Abstract

Resolution ANEEL 505 made voltage supply limits even stricter. As a result, voltage regulation in some of ELEKTRO’s facilities was difficult, and satisfactory results were not being obtained with conventional regulation relays due to the association with long lines feeding industrial and residential loads, which may lead to low voltages when the industrial sector is operating in light load regime and the residential segment, operating at full load, such as on Sunday peak times.

This paper presents the results of a partnership between ELEKTRO and a relay manufacturer that resulted in the development of a relay that operates with multiple adjustment groups, programmed for automatic operation on specific times of the day and for certain days of the week ranges.

The paper brings the experiences with this relay in the regulation system for a 138/34.5kV S/S, which feeds a 34.5/13.8kV 18.75MVA S/S approximately 20km away which features fixed tap adjustment, fact which makes creates complex regulation at the 34.5kV bus bar. By analyzing the system’s load curves, 6 adjustment groups were defined for actuation on specific days and times, thus achieving more accuracy in voltage levels and reduction of approximately 40% in the on-load tap operations.

With a concession area formed by 223 municipalities in the State of São Paulo and 5, in Mato Grosso do Sul, addressing 1.9 million clients, ELEKTRO started its operations in 1998, during the privatization of CESP’s energy distribution area.

ELEKTRO covers an area of more than 120 thousand square kilometers, equivalent to 37% of the total area of the State of São Paulo, so that the company client profile ranges from industries, commercial ventures to homes often geographically distant but linked to the same distribution mains. As a result, the consumption characteristics show major variations as a function of time of the day and day of the week, including changes in the daily load profile and significant displacements of load centers.

In this context, resolution 505 of ANEEL¹, published on 26/11/2001, targets assuring every client along the electric energy distribution network delivery of voltage within adequate limits, and to this end, defines the levels considered acceptable, precarious or critical for the voltage delivered (Service Voltage), in addition to indicators and limits for the time voltage falls outside the acceptable boundaries. These indicators become stricter at every year, until the definitive minimum values are reached in 2007.

The approach traditionally deployed by ELEKTRO to endeavor to address the dispositions set out in resolution 505 was using LDC – Line Drop Compensation, present in nearly 100% of the voltage regulation relays found on the market today. In some situations, however, this approach did not offer satisfactory results, due to the seasonal changes resulting from the consumer profile or changes in the configurations of the electric energy system in maneuver situations or yet, due to specific load characteristics for typical days when, during the day, the load is strongly industrial in certain portions of electrical regions (industrial districts) and, at peak times, materially residential in other parts of the same electrical region (residential districts).

As a function of this, ELEKTRO realized the need for a voltage regulation system capable of addressing their distribution network’s seasonality features, which was achieved in 2004 through work done in partnership with a manufacturer of voltage regulation relay (Treetech). Under this aspect, this article will present the experiences and results obtained both in the initial attempts at solving the problem, as well as application of the new voltage regulation system, which operates based on consideration for load seasonality features.

Voltage regulation relays operate based on programming for a few basic parameters, as shown in illustration 1.

Illustration 1 – Basic operation parameters for voltage regulation relay

The parameters are:

• Rated or reference voltage, which can be referred to the system’s actual operating voltage (kV) or to the secondary of the PT (V),
• The insensibility, which is the acceptable tolerance range for voltage above and below the reference voltage, usually defined as a percentage of it,
• Timing to perform the first tap change after the voltage measured remains for a given time, usually in seconds, above the upper or below the lower limits.

The timing mentioned above can also be programmed for operation by:

• Time Set, where the delay in changing taps is always the same (the value adjusted in seconds), regardless of the magnitude of the voltage deviation, or
• Inverse Time, where the delay in changing taps (t) is the same as the adjusted value (T) multiplied by a reduction factor inversely proportional to the magnitude of the voltage deviation (∆U) in relation to the adjustment for insensibility (Ins), as shown in equation 1:

t = T . ( Ins / ∆U )      (1)

Programming the parameters above would already be enough to ensure good voltage regulation at the substation bus bar and for any loads located geographically close to it. However, for more distant loads, drops in resistive and inductive resistances in conductors may cause final voltage levels to fall below the lower threshold allowed, especially when load currents are higher, as shown in illustration 2. For this reason, most voltage regulation relays are also equipped with load current measuring devices with adjustments for line voltage drops.

Illustration 2 – Line voltage drop and load voltage

Since the ideal situation shown in the illustration above is not found on most practical applications, with the load being concentrated on a single point, the usual procedure implies in adopting a fictitious load center, which is a point along the line where it is considered that all the loads are concentrated. Based on the location attributed to this point line voltage drop compensation parameters are then calculated. The result is such that, at this specific point, the voltage will be calculated by the regulation relay and maintained close to the rated value. For the other points in the network, there may be variations, however, if the load center is adequately chosen (and if the network characteristics allow it) these variations will remain within the limits defined by the upper and lower threshold limits.

As mentioned above in item 2.1, voltage regulation relays operate based on tap changes in the commuter after a deviation is detected between the voltage measured and the rated voltage higher than the level of insensibility set, to which we must add a time lag before tap changes in order to avoid unnecessary changes caused by instant voltage deviations. During the periods when the load remains constant or varies slowly, this approach has proven to be suitable.

However, at peak times, when loads tend to shift fairly quickly, both increasing and decreasing, there is what is known as the “drag” effect, shown in illustration 3. The chart shows what happens when average voltage remains below rated voltage when the load is rising, which begins around 5:30 pm, exactly when a higher voltage would be required, and the opposite occurs when the load decreases, when average voltage remains higher than rated voltage. In addition to this, we can see an increase in periods when voltage is higher or lower than the permissible thresholds, which contributes to worse quality indicators than values established in ANEEL’s resolution 505.

Illustration 3 – Drag effect during peak times

One possible solution for this problem would be to reduce the settings for the insensibility and timer values for the voltage regulation relay, in order to make their actuation more agile in these periods of fast variations in load levels. However, this solution has the severe inconvenience of increasing sharply the number of operations performed by the on-load tap changer throughout the day, even in periods where these changes would not be necessary. Consequences of this include higher maintenance cost for the equipment, since maintenance intervals are determined by the number of operations, increased equipment unavailability periods due to increased maintenance frequency and increased failure risk, considering that the on-load tap changer is one of the main sources of defects in transformers.

As mentioned in item 2.1, the use of compensation for line voltage drop requires definition of a load center which represents the load connected to the distribution network. This process can be applied successfully in networks where the load center remains practically unchanged over time. This hypothesis has good chances of being true when the majority of the loads hooked to the network are of similar natures, such as for instance, exclusively residential sector, where maximum load with a given threshold occurs between 6 and 8 pm, or else exclusively industrial, where usually maximum loads at another threshold occur between 7 am and 5 pm.

However, for distribution networks with mixed loads, mixing, for example, residential and industrial loads, the load center can undergo significant displacements depending on the day of the week or even of the time of the day. In these cases, the use of conventional voltage regulation relays and line voltage drop compensation, described in item 2.1 above, do no bring about satisfactory results, bearing in mind that calculations and parameter definitions are carried out considering predominance of one type of load (industrial, for example) which will cause excessively high or low voltages when the load type changes.

It may also happen in cases where the industrial load is higher than the residential one, that total demand during the time in which industries are active, i.e., up to about 5 pm, is higher than for the period when the residential load is predominant, from 6 to 8 pm. In this period, which is when residential loads reach their consumption peak, voltage at the substation bus bar should be kept high, bearing in mind that secondary networks are more loaded and therefore with the highest level of voltage drop. However, since total demand was reduced, the compensation for line voltage drop will reduce the voltage at the substation bus bar, so that residential loads might then receive voltage below minimum permissible values.

ANEEL resolution 505, in addition to specifying the acceptable voltage supply limits, also establishes indicators and their values for the time that the voltage remains at precarious and critical levels.

Since voltage levels considered precarious or critical are different for voltages above and below rated, so can the times permissible for voltage deviations be different dependent on direction of deviation. This way, conventional relays would force timer programming according to the shortest of these times, which would lead to the commuter performing unnecessary operations in the opposite situation, with the same inconveniences already previously mentioned in terms of increased maintenance costs for the equipment, increased equipment unavailability periods due to increased maintenance frequency as well as increased failure risk.

One important benefit stemming from using the new voltage regulation relay with multiple parameter definitions, in addition to improved voltage regulation, is the possibility of concentrating the operations of the OLTC on given days and times, following the concept of saving in some moments to spend in others where tap changes are really necessary.

This can be seen in table 3 below, where part of the parameter definition used for the relay is displayed. During the times where there is more load variation, when regulation must occur quickly and accurately, which fall from Monday to Saturday from 5:30 to 11:59 pm and on Sundays from 5:30 to 10:59 pm, the relay operates with insensibility (dead range) of 0.9% and timing of 70 seconds to raise or lower voltage. On the other times, that is from Monday to Saturday from 0:00 to 6:59 am and n Sundays from 11:00 pm to 5:29 pm, the relay operates with increased insensibility and timing of 1.1% and 110 seconds, respectively.

Parameter definition for the new voltage regulation relay at BJP SS

The results achieved by this setting adjustment philosophy can be seen in table 4, where the monthly tap change operation survey results are displayed. From December 2004 to January 2006, when regulation was performed by a conventional relay, the number of monthly tap changes was never less than 738, reaching peaks of nearly 1500 operations/month in the last year. On the other hand, the latest data sets, after installing the new regulation relay in February 06, 2006, indicates a reduction in the number of tap changes down to around 420, a reduction on the order of 40%, even in relation to the best result obtained previously with conventional regulation.

Number of tap changes using a conventional relay and with a multiple parameter relay

The experience obtained from applying the new voltage regulation relay with multiple parameter definitions has shown that adopting this new philosophy can bring about material gains in different aspects.

First, there were gains for clients in the quality of the voltage regulation process, which in the case in point was the result of the possibility offered by specific parameter definition for each time slot. This allowed the voltage regulation relay to operate on different voltage thresholds according to the load situation of the residential portion, even with the industrial portion operating at light load regime, like for instance peak time on Sundays.

Additionally, as a result of the flexibility afforded by defining parameters for the operation of the relay as a function of days of the week and times of the day, it will be possible to reduce the wear on the OLTCs for the source transformers by significantly reducing the number of operations without sacrificing the quality of the voltage regulation. This will allow programmed maintenance intervals for power transformers to be extended, due to a lower number of changes, simultaneously reducing the failure risk for these transformers, considering that statistically the on load tap changer is by far the biggest source of defects in transformers.

Finally, with the intelligent process of parameter definition suited to the peculiarities of loads characteristics, gains were also obtained by avoiding the need for investments with the replacement of the existing OLTC-less transformers at the 34.5kV S/S, thus also avoiding costs incurred in maintenance for the new equipment with OLTCs.

Good management of voltage regulation for the electrical system, through the functionalities of the voltage regulation relay with multiple parameter definition capability developed here, contributes to moderate tariff setting, helping to ensure fair tariffs and in particular defending the quality of the service, maintaining voltage supplied to clients within the ranges defined in ANEEL resolution 505/2001.

It is worth highlighting here that the cost for the voltage regulation relay developed here is similar to those for conventional relays, since the improvements were implemented by way of changes in the operating logic, without any significant hardware alterations.