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C - Connection to the HV publicdistribution network

5.1 General

FunctionsThe substation

According to the complexity of the installation and the manner in which the load isdivided, the substation:c Might include one room containing the HV switchboard and metering panel(s),together with the transformer(s) and low-voltage main distribution board(s),c Or might supply one or more transformer rooms, which include local LV distributionboards, supplied at HV from switchgear in a main substation, similar to thatdescribed above.These substations may be installed, either:c Inside a building, orc Outdoors in prefabricated housings.

Connection to the HV network Connection at HV can be:c Either by a single service cable or overhead line, orc Via two mechanically interlocked load-break switches with two service cables fromduplicate supply feeders, orc Via two load-break switches of a ring-main unit.

MeteringBefore the installation project begins, the agreement of the power-supply utilityregarding metering arrangements must be obtained.

A metering panel will be incorporated in the HV switchboard. Voltage transformersand current transformers, having the necessary metering accuracy, may be includedin the main incoming circuit breaker panel or (in the case of the voltage transformer)may be installed separately in the metering panel.

Transformer roomsIf the installation includes a number of transformer rooms, HV supplies from the mainsubstation may be by simple radial feeders connected directly to the transformers, orby duplicate feeders to each room, or again, by a ring-main, according to the degreeof supply availability desired.

In the two latter cases, 3-panel ring-main units will be required at each transformerroom.

Local emergency generatorsEmergency standby generators are intended to maintain a power supply to essentialloads, in the event of failure of the power supply system.

CapacitorsCapacitors will be installed, according to requirements:c In stepped HV banks at the main substation, orc At LV in transformer rooms.

TransformersFor additional supply-security reasons, transformers may be arranged for automaticchangeover operation, or for parallel operation.

One-line diagramsThe diagrams shown in Figure C29 next page represent:c The different methods of HV service connection, which may be one of four types:v Single-line servicev Single-line service (equipped for extension to form a ring main)v Duplicate supply servicev Ring main servicec General protection at HV, and HV metering functionsc Protection of outgoing HV circuitsc Protection of LV distribution circuits

5 The consumer substationwith HV metering

A consumer substation with HV metering is an electrical installation connected to a utility supply system at a nominal voltage of 1 kV - 35 kV and generally includes a single HV/ LV transformer which exceeds 1,250 kVA,or several smaller transformers.The rated current of the HV switchgear does not normally exceed 400 A.

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Power s upplys ys tem

Service connection HV protectionand meterin g

HV di s tribution and protectionof out g oin g circuit s

LV di s tributionand protection

Su pplier/con su merinterf a ce

Down s tre a m termin a ls ofHV isola tor for the in s ta lla tion

LV termin a ls oftrans former

Duplica te-

su pplys ervice

Ring-m a ins ervice

S ingle-line s ervice(e qu ipped forexten s ion to forma ring m a in)

Protection+ au tom a ticch a ngeoverfea ture

Autom a tic LVs tand by s ource

Protection

ProtectionLV

Autom a tic LV/HVs tand by source

I nomin a l oftrans former u 45 A

A s ingle tr ans former

S ingle-line s ervice

Fig. C29 : Consumer substation with HV metering


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5.2 Choice of panels

A substation with HV metering includes, in addition to the panels described in 4.2,panels specifically designed for metering and, if required, for automatic or manualchangeover from one source to another.

Metering and general protectionThese two functions are achieved by the association of two panels:c One panel containing the VTc The main HV circuit breaker panel containing the CTs for measurement andprotection

The general protection is usually against overcurrent (overload and short-circuit) andearth faults. Both schemes use protective relays which are sealed by the power-supply utility.

S ubstation including generatorsGenerator in stand alone operation

If the installation needs great power supply availability, a MV standby generator setcan be used. In such a case, the installation must include an automatic changeover.In order to avoid any posssibility of parallel operation of the generator with the powersupply network, a specific panel with automatic changeover is needed (see Fig. C30 ).c ProtectionSpecific protective devices are intended to protect the generator itself. It must benoted that, due to the very low short circuit power of the generator comparing withthe power supply network, a great attention attention must be paided to protectiondiscrimination.c ControlA voltage regulator controlling an alternator is generally arranged to respond to areduction of voltage at its terminals by automatically increasing the excitation currentof the alternator, until the voltage is restored to normal. When it is intended that thealternator should operate in parallel with others, the AVR (Automatic VoltageRegulator) is switched to parallel operation in which the AVR control circuit isslightly modified (compounded) to ensure satisfactory sharing of kvars with the otherparallel machines.

When a number of alternators are operating in parallel under AVR control, anincrease in the excitation current of one of them (for example, carried out manuallyafter switching its AVR to Manual control) will have practically no effect on thevoltage level. In fact, the alternator in question will simply operate at a lower powerfactor (more kVA, and therefore more current) than before.

The power factor of all the other machines will automatically improve, such that theload power factor requirements are satisfied, as before.

Generator operating in parallel with the utility supply network To connect a generator set on the network the agreement of the power supply utilityis usually required. Generally the equipement (panels, protection relays) must beapprouved by the utility.The following notes indicate some basic consideration to be taken into account forprotection and control.c ProtectionTo study the connection of generator set, the power supply utility needs some dataas follows :v Power injected on the networkv Connection modev Short circuit current of the generator setv Voltage unbalanced of the generatorv etc.Depending on the connection mode, dedicated uncoupling protection function arerequired :v Undervoltage and overvoltage protectionv Underfrequency and overfrequency protectionv Zero sequence overvoltage protectionv Maximum time of coupling (for momentary coupling)v Reverse real power

For safety reasons, the switchgear used for uncoupling must also provided with thecharacteristics of a disconnector (i.e total isolation of all active conductors betweenthe generator set and the power supply network).

Fig. C30 : Section of HV switchboard including standby supply panel

From s tand by gener a torP i 20,000 kVA

HV dis tribu tionpanel s forwhich s tand by

su pply i s re qu ired

Autom a tic

ch angeoverpa nel

Busba r

tra ns itionpa nel

To rem a inderof the HVswitch boa rd


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c ControlWhen generators at a consumer s substation operate in parallel with all thegeneration of the utility power supply system, supposing the power system voltage is

reduced for operational reasons (it is common to operate HV systems within a rangeof 5% of nominal voltage, or even more, where load-flow patterns require it).An AVR set to maintain the voltage within 3% (for example) will immediatelyattempt to raise the voltage by increasing the excitation current of the alternator.

Instead of raising the voltage, the alternator will simply operate at a lower powerfactor than before, thereby increasing its current output, and will continue to do so,until it is eventually tripped out by its overcurrent protective relays. This is a well-known problem and is usually overcome by the provision of a constant power-factor control switch on the AVR unit.By making this selection, the AVR will automatically adjust the excitation current tomatch whatever voltage exists on the power system, while at the same timemaintaining the power factor of the alternator constant at the pre-set value (selectedon the AVR control unit).In the event that the alternator becomes decoupled from the power system, the AVR

must be automatically (rapidly) switched back to

constant-voltage

control.

5.3 Paralell operation of transformers

The need for operation of two or more transformers in parallel often arises due to:c Load growth, which exceeds the capactiy of an existing transformerc Lack of space (height) for one large transformerc A measure of security (the probability of two transformers failing at the same timeis very small)c The adoption of a standard size of transformer throughout an installation

Total power (kVA)The total power (kVA) available when two or more transformers of the same

kVA rating are connected in parallel, is equal to the sum of the individual ratings,providing that the percentage impedances are all equal and the voltage ratios areidentical.Transformers of unequal kVA ratings will share a load practically (but not exactly) inproportion to their ratings, providing that the voltage ratios are identical and the per-centage impedances (at their own kVA rating) are identical, or very nearly so. Inthese cases, a total of more than 90% of the sum of the two ratings is normallyavailable.

It is recommended that transformers, the kVA ratings of which differ by morethan 2:1, should not be operated permanently in parallel.

Conditions necessary for parallel operationAll paralleled units must be supplied from the same network.The inevitable circulating currents exchanged between the secondary circuits of

paralleled transformers will be negligibly small providing that:c Secondary cabling from the transformers to the point of paralleling haveapproximately equal lengths and characteristicsc The transformer manufacturer is fully informed of the duty intended for thetransformers, so that:v The winding configurations (star, delta, zigzag star) of the several transformershave the same phase change between primary and secondary voltagesv The short-circuit impedances are equal, or differ by less than 10%v Voltage differences between corresponding phases must not exceed 0.4%v All possible information on the conditions of use, expected load cycles, etc. shouldbe given to the manufacturer with a view to optimizing load and no-load losses


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Common winding arrangementsAs described in 4.4 Electrical characteristics-winding configurations therelationships between primary, secondary, and tertiary windings depend on:c Type of windings (delta, star, zigzag)c Connection of the phase windingsDepending on which ends of the windings form the star point (for example), a starwinding will produce voltages which are 180 displaced with respect to thoseproduced if the opposite ends had been joined to form the star point. Similar 180 changes occur in the two possible ways of connecting phase-to-phase coils to formdelta windings, while four different combinations of zigzag connections are possible.c The phase displacement of the secondary phase voltages with respect to thecorresponding primary phase voltages.As previously noted, this displacement (if not zero) will always be a multiple of 30 and will depend on the two factors mentioned above, viz type of windings andconnection (i.e. polarity) of the phase windings.

By far the most common type of distribution transformer winding configuration is theDyn 11 connection (see Fig. C31 ).

Fig. C31 : Phase change through a Dyn 11 transformer

1

23

V12

1

2

3

N

1

2

3

Volt age vector s

V12 on the prim a ry winding prod uce s V1N in thesecond a ry winding and so on ...

Winding scorre s pondence

1

2

3

N


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