Components of Online Double Conversion UPS– Part One ~ Electrical Knowhow

A double conversion UPS converts the incoming alternating current (AC) to a direct current (DC), so it can power the system’s battery, and then inverts the DC back to AC for powering equipment – hence the name “double conversion.”

We need to know how a UPS’s components work together so we can better understand the UPS system and ensure that the critical load remains online.

As shown in the Fig.1, an online double Conversion UPS has (4) major components:

Rectifier,
Inverter,
Energy Storage (Battery),
Static Switch.

Fig.1 Components of Online Double Conversion UPS

1- Rectifier

The rectifier acts as a load to the electrical mains. The primary objective of the rectifier is to:

A- Convert the incoming power supply (AC) to DC

This DC voltage is used to support what is commonly called a DC Bus, which is the connection between the rectifier, the stored energy device, and the inverter.

B- Charge the battery

All UPS systems need a battery charger—a type of rectifier—to keep batteries charged and recharge them after a power outage. Some rectifier systems use what is called a walk-in circuit. This circuit ‘walks’ up its output, or the DC Bus, slowly. Walking the DC Bus up gradually accomplishes several useful things, including protecting the DC filter caps and reducing inrush current to the system.

Some models of UPS systems will use a separate battery charger that can be turned off when the batteries are fully charged. It will then monitor the batteries and when they need to be charged it w ill turn back on.

For a double-conversion system with separate battery chargers, there needs to be a rectifier to support the DC Bus during normal operation.

C- It also has a hidden objective which is to draw a sinusoidal current from the mains and also to ensure the current drawn is in phase with the voltage waveform so that the current harmonic distortion injected on the mains is less and the power factor is better.

Therefore, Other names used for rectifiers are battery charger and AC to DC converter.

The rectifier in a three phase UPS is designed to operate under:

Nominal input voltage of 415V and
Frequency of 50Hz.

Taking into consideration voltage fluctuations, the rectifier is typically designed to operate with:

An input specific voltage range of ±15% and
Frequency range of ±6%.

In general, the best rectifier topology should have:

High efficiency,
High power factor (PF) and
Low current distortion (THDi).

This will ensure good compatibility with Generators and also reduce the need to oversize the DG set, incoming transformer and cable sizing for supporting the UPS.

The technology of the UPS has evolved and different technologies are being used in the rectifier of the UPS. A short comparison of different rectifier technologies is given in Fig-2.

Fig-2 Comparison of Different Rectifier Technologies

2- Inverter

The primary objective of the inverter is to convert DC power to AC power and to support the loads.

The DC power can be either from the rectifier or from the battery connected to the DC bus of the UPS System.

The inverter is a critical component as this acts as a source to the critical loads connected to it. As a source, the inverter has to support the loads with sinusoidal voltage waveform under below conditions:

Zero break power from mains to battery mode,
Static and dynamic loading conditions,
Overload conditions,
Linear and Non-Linear loading conditions,
Faster fault clearing,
Overload handling capability.

There are two main Inverter topologies namely with:

Transformer in the inverter output and
Transformer less inverter topology.

We already explained these topologies in article “Classification and Types of UPS – Part Seven”

Inverter typologies

As already seen for the rectifiers, also the inverters can be separated in different typologies, according to the constructive solution chosen (TYPE 1, TYPE 2 & TYPE 3)

TYPE 1

It’s the single-phase inverter, with the following manufacturing features:

Use of two power components, each containing two IGBTs
Installation on a single heatsink
Forced cooling with single fan
Power connections carried out through interface card IBPC-7 (PB120), which includes the DC capacitors and the Hall effect CT.

TYPE 2

It’s the three-phase inverter, with the following manufacturing features:

Use of two power components, each containing two IGBTs
Installation on a single heatsink
Forced cooling with single fan
Power connections carried out through interface card IBPC-7 (PB120), which includes the DC capacitors and the Hall effect CT.

TYPE 3

It’s the three-phase inverter used starting form the 40kVA. The power components are connected with cables and/or copper bars, without interface card. Double IGBT packs (that is a single component containing two IGBTs) are generally used up to 160kVA range

3- Energy Storage

When electrical service is disrupted (i.e., mains failure), the UPS continues to support the load connected to it through its energy storage system. A stored energy source provides DC power to the inverter when the normal AC power is not available. This normal AC source could be utility or generator power.

The UPS may provide power for durations ranging from 10 to 20 seconds to several hours. Shorter duration UPSs are designed to carry the load during the start-up of back-up electrical generators, typically diesel engine driven generators, and to enable a smooth transition to the generator as the power source.

In many cases, the UPS is designed to provide power for 5 to 30 minutes. The purpose is to enable an orderly shutdown of operations thereby avoiding an abrupt shutdown, which would otherwise cause equipment damage, product/work losses or a security/safety hazard. The under-desk UPS for PCs is an example.

UPS with enough energy to provide power for several hours are somewhat rare. A key reason is that, in most situations, it is less expensive to store energy in the form of diesel fuel (for generators) if backup power is needed for several hours.

There are different technologies of energy storage solution available in the market like:

Battery,
Flywheels,
Ultra capacitors,
Hydrogen Fuel Cells.

The selection of right energy storage system depends on:

Required runtime/backup time,
Power density/footprint,
Weight,
Lifespan / cycle count,
Reliability,
Cost of Ownership (Initial cost /Maintenance cost),
Operating temperature.

3.1 Energy Storage system - battery

Battery is the most critical component in the UPS and is also considered as heart of the UPS System. Without, battery the UPS is just a power conditioner.

The purpose of the battery is to provide the energy necessary to supply the load when the mains supply in not available.

Cost of battery is a major component on the final price of the UPS solution proposed to the customer.

A battery is an electrochemical device that stores energy at one time for use at another. The battery uses electrical energy to store energy in chemical form which is converted to electrical energy during the discharge of the battery (see Fig.3)

Fig.3 battery is an electrochemical device

The UPS battery may furnish power to the inverter for a few seconds, many minutes, or hours. The battery capacity is determined by the amount and duration of power the inverter has to deliver to the load from the battery.

Types of Battery

Three common varieties of battery chemistries popularly used in UPS applications (Fig.4) are:

Lead Acid,
Nickel Cadmium,
Lithium Ion.

Fig.4 Types of Battery

3.1.1 Lead Acid Battery

It is very commonly used as the storage battery or secondary battery where electrical energy can be stored as chemical energy and this chemical energy is then converted to electrical energy as and when required.

The conversion of electrical energy into chemical energy by applying external electrical source is known as charging of battery.

Whereas conversion of chemical energy into electrical energy for supplying the external load is known as discharging of secondary battery.

During charging of battery, current is passed through it which causes some chemical changes inside the battery. This chemical changes absorb energy during their formation.

When the battery is connected to the load, the chemical changes take place in reverse direction, during which the absorbed energy is released as electrical energy and supplied to the load.

The principle working of lead acid battery

The main active materials required to construct a lead acid battery are:

Lead peroxide (PbO2 ).
Sponge lead (Pb)
Dilute sulfuric acid (H2 SO4 ).

The positive plate is made of lead peroxide. This is dark brown, hard and brittle substance.

The negative plate is made of pure lead in soft sponge conditions.

Dilute sulfuric acid used for lead acid battery has ratio of water to acid = 3:1.

During discharging:

Both of the plates are covered with PbSO4
Specific gravity of sulfuric acid solution falls due to formation of water during reaction at PbO2 plate.
As a result, the rate of reaction falls which implies the potential difference between the plates decreases during discharging process.

During charging:

Lead sulfate anode gets converted into lead peroxide.
Lead sulfate of cathode is converted to pure lead.
Terminal potential of the cell increases.
Specific gravity of sulfuric acid increases.

Advantages and Limitations of Lead Acid Batteries

Advantages

Inexpensive and simple to manufacture — in terms of cost per watt hours, the VRLA Battery is the least expensive.

Mature, reliable and well-understood technology — when used correctly, the VRLA Battery is durable and provides dependable service.

Low self-discharge —the self-discharge rate is among the lowest in rechargeable battery systems. • Low maintenance requirements — no memory; no electrolyte to fill.

Capable of high discharge rates

Limitations

Cannot be stored in a discharged condition.

Low energy density — poor weight-to-energy density limits use to stationary and wheeled applications.

Allows only a limited number of full discharge cycles — well suited for standby applications that require only occasional deep discharges.

Environmentally unfriendly — the electrolyte and the lead content can cause environmental damage.

Transportation restrictions on flooded lead acid — there are environmental concerns regarding spillage in case of an accident.

Thermal runaway can occur with improper charging.

The lead acid battery is further classified as:

Sealed Maintenance Free (SMF) VRLA Battery,
Tubular/Flooded Battery,
Tubular Gel VRLA.

3.1.1.A SMF (Sealed Maintenance Free) battery

It is a battery that doesn't require topping up due to negligible water loss. It is designed in such a way that it cannot be opened or refilled. These batteries are safe, maintenance free and are suitable for most UPS applications. The SMF battery will have an additional safety valve which release the excessive formation of hydrogen, as a result of overcharging, in to the atmosphere.

SMF battery works on a recombination technology where the hydrogen gas evolved during the charging process, is converted to water with the help of oxygen present inside the battery container.

The typical cyclic performance of the battery is less and is limited by the operating temperature and the charging profile.

The SMF battery delivers higher power at higher temperatures but the life of battery comes down significantly The SMF battery needs to be installed in a controlled environment to maintain the temperature at 25-27 deg C and an additional hydrogen sensor in the battery room is recommended for installation.

3.1.1.B Tubular/Flooded Battery

Tubular Batteries have openings at top to add distilled water for maintenance and safe running. These batteries are very rugged and used in Cyclic application.

These batteries last longer due to robust design and are suitable for harsh environment applications. The tubular battery can be installed in any environment (other than closed air conditioner room) with proper ventilation and air exchanges as hydrogen evolution from the battery is higher when compared with SMF buttery.

3.1.1.C Tubular Gel VRLA

Tubular Gel batteries require no topping of water and is a sealed, valve regulated lead-acid deep cycle battery that uses a gel electrolyte.

These type of batteries are rugged and suitable for cyclic applications but are maintenance free compared to flooded tubular batteries.

3.1.2 Nickel cadmium cell (Ni-Cd)

The active components of a rechargeable Ni-Cd battery in the charged state consist of:

Nickel hydroxide (NiOOH) in the positive electrode,
Cadmium (Cd) in the negative electrode,
For the electrolyte, usually caustic potash solution (potassium hydroxide) is used.

Due to their low internal resistance and the very good current conducting properties, Ni-Cd cells can supply extremely high currents and can be recharged rapidly.

These cells can operate over a large temperature range, from +60°C down to -20°C.

The selection of the separator (nylon or polypropylene) and the electrolyte (KOH, LiOH, NaOH) is also of great importance. These elements influence the voltage conditions in the case of a high current discharge, the service life and the overcharging capability of the cell.

In the case of misuse, a very high-pressure may arise quickly. For this reason, these cells are equipped with a reversible safety valve, which can act several times.

NiCad cells offer a long service life (depending on the type of application and charging unit up to 2000 cycles).

Advantages and Limitations of Ni-Cad Batteries

Advantages

Fast and simple charge — even after prolonged storage. High number of charge/discharge cycles — if properly maintained, the Ni-Cd provides 2000 charge/discharge cycles.

Good load performance — Ni-Cd allows recharging at low temperatures. • Long shelf life – in any state-of-charge.

Simple storage and transportation — most air freight companies accept the Ni-Cd without special conditions.

Good low temperature performance

Forgiving if abused — the Ni-Cd is one of the most rugged rechargeable batteries.

Economically priced — the Ni-Cd is the lowest cost battery in terms of cost per cycle.

Available in a wide range of sizes and performance options — most NiCd cells are cylindrical

Limitations

Relatively low energy density — compared with newer systems.

Memory effect — Ni-Cd must periodically be exercised to prevent memory affect.

Environmentally unfriendly — Ni-Cd contains toxic metals. Some countries are limiting the use of Ni-Cd battery.

Has relatively high self-discharge — needs recharging after storage.

Comparing Different Types of Battery (see Fig.5)

Fig-5 Comparing Different Types of Battery

3.1.3 Lithium Ion battery

Lithium-ion batteries offer several advantages over traditional valve-regulated, lead acid batteries as follows:

Commonly used in UPSs today,
A much longer life span,
Smaller size and weight,
Faster recharge times,
Declining prices.

Therefore, lithium-ion batteries become an appealing energy storage technology option for energy storage.

Similar to the lead- and nickel-based architecture, lithium-ion uses a cathode (positive electrode), an anode (negative electrode) and electrolyte as conductor, it consist of:

The cathode is a metal oxide and
The anode consists of porous carbon.

During discharge:

The ions flow from the anode to the cathode through the electrolyte and separator.

During Charging:

It reverses the direction and the ions flow from the cathode to the anode.

When the cell charges and discharges, ions shuttle between cathode (positive electrode) and anode (negative electrode). On discharge, the anode undergoes oxidation, or loss of electrons, and the cathode sees a reduction, or a gain of electrons. Charge reverses the movement.

All materials in a battery possess a theoretical specific energy, and the key to high capacity and superior power delivery lies primarily in the cathode. For the last 10 years or so, the cathode has characterized the Li-ion battery.

Common cathode material include:

Lithium Cobalt Oxide (or Lithium Cobaltate),
Lithium Manganese Oxide (also known as spinel or Lithium Manganate),
Lithium Iron Phosphate,
Lithium Nickel Manganese Cobalt (or NMC),
Lithium Nickel Cobalt Aluminum Oxide (or NCA).

Advantages and Limitations of Li-ion Batteries

Advantages

High energy density — potential for higher capacities.

Relatively low self-discharge — self-discharge is less than half that of Ni-Cd and NiMH.

Limitations

Requires protection circuit — protection circuit limits voltage and current. Battery is safe if not provoked.

Subject to aging, even if not in use — storing the battery in a cool place and at 40 percent state-of-charge reduces the aging effect.

Moderate discharge current.

Subject to transportation regulations — shipment of larger quantities of Li-ion batteries may be subject to regulatory control. This restriction does not apply to personal carry-on batteries.

Expensive to manufacture — about 40 percent higher in cost than Ni-Cd. Better manufacturing techniques and replacement of rare metals with lower cost alternatives will likely reduce the price

Not fully mature — changes in metal and chemical combinations affect battery test results, especially with some quick test methods.

Different Technologies of Lithium Ion Battery (see Fig.6)