DC power system training

The purpose of this presentation is to give an introduction to DC power systems and DC power

fundamentals.

DC Power System

Scope of training

• Why do we user DC power?

• DC system – Water analogy

• Definition of electrical terms

• Electrical basic equations

• DC power system – Elements and fundamental operation

• Battery management

DC power system

Why DC Power?

• Provide a reliable power supply for telecom industry.

• Telecom equipment must operate when AC supply is absent.

• DC energy can be stored on batteries and used when AC power is absent.

• A DC power system is 100% uninterruptible.

• The DC power supply is a filter for noise (EMC, surge etc) that is present on the mains supply.

DC power system

Water analogy

• A dam in a river is a god representation of a battery. The pressure across the dam represents the voltage on the battery.

• A pump that pumps water into the dam is a god representation of a rectifier that pumps current into a battery.

• The water represents the current in an electrical circuit.

• The pipe that the water flows through is a good representation of resistance.

DC power system

PUMP = RECTIFIER

DAM = BATTERY

Pipe = Resistance

Water = Current

DC power system

Definition of electrical terms

Current: A flow of electrical charge caused by an potential difference. Current is measured in Amps (A).

Voltage: The differences in potential between two points that causes electrical charge to flow. Voltage is measured in Volts (V)

Resistance:

The opposition to the flow of current. Resistance is measured in ohms ()

Power: The time rate at which energy is transmitted to the time rate of doing work. Power is measured in Watts (W)

DC power system

DC power system

Energy: The ability to do work. Energy is measured in joules (J)

Efficiency: The ratio of the output power to the input power. Usually measured in percentage (%)

Power Factor:

The derivation between the AC current and the AC voltage. Power factor is a number between 0 and 1. The power factor or any equipment should be close to 1.

Electrical basic equations

• Ohms law:

Voltage = Current x Resistance

• Power calculation:

Power = Voltage x Current

• Energy calculation:

Energy = Power x time

DC power system

DC power system

SURGE PROTECTION

POWER DISTRIBUTION

LVBD

BATTERY DISTRIBUTION

UPSAC/DC

BATTERY DC/AC

TELECOM EQUIPMENT

BATTERIES

INVERTERDC/AC

AC LOAD

AC INPUT PROTECTION

ControlModule

RECTIFIER #1

RECTIFIER #2

RECTIFIER #n

REDUNDANT RECTIFIER

SYSTEMAC/DC

G

DIESELGENERATOR

AC switch board

Power system building blocks

DC power system

AC Switch board:

The AC input to the power system is taken from the switch board

Diesel generator:

The diesel generator is a small power plant that generate AC current. The generator is used as a standby power, that will be connected when mains fail. The generator can be connected manually or automatically.

AC input protection:

Circuit breakers for protection of individual rectifiers.

Surge protection:

Protection against surge and voltage spikes on the AC supply.

The elements in a Power system

DC power system

DC rectifier system:

The rectifier system converts the current from AC to DC. The system consist of rectifiers and a control and monitoring unit

Rectifiers: The rectifiers converts AC to DC.

Control and monitoring unit:

The control unit monitors all the essential parameters in the system (voltage, current and alarms). Parameters can also be changed from the unit.

Power distribution

Fuses of circuit breakers for distribution of DC power to the telecom equipment.

ACDC

DC power system

Batteries The batteries stores the energy for use when the AC mains fail

LVBD Low voltage battery disconnect (LVBD) is used for disconnecting the batteries before they are damaged due to deep discharge.

Battery distribution:

Fuses for protection of individual battery banks against short circuit.

UPS Converts power from AC to DC and back to AC again. The UPS usually have a small battery bank on the DC side

Inverter Converts from DC to AC. Used for AC load that require long battery backup.

System operation

Current flow in a system – Normal operation

• MAINS INPUT OK

• LOAD SUPPLIED BY THE RECTIFIER SYSTEM

• BATTERIES ON FLOAT CHARGE.

RECTIFIER SYSTEM

TELECOM EQUIPMENT

BATTERY BANK

AC

INPUT

System operation

Current flow in a system – Mains fail

• MAINS INPUT FAILED (ABSENT)

• LOAD SUPPLIED BY THE BATTERIES

• BATTERIES ARE DISCHARGING

RECTIFIER SYSTEM

TELECOM EQUIPMENT

BATTERY BANK

AC

INPUT

System operation

Current flow in a system – Mains restored

• MAINS INPUT OK

• LOAD SUPPLIED BY THE RECTIFIER SYSTEM

• BATTERIES ARE RECHARGING

RECTIFIER SYSTEM

TELECOM EQUIPMENT

BATTERY BANK

AC

INPUT

System operation

Mains failureU

54.5

43.2

Float charge

Battery discharge

Battery recharge

t- - - w/current limitation

Disconnect Voltage

Mains resume

Voltage sequence:

Constant output power

• Modern telecom equipment behaves as a constant power load

• A constant power rectifier system will regulate the voltage – current ratio to give constant power output.

• Constant power rectifiers will a increased recharge capacity for the batteries (15- 20%).

Constant power

Constant power

Constant output power rectifier and constant current rectifier.

0

10

20

30

40

50

60

0 50 100 150

Load current (%)

Lo

ad

Vo

lta

ge

(V

)

Standard

Constant power

Battery management

Float voltage:

The recommended charge voltage to be applied to a battery under normal conditions

Battery string

A group of battery cells linked in series to produce the operating voltage.

Capacity Is the electrical energy stored in the battery and is measured in ampere hours (Ah)

C10 The nominal capacity referred to a 10 hours discharge rate

Battery management – definitions

Battery management

Equalising batteries:

A high voltage is applied to the batteries in order to equalise the cell voltage on a string of batteries

Boost voltage Is a voltage higher then the float voltage for fast charging or equalising of batteries

Temperature compensation

A technique that change the battery voltage as a function of battery temperature. This technique will increase the lifetime of the batteries and the rate of change is usually specified by the battery manufacturer

Battery management - why

• Ensure long lifetime of batteries. This is done by correct float voltage, temperature compensated charging and temperature controlled environment.

• Prevent batteries form being damaged due to deep discharge. Low voltage battery disconnect is used to prevent deep discharge & damage to the load equipment

Battery management

Battery management

Open Circuit Voltage in relation to state of charge of the cell.

25 50 75 100

VDC

1.90

Op

en C

ircu

it V

olta

ge (

per

cel

l)l

1.95

2.00

2.05

2.10

2.15

0 State of charge (%)

Battery management

Temp (ºC)

VDC

2.22

Vol

t p

er c

ell

2.24

2.26

2.28

2.30

2.32

2.34

2.36

2.38

-10 0 10 20 30 40

Float Voltage versus temperature

Battery management

Temp (ºC)

VDC

70

Ava

ilab

le c

apac

ity

(%)

75

80

85

90

95

100

105

110

5 10 15 20 25 30

Battery capacity versus temperature