Impacts of Distributed Generation on Power System Transient Stability
J.G. Slootweg Member IEEE W.L. Kling Member IEEE
Electrical Power Systems, Faculty of Information Technology and Systems, Delft University of Technology
P.O. Box 5031, 2600 GA Delft, The Netherlands
Phone: +31 15 278 6219, Fax: +31 15 278 1182, e-mail: j.g.slootweg@its.tudelft.nl
Abstract - It is expected that increasing amounts of new generation technologies will be connected to electrical power systems in the near future. Most of the technologies are of considerably smaller scale than conventional synchronous generators and are therefore connected to distribution grids. Further, many are bad on technologies different from the synchronous generator, such as the squirrel cage induction generator and high or low speed generators that are grid coupled through a power electronic converter.
When connected in small amounts, the impact of distributed generation on power system transient stability will be negligible. However, if its penetration level becomes higher, distributed generation may
start to influence the dynamic behavior of the power system as a whole. In this paper, the impact of distributed generation technology and penetration level on the dynamics of a test system is investigated. It is found that the effects of distributed generation on the dynamics of a power system strongly depend on the technology of the distributed generators.
Keywords: distributed/disperd generation, transient stability, power system dynamics, modeling, simulation
I. INTRODUCTION
It is expected that increasing amounts of new technologies for electrical power generation will be introduced in electrical power systems in the near future. A number of reasons for this development exist. The emphasis on power generation from renewable sources in order to reduce the environmental impact of power generation leads to the development and the u of technologies for renewable power generation, such as solar panels, wind turbines and wave power plants. The goals of decreasing the cost associated with electrical power transmission in liberalized markets and increasing the efficiency of primary fuel u by using combined heat and power generation (CHP) lead to the installation of generation equipment at consumer sites.
Many of the new technologies do not u a conventional grid coupled synchronous generator to convert primary energy into electricity. Instead, they u a squirrel cage or doubly fed induction generator (some wind turbine concepts) or a synchronous or squirrel cage induction generator that is grid connected through a power electronic converter (other wind turbine concepts, small scale high speed gas turbines). In the ca of solar panels and fuel cells, it is not even mechanical power that is converted into electricity.A further difference between the new technologies and conventional means of electrical power generation is that many of them are of considerably smaller size than conventional thermal, nuclear and hydro units that up to this moment deliver the majority of the electrical power consumed worldwide. Therefore, they are often connected to low and medium voltage grids and not to the high voltage transmission grid, resulting in longer electrical distances.
As long as the penetration level of the new technologies in power systems is still low and they only cover a minor fraction of the system load, they hardly impact the dynamic behavior of a power system. Therefore, in power system dynamics and transient stability studies, they are normally considered as negative load and their intrinsic dynamics and their controllers, if prent, are not taken into account. However, if the amount of new generation technology introduced in a power system becomes substantial, it may start to influence the overall behavior of this system [1,2]. Up to t
his moment, it is unknown at which penetration level and in which way distributed generation influences power system dynamic behavior, which is partly caud by the lack of adequate models of the new technologies.
The goal of the rearch prented here is to acquire insight in the impact of distributed generation on power system transient stability. To this end, various technologies and penetration levels are studied. The paper is structured as follows. First, the rearch approach is described and the preparation of the test system and the modeling of the various distributed generation technologies is commented upon. Then the results of the investigations are prented and discusd.
II. RESEARCH APPROACH
For the rearch, a well known dynamics test system was chon and adapted to the needs of this study. In the ba ca, this test system does not contain any distributed generation. The whole load is covered by the ten large scale synchronous generators prent in the system. This system has been widely ud for various kinds of power system dynamics studies. The considerations leading to the choice of this test system and the development of the models ud in the rearch will be commented upon in more detail in the next paragraph, in which the preparation of the test system is described.
0-7803-7519-X/02/$17.00 © 2002 IEEE
Fig. 1. Transient stability indicators: maximum rotor speed deviation and oscillation duration
Five different distributed generation technologies are studied below, namely:
&
squirrel cage induction generator &
uncontrolled synchronous generator &
synchronous generator with grid voltage and frequency control
&
uncontrolled power electronic converter &
power electronic converter with grid voltage and frequency control
The penetration level of each of the technologies was varied in the following way:
&
A distributed generator was connected to each load bus by an impedance of 0.05j p.u. on the 100 MVA system ba.
&
Both real and reactive power of the loads were incread in steps of five per cent until the load was incread to one and a half times that of the ba ca.&
The distributed generator connected to each of the load bus was adjusted in order to generate an amount of active power equal to the increa of the real power consumed by the load at that bus.
Thus, the active power generated by the large scale generators was not changed, except for the active power of generator number 2, which acts as the swing bus or slack node. Its power is changed during the solution of the load flow to make up for changes in the network loss, if any. Note that a load increa of 50 per cent that is covered by distributed generation corresponds with a distributed generation penetration level of 33 per cent.
It is not considered realistic to further increa the distributed generation penetration level. Higher penetration levels might give ri to the necessity to reconsider the contemporary electrical power systems concept and compare it with other solutions, such as DC links or (mi-)autonomous systems.Therefore, it ems not uful to investigate the impact of such high penetration levels of distributed generation on the dynamics of a classic interconnected AC power system.
It was assumed that the uncontrolled distributed generation technologies operated at unity power factor, whereas the reactive power of the controlled technologies was able to vary between zero and half the active power (cos 1 ±0.9) and controlled in such a way that the terminal voltage was kept as near to its nominal value as possible without exceeding the reactive power limits of the generators.
To investigate the transient stability of the test system, a fault was applied to the transmission line between the bus 15 and 16 of the test system, which was carrying 315 MW and 150MVAr in the pre fault ba ca scenario. After 150 ms, the fault was cleared by tripping the faulted line. It is assumed that none of the centralized and distributed generators bad on synchronous or asynchronous machines are disconnected during the applied fault.
To asss the impact of distributed generation technology and penetration level on the transient stability, an indicator for the transient stability is needed. Here, two indicators have been applied to the rotor speed oscillations of the large scale generators in the test system that occur after application of the
fault, namely:
&
maximum rotor speed deviation &
oscillation duration The meaning of the maximum rotor speed deviation is obvious. It is the maximum rotor speed deviation reached by the generator during or after the fault. The oscillation duration is defined as follows:
The oscillation duration is equal to the time interval between the application of the fault and the moment after which the rotor speed stays within a bandwidth of 1 10-4 p.u. during a time interval longer than 2.5 conds.
Thus, the oscillation duration is a measure for the time span that is needed to reach a new equilibrium after a disturbance.Both measures are depicted in figure 1. The lower the value of each of the indicators for a given ca, the better the transient stability is.
III. TEST SYSTEM PREPARATION
To investigate the impact of distributed generation on power system transient stability, a model of a power system of a certain scale is necessary in order to arrive at results that can be considered reprentative. On the other hand, the system should not be too large, in order to reduce the computation time and to limit the number of possible scenarios.
Keeping this in mind, the widely ud New England test system has been chon as the test system to be ud here [3].The system is depicted in figure 2 and some of its characteristics are given in table 1. To arrive at a full dynamic model of the system, reprentative values for the parameters of the generators and the exciters and governors have been taken from other sources [4,5]. The loads
are equally divided in constant impedance, constant power and constant current.As already discusd above, in this paper five types of distributed generation are distinguished. For the synchronous and squirrel cage induction generator, as well as for the governors and exciters of the distributed generation formed by controlled synchronous generators, standard models for u in power system dynamics simulations can be found in the literature [4,5].
The power electronic converter is modeled as a source of active power (P) and reactive power (Q). This is necessary becau the grid reprentation in power system dynamics simulation software and the typical time step ud do not
allow detailed modeling of power electronics and its
Fig. 2. One line diagram of the New England test system [3]
Table 1. Characteristics of the New England test system
System characteristic Value
# of bus39
# of generators10
# of loads19
# of transmission lines46
Total generation6140.7 MW / 1264.3 MVAr Total reactive power compensation1408.9 MVAr
Total load6097.1 MW / 1408.7 MVAr controllers for both theoretical and practical reasons [4, 6-8]. In PSS/E, no standard model for reprenting power electronics is available. Therefore, a so-called ur-written model of a power electronic converter was developed and integrated into this program [5].
The prime mover of the synchronous and asynchronous generators and the primary side of the generators grid coupled through a power electronic converter is not taken into account, becau they are not of major importance in this study. Details on how to model wind turbines, one of the most frequently occurring distributed generation technologies including their prime mover, can be found in [9, 10].
IV. RESULTS
A. Overview of results
In tables 2 and 3, the simulation results are summarized. In the first row for each of the penetration levels studied, the maximum, and thus worst values of the two stability indicators ud, namely the maximum rotor speed deviation and the oscillation duration, are displayed, together with the numbers of the generators at which the given values occur in parenthesis. The numbers correspond to figure 2. In each cond row, the relative change compared to the ba ca is given in per cent.
In the left column, the distributed generation penetration level is indicated. In the cond row, the distributed technologies are indicated. ASM means asynchronous squirrel cage Table 2. Results for the maximum rotor speed deviation as stability indicator for various distributed generation technologies and penetration levels. The number of the generator at which the value occurs is indicated in parenthesis.
Dist.
pen.
level
[%]
Maximum rotor speed deviation [ 10-3 p.u.] and relative
change [%]
ASM SM SMC PE PEC
0 6.4 (6) 6.4 (6) 6.4 (6) 6.4 (6) 6.4 (6)
00000
9 6.1 (6) 6.1 (6) 6.1 (6) 5.9 (6) 5.9 (6)
-4.7-4.7-4.7-7.8-7.8
17 6.1 (1) 5.9 (6) 6.0 (6) 5.5 (6) 5.5 (6)
-4.7-7.8-6.3-14.1-14.1
23 6.1 (1) 5.8 (6) 5.8 (6) 5.1 (6) 5.1 (6)
-4.7-9.4-9.4-20.3-20.3
29 6.1 (1) 5.6 (6) 5.7 (6) 4.7 (6) 4.7 (6)
-4.7-12.5-10.9-26.6-26.6
33 6.3 (10) 5.5 (6) 5.5 (6) 4.3 (6) 4.3 (6)
-1.6-14.1-14.1-32.8-32.8
Table 3. Results for the oscillation duration as stability indicator for various distributed generation technologies and penetration levels. The number of the generator at which the value occurs is indicated in parenthesis.
Dist.
pen.
level
[%]
Oscillation duration [s] and relative change [%]
ASM SM SMC PE PEC
0 5.8 (5) 5.8 (5) 5.8 (5) 5.8 (5) 5.8 (5)
00000
9 5.9 (5) 5.7 (5) 6.7 (9) 5.6 (9) 5.5 (9)
+1.7-1.7+15.5-3.4-5.2
17 5.9 (9) 6.1 (7) 6.2 (7) 6.2 (9) 5.1 (2)
+1.7+5.2+6.9+6.9-12.1
23 6.5 (3) 5.9 (9) 6.1 (7)7.4 (9) 5.1 (9)
+12.1+1.7+5.2+27.6-12.1
29 6.1 (9) 6.9 (9) 6.2 (7)8.4 (9) 5.1 (9)
+5.2+19.0+6.9+44.8-12.1
33 5.7 (3)7.4 (10) 6.3 (2/3)8.5 (9) 5.1 (9)
-1.7+27.6+8.6+46.6-12.1
induction generator, SM synchronous machine, PE stands for distributed generation which is grid connected through a
power electronics interface and if a C is added to the
Fig. 3. Rotor speed deviation of generators 1 (upper graph) and 6 (lower graph). The solid line corresponds to the ba ca, the dashed and dotted lines to a distributed generation penetration of 20 and 33 per cent
respectively.
Fig. 4. Terminal voltage of generator 6 when varying amounts of distributed generation bad on uncontrolled (upper graph) or controlled (lower graph)synchronous generators are connected to the system. The solid line corresponds to the ba ca, the dashed and dotted lines to a 20 and 33 per cent distributed generation penetration respectively.
acronym, the distributed generators are equipped with voltage and frequency control.
From tables 2 and 3, it can be concluded that it is difficult to derive an overall conclusion with respect to the impact of distributed generation technology and penetration level on power system stability, becau the results are quite mixed.Therefore, each of the three main technologies, namely the squirrel cage induction generator, the synchronous generator and the power electronics interface will parately be commented upon.
B. Asynchronous generator
From tables 2 and 3, it can be en that distributed squirrel cage induction generators have an ambiguous influence on the two stability indicators. Both the maximum rotor speed deviation and the
oscillation duration are not very much effected and the maximum value occurs at various generators when the distributed generation penetration level changes. The shape of the oscillation is not very much influenced as well, as can be en from figure 3. In this figure, the rotor speed deviation of the generators 1 and 6 is depicted for the ba ca and for a distributed generation penetration level of 20 and 33 per cent respectively. By inspection, it can be en from this figure that the oscillation duration is in the range of 6 conds. This is in the same range as the longest oscillation duration which occurs at generators 5, 3 and again 3respectively for the depicted distributed generation penetration levels, according to table 3. The obrvations lead to the conclusion that distributed squirrel cage generators do not have very much influence on the transient stability of an electrical power system.
This result can be explained as follows. As discusd in [11],the effect of squirrel cage induction generators on power system stability depends on their distance to the synchronous generators. If they are located near synchronous generators and the latter speed up during a fault, the stator frequency of the asynchronous generators increas. This leads to a decrea in the slip frequency and thus in generated power,
which in turn slows down the speeding up of the synchronous generators. When the asynchronous g
enerator is at a larger distance and more weakly coupled to the synchronous generator, its speeding up during the fault will result in an increasing reactive power demand. This will result in a lower terminal voltage at the remote synchronous generator and thus in a decrea of synchronizing torque and a faster increa in rotor speed.
In the cas studied here, both the synchronous generators and the distributed asynchronous generators are spread through the system. The finding that distributed generation bad on squirrel cage induction generators ems not too have much effect on the transient stability of power systems can therefore probably be explained by noticing that both effects occur simultaneously and counterbalance.
C. Synchronous generator
It can be concluded from table 2 that an increasing penetration level of distributed generation bad on synchronous generators either with or without voltage control leads to a decrea in the overspeeding of the synchronous generators during the fault. This result can be explained by realizing that the distributed synchronous generators are equipped with an excitation winding on the rotor, which keeps the generators excited during the fault. As a result of this, the distributed generato
rs supply a fault current and the voltage during a fault does not drop as far as with no distributed generators prent. As a result, the synchronizing torque at the generators remains higher.
This reasoning is supported by simulation results depicted in figure 4. In this figure, the terminal voltage of generator 6,which has the highest overspeeding during the fault as can be concluded from table 2, is given for a penetration level of 0,20 and 33 per cent of both uncontrolled and controlled synchronous generators. It can be en that the higher the penetration level of distributed synchronous generators, the
higher the terminal voltage of generator 6. The respon is
Fig. 5. Rotor speed deviation of generator 10. The solid line corresponds to the ba ca, the dotted line to a 33 per cent penetration level of uncontrolled synchronous generators.
Fig. 6. Node voltage of bus 4 (upper graph)and 8 (lower graph). The solid line corresponds to the ba ca, the dashed and dotted lines to a 20 and 33per cent distributed generation penetration respectively.
similar for distributed synchronous generators without and with voltage and frequency control, which was to be expected, becau both have a similar effect on the amount of overspeeding.
With respect to the oscillation duration, it can be concluded from table 3, that it is incread when synchronous distributed generation is connected to the system and that, in opposition to what applies to the maximum rotor speed deviation, there is a difference in the oscillation duration in the ca of controlled and uncontrolled synchronous generators.
In figure 5, the rotor speed deviation of generator 10 with a 33 per cent penetration level of uncontrolled synchronous generators is depicted together with the ba ca. It can be en that the damping of the oscillation is reduced by the distributed synchronous generators. This obrvation might be caud by the interarea oscillation phenomenon [4, 12].However, this conjecture can not be
confirmed or denied by the results alone and requires more rearch. Further, it was difficult to conclude whether this result occurs only in the test system ud or in general. Therefore, further rearch on the impact of synchronous distributed generators on the damping of power system oscillations is necessary.
D. Power electronic converter
As can be en in table 2, the connection of distributed generation grid coupled through a power electronics interface results in a decrea in the maximum rotor speed deviation of the synchronous generators. This is caud by the fact that distributed generators are quickly disconnected by their power electronic converters when a fault occurs, becau of the terminal voltage drop. Thus, a lot of generation is lost in ca of a fault, reducing the acceleration of the rotors of the synchronous generators. This also explains why the overspeeding is further reduced, when more distributed generation is connected: the amount of generation lost at the fault increas, resulting in a decreasing rotor acceleration. The obrvation that the maximum rotor speed deviation is the same in the cas of controlled and uncontrolled distributed generators can be explained as follows. The maximum rotor speed deviation is reached during the fault.However, during the fault, both uncontrolled and controlled distributed generators grid coupled through a power electronics interface
are disconnected. Therefore, neither the uncontrolled nor the controlled generators are on line during the fault and as a result, the maximum rotor speed deviation is the same in both cas.
Although the obrved reduction in rotor speed deviations is as such desirable, the price paid for it is a large voltage drop at some nodes due to the disconnection of the distributed generators when the fault occurs. This can be en in figure 6,in which the voltage of the bus 4 and 8 is depicted for the ba ca and for a 20 and 33 per cent penetration level of uncontrolled power electronics. It can be en that the voltage drop becomes increasingly vere when more distributed generation is connected to the system. This may lead to other problems, such as tripping of generators and/or loads.Therefore, it might be questionable if the current operating practice, in which power electronic converters are disconnected in ca of a voltage drop in order to protect the miconductor switches and to fulfill utility requirements, is optimal in the long run if the penetration level of this technology will increa.
The oscillation duration time shows substantial differences between uncontrolled and controlled power electronics interfaces. This might be caud by the governor models fitted to the synchronous generators, as can be concluded from figure 7. In this figure, the rotor speed deviation of generator 9 is depicted for the ba ca and for a 20 and 33per cent penetration of both uncontrolled and contr
olled distributed generation bad on power electronic converters.It can be en that in ca of uncontrolled distributed generation, it takes a long time until the rotor speed stays within a bandwith of 1 10-4 p.u. as required according to the definition. This is not only caud by the oscillations, but as well by the long time it takes until a new equilibrium has been reached. This may be caud by the synchronous generator governor model and its parameters and will perhaps be different when governor models that are able to change the mechanical power more rapidly are ud. Further rearch must therefore be devoted to the impact of uncontrolled power electronics on power system transient stability.
The figure also shows that when controlled power electronics is connected, the oscillations damp out quickly, which is in
agreement with results prented in the literature [2].
