|Home | Articles | Forum | Glossary | Books|
All but the smallest substations include auxiliary power supplies. AC power is required for substation building small power, lighting, heating and ventilation, some communications equipment, switchgear operating mechanisms, anti condensation heaters and motors. DC power is used to feed essential services such as circuit breaker trip coils and associated relays, supervisory control and data acquisition and communications equipment. This section describes how these auxiliary supplies are derived and explains how to specify such equipment.
TBL. 1 Comparison Between Various Battery/Battery Charger Combinations
1. Single 100% battery and 100% charger
2. Semi-duplicate 2350% batteries and 23100% chargers
3. Fully duplicate 23100% batteries and 23100% chargers
Low capital cost
Medium capital cost
Standby DC provided which is 100% capacity on loss of one charger Each battery or charger can be maintained in turn Each battery can be isolated and boost charged in turn without affecting DC output voltage
Full 100% standby DC capacity provided under all AC source conditions and single component (charger or battery) failure
No standby DC System outage for maintenance Need to isolate battery/ charger combination from load under boost charge conditions in order to prevent high boost voltages appearing on DC distribution system b
50% capacity on loss of one battery during AC source failure
High capital cost Greater space requirement Increased maintenance cost
Not all batteries have a boost facility.
It’s possible to specify the voltage operating range of auxiliary devices to match any possible imposed voltages.
Note: Capital cost and reliability objectives must first be considered before defining the battery/ battery charger combination to be used for a specific installation. The comparison given describes the advantages and disadvantages of three such combinations.
2. DC SUPPLIES
2.1 Battery and Charger Configurations
Capital cost and reliability objectives must first be considered before defining the battery and battery charger combination to be used for a specific installation. The comparison given in TBL. 1 describes the advantages and disadvantages of three such combinations.
FIG. 1 details the main electrical features associated with these battery and charger combinations. Charger units are used to supply either just a battery to provide an autonomous DC supply or a battery/inverter combination to provide an autonomous AC supply. The level of 'autonomy' is usually defined in terms of the number of hours or minutes the equipment will enable a specified load to function correctly after loss of input mains AC supply. The capacity of the charger must also be such that after a severe discharge it has the capacity to supply the full DC system load current and the full charging cur rent simultaneously. The technique used for battery charging is called 'float' charging and involves the battery being permanently connected to the load (possibly via an inverter) in parallel with a charger. Therefore the charger must satisfy the requirements of both the battery and the load. The exact charger functional requirements will depend upon the type of battery (lead acid, nickel cadmium _ NiCad, sealed recombination, etc.) being used and this is discussed in Section 4.3. In general the charger must provide a combi nation of constant voltage and constant current charging profiles within close tolerances. For some battery types it must also be able to be switched to a 'boost' charge function that will apply a larger voltage to the battery in order that the charging period may be reduced. The control unit is relatively complicated but may be seen as an analogue feedback loop which samples the output voltage and current and uses these signals to control a single or three phase thyristor bridge rectifier. Switched mode power supplies are also employed in the smaller units and by using an oscillator frequency of around 20 kHz small wound components help to reduce charger size and weight.
The simple single battery/single charger combination is suitable for the small distribution substation where, with perhaps only a few meters between the switchgear and the DC distribution board; 30 V DC was often specified to operate trip coils and relays in the past _50 V is more common now. A useful low-cost addition to such a simple system would be a facility to connect an emergency (or 'hospital') battery via the DC distribution.
The option of using 2x50% batteries and 2x100% chargers may be used for primary substation applications where this is the practice of the sup ply authority or where costs are to be kept to a minimum in keeping with high reliability. It’s very important to specify clearly the operating regime for such a system before going out to tender as manufacturers will need to understand fully the interlock requirements involved. A DC supply float of 125 V is a typical IEC standard voltage for such applications with 110 V nominal system voltage.
For the larger substations, the cost of the DC supply will be small in comparison with the total substation, and the full 23100% battery and 23100% charger combination is usually chosen. Separate systems are often used for substation switchgear control and communications equipment.
As an alternative to two separate 50% batteries, a single battery composed of two strings of batteries in parallel can be employed. This has the advantage of enabling limited service to be maintained in the event of one cell failing in open-circuit mode. Further, as modern chargers can often be quickly returned to service after failure by simply replacing an electronic 'card', a single alarmed charger in conjunction with a two string battery is often considered satisfactory, depending on the risk assessment for the installation in question.
In making such assessments, which must consider the overall reliability of the installation, it’s essential to consider also the frequency of inspection and the extent of remote supervision and alarms. Even a fully duplicated system is at risk of collapse if a single, initially non-critical, failure is left untended for a considerable time.
2.2 Battery Charger Components
The function of the different components shown on the block diagrams in FIG. 1 is as follows.
2.2.1 Interlocks and Cross Connecting Batteries and Chargers
The interlocks between the battery/battery charger combination and the DC distribution board are necessary to prevent boost charging voltages appearing on the DC distribution system which could exceed the ratings of trip coils and other equipment. For NiCad batteries approximate voltages would be:
In the semi-duplicate system the interlocks must ensure:
(a) Only one battery/charger can be selected to boost charging at any one time.
(b) Busbars have to be interconnected prior to boost charging commencing.
(c) Boost voltage is not to be applied to the DC distribution busbar and system.
A busbar-section switch is used to achieve this.
End cell tapping is a low-cost method used to prevent boost voltages appearing on the DC system. However, it has the disadvantage of reduced reliability owing to additional switching components and series cells with differing states of charge. Alternatively for low power chargers (,1kW) DC series regulation may be used with low output impedance common collector transistor/zener diode combinations. The disadvantages here are the costs involved for high current systems, heat losses and again reduced reliability.
The fully and semi-duplicated systems may also be specified such that the batteries and chargers may be either manually or automatically cross connected so either battery may be charged from either charger. This improves the availability of the DC supply but does so at the expense of increasing complexity. Failure of the cross connecting switches at a point of common connection could reduce reliability.
2.2.2 Anti-paralleling Diodes
These are intended to prevent high circulating currents in the duplicated and semi-duplicated systems. Should one battery be faulty, the fully charged battery should not be allowed to discharge into it. Such diodes have very high reliability with low forward voltage drop. They are only likely to fail to short circuit and therefore will maintain a connection between the battery and the DC distribution system.
2.2.3 Battery Fuses
These are positioned in both the positive and negative battery leads so as to minimize unprotected cable or equipment and should be accessible so as to provide an easy method of battery isolation for maintenance. The fuses are intended to protect against fire and to limit fault durations. The fuse rating for normal lead acid or NiCad cells may need to be at least three times the maximum battery demand current at the highest boost charge voltage. In assessing this maximum demand, take account of any short term requirements (e.g. motor starting cur rents). It’s important for the designer to ensure the positioning or type of fuse presents no danger of gas ignition upon fuse operation.
2.2.4 Radio Frequency Interference Suppression
The steep wave fronts associated with fast thyristor switching are rich in harmonics. The system design engineer must therefore satisfy Electro Magnetic Compatibility (EMC) requirements (typically to EN5022 or BS6527 Class B conducted and Class A radiated levels). Simply specifying DC output ripple (to be typically 5_10%) and noise levels is insufficient if sensitive electronic equipment is involved in the substation installation. Adequate filtering will involve radio frequency chokes (RFCs) in the supply source and load connections together with bypass capacitors to short RF to earth and adequate screening. Refer also to Section 24. In the UK, Engineering Recommendation G5/4 sets limits on the connected pulsed rectifiers on the public supply network.
2.2.5 Protection and Alarms
Typically some of the following may be specified:
AC fail Battery fault Charger fail DC voltage AC earth leakage
Overload Reverse polarity Loss of AC supply detection Voltage per cell or string of cells monitored a Output ripple, firing pulse fault or output tolerance Voltage high/low detection and tripping
Earth leakage module
Shut down and auto reset as temperature reduces Overcurrent limiting Tripping
2.2.6 Metering and Controls
Typically, some of the following may be specified with remote monitoring connections as required:
AC supply present
DC voltage DC current Isolation Float/boost
Lamp or AC voltmeter with or without phase selection
Local or remote combined or individual indication
Battery voltage and/or DC system voltage a Battery charging current and/or system load current
AC source and DC supply
Monitoring of battery condition through impedance measurement is sometimes used on critical installations. If recorded on a historic basis, the technique can give warning of imminent risk of failure, rather than just alarm after failure. For further guidance on monitoring lead acid batteries see IEC/TR62060 _ user guide on monitoring lead acid stationary batteries.
2.2.7 DC Switchboard
The DC switchboard should comply with the requirements of IEC 60439.
Double pole switches and fuses, switch fuses or MCBs (miniature circuit breakers) may be used for incomers and outgoing ways to the DC distribution system.
Links or switches may be used to sectionalize the busbars as necessary.
The complete charger, battery and DC distribution board may be housed in a single cabinet for the smaller units. The danger of vented gas causing corrosion problems or gas ignition is small if sealed recombination cells are correctly used, but in very critical locations the probability of a number of co-incident failures (e.g. cell sealing plus charge rate control plus inspection oversight) must be assessed and the risk mitigated to an acceptable level.
Larger installations require separate battery racks with combined or separate charger/DC distribution board combinations.
2.2.8 DC Distribution Supply Monitoring
A healthy DC supply is essential for the correct operation of the substation controls, relays and circuit breakers. A regime of DC distribution supply monitoring must therefore be defined so that immediate remedial action may be taken should the DC supply fail. In addition to the alarms on the battery/ battery charger combination itself alarms may be derived from failure within the DC distribution. A typical scheme is shown in FIG. 2. In this case the DC supply is duplicated to each control and relay panel by sectionalizing the DC distribution board and having separate feeders to each panel. Each relay and/ or control panel DC circuit associated with each power substation circuit is also monitored for loss of DC supply. Since DC failure could in itself prevent alarms from operating small DC/DC converters may be specified to drive the annunciator modules.
2.3 Installation Requirements
Since acid or alkaline liquids and vapors are toxic, a separate battery room is traditionally provided in the substation control building to house the battery banks. The room has to have adequate ventilation (possibly forced), an acid resistant concrete or tiled floor and sink unit with running water and eye wash facilities. Division II explosion-proof lighting and ventilation fan installations are required for large vented battery installations. In addition notices must be displayed about the corrosive materials and to prohibit smoking. Most lead acid and NiCad batteries are now manufactured in enclosed containers with special plugs to permit ventilation without excessive loss of electrolyte. A typical battery room as built for the Channel Tunnel Main 132 kV/25 kV/21 kV Intake Substation at Folkestone, UK, is shown in FIG. 3.
The ventilation requirements for other than the sealed recombination type cells is determined from manufacturers' literature. It can be shown that in the case of a lead acid battery 1 gram of hydrogen and 8 grams of oxygen will be evolved with an input of 26.7 ampere hours to a fully charged cell.
One gram of hydrogen will occupy 11.2 liters, or 0.0112 m3. The volume of hydrogen produced by a battery will therefore be equal to:
no: of cells x charge current x 0:0112 /26:7
no: of cells3charge current30:0004194 m^3
This value may be expressed as a percentage of the total volume of the battery room and assuming that a mixture of 2% hydrogen and air is a safe limit (based on 50% of the 'Lower Explosive Limit' of 4% for hydrogen), the number of air changes per hour to keep the concentration of hydrogen within this limit can be calculated. A typical small battery requiring a charging cur rent of 17 amps will require about three changes of air per hour if installed in a 43232.5 m room.
As an indication of the amount of air to be replaced in order to consider the battery room to be adequately ventilated the following practical formula is used:
Q=55xN x I liters=hour where
Q=volume rate of air replacement (liters per hour) S=factor for allowable air and hydrogen volume plus a safety factor (per A h)
N=number of battery cells
I=charging current causing formation of hydrogen gas (A) (Note:7 amps per 100 A h battery capacity typical) Therefore a 110 V lead acid, 400 A h capacity substation battery will consist of approximately 54 cells and Q=83,160 liters/hour (83 m^3 /hour). An equivalent NiCad system would have more cells and a slightly greater ventilation requirement.
The amount of hydrogen quoted above that is released during charging is appropriate only to the period near the end of a boost charge. Therefore full forced ventilation will strictly only be necessary for a few hours every 1 or 2 years and it’s important not to get this problem out of perspective (but remember that a charger fault or abnormal ambient temperatures can also affect gas generation). In installations with vented lead acid batteries of the order of or greater than 20 kVA h capacity the hydrogen production and temperature rise during boost charging makes the provision of a separate ventilated room mandatory.
Since temperature affects battery performance, temperature effects must also be considered in designing the ventilation system.
2.3.3 The Installation Process
Designs must take account of the need to safely install, maintain and dismantle a battery installation. Some battery units come in multi-cell blocks, and can be very heavy. Racks _ either open or in-cabinet _ must enable safe handling, taking account of human weight handling limits. If handling trolleys are used, access routes must be designed accordingly.
2.4 Typical Enquiry Data _ DC Switchboard
1. Maximum physical dimensions _ widthxdepthxheight (mm)
2. Enclosure IP rating (IEC 60529)
3. Single line diagram drawing number
4. Unequipped spare ways
5. Equipped spare ways
6. Relevant standards DC distribution boards IEC 60439 Molded case circuit breakers
IEC 60157 Fuses IEC 60269 Contactors IEC 60158 Isolators and earth switches EN 60129
7. Busbar maximum current rating (A)
8. Switchgear type c
10. Manufacturer's drawings
11. Metering, alarms and protection e
12. Boost charge contactors
13. Anti-paralleling diodes
Including possible provision of 'hospital battery' connection.
Recommend P2 category for repeated short circuit capability.
Metal-clad, metal enclosed, etc.
To be completed by the manufacturer.
To be clearly indicated upon enquiry drawing or detailed circuit by circuit here. Quiescent and operated power consumption should be noted.
Maximum current rating, coil rating and method of interlocking if applicable. t and reverse blocking voltage diode details.
Batteries consisting of a series of individual cells are used to store electricity and are relied upon to provide the required power for a specified period within specified voltage limits. Different battery types have different characteristics best suited to different applications. The choice for substation auxiliary supplies lies between lead acid and nickel cadmium cells and variants within these categories. At the time of writing, alternatives such as fuel cells don’t provide economic or sufficiently proven options for this use.
3.2 Battery Capacity
The capacity of the battery is determined by the capacity of the individual series connected cells. Parallel connection of cells can be made to increase capacity. In the past this practice was discouraged because a weak or defective cell in one of the batteries means that this battery on discharge does not carry its share of the total load. Also, on charge the battery with a defective cell tends to accept a greater share of the available charging current to the detriment of healthy cells in parallel with it. However, provision of parallel strings provides continuity of service, albeit with reduced capacity, in the event of one cell failing in open-circuit mode _ something not detected by the simpler battery voltage monitoring alarms. Selection of approach must depend on the manufacturer's data as to failure mode probability, and the required installation reliability.
Capacity is expressed in ampere-hours (A h) and is a measure of the electricity that the battery is able to deliver. The following factors affect its capacity:
1. The rate of discharge. If a lead acid battery has a capacity of 100 A h at a 10 hour discharge rate it can deliver 10 A for 10 hours while maintaining the load voltage above a certain value. Rapid discharge over a 1 hour period will reduce its capacity to typically 50 A h, i.e. a constant current of 50 A for 1 hour. This effect is not so severe with NiCad batteries.
2. The output voltage reduces as the battery is discharged. It’s therefore necessary to specify required current delivery over a given period within voltage limits. In particular the required 'end voltage' at the end of the discharge period must be detailed when specifying battery capacity.
3. Battery capacity varies with temperature. The maximum and minimum temperature range at which the battery will be expected to supply the required capacity must be specified. A battery with 100 A h capacity at 15 _C might have a capacity of 95 A h at 10 _ C. Typically, the variation in capacity with temperature is as follows:
0:6% increase per _C from 0_ Cto 130_ C 1:5% decrease per _ C from 0_ Cto _20_C
_1% increase per _ C from 0_ Cto 160_ C 1:5% decrease per _ C from 0_ Cto _10_ C
3.3 Characteristics of Batteries
The characteristics of different battery types and their relative advantages and disadvantages for different applications are given in TBL. 2.
Essentially NiCad battery banks maintain their capacity better at lower temperatures. NiCad life expectancy is good (typically 15 years), better than the standard pasted or tubular lead acid battery (typically 12 years) but not quite as long as the rugged lead acid Plante ´ cell (typically 20 years). NiCad batteries lose their capacity over time under float charge conditions more so than lead acid types. NiCad battery chargers can therefore be programmed automatically to boost charge the NiCad battery bank at regular intervals. Sealed gas recombination batteries have lower life (typically 10 years) and require a strict charging regime. They may be of either lead acid or NiCad type and have the advantage of not requiring special battery room provisions. There is a huge range of international codes governing batteries; it includes IEC 60623 _ Specification for vented nickel-cadmium rechargeable single cells (see also BSEN 2570) and IEC 60896 _ Specification for stationary lead acid batteries. Valve-regulated lead acid batteries perform similarly to sealed batteries, but precautions must be taken to deal with the limited level of gas generation under emergency conditions (e.g. if overcharged). Note that ambient temperatures above the design specification (typically 20 _ C) won’t only affect capacity (see Section 4.2.2) but will significantly reduce design life. If this is likely to be a problem, careful consultation with the manufacturer is advised, and engineers should ensure that specifications clearly define conditions to be encountered. Increased life can be achieved by over specifying the A h requirement, which has a cost implication, and so, as with other aspects, the cost saving due to increased life has to be weighed against increased initial capital cost.
The discharge period of the battery is the time required before a full capacity battery becomes discharged to a specified end voltage which will still ensure correct equipment operation. A comparison of discharge characteristics for different types of lead acid cells together with the characteristics for a 110 V DC substation battery system using NiCad cells is given in FIG. 4. Superimposed upon the substation NiCad characteristic are the maximum and minimum circuit breaker closing coil voltage tolerance limits (615%), the minimum relay operating voltage limit (_20%) and the mini mum trip coil operating voltage limit (_30%) around the 110 V nominal 110 V DC level.
One aspect of battery comparison not covered in TBL. 2 is the environmental impact. This is a rapidly changing situation, and specifiers should check current regulations in the region of installation. For example, lead, cadmium and mercury may all be used in one or another cell design, and may all be affected by environmental legislation.
TBL. 2 Characteristics of Different Battery Types
Description Lead Acid_Plant e´ Lead Acid_Pasted Nickel_Cadmium | Sealed Gas Recombination (Lead Acid in This Case _ NiCad Also Available)
3.4 Battery Sizing Calculations
3.4.1 Capacity and Loads
The required battery capacity is calculated by determining the load which the battery will be expected to supply, the period for which the supply is required and the system voltage limits. Reference is then made to manufacturers' tables of capacity, discharge current capability and final voltages. This should take account of the expected temperature range -- see Section 3.2 item 3.
The load on the battery is calculated from the power consumption characteristics of the loads taking into account their nature:
_ Continuous _ (indicating lamps, relays, alarm systems or other items that draw steady current over the whole battery discharge period).
_ Time limited _ (motors, emergency lighting or other systems which consume power for longer than 1 minute but shorter than the battery discharge period).
_ Momentary _ (particularly the power needed to close or trip switchgear).
Good design practice is to adopt common nominal voltages for substation loads in order to avoid additional batteries or voltage tappings on the battery bank. Standard voltages used are 24, 30, 48 and 110 V. A 48 V DC supply to control and communications equipment is often used and is physically separated from other 110 V DC substation switchgear, control, relay and ser vices load supplies. The control and communications equipment is more locally confined, more suited to a lower operating voltage, voltage drop is not such a problem and different maintenance personnel are involved.
Some typical substation loads are listed below:
Trip coils --- Load requirements approximately 150 W for less than 1 second. Note that in complex protection schemes (e.g. busbar protection) several trip coils may be simultaneously energized and the sum of the individual loads must therefore be used in the battery sizing calculations.
Controls/relays --- Continuous loads such as indicator lamps will contribute to battery discharge on loss of mains supply.
Closing coils ---Older oil circuit breaker coils may take 10_30 kW depending upon design, for less than 1 second at 110 V. More modern vacuum or SF6 circuit breaker motor wound spring charged mechanisms and solenoid closing coils have 300_600W ratings.
DC motors --- Diesel generator 'black start' pump and cranking, isolator or switchgear drives, air blast circuit breaker air compressor motor drives.
The standby period or autonomy varies according to the particular power supply authority standards. For industrial consumers 30 minutes is typical, power utilities 60 minutes and 120 minutes minimum on major installations.
Where standby generation is also available the battery standby period may be reduced to say 15 minutes after which it’s assumed that the local diesel generator will have successfully started automatically.
3.4.2 Practical Example
A distribution substation having 17 No. 13.8 kV oil circuit breakers is to be refurbished with a new battery/battery charger configuration comprising 100% 110 V NiCad battery and 100% charger unit for 3 hour autonomy with the following duties:
1. Momentary loads
Switchgear closing 13.8 kV breakers, 15 kW each _ consecutive load switchgear closing current515 kW/110 V5136 A 20 No. 380 V breakers, manual close.
1. 7 13.8 kV breakers, 150W each _ simultaneous or 20 380 V breakers, 100 W each _ simultaneous.
Take maximum switchgear tripping current from either the 13.8 kV or the 380 V breakers. 17 No.3150W/110 V523 A approx.
2. Time limited/continuous loads
Control and switchgear building emergency lighting Indicator lamps Trip circuit healthy Control panel transducers Relay panel components Total time limited/continuous load
Capacity of time limited=continuous load = wattsxperiod of autonomy hours / voltage
Average continuous load =16 Amps
Allowance for future expansion =25%
Maximum momentary load =136 Amps
(In this case occurs on switchgear closing. Switchgear tripping only presents a small load in comparison and may be ignored.) Allowance for future expansion 55%
From manufacturers' tables a suitable battery may be selected with the most onerous of the calculated capacity, maximum current or continuous load current taking precedence, after taking account of any ambient condition limitations.
3.5 Typical Enquiry Data
It’s normal practice for both the batteries and the charger units to be purchased from the same supplier in order to ensure correct compatibility. The following enquiry forms may be used to assist the vendor to understand fully the requirements for the particular installation. If battery or charger life, or reliability levels (mean time between failures5mtbf) are critical these must also be scheduled.
Type of battery and relevant IEC standard Electrolyte Nominal system voltage (V) Ambient temperature maximum, minimum and average (_C) Number of cells Float voltage per cell (V) Normal system float voltage required (V) Normal float charging current required (A) Minimum recommended battery voltage (V) Recommended boost charging voltage per cell (V) Recommended boost charging current (A) Dimensions of cells _ widthxdepthxheight (mm) Overall dimensions of battery bank _ widthxdepthxheight (mm) Overall weight of battery bank (kg) Weight of individual battery cells or blocks (kg) Material of battery cases Battery capacity at hour discharge rate (A h) Duty cycle requirements
Battery voltage at end of duty cycle (V) Normal standing load (A) Maximum DC current capability (short circuit) (A) Battery mounting b Connections
Volume of hydrogen produced during boost charging (l) Manufacturer, type reference and manufacturer's drawings
To be clearly specified in the tender documents.
Wood or metal stands or racks, internal batteries to charger, access for topping up, etc.
Markings, connecting links and cabling, etc.
3.5.3 Battery Charger
Maximum physical dimensions _ widthxdepthxheight (mm)
Enclosure IP rating b (IEC 60529) Ambient temperature maximum, minimum and average (_C) Charger to suit following type of battery (cell type) (separate rack or internal to charger) AC input supply for which specified output must be maintained (No. phases) (1/3 ph voltage) (V) (voltage tolerance)6 (%) (frequency) (Hz) (frequency tolerance)6 (%) (AC input) (kVA) DC output (Float voltage) (Boost voltage) (Float current) (Boost current) (Ripple) (DC output)
Psophometric output noise level (for loads between 0% and 100% to CCITT Regulations) Noise level limit Current limitation range 6 Voltage limitation range 6 Time to recharge battery to 90% capacity from fully discharged state Charger efficiency Overload protection Controls, indications and alarms
Manufacturer and type reference h
a Add details of gland plate, top or bottom cable entry, etc. as required.
b Often best left to manufacturer unless specific housing conditions are required. For example a high IP rating could necessitate forced air cooling which in turn could reduce overall reliability.
Alternatively, specify ambient conditions which might affect electronic charger components (e.g. humidity level, high dust content).
c Subject to supply kVA limits as specified, manufacturer to indicate maximum AC current required (in amperes).
d Output voltage range as per IEC 62271-100 and IEC 60694 for nominal switchgear DC voltage and shunt trip coil voltage ranges.
e Manufacturer also to confirm compliance with relevant limits on harmonics generated back into the AC supply.
f See Sections 2.2.5 and 2.2.6.
g For example IEC 60146 for semiconductor rectifier equipment.
h To be completed by manufacturer unless nominated supplier sought.