reading / British practice / Vol D - 1990 (ocr) ELECTRICAL SYSTEM & EQUIPMENT

2.41 Ambient temperature

Since battery capacity and performance is reduced by low temperature, a minimum electrolyte temperature of 5 ° C is maintained as a general rule. However, in nuclear power stations, where reduced capacity could affect the safe shutdown of a reactor, a minimum temperature of 15 ° C is maintained by thermostatically controlled heaters of the totally-enclosed tubular type.

Whilst batteries are capable of operating in an ambient temperature of 35 ° C, with occasional excursions to 40 ° C, these high temperatures are avoided, as they

Connections within the battery rooms and to the outside are made of suitably-sized solid copper rod identified at intervals with acid-resistant red/black paint (or tape) for positive/negative connections, respectively. They are supported on suitable stand-off insulators mounted on battery room walls or on extensions of battery stands. The supports are designed to withstand the electromagnetic forces experienced in the event of an inadvertent short-circuit.

The main connections to the DC switchgear are taken from the battery via through wall bushings to a top-

ROOF SLOPED TO AID VENTILATION

8,7

BATTERY

SINGLE ROW

DOUBLE TIER

At_

1900 MIN

SINK:BENCH

BATTERY ROOM PLAN

FIG. 9.3 Typical layout and environmental requirements of a battery room

VENTILATION AIR

TO ATMOSPHERE //1.-

entry fuse switch unit mounted immediately outside the battery room. Cable connections to the respective chargers or distribution boards are all made at the bottom of these fuse switch units.

2.4.6 Access to battery rooms

Access doors to battery rooms should be locked from the outside of the room at all times, except when work is carried out within the room. Emergency exit doors must be provided with a quick release device on the inside, operative at all times, even when locked from the outside.

To prevent damage to cells, battery rooms must not be used as access routes to other areas.

2.5 Initial tests, charging, maintenance and site testing

The capability of a battery system to meet emergency demands throughout its life depends on the condition and state-of-charge of the battery installation. CEGB guidelines ensure that operators can have confidence in the capability of a battery to meet an emergency demand.

Plante cells basically deliver 100% capacity after initial preparation and charging. Their end of life is not predictable by relating capacity to years of service. They would normally deliver 100% capacity throughout their life which, if they are on charge/discharge cycling operation, can be in the order of 500/600 full cycles. On float charge service, the life is usually dependent on the manner in which the cells are managed. It can be judged from the physical state of the battery, coupled with knowledge of cell electrolyte, specific gravities, voltages and related data. Such data can best be assessed by a specialist from by a manufacturer.

2.5.1 Tests in manufacturer's

Tests on batteries in manufacturer's works can be divided into two categories:

•Type tests.

•Routine tests.

the emergency loads specified for the particular battery, During the discharge, records of voltage, temperature and specific gravity are taken and plotted against time At the end of the discharge, the cell voltage must be such that, for a complete battery of such cells, th e

total voltage must not be less than that specified. The cells used above are then charged at the specified

starting rate to a cell voltage of 2.3 V and from thi s voltage at the specified finishing rate to a cell voltage of 2.7 V. Voltage, temperature and specific gravity are recorded and plotted against time. The time required to charge the battery must not be longer than that spe, cified. Starting and finishing rates are given in Section 2.5.3 of this chapter.

During the foregoing tests the specific gravity must be maintained within specified upper and lower limits. Electrolyte must not be added during discharge. After charging the specific gravity may be adjusted to the specified normal figure.

In order to ensure that the cell lid is correctly bonded to its container, an air leakage test at an appropriate pressure is carried out. No loss of pressure must take place over a specified period.

Routine tests

Apart from a visual examination, the only routine test carried out on each cell in the manufacturer's works is the air leakage test described above.

2.5.2 Tests at site

After erection and initial filling with electrolyte and charging, a discharge test to demonstrate the 3 h rated capacity is carried out on each battery. For this, the manufacturer provides a resistive load frame, as it is unlikely that load is available in the power station at that stage. A discharge characteristic of voltage against ti me is plotted. On completion, the battery is fully

recharged.

Further tests are carried out in conjunction with the associated chargers and are described in Section 4.4.4. of this chapter.

2.5.3 Charging

Float charging

Type tests are usually carried out on a representative sample of cells of a new design and provide a means of proving the design characteristics. Routine tests are less searching in character and are usually confined to establish that a cell has been assembled correctly.

Type tests

Dependent on the total number of cells, one or more cells of each type and size are fully charged. After a period of not less than 12 h and not more than 18 h during which the cells are disconnected from the supply, they are subjected to a discharge appropriate to

The method of operation for all the battery systems is known as 'float charging' which means that the charger, the battery and the DC standing (i.e., continuous) load are all connected in parallel. The charger output voltage is such that the battery voltage is held constant at a voltage of 2.25 V per cell. This is the optimum floating voltage, resulting in the longest possible life allied to the minimum maintenance requirements. Assuming the battery is fully charged to begin with, it will take sufficient 'trickle' charge from the charger at the above voltage to ensure it is maintained in a fully charged condition.

is required when carrying out such examinations, since it is not unknown for the riser to appear normal on visual inspection, but for it to collapse and fail when disturbed.

Corrosion of terminals and connectors

Corrosion of terminals and connectors can occur, but is avoidable since they are accessible for cleaning, inspection, preservation and continuity testing.

Failure of the cell container

Safeguards against such failures are:

•Regular inspection of pillar seals and lid/case seal.

•Administrative control of door locks to reduce the risk of accidental damage, e.g., by battery rooms being used as access routes.

Contamination of electrolyte

Electrolyte consumables, hydrometers, etc., must be subject to quality assurance control. Procedures are specified and care must be taken to adhere to them.

Progressively increasing need for make-up water

A change in the make-up water rate is indicative of a change in cell chemistry and serves as an indicator for detailed examination.

An inspection routine is usually based on the following:

•Daily Check overall battery voltage.

•Weekly Check voltage and specific gravity on se-

lected pilot cells per battery, always using the sam e cells. Electrolyte levels are noted and recorded and topped-up, if necessary. A minimum of two pil ot cells per battery are used dependent on the size of the battery and are evenly spaced throughout th e total number of cells, e.g., 1, 30, 60 and so on.

Check voltage and specific gravity, and visually inspect every cell.

• Annually Employ the services of a specialist to assess battery and experience over the year.

• Six yearly See Section 2.5.7 of this chapter.

2.5.6 CEGB experience

It is CEGB policy to follow a pattern of routine inspection, but the exact routine varies from station to station. Some stations employ manufacturers to assist in the assessment of battery conditions.

Generally, the CEGB has followed the recommendations of the specialists regarding battery replacement and overall experience has shown this to be a conservative policy in that there is no record of a battery failing to fulfil its required duty. Equally, the record of battery life of up to 30 years has been satisfactory.

Degree of undercharging or overcharging

Correct battery voltage is necessary to obtain maximum life from an installation. Chargers are checked regularly to ensure that the correct float voltage is maintained.

Boost charging

The condition at any time and the ultimate life of an emergency supply battery is influenced by its state of maintenance. A boost charge every 2-3 years is usual. The rate and duration of a boost charge follows the particular manufacturer's recommendations. Details of the boost charge schedule are included in the operating instructions.

Duty cycles

The number of charge/discharge cycles in service as ‘vell as the type of duty can determine the ultimate life of a battery installation, which has otherwise been maintained correctly.

2.5.5 Inspection

The state of the battery is a good indicator of general conditions of the DC system and its associated chargers. Regular visual inspections are a reliable guide to battery condition and its ability to perform correctly when called upon in an emergency.

2.5.7 The case for testing

Experience with Plant cell batteries has been sound and it is known that cell capacity does not drop until end of life. Nevertheless, it can be argued that discharge testing to establish the capacity of a battery is a more certain demonstration of its ability to perform satisfactorily. However, testing only demonstrates this at the time of test and at all other times reliance has to be placed on the inspection routines. In view of this, the CEGB does not consider it worthwhile to conduct routine discharge tests, for the sake of the battery alone (see Section 2.5.10 of this chapter for DC system tests).

It is important, therefore, to ensure that the inspection routines previously outlined, are sufficiently rigorous and are capable of accurately determining the state of the battery. This can be attained more positively by removing one or two cells for dismantling, followed by closer inspection of all the parts by a specialist. The CEGB recommends that two sample cells are removed from a battery at six-yearly intervals (or at more frequent intervals, e.g., annually, if the normal inspection shows that unusual deterioration has taken place and that such a check would be prudent). These removed cells will be unserviceable after the inspection and therefore new replacement cells will be required to complete the battery.

CI is the half-hour discharge rate and C 10 is the ten-hour discharge rate

Note 2: Thickness growth will be detected as a mode of failure under items 3 and 5_

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emergency lighting, emergency DC drives, etc., because He solenoid operating range is between +5% and

- 15n of the rated voltage, whereas the voltage range of emergency lighting is ±20% and of motors ± 10% continuous; —20% for 30 minutes. If the whole DC were designed around the closing solenoids, then the battery capacity would need to be considerably

higher, typically twice the capacity without the closing

'solenoids since the designed voltage range would be much less. It is therefore generally more economical

to provide dedicated batteries for switchgear closing

.olenoids.

Since BS5311 gives 220 V and 250 V as preferred soilage ratings of solenoids, the CEGB chose the kisser, permitting 105 cells to be used for the 220 V 'solenoids instead of 120 cells at 250 V.

A dedicated battery for switchgear solenoids also offers a distinct advantage with respect to iblackstare ,apability, i.e., complete loss of AC supplies to the ,hargers as the result of a Grid failure.

Under these conditions, switchgear could be operated

or many hours, or even several days as no standing loads are connected to the battery. A shared batter!, would usually only be designed for a 30-minute capacity.

The system is designed to meet the following criteria:

(a)With the battery on 'float charge', it must be possible to close any two circuit-breakers simultaneously.

(b)With the battery on 'float charge', it must be possible to close up to 100 circuit-breakers each day. There must be sufficient spare capacity in the battery rating to avoid having to boost-charge it to meet this duty.

(c)Following a loss of AC supply to the charger, the battery must be capable of supplying the solenoid of the highest current-rated single 11 kV circuitbreaker continuously for 30 s or two 3.3 kV circuit -

breakers simultaneously at the commencement of battery discharge.

Additionally, it must be capable of closing all other designated circuit-breakers, such as interconnectors and emergency pumps, to shut down the station in a safe manner and to restore the supplies following Grid reconnection. The minimum discharge voltage at the end of this duty must not fall below 199 V at the battery fuse switch terminals.

Following this duty and after restoration of the charger supply, it is assumed that no loads will be connected for 10 hours in order to achieve full readiness to meet duties (a) and (b) above.

It should be noted that this system is not necessary if switchgear closing solenoid mechanisms are replaced by motor-wound spring-charged closing mechanisms. The advantages and disadvantages of this choice are discussed in Chapter 5.

3.2.2 110 V DC systems for switchgear control, protection and interlocks

These systems provide a secure DC supply for essential loads, such as:

•Switchgear and controlgear tripping.

•Switchgear closing, where spring-charged closing mechanisms are used.

•Interlocks and protection.

•Local control equipment and essential instruments.

Each battery is designed to be capable of supplying its designated standing and emergency loads for a discharge period of 30 minutes following complete failure of all incoming AC supplies,

Additionally, each battery is also capable of supplying one-half of its designated standing load for a

discharge period of one hour following loss of a charger. This permits a standby charger to be connected manually.

The minimum discharge voltage at the end of these duties must not fall below 102 V at the battery fuse switch terminals.

3.2.3 4-8 V DC systems for telecommunications, plant control and alarms

These systems provide secure supplies for two distinct categories of equipment:

(a)Telecommunications equipment such as:

•Private automatic exchange (PAX)

•Private automatic branch exchange (PABX)

•Direct wire telephone system (DWTS)

•Radio system controllers

•Control desk telephone equipment

•System operation telecommunications equipment.

(b)DC supplies for essential loads, such as:

•Automatic sequence control equipment

•Alarms and indications at central control room

•Manual control from central control room.

Each battery is designed to be capable of supplying its designated standing and emergency loads for a discharge period of 30 minutes following complete failure of all incoming AC supplies.

Additionally, each battery is also capable of supplying one-half of its designated standing load for a discharge period of one hour following loss of a charger. This permits a standby charger to be connected manually.

The minimum discharge voltage at the end of these duties must not fall below 46 V at the battery fuse switch terminals.

3.2.4 250 V DC systems for emergency lighting and emergency drives

These systems provide secure DC supplies for loads, such as:

•Emergency lighting (DC luminaires).

•Emergency auxiliary drives, such as lubricating oil pumps for turbine-generators, gas circulators, etc.

•Emergency valve operation.

•Fire sirens.

Each battery is capable of supplying the designated total load of all emergency auxiliary drives and the designated total emergency lighting load, excluding plant buildings remote from the main station building, for a discharge period of 30 minutes following complete failure of all

The minimum discharge voltage at the end of the se duties must not fall below approximately 211 V at the battery fuse switch terminals.

In the unlikely event that AC supplies are not restored by means of diesel generators, it is necessary to disconnect loads no longer required to permit the hydrogen-cooled generator emergency seal-oil pump s to continue to operate for a further 1.5 hours.

Sufficient spare capacity is allowed in the rating of the battery to avoid having to boost charge it after a 30-minute discharge period with loads as described above.

3.3 Duplication of battery/charger systems

To allow boost charging and maintenance to be carried out, which requires isolation of the battery/charge r combination from the DC load, they are provided in duplicate with suitable switching facilities at the DC switchboard. These permit the two battery/charger systems to be paralleled before taking one off-load.

3.4 DC system voltage limits

Power station DC systems must be designed to lake account of limits of the load voltage. For good battery operation it is necessary to charge at the float voltage, but it is also necessary to discharge the battery down to a sufficiently low voltage, if the full capacity is to be released from the cells.

In order to ensure that the conditions at the terminals of any DC loads are consistent with the requirements of Chapter 1, the battery parameters as shown in Table 9.3 are used.

4 Chargers

4.1 Introduction

From the remarks made in Section 2.5 of this chapter, it will be evident that a correct charging procedure is most important to ensure satisfactory battery life and performance. Modern charging equipment, using AC supplies, provides safe and flexible control of battery