comparison of practical fault ride-through capability for mv - PSCC

COMPARISON OF PRACTICAL FAULT RIDE-THROUGH CAPABILITY 

FOR MV-CONNECTED DG-UNITS 

Edward Coster 

ENECO NetBeheer/Eindhoven University of Technology, 

Rotterdam/Eindhoven, The Netherlands 

e.j.coster@eneco.nl / e.j.coster@tue.nl 

Johanna Myrzik 

Eindhoven University of Technology, 

Eindhoven, The Netherlands 

j.m.a.myrzik@tue.nl 

Abstract – Nowadays amount of distributed generation 

(DG) units is increasing rapidly. Most of them are represented 

by combined heat and power (CHP) plants and 

wind turbines. At this moment there are no requirements 

defined for short-circuit behavior of such generators connected 

to MV-grid. However, in the future this situation 

will be not acceptable anymore, since with the present 

protection settings of DG a fault in transmission network 

may lead to disconnection of DG over large geographic 

areas. In this paper the fault ride-through capabilities of 

wind turbines as well as CHP-plants are investigated based 

on transient stability analysis (calculation of critical clearing 

times (CCT) curves). Different types of DG have different 

fault ride-through capabilities. It is shown that adjustment 

of the protection settings according to these capabilities 

can lead to significant improvement of DG-units availability 

without loosing stability of generators. 

Keywords: distributed generation, fault ridethrough, 

power system dynamics, protection 

1 INTRODUCTION 

The nature of today's distribution grids is changing 

from passive to active. The penetration level of distributed 

generation units (DG-units) is still increasing and it 

is expected that this growth will continue the next few 

years. At the moment not only the amount of DG-units is 

increasing, but in specific area also the sizes of the DGunits. 

In this way in the near future DG may generate a 

substantial part of the electricity consumption. 

Currently wind turbines have a large share in the generation 

of electricity by DG-units. Originally, the small 

wind turbines were connected to low voltage networks. 

The increase in wind turbine size have lead to large 

groups of wind turbines, which are connected to the 

medium voltage grid (MV-grid) and high voltage grid 

(HV-grid). In [1, 2, 3] it is stated that large wind farms 

must be treated like conventional power plants. This 

consideration has lead to the fact that grid operators 

have established requirements for the connection of 

large wind farms. 

In the Netherlands local authorities have designated areas 

where greenhouse activities can be developed. Each 

greenhouse contains a CHP-plant in the range of 1-3 

MVA and the size of these units needs connection to the 

Anton Ishchenko 



a.ishchenko@tue.nl 

Wil Kling 



w.l.kling@tue.nl 

local MV-grid. Due to the high density of greenhouses in 

such areas, the penetration level of CHP-plants in the 

MV-grid is high which in these areas can lead to a total 

generated power of 100 MW or more. The load in these 

areas is less than the total amount of generated power. 

Due to the small size of the CHP-plants, grid operators 

do not have any requirements for connecting such 

units to the grid. The current practice is to disconnect 

the CHP-plants when the voltage drops to 80% of the 

nominal voltage. With the increasing penetration level 

of CHP-plants it can become important to keep, besides 

the wind farms, the CHP-plants connected to the grid as 

well [2]. To do so the ride-through capabilities of the 

DG-units have to be known. The Critical Clearing Time 

(CCT) is related to the ride-through capabilities of the 

DG-units since DG-units cannot be kept connected when 

the generators become unstable. In this paper the CCT is 

taken as an indicator to assess the ride-through capabilities 

of DG-units. 

Besides the stability indicator, CCT, there might be 

other limitations on ride-through capabilities, such as 

limits of power electronic converters, mechanical stress 

on wind turbine axes as well as turbine blades. These 

limitations are not considered in this paper. The objective 

of this paper is to set up a comparison between the 

ride-through capabilities of a wind turbine having a 

Doubly Fed Induction Generator (DFIG) and a CHPplant 

having a synchronous generator. 

The paper is organized as follows: First the dynamic 

models of the CHP-plant and the DFIG are described 

briefly. Then, with the aid of a test grid and the dynamic 

models, for several voltage sags the CCT of both DGunits 

are determined. The paper ends with a case study 

of an existing power system including large penetration 

of DG-units. 

2 MODELING OF DG-UNITS 

The ride-through capabilities of DG-units are determined 

with the aid of dynamic models. These dynamic 

models are implemented in simulation software such as 

Matlab/Simulink and DIgSILENT's PowerFactory. In 

this section the models of the CHP-plant and the DFIG 

wind turbine are discussed briefly. 

16th PSCC, Glasgow, Scotland, July 14-18, 2008 Page 1

2.1 Dynamic Model of a CHP-plant 

Modern CHP-plants have a turbo charged gas engine 

as a prime mover. When studying the ride-through capabilities 

of such CHP-plants it is necessary to study the 

dynamics of the gas engine. Gas engine modeling is 

very complex due to the difficulties in describing the 

internal processes of the engine by equations. Most 

models which are available in the literature are too specific 

and not developed for power system dynamical 

studies. In [4] a model is developed, which is suitable to 

represent the gas engine dynamics. In Figure 1 a block 

scheme of the gas engine model is given. This block 

scheme is implemented in Matlab/Simulink and dynamic 

simulations are performed. In [4] a detailed description 

of the applied model as well as the used figures can be 

found. 

Throttle 

α 

1-α 

Engine 

Turbo 

Charger 

Figure 1: Block scheme of a turbo charged gas engine 

Torque 

The dynamics of the gas engine is demonstrated with 

a simulation of a torque step. At t = 0 s the engine is 

started up and at t = 5 s a torque step from 0.8 p.u. to 0.3 

p.u. is applied. The result is depicted in Figure 2 and 

shows that the dynamics of the gas engine are mainly 

determined by the dynamics of the throttle. 

2.2 Dynamic Model of a DFIG 

The scheme of a DFIG used for the simulations is 

shown in Figure 3. This concept includes a wound rotor 

asynchronous generator with the stator directly connected 

to the grid, while the rotor is coupled with the 

network using an AC to AC power converter with a DClink. 

Wind 

Turbine DFIG 

1.5 MW 2 MVA 

575 V / 10 kV 

Pitch 

Control 

RSC 

~ 

RSC 

Control 

GSC 

~ 

GSC 

Control 

150 kvar 

Figure 3: Model of the DFIG for stability assessment 

The simulations of the DFIG dynamic behavior were 

performed with the Matlab/SimPowerSystems toolbox 

using the DFIG model available there [5]. The model 

includes the following main features: 

• The wind turbine is modeled using steady-state 

aerodynamic relationship between wind speed 

and the mechanical power of the turbine. 

• The power electronic converters are represented 

by equivalent voltage sources generating the AC 

voltage averaged over one cycle of the switching 

frequency. 

• Pitch control, rotor-side converter (RSC) and 

grid-side converter (GSC) controls are modeled 

in full detail. During the simulation the RSC control 

operates in power factor regulation mode 

keeping the generator reactive power exchange 

equal to zero. 

Figure 2: Dynamic response of a gas engine 

In comparison with typical fault clearing times of the 

protection systems and critical clearing time of generators, 

the dynamics of the gas engine are slow. Due to 

these slow dynamics the gas engine is represented by its 

inertia only. This inertia is incorporated with the generator 

inertia. 

The generator model of the CHP-plant, used in the 

dynamic simulations, is a regular model of the synchronous 

machine which is available in PowerFactory. 

• The wound rotor asynchronous generator is represented 

by 5th order model in the dq-reference 

frame. 

• The model does not include crowbar protection. 

3 TRANSIENT STABILITY OF DG-UNITS 

The aim of ride-through criteria is to keep DG-units 

connected to the grid during the disturbance and recover 

the power balance after fault clearing. Keeping DG-units 

connected to the grid means that the units have to withstand 

certain voltage dips. The effect of voltage dips on 

stability of CHP-plants and wind turbines is studied by 


means of a study grid having a variable voltage source. 

The voltage source is able to produce a voltage dip of a 

certain depth and duration. The defined dynamic models 

are implemented in the test grid and the stability of the 

DG-units as a function of the depth and duration of the 

voltage dip is determined. 

In Figure 4 the study grid is shown. It consist of two 

10 kV busbars, a swing-bus which represents the external 

grid, a 3 km XLPE 630 mm 2 Al cable connection 

and a generator. Parallel at the swing-bus the variable 

voltage source is connected. 

Variable 

Voltage 

Source 

Swing 

Bus 

V 

Bus 1 

Bus 2 

Figure 4: Study grid including a variable voltage source 

G 

3.1 Dynamic Behavior of CHP-plants 

With the aid of the defined test grid the dynamic behavior 

of the CHP-plant is studied. In the simulations 

the rotor angle is used as a stability index. To find the 

stability limit at a certain depth of the voltage dip the 

duration of the voltage dip is adjusted. In this way the 

CCT as a function of the voltage dip can be found. In 

Table 1 the figures for which the simulations are carried 

out are displayed. The results of the simulation are depicted 

in Figure 5. 

S nom [MVA] P [MW] p.f. H [s] 

3 2.4 1.0 2 

3 2.4 0.8 2 

3 2.4 1.0 4 

3 2.4 0.8 4 

Table 1: Simulation figures of the CHP-plant 

u [p.u.] 

0.5 

0.45 

0.4 

0.35 

0.3 

0.25 

0.2 

0.15 

0.1 

H = 2 s, p.f. = 1 

H = 2 s, p.f. = 0.8 

0.05 

H = 4 s, p.f. = 1 

H = 4 s, p.f. = 0.8 

0 

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 

t [s] 

Figure 5: CCT of a CHP-plant as a function of the voltage 

dip (remains voltage) 

3.2 Dynamic Behavior of a DFIG 

Transient stability of a DFIG can be evaluated by 

monitoring the DC-link voltage. Its behavior for threephase 

fault durations of 100 and 300 ms at wind speed 

of 10 m/s (voltage dip to 0.6 p.u. of the nominal voltage) 

is illustrated on Figure 6. 

2800 

2600 

2400 

2200 

2000 

1800 

1600 

1400 

1200 

Fault duration = 300 ms 

Fault duration = 100 ms 

DC-link voltage Vdc [V] 

1000 

0 0.1 0.2 0.3 0.4 0.5 0.6 

t [s] 

Figure 6: DC-link voltage of a DFIG for different fault durations 

In an unstable situation the DC-link voltage starts to 

increase significantly immediately after the fault. The 

other variables (active and reactive powers, mechanical 

speed of the machine, etc.) exhibit not so clear unstable 

behavior, although as it can be seen from Figure 7, active 

and reactive powers start to oscillate, and the mechanical 

speed begins to grow slowly. 

2 

0 

Active power [p.u.] 

-2 

0 0.1 0.2 0.3 0.4 0.5 0.6 

1 

0 

Reactive power [p.u.] 

-1 

0 0.1 0.2 0.3 0.4 0.5 0.6 

1.11 

1.1 

1.09 

Fault duration=300 ms 

Fault duration=100 ms 

Mechanical speed [p.u.] 

0 0.1 0.2 0.3 0.4 0.5 0.6 

t [s] 

Figure 7: Mechanical speed, active and reactive powers of a 

DFIG for different fault durations 

To determine the CCT of the DFIG the same approach 

as described in section 3.1 is used. The simulations 

are carried out for various wind speeds which lead 

to the results depicted in Figure 8. 


u [p.u.] 

0.5 

0.45 

0.4 

0.35 

0.3 

0.25 

0.2 

0.15 

0.1 

Wind speed = 9 m/s 


0.05 



0 

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5 

t [s] 

Figure 8: CCT of a DFIG as a function of the voltage dips 

Addition of the crowbar allows to prevent the growth 

of the DC-link voltage during a short-circuit and significantly 

increase the CCT, which becomes related to the 

large extent only with the mechanical part of wind turbine 

having a very large inertia. Switching between 

different control strategies in case of normal operation 

and fault conditions improves the shape of the DFIG 

transient response and prevents transient stability problems 

as illustrated in [6]. 

4 CASE STUDY 

The frequency of disconnections, initiated by the undervoltage 

protection can be estimated by means of a 

voltage dip profile. The dip profile describes, for a certain 

location in the grid, the magnitude, duration and 

frequency of the dips. In a case study, the developed 

CCT-curves in combination with the voltage dip profile 

are used to study the stability of the DG-units when 

disconnection by the undervoltage protection is prevented. 

The dip magnitude is determined by the network 

impedances and the type of fault [7]. When considering 

voltage dips caused by faults, the fault clearing time of 

the protection specifies the dip duration. The fault frequency 

determines the number of voltage dips. 

For the power system of the province Zuid Holland, 

the Netherlands, the voltage dip profile at the point of 

connection of the DG-units is assessed. 

4.1 Examined Grid 

The province of Zuid Holland has a 150 kV transmission 

grid, which is connected to the 380 kV grid at 3 

locations. The layout is given in Figure 9. From the 150 

kV nodes (HV/MV substations) the MV-networks are 

fed. Large generators are connected to the transmission 

grid. 

In the province of Zuid Holland there are three large 

areas having a high penetration of DG-units. In Table 2 

an overview of the location, type and amount of DG is 

given. 

Substation Type of DG S tot [MVA] 

Sub 3 Wind Turbines 70 

Sub 4 Wind Turbines 40 

Sub 6 CHP-plants 200 



Table 2: Location, type and amount of DG 

In the case-study substation 6 is taken as an example 

to study the voltage dip propagation through the transmission 

and distribution grid as well as the effect of the 

voltage dips on the DG-units. Therefore the MV-grid 

fed by substation 6 is modeled in detail including DGunits. 

The MV-grid is schematically shown in Figure 10. 

The generic fault clearing times of the MV-grid 

shown in Figure 10 are listed in Table 3. 

Figure 10: Overview of MV-grid having large penetration of 

DG-units 

Figure 9: Transmission grid of the province of Zuid Holland, 

the Netherlands 

Voltage level Protection Fault clearing time 

380 kV distance 150 ms 

150 kV distance 150 ms 

25 kV overcurrent 600 ms 



Table 3: Generic fault clearing times 


4.2 Voltage Dips Profile 

As mentioned earlier the fault frequency is based on 

the failure rate of the components in the system. Based 

on [8] for HV-lines and MV-cables respectively λ = 

0.54 [times per annum/100 km] and λ = 1.02 [times per 

annum/100 km] is used. In Table 4 the fault type distribution 

is given. 

Component 3 ph 1 ph 2 ph 2 ph-ground 

HV OH-line 30% 60% 5% 5% 

MV cable 30% 70% – – 

Table 4: Failure rate and fault distribution 

In PowerFactory a tool is available to calculate voltage 

dip tables. With the aid of this tool, the failure rate 

and fault distribution voltage dip profiles are constructed 

for a generator connection in the 10 kV grid of 

Figure 10. The results are reported in two distinct ways: 

1. Dip profile including fault type distribution. 

2. Dip profile including voltage level distribution. 

The first profile is depicted in Figure 11 and shows 

on the x-axis the depth of the dip (remains voltage level) 

experienced by the generator and on the y-axis the frequency 

of occurrence. The undervoltage protection 

measures the voltage between the phases so the voltage 

dip profiles are based on the line-line voltage. According 

to [7, 9] in case of a single phase to ground faults the 

zero sequence voltage does not propagate through a Dy 

and Yd transformer. When Z0 ≫ Z1 

the sagged voltage 

remains high. All voltage levels smaller than 0.8 p.u. 

are caused by three phase faults. 

1.4 

single phase to ground faults and the dips do not really 

propagate through the Dy and Yd transformers. 

Times per year 

1.4 

1.2 

1 

0.8 

0.6 

0.4 

0.2 

0 

10 kV 

20 kV 

25.6 kV 

150 kV 

380 kV 

0.2 0.4 0.6 0.7 0.8 0.85 0.9 0.95 

U [p.u.] 

Figure 12: Voltage dip profile based on different voltage 

levels at generator terminals 

5 RESULTS 

In this section the results of the case study are discussed. 

The aim of the case study is to demonstrate 

whether the DG-units can stay connected to the grid and 

maintain stable operation. Therefore the worst case 

CCT-curve of the CHP-plant and the worst case CCTcurve 

of the wind turbine are depicted in Figure 13. To 

judge if the DG-units stay in stable operation, the duration 

of the dip has to be determined. This is done by 

combining the results of Figure 12 and the fault clearing 

time of the protection of the MV-grid (Table 3). It is 

assumed that all faults are cleared by the primary protection. 

This leads to the red dots in Figure 13. 

1 

0.9 

Times per year 

1.2 

1 

0.8 

0.6 

3 phase 

2 phase 

2 phase-ground 

single phase-ground 

u [p.u.] 

0.8 

0.7 

0.6 

0.5 

0.4 

0.3 

0.4 

0.2 

0 

0.2 0.4 0.6 0.7 0.8 0.85 0.9 0.95 

U [p.u.] 

Figure 11: Voltage dip profile based on fault distribution at 

generator terminals 

In Figure 12 the second dip profile is depicted and 

shows the experienced voltage dip caused by disturbances 

at a different voltage level. In the figure it can be 

seen that voltage levels smaller than 0.8 p.u. are caused 

by faults in the 25 and 20 kV grid. At the generator 

terminals dips caused by disturbances in the HV-grid are 

hardly noticed. As discussed above most of the faults are 

0.2 

0.1 

CHP 

DFIG 

0 

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 

t [s] 

Figure 13: Comparison of fault clearing times and CCTcurves 

of DFIG and CHP-plant 

Most dots are above both CCT-curves and those dips 

do not lead to unstable operation of either the wind 

turbine or the CHP-plants. The dots under the CCTcurves 

are faults in the 20 kV grid that have a clearing 

time, which is larger than the CCT of the DG-units. In 

this situation, keeping the DG-units connected lead to 

unstable operation. Preventing instability of the DGunits 

can be done by reducing the fault clearing time. 


Reduction in fault clearing time can be reached by reducing 

time grading of two successive relays or applying 

differential protection. 

The nowadays undervoltage protection settings in the 

Netherlands are such that at voltage levels below 0.8 

p.u. all DG-units are disconnected. Comparing the current 

settings with the CCT-curves displayed in Figure 

13, it can be concluded that the settings of the undervoltage 

protection can be altered without instability of 

the DG-units. 

Based on the data in Figure 12 with the current settings, 

the frequency of disconnection is 0.37 times per 

annum or 9.3 % of all the voltage dips (calculated by 

summing up all the frequencies of the voltage dips below 

0.8 p.u.). By applying the developed CCT-curves in 

the undervoltage protection the frequency of disconnection 

can be reduced to 0.02 times per annum or 0.5 % of 

all the voltage dips. This figure holds for the wind turbine, 

while for the CHP-plant there are no disconnections 

at all. Thus, the availability of the DG-units can be 

increased significantly by adjusting the under-voltage 

protection in accordance with the CCT-curves. 

The fault ride-through capabilities of the wind turbine 

can be improved by adding a crowbar to the DFIG. The 

crowbar separates the rotor side converter from the rotor 

circuit. In [10] the operation of the crowbar is simulated 

and it is demonstrated that fault ride through during and 

after a deep voltage dip (0 V remains, 150 ms) is possible. 

6 CONCLUSION 

In this paper the ride-through behavior of a CHPplant 

and a DFIG-based wind turbine is studied. Based 

on the data of an existing power system including reliability 

data a voltage dip profile at the generator terminals 

is established. This profile is combined with the 

fault clearing times of the test system which have led to 

a plot where the depth and duration of the voltage dips 

are indicated. Using the single machine infinite network 

scheme the CCT-curves of the DG-units are derived. 

From stability point of view these curves show that it is 

not necessary to disconnect the DG-units at a voltage 

level of 0.8 p.u. (the present setting of DG undervoltage 

protection in the Netherlands), but at lower levels, which 

are dependent on the critical clearing times. These CCTcurves 

can be implemented in the undervoltage relay 

and prevent switching off during a voltage dip caused by 

a fault in a HV- or neighboring MV-grid. In some cases 

disturbances in the 20 kV grid lead to unstable operation. 

This can be prevented by adjusting the fault clearing 

time or applying differential protection. For DFIG 

wind turbines transient stability can be significantly 

improved by introduction of crowbar protection and 

switching between different control strategies for normal 

operation and fault conditions. 

The method presented in this paper can be followed 

in all locations of the grid and prevent ambitious disconnection 

of DG. 

REFERENCES 

[1] B.M. Buchholz, “Dynamic Simulation of Renewable 

Energy Sources and Requirements on Fault Ride 

Through Behaviour”, Power Engineering Society 

General Meeting, Montreal, Canada, June, 2006 

[2] F.M. Gatta, F. Ilicento, S. Lauria and P. Masato, 

“Behaviour of Dispersed Generation in Distribution 

Networks During System Disturbances. Measures to 

prevent disconnection”, CIRED 17th International 

Conference on Electricity Distribution, Barcelona, 

May 2003 

[3] I. Erlich and U. Bachmann, “Grid Code Requirements 

Concerning Connection and Operation of 

Wind Turbines in Germany”, IEEE Power Engineering 

Society General Meeting, San Francisco, Californie, 

USA, 2005 

[4] G. Daniëls, “Betrieb von Mittelspannungsnetzen mit 

hoher Blockheizkraftwerkeinspeisung”, Band 39, 

dissertation Rheinisch-Westfälischen Technische 

Hochschule Aachen, 1996 (in German) 

[5] “SimPowerSystems User's Guide”, The Mathworks, 

2007 

[6] A.D. Hansen and G. Michalke, “Voltage grid support 

of DFIG wind turbines during grid faults”, 

European Wind Energy Conference & Exhibition, 

Milan, Italy, May 2007 

[7] J.F.G. Cobben, “Power Quality: Implications at the 

Point of Connection”, PhD Thesis, ISBN: 978-90- 

386-1030-6, Eindhoven University of Technology, 

June 2007 

[8] R. Luiten et al, “Reliability of Electricity Networks 

in the Netherlands for 2006”, KEMA, May 2007 (in 

Dutch) 

[9] M.H.J. Bollen, “Understanding Power Quality Problems, 

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NJ: IEEE press, 1999 

[10]I. Erlich, H. Wrede and C. Feltes, “Dynamic Behavior 

of DFIG-Based Wind Turbines during Grid 

Faults”, PCC Power Conversion Conference, Nagoya, 

Japan, April, 2007

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