IEEE 34 bus intermittent sources.pdf

7/25/2019 IEEE 34 bus intermittent sources.pdf

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Modeling of a Small Distribution Grid with Intermittent Energy

Resources Using MATLAB/SIMULINK

Liviu Mihalache, Member, IEEE, Sindhu Suresh, Member, IEEE,

Yaosuo Xue, Member, IEEE, and Madhav Manjrekar, Member, IEEE

AbstractRenewable Energy resources are growing exponen-tially, demanding for more studies in the field of integration.The penetration of these resources has grown to a level whichdemands structural and functional changes to the grid systemto accommodate variable energy resources resulting in smartgrid topology. Behavior of the grid depends upon the type ofintermittent energy resources being added, point of couplingimpedance and the load distribution along the system. As mostof these integration activities take place on the distribution sideof power network, it is desired to conduct a comprehensiveanalysis at the low voltage level. This paper presents the resultson modeling and dynamic analysis of a distribution grid systemwith different levels and types of renewable energy resources

using IEEE 34 bus system as a candidate testbed. The completesystem is modeled and analyzed using Matlab/Simulink.

Index TermsConstant Load, Distributed Generation (DG),Intermittent Energy Resources, IEEE 34 Bus, Split Phase Trans-former

I. INTRODUCTION

Twenty-first century is witnessing a revolution of a fullycontrolled, flexible grid system with bidirectional power and

information flow called Smart Grid. This architecture provides

the flexibility to adapt to a changing mix of demand-side

resources, including varying load, dispatchable distributed

generation and storage, as well as variable-output local gener-ation such as wind and solar. In US, distributed generation

system (DG) constitutes 1.6% of the summer peak as of

2007, and is projected to grow at an exponential pace [1]

that demands for a total analysis of the system dynamics.

Modern grid system can be considered as a combination of

power electronic devices and intermittent energy resources

alongside with the old downstream flow grid. For achieving

bidirectional power flow in the distributed grid system, it is

required to integrate more solid state switching elements [2].

Power electronic devices with sophisticated control circuitry

are able to enhance the performance of the grid by improving

its reliability and stability. Unlike the fossil fuel generation

system, most of the distributed energy resources (DER) are

intermittent in nature which calls for a detailed analysis of

its impact on the grid system, especially when wind power

combines with solar and storage devices [4], [7], [9]. To

understand these characteristics in detail it is preferable to use

a small distribution grid model as its size can be modeled

in Matlab without a significant increase in the simulation

-

Authors are with Siemens Corporate Research, Princeton, New Jersey -

08540, USA. (email: [email protected])

Fig. 1. Conceptual architecture of the proposed system.

and computational time. Several studies indicate that IEEE-

34 bus system with application-specific modification would

be a suitable testbed for the analysis of DG [3], [5], [6], [8].This paper presents the results of modeling and analysis of

a 2.5MW grid system with an emphasis on the nature of the

integrated energy resource, the point of coupling, penetration

level and the nature of the load. Conceptual architecture of the

candidate system is given in Fig. 1. The dotted lines indicates

the work which will be reported in the future.

This paper is organized as follows. Section II describes the

modeling of the IEEE 34 bus system with split-phase AC and

the results of the dynamic studies are then compared with

equivalent results of the IEEE benchmark system. Modeling

of the system components such as load, DG and converters

are presented in section III. Section IV presents different casestudies and corresponding results. Lastly, section V presents

conclusions and lists the scope for future work.

II. MODIFIED IEEE 34 NOD ET EST FEEDER

The testbed chosen for distribution system analysis is the

modified version of standard IEEE 34 bus test feeder as shown

in Fig. 2. This feeder mimics an actual feeder in Arizona

with all its electrical characteristics. From a macroscopic level

of conducting operational studies at certain point of coupling

(PCC) due to the integration of renewable energy resources

978-1-4577-1002-5/11/$26.00 2011 IEEE


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recommends a modified model. The original feeder is mod-

ified with a center-tapped transformer of 120/240V, which

is required for the residential type connection requirements.

Constant current and power models are detailed in section

III. The original IEEE 34 bus system model was developed

Fig. 2. Modified IEEE 34 bus system.

before the exponential increase of interconnection of all DGs

justifying the need for modification. The characteristics of the

newly developed testbed are given below. The (N-0), (N-1)

and (N-2) contingency analysis as recommended by NERC

for transmission systems [12] were adopted for the distribution

system reliability analysis. All the three cases were analyzed

in detail before integrating the renewable energy sources and

are detailed in this section.

The feeder is modeled by using three-phase mutual

inductance blocks where the mutual coupling between

phases are calculated using Carsons equation [13]. The

feeder model is given in Fig. 3

Fig. 3. Mathematical model of the feeder.

Constant power and constant current blocks are intro-

duced and will be discussed in detail in section III.

Distributed loads are connected at the center of each

segment. This leads to an increase in the number of

feeders in the Matlab-Simulink model.

A single-phase center-tapped transformer of 120/240 V

is introduced into the system between the lateral nodes

and the loads are distributed accordingly. A long cable of varying length is introduced on the

secondary side of the transformer to observe the impact

of voltage fluctuations using Carsons equation [14].

A. Case Study (N-0) No Fault

For the (N-0) analysis, the secondary of the center tapped

transformer is subjected to balanced and unbalanced load

conditions. A long cable introduced in the secondary side of

the transformer creates voltage drop impact. Fig. 4 gives the

transient response for both center tapped transformer secon-

daries with an intermittent load switched on at 0.2 seconds.

For given cable length and load, the secondary voltage drops

to 103.5V which is below the ANSI standard range A-normal

steady state voltage (110V) and is also below the B-Emergency

steady state value (107V).

Fig. 4. Transient response with unbalanced load on the secondary of thetransformer.

B. Case Study (N-1) With Single Fault

Contingency analysis is carried out by creating single line

and three phase fault with normal clearing time. This is tested

under different scenarios, for instance injecting a three phase

short circuit downstream from the center tapped transformer

at node 858 and removing the fault after 10 cycles. Some

scenarios especially the ones which have a drastic effect on

the grid system are explained in detail here. A single-phase

to ground fault is introduced at node 890 and its effect is

analyzed at node 844 where the power factor capacitors are

installed. A fault at node 890 has a smaller impact at node 844

as compared to the case where the fault is located near by. This


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is due to the large distance from the fault and the presence

of 500kVA transformer on the 4.16kV line, which dampens

out the oscillations. Moreover, the power factor capacitors

are discharging through a large feeder impedance. Current of

phase A does not experience a large overshoot and the voltage

dip on phase A and swells on other two phases are limited as

shown in Fig. 5.

Fig. 5. Single-phase fault to ground (Node 890, phase A).

The effect of single phase line to ground fault is simulated

at the same node but on different phases other than A. An

asymmetric fault on phase C results in an increase in voltage

on other two phases. Because the center-tapped transformer is

located on phase A, the voltage boost cancels out the voltage

drop due to the secondary side line impedance, however, this

may not be the situation with light load.C. Case Study (N-2) With Multiple Fault

Test cases were generated to see the impact of losing the

largest consumer with and without fault on the grid system.

As expected, a temporary loss of a load results in an increase

of voltage, especially for the node 844 with power factor

capacitors, leading to voltage levels of more than 1p.u. A

fault occurring along with loss of an element has a significant

impact leading to an emergency state. Test cases were con-

ducted for different combinations. Nodal voltage matrices were

cross-checked with the results from the standard softwares

for distributed generation like Wind-Mill. Two scenarios with

multiple faults are discussed in detail here. In the first case,

the short circuit at 4.16kV transformer on phase C is followed

by a fault on node 858 at phase A. The effect on node 844

where the power factor capacitors are located is shown in

Fig. 6. In this case both faults on the system overlap for a time

period from 0.3 to 0.515s. Presence of the short circuit fault on

two different phases lead to an over-voltage on the unaffectedphase. In this particular case, due to large impedance, the

transients are dampened out fast. Fig. 7 shows the voltages

on the secondaries of the center tapped transformer due to

a combined loss of the largest load (150+j75) at node 890

and a single phase fault on node 858. Neither the loss of an

element nor the short circuit does not lead to a voltage drop

below minimum utilization voltage set by the ANSI emergency

standard steady state range of (107V). The root mean square

voltage on the secondary of the transformer varies dynamically

under different system conditions and is represented on the

Fig. 7 with various modes the system has gone through.

Fig. 6. Single-phase fault at 4.16KV transformer (phase C) and at node 858(phase A).

III . MODELING OF THES YSTEMC OMPONENTS

Dynamic behavior of the distribution grid system is an-

alyzed by introducing constant power and current load as

specified in the IEEE 34 bus system model along with the

intermittent generating units. This in turn helps to understand

the effect of adding renewable power sources to a grid. The

blocks modeled for these are detailed in this section.


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Fig. 7. Loss of an element followed by a single line ground at node 890.

A. Constant Load Model

Matlab/Simulink does not have constant power and constant

current blocks. The static load model that represents the power

relationship to voltage as an exponential equation is [10],

usually expressed as in the following form.

PL= PLoV (1)

QL= QLoV (2)

Where, is the active power exponent, is the reactive power

exponent, PL0 is the active power operating point, QL0 is

the reactive power operating point, V is the per unit value of

voltage. The current absorbed by load is given by the equation

IL=(PL jQL)

(3)Vnode(3)

the exponents are set to 0,1 the load can be represented as

constant power and current respectively. The Phase Locked

Loop was modified in order to avoid use of algebraic loops

in Matlab. The model block and the simulation results for

constant current are shown in Fig. 8 and Fig. 9 respectively.

The constant power model block is shown in Fig. 10.

B. Wind Model

A 10% (260kW) wind power source is modeled based on

an existing Simulink wind turbine model. The wind speed

is set such that the power generated is 260kW. The main

parts of the system are the wind turbine, the doubly-fed

induction generator, the back to back PWM converter and

the electromagnetic filter. This model is incorporated with

additional control circuitry for grid and rotor side of the

converter along with the wind turbine.

The reactive power demand for the wind turbine is assumed

to be zero for the initial phase of study. Electromagnetic

torque controller [11] has a closed loop algorithm which

controls the phase voltage to the rotor side based on the

Fig. 8. Constant current mode.

Fig. 9. Constant current model output.

wind speed, active and reactive power, direct and quadrature

axis current and voltage. Grid side controller regulates the

voltage of the DC link. It consists of voltage regulator on

the outer loop and current regulator assisted by feed forward

loop which regulates the grid side voltage generated by the

converter. Electromagnetic filters with Q value of 50 are used

for removing the harmonics of the order 3 and above. Theturbine and drive train is a closed loop system which controls

the torque generated by the drive mechanism based on the

nominal speed of the generator. The output of the wind based

converter is given by Fig. 11.

C. Photovoltaic Power

A 10% PV power source is developed assuming maximum

power point tracking. The PV inverter is current controlled at a

2.6KHz switching frequency and uses Direct-Quadrature axis

(DQ) PI regulator in a rotating frame synchronized with the

grid via Phase Locked Loop. The base power of the inverter is


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Fig. 10. Constant power model.

Fig. 11. Wind power 260kW.

500kW and delivered power to the grid is 250kW depending

on the control circuit set point. The model is given by Fig. 12.

IV. ANALYSIS OF R ENEWABLEI NTEGRATEDDISTRIBUTIONG RI D S YSTEM

A. 10% Wind Power (N-0) No Fault

The wind power is connected to Node 890 which has

the maximum voltage drop, nearly 10%. Penetration of wind

power at node 890 provides an almost 8% increase in the

voltage profile. At this node the addition of the wind power has

a significant effect to the nearby nodes leading to an increase

in voltage of about 1.05p.u. and it fades away as the nodes

are located further down. For some of these remotely located

nodes, the voltage boost is only between 1-2%. If possible, it

Fig. 12. Photovoltaic power model.

is better to integrate wind resources where the voltage is at its

minimum, in order to provide maximum local compensation

and minimum disturbances to other nodes. If the load at node

890 becomes lighter, the voltage profile will increase signif-

icantly leading to an over-voltage situation without voltage

regulators. Similar simulation was carried out by connecting

the wind power at node 848. In this particular situation the

impact of addition of wind power has less significance on node890, as expected. The model is able to detect the voltage on

secondary 1 of the center-tapped transformer as it approaches

minimum utilization voltage according to ANSI standard A

steady state with wind power on the grid system.

B. 20% Renewable Energy from Wind and Solar (N-0) No

Fault

Node 890 was chosen again due to the maximum voltage

drop, nearly 10%. 20% penetration leads to an increase in

voltage around 1.15p.u. nearly doubling the effect with wind

power alone. In general, the same conclusions from the previ-

ous case remain valid. This scenario analysis clearly indicates

that voltage regulators are mandatory in order to prevent over

voltage. As the time required for the tap-changers to change is

more it leads to a potential tripping of the system due to failure

in triggering the critical protection devices. The total active

power at the input of the IEEE 34 bus system is reduced due to

the reduction in power injected by the 20% renewable energy

source. In this situation if the reactive power of the system

remains relatively constant can lead to a lower power factor

of operation. In such situations reactive power compensator

are recommended to keep the system power factor at a higher

level. Addition of 20% renewable energy at node 890 has little


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effect under normal conditions on the unbalanced load of split

phase AC transformer located between node 818 and 820. The

model is able to detect that voltage on secondary terminal

which approaches minimum utilization voltage according to

ANSI standard A - steady state voltage, shown in Fig. 13.

Fig. 13. Voltage characteristics at the secondary terminal of center tappedtransformer with 20% renewable energy at node 890.

Table. 1 shows the characteristics of the grid when renew-

able sources are coupled at node 890.

TABLE IPHASE A VOLTAGE NODES COMPARISON WITH RENEWABLE SOURCES AT

NODE 8 90 O F IEEE 34 BUS SYSTEM.

Node(phase A) IEEE 34 with 10% Wind 20% (Wind + Solar )

860 1.0303 1.0519 1.0741

836 1.0309 1.0532 1.0734

840 1.0317 1.0532 1.0734

862 1.0303 1.0532 1.0734

890 .9015 .977 1.0514

848 1.0310 1.0539 1.0741

832 1.0359 1.0588 1.0796

834 1.0310 1.0539 1.0741

844 1.0303 1.0532 1.0741

C. Fault Analysis with 10% Wind Power (N-1)

A three-phase short circuit fault downstream (node 858)

leads to a collapse of the transformer voltage 50% below the

ANSI standard A - normal steady state and the emergency

steady state irrespective of the presence of a 10% wind power

at 890, as seen in Fig. 14. The same fault leads to a more

severe fault condition at node 890. Under voltage situation

will activate the protection device to trip and disconnect the

wind source from the grid system. Three phase fault is injected

into different nodes and the impact of the system as a whole is

studied. The whole procedure is carried out for the analysis of

single phase fault at node 858, which is close enough to 890.

The asymmetrical fault leads to voltage increase in other two

phases there by activating both under-voltage and over-voltage

state leading to anti-islanding mode.

Fig. 14. Three phase fault at node 858 as reflected at the center-tappedtransformer with 10% wind power.

D. Fault Analysis with 20% DG Power (N-1)

The procedure used for the 10% wind scenario is now

extended for 20%. The characteristic behavior of the grid

system remains as that of the 10% integration but the differ-

ences in the severity or gravity of operation of the protection

devices increase due to the magnitude of change in the system

behavior. In this section we will be detailing more on single

phase fault at node 858. The effect of single phase short

circuit to ground fault at node 858 on phase A is reflected

at the secondary of the center tapped transformer. This faultleads to a collapse of the transformer voltage below the

minimum utilization level of ANSI standards both for normal

and emergency state, shown by Fig. 15. Impact of the fault on

node 890 is not so severe as that with 10% wind power but it

is significant enough to cause the protection device to trigger.

A typical scenario of losing a customer at node 890 is

simulated with 20% wind power. Removal of load triggers

over-voltage mode and the turbine is disconnected from the

system. When the load of 450kW is reconnected there are

heavy oscillations and over voltages for seconds which again

lead to the failure of interconnection.E. Fault Analysis with 10% Wind Power (N-2)

Multiple fault contingency analysis is carried out for differ-

ent combinations. Loss of element followed by short-circuit

fault, multiple short circuits at different nodes at the same

time, loss of an element and fault are the different scenarios

analyzed. Here loss of an element and single line to ground

fault is described in detail. Fig. 16 shows the impact of

simultaneous unusual incidents at node 890 namely losing a

customer at node 890 and a single phase line to ground fault

on phase C. Removal of the load results in over voltage which


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Fig. 15. Characteristics at the center tapped transformer with a single phasefault at node 858 with 20% renewable energy at node 890.

in turn will trigger the anti-islanding mode. Voltage drop onphase C due to the short circuit will lead to the disconnection

of wind power from the grid.

Fig. 16. Loss of an element and single line to ground fault as reflected tothe wind power source.

F. Fault Analysis with 20% Renewable Energy (N-2)

Similar scenario as explained above for 10% wind power iscarried out for 20% integration. To have a better understanding

of a different fault mode, the loss of an element followed

by single-phase short-circuit is depicted in Fig. 17. Once

the load is removed, the voltage swelling may trigger over

voltage protection and disconnect the wind power source.

Heavy oscillations and over voltage when the load is connected

back can again force the DG to go off-line.

G. Low Voltage Ride Through (LVRT) using the PV Inverters

Depending on the grid code system the renewable generator

should be able to withstand voltage fluctuations for small time

Fig. 17. Characteristics at the node 890 with loss of an element followedby single line fault with 20% renewable energy at node 890.

periods. Although mainly designed to provide active power,

these converters can also be used to provide additional utility

functions also known as ancillary services. The IEEE standard

1547 defines ancillary services as frequency regulation, voltage

regulation, reactive power supply, spinning and non spinning

reserves. Reactive power control is the most important among

the listed ones as it helps to protect the loads from voltage

fluctuations.

The extent to which the PV inverter can stabilize the voltage

fluctuations depends on different characteristics like maximum

current rating of the inverter, coupling impedance, inverter linefilter, existing load at the point of coupling and the ability to

detect the voltage dip and swell in a short period of time [15].

For a given coupling impedance and existing load the change

in reactive power required to compensate the voltage variation

is approximated by the equation given below by equating Vto zero and rearranging for the reactive power injection.

V P.R

V Q.

X

V (4)

Q P.R

X (5)

Addition of more inverters can increase the voltage boost

and totally compensate the grid voltage variation. Fig. 18

shows how the multiple inverter system compensates the

voltage drop. Grid voltage starts dropping at 0.2s. Inverter

1 with a rating of 10% of the total grid load is switched on

at 0.4s which compensates for the drop to an extent but not

completely. After a time interval of 0.5s, the second inverter is

activated along with the first one leading to complete voltage

compensation.

The PV inverter has two modes of operation under LVRT.

Fig. 19 shows a 90 degree phase shift between voltage and

current confirming the switching of the PV inverter from active

mode to reactive mode. Under normal mode of operation, the


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Fig. 18. Low-voltage Ride Through with grid voltage supported by reactivepower injection (Two Inverter).

Fig. 19. Characteristics of current and voltage under LVRT for the PVInverter.

PV inverter acts like an active power module, while under grid

fault condition the PV inverter switches in the grid support

mode and delivers only reactive power. Fig. 20 details the

operation and transition of the PV-inverter from active to

reactive mode.

Fig. 20. Power delivery of PV inverters under LVRT.

V. CONCLUSION

This paper reports the work using Matlab/Simulink to model

a small distribution grid with up to 20% renewable energy

resources (10% wind and 10% solar) based on a modified

IEEE 34 bus system which included a center-tapped low

voltage transformer typically used in residential areas. The

model is then analyzed for various fault scenarios with and

without the renewable 20% energy sources and their impactin various points across the IEEE 34 bus system. The model

developed was also used to analyze a LVRT scenario with

multiple PV inverters. Overall the model has proved a viable

tool that can be used for small distribution grid analysis. It can

also be utilized as a test environment to study various detailed

control strategies applicable to both wind and solar based

PWM converters. Future work will be extended to the design

and application of Distributed Grid Management Controller

System based on DSP and FPGA platforms.

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