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Procedure for the establishment of a new substation

Procedure for the establishment of a new substation

Large consumers of electricity are invariably supplied at HV. On LV systems operating at 120/208 V (3-phase 4-wires), a load of 50 kVA might be considered to be “large”, while on a 240/415 V 3-phase system a “large” consumer could have a load in excess of 100 kVA. Both systems of LV distribution are common in many parts of the world. As a matter of interest, the IEC recommends a “world” standard of 230/400 V for 3-phase 4-wire systems.

This is a compromise level and will allow existing systems which operate at 220/380 V and at 240/415 V, or close to these values, to comply
with the proposed standard simply by adjusting the off-circuit tapping switches of standard distribution transformers.

The distance over which the load has to be transmitted is a further factor in considering an HV or LV service. Services to small but isolated rural consumers are obvious examples. The decision of a HV or LV supply will depend on local circumstances and considerations such as those mentioned above, and will generally be imposed by the utility for the district concerned.

When a decision to supply power at HV has been made, there are two widely followed methods of proceeding:

  1. The power-supplier constructs a standard substation close to the consumer’s premises, but the HV/LV transformer(s) is (are) located in transformer chamber(s) inside the premises, close to the load centre
  2. The consumer constructs and equips his own substation on his own premises, to which the power supplier makes the HV connection

In method no. 1 the power supplier owns the substation, the cable(s) to the transformer(s), the transformer(s) and the transformer chamber(s), to which he has unrestricted access. The transformer chamber(s) is (are) constructed by the consumer (to plans and regulations provided by the supplier) and include plinths, oil drains, fire walls and ceilings, ventilation, lighting, and earthing systems, all to be approved by the supply
authority.

The tariff structure will cover an agreed part of the expenditure required to provide the service. Whichever procedure is followed, the same principles apply in the conception and realization of the project. The following notes refer to procedure no. 2.

Preliminary information

Before any negotiations or discussions can be initiated with the supply authorities, the following basic elements must be established:
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Maximum anticipated power (kVA) demand

Determination of this parameter is described in Chapter B, and must take into account the possibility of future additional load requirements. Factors to evaluate at this stage are:

  • The utilization factor (ku)
  • The simultaneity factor (ks)
    .

Layout plans and elevations showing location of proposed substation

Plans should indicate clearly the means of access to the proposed substation, with dimensions of possible restrictions, e.g. entrances corridors and ceiling height, together with possible load (weight) bearing limits, and so on, keeping in mind that:

  • The power-supply personnel must have free and unrestricted access to the HV equipment in the substation at all times
  • Only qualified and authorized consumer’s personnel are allowed access to the substation
  • Some supply authorities or regulations require that the part of the installation operated by the authority is located in a separated room from the part operated by the customer.
    .

Degree of supply continuity required

The consumer must estimate the consequences of a supply failure in terms of its duration:

  • Loss of production
  • Safety of personnel and equipment

The utility must give specific information to the prospective consumer.

Project studies

From the information provided by the consumer, the power-supplier must indicate:

The type of power supply proposed and define

  • The kind of power-supply system: overheadline or underground-cable network
  • Service connection details: single-line service, ring-main installation, or parallel
    feeders, etc.
  • Power (kVA) limit and fault current level
    .

The nominal voltage and rated voltage

(Highest voltage for equipment) Existing or future, depending on the development of
the system.
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Metering details which define:

  • The cost of connection to the power network
  • Tariff details (consumption and standing charges)
    .

Implementation

Before any installation work is started, the official agreement of the power-supplier must be obtained. The request for approval must include the following information, largely based on the preliminary exchanges noted above:

  • Location of the proposed substation
  • One-line diagram of power circuits and connections, together with earthing-circuit
    proposals
  • Full details of electrical equipment to be installed, including performance
    characteristics
  • Layout of equipment and provision for metering components
  • Arrangements for power-factor improvement if eventually required
  • Arrangements provided for emergency standby power plant (HV or LV) if eventually
    required

The utility must give official approval of the equipment to be installed in the substation, and of proposed methods of installation.

Commissioning

When required by the authority, commissioning tests must be successfully completed before authority is given to energize the installation from the power supply system.

After testing and checking of the installation by an independent test authority, a certificate is granted which permits the substation to be put into service.

Even if no test is required by the authority it is better to do the following verification tests:

  • Measurement of earth-electrode resistances
  • Continuity of all equipotential earth-and safety bonding conductors
  • Inspection and testing of all HV components
  • Insulation checks of HV equipment
  • Dielectric strength test of transformer oil (and switchgear oil if appropriate)
  • Inspection and testing of the LV installation in the substation,
  • Checks on all interlocks (mechanical key and electrical) and on all automatic
    sequences
  • Checks on correct protective-relay operation and settings
    .
    It is also imperative to check that all equipment is provided, such that any properly executed operation can be carried out in complete safety. On receipt of the certificate of conformity (if required):
  • Personnel of the power-supply authority will energize the HV equipment and check
    for correct operation of the metering
  • The installation contractor is responsible for testing and connection of the LV installation
    .
    When finally the substation is operational:
  • The substation and all equipment belongs to the consumer
  • The power-supply authority has operational control over all HV switchgear in the substation, e.g. the two incoming load-break switches and the transformer HV switch (or CB) in the case of a MV switchgear, together with all associated HV earthing switches
  • The power-supply personnel has unrestricted access to the HV equipment
  • The consumer has independent control of the HV switch (or CB) of the transformer(s) only, the consumer is responsible for the maintenance of all substation equipment, and must request the power-supply authority to isolate and earth the switchgear to allow maintenance work to proceed.
    .
    The power supplier must issue a signed permitto- work to the consumers maintenance personnel, together with keys of locked-off isolators, etc. at which the isolation has been carried out.

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Power quality

Power quality

The term power quality seeks to quantify the condition of the electrical supply. It not only relates largely to voltage, but also deals with current and it is largely the corrupting effect of current disturbances upon voltage. Power quality can be quantified by a very broad range of parameters, some of which have been recognized and studied for as long as electrical power has been utilized. However, the advent of the term itself is more modern and it has created a useful vehicle for discussing and quantifying all factors that can describe supply quality. Power quality is yet another means of analysing and expressing electromagnetic compatibility (EMC), but in terms of the frequency spectrum, power quality charac- terizes mainly low-frequency phenomena. Perhaps because of this and because of the manner in which it affects electrical equipment, power quality has largely been dealt with by engineers with electrical power experience rather than those with an EMC expertise. In reality, resolving power problems can benefit from all available expertise, particularly since power quality disorders and higher frequency emissions can produce similar effects.

In 1989, the European Community defined the supply of electricity as a product, and it is therefore closely related to the provisions and protection of the EMC Directive (89/336/EEC), but in drawing a comparison between electricity and other manufactured product it is essential to recall a significant difference.

Electricity is probably unique in being a product which is manufactured, delivered and used at the same time. An electricity manufacturer cannot institute a batch testing process for example and pull substandard products out of the supply chain. By the time electricity is tested it will have been delivered and used by the customer whether it was of good quality or not.

Key parameters

The parameters that are commonly used to characterize supplies are listed in Table 1 together with the typical tolerance limits which define acceptable norms. Within Europe these power quality limits are defined by the EN 61000 series of standards in order to be compatible with the susceptibility limits set for equipment.

Table 1: Summary of power quality levels defined by EN 50160
.Power frequency (50Hz).Interconnected systems
.±1% (95% of week)
.+4% (absolute level)
.−6% (absolute level)
.Supply voltage variations on 230V nominal.±10% (95% of week based on
.10 min samples, rms)
.Rapid voltage changes.±5% Frequent
.±10% Infrequent
.Flicker.Pk=1.0 (95% of week)
.Supply voltage dips.Majority
.Few 10s
.Duration <1s
.Depth <60%
.Some locations
.Few 1000 per year of <15% depth
.Short interruptions.20–500 per year
.Duration 1s of 100% depth
.Long interruptions.10–50 per year
.Duration >180s of 100% depth
.Temporary power frequency overvoltage.<1.5kV
.Transient overvoltages.Majority
.<6kV
.Exceptionally
.>6kV
.Supply voltage unbalance.Majority
.<2%(95% of the week)
.Exceptionally
.>2%, <3%(95% of the week)
.Harmonic voltage distortion.THD <8%(95% of the week)
.Interharmonic voltage distortion.Under consideration
.Mains signalling.95 to 148.5kHz at up to 1.4Vrms (not in MV)

The more a supply deviates from these limits, the more likely it is that malfunction could be experienced in terminating equipment. However, individual items of equipment will have particular sensitivity to certain power quality parameters while having a wider tolerance to others. Table 2 provides examples of equipment and the power quality parameters to which they are particularly sensitive. Table 2 shows a preponderance of examples with a vulnerability to voltage dips. Of all the power quality parameters, this is probably the most troublesome to the manufacturing industry; and in the early 1970s, as the industry moved towards a reliance on electronic rather than electromagnetic controls, it was commonly observed how much more vulnerable the industrial processes were to supply disturbances.

Supply distortion (characterized by harmonics) is another power quality parameter that has received enormous attention, with many articles, textbooks and papers written on the subject. However, the modern practices that will be discussed later have reduced the degree to which this currently presents a problem. Other parameters tend to be much less problematic in reality, although that is not to say that perceptions sometimes suggest otherwise. Voltage surge and tran- sient overvoltage in particular are often blamed for a wide range of problems.

Table 2: Examples of sensitivity to particular power quality parameters
Equipment typeVulnerable power quality parameter Effect if exceededRang
.Induction motor .Voltage unbalance.Excessive rotor heating.<3%
.Power factor correction .capacitors.Spectral frequency .content.This is usually .defined .in terms of harmonic .distortion.Capacitor failure due to .excessive current flow or .voltage.Most sensitive if .resonance occurs.In resonant .conditions
.PLCs .Voltage dips.Disruption to the programmed .functionality.V tr
.Computing systems .Voltage dips.Disruption to the programmed .functionality.V
.Variable speed drives, .motor starters and .attracted .armature control .relays .Voltage dips.Disruption to the control system .causing shutdown. V
.Power transformers .Spectral frequency content of .load current.This is usually .defined in terms of harmonic .distortion.Increased losses leading to excessive temperature rise.At full load
.Devices employing .phase .control, such as .light .dimmers and .generator .automatic .voltage regulator .(AVRs) .Alteration in waveform zero .crossing due to waveform .distortion, causing multiple .crossing or phase asymmetry.Instability.Will depend .upon the r
.Motor driven .speed-.sensitive plant .Induction and synchronous .motor shaft speed are .proportional to supply .frequency. Some driven loads .are .sensitive to even small .speed variations.The motors themselves are .tolerant of small speed .variations.At high supply .frequencies (>10%) shaft .stresses may be excessive due .to high running speeds.Limits .depend on .the .sensitivity

However, very often this is a scapegoat when the actual cause cannot be identified. Even when correlation with switching voltage transients is correctly observed, the coupling introduced by poor wiring installations or bad earth bonding practices can be the real problem. Unlike the other power quality parameters, voltage transients have a high frequency content and will couple readily through stray capacitance and mutual inductance into neighbouring circuits. Coupling into closed conductor loops that interface with sensitive circuits such as screens and drain wires can easily lead to spurious events.

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