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

In the previous article “ Components of Online Double Conversion UPS– Part One”, We showed that an online double Conversion UPS has (4) major components As shown in the Fig.1:

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

Fig.1 Components of Online Double Conversion UPS

We explained the first three components and indicated that the energy storage system has many types as follows:

Battery,
Flywheels,
Ultra capacitors,
Hydrogen Fuel Cells.

We explained the battery and today we will continue explaining some related topics of battery and other energy storage systems in addition to this we will explain the earthing principles of UPS Systems.

1- Battery Configurations

The most used battery configuration are:

Serial Strings,
Parallel Strings.

Notes:

The configurations in above are samples – on-site arrangements will obviously differ from site to site.
In any battery configuration, all of the cells used in a serial string must be identical to each other.

1.1- Serial Strings

A serial string is a single series of blocks connected ‘end-to-end’ to form the battery. The positive terminal of the first block is connected to the negative terminal of the second block, the positive terminal of the second is connected to the negative of the third, etc. (see Fig.2)

Fig.2 Serial Strings

The overall voltage of the battery is the sum of the individual block voltages and must be arranged to match the float voltage setting of the UPS.

The capacity of the battery is unchanged with this arrangement, being the same as each individual block.

For example:

If 12 nos. x 12V 10Ah blocks are connected in series, the resulting battery is 144V with a 10Ah capacity.

1.2- Parallel Strings

A parallel string is a combination of two or more serial strings, and each string must contain the same number of blocks. Batteries are paralleled for two main reasons:

Either to increase the resilience of the battery bank so that a single faulty battery will not cause all of the batteries to be unavailable to the UPS, or
To increase the total capacity (Ah rating) of the battery bank.

The positive terminal of the first battery string is connected to the positive terminal of the second battery string, the positive terminal of the second is connected to the positive of the third, etc. (see Fig.3)

Fig.3 Parallel Strings

The negative terminal of the first battery string is connected to the negative terminal of the second battery string, the negative terminal of the second is connected to the negative of the third, etc.

The overall voltage of the battery is the same as the voltage of each string.

The capacity of the battery is the sum of the capacities of the individual strings.

For example:

If 3 strings of 12 nos. x 12V 10Ah batteries are connected in parallel, the resulting battery is 144V with a 30 Ah capacity.

Note:

Due to potential equalization problems (i.e. unequal charge being taken on by individual batteries), it is unusual for more than six battery strings to be connected in parallel.

2- Battery Size and Location

UPS suppliers offer a range of standard batteries, all designed to support the full UPS load but with various back-up/autonomy times.

UPS suppliers often accommodate the batteries in different types of steel structure as follows:

UPS cabinet, (see Fig.4)
Additional cabinets: which match the UPS, are usually available for larger battery installations – these may need to be built to order,(see fig.4)
Open type racks: they usually require the batteries to be kept in a separate battery room with controlled access arrangements. They are used for Larger or ‘non-standard’ installations or to suit a particular installation, (see fig.5)
Cladded racks: They are used for Larger or ‘non-standard’ installations or to suit a particular installation. (see fig.5)

Fig.4 UPS cabinet & Additional cabinets

Fig.5 Open type racks & Cladded racks

The table below gives some sample battery sizes and weights.

UPS Rating (kVA)	Autonomy (Minutes)	Size Inc. UPS H x W x D (mm)	Weight (kg) including UPS
5	5 45	One off*- 690 x 200 x 690 Two off - 690 x 200 x 690	77 226
15	20	Two off - 690 x 200 x 690	353
30	6 25	One off*- 1400 x 580 x 750 Two off - 1400 x 580 x 750	490 1055
60	8	Two off - 1400 x 580 x 750	1060
120	15	Three off - 1800 x 580 x 750	2960

* Batteries located within the UPS cabinet

3- Battery Transition Boxes

Transition boxes (see fig.6) are used to simplify the connection of a battery to a UPS. In addition to providing space for the correct termination of battery cables.

they also contain suitable fuses to protect the individual battery strings and associated cabling.

Fig.6 Battery Transition Boxes

When two or more parallel battery strings need to be connected to the same UPS, it is common to use transition boxes. Fused transition boxes allow individual battery strings and cables to be protected and also enable an individual battery string to be safely isolated for maintenance or repair without completely disconnecting the UPS equipment.

It is important to keep the lengths of cables within each battery string approximately the same to ensure that the impedance (and hence the current share) of each battery string is approximately the same

4- Battery Monitoring

Equalization problems can be minimized using a modern battery monitoring application, such as the patented Battery Analysis & Care System (BACS), which can provide integrated battery monitoring and management over an Ethernet network.

Using web-management technology, the system sequentially checks the internal resistance, temperature and voltage of each individual battery block and corrects its charging voltage as required to obtain a balanced charging condition across the battery string.

By constantly monitoring and controlling the individual charging voltages for each battery block it ensures they are kept in their optimal voltage operating range and guarantees the availability of the battery at all times.

Other benefits from using the BACS equalization system include:

1- Avoid overcharging:

Through the equalization process the unnoticed overcharging of individual batteries (gassing, dry-out, thermal runaway) is prevented.

2- Avoid undercharging:

Through the equalization process the unnoticed undercharging of individual batteries (sulphation, loss of capacity) is prevented.

3- Indication of battery problems:

Typical battery problems such as sulphation, corrosion, gassing, dryout, thermal runaway etc. are visible through a rise of impedance and temperature.

4- Avoid sulphation :

Sulphation is a typical problem for UPS batteries because they are consistently held at a float charge level for a long time. Its not guaranteed that ALL batteries have really been fully charged when the UPS charge switches from boost charging to float charging. The result maybe that some batteries are overcharged, while others have never been fully charged. Equalization avoids sulphation through the process of bringing the overcharged and undercharged accumulators to a balanced voltage level.

5- Show stratification:

BACS warns of a possible stratification of the electrolyte through detecting increasing impedance and drifting voltages. The stratification can be removed through a discharge process and the BACS will indicate this effect through a lower impedance and improved equalizing.

6- Early warning to replace batteries:

Through impedance trending you can see in the early stage that some battery blocks are damaged or simply weaker than others. The earlier accumulators are replaced the better for an increased lifetime of the complete battery system.

7- Extension of service life up to 30%:

The service life of all the batteries in a string depends on the weakest member – i.e. the weakest battery. By equalizing, all batteries are kept constantly in their ideal voltage window so that all negative influences of wrong charging voltages and currents within the string are eliminated.

8- Improved maintenance:

A BACS system improves the service quality by providing remote monitoring through Internet, VPN or other network for downloading real time data and battery history for analysis. Single, individual battery tests are now possible without the need to disconnect batteries from the group. Maintenance and battery testing are able to take place at any time, under real operating.

Second: Energy Storage System - Flywheel

Flywheel stores electrical energy in the form of kinetic energy during charging process and during the discharging the kinetic energy is converted into electrical energy.

A typical system consists of (see fig.7):

A rotor suspended by bearings inside a vacuum chamber to reduce friction, connected to a combination of electric motor/electric generator.

Fig.7 Flywheel

First generation flywheel energy storage systems use a large steel flywheel rotating on mechanical bearings. Newer systems use carbon-fiber composite rotors that have a higher tensile strength than steel and are an order of magnitude lighter.

During charging process, the motor rotates at 1000rpm in clockwise direction to store the electrical energy in the form of kinetic energy. During discharge the motor acts as a generator and will convert the kinetic energy back to electrical energy

Magnetic bearings are necessary; in conventional mechanical bearings, friction is directly proportional to speed, and at such speeds, too much energy would be lost to friction.

The flywheel used mainly in rotary UPS systems using rotating mass for energy storage, rotary UPS systems have more mechanical moving pieces than a static UPS. The spinning flywheel of a rotary system can typically only provide AC power for 5–15 s at full load, so a backup source such as a diesel generator may be coupled to the generator to provide longer runtimes (Fig.8).

Fig.8 Diesel Generator with Rotary UPS & Flywheel

Advantages and Limitations of Flywheel

Advantages

• High energy density — potential for higher capacities.

• Low Maintenance — no periodic discharge is needed; no memory.

• Flywheels are not affected by temperature changes unlike chemical rechargeable batteries

• Shorter time to recharge

• Long Life >20 years

Limitations

• Can be used only for a short backup time, in few seconds

• High power applications with shorter backup time

Third: Energy Storage system – Super Capacitors

SuperCaps (also known as ultra-capacitors or electric double-layer capacitors) (see fig.9) provide an alternative source of DC power to traditional rechargeable batteries. Super capacitors are high density energy storage devices with a capacitance (energy density) of up to 10,000 times that of conventional electrolytic capacitors.

Fig.9 SuperCaps

Super capacitors or double layer capacitor store energy much in the same way as a conventional capacitor, hence the amount of stored energy can be described by: A double layer capacitor consists of two electrodes, a separator, electrolyte, two current collectors and housing.

A very high capacitance is obtained in this way. Super capacitors are suitable for high power applications and offer very quick response times and high efficiency.

Disadvantages are comparatively low energy density, high self-discharge and high cost.

Small units exists, larger sizes are under development.

Typical power ratings are 1kW-250 kW and efficiencies in the ranges of 85-98%

Advantages and Limitations of Super Capacitors

Advantages

• Short duration runtime critical applications

• Compact foot print and power density

• High working ambient temperatures

• ECO friendly low environmental impact

• High energy efficiency and low running costs

• Lower Total Cost of Ownership (TCO)

Limitations

• High self-discharge

• Ride through for shorter power outages in seconds.

Forth: Hydrogen Fuel Cells

When hydrogen and oxygen combine to produce water a chemical reaction occurs, one by-product of which is electrical energy. Hydrogen fuel cells can therefore be said to convert hydrogen gas into electrical energy.

Fig.10 Hydrogen Fuel Cells

Each hydrogen fuel cell (see fig.10) has two electrodes, an anode and a cathode, that are separated by a polymer electrolyte membrane. Oxygen is passed over the cathode and hydrogen is passed over the anode. The hydrogen molecules are converted into electrons and protons when they pass over a catalyst (typically platinum) on the anode. The electrons flow out of the fuel cell as electrical energy whilst the protons flow through a membrane to the cathode where they combine with the oxygen to produce pure water. Figure 8.2 shows a typical hydrogen fuel cell.

Hydrogen fuel cells are significantly more expensive than batteries and, because hydrogen is an explosive gas, great care has to be taken with its storage. Also, because hydrogen is currently manufactured from natural gas and energy is required to make the hydrogen the “environmentally friendly” credentials of hydrogen fuel cells are currently questionable.

Hydrogen fuel cells are smaller and lighter than batteries and the research and development currently taking place into the use of hydrogen fuel cells in automotive applications will have spin-off benefits for “standby” fuel cell applications.

4- Static switch

Static Switch The static switch is the last major component in a traditional double-conversion UPS system. The purpose of the static switch is to provide a method to transfer the critical load from the inverter to a bypass source without interruption. If bypass power is available and acceptable, the system will transfer the load to the bypass source.

It’s important to understand that bypass power is not UPS protected power. If the input source is lost, the critical load will lose power. The static switch is critical when a UPS has a failure or maintenance is required.

The static switch of an on-line UPS has two operational states, ‘on UPS’ (the normal condition) and ‘on bypass’. When the UPS is operating on bypass there will be an accompanying alarm or warning condition as in this state the critical load is not protected from mains disturbance or interruption. In both cases it is the job of the static switch to provide a very fast, break-free, transfer between the inverter output and the bypass.

The static switch can be considered to be an intelligent switch that decides whether to use the UPSs inverter output voltage or the raw mains to supply the load. The decision is made by the static switch’s own and/or the UPSs control logic which continually monitors the bypass (raw mains) and inverter voltages.

The control logic typically controls the phase and frequency of the UPS inverter(s) to ensure that the bypass and inverter voltages are in phase (synchronized) with each other. Bi-directional, break-free transfer between the two supply sources is only possible when the bypass and the inverter are ‘synchronized’.

As we mentioned before in Article “Classification and Types of UPS – Part Five” that there are two types of bypasses (see Fig-11):

Static bypass,
Maintenance bypass.

Fig.11 UPS bypass switches

1- Static bypass (internal Maintenance Bypass):

It gets its name from the original UPS systems using a “static switch” to bypass the rectifier and inverter when a fault occurs. This allows the load to continue to operate from unprotected power.

The switch is called static because it is an electronic switch that is solid state, not mechanical.

The static switch provides a method to bypass the rectifier and inverter when maintenance must be completed without shutting down the load.

Note:

Although the UPS power blocks can be totally isolated while the load is powered through the maintenance bypass supply, making it safe to carry out maintenance procedures etc., there will still be live power within the UPS at its power isolators and input/output terminal connections.

2- Maintenance bypass (External Maintenance Bypass):

It Is external manual switch separated from the UPS, It is usually a wrap-around switch either of rotary or MCB design , this switch ensures a continuous flow of power while your UPS is undergoing maintenance or repair. Without it, your network may suffer significant downtime, leading to lost revenue and productivity.

Bypass Interlocking

Interlocking between the maintenance bypass and UPS isolators is required to ensure that the load is transferred between the two power sources in a controlled manner. This is necessary to ensure that the load is uninterrupted during the transfer, and the UPS is not damaged by back-feeding from maintenance bypass supply into the UPS output terminals while the inverter is on load.

The power isolators within the UPS are invariably electrically interlocked to prevent such problems occurring. However, when an external maintenance bypass circuit is employed, additional electrical or mechanical interlocking devices are usually required.

Static bypass Types (fig.12)

1- Single-phase static switch

It’s composed by two pairs of thyristors, connected in anti-parallel, that interrupt the phase conductors (inverter/bypass)

The bypass component is protected by a fast-acting fuse

In order not to modify the grounding system the neutral conductor is not interrupted

2- Three-phase static switch

It’s composed by six pairs of thyristors, connected in anti-parallel, that interrupt the phase conductors (inverter/bypass)

Fig.12 Static bypass Types

Static switch typologies

There are 3 Static switch typologies, The three types use different firing cards, that vary on the basis of the components layout.

TYPE 1

It’s the single-phase static switch.

TYPE 2

It’s the three-phase static switch that uses compact type thyristors. (SemiPack)

TYPE 3

It’s the three-phase static switch that uses disc-type thyristors (used only on the 500kVA and 650kVA).

Earthing Principles of UPS Systems

First: For Transformer-based/Transformer-less UPS with bypass

In this system Fig.13, the UPS Neutral Should not be bonded to the grounding conductor. This earthing configuration is similar for both transformer based / transformer-less UPS and This earthing configuration has the following functions:

Does not change the upstream and downstream earthing system
Does not provide any galvanic Isolation or
Does not provide Common Mode Noise attenuation.

Fig.13 Earthing System-UPS with bypass

Second: For Transformer-based UPS without bypass

In this system Fig.14, the UPS neutral should be bonded to the grounding conductor. This earthing configuration has the following functions:

Change’s the upstream and downstream earthing system
Provides galvanic Isolation
Provides Common Mode Noise attenuation.

Fig.14 Transformer Based UPS without Bypass

Third: For Transformer-less UPS without bypass

In this system Fig.15, the UPS neutral should not be bonded to the grounding conductor. This earthing configuration is similar for both transformer based / transformer-less UPS with bypass and This earthing configuration has the following functions:

Does not change the upstream and downstream earthing system
Does not provide any galvanic isolation or
Does not provide Common Mode Noise attenuation.

Fig.15 Transformer-less UPS without bypass

Forth: For UPS with Isolated bypass

In this system Fig.16, a transformer is introduced in the bypass path of the UPS. The output neutral of the UPS should be bonded to the grounding conductor. The UPS acts as a seperatively drived source. This earthing configuration has the following functions:

Change’s the upstream and downstream earthing system
Provides galvanic Isolation
Provides Common Mode Noise attenuation.

Fig.16 UPS with Isolated bypass

Points to be taken care while earthing UPS

Using of 4 Pole breakers at the input is prohibited as it breaks the input neutral which results in the floating of output neutral.

The output neutral of the UPS should not be earthed unless there is a transformer in the bypass or global input.

The bypass mains and the rectifier mains has to be supplied with power from the same earthing system. If the earthing systems are different a transformer needs to be considered in bypass or rectifier input.

In TNS system, the neutral earthing should be done only at one point(at source) only, multiple earthing is prohibited as per standards.

Selecting the Earthing Conductor Size

Please see below the Earthing conductor size requirement. This is based on the standard IEC 60364-5-54

Cross Sectional Area of the phase conductor (mm2)	Minimum cross section area of PE conductor (mm2)
Sph ≤ 16	16 sq mm
16 < Sph ≤ 25 25 < Sph ≤ 35	16 sq mm
35 < Sph ≤ 50 Sph > 50	Sph/2