978-1-4244-7467-7/10/$26.00 ©2010 IEEE

2010 IREP Symposium- Bulk Power System Dynamics and Control – VIII (IREP), August 1-6, 2010, Buzios, RJ, Brazil

On the Assessment of Voltage Ride-Through Needs of the Power Transmission Grid

Luiz Felipe W. Souza Tatiana M. L. Assis Igor F. Visconti

CEPEL UFF CEPEL

Abstract – This paper discusses methods and criteria for the assessment of the low voltage ride-through (LVRT) needs of the power grid. Simulation techniques are used to investigate both how voltage dips caused by short-circuits spread over the grid and what impact do they cause in the operation performance of power systems with wind generation. The analysis is performed for wind turbines (WT) with different LVRT characteristics. It is shown that, depending on system’s topology characteristics and the voltage level of WT’s point of connection, ride-through needs may be different throughout power systems, especially those of continental countries as Brazil. As a conclusion, it is possible to have less severe LVRT requirements without risking system operation security.

Keywords – Wind power, voltage dips, low voltage ride-through requirements.

Introduction Voltage dips have traditionally been a problem for end-users and their sensitive loads. Industrial customers have particularly been concerned with the effects of voltage dips on their complex automated processes. Dip failures may result in considerable economic impact due to loss of production [1]. Meanwhile, for the utilities, voltage dips were much more a business than a technical problem, in a sense that unsatisfied customers might look for another power dealer. That has been the main reason for transmission and distribution utilities to monitor and try to solve problems caused to customers by voltage dips. In recent years, wind power generation experienced a large increase throughout the world. Although wind power has major benefits to the environment, it has also some particular characteristics that impose new challenges for the power system operation. Among those challenges is the fact that wind turbines (WT) may be highly sensitive to voltage dips. As the percentage of wind power generation is getting higher, a voltage dip may cause a subsequent loss of wind power generation [2], resulting in stability troubles that may jeopardize the

system reliability. In other words, voltage dips are now also a technical problem for utilities and system operators. In order to cope with this problem, many utilities and system operators are issuing grid codes with low voltage ride-through requirements (LVRT) for WT [2][3]. Some manufacturers complain that those ride-through requirements are too rigid, resulting in costly solutions. Many researchers are working on the detailed modeling of WT to study their behavior when submitted to voltage dips and propose enhancements of their ride-through characteristics. A large amount of technical literature is available on this subject, e.g. [4]. On the other hand, few papers ask how system operators and planners established those ride-through requirements. The aim of this paper is to discuss methods and criteria for the assessment of the low voltage ride-through needs of the power grid. The problem is investigated using two different approaches: • analysis of the voltage dip behavior of the power

system; • analysis of the effect of voltage dips on the

performance of a power system with wind power generation.

For the first topic, a short-circuit analysis software is used. The well-known technique of vulnerability area [1] evaluation through systematic short-circuit simulations is extended to include, simultaneously, multiple substations, doing the so-called superposition of vulnerability areas. As for the impact of voltage dips on the power system, a dynamic simulation tool is used to examine the impact of voltage dips on the power system operation, aiming at aspects such as voltage [5] and transient stability [6]. The paper begins by briefly reviewing technologies of wind turbines, giving emphasis to their sensitivity to voltage dips. Low-voltage ride-through (LVRT) curves are then introduced, and the requirements of some Grid Codes around the world are compared. Next, short-circuit analysis software is used for the assessment of voltage dip severity and their propagation in the grid. Protection time

and how it affects LVRT curves are examined. Dynamic simulations of the Brazilian interconnected system, modeled in detail and including wind power generation, are performed. Some discussions on LVRT criteria based on the needs of the power system precede the final conclusions of the paper. Wind Turbines Technologies : a Brief Review As stated in the introduction, the focus of this paper is in the power system and its needs related to LVRT requirements ther. It is beyond the scope of the paper to make an in-depth analysis of WT technologies and the whys and hows of their sensitivity to voltage dips – for this, the reader is referred for other good papers available, e.g., [4][5]. However, in order to have a good understanding of what makes the WT so sensible to voltage dips – and perhaps being somewhat didactic too – this session presents a brief review of some WT technologies. The key point here is how they behavior when subjected to voltage dips. There are different types of WT with various control philosophies, forms of operation and connection to the grid. Usually, WT are classified as fixed or variable-speed devices. The fixed-speed turbines employ induction generators (squirrel cage) directly connected to the electrical grid. They operate within a narrow range of speed slightly higher than the synchronous speed, so they are called fixed speed wind turbines. This technology shows advantages when compared to variable speed device, including simplicity, mechanical strength, cost and easy of operation. On the other hand, they have less control flexibility. Variable-speed WT employ synchronous or asynchronous machines connected to the grid through power electronic converters. A lot of advantages of this type of generator can be mentioned when it is compared to fixed-speed machines, such as the reduction of mechanical stress, the improvement of power quality and the increase in the overall system efficiency. Those advantages are achieved through a higher cost per MW. WT using synchronous machines are usually connected to the grid through a rectifier and a PWM voltage-source converter (VSC). Typically there is a chopper (DC-DC converter) between the rectifier and the VSC, in order to control the DC capacitor voltage, as it varies with the wind speed. This is necessary for the correct operation of the VSC. The synchronous generator's rotor may have DC windings or permanent magnets. Due to its high flexibility and the decreasing costs of power electronics

devices, this technology tends to increase its participation in the wind generation market in the future, being the solution adopted by many manufacturers, specially for higher MW turbines (see Table 1). Doubly-fed induction generators (DFIG) wind turbines are classified as adjustable speed machines. In doubly-fed induction generators, the stator is directly connected to the grid, as in conventional squirrel-cage induction generators. The rotor is composed by a three phase winding that is connected to the grid through a back-to-back converter (Fig. 1). The four-quadrant AC-AC converter is based on insulated gate bipolar transistors (IGBT) and operates in a high switching frequency using the PWM control technique. The transient stability simulations results presented in the end of the paper were obtained for wind farms with DFIG. Details about this technology control strategies can be found in [7].

InductionGenerator

Rotor side converter Grid side converter Fig. 1– Doubly-fed induction generator Table 1 shows a summary of powers and technologies of WT of different manufacturers. Data was obtained from the manufacturers' websites.

Table 1 – MW of WT and technologies per manufacturer

Fix. Speed, Ind. Gen.

Var. Speed, Ind. Gen.

Var. Speed, Sync. Gen.

Enercon - - 0.85, 1.65,

1.8, 2, 3

Vestas - - 0.8, 0.9, 2 2.3, 3, 7.5

Suzlon 0.6, 1.25,

1.5, 2 - -

Gamesa - 0.85, 2 4.5 GE - 1.5 2.5

Each of these technologies is affected in some different way by voltage dips and requires a different solution for improving LVRT capability [5]: • Directly Connected Induction Generator: during

voltage dips, the electrical power output decreases. As the pitch control usually is not fast enough, the mechanical power input of the wind does not change,

thus the speed of the rotor increases and the machine must be disconnected. Solutions for LVRT improvement include increment of the machine inertia and the use of devices to transiently support the voltage in the machine terminals, such as SVC.

• Variable Speed Synchronous Generator: during voltage dips, although the VSC is able to keep control of the machine, there is an overvoltage in the DC capacitor that ultimately can lead to WT disconnection. Solutions for LVRT improvement include the use of power electronics circuitry to damp the voltage in the capacitor or the adoption of some kind of energy storage technology.

• Variable Speed Induction Generator: during voltage dips, rotor and converter currents may dangerously increase. A crowbar circuit [5] is used to improve LVRT capability.

The solutions described in the previous bullets, although technically feasible and commercial available, increase the cost of the WT. Low-Voltage Ride-Through and Grid Codes Grid codes are documents that contain technical procedures and responsibilities regarding power system planning, operation, maintenance and protection. Recently, the protection system of wind generators switched off the machines when a fault occurred producing a low voltage at the point of common coupling. Typically, voltages less than 0.8 pu would result in automatic wind plant disconnection and it would be turned on when the network was fully restored. As the amount of wind power was low when compared to other forms of power generation, the grid would not suffer significant impact due to their loss during transients [7]. The Low-Voltage Ride-Through Curve The increase of wind power penetration has brought concern about this protection philosophy since the amount of disconnected power may be too large, causing a loss of generation big enough to eventually lead the power system to instability. In this way, the grid codes around the world have been changed in order to prevent such problem and guarantee system security. Among the requirements currently incorporated in the grid codes is the ability to enable wind generators to withstand voltage dips due to short circuits. Such requirement is known as “low voltage ride-through” (LVRT) and it is expressed by a voltage vs. time curve. Fig. 2 shows a typical LVRT curve with most notable characteristics.

Vi

Ti

Voltage

time

Vf

Tx

Vx

Tf

Wind plant must not disconnect

Emergency Beginning

Dynamic Voltage RecoveryProtection

System Time

Fig. 2– Typical LVRT curve In the LVRT curve, the vertical axis is the voltage in the WT terminal and the horizontal axis is the time span, considering the beginning of the fault/event as the instant t = Ti. To cope with LVRT requirements, wind generators must not disconnect from the grid if their after-fault voltage profile keeps above the limiting curve. In contrast, they can be disconnected if the terminal voltage drops bellow the limiting curve. The voltages Vi, Vf and Vx of Fig. 2 represent, respectively, the pre and post fault steady-state voltages and the minimum value of the voltage dip. Vx can be estimated for different faults types and locations using short-circuit software. Fig. 2 also points out two important periods after emergency beginning. The first one corresponds to the fault duration, which depends on the protection system actuation time. During this period, there is a voltage dip. The second important period corresponds to the dynamic voltage recovery time which depends on voltage control systems and power system dynamics as a whole. After fault clearing, dynamic voltage recovery in each system bus can be studied using a full time-domain simulation tool with detailed system modeling, including machines, voltage and frequency regulators. Grid Codes and LVRT Requirements Minimum LVRT requirements were firstly established by Germany [7]. Later, other companies have adopted similar rules. Fig. 3 illustrates some LVRT characteristics adopted in different countries [8][9].

1.00

0 0.15 1.5

V [pu]

time [s]

Germany(E.ON)

0.90

1.00

0.20

0 0.5 2

V [pu]

time [s]

Italy

0.8

0.75

0.90

1.000.950.80

0.20

0 0.5 1 15

V [pu]

time [s]

Spain

1.00

0.15

0 0.625 3

V [pu]

time [s]

USA/Ireland/Canada (AESO)

0.90

1.00

0.25

0 0.2 0.75

V [pu]

time [s]

Brazil

0.95

1.00

0.15

0 0.15 2.5

V [pu]

time [s]

United Kingdom

1.2

0.800.85

Fig. 3– LVRT requirements in different countries In these curves, it can be seen that each country adopts a different approach for minimum voltage during the event, protection system time and voltage recovery. Germany and the UK grid operators issue LVRT curves with hard constraints on minimum voltage: WT thereto connected have to withstand zero voltage for a period of time. USA/Ireland/Canada, Spain and Italy are more severe on the protection time duration. In six different LVRT curves presented, there are four different profiles for the voltage recovery period. Even for the three countries with similar profiles, slopes, Vf and recovery times are different from each other. With such a variety of curves and different approaches adopted by each country, an overall comparison of the requirements is not easy to perform. Fig. 4 shows two graphs with different forms of comparisons of the LVRT curve. The left graph shows the area above the LVRT curve, in pu.s. This can give a rough measurement of the rating requisites of RT solutions for WT that should comply with each curve. The right graph simply shows the minimum voltage. In each graph, countries are ordered from left to right in order of increasing severity of LVRT requisite, according to the proposed indexes.

Br.

UK Ger

.

Ita.

USA

Spa.

00,20,40,60,8

11,21,41,61,8

Area

(pu.

s)

Br.

Ita.

Spa.

USA UK G

er.

0

0,05

0,1

0,15

0,2

0,25

0,3

Low

er V

olta

ge (p

u)

Fig. 4– Different measurements of severity of LVRT requirements of Grid Codes: voltage dip area to ride through and minimum voltage It is hard to imagine one single power system event that causes a voltage profile similar to some of those LVRT curves, as pointed out by [7]. In Fact, LVRT curves are envelopes that contain a family of events that may occur in the power system and for which the WT should keep connected to the grid. As seen in the comparison of LVRT from different countries, each one adopted different curves that would somehow fit the needs of their grids. The criteria that may be adopted for build LVRT curves are discussed in the following sections. Assessment of Voltage Dips Vulnerability Area The vulnerability area technique is widely adopted in power quality studies to estimate the exposure of a load to voltage dips [1]. This analysis is made possible by software that may calculate voltages in power system substations for different types and locations of short-circuits applied throughout the system. Referring to Fig. 2, short-circuit analysis will help to determine the Vx of the LVRT curve. Sliding short-circuits are systematically applied in the grid and the resulting voltages are calculated in the substations of interest. The results may be compiled in different ways, that may show the total amount of the network in which a short-circuit will cause the voltage to drop below a certain level. This concept is known as vulnerability area. If there is information on statistics of fault occurrence per km of transmission line, the expected number of voltage dips may be estimated from the vulnerability area. In order to illustrate the technique and to discuss how it may help to establish LVRT requirements, an example will be presented. The example uses data from the Northeastern Brazilian Grid, shown in Fig. 5. This region has a good potential for wind power generation and many plants are already installed or will be installed in the near

future. Five different locations were chosen to have large wind farms: Delmiro Gouveia 230 kV, Delmiro Gouveia 69 kV, Bongi 230 kV, Rio Largo 230 kV and Jacaracanga 230 kV.

Rio Largo, 69 kVBongi, 230 kVJacaracanga, 230 kVDelmiro Gouveia, 230 kV

Delmiro Gouveia, 69 kV

Fig. 5– Northeast region in Brazil and analyzed points

The sliding short-circuit technique was applied to Brazilian network, with those five substations as monitoring sites. The automatic sliding short-circuit calculation was performed using the software Anafas, which is developed by Cepel. Fig. 6 shows the results obtained for the vulnerability areas of the 500 kV network, for each site, considering three-phase faults.

70 60 50 40 30 20 10Jacaracanga 230 kV

Bongi 230 kVDelmiro Gouveia 230 kV

Rio Largo 69 kVDelmiro Gouveia 69 kV

0

200

400

600

800

1000

1200

1400

1600

Vul

nera

bilit

y A

rea

[km

]

Voltage [%]

Fig. 6– Vulnerability area: 500kV system short-circuits

The vulnerability area gives some interesting information about voltage dips in the system. For instance, short-circuits in about 700 km of 500 kV transmission lines will cause voltage dips below 70% in Jacaracanga 230 kV. Meanwhile, faults in about 1400 km of 500 kV lines may cause 70% dips in Delmiro Gouveia 230 kV. Similar analysis may be performed for other substations and levels of voltage dips.

A WT that should withstand a minimum voltage of 20% without disconnecting from the network, as proposed in Spanish Grid Codes, will be exposed to 35 km of 500 kV transmission lines in Jacaracanga, in which a short-circuit will cause the voltage to drop below 20%. At Delmiro Gouveia, the same WT will be exposed to an area twice as big as the first one. If a constant fault rate per km is assumed for the whole 500 kV grid of the region, a WT connected in Delmiro Gouveia is expected to be disconnected from the grid two times more in a given period. So, depending on the voltage dip behavior of the substation, WT following the same LVRT requirements will have different numbers of disconnections from the network. This is particularly important for the connection of large wind farms to the grid. A poor choice of the substation, regarding voltage dips, may cause an unacceptable number of disconnections of the WT. The vulnerability areas give a good picture of the expected number of voltage dips in a substation. However, they are not much helpful alone in determining the impact of a short-circuit to different substations and to the power system as a whole.

Superposition of Vulnerability Areas: how voltage dips spread over the grid

An extension of the concept of vulnerability area may help to estimate how a short-circuit will simultaneously impact WT in different substations of the network. Sliding short-circuits are applied in the grid, but now voltages are monitored simultaneously in two or more substations. This technique, known as superposition of vulnerability areas, permits to assess how a voltage dip spread over the grid. Fig. 7 shows a map with a sketch of part of the superposition of vulnerability areas of Rio Largo and Jacaracanga substations, for 50% and 70% voltage dips. The map shows that, for the region represented, there is no part of the grid where a fault will cause voltage dips below 50% simultaneously in Rio Largo and Jacaracanga. As the main concern that leads to the imposition of LVRT limits was the loss of a large amount of wind generation during a voltage dip, the technique of superposition of vulnerability areas may be very helpful in determining LVRT requirements. For instance, once the maximum number of wind farms allowed to disconnect from the grid due to a voltage dip is defined, the technique may be used to determine which voltage dip may occur simultaneously in that number of substations. The value of simultaneous voltage dip obtained will give the Vx limit of the LVRT curve.

Table 2 presents the voltage level in each point, considering faults located in four different regions of the superposed vulnerability area illustrated (a, b, c and d).

70%

Rio Largo

Jacaracanga

50%

bc

d

a

Fig. 7 – Example of superposition of vulnerability areas

For instance, a fault at PAF 500 kV bus (region b) would result in a 41% voltage at Rio Largo and a 64% voltage at Jacaracanga. In this case, depending on the amount of generation in each point and the dynamic security criteria of the system, only part of the WT would be required to remain connected.

Table 2 – Voltage for different short-circuit locations

Region Fault Location Voltage

Rio Largo Jacaracanga

a Xingó 500 kV bus 35% 74% b PAF 500 kV bus 41% 64% c LT 500 kV PAF-LGZ 51% 69% d Ford 230 kV bus 92% 40%

Voltage dips in practice Power Quality monitoring may help to identify minimum voltage levels and duration of voltage dips registered in a given substation. Fig. 8 shows Minimum Voltage vs. Duration curve for a 69 kV bus. Fig. 9 shows the same curve for a 230 kV bus. In both cases, power meters were adjusted for registering voltage dips below 90% and voltage dips as low as 0,2 pu were recorded. This value alone is not of much important, as it is necessary to establish how these dips spread over the system – a procedure similar to the superposition of vulnerability area may be adopted for data processing of measurements.

0 0 0,01 0,1 1 100

0,2

0,4

0,6

0,8

1

Voltage Dip Duration (s)

Min

imum

Vol

tage

(pu)

Fig. 8– Voltage dips recorded in a 69 kV substation during a year

0 0 0,01 0,1 1 100

0,2

0,4

0,6

0,8

1

Voltage Dip Duration (s)

Min

imum

Vol

tage

(pu)

Fig. 9–Voltage dips recorded in a 230 kV substation during a year

But the important point to notice is that in the 230 kV substation the 0,2 pu dip lasted for no more than 100 ms. However, voltage dips of 20% with durations up to about 500 ms were recorded in the 69 kV substation. The duration of the voltage dip will depend on the protection time, that will be higher for lower voltage levels. So, a LVRT curve that should be adopted for different voltage levels may be too severe for the highest voltage or too loose for the lower voltage. This aspect is discussed in the following section. The statistics of voltage dips usually represent each event by two indexes: voltage magnitude and duration. The voltage dip is implicitly modeled by a rectangular-shaped curve. This is a good approximation for distribution systems. On the other hand, voltage dips in transmission systems look more like the example of Fig. 10. During the fault, the voltage drops for a few cycles, before reaching its minimum value. After the fault, there is a period of voltage recovery.

0,45 0,5 0,55 0,6 0,65 0,7 0,75 0,8 0,850,8

0,85

0,9

0,95

1

Time (s)

Volta

ge (p

u)

Fig. 10– A voltage dip recorded in a 138 kV substation

Protection influence on LVRT Curves The speed of protection depends on several aspects that include the protection philosophy, communication systems, relays and circuit breakers technologies. In general, the higher the voltage level, the smaller the protection time. This fact can be observed in Brazilian grid codes that establish criteria adopted in planning studies. One of those criteria is the protection time used in dynamic simulations as shown in Table 3 [10].

Table 3 – Protection time used in Brazilian studies Nominal Voltage

[kV] Clearing Time

[ms] 765 80

345, 440, 500 and 525 100 138 and 230 150

138 150 88(*)

and 138(*) 450 69(*) 800

(*)Without teleprotection

The table presents the total protection time that is considered in stability studies. The indicated values do not take into account circuit breaker failure. Being conservative, the LVRT requirement should consider the higher protection time, so the wind farms would remain connected even for short-circuits at low voltage levels. However, the imposition of such severe requirement should take into account the point of connection of each wind farm. Depending on wind plant location, such rigorous situation may never occur. For instance, if a fault in the 69 kV network occurs, a WT connected in a 230 kV substation may experiment a voltage dip with a duration equal to the protection time of 69 kV, but the dip will be shallow, i.e., the minimum voltage during the dip will not be too low. Considering a minimum voltage corresponding to short-circuits in the 230 kV network and the protection time of 69 kV is unrealistic and over conservative.

Voltage Dips Impact in Power Systems with Wind Generation As already discussed in this paper, the major concern of voltage dips is that they cause a large number of disconnection of WT that, in the end, may cause stability problems to the system. Thus, it is important to define the maximum amount of wind generation power disconnection that the power system can tolerate. This task is performed using transient analysis simulation. Also, the voltage recovery profile will be defined from these time-domain simulations.

Differently from short-circuit analysis, where a systematic technique may be adopted to evaluate the behavior of selected substations, transient stability analysis should be thoroughly carried on to identify critical situations. The task is quite similar to that traditionally performed in transient stability analysis: a collection of cases is selected and simulated. The experience of power engineers and their knowledge of the power system is very important for the cases selection and the analysis of the results. An additional complication comes from the fact that in order to define LVRT needs of the network, engineers have to consider not only the present but also the future network. For this, they have to foreseen the characteristics of the wind generation in years to come, including their location, amount of total power and size of individual plants. This is only achieved by taking into account the wind potential of the region and the grid reinforcements planned. Modeling Issues The network should be detailed modeled for power system simulations for the assessment of LVRT needs. This includes detailed models of traditional power system components, such as generators, excitation systems, speed regulators, SVC, etc. Also, WT should be represented in a high degree of detail. A collection of WT models is presented in [11], while [12] presents details of the models implemented in Anatem, the transient stability analysis software adopted in this work. A key issue when it comes to voltage response analysis is the load dynamics [5]. Traditionally, loads are represented by ZIP models that are helpful in identifying stability problems but fail in reproduce the voltage profile. In order to have faithful representations of the voltage behavior, a dynamic model of load should be implemented, at least near the wind farms locations. Fig 11 shows a comparison of voltage profiles in a 69 kV load bus for two different load models: ZIP and dynamic. The dynamic model adopted is linear of 2nd order, relating voltage input to the power output. Its parameters were obtained by measurements. Details on this model may be found in [13]. From the comparison of the results, it is easy to see that the ZIP model fails to reproduce the voltage recovery, after the fault clearing.

Fig. 11– Anatem simulation results: voltage profile in a 69 kV bus for ZIP and dynamic load model

LVRT characteristics as affecting power system performance – A Case Study A case study of the impact of voltage dips to the power system is presented in this section. The whole Brazilian Grid was modeled. Wind power plants of 400 MW were connected in the five locations shown in Fig. 5. Each plant has variable speed induction generation WT (DFIG). A short-circuit was applied in the 500 kV network near Fortaleza with the subsequent opening of two 500 kV lines. This event causes a severe voltage dip in the wind farms connected to Delmiro Gouveia 230 kV and 69 kV substations. The three other wind farms, located in Rio Largo, Bongi and Jacaracanga, experience shallow voltage dips. Three different situations were considered for LVRT: no LVRT at all, mild and rigorous LVRT requirements. For no LVRT requirement, all wind farms, even those three subjected to shallow voltage dips, disconnect from the system and a total of 2000 MW of generation is lost. For rigorous LVRT requirements, all five wind farms remain connected after the fault. If a mild LVRT characteristic is considered, only the wind farms in Delmiro Gouveia, that experience a severe voltage dip, are disconnected. In that case, 800 MW of wind generation are lost. Fig. 12 illustrates each LVRT characteristic considered. The rigorous requirement is similar to the one adopted in Brazil. The figure also shows voltage profiles of Delmiro Gouveia 230 kV substation for the three situations described. Fig. 13 shows voltage profiles of Bongi substations for the three situations. It can be seen that when the WT remain connected, the voltage profile remains virtually flat. Even if the WT disconnects, the resulting voltage dip is negligible.

Fig. 12– Delmiro Gouveia 230 kV voltage profiles and LVRT characteristics considered

Fig. 13– Bongi 230 kV voltage profiles for different LVRT curves

The impact of the WT disconnections to the power system may be evaluated in the following figures. Fig. 14 shows the north-northeast power flow for the three situations. One may see that the loss of wind power in the Northeast, for the case analyzed, is compensated by the increment of the power flow imported from the North.

Fig. 14– Power flow from North to Northeast

Fig. 15 to 17 show the angles of power plants located in the Northeast (Xingó), North (Tucuruí), South (Itaipu) and Southeast (Angra), for the three situations described. While in the rigorous LVRT the oscillation of machines is negligible, in the no LVRT case the machines experience large oscillations. However, in both cases, the system is stable.

Fig. 15– Machine angles for Rigorous LVRT case

Fig. 16– Machine angles for Mild LVRT case

Fig. 17– Machine angles for No LVRT case

Criteria for LVRT – Some Ideas In previous sections, the problem of WT sensitivity to voltage dips, its impact on power systems and the rationale for LVRT requirements were presented. Techniques to assess both dip performance and power system response to them were discussed. This section summarizes some important thoughts and ideas that may serve as criteria for LVRT definition. The first important issue is to always keep in mind that WT are sensitive to voltage dips, but this only becomes a problem for power systems when a large number of wind generators is disconnected due to voltage dips. That is the only reason for system operators and utilities to impose LVRT requirements to WT. The intention is to protect the power system and maintain its secure operation. An efficient LVRT requirement should avoid the disconnection of the amount of wind generation that would jeopardize system stability. On the other hand,

ride-through enhancement solutions increase the overall cost of WT. A fair LVRT requirement should avoid unnecessary exigencies to the WT. Using the concept of vulnerability areas, it can be seen that severe voltage dips will occur only for short-circuits in small portions of the transmission grid. With the superposition of vulnerability areas, it can be seen that the more severe a voltage dip gets, the smaller the superimposed vulnerability area is. Depending on the substations of interest, it may be impossible for a voltage dip of a given magnitude to occur simultaneously in all sites. The minimum voltage dip that may occur simultaneously in the sites of interest may be used as the minimum level of an LVRT requirement, if the goal is to avoid that WT in those sites disconnect at the same time. Transient stability simulations are of paramount importance in the definition of which level of wind generation loss is admissible after a fault. There is no rule of thumb here, as a lot of detailed simulations of the full system should be executed. Different configurations of the present and the future power grids, submitted to different events, should be evaluated. Also, the possible WT technologies and the foreseen wind penetration should be considered. The voltage recovery portion of the LVRT curve is obtained from these simulations. Here the same assumptions of superposition of vulnerability apply. A conservative limit would consider the worst voltage recovery profile among those of the substations of interest. However, it would be more reasonable to determine which amount of wind generators will cause stability problems when concurrently disconnected during the voltage recovery period. Then, the voltage recovery profile should be correspondently defined. Some Grid Codes adopt a minimum voltage of zero for LVRT curves. This is over conservative and probably based on an unreal assumption. Using the concept of vulnerability area, it is possible to conclude that a zero voltage dip in a power system busbar will only happen for a solid short-circuit in that same busbar. Unless there is one single large wind farm connected to a substation that causes stability troubles to the system if disconnected, there is no reason for the adoption of such a low limit. Protection time plays an important part in the definition of the duration of LVRT curve, prior to the voltage recovery period. An envelope that includes both the longer protection time system of lower voltage transmission systems and the minimum simultaneous voltage dip magnitude of higher voltage transmission systems makes no sense. The level of expected wind power penetration in a specific area of the system is a fundamental aspect for LVRT requirement definition. The adoption of the longer protection times of 69 kV systems and the lower

simultaneous voltage dips of 230 kV systems could only be justifiable if there are large amounts of wind generation at the 69 kV and the 230 kV at the same time and the loss of generation at each of these voltage levels alone would risk system security. But the system operator has always the option of defining different LVRT requirements for different voltage levels. Conclusions The problem of wind turbine sensitivity to voltage dips and its hazards to power system security was the point of departure to discuss low-voltage ride-through needs of the power system. Techniques of power system simulations and how they may help to assess these needs were examined. It was shown that substations with different voltage levels and sited in different geographical regions may require different ride-through characteristics of WT. Also, it was shown that the voltage dip behavior of a single substation is lesser important than the concurrent behavior of many substations. This is the key aspect of the definition of LVRT needs of the power system: how a voltage dip will spread over the network, as the wider the area it affects, the higher is the number of WT possibly disconnected from the grid. Suggestions for a reasonable definition of LVRT requirements were given. It was pointed out that it is not necessary to have a single ride-through curve that is representative of the whole power grid. Rather, the use of a set of ride-through curves may enhance the coordination of power grid and WT and avoid excessively rigorous requirements, eventually resulting in benefits for both manufacturers and power generators without risking power system reliability.

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