Substation Auxiliary Power Supplies (part 2)

4. AC SUPPLIES

1. Power Sources

FIG. 5 Derivation of substation auxiliary low voltage (LVAC) supplies: (a) Distribution substation; (b) primary substation with AC auxiliary supplies derived from earthing transformer(s); (c) dedicated and duplicated auxiliary transformers (courtesy of Balfour Beatty Projects and Engineering Ltd).

Substation auxiliary AC supplies may be derived from dedicated sources or from additional circuits on low voltage distribution switchgear forming part of the substation's outgoing distribution system. Three examples are given in FIG. 5:

_ Simple 380-415 V three phase circuit allocations fed by the distribution substation transformer(s).

_ Tertiary windings on substation main transformer(s) or from earthing transformer (zigzag star-star) windings.

_ Dedicated substation auxiliary transformers and switchgear.

The essential factors to be considered are the level of security of supply required (duplicated transformers, independent source of supply, LVAC sectionalized switchboard, key interlocks, etc.), the fault level of the LVAC switchgear (possible high fault levels at primary substation sites) and allowances for future substation extensions (additional future switchgear bays, future use of presently equipped or unequipped spare ways).

2. LVAC Switchboard Fault Level

The substation auxiliary LVAC switchboard will typically be fed by auxiliary transformers in the range 100_630 kVA. Transformers in this range normally have impedance values of the order of 4_5% and will therefore act as the main fault limiting element in the system between generation source and substation LVAC switchboard. Neglecting source impedance this implies auxiliary transformer secondary fault levels of some 12 MVA without having transformers in parallel. Key-interlock systems are usually employed to pre vent paralleling of substation auxiliary transformers and thereby avoid exceeding the fault rating of the switchgear. Air circuit breakers are often employed as incomers and bus-section switches on the LVAC switchboard.

They can be specified to cater for high fault levels and load currents over a wide temperature range in withdrawable format and as an integral part of a larger switchboard.

3. Auxiliary Transformer LV Connections

A single auxiliary transformer is normally connected to the LVAC distribution switchboard earth and neutral busbars via links as shown in Figs. 4.6a_c.A current transformer (CT) associated with secondary unrestricted earth fault protection is located on the earth side of the neutral to earth link. In this way the CT is in the path of the earth fault current. At the same time unbalanced or harmonic currents involving the neutral (3rd harmonics and multiples) won’t be 'seen' by the CT in this position. This is the 'classic' standby earth fault (SBEF) protection CT location and transformer connection to the LVAC switchboard for a single transformer source of supply. This arrangement is unsatisfactory when applied to a multi-source supply system.

Consider the case of two auxiliary transformers, A and B, used to derive the substation auxiliary LVAC supply. FIG. 6b shows such a system with a key-interlocked normally open LV busbar-section circuit breaker and the same 'classic' SBEF CT location. For an earth fault, IF 5IFA 1IFB, on the left-hand side LV switchgear busbar fed by transformer A a proportion of the fault current, IFB, may return to the neutral of transformer A via the earthing path of transformer B. This fault current could therefore cause the relay associated with transformer B to operate. If the earth fault current path is particularly unfavorable it’s possible for the LVAC switchboard incomer from transformer B to be tripped, thereby losing the healthy side of the switchboard. In addition it should be noted that the IFB proportion of the fault current will bypass, and not be summated by the neutral CT associated with the transformer supplying the fault.

FIG. 6 Auxiliary transformer LV connections: (a) Single transformer LVAC auxiliary supply source and simplistic single line diagram representation (inset); (b) substation auxiliary supply derived from duplicate transformer source with incorrect SBEF CT location; (c) substation auxiliary supply derived from duplicate transformer source with correct SBEF CT location.

IF = busbar earth fault current. IFA =component of earth fault current returning via transformer 'A' neutral to earth link. IFB 5component of earth fault current returning via transformer 'B' neutral to earth link. _5open bus-section circuit breaker.

The auxiliary transformer connections to the LVAC switchboard shown in FIG. 6c involve relocating the SBEF CTs close to the transformer neutrals.

Even with both earth fault components IFA and IFB present, the SBEF CT associated with transformer A summates the currents to operate the appropriate relay and in turn correctly disconnects transformer A from the faulty bus bar. At the same time maloperation of transformer B protection is avoided and transformer B continues to supply the healthy busbar and associated sub station LVAC loads. The disadvantage of this connection arrangement is that the SBEF CTs will now register 3rd harmonic or out of balance load cur rents. Relays with harmonic restraint filters can be employed in cases where the harmonic component of the load (such as with discharge lighting) is high.

4. Allowance for Future Extension

It’s good engineering practice to formulate a policy for spare capacity on auxiliary LVAC transformers and associated switchgear in keeping with cap ital cost constraints.

This is especially true in developing countries and a typical policy guide might be to allow an overall 25% spare switchboard capacity with 10% equipped spare ways and 15% unequipped spare ways within the switch board physical dimensions.

5. Typical Enquiry Data

The table given below describes the essential characteristics for a substation auxiliary LVAC distribution board. This type of enquiry data sheet should be used in conjunction with a full enquiry specification of requirements which details all general and specific requirements (LVAC supply characteristics, etc.):

Maximum physical dimensions _ widthxdepthxheight (mm) Enclosure IP rating (IEC 60529) Single line diagram drawing number

Unequipped spare ways Equipped spare ways Operating voltage (max) (V) 1 min power frequency voltage (kV rms) System frequency Phases (Hz) Short time current (3 s or 1 s as appropriate) Floor mounting/free standing, etc. (kA) Front access/rear access Busbars and switchboard allowable for future expansion Painting finish (Yes/No) Earth bar (internal, full size, etc.)

Panel indicators Panel anti-condensation heaters Wiring (Yes/No) Standard Control wiring size (mm^2) CT wiring size (mm^2) Ferrule/cable core identification standard Relevant standards AC distribution boards IEC 60439 Molded case circuit breakers

b IEC 60157 Fuses IEC 60269 Contactors IEC 60158 Isolators and switches IEC 62271-102 and IEC 60265 Busbar maximum current rating (A) Terminal details Switchgear typec Manufacturer

Manufacturer's drawings

Metering, alarms, remote indication outputs and protection ne Remote control facilities Auxiliary power supplies

Include method of interlocking (mechanical key interlocks) if applicable for incoming supply with switchgear bus-section circuit breaker.

Recommend P2 category for repeated short circuit capability.

Metal-clad, metal enclosed, withdrawable fuse carriers, circuit pad-lock arrangements, gland plate details, labeling, ACB incomer details, MCCBs, MCBs, fuses, etc.

To be completed by the manufacturer unless nominated supplier required.

To be clearly indicated upon enquiry drawing or detailed circuit by circuit here. Quiescent and operated power consumption should be noted.

6. Earthing Transformer Selection

It’s often necessary to derive the substation LVAC supply from the main power transformers. The lowest primary substation distribution voltage level (10 kV, 20 kV, etc.) is also often provided by a delta secondary. Provision of a medium voltage earthing point is necessary in order to limit and better control the medium voltage earth fault level. This earthing point and derivation of a useful LVAC substation auxiliary power source may be provided by using an earthing transformer. Refer to Section 14, Section 14.5.6.

The options available are:

_ interconnected star/star;

_ star/interconnected star;

_ star/delta/interconnected star.

The zero sequence impedance on the MV side must limit the earth fault cur rent to a specific value of typically 1,000 A. The earthing transformer must exhibit low positive and zero sequence impedance on the LV side in order to permit unbalanced loads and minimize voltage regulation difficulties. The relative merits of these different earthing transformer connections are described when fed from the delta-connected secondary of a primary substation power transformer.

FIG. 7 Interconnected star/star winding: (a) 11 kV earth fault; (b) LV earth fault single phase loading.

1. Interconnected star/star

An interconnected star winding on its own has sufficiently low reactance to provide an MV earthing point in conjunction with a main delta-connected power transformer secondary winding.

FIG. 8 Star/interconnected star winding: flux diagrams.

FIG. 9 Star/delta/interconnected star winding flux diagrams.

FIG. 10 Uninterruptible power supply (UPS).

FIG. 11 Conventional vented lead acid battery room showing battery bank, sink, tiles and ventilation arrangements (courtesy of Balfour Beatty Major Projects).

Figures 7a and b show the winding connection/flux diagram for an 11/0.415 kV interconnected star/star earthing transformer under MV earth faults and under LV unbalanced load or earth fault conditions respectively. An ampere-turn balance is achieved for the external 11 kV earth fault condition and so the earthing transformer presents a low reactance to such faults. An unbalanced LVAC load or a phase-to-neutral earth fault on the secondary side of the earthing transformer produces no corresponding ampere-turn balance with this vector grouping. Therefore the magnetic circuit to secondary zero sequence currents must pass out of the core, returning via the air/oil interface to the tank sides. In practice, for the usual 3-limb core arrangement, the resulting zero sequence impedance is sufficiently low to allow limited unbalanced loading. However, a transformer design that does not rely on external flux paths for certain loading or fault conditions can be more precisely designed. If a 5-limb or shell type arrangement is used the resulting magnetizing current would be very low and unbalanced loading impossible. Interconnected star/star earthing transformers for substation auxiliary LVAC loads in the range 250_500 kVA are perfectly feasible. However, as the transformer rating increases so does the percentage reactance and load regulation becomes difficult.

2. Star/interconnected star

Figures 8a and b show the winding connection/flux diagram for this vector grouping again under MV earth fault and LV unbalanced loading or earth fault conditions, respectively. For the 11 kV earth fault case the ampere turns in the earthing transformer star winding are not balanced against the delta connected primary substation transformer secondary. The interconnected star earthing transformer secondary winding has no effect in providing balancing ampere-turns for this fault condition. Therefore the earthing transformer presents a high reactance to 11 kV earth faults and is not particularly useful for this substation application. Under LVAC unbalanced load conditions an ampere-turns balance is achieved and the earthing transformer presents a low reactance to out-of-balance secondary loads.

3. Star/delta/interconnected star Refer to Figs. 4.9a and b for the winding connection and flux diagrams for this vector grouping again under MV earth fault and LV unbalanced loading or earth fault conditions, respectively. Under 11 kV earth fault conditions balancing ampere-turns are provided by the circulating current in the earthing transformer delta winding. The earthing transformer therefore provides a low reactance to 11 kV earth faults. An ampere-turns balance is also achieved for the LVAC out-of-balance or earth fault conditions such that the earthing transformer with this vector grouping presents a low reactance. The cost of the additional third delta winding makes this earthing transformer connection less economic than the simple and more common interconnected star/star type. However, the connection offers greater flexibility in the design of satisfactory impedances and as the earthing transformer LVAC load requirement increases this connection offers better regulation than the interconnected star/star arrangement.

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TBL. 3 Uninterruptible Power Supply Technical Particulars and Guarantees

Description | Type and Requirement or Manufacturer's Guarantee

Manufacturer Standards Source supply Voltage (rms) (V6) Frequency (Hz6) No. of phases Minimum power factor UPS output Voltage (rms) (V6) Frequency (Hz6) No. of phases Minimum load power Factor Maximum load current (A) Types of output switching devices Indications and controls Remote control or indication requirements Rectifier output current range Float (A) Boost (if applicable) (A) End boost (A) Battery charging time from fully discharged (end) condition to 90% fully charged capacity (h) Battery capacity (rating) (A h) Cell type Cell range/operating Voltage

Float Boost Commission Cell voltage when fully discharged Equipment function Load duty Operation mode Continuously connected without static bypass switch Continuously connected with mechanical bypass switch Continuously connected with static bypass switch Active standby mode with no-break static transfer switch UPS space requirement (L3W3D) (mm) UPS weight (kg) Battery space requirement (if separate) (L3W3D) (mm) Weight of one cell and total battery (filled) Bank (kg and kg) Ventilation requirements Environmental conditions Temperature (minimum, average and maximum) Relative humidity range

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Statements about changeover times, transient behavior under changeover and supply side interruptions, undervoltage, overvoltage and voltage spike conditions Continuously conditioned power to load even in event of mains failure. Especially suitable when different input and output voltages and frequencies are required Load is normally supplied from the UPS with an electro mechanical contactor for short break changeover to the alternative supply when required.

Useful if supply is poor quality but not a true no-break system Uninterrupted changeover in the event of a fault or overload condition. Often specified for computer power supplies Useful configuration if mains supply variations are acceptable to the load Rectifier maintains the battery in charged condition and UPS used immediately upon mains failure

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7. Uninterruptible Power Supplies

7.1 Introduction

Static uninterruptible power supply (UPS) units producing a secure AC (or DC) output usually consist of an AC to DC rectifier, battery unit and (for an AC output) a DC to AC inverter as shown in FIG. 10. As well as being used to provide supply security, it may also be installed to provide power of controlled quality to sensitive electronic equipment.

FIG. 12 Combined sealed lead acid battery and charger unit.

7.2 Operation

The rectifier float or boost charges the battery bank. The battery is sized for a given autonomy of supply under mains power failure conditions in the same manner as described in Section 3 above. The autonomy may be specified as typically between 15 minutes and 3 hours under full load conditions.

The inverter produces, from the stored energy contained within the battery, an independent AC supply with very close tolerances. It’s usually an isolated supply, and the UPS may be considered and treated as a generator in many respects. For example, it’s vital to examine the earthing arrangements, and also safety isolation when working on the system.

The unit may be continuously connected in circuit. This configuration is particularly applicable where different input to output voltages and/or frequencies are required. Alternatively a very fast acting solid state transfer switch (SBS) may be used in conjunction with a voltage-sensing electronic control circuit to connect the unit upon brief mains supply voltage dips, spikes or longer-term interruptions. This ensures that the load supply is maintained with an 'uninterrupted' changeover in the event of a fault or an overload condition. Such systems are usually specified for computer power supplies.

7.3 Particular Requirements

Apart from the autonomy required special consideration must be given in the UPS specifications to the speed of changeover (fraction of a cycle) achievable by the static bypass transfer switch. The various parts of IEC 60146 detail methods for specifying the UPS. The tolerance of the connected load to voltage disturbances must also be matched with those likely to be caused with the UPS in service. In particular, the specifications must cover limitations to harmonic disturbances caused by the solid state rectifier and inverter units, both as regards to the load and also in the supply network ( FIG. 11).

A typical technical data sheet for use at the enquiry stage for UPS systems is detailed in TBL. 3.

Bearing in mind that a major reason for installing a UPS facility is reliability of supply, it’s worth mentioning again that the reliability and availability of the UPS unit itself should be carefully investigated. Availability depends, among other things, on time to restore after failure, and that in turn depends upon speed of detection of failure. Choice of test facilities _ local or remote, automatic or manual _ and frequency of test are significant. For AC output units, a manual bypass facility (preferably external), to allow supply continuity during maintenance or repair, is strongly recommended ( FIG. 12). For further treatment of the subject of reliability, see Section 23.