GENERATORS AND LOADS MODELS TO INVESTIGATE...

23rdInternational Conference on Electricity Distribution Lyon, 15-18 June 2015

Paper 0780

CIRED2015 1/5

GENERATORS AND LOADS MODELS TO INVESTIGATE UNCONTROLLED

ISLANDING ON ACTIVE DISTRIBUTION NETWORKS

Paolo MATTAVELLI,

Riccardo SGARBOSSA

Roberto TURRI

University of Padova – Italy

[email protected]

[email protected]

[email protected]

Gianluca SAPIENZA,

Giovanni VALVO,

Cristiano PEZZATO,

Alberto CERRETTI

Enel Distribuzione – Italy

[email protected]

[email protected]

[email protected]

[email protected]

Ettore DE BERARDINIS

CESI – Italy

[email protected]

ABSTRACT

The number of distributed energy resources (DERs)

connected to low voltage (LV) distribution networks has

increased the concern on the unintentional/uncontrolled

islanding operations. To evaluate the effect of the

Distributed Generation (DG) on the uncontrolled

islanding events in LV network portions independent

studies have been performed, considering different LV

load dynamic characterization and inverter based models

for a correct evaluation of the islanding issue. P/f and

Q/V capabilities and regulation laws required by the

DERs stated by the most relevant standards have also

been considered. Field measurements, simulations in

different simulating environments as Real Time Digital

Simulator (RTDS), Simulink/Power System Blockset and

DIgSILENT Power Factory are reported.

INTRODUCTION

To evaluate the effect of the most relevant capabilities

and regulation laws required to distributed energy

resources (DERs) [1-2] on uncontrolled islanding in

distribution networks, two completely independent

studies have been performed, based on:

1) field measurements to assess LV load dynamic

behavior;

2) digital simulations performed in different simulation

environments (RTDS, Simulink/DigSILENT).

Suitable models for the inverter based generators and for

the loads are essential for a correct evaluation of the

phenomena. This need has recently led to the Joint

Working Group C4/C6.35/CIRED: “Modelling and

dynamic performance of inverter based generation in

power system transmission and distribution studies”.

Considering that inverter based models in standard

libraries do not include the several new DER capabilities

and may be not adequate for dynamic simulations (EMT-

time domain), additional models have been developed.

More precisely:

• University of Padova realized models in the time

domain (Power System Blockset), based on switching

inverter operation. Subsequently, porting of the model

in an equivalent RMS model to be adopted in

DigSILENT was performed by simplification firstly

to average models and then to RMS-phasor models, in

order to be used in dynamic simulation, together with

other network components (load, generator, etc).

• CESI and ENEL Distribuzione implemented in a Real

Time Digital Simulator (RTDS) digital models of PV

inverters, synchronous generators, loads and

protection systems.

The correct dynamic representation of the LV load

depending on the voltage and the frequency [3-4] is

important for the assessment of unintentional islanding

operation. Considering the data in [3-4] outdated, ENEL

decided to set up two field measurement campaigns in

order to assess the current typical LV equivalent load

behavior and to define updated sets of load model

parameters. In any case, in order to cover different

scenarios for uncontrolled islanding operation, analysis of

islanded operation has taken into account both load

representations, i.e. the old ones with the coefficient

reported in 1993 [3-4] and the new ones as resulting from

the two field measurement campaigns.

STANDARDS AND CONNECTION RULES

Recently, reference technical rules have been revised by

standards, introducing for DERs protection interface

systems and local control strategies, in order to integrate

the growing number of DGs [1-2]. Permissive thresholds

for voltage and frequency have also been introduced in

compliance with the Fault Ride Through (FRT)

philosophy [3- 4], as shown in Fig. 1.

a) b)

Figure 1-a) Frequency permissive and restrictive thresholds.

b) voltage FRT thresholds.

Moreover, local controls set by standards are required:

active and reactive power exchanged by DGs are function

of the frequency and voltage levels, as shown in Fig. 2,

introducing the droop characteristic Q=f (V) and P=f (f).

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These control strategies are aimed at facilitating the

electrical system stability but they can potentially

increase the risk of uncontrolled islanded network

operation.

a) b)

Figure 2 - a) Inverters could be required to participate to

the voltage regulation using Q injections from a minimum of

-0.4843 to the maximum of 0.4843 with respect to the rated

power; b) gradual limitation of the generated P according to

an over-frequency statism so=2.4% [1-2].

LV LOAD CHARACTERIZATION

For the load static and dynamic characteristics, the

approach proposed in [3-4] has been adopted, where the

active Pout and the reactive Qout load powers depend both

on frequency and amplitude of the voltage waveform,

i.e.:

{𝑃𝑜𝑢𝑡 = 𝑃0 {[∆𝑓 (

𝑘𝑝𝑓

1+𝑠𝑇1) + 1] (

𝑢𝑝

𝑢𝑜)

𝑘𝑝𝑢}

𝑄𝑜𝑢𝑡 = 𝑄0 {[∆𝑓 (𝑘𝑞𝑓

1+𝑠𝑇1) + 1] (

𝑢𝑝

𝑢𝑜)

𝑘𝑞𝑢} (1)

where 𝑢𝑝 = [∆𝑢 (1

1+𝑠𝑇1) + 𝑢𝑜] and Po and Qo are the

active and reactive power respectively absorbed at

nominal frequency f0 and nominal amplitude u0.

Moreover, kpf , kpu, kqf and kqu are parameters that describe

different type of loads (residential, industrial, agriculture)

and the voltage and frequency deviation from their

nominal parameters are denoted as ∆u = u − uo and ∆f =

(f − fo ) /fo , respectively.

The above-mentioned parameters describe the variation

of the load as a function of voltage and frequency,

therefore the campaign has been realized by using a

suitable synchronous generator able to supply the LV

network independently from the MV/LV transformer, as

shown in Fig. 3.

Figure 3 – Test field realization

Tests have been carried out at fixed voltage to determine

kpf , kqf, at fixed frequency to find kpf ,kqf, according to the

following equations:

𝑘𝑝𝑢 =log

𝑃

𝑃0

log𝑢𝑝

𝑢0

𝑘𝑞𝑢 =log

𝑄

𝑄0

log𝑢𝑝

𝑢0

𝑘𝑝𝑓 =1

𝑓∙ (

𝑃

𝑃0− 1) =

𝑓0

𝑓 − 𝑓0∙ (

𝑃

𝑃0− 1)

𝑘𝑞𝑓 =1

𝑓∙ (

𝑄

𝑄0− 1) =

𝑓0

𝑓 − 𝑓0∙ (

𝑄

𝑄0− 1)

(2)

where P0, Q0, U0, f0, are the initial active power reactive

power, supply voltage, frequency, for each test.

The initial and final values for the active/reactive power

at the corresponding values for the voltage or frequency

have been determined by interpolating several

measurements, for the kpu, kqu characterization, as shown

for the sake of example in Fig. 4.

Figure 4 – Example of P and Q variation as a function of up

Table 1 shows the mean values measured during these

tests. The values are quite different from the IEEE

campaign and they refer to a limited number of tests. This

difference may be justified by the different type of

electrical equipment currently used.

Residential loads Other loads

kpf

0.7 ÷ 1.0 -0.8 2.6 -0.7

kqf

-2.1 ÷ -1.5 -6.7 1.6 -4.0

kpu

0.9 ÷ 1.7 1.2 0.1 1.7

kqu

2.4 ÷ 2.6 6.1 0.6 5.3

Table 1 – Field tests results (average values).

V20/0.4 kV

G~

Other lines

Residential loads

Other load

MV Network

Synchronous generator


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The T1 parameter has been determined by applying step

voltage variations using the tap changer of the Primary

Substation HV/MV transformer. T1 value has been

confirmed to be about 0.1 s.

INVERTER AND LV NETWORK

DISTRIBUTION MODEL

DIgSILENT inverter model RMS vs

Simulink/Time domain model (Case A)

In this study a detailed inverter model, with embedded

voltage and current regulations typical for photovoltaic

applications, has initially been developed. In order to

study the uncontrolled islanding operations the average

values and the phasors representations are superior to the

detailed model in terms of computational requirements

and numerical convergence issues. Although different

conversion module configurations can been considered

[5], the analysis has focused on the single-stage system

and on the two level Voltage Source Inverter (VSI). A

three-phase inverter with EMI filter in addition to the

inductive output filter, as shown in Fig. 5, has been

considered.

Figure 5 Three phase inverter with L1,2,3 inductive filter and

EMI filter composed by Lf1,2,3 ,Cf1,2,3 and RCf1,2,3 as dumping

element of the filter (if present).

The switching function approach has been initially used

to fully represent the switching operation of the inverters,

as depicted in Fig. 6. However, for most of the islanded

analysis, the detailed switching operation has been

neglected, in order to reduce simulation time and

facilitate convergence, and an average model, where each

state variable is averaged over the PWM switching

period, is adopted, as generally shown in Fig. 6. The

averaging model is more efficient and with less numerical

convergence problem than the detailed switching model.

Moreover the presented average model has been

transferred in the d,q coordinates using a Park

Transformation, and completed with the required current,

voltage and Phase-Looked Loop (PLL) regulations and

with the required protection functions. At last we obtain a

dynamic model expressed in amplitude and phase of

voltages and currents instead of instantaneous sinusoidal

values.

Figure 6- Switching and average inverter model

The representation has been developed in

Maltab/Simulink environment and compared with

DIgSILENT software using RMS simulations. The

results show a good match between the two

environments.

With this modeling approach, the tested LV network is

shown in Fig. 7. It presents a DG unit, three loads, a

MV/LV Transformer and cables lines, whose lengths are

shown in Fig. 7, and characterized by R=0.164 Ohm/km,

X=0.0691 Ohm/km and B=185.35 µS/km. The switching

of the breaker starts the islanding events.

RTDS model (Case B)

After CESI residential loads characterization, a large 4-

wires unbalanced LV grid has been implemented by

ENEL-CESI in the Real-Time Digital Simulator (RTDS)

installed in the ENEL Smart Grid Test Center of Milano.

The RTDS allows to simulate a large number of power

ai

bi

civ

BC

C f 1 C f 2C f 3

RCf 1RCf 2 RCf 3

L1

L2

L3

L f 1

L f 2

L f 3

vAB

vCA

Figure 7 - LV distribution network used Case A


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system components and control model in real-time, in the

time domain, using the “Dommel algorithm”.

The LV grid, represented in Fig. 8, is composed by three

feeders (urban, rural, and urban-rural mixed feeder),

single and three-phase loads (with the caracteristics and

dynamics derived by the CESI load caracterization) and

18 PV inverter-based distributed generators (6 three-

phase 10 kWp, 3 three-phase 20 kWp, 4 single-phase 6

kWp, 5 single-phase 3 kWp).

Each generator is regulated in P,Q mode, where the P set-

point is generated using the P-f characteristic represented

in Figure 2 b). Indeed, the Q set-point is generated using

the Q-V characteristic represented in Fig. 2a. Generators

are synchronized to the grid voltage using a PLL and the

coupling to the grid is performed using a transformer

with short-circuit impedance equal to 6%. Finally, the

interface protection and the capability curve has been

modeled for each generator. The LV grid is fed, from the

MV grid, by a Dy11 MV/LV transformer. The grid

neutral wire is connected to the transformer secondary

winding neutral point. Lines are represented using the PI

section model, where the zero-sequence impedance takes

into account the neutral wire effect.

Figure 8 - RTDS model of the LV grid (Case B)

EXAMPLES OF SIMULATION RESULTS

In this work parametric analyses considering various

combinations of Q(V) and P(f) regulations have been

performed, taking into account the inertia of rotating

machines and the initial active and reactive power

unbalance, in order to identify the role of the different

parameters in determining islanding conditions. In the

following, few samples of the results obtained are

presented and discussed.

An Example of Results for Case A

The inverter model has been tested with dynamic load

models, including the residential and industrial model

parameters of [3]. Results show how uncontrolled

islanding operations occur in a LV network. Furthermore,

the influence of the standards regulation requirements

leads to islanding events starting from different power

balance conditions between loads and generator. Due to

space constraints, only results performed with

DIgSILENT Power Factory are reported here, but similar

results were obtained with the switching and average

Simulink models. The tests have been carried out

modifying the loads and inverter rated powers, in order to

analyze the islanded grid behavior under different initial

conditions.

Pg [kW] PL [kW]

Pg = PL 50 50.2

Pg < PL 50 59.1

Pg > PL 50 30.1

Fre

qu

ency

[H

z]

Time [s]

Vo

ltag

e p

.u.

Time [s] Figure 9 - Frequency and voltage at the LV Busbar 0.4 kV

Fig. 9 shows frequency and voltage of the islanded LV

grid portion between the standards imposed thresholds.

The uncontrolled islanded condition is facilitated by the

regulation of active and reactive power, but also due to a

regulation effect of the dynamic load model considered.

Examples of Results for Case B

RTDS simulation results are shown in Table 2. In this

case the load model parameters are based on the updated

measurements reported in Table 1.

It may be observed that:

1. With only PV static generation, without any Q(V) and

P(f) regulation, islanding is not possible even in case

of small power unbalance. The same occurs, if part of

PV generation is substituted by synchronous

machines.

2. With only PV static generation, with Q(V) and P(f)

regulation with hysteresis (according to CEI 0-21

Italian rules), islanding is not possible even in case of

small power unbalance. Instead, if hysteresis is

excluded for P(f) regulation (like requested in RfG

rules from ENTSOE), stable islanding operation is


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possible only if the generation is greater than the load.

This is possible because the P(f) may reduce the

generation power, according to the frequency growth,

until a new balance condition will be restored. The

same occurs, if part of PV generation is substituted by

synchronous machines.

3. With only PV static generation, with or without any

Q(V) and P(f) regulation, transient islanding is always

possible even in case of small power unbalance. In

this case the generation could be disconnected before

fast reclosing (operated on MV side) only if narrow

frequency thresholds are activated. On the other hand,

if part of PV generation is substituted by synchronous

machines, it is possible to disconnect the generation

during the fast reclosing only if a large power

unbalance is present. Again this phenomenon is due to

the inertia increment of rotating machines.

Of course, the above mentioned results have to be

considered as qualitative and not quantitative due to the

variability and uncertainty of all factors influencing the

transient to reach the island operation and during the

island operation itself: a) V and f measurement methods;

b) actuation periods of Q(V) and P(f) regulations; c)

loads behavior in function to V and f; d) power factor; e)

generators and loads inertia. These factors are not

completely determinable because, for some of them, a

reference to standards does not exist.

CONCLUSIONS

Before new network code on Requirements for

Generators (RfG), “narrow” band frequency thresholds

hindered stable island and guaranteed, with very high

probability, disconnection before fast reclosing operated

by the circuit breaker at the beginning of MV feeder.

After RfG requirements (partly introduced in some

Countries), the possibility of uncontrolled islanding

operation has significantly increased.

The conclusions of the present study are, anyway,

qualitative, because studies are valid only for the used

loads models and generators models; in particular, the

different dynamic response of the loads intentionally

adopted by the two studies, highly influences the

behaviour of the phenomena. In particular, it should be

better investigated the dynamic characterization of loads

(P and Q variation function of V and f, inertia, cos , etc)

in order to achieve sufficient level of details, not below

that of IEEE 1993 report [3] which is old and focused on

the creation of aggregations at HV level. With regards to

generators, different generators capabilities strongly

influence the system behaviour.

Finally, further potential influencing factors/elements are

not still fully investigated: fast voltage support, power

system stabilization, synthetic inertia, power factor

regulation [6], and energy storage systems. Such

elements should be included as well in dynamic studies,

in order to assess their effects on the system.

REFERENCES [1] CEI Comitato Elettrotecnico Italiano, “Standard-CEI 0-16,

Reference technical rules for the connection of active and

passive consumers to the HV and MV electrical networks of

distribution Company”, 2011-12.

[2] CEI Comitato Elettrotecnico Italiano, “Standard-CEI 0-21,

Reference technical rules for the connection of active and

passive users to the LV electrical Utilities”, 2011-12.

[3] IEEE Task Force on Load Representation for Dynamic

Performance, 1993 “Load Representation for dynamic

performance analysis”, IEEE Transactions on Power

Systems, Vol. 8, No.2.

[4] K. Tomiyama et al., 2003, “Modeling of Load During and

after System faults based on Actual Field

Data”,Proceedings IEEE Power Engineering Society

General Meeting

[5] Y. Xue, L. Chang, S. Bækhøj Kjær, J. Bordonau, and T.

Shimizu, 2004, " Topologies of Single-Phase Inverters for

Small Distributed Power Generators:An Overview", IEEE

Trans. on Power Electronics, Vol.19,No. 5, pp. 1305 - 1314.

[6] L. Cocchi , A. Cerretti et al. “Influence of average power

factor management on distribution network power losses"

CIRED 2015 Lyon conference.

Table 2 – RTDS simulation results.

Generation

type

No regulation

Q(V), P(f)

Regulation Q(V),

P(f) with histeresys

CEI 0-21

Regulation Q(V),

P(f) without histeresys

RfG

Stable island

NO NO YES (soglie strette non attive)

P%>0

NO NOYES (soglie strette non attive)

P%>0

Transient island,

not permanent

operation

YES

• Narrow tresholds enabled:<0.5 s

• Narrow tresholds disabled: <0.7 s, scatto

V

YES

• Narrow tresholds enabled: <0.6

s,

• Narrow tresholds disabled: <5 s,

trip V

YES

• Narrow tresholds enabled: <0.6 s

• Narrow tresholds disabled:<5.2 s,

large treshold trip or trip V

YES


• Narrow tresholds disabled: <1 s, scatto V

YES



large strshold trip

YES

• Narrow tresholds enabled:<3 s


large strshold trip

Disconnection

before FR

(0.6 s)

Frequency narrow tresholds if P%> 0%Frequency narrow tresholds if

P%> 10%

Frequency narrow tresholds if P%>

10%

Frequency narrow tresholds

If P%> 45%


If P%>99%


if P%> 99%

+

+

+

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