Costs Incurred by the Utility to Mitigate PQ Issues

For utilities, quality of supply generally covers continuity and low voltage, and it is for these cases that investments are made, rather than for harmonics/dips/swells etc. It is also the case that network investment for the purposes of providing increases in capacity will also (as a byproduct) improve continuity and voltage regulation.

The types of network reinforcement possible can be assembled into different groupings with their main drivers, which will also dictate where their costs are allocated; e.g. surge arresters are installed with all pole-mounted transformers to protect the transformer and may also result in an improvement in voltage swells/dips, but they are not installed for power quality improvement, and their costs will be allocated to new supplies or network refurbishment rather than power quality.

Harmonics

Harmonic limits are set on the system by national standards, and in order that these are not breached, lower limits are imposed on customer connections. If the customer’s connection is likely to result in a harmonic limit breach, then the method of connection is either changed or the customer installs mitigation measures at their own expense.

Harmonic filters would usually be installed by customers rather than the utility. If background harmonics were excessive, the utility might be required to install harmonic filters. Zig-zag transformers can be used to cancel out harmonics. However, on an HV system, a delta winding is installed on transformers giving the same or similar effect. The extra cost of providing the delta winding is minimal when compared to the total cost of the transformer.

The neutral in a balanced three-phase system should be lightly loaded, but with increased amounts of loads such as office fluorescent lighting and switched-mode power supplies, the harmonic currents in the neutral become much larger. Neutral currents can be up to 170% higher than the phase currents [102].

The utilities that had undersized their neutral might be forced to up-rate it to the same size as the phase conductor, except where it can be shown that a smaller conductor will suffice. This extra cost would amount to about €2 per meter on a main incoming LV cable.

Direct Cost Calculation

The presence of harmonics on a network can have a detrimental effect on assets in the medium and long term; these include but are not limited to:

• Equipment is subjected to voltages and currents at frequencies that it was not designed to withstand.

• Derating of network equipment, such as cables and overhead lines, due to the additional harmonic load.

• Derating and overheating of transformers, particularly due to saturation effects in the iron core.

• Premature aging of network equipment, e.g. insulation materials and electronic components.

• Neutral conductor overload.

• Additional losses in the conductors and transformers.

Joule losses in aerial and underground power networks can be calculated according to the following simplified method:

E t L I R P

n n n

CU

3 · · · · ·

2

∑

^∞ 2

=

= (euros) (3-2)

where:

PCU = power losses (quantified in euros/year) Rn = impedance at harmonic n (Ω/km) In = averaged current at harmonic n (A) L = total lengths of line (km)

t = hours

E = price energy (euros/kWh)

The power losses can be determined for different areas of the network and added together to get an indication of the system losses due to harmonics. For DNOs that are regulated, the value of these losses to the company will be determined by the regulatory framework. In the UK, DNOs are not penalized for losses, but they are rewarded for reducing them. The cost of reducing harmonics would therefore need to be less than the reward available for a DNO. The costs associated with premature aging and derating of assets are not easily quantified because DNOs do not generally maintain records of operating

temperatures and harmonic levels at their assets. The effect on the asset is therefore unknown.

More detailed calculation methods of are described in detail in Appendix 2-K.

Flicker

Voltage fluctuations in the power systems cause a number of harmful effects of technical and ergonomic nature. Both kinds of effects may involve additional costs in the production process. Several selected adverse effects of voltage fluctuation are shortly described. Also, frequently occurring, irregular operation of contactors and relays should be mentioned, because their economic effects could be damaging.

• Electric machines: Voltage fluctuations at the induction motor terminals cause changes in torque and slip; as a consequence, they influence technical processes. In the worst of cases, they may lead to excessive vibrations and therefore to a reduction of mechanical strength and shortening the motor service life. Voltage fluctuations at the terminals of synchronous motors and generators give rise to hunting and premature wear of rotors; they also cause additional torque, changes in power, and increase in losses.

• Static rectifiers: A change of supply voltage in phase-controlled rectifiers with DC side parameters control usually results in a lower power factor and generation of non-characteristic harmonics and interharmonics. In the case of a drive braking in an inverter mode, it can result in a switching failure, with consequent damage to the system components.

• Electrolysers: Here the equipment useful life can be shortened, and the efficiency of technical processes can decrease. Elements of the high-current line become significantly degraded, and there exists a real risk of increased maintenance and/or repair costs.

• Electroheat equipment: In this case, the efficiency is lessened—for example, with the arc furnace, due to a longer melt time—but it is noticeable only when the magnitude of a voltage fluctuation is significant.

• Light sources: A change in the supply voltage magnitude results in change of the luminous flux of a light source, known as flicker. It is a subjective visual impression of unsteadiness of a light flux, whose luminescence or spectral distribution fluctuates with time. Excessive flicker can cause migraines and is responsible in some instances for the so-called “sick building syndrome.”

The voltage of an electrical network varies all the time under the influence of various switching on-and-off operations of electrical equipment connected to the supply network. The voltage variation can be slow or fast, depending on whether it is a progressive variation of the total load supplied by the grid, or it is an abrupt variation of a large load. The level of voltage variations

emitted by an electrical equipment into the supply network to which it is connected depends on the network impedance. With increasing impedance, the level of voltage variations will increase. The variations of the voltage create flicker, a perturbation that affects the lighting equipment and creates a impression of unsteadiness of the visual sensation.

Complaints due to flicker are usually a localized problem. As a consequence, routine measurement campaigns are not carried out often. On the other hand, the available data confirm that the long-term and short-term flicker levels are commonly below those levels that might give rise to complaints. Values in excess of the EN 50160 value do occur, however.

Especially in remote areas, flicker levels might increase up to critical values, as demonstrated by other measurement results. Given the localized nature of complaints arising from flicker, excessive flicker values tend to be found in the framework of measurements that are targeted specifically at areas of complaint.

Given the immediate visibility of the phenomenon and the severe human discomfort that can be caused, each case of complaint must be taken very seriously. In order to prevent flicker from becoming a widespread problem, appropriate emission limitation is essential, with due allowance for the cumulative effect across the network levels (different from the harmonic cumulative effect) .

SDR O

L n L

i

i i i

ex

= ∑ × ⁽ + ⁾ ×

Underground Cables

Underground cables are less prone to transient faults than overhead, but it would not be effective unless the remainder of the network was also underground. UG cables are installed in areas where it is not feasible to install overhead lines, or where it is more economic to do so because it is a site that has not previously been developed and the ground is already open. However, it is worth noting that in cases where the underground cable is old and fault-prone, it can give rise to a significant impact on continuity as it is usually located in urban areas feeding large numbers of customers. Usually the proportion of faulty cable is low (depending on life cycle stage) and may be deemed poor enough to warrant replacement because of its condition.

It is not possible to give an indicative cost for replacing overhead lines with underground cable, but it is likely to be prohibitively expensive in the vast majority of cases given the need to bury the entire network for transient faults to be reduced. According to some reports, 100% of underground cable would reduce the occurrences of dips by 67%, but due to higher losses of supply the end costs would be reduced by only 1%.

Increased Sectionalizing

Increased sectionalizing reduces the number of customers impacted by a fault and thereby increases overall reliability. Where networks are in close proximity and can be interconnected and sectionalized, this is done as a matter of design to improve continuity. There is also the spin-off benefit in reducing the impact of dips. Dividing a network into two halves would result into a reduction of voltage dips by 50%

each. However, this would also result in reduced redundancy, increased restrictions on switching of network parts, and therefore reduced security of supply [113]. Indicative costs for (statistically) avoiding one voltage dip by splitting a network is given in the following example:

MV network Length: 4539 km

Number of customers: 616,000

Measurements conducted in 30 substations on this network showed an average of 21.2 voltage dips per substation per year and a statistical occurrence of 0.14 disturbances per km.

Two-busbar operation enables a reduction of occurring voltage dips to half, i.e. 10.1 per substation a year.

This modification would cost around €15.5million for 30 Peterson coils and 8 transformers. This would

result in (statistically 10.1 x 30) 318 voltage dips per year being avoided. Therefore, the cost per voltage dip avoided would be approximately €50,000.

Insulate Overhead Lines

Insulation of an overhead line can provide protection against transient earth faults caused by

trees/branches rubbing against the line. In isolated neutral circuits, the line will not trip and the dip will be minor, but in directly earthed circuits there will be a much more significant impact on continuity.

There are four ways in which this can be addressed:

1. Vegetation management

2. Installation of an arc-suppressed system

3. Installation of faulty phase earthing on current direct earthed 20-kV system 4. Insulation of covered conductors

Vegetation management has low yearly costs even when very extensive work is required and is effective at reducing dips and outages caused by trees.

Installation of an arc-suppressed system would depend on the suitability of the network to accommodate it.

Use of insulated conductors on new lines is expensive and unnecessary where the line is clear of vegetation. However, most lines are not new, just extensions of existing circuits, so there is often little point in insulating one area when another is left open. Restringing (reconductoring) lines in

covered/insulated conductors would be a major exercise, and to be effective in improving continuity would have to be done for the full circuit, assuming that the dip on the busbar feeding station for a fault on adjoining lines was not severe enough to offset the benefits.

Conductor Spacing Modification/Animal Guards

A network is normally designed in such a way that conductor clashing does not occur and thereby avoids unnecessary outages. Typically, in order to have faults due to conductor spacing, the line spacing must be such that it can be bridged by a bird’s wingspan, or in the case of bushings on pole mounted equipment, that they are close enough for vermin to bridge the gap between bushings.

These possibilities are usually foreseen in the design, so that conductor spacing is adequate, and where the line is in the path of migrating birds with large wing spans, bird guards can be fitted on the erection or at a later date at low cost, because they simply clip onto the line and can be installed from the ground using a hotstick.

For equipment such as transformers or reclosers mounted on poles, guards are placed over the bushings where the clearances are close. The installation of bird guards is a relatively low-cost solution that is adopted to reduce faults, which are normally sever in that they result in fuse blowing and power cuts.

Lightning Protection

Lightning causes problems either by hitting a line, in which case permanent damage is caused and cannot be prevented, or striking near the line, causing induced currents, whose effects can be mitigated to some extent. The probability of lightning strikes depends on location, but within location is strongly dependent on the structure’s height—the higher the structure the greater the likelihood of being affected by lightning strikes.

High-voltage lines are much taller than LV or MV lines, so there is a higher probability of such lines being affected. There are a number of mitigation measures that can be taken, the most common being the installation of ground wires above the phase conductors, which results in the lightning strike being earthed via numerous towers so that the rise in voltage at each tower is insufficient to backflash onto the phase conductor and cause a dip in voltage.

For lines held by wood pole construction, accommodating a ground wire costs about 20% more, as well as resulting in a structure that is visually intrusive and less acceptable to the local population. An

alternative solution that can be used is to place surge arresters on each phase so that a lightning strike causes a short dip in voltage but is less likely to damage equipment, meaning that the circuit will still be available after the strike. Pole and tower grounding are important in order to avoid tower voltage rise and risk of flashover, which would cause a voltage dip in ground-wire-protected lines.

For an HV network, the precautions against lightning strikes arise from the need to protect equipment from damage and have the spin-off effect of reducing the impact of voltage dips. HV networks are optimized at design so that usually there is little improvement that can be made if the line has been properly designed from the start.

MV lines tend not to have ground wires due to excessive cost and the lesser impact and probability of a lightning strike. Using surge arrestors on MV lines is standard practice at each transformer and at other pole-mounted

equipment such as reclosers, as well as at single-phase tee off’s and cable line interconnections. If a lightning strike occurs, the surge arrestor would conduct, resulting in a voltage dip. In such cases, there is no method to prevent against dips from lightning strikes because the mechanism is there to protect against equipment damage.

Fast Switching with Instantaneous Protection Fast switching requires sophisticated relays and circuit breakers that can also operate quickly. It

also requires a meshed system or else the fast switching would result in an outage. On transmission networks, fast switching would be normal, but not at lower voltages, where the network is usually radial.

Typical costs for the equipment involved are:

Static switch with backup feeder $100/kVA for low-voltage applications.

$60/kVA for medium-voltage applications

Static switches $600k-$700k for very fast TS: within 0.25 cycles (11 kV, 10 MW)

$125k for fast TS: within 2 cycles

$75k for regular TS: within 6-7 s (10 MVA)

Solid-state transfer switch (SSTS) From $75k (several seconds) to $700k (1/4 cycle) Fuse Replacements

Correctly sizing fuses so that faults are confined to the section with the fault is good design. However, the closeness to which fuses can be coordinated with each other can be limited by the network. There are new intelligent fuses that are actually single-phase reclosers that can be more correctly set so that they trip in the event of a fault before the recloser on the main line trips. These cost around €2,000 per installation.

Fault-Current Limiting

These are reactors that are installed so as to limit the fault current and hence reduce voltage dips. In new installations, there is little difficulty in accommodating them, but in existing installations, it would often be difficult to find room for them and to make the necessary connections, particularly if compact metal-clad switchgear is used.

Reactors can be installed in the coupler bay between two transformers; they have little current through them and so low losses. They do not come into play unless there is a fault on either transformer,

whereupon the fault current from the second is limited. Reactors can also be placed on individual lines so as to limit the fault current so that the voltage dip on other lines is limited. It does mean, however, that the voltage dip on the line with the reactor will be greater, although the dip on adjoining lines will be less.

The cost of a 10% impedance reactor on the MV side of a 20-MVA transformer would be about €50,000 installed in a new build situation.

Capacitors for Voltage Regulation

Voltage regulating transformers rather than capacitors are used for voltage regulation because they are active and can either reduce or boost the voltage around a given set point. They cost around €25,000 per set and two sets are used in open delta for regulation on MV.

Improving the power factor of the line by supply Vars is one way in which shunt capacitors can be used, but suitable tariffs that require the customer power factor to be between 0.95 and 1 are more effective, as these are spread over the network and the Vars supplied locally. Placing capacitors in series with the line to reduce inductance is not done at MV because overvoltages can arise, which damage the capacitor.

Protection Coordination Modifications

Regular reviews of protection settings take place to ensure that proper protection coordination is achieved to ensure that downstream devices (circuit breakers/fuses) operate before upstream devices to ensure that the section of network effected by the fault is minimized. At the main feeding substations, the individual feeder breakers should trip before the transformer breakers and similarly the protection devices on the network should trip before the upstream protection on the feeder trips. However, it is also recognized that protection systems are complex and attempts to optimize too closely could result in reducing the impact of the protection.

Replacement of Old Feeders/Transformers

Transformers normally operate correctly (excluding tap changer) until they fail; i.e., they do not malfunction—

they catastrophically fail. So unless there is an on load tap changer issue, the only reason to change a transformer is to avoid the risk of it failing unexpectedly. It will operate correctly up to the time it fails, without giving rise to dips/poor voltage.

Replacement of old overhead feeders can be effective as problems due to cracked insulators are

Replacement of old overhead feeders can be effective as problems due to cracked insulators are

In document CIGRE 467 - Economic Framework for Power Quality (Page 40-46)