C I R E D 20th International Conference on Electricity Distribution Prague, 8-11 June 2009

Paper 0892

CIRED2009 Session 5 Paper No 0892

POWER FACTOR, VOLTAGE VARIATIONS AND LOSSES: A DETAILED ANALYSIS OF THE ITALIAN MV NETWORKS

Maurizio DELFANTI Marco MERLO

Mauro POZZI Marco Savino PASQUADIBISCEGLIE Luca LO SCHIAVO

Politecnico di Milano – Department of Energy - Italy AEEG - Italy

mauro.pozzi@polimi.it lloschiavo@autorita.energia.it

ABSTRACT1 This paper reports on a study, performed on a significant sample of Italian MV networks, aimed at the estimating the benefits deriving from the application of different limits (in terms of power factor to be respected by MV and LV final customers). The benefits obtained are evaluated w.r.t. network real losses and to power quality (in particular, supply voltage variations). Finally, a cost/benefit analysis is provided. The results show that a better reactive compensation could be a practicable way to enhance distribution networks performance. The real application of the new reactive constraints can be envisaged only within a suitable regulatory framework.

INTRODUCTION The attention to a better power quality, and to an efficient operation of distribution networks is becoming higher and higher all over the world; this interest is increasing also in Italy, where the high energy prices push in the direction of fostering efficiency as much as possible. This work describes a research aimed at simulating the behavior of a significant sample of MV distribution networks (representing about 5% of the whole number of Italian MV networks), in presence of different limits on the power factor of customers’ load. Currently, the power factor (hereinafter, pf) limit adopted for both MV and LV customers connected to the Italian networks is 0.9 (in terms of monthly average value). The study consists in simulating a year of operation for each available network (hereinafter, network indicates a set of MV feeders connected to a primary busbar, along with the relevant HV/MV transformer, see Fig. 1).

Fig. 1 – Network model adopted

M. S. Pasquadibisceglie is now with Autorità per l’energia elettrica e il gas. Views expressed in this paper do not necessarily reflect those of the institution he and L. Lo Schiavo work for.

The aim of the paper is to estimate the benefits deriving from the application of different limits (in terms of power factor to be respected by MV and LV final customers). The benefits associated to each study case are evaluated with a twofold aim:

• reduction in active losses; • enhancement of voltage regulation (i.e. fulfillment

of limits related to supply voltage variation). The paper is organised as follows: after this introduction, the grid dataset is described; then, we focus on the features of the procedure used to simulate the networks in presence of different load power factor limits. We present some results of simulations, for both the efficiency and the power quality issues. Finally, some concluding remarks are provided.

GRID DATASET AND MODEL OF THE ITALIAN MV DISTRIBUTION SYSTEM The model adopted for this paper is derived from a huge data sample, consisting of about 60000 MV busses, belonging to the MV lines fed by about 400 MV primary busbars (i.e. busbars directly fed by a HV/MV transformer, like the ones shown with red color in Fig. 1). As the overall MV Italian system consists of about 4000 MV primary busbars, the complete data set covers 10% of Italian MV networks [1]. The primary busbars selected are the same equipped with the monitoring system QUEEN (QUality of Electrical ENergy), promoted by the Italian Regulatory Authority for Electricity and Gas [2]. This will allow for some comparisons between the simulation and the real behavior of the network. The data were supplied from many distributing companies. ENEL Distribuzione is the most important distribution company in Italy: the records relevant to this company are about 50000 on a total number of 60000. The remaining data are relevant to other distributing companies, each of them serving more than 100000 LV customers. Considering that every MV line has a radial structure, the data were organized by means of a procedure of acquisition based on the link between a bus and the relevant upstream bus, up to the primary busbars. For the aim of this paper, a reduced data set was employed: it covers about 6.69% of the whole set of the Italian MV networks.

C I R E D 20th International Conference on Electricity Distribution Prague, 8-11 June 2009

Paper 0892

CIRED2009 Session 5 Paper No 0892

NEW POWER FACTOR LIMITS: IMPACTS ON LOSSES AND SUPPLY VOLTAGE As the data collected in [1] did not contain any information about load demand at every bus, for the purpose of this analysis, it was necessary to suitably model the yearly load profile of the load bus connected to each grid [3]. The sample exploited is made up by 265 networks; the overall number of MV feeders is 1767. As shown in Fig. 1, each load bus can represent:

• a MV customer (load type 1: in this case, the contractual power is given);

• a MV/LV transformer (load type 2, distribution secondary station, the rated power is given).

Once the load has been suitably modeled (both in terms of level of power consumption and yearly profile, as pre the simplified approach detailed in [3]), the study consisted in simulating a year of operation of each network, by applying:

a) a pf limit of 0.9 (base case – current situation); b) a pf limit varying from 0.91 to 1 (ten study cases).

As for the current situation, in particular, all MV customers have to respect the limit given in a), while for LV connections, such limit applies only for customers with a contractual power exceeding 25 kW. As a consequence, from a general point of view, it is possible to apply the new pf limits on both type 1 and type 2 busses. In a first stage, the enforcement of new (more stringent) pf limits seems easier to apply on MV customers only: MV consumers have already installed reactive compensation devices, that can easily be enhanced in order to match the new limits. Hereinafter, the study will consider only this application, i.e. the enforcement of new limits on type 2 load busses. The losses explicitly considered in the model, and calculated by the procedure, are:

1. losses on HV/MV transformer; 2. losses on MV feeders.

Further losses are separately considered, and determined by the conventional coefficient defined by the Regulator [4]

3. losses on HV (transmission, 380/220 kV; subtransmission, 132/150 kV);

4. losses on MV/LV transformers; 5. losses on the LV network of MV customers.

Figures related to losses types 3, 4, and 5 are estimated too, but they are considered separately: in fact, the primary aim of the study is to determine the improvement in performances and the economic advantage for the MV Distribution network in the presence of the new constraints on pf. In particular the economic advantage, if only type 2 busses are considered for the application of the new pf constraint, is related to points 1, and 2, that represent costs saved by the distributor. On the other hand, losses related to point 3 are of interest from a system point of view, while point 4 and 5 represent only a benefit for the customer. As for voltage regulation, a suitable procedure has been set up in order to simulate the operating conditions of each network, that were compared with field measurement data.

Voltage is controlled by means of a fixed voltage set-point in the MV busbar of the HV/MV transformer, where an on load tap changer is installed. In our simulation, the set-point is determined in the condition of maximum load: the minimum value of voltage at MV primary busbars, which allows to fulfil lower voltage limits in all busses of the MV feeder is chosen. This procedure is coherent with the practice adopted by ENEL,(the largest Italian DNO) on most HV/MV interfaces. The analysis consisted in calculating the difference between primary busbars’ voltage and the actual voltage at every MV bus, and in checking how this quantity is affected by the different pf adopted.

IMPROVEMENTS IN NETWORK OPERATION As previously stated, the improvements expected in network operation are mainly related to two aspects:

• reduction in active losses; • enhancement of voltage regulation (i.e. fulfillment

of limits related to supply voltage variation). Reduction in active losses; As for losses, a comparison is made between the base case level and the level achieved in each subcase. A preliminary observation on the above study cases (see) led to the opportunity of considering in a further detail only the test case related to pf = 0,95.

Fig. 2 – Percentage of energy saved with new pf limits

This opportunity can also be derived by observing similar experiences in other European countries. Focusing on the difference in real losses between base case and study case with pf = 0.95, Fig. 3a reports the percentage of energy that can be saved (average on the whole sample).

Fig. 3 – Percentage of energy saved with the new pf limit set at 0.95;

breakdown of the energy saved

C I R E D 20th International Conference on Electricity Distribution Prague, 8-11 June 2009

Paper 0892

CIRED2009 Session 5 Paper No 0892

Fig. 3b shows the breakdown of the energy saved among the different network components modeled. The data obtained by the calculation procedure will be used in a successive section for a Cost Benefit Analysis. Enhancement of voltage regulation As for voltage regulation, the metric adopted consists in calculating, for each feeder under analysis, the voltage drop between primary busbars and every single bus in the feeder itself in the base case (pf=0,9). DVi = Vpb – Vi

The highest DVi (DVmax) is chosen as an indicator of the “criticality” of the voltage regulation of the whole feeder. The following Fig. 4 depicts the behavior of all the feeders in the sample, by reporting 1767 DVmax (one for each feeder) ordered increasingly (blue points).

0 500 1000 1500

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14Differenza tensioni (Carico massimo)

Diff

eren

za te

nsio

ni [p

.u.]

Fig. 4 – DVmax for each feeder with pf = 0.9 and pf = 0.95

Red points represent, for every feeder, the same DVmax obtained in the study case with pf = 0.95: it can be seen that the new DVmax obtained do not differ very much from the base case. This is due to the fact that the base case voltage profile shows, in the vast majority of feeders, a very limited voltage drop. As expected, the enhancement in voltage profile (i.e., the diminution in the maximum voltage drop DVmax) is observed for the feeders that show a higher DVmax in the base case. The situation can be better described by the following Fig 5: data are ordered with the same criterion of Fig. 4, and highlight that the enhancement obtained is higher for the networks that are most critical (i.e., that are closer to voltage limits in the base case). This result shows that stricter limits on pf could be in principle helpful for the Distributor in order to postpone network investments: even if absolute enhancements obtained are small, they are more significant for critical situations.

0 500 1000 1500

0

2

4

6

8

10

12x 10-3 Delta (Carico massimo)

Fig. 5 – Enhancement (diminution) in DVmax for each feeder from pf = 0.9 to pf = 0.95. Data ordered on max voltage drop with pf=0,9

COST BENEFIT ANALYSIS The study of the improvements associated to a pf greater than 0.9 may be further emphasized by performing an economical analysis of the associated costs and benefits for the DNO. In Italy network operators are recognized losses up to a standard level given by the adjustment factors set by the Regulator [4]; the difference between actual and standard losses, either positive or negative, is entirely borne by the DNOs [5] and eventually included in the distribution tariff every four years. In case of adoption of a greater pf, the resulting losses reduction would reduce the aforementioned difference borne by the DNOs, thus representing an economical saving. For each considered pf applied to final customers, Fig. 6 provides the estimated economical savings. In Italy the mean energy price is about 85 €/MWh: for the sake of simplicity, since losses are mainly concentrated in peak hours, an energy cost equal to 100 €/MWh is assumed.

Fig. 6 – Economic savings vs. p.f

In general the investment to apply a greater pf would be entirely borne by final customers, while the economical advantages would benefit only the DNO, at least in the short time (distribution tariffs are determined on a four year basis). Nonetheless, since in the medium and long time the benefit would pass to final customers through the reduction

C I R E D 20th International Conference on Electricity Distribution Prague, 8-11 June 2009

Paper 0892

CIRED2009 Session 5 Paper No 0892

of the distribution tariff, a comparison between the benefits associated to losses reduction and the total investment made by final customers to improve their pf is possible. Fig. 7 presents the different investment costs required to increase the pf: static compensators are chosen due to their diffusion and modularity and an indicative cost of 35 €/kvar is assumed. This represents the marginal cost the customers would face to increase the reactive compensation already in place.

Figure 7– Investment costs in reactive compensators vs. pf

In general a detailed cost benefit analysis would entail the computation of discounted cash flow taking into account the interest rate. For sake of simplicity in this paper, for each pf the return time of the investment is estimated by dividing the reactive compensators costs by the global savings associated to the losses reduction. The results are depicted in Fig. 8.

Figure 8– Return time of the investment vs. p.f

The adoption of a pf limit equal to 0.95 (vs the actual limit = 0,9) would have an indicative return time equal to five years: assuming greater pf would not be useful since the return time underwent a relevant increase, making the global investment less convenient. Moreover it is worth noticing that the return time above presented does not take into account the potential further benefits associated to the losses reduction in customers’ grids. In particular MV customers usually install reactive compensators in LV busbars (either concentrated in the MV – LV transformer busbar or distributed among the different loads): the application of a greater pf therefore would lead to a losses reduction at least in the MV – LV transformer, thus reducing the global energy withdrawn from the distribution network. This aspect, not relevant for the point of view of the DNO, focused on system benefits, would

have however a not negligible impact on the customer’s economical balance. Finally some further remarks are useful from the customer point of view. Even if a benefit would be got on a medium or long term basis, in the short time investments regarding the application of greater pf would not willingly be made by the MV customers: money, in fact, would be spent today, while benefits in terms of reduced tariff would be seen only after some years. These investments should, therefore, be encouraged in the short term. Actually customers not satisfying the 0.9 limit on pf are subjected to a reactive energy tariff: dually customers with a pf >0.95 might be entitled to receive a compensation for the reactive energy they would not withdraw thanks to the greater pf they applied. Alternatively a fraction of the cost borne by final customers making investments in improved reactive compensators might be covered by an economical incentive.

CONCLUSIONS In general imposing a greater pf to final customers could result in both technical and economical improvements. Voltage profiles could be slightly enhanced, but the most important contribution could be given by the losses reduction, with benefits that are primarily related to DNOs’ economics. Since the investments would be entirely borne by final customers, a suitable mechanism should be designed in order to encourage the installation of new compensation devices. An economical incentive or an opportunely defined compensation for the lower withdrawn reactive energy might represent possible solutions.

REFERENCES [1] M. Delfanti, M. Merlo, M.S. Pasquadibisceglie, M. Pozzi, L. Lo

Schiavo: “Assessing Italian MV network performance: a detailed analysis of short circuit power levels”, Paper 0898, Proceedings CIRED 19th Conference on Electricity Distribution, Vienna, 21-24 May 2007, pp 1- 5.

[2] F. Villa, A. Porrino, R. Chiumeo, S. Malgarotti, “The power quality monitoring of the MV network promoted by the Italian regulator. Objectives, organisation issues, 2006 statistics”, Paper 0042, Proceedings CIRED 19th Conference on Electricity Distribution, Vienna, 21-24 May 2007.

[3] M. Delfanti, M. Merlo, V. Olivieri, M. Pozzi, M. Gallanti, “Power flows in the Italian distribution electric system with dispersed generation”, Paper 0880, Proceedings CIRED 20th Conference on Electricity Distribution, Prague, 8-11 June 2009, pp. 1-4.

[4] AEEG, 2006, “Condizioni per l’erogazione del pubblico servizio di dispacciamento dell’energia elettrica sul territorio nazionale e per l’approvvigionamento delle relative risorse su base di merito economico, ai sensi degli articoli 3 e 5 del decreto legislativo 16 marzo 1999, n. 79”, Order 111/06, Gazzetta Ufficiale della Repubblica Italiana n. 158 del 4 luglio 2006 (in Italian only).

[5] AEEG, 2007, “Approvazione del Testo integrato delle disposizioni dell'Autorità per l'energia elettrica e il gas per l'erogazione dei servizi di vendita dell'energia elettrica di maggior tutela e di salvaguardia ai clienti finali ai sensi del decreto legge 18 giugno 2007, n. 73/07”, Gazzetta Ufficiale della Repubblica Italiana n. 161 del 17 luglio 2007 (in Italian only).