• Isolation in UPS Systems
• The Impact of Leading Power Factor of Blade Servers on Derating of UPS Systems
• Measuring High Availability Power Protection Systems
• Looking After The Battery
• Reliability of parallel redundant UPS
• The Real Cost Of Protecting Investment In IT

Isolation in UPS Systems

There are three distinct types of isolation in a UPS system: the galvanic isolation between input and output, the input isolation between mains and battery, and the isolation between the dc circuit and the UPS output. It is important to understand the distinction between these types of isolation in order to avoid misinterpretation of specifications.

Galvanic isolation input/output

In transformer-based UPS systems the transformer is used to step up the voltage at the output of the inverter to a voltage compatible with the utility or generator supply voltage. A common misconception is that the transformer is also used to provide galvanic isolation, which is not the case. In transformer-based UPS systems, the neutral line passes through the bypass line and therefore no galvanic isolation between UPS input and output is provided. If a true and total galvanic isolation is required in transformerbased or transformerless UPS, an additional transformer is necessary at the output of the UPS, so that a galvanic isolation from the load is provided for both the inverter and bypass.

Input isolation between mains and battery

In the early 60s, when only open lead-acid batteries where available, galvanic isolation was required for safety reasons. Since the late 80s, when the maintenance-free lead-acid or nickel-cadmium batteries came into use, input galvanic isolation was abandoned. Today this isolation is very rare.

DC-component output isolation

Transformer-based technology

As mentioned above, in transformer-based UPS systems the transformer is used to step up the voltage at the output of the inverter to a voltage compatible with the utility or generator supply voltage. Furthermore, the transformer isolates DC components, and therefore the inverter transformer isolates the DC circuit from the output load. Fig. 1 shows a block diagram of a transformer-based, double-conversion UPS system. It can be seen that the transformer is on the output of the inverter and not on the output of the UPS.

There are two possibilities for the DC component to pass from the UPS to the load – when there is an inverter IGBT fault or a bypass thyristor fault.

In the event of an inverter IGBT fault – if, for example, IGBT 2 of the inverter does not conduct – a DC component will be generated, and in the transformer-based UPS the output inverter transformer will isolate the inverter DC component from the load (Fig. 2).

In the event of a bypass thyristor fault – if, for example, one of the thyristors does not conduct – a considerable DC component will feed the load as the transformer does not isolate the bypass. The transformer-based UPS does not control this DC component (Fig. 3).

Transformerless technology

As transformerless UPS technology (Fig. 4) does not provide an inverter output transformer, the DC-component issue must be handled differently. The DC component is blocked at the output by hardware and software regulation and control so that it cannot be fed to the load. The transformerless UPS behaves as follows in the two cases.

In the case of an inverter IGBT fault – if, for example, IGBT 2 of the inverter does not conduct – a DC component will be generated. Transformerless UPS technology handles the DC component by means of a fully-redundant EDCP (electronic dc protection) system, so that the probability of a DC component appearing at the inverter output is practically zero (Fig. 5). How does the EDCP system in PowerWAVE transformerless UPSs work?

The PowerWAVE UPS is provided with a fully-redundant EDCP system on the inverter side consisting of three parts. Firstly, redundant DC-component regulation continuously detects (double, redundant detection) and regulates (double, redundant regulation) the DC component within a tolerance of ±10mV. Note that a normal mains supply to which all non-protected equipment is exposed has a DC-component tolerance of ±300mV.

Secondly, redundant DC-component control continuously detects (double, redundant detection) the DC component, and if it is higher than 4V the DC-component control circuit (double, redundant control) will automatically and instantly transfer the load to bypass. The inverter, rectifier and booster will be switched off, and the battery will be disconnected. The alarm DC-COMPONENT FAULT will appear. To make sure the DC component does not appear on the load side, the EDCP system operates at all times, even if the UPS is in LOAD-OFF mode. Bearing in mind that the DC-component detection, regulation and control circuits are redundant, this makes the EDCP system very safe and secure.

Thirdly, a DC component may appear on the output if one IGBT fuse blows and the other IGBT continues to conduct. The PowerWAVE inverter bridges are designed in such a way that if one of two vertical fuses (F1 or F2) blows the other fuse will also automatically blow, preventing the DC component flowing to the load.

Using advanced electronic technology, the EDCP system is extremely reliable. The probability of a DC component passing through a transformerless EDCP system is no higher than the probability of a transformer going short circuit (and allowing the DC component to pass). Furthermore over 6000 PowerWAVE three-phase transformerless UPS systems are operating and being protected by the EDCP system, without a single case of a DC component appearing on the load.

In the case of a bypass thyristor fault, PowerWAVE UPS are provided with an additional EDCP system on the bypass side, which detects if one of the static bypass SCRs is not conducting. In this event, the load will be automatically transferred to inverter within 2 to 5ms in order to avoid a DC component on the load side.

High reliability

In the PowerWAVE transformerless UPS, the addition of the EDCP system on the bypass side provides more effective overall protection from the DC component than simply having a transformer on the inverter side.

Further Information

Uninterruptible Power Supplies Limited is the UK’s foremost independent provider of power support solutions. For further information about its products and services visit www.upspower.co.uk

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The Impact of Leading Power Factor of Blade Servers on Derating of UPS Systems

Data centres are dynamic computer environments. In recent years the increasing mix of old and new computer technologies is causing the overall power factor of the computers/servers to shift towards unity. Furthermore with the introduction of powerful blade servers the overall power factor may even become leading.

This server evolution is becoming a big challenge for IT managers as most of the installed legacy UPS systems, with PWM (pulse width modulated) inverter switching, are designed to provide maximum power at lagging power factors. These UPS systems are approaching their kW power limits due to the change of loads from lagging to leading power factors, or may even shift into an overload condition. The majority of legacy UPS topologies that are installed in IT environments experience a typical derating up to 30% compared with modern Transformerless topologies.

Derating of UPS topologies with leading loads

Legacy UPS topologies are designed to provide maximum kW power for lagging loads, typically at PF = 0.8. If the load shifts from lagging to leading PF, legacy doubleconversion UPSs will derate substantially and hence reach or exceed their rated power. The PWM inverter switching in most transformer-based UPS systems is slower and does not manage to avoid derating when supplying loads with leading PF. Modern Transformerless UPSs with adaptive inverter switching experience a limited derating at unity and leading PF.

Fig. 1 shows typical values of power versus load power factor for both modern Transformerless and legacy UPS topologies. Legacy UPS topologies (300kVA) typically provide 214kW at PF = 0.95_lead, which corresponds to 11% derating with respect to the rated power at PF = 0.8_lag, and 182kW at PF = 0.90_lead, which corresponds to 24% derating.

Transformerless UPSs (300kVA) experience no derating up to PF = 0.95_lead with respect to the nominal power at PF = 0.8_lag, and have a derating of approximately 3% at PF = 0.90_lead.

Fig. 2 shows that the Transformerless UPS can provide substantially more power than equivalent legacy UPSs. The 300kVA Transformerless UPS provides up to 26kW more power for a 200kW load with PF = 0.95_lead, or up to 50kW more power for a 200kW load with PF = 0.90_lead, than equivalent legacy UPSs, which corresponds to 25% of the total load value.

If a legacy UPS is supplying a load with traditional servers and the IT manager decides to introduce blade servers to achieve higher computing density, the power demand will grow and the legacy UPS may reach its power limits or may even be in overload. In this case, there are various ways to overcome the problem.

1. Replace existing legacy UPS with a legacy UPS of higher output power rating. This may cause changes in the power distribution and installation, and is a high cost solution.
2. Add another parallel UPS to the existing legacy UPS. This may cause changes in the power distribution and installation and is a very high cost solution.
3. Replace legacy UPS with a Transformerless UPS which provides more power for leading loads. This is a lower-cost solution.

When new data centre power requirements are assessed it is very important to evaluate the power that the specified UPSs can provide at leading power factors. The shift to leading power factors gives a clear advantage to Transformerless UPSs with respect to legacy UPSs. Due to the substantial derating of legacy UPSs when powering loads with leading power factors, it will be possible in many cases to specify a smaller Transformerless UPS against a larger legacy double-conversion UPS.

Meeting the challenge

Fig. 3 shows how two typical UPS topologies cope with blade servers with leading input power factor, which represents a major challenge for legacy double-conversion UPS.

The shift to data centres with leading power factors opens the door to the competitive advantages of Transformerless UPS, particularly in the output power range from 160 to 300kVA, single and parallel configurations.

Uninterruptible Power Supplies Limited is currently involved in the development of data centres where the introduction of blade servers is presenting a challenge. With the company’s PowerWAVE Transformerless UPS topology, the data centre manager can respond to most of the power growth demand that cannot always be accommodated by legacy equipment. PowerWAVE Transformerless UPS topology offers these advantages for data centres:

• low derating for leading computer power factors up to PF = 0.9_lead
• lower installation costs than those associated legacy UPS
• no need for over-sizing of the generator thanks to low THDi
• less energy loss and less heat dissipation (meaning lower air conditioning costs) thanks to high efficiencies.

Further Information

Uninterruptible Power Supplies Limited is the UK’s foremost independent provider of power support solutions. For further information about its products and services visit www.upspower.co.uk

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Measuring High Availability Power Protection Systems

The Power Availability Index

High availability is one of the most important issues in computing today. Understanding how to achieve the highest possible availability of systems has been a critical issue in mainframe computing for many years, and now it is just as important for IT and networking managers of distributed processing. A certain amount of mystery surrounds the topic of power availability, but consideration of just a few important points leads to a metric which IT managers can use to increase their systems and applications availability and make a rational price/performance purchase decision.

The importance of high systems availability

Availability is a measure of how much time per year a system is up and available. Usually, companies measure application availability because this is a direct measure of their employees’ productivity. With critical applications, or parts of critical applications, physically distributed throughout the enterprise, and even to customer and supplier locations, IT managers need to take the necessary steps to achieve high applications availability throughout the enterprise.

Power availability is the largest single component of systems availability and is a measure of how much time per year a computer system has acceptable power. Without power, the system, and most likely the application, will not work. Since power problems are the largest single cause of computer downtime, increasing power availability is the most effective way for IT managers to increase their overall systems availability. Power availability, like both systems and applications availability, has two components: mean time between failures (MTBF) and mean time to repair (MTTR). The two most important issues in increasing power availability are therefore increasing the MTBF and decreasing the MTTR of the power protection system.

Increasing MTBF

MTBF is the average number of hours it takes for the power protection system to fail. The MTBF of the system can be increased in two ways: by increasing the reliability of every component in the system, or by ensuring that the system remains available even during the failure of an individual component. There is a finite limit to how reliable individual components can get, even with increased cost. Today, typical power protection systems that rely only on high component reliability achieve MTBF between 50 000 hours and 200 000 hours.

By adding a level of redundancy to the system it is possible to achieve a three- to six-fold improvement in MTBF for power protection devices. Redundancy means that a single component of a power protection system can fail and the overall system will remain available and protect the critical load.

Of course, component reliability is a requirement of any system. However, Fig. 1 shows the diminishing returns of increasing component reliability. Line 1 shows the plateau that occurs when MTBF is increased by using more reliable (and therefore more costly) components. Line 2 shows how redundancy, in addition to component reliability, can raise MTBF to the next plateau.

Decreasing MTTR

One way that systems downtime can occur is when both the power protection system and the utility power fails. A shorter MTTR can decrease the risk that both of these events will occur at the same time. By driving the MTTR towards zero, it is possible to essentially eliminate this failure mode.

Adding hot-swappability to a power protection system is the most effective way of decreasing MTTR. Hot-swappability means that if a single component fails, it can be removed and replaced by the user while the system is up and running. When hotswappability is used in conjunction with a redundant system, MTTR is driven close to zero, since the device is repaired when there is a component failure but before there is a systems failure.

The Power Availability (PA) Chart

The relationship between power availability, redundancy, and hot-swappability is easily explained by using the PA Chart, which categorises power protection systems in quadrants according to how well they meet the requirements of high power availability – redundancy and hot-swappablity. As more components in a system become hotswappable, the system moves from the bottom to the top of the graph (Fig. 2), and as more components become redundant, it moves from the left to the right of the graph. IT managers can choose the solution that is right for them, depending on the need for high availability and the amount of money they want to spend.

The PA Chart corresponds to the types of power protection systems available today as shown in Fig. 3. The standalone UPS is neither hot swappable nor redundant. As shown in the table, a standalone UPS provides normal power availability because uptime is dependent on the reliability of the UPS itself.

The fault tolerant UPS is sometimes described as providing affordable redundancy. Systems of this type have redundant components but not all of the major components are hot-swappable. This type of system offers high power availability because the power protection system will continue to protect the load when a component fails. But because a failed component often results in the entire UPS needing replacement, this type of system can have serious drawbacks, including expensive and time-consuming repair with both systems downtime and a major inconvenience for IT managers. Fault tolerant UPS systems may have some hot-swappable components, such as batteries and a subset of power electronics, but in most cases a high number of critical components, such as the processor electronics, will not be hot-swappable. The more components that are not hotswappable, the lower the power availability.

Like fault-tolerant UPS, modular UPS offer high power availability. Modular UPS have multiple hot-swappable components and are typically used for multiple servers and critical applications equipment. Many modular UPS also have redundant batteries. Their main advantage over fault-tolerant UPS is that all of the main components which can potentially fail can be hot-swapped, eliminating planned downtime due to a service call.

The PowerWAVE range of modular UPS offers the highest level of power protection currently available in the UPS market. In a PowerWAVE modular UPS the power electronics, batteries, and processor electronics are both redundant and hot-swappable. This system provides very high power availability and the highest level of protection for IT managers’ critical loads. A PowerWAVE modular UPS may cost a little more than a similarly-rated standalone UPS, but the increased system reliability and availability are invaluable to the IT manager.

The Power Availability (PA) Index

The different types of power protection systems in the PA Chart can be measured linearly with the PA Index, according to how much power availability they provide. The PA Index serves as a tool to explain the difference between power protection systems. Fig. 4 shows each of the quadrants from the PA Chart mapped into a level of the PA Index. Fig. 5 shows the relative power availability provided by each type of system. The PA Index maps directly into the PA Chart and makes the different characteristics of high availability power protection systems clear.

Conclusion

In conclusion, IT managers can use the PA Chart and the PA Index to help them choose the right power protection system for their high availability applications. The standalone UPS, the modular UPS, and the PowerWAVE 9000 Series modular UPS all offer real benefits in terms of power availability versus cost. Although fault-tolerant UPS offer high power availability – and are marketed as such – they introduce serious drawbacks including a high MTTR and potentially significant inconveniences for IT managers.

Further Information

Uninterruptible Power Supplies Limited is the UK’s foremost independent provider of power support solutions. For further information about its products and services visit www.upspower.co.uk

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Looking After The Battery

The Key Component In UPS Systems

The battery plays a key role in the overall reliability and availability of a power protection system. It supplies the energy required by the critical load in the event of a mains utility failure, or when the input mains voltage and frequency are outside the acceptable values. Moreover, the battery represents an important share of the total cost of the UPS, and therefore battery care and management are of paramount importance when a UPS is designed.

Battery care can be defined as ‘protection from life-shortening factors’, and battery management as ‘correct battery charging and advanced battery diagnostics’. Improper care and management of batteries is the number one cause of system downtime. In order to optimise battery lifetime, runtime and performance, UPSs require a battery management system and battery protection features.

What factors affect battery life?

The two factors with major impact on battery lifetime are temperature and the number of battery discharge-recharge cycles. Battery life expectancy will dramatically decline with increasing ambient temperature, as shown in Fig. 1. Every battery discharge-recharge cycle reduces the lifetime of the battery – the higher the percentage of total available energy that the battery delivers during a discharge, the greater the reduction in battery life. The main reason for this is the deterioration of the battery contacts (poles) with the increasing number of discharges-recharges.

Frequent discharging of batteries in UPS systems causes them to reach end of life long before the expected end of life in float service. In addition, discharge cycles which occur for most large step load changes on the UPS output may be of short duration but should still be considered as battery discharge cycles.

Key features of a battery management system

The goals of a battery management system are to protect the battery from negative environmental impacts, such as high temperature and misuse, and to avoid curtailing battery life by advanced management of battery charging and failure-preventing diagnostics. If these goals are achieved, the enduser will be required to replace batteries less often, with financial and environmental benefits. A well protected and managed battery is a healthy battery, and hence it enhances the overall availability of the UPS system.

It is important to note that in a UPS, the design of the DC-circuit is decisive with regard to the battery lifetime. Fig. 2 compares traditional DC-circuit technology with the advanced FBM (Flexible Battery Management) technology found in PowerWAVE UPS. In the FBM design, the introduction of SCRs protects the battery by blocking all the negative impacts (AC-ripple and unwanted disturbances) that are generated by the rectifier/charger and/or inverter and traditionally reach the battery.

AC-ripple free battery charging

AC-ripple (Fig. 3) generated by the rectifier/charger and/or inverter is the major cause of internal battery temperature increase and deterioration of battery poles. Most UPS systems in use today have DC circuits which are rich in AC-ripple. The SCR protective circuits in PowerWAVE UPSs provide battery charging which is free of AC-ripple, reducing the internal battery temperature and protecting the battery poles. In addition, an algorithm optimises the battery charge voltage to the ambient temperature. Both of these features help to prolong battery life.

Reducing the number of discharge cycles

Frequent discharging cycles and high discharge currents cause the battery poles to deteriorate dramatically. Many batteries used in UPS applications tolerate a relatively low number of discharges before they must be replaced. In order to reduce the number of battery discharges, PowerWAVE UPSs provide a wide input voltage range (-23%/+15% @100%load, -40%/+15% @60% load) and a wide frequency range (35 to 70Hz).

In traditional UPS designs where the rectifier/charger and inverter are directly linked to the battery, discharge cycles occur in the event of load jumps at the output of the UPS.These load jumps cause a decline in the DC-link voltage, and the battery will be discharged. Even though the discharge time is typically only 150msec, the high discharge currents (Fig. 4a) will cause chemical reactions which will again degrade the battery poles. In PowerWAVE UPSs, the SCRs separate the battery charger from the battery, and prevent a DC-link voltage decline, or battery discharge, in the event of load jumps (Fig. 4b).

Protection from misuse and inadequate charging voltages

In order to proactively protect the battery from misuse or from low and high charge voltages, PowerWAVE UPSs are equipped with protective controls which are performed during the start-up procedures and during normal operation.

During the system start-up, a configuration and polarity control procedure performs a cross verification between the battery parameter values configured in the UPS and the real battery voltage. An alarm is generated if there is any discrepancy. During operation, a battery fuse check is executed every 60 minutes to monitor battery presence, and a further control is executed every 30 seconds to verify that the battery voltage doesn’t exceed the permissible limits.

In special circumstances, the floating voltage can be fine-tuned, depending on the number of battery blocks or whether an external battery temperature probe is connected to the UPS. In some circumstances it may be useful to adjust the minimum battery voltage in order to increase the autonomy time and define the level at which the inverter will turn off during battery operation. This value depends on the number of battery blocks, and can vary depending on the instantaneous load.

Proactive battery failure detection

Faulty batteries are typically discovered when the UPS transfers to battery mode in response to a utility failure and the critical load crashes. In order to help prevent such a serious situation, PowerWAVE UPSs are equipped with two kinds of battery test in order to proactively detect a faulty battery. A battery test can be performed manually or automatically at defined intervals, bearing in mind the desirability of minimising the number of discharge cycles. The test simulates a battery discharge for one minute and, if the result is positive, allows a real battery discharge for one minute. At either stage the test is aborted and an alarm generated if the battery voltage falls below a pre-configured level.

A deep battery discharge test safely discharges the battery completely in order to check the actual battery capacity status. This test should only be performed under manual control, and must be adaptable to parallel UPS configurations.

Flexible configuration for least-cost autonomy

Because PowerWAVE UPSs can operate with variable battery voltages, various battery configurations are allowed per UPS model. Flexibility in respect of battery capacity and the number of battery blocks can exactly match runtime requirements at lowest cost.

A new level of confidence

Detection of battery faults is implemented in most three-phase UPS systems. An advantage in PowerWAVE UPSs is that the FBM algorithm, which is controlled by the CPU microprocessor, continuously monitors the batteries through an automatic and adjustable test system which detects battery faults and generates an early warning. The battery charge current is adjustable and limited, and is dependent on the battery temperature. There is also battery minimum and battery maximum voltage control and limitation.

Advanced FBM implemented in PowerWAVE UPSs offers the highest confidence that UPS batteries are being protected and managed in order to realise best performance and longest lifetime. A well-protected and well-managed battery is a healthy battery, enhancing the overall availability of the UPS system and critical applications.

Further Information

Uninterruptible Power Supplies Limited is the UK’s foremost independent provider of power support solutions. For further information about its products and services visit www.upspower.co.uk

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Reliability of parallel redundant UPS

The primary objective in the implementation of a UPS system is to improve power reliability to the limits of technical capability, the ultimate aim being to totally eliminate the possibility of any power disturbance or downtime. When they appeared in the 50s, the first static UPS systems comprised a rectifier, battery and inverter, and were used to stabilise the output power and to continue to supply the load for a short period of time in the event of a rectifier failure. The reliability of this simple UPS chain depended predominantly on the inverter reliability. An inverter failure meant an immediate load.

In the early 60s the static bypass switch was introduced to enable an interruption-free load transfer to the standby mains in the event of an inverter failure or overload. The standby mains, although far less reliable than the UPS, serves as a reserve power supply in the event of an inverter failure, enabling continuation of the power supply to the load while the inverter is being repaired. This new architecture substantially improved the overall reliability, which no longer depended predominantly on the inverter reliability. The reliability of the new UPS with static bypass depended on the quality of the mains (MTBF_MAINS), the time-to-repair of the UPS (MTTR_UPS), and on the reliability of the static switch.

However, the activities which are dependent upon computer-controlled real-time information systems have grown exponentially in recent years and the highest reliability UPS configurations have become an everyday requirement. Very critical loads cannot rely on a power supply configuration of a single UPS with static bypass system; the need for (n+1) redundant parallel UPS configurations is becoming standard. A comparison of the reliability for various UPS configurations is based on the reliability figures presented in MIL-HDBK-217 F (Not.2, 1995). The following calculations were implemented on the PowerWAVE Series UPS and have been confirmed by field statistics.

Single UPS without static bypass switch (SBS)

The reliability of a single UPS without bypass depends on the reliability of the rectifier, battery and inverter (see Fig.1). For example, in the event of an inverter fault, the load would crash.

Calculation of MTBF (MRBF_SU)

(MTBF_SU is the mean time between failures of single unit without static bypass
λUPS is the failure rate of single unit without static bypass switch
λRECT is the failure rate of the rectifier
λBATT is the failure rate of the battery
λINV is the failure rate of the inverter)

MTBF_UPS = 1/λUPS

λUPS = λRECT + λBATT + λINV

Figures from statistical failure analysis show λRECT = 20 per million hours,
λBATT = 10 per million hours, λINV = 20 per million hours. If these figures are applied in the equation, MTBF_UPS for a UPS system without static bypass switch will be 20 000 hours.

Single UPS with static bypass switch

The reliability of a single UPS can be increased significantly by introducing a redundant mains power source and linking it to the main UPS supply source by means of a static bypass transfer switch (Fig. 2). For example, in the event of an inverter fault the load will not crash, but will transferred to mains without interruption.

Calculation of MTBF (MTBF_SU+SBS)

(For all subsequent calculations:

MTBF_UPS+SBS is the mean time between failures of single unit with static bypass switch
MTBF_M is the mean time between failures of the mains
λ_UPS+SBS is the failure rate of a single unit with static bypass switch
λSBS is the failure rate of the static bypass switch with control circuit
λPBUS is the failure rate of parallel bus (only for parallel systems)
λM is the failure rate of the mains
μSU is the repair rate of the static bypass switch (μSU = 1/MTTR_UPS)
μM is the repair rate of mains (μM = 1/MTTR_M)
MTTR_SBS is the mean time to repair of static bypass switch
MTTR_M is the mean time to repair of the mains)

MTBF_UPS+SBS = 1/λ_UPS+SBS

λ_UPS+SBS = λUPS//λM+λSBS

λUPS/λM = λUPS λM (λUPS + λM + μUPS+μM)
λUPS λM +μUPS μM +(λUPS+λM) (μUPS+μM)+λ²UPS+λ²M

= λUPS λM (μUPS+µM) = 6 per million hours
μUPS μM

Note that all calculations are performed using the following constants:

MTBF_M = 50 hours, this figure represents a ‘‘good quality’ mains

MTTR_UPS = 6 hours

MTTR_M = 0.1 hours.

Furthermore, from statistical failure analysis, the figures for the failure rates of the static bypass switch for the power part and the control electronics part give λSBS = 2 per million hours.

Using these results:

λ_UPS+SBS = λUPS//λM + λSBS = 6 + 2 per million hours = 8 per million hours, or

MTBF_UPS+SBS = 125 000 hours.

In the above formula it can be seen that the reliability of the UPS with static bypass switch (MTBF_UPS+SBS) depends largely on three parameters: the reliability of the mains, the MTTR of the UPS and the reliability of the static bypass switch. This dependence is illustrated in Fig 3.

Parallel redundant UPS with static bypass switch

The reliability of a single UPS can be increased significantly by introducing a redundant parallel configuration (Fig. 4).

Calculation of MTBF for an (n+1) redundant parallel UPS system (MTBF_(n+1)UPS+SBS)

Failure rate, λ_(n+1)UPS+SBS = (λUPS1//λUPS2……..//λUPS(n+1)) + (n+1)λPBUS + (λSBS1//λSBS2……..//λSBS(n+1)) ~ (n+1)λPBUS

Reliability, MTBF_(n+1)UPS+SBS = 1/λ_(n+1)UPS+SBS

Availability A_(n+1)UPS+SBS = MTBF_(n+1)UPS+SBS
MTBF_(n+1)UPS+SBS + MTTR_UPS

The reliability of an (n+1) parallel redundant system depends largely on the failure rate of the parallel bus, which is the only single point of failure. The UPS parallel redundant chains, the static bypass switches and their control electronics, as well as the mains power lines are all redundant and have therefore minor or even negligible impact on the overall reliability.

The following constants are used in the calculations (Fig. 5):

MTBF_M = 50 hours, this figure represents a ‘good quality’ mains

λPBUS = 0.4 per million hours.

Comparison of UPS configurations

In the single UPS chain (rectifier, battery and inverter), the reliability of the UPS largely depends on the reliability of the inverter.

By introducing the static bypass switch, that is a reserve mains power supply, the reliability will increase by a factor of six if the mains MTBF is 50 hours (good quality) and the MTTR of UPS is six hours. This reliability level is unfortunately not sufficient, because it still depends substantially on the reliability of the raw mains and on the quality of the UPS aftersales organization (response time, travelling time, repair time, etcetera). Modern critical loads cannot rely on the mains quality and on longer repair times.

To overcome the dependance on the raw mains, n+1 redundant parallel UPS configurations are recommended. The disadvantage of traditional standalone n+1 redundant configuration is the relatively long repair time of the UPS (typically six to 12 hours). By implementing modular, hot-swappable, n+1 redundant parallel systems based on modular PowerWAVE technology, the critical load will be completely mains independent. A faulty UPS module may be replaced without the need to transfer the rest of the UPS modules to raw mains. Furthermore the replacement of the modules takes at most 0.5 hours, which dramatically decreases the time-to-repair in comparison with traditional parallel systems.

The following example shows the impact on the reliability and availability of the choice of UPS configuration, comparing a traditional (1+1) redundant UPS configuration and a modular (4+1) redundant UPS configuration.

Fig. 6 shows a block diagram of two redundant UPS configurations. The system on the left represents a (1+1) redundant configuration with traditional standalone UPSs, whereas the system on the right side represents a (4+1) redundant configuration with modular hot-swappable UPSs.

Availability (A) is an important parameter when evaluating the reliability of UPS configurations. A is defined as:

A = MTBF_UPS
MTBF_UPS + MTTR_UPS

Fig. 7 compares the availability of the configurations shown in Fig. 6. Note that the MTBF_UPS figures are taken from Fig. 5. Two cases are considered:

Case 1: both UPS configurations have the same MTTR_UPS, six hours

Case 2: the traditional standalone UPS configurations has MTTRUPS of six hours, whereas the modular UPS configuration with hot-swappable modules has MTTR_UPS of 0.5 hours.

In Case 1, the availability of the (1+1) redundant configuration is higher than the availability of the (4+1) redundant configuration if the MTTR is the same for both configurations. This is due to the fact that the MTBF of a (1+1) redundant configuration is higher than the MTBF of a (4+1) redundant configuration.

In Case 2, the availability of a (1+1) redundant configuration with longer MTTR may be lower than the availability of a (4+1) redundant configuration with a shorter MTTR.

Conclusion

These cases show how important MTTR is for reaching high availabilities. If in one of the above redundant configurations one of the UPS is faulty, there will be no redundancy left (low-availability regime) and the faulty part/module must be repaired or replaced as quickly as possible in order to restore redundancy (high-availability regime). With PowerWAVE modular UPS, the shortest MTTRs are achieved and, consequently, the highest availabilities, even if a larger number of modules are paralleled.

Further Information

Uninterruptible Power Supplies Limited is the UK’s foremost independent provider of power support solutions. For further information about its products and services visit www.upspower.co.uk

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The Real Cost Of Protecting Investment In IT

In making a decision to purchase or to invest, it is important to measure the total cost of ownership (TCO) in order to predict how the investment will be paid back. In the case of a data centre, the individual cost factors involved in protecting the investment include the necessary infrastructure for providing power, cooling and IT equipment protection.

A methodology for determining TCO (excluding that of the IT equipment itself) is illustrated by considering the cost factors of a UPS system investment for a data centre with an 80kVA load requirement. It is assumed that 100 percent of the 80kVA load is required.

Total cost of investment in a UPS system depends not only on the purchase price but also on

• capital cost including the purchase price and transportation costs determined by weight and volume
• building/footprint costs including installation cost, power density (kVA/sq m), and security concept (redundancy, availability)
• operating costs, including energy costs related to technology-dependent UPS losses, additional costs associated with cooling system energy losses, the cost of maintenance, training for maintenance, and spare parts stock, logistics and handling
• upgrade costs dependent upon flexibility and capability to upgrade without load interruption.

The major contribution to the total cost of a data centre is usually an oversized or inefficient UPS system. Taking the case of a UPS system with a load of 80kVA, total costs and performance of a traditional UPS system are compared with those of an advanced modular UPS system. It is assumed that for optimum availability, a parallel redundant solution (n+1) is selected. A traditional parallel configuration of two 80kVA UPS is therefore compared with an advanced modular parallel configuration of three 40kVA UPS (Fig. 1).

Capital cost

Purchase price

The purchase price of a traditional UPS system is typically 10 to 15 percent less than that of an advanced modular ups system. However, the purchase price is not the only decisive factor when considering overall costs. The lower purchase price of traditional UPS technology must be offset against significantly higher operating costs in comparison with a modular system based on technology which reduces energy loss costs. The higher cost of the modular system is recovered within the first year of operation. A comparison of additional long-term costs also favours modular technology.

Transportation cost

A traditional UPS is usually built with an output transformer, which implies a total weight up to two or three times higher than that of a transformerless UPS system. This weight difference can increase transport cost by 50 percent or more (Fig. 2).

Building/footprint costs

Installation costs and power (in kVA) per footprint surface

The traditional UPS system based on two UPS units typically needs two to three times the amount of floorspace in m2 required for an advanced modular UPS system such as the PowerWAVE 9000 Series UPS (Fig. 3).

Security concept (redundancy, availability)

System availability is dependent on the mean time between failures (MTBF) and, even more, on time to repair in the event of a failure, mean time to repair (MTTR). In modular UPS systems, MTTR can be up to 12 times less in comparison with traditional UPS systems because a module can be quickly exchanged without load interruption, increasing the total availability of the UPS system to 0.999999. Fig. 4 illustrates how system MTBF and MTTR affect the availability of two seemingly equivalent systems.

In Example 1, the MTBF of the non-modular system is shown as being higher than the modular system simply because it comprises of two rather than three UPS. If the MTTR of the non-modular and modular systems were the same (for example, six hours) then the non-modular system would have the higher availability (because of the higher MTBF). However, Example 2 shows how the modular system’s availability is significantly increased when its true MTTR of 0.5 hours is considered.

Operating costs

UPS and cooling system energy losses

Energy costs are directly proportional to a complete system’s efficiency at a defined load. UPS systems do not generally work under the ideal circumstances of 100 percent load, but tend to work under partial load conditions. It is therefore important to pay attention to the inner architecture of a UPS system, especially in relation to performance and efficiency under partial load conditions.

For most mission critical loads, parallel redundant systems deliver less than 50 percent of the load power per UPS. This implies lower efficiency than for full load conditions. With the PowerWAVE 9000 modular UPS, smaller power units are configured in parallel, for example three small modules instead of two large standalone systems. This also achieves redundancy, but with the advantage of a better performance and higher efficiency in partial load (now 66.7% loaded) conditions (Fig. 5). Fig. 6 shows the corresponding energy savings.

Maintenance costs

Maintenance of a traditional UPS system, with its greater volume and costly construction of individual components, is much more time consuming than that of its modular counterpart. The maintenance costs of a modular system are up to 30 percent lower compared with a traditional system. Individual components of modular systems are smaller and easier to manage, and therefore easier to replace.

Spare part stock, logistics and exchange

Traditional UPS systems are not built as system-modules and therefore it is very difficult to propose a cost-efficient spare part package. For security reasons, often the most extensive and expensive spare parts kit will be selected. Even then, there is no guarantee that the spares kit will be effective or contain the part required for any or all failures which could arise, and there is a time overhead for stock management and logistics.

The hot-swappable technology of the modular system eliminates the complication of choosing the right spare parts kit. All that is required is a single replacement module, and even when there are different power ranges in operation, holding the highest kVA-rated module as a spare covers all eventualities. Personnel can swap modules within 30 minutes with little training, minimal effort, lowest expenditure and smallest footprint. Through the use of spare modules, it is possible to save up to 50 percent on logistics and stock management costs (Fig. 7).

Training costs

If there are many different types of UPS systems within a company, maintenance training for each individual system is time consuming and costly. In contrast, modular systems over a wide range of output powers will have the same board layouts and architecture. The know-how gained by training on one UPS module can be applied to other UPS models without additional training. Because it is no longer necessary to have systembased specialists, savings related to the training of maintenance personnel can be up to 67 percent.

Upgrade cost

Upgrading a traditional UPS demands extra space and costly cabling. In addition, the existing UPS needs to be shut down during the upgrade.

With PowerWAVE modular UPS, the upgrade is performed by simply inserting the additional power module/s into the rack. For example, three 20kVA modules may be replaced by three 30kVA modules, provided the system’s distribution and frame has been specified for the maximum foreseeable requirement. Such upgrades can be performed without any interruption to the load, without increasing the footprint, and with no additional work on site. This unique flexibility makes upgrading a system very easy, and implemented with only five to 10 percent additional costs.

Summary of cost comparison

Fig. 8 compares typical first-year values for a traditional configuration of two 80kVA UPS for a total load of 80kVA (50 percent per system unit) with a modular configuration of three 40kVA for a total load of 80kVA (66.7 percent per module). First-year costs for traditional technology are expressed as a percentage of the costs for a modular configuration (100 percent).

Fig. 9 shows a breakdown of costs for the first year of operation, showing that savings on a modular configuration as the highest item, exceeding purchase cost and far outweighing the differential between the costs of traditional and modular systems.

Further Information

Uninterruptible Power Supplies Limited is the UK’s foremost independent provider of power support solutions. For further information about its products and services visit www.upspower.co.uk