms and in second stage in 100 ms. The limit value of the minimum voltage protection has been fixed equal to the 85% of the rated one.

Considering 54 wind farms, after a fault on a 400 kV line about the 80% of the wind power plants disconnect themselves from the mains, that correspond to the 82% of the total produced energy. All the disconnections are due to the trip of the minimum voltage relays.

In the following it is considered as example the behaviour of two close wind farms of different rated power: 6MVA and 20MVA.

The speed curves reported in Figure 2 show that, in the two cases, the maximum allowed speed has not been reached and that the two generators are not in the instable condition. However the 20 MVA power plant disconnects itself from the grid, while the other one remains connected.

Figure 2: Speed curves of two wind farms after the fault

The voltage curves represented in Figure 3 show that the disconnection is due to the trip of the minimum voltage relay. Indeed, after the fault, the voltage of the 20 MVA power plant slowly reaches its nominal value, remaining under the 85% Vn for a time longer than the protection system restoring one (1s), causing the permanent disconnection from the net.

Figure 3: Voltage curves of two wind farms after the fault

Considering the same above conditions, different faults on 225 kV lines have also been simulated. Also in this case, all the disconnections are due to the trip of the minimum voltage relay, but the faults on these lines cause a wind generation loss lower then the case of 400 kV lines. The worst 225 kV fault causes the disconnection of 32 wind farms (approximately the 60% of the wind network), while the worst 400 kV one determines the lose of 49 wind farms, that means almost the entirety of the network. In fact, a fault isolation on a 400 kV line means the trip of such line and than the overloaded of all the 225 kV net.

A really influent factor is the trip time of the minimum voltage relay. From the many simulations carried out, it is possible to assure that it is advisable to fix this time at least equal to 4s instead than the 1s simulated above, in order to avoid the disconnection of farms that remain stable. B. Wind system with fix rotation speed with normal second stage

All the 400 kV lines are provided with the accelerated second stage, while some 225 kV lines have still protections with a normal second stage.

The faults on lines with this kind of protection are backwardly isolated and so they are disadvantageous for the wind farm stability. Contrary to the previous case, in this one the disconnections are not due to the trip of the minimum voltage relay but of the maximum speed protection. The percentage of wind generation lose in this case is really high and it is close to the almost totality of the network.

In Figure 4 is reported the voltage curve of a wind farm after a fault on the 225 kV line at 10% of the length line.

Figure 4: Voltage curve of one wind farm after the fault

It is possible to note a long and deep voltage sag following the fault, due to the backwardly isolation of the fault in second stage (728 ms), that causes the fast lose of the stability and than the trip of the maximum speed protection, as reported in Figure 5.

Figure 5: Speed curve of the wind farm after the fault

In general, considering systems at fix rotation speed, after the fault isolation, a wind farm absorbs reactive power, extending the voltage sag generated by the fault.

C. Wind system with partially variable rotation speed with accelerated second stage

When the fault happens, the voltage at the terminals of the wind power plant falls down the protection limit causing the trip of the voltage relay, as reported in Figures 6 and 7.

Figure 6: Voltage curve of one wind farm after the fault

Figure 7: Speed curve of the wind farm after the fault

The wind farm remains connected to the mains until the trip of the minimum stator voltage relay. In this situation the system is in instable condition and absorbs a greater amount of reactive power determining a deeper and longer voltage sag. D. Wind system with partially variable rotation speed with normal

second stage Also in this case many simulations have been performed

considering the different protection settings. In particular the case with low limit setting is reported in Figures 8 and 9.

Figure 8: Voltage curve of one wind farm after the fault

Figure 9: Speed curve of the wind farm after the fault

This limit value permits to reduce the overcurrents that flow in the power converters following the fault, without influencing the system stability.

It is possible to assure that in this case the minimum voltage value reached during the sag is lower than the one

obtained considering the system without any protections, therefore the minimum voltage relay does not trip. However, there is the disconnection of this wind farm from the net, due to the excessive duration of the voltage sag.

V. CONCLUSIONS

The studied reported in this paper has furnished information about the dynamic behaviour of the wind generation networks and the lose of wind energy production following faults normally isolated on a 400/225 kV net.

The study has demonstrated the different dynamic behaviour of the two wind generator typologies more used today: with fix and with partially variable rotation speed.

The influence of different factors on the wind generation lose due to faults, has been studied. In particular it has been considered the kind of technology used, the fastness in isolating the fault, the sizing and the production level of the wind network.

The study has mainly highlighted that the wind production lose following the faults strongly grows with the increasing of the connected wind power.

Moreover, the comparison between the use of normal and accelerated second stage has demonstrated how the second case can introduce a clearly improvement. In fact, the faults on the lines with normal second stage are the most critic for the wind system stability and for the lose of wind production. This criticality is due to the trip delay of the circuit breaker on the second stage. These considerations are valid both for systems with fixed and partially variable rotation speed.

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