UPS Systems – What Does On-Line Double Conversion Mean?

UPS Systems – What Does On-Line Double Conversion Mean?

By G Bennett, Standby Systems

Abbreviations:

IGBT – Integral Gated Bipolar Transistor
PWM – Pulse Width Modulation
THD – Total Harmonic Distortion
UPS – Uninterruptible Power Supply

A UPS is there for one primary reason – to provide uninterrupted power to the load. We recognise reliability as a primary consideration [1], but must also appreciate that there are preferred topologies. This article makes the case for online double conversion.

There are several different types of static UPS systems with maximum reliability and quality of power being achieved by using the on line double conversion principle. All of the leading UPS suppliers from Riello Aros, Liebert, APC Schneider, Powerware and Meissner offer these products.

An online double conversion system is a system which is connected between mains and a sensitive load, guaranteeing a safe and reliable power supply to this load, regardless of the condition of the mains. It contains no moving parts and is equipped with a battery to supply backup power to the load for a specified time.

What Is Online Double Conversion?

The words ‘online double conversion’ in terms of a UPS system means what it says. The UPS consists of five major components:

  • A rectifier – first conversion;
  • An inverter – second conversion;
  • A static bypass switch;
  • A manual bypass switch;
  • A battery.

The double conversion takes place in the conversion of AC mains to DC via a rectifier, this DC charges the battery and supplies the inverter which converts the DC to AC mains again by means of PWM. Hence the double conversion. The on line comes from the fact that the UPS is on all the time giving the load complete isolation from the mains supply voltage and frequency fluctuations.

Figure 1 clearly shows the mains or generator supply fed into the rectifier (first conversion), via the battery bus to the inverter (second conversion). It also shows a static and manual bypass switch.

Types of UPS or uninterruptible power supplies
Figure 1: Schematic diagram of a double conversion UPS

System Components

Rectifier

The rectifier usually consists of a controlled thyristor bridge, which converts the AC mains into DC. The purpose of the rectifier is to charge the batteries while at the same time supplying the full inverter load as well as accommodating for machine losses.

During the operation of a rectifier, harmonics are generated. These high frequencies can have a detrimental effect on the mains supply. Typical effects of harmonics are overheating of cables and circuit breakers, non-operation of power factor correction equipment resulting in higher electricity bills and other spurious problems.

Although a UPS rectifier will contribute to the harmonic content of the mains supply, it is not the only cause of harmonics. Harmonic content in a mains supply will vary from site to site and is largely dependent on the supply impedance and what types of loads are connected to the supply.

In most cases a standard six pulse rectifier will generate in the region of about 30% THD. This can be enough to cause problems with a mains supply. The best option is to eliminate the UPS harmonic generation by the installation of a filter or a 12 pulse rectifier / IGBT rectifier, and thus ensure that the UPS harmonic content generated is removed from the overall sum of harmonics.

After the rectifier has generated DC, this area is known as the DC link. During generation of the DC, the rectifier will produce an AC ripple, which is super-imposed on the DC.

Ripple can have serious effects on the battery’s life expectancy and performance, and is thus filtered out.

The rectifier should be equipped such that it will recharge a battery in the shortest possible time, but at a rate recommended by the manufacturer. The recharge rate is critical as too fast or too slow a recharge will have an effect on the battery’s life expectancy and performance.

Inverter

The UPS inverter is basically the reverse of a rectifier. The DC power available from the DC link is chopped up by a process known as PWM. Basically the DC is switched on and off via IGBTs, in short bursts of different amplitude and for different time periods, thereby forming a harmonic rich sine wave. The wave is then filtered and a pure sine wave output is produced.

The inverter is one of the most important parts of any UPS system, as its ability to perform can have a detrimental effect on the load. A UPS inverter should have as a minimum the following characteristics:

  • Overload – 110% for one hour, 150% for 30 s – this allows the UPS to accept load surges on switch on off loads, and will prevent the load from being dropped in a case when mains is not available;
  • Step load – ±5% for 100% load step – the UPS should be capable of accepting the application of a full inverter load in one step without the mains varying beyond the maximum and minimum voltage limits of the load;
  • Crest factor – 3 to 1 at 100% rated inverter load – the crest factor is the ratio of peak to average current. Typical UPS loads are switch mode power supplies in pcs. These power supplies will draw very high currents (peak currents) for short periods during the rectification process. When a UPS is fully loaded, its ability to accept such loads becomes very important as if it cannot, it will lock out in bypass.

Rating

A UPS’s rating is very important and should be understood:

In terms of electrical theory: Real Power (kW) = Apparent Power (kVA) x Power Factor

Let’s assume a UPS of 200kVA as a working example:

Thus 200kVA at unity power factor is 200kW and 200kVA at 0.8 power factor is 160kW 2In the modern world loads such as IT, instrumentation and control, lighting etc. run at power factors of between 0.6 and 0,8 lagging. Due to this fact the UPS industry has downgraded machines and all UPS units will have a power factor attached to their rating.

A 200kVA UPS is typically sold as a 0.9 power factor machine i.e. 200kVA 0.9 power factor = 180kW

If the load is 200kW, then a 200kVA machine will not do the job. During sizing of the system a 200/0.9 or 250kVA machine will be required.

In practise, always divide the load sizing by 0.9 to get the UPS sizing and be careful, some manufacturers produce machines with 0.7 or 0.6 power factor ratings as well. Therefore you should know the power factor of the section of your plant to be supplied.

Static Bypass

The static bypass in a UPS is a very fast electronic switch.

When the UPS runs normally, power flow is via the rectifier and inverter to the load. Power is also available to the input of the static and manual bypass. The inverter looks at the mains power available at the static bypass input and stays synchronised to the mains provided it remains within the set parameters for voltage and frequency

Should the inverter develop a fault or should any problem occur within the UPS then the inverter will switch off and at the same time the static switch will be fired. Thus a no break transfer will occur from inverter to mains, and the load will be fed by mains. This also occurs on excessive overload.

Similarly, when the inverter is repaired and switched on again then the inverter will start up and synchronise to the mains. Once locked onto mains frequency the static switch will be switched off and the inverter takes over the load – again without a break.

Manual Bypass

The manual bypass is a mechanical switch which should only be used when the static bypass is in operation. The main reason for this device is to manually and mechanically connect mains to the load. Once this has been done, the rectifier, inverter, DC link, batteries and static switch should be able to be isolated if they are fitted with mechanical isolation devices.

Software

Most UPS systems nowadays are available with several forms of monitoring and shutdown software. The software interfaces whether RS232, RS485, Ethernet, SMS link or normal contact interface can be a useful tool to both the user of the UPS and the supplier. Care should however be taken not to buy the software instead of the product.

Conclusion

The online double conversion UPS design principle offers maximum protection for your loads. It does have disadvantages like harmonic generation into mains and efficiencies of about 2% less than other methods – but when it comes to a UPS, load protection remains the main priority.

#1 References

[1] Bennett G. “UPS systems and their effects on reliability”. Electricity+Control, May 2001.