Electrical noise and mitigation

Introduction

Process control/SCADA systems used in industries and computers/communication equipment deployed in offices are sensitive to power interruptions and voltage/frequency excursions and require an uninterrupted power supply of good quality free from harmonics, voltage irregularities, etc. In this section, we will take a look at different uninterrupted power supply options available and in particular solid-state inverter-based systems. We will also touch on the issues that are important in selecting small and medium capacity UPS systems that one finds in common use these days in many premises. The issues relating to the grounding of these systems, as separately derived sources, will also be discussed from extensive references available in IEEE: 142 publication.

Power quality issues

Power supply systems in themselves cannot guarantee 100% reliability. Nor can they ensure that power supply parameters stay within the stipulated limits all the time. This is so because the point of generation and the point of usage are separated by large distances from a few miles to a few hundred miles. The power supply to consumers comes from utility grids with scores of large generators interconnected by long transmission lines, step up and step down transformers, distribution lines and switchgear at different points along the transmission and distribution chain. The transmission and distribution lines mostly are through overhead lines, which are prone to faults by lightning, failure of insulators, and so on. Even the failure of a single line can have a cascading effect by causing system instability due to sudden load shifts and can cause much more extensive disruption.

Generating equipment can also have outages due to various reasons and in turn can cause frequency excursions in the system they feed to, as it struggles to cater to the increased demand on the remaining generators. Availability of adequate spinning generation reserve mitigates the problem to a large extent.

Tripping of lines or generators can alter the flow of power in a grid system resulting in sustained outages. The process of detection and clearing of a fault is accompanied by momentary voltage drop in the affected and adjacent parts of the transmission network.

Once the fault is cleared, the resulting power flow changes can cause sustained voltage drop due to overloading of the healthy lines. Disturbances are also caused by abnormalities in the distribution system closer to the consumer. As a matter of fact, close short circuit faults can cause severe voltage dips in the consumer supplies and can cause resetting of computer hardware since their switched mode power supply cannot ride through all but very brief voltage dips. Motors fed from the affected distribution systems can be disrupted, as the contactors feeding them tend to drop out during these dips. Also, sometimes the motors themselves pull out of step, as the torque generated during reduced voltage situations cannot sustain the torque required by the mechanical loads they drive.

In this section, we will mainly consider voltage amplitude disturbances, which can be corrected by UPS systems. Frequency disturbances and harmonic voltages require other corrective equipment, which are beyond the scope of this discussion.

FIG. 1 shows the results of a study by EC&M magazine regarding occurrences of voltage disturbances and gives us an idea with the frequency of such occurrences in any supply network.

FIG. 1 Power supply disturbances - a breakup

Definitions of abnormal voltage conditions

Sag

A sag is a temporary reduction in the normal AC voltage. A momentary sag is a variation, which lasts for a period of 0.5 cycle to about 2 s usually the result of a short circuit somewhere in the power system. Instances of longer duration of low voltage are called sustained sags (see FIG. 2).

FIG. 2 Sag - momentary and sustained

Swell

Swell is the opposite of sag and refers to the increase of power frequency voltage. A momentary swell lasts from 0.5 cycles to 2 s. A sustained swell lasts for longer periods (see FIG. 3).

FIG. 3 Swell - momentary and sustained

Surge

Surge is a sub-cycle disturbance lasting for a duration of less than half a cycle and mostly less than a millisecond. The earlier terminology was transient or spikes. The decay is usually oscillatory. Surges generally occur due to atmospheric disturbances such as lightning or due to switching of large transformers, inductors or capacitors (see FIG. 4a and b For examples).

FIG. 4a Surge voltage with oscillatory decay

FIG. 4b Surge caused by lightning

Interruption

Interruption means the complete loss of voltage. A momentary interruption lasts from half- cycle period to less than 2 s. Longer interruptions are called sustained interruption.

Momentary interruption is usually the result of a line outage with the supply being restored automatically from another source or by auto-reclosing operation. Refer FIG. 5 for illustration. An interruption can be instantaneous or of slowly decaying type.

FIG. 5 Examples of supply interruption

In FIG. 5, the one at the top shows the RMS voltage value during a momentary interruption. The FIG. on the lower left depicts the waveform of a sustained interruption where the voltage drops to zero almost instantaneously. The waveform on the lower right shows an interruption where the voltage decays slowly.

Susceptibility and measures to handle voltage abnormalities

Many modern office equipment including desktop computer systems can tolerate voltage fluctuations to some extent by virtue of large capacitances (in relation to load currents) and internal regulation circuitry and can ride through a voltage sag or even a momentary interruption. The tolerance range for voltage fluctuations can be -13 to +6% for slow/sustained variations (sag/swells) and greater for short time disturbances. However, longer disturbances call for measures to counter their effects and apply corrections. Sustained low voltages can be corrected by voltage-regulating transformers. Interruptions on the other hand can be tackled by standby sources. Since engine-operated power sources generally need time to start up and pick up load some other type of source, which can feed the load without there being a break of supply will be required. Both mechanical and electrical no-break sources are available in the market and can be used depending on applications.

We will briefly review these devices in the next paragraphs.

Regulating transformer

Slow RMS voltage variations can be effectively tackled by regulating transformers or transformers with on-load tap changers. These devices can be made to correct voltage fluctuations using voltage sensors and circuitry to effect corrections. One type of regulating transformer (a single-phase variety) is shown in FIG. 6.

FIG. 6 Regulating transformer

In this device, a continuously variable auto-transformer with a center tap is connected to the primary of a boost-buck correction transformer. The variable arm of the auto transformer is operated by a servomotor, which is driven by a control circuit which compares the output voltage against a set point value. The amount and polarity of correction is decided by the voltage feedback. Such devices are also called as servo stabilizers.

Three-phase systems employ a similar device called an induction regulator. A typical scheme is shown in FIG. 7.

FIG. 7 Three-phase induction regulator

This device is a transformer constructed like an induction motor. The stator of the device is connected to the power source and forms the primary of the regulating transformer. The secondary winding, which is similar to a wound rotor of an induction motor is connected in series with the supply. The secondary voltage induced in the rotor is of constant magnitude but the phase angle varies with the position of the rotor vis-à-vis the stator. The regulating circuit senses the voltage output, compares it with a set value and sends its output to a servo-motor. The servo-motor adjusts the rotor position based on the command from the regulating circuit. The output voltage is thus adjusted to the set value.

The vector diagram shown in FIG. 8 explains how regulation is obtained.

The induction regulator is essentially an electromechanical device and can correct sustained sags or swells. Momentary variations as well as surges are let through without correction as the device is too slow to react to these fast changes.

Transformers and auto-transformers with multiple taps and on-load tap changers can also be employed for correcting sustained voltage variations. Their time of operation, however, is even greater than servomotor-operated regulators. Also, regulators give step less correction whereas tap changers essentially correct in steps of the order of 1% or more.

All these devices correct voltages within a predetermined band of limits beyond which corrections cannot be made. They are also not a solution for voltage interruptions.

Standby sources

Important installations are normally provided with alternate supply source to ensure continuity of power supply in the event of failure of the normal source. This can be in the form of a second power feeder or a standby source itself. Refer FIG. 9, which illustrates this arrangement.

In the first case of a standby or duplicate feeder as it’s sometimes called, the chance of a total failure cannot be ruled out as both feeders may be from a common source of the same power utility and therefore may fail simultaneously. The second case with an independent standby source is more reliable.

FIG. 8 Vector diagram of induction regulator

It should however be noted that the changeover from normal source to standby source involves a delay of up to 1 s even though this is done using automatic circuit breakers. In the case of the independent source, which is usually an engine-driven alternator, several seconds may elapse before the engine starts and takes over the load. Thus, they are not useful where even momentary interruptions cannot be tolerated.

It’s possible to run the sources in parallel. In the case of the independent source option, this will mean running the alternator synchronized with the utility power source.

Paralleled source configurations have to be designed carefully to ensure that both sources don’t trip in the event of short circuits or other faults. Unless this is achieved, the use of standby sources cannot ensure uninterrupted power.

FIG. 10 with duplicate incoming feeders illustrates how a fault F in one of the incoming lines causes short circuit currents to flow in both feeders and is likely to cause tripping of both sources. The design in this case will call for time-coordinated directional current protection schemes to prevent such maloperation.

FIG. 9 Standby power source

FIG. 10 Current flow in duplicate feeders under fault conditions

FIG. 11 illustrates a case of using a standby source, which is synchronized with the utility source and sharing the load. In case of a major failure in the utility system, the generator has to take the full load and therefore can get overloaded. Under certain conditions the standby source may send power back into the utility grid, causing the generator and engine to sink due to heavy load application all of a sudden. Such an eventuality has to be protected using relays to sense low frequency/rate-of-change of frequency or reverse power flow or both. Also, some of the in-house loads which are less critical may have to be disconnected to relieve the load on the generator.

FIG. 11 Load flow in standby engine source under utility fault condition

Such elaborate systems are possible only in large industrial plant power systems.

Even in these systems problems could occur in downstream distribution, which can disrupt power to critical LV loads. Other measures at LV level are therefore necessary to ensure uninterrupted power supply to critical equipment. Such uninterrupted systems can be electromechanical or electronic. We will discuss them in detail in the coming paragraphs.

Electromechanical UPS systems

Various electromechanical systems employing motor generators with an energy storage mechanism and a standby engine drive can provide an excellent uninterrupted power supply source in situations where large motor and electronic loads of several hundred kilowatts is to be fed. FIG. 12 illustrates a typical system.

FIG. 12 Electromechanical UPS system

In this system, SMG is a machine, which can run both as a generator and a synchronous motor. It operates normally as a motor and keeps the energy storage system, normally a flywheel or a hydraulic/pneumatic accumulator fully charged. When the power fails, the system starts slowing down and using a sensor, the engine can be started with the accumulator providing the starting power. Usually an electromagnetic clutch (not shown in the diagram) is part of the scheme to keep the engine disconnected from the SMG shaft. Once the engine starts, the machine goes to alternator mode and starts supplying power to the loads. There will be a fall of voltage and frequency due to the slowing down of SMG under loss of power condition, but it’s unavoidable in this configuration. Also, it should be noted that under normal mode, the power comes from the primary source and no power quality improvement takes place in the supply system as such. FIG. 13 shows an arrangement where this problem is addressed.

This system is similar to the earlier one except for an additional generator mounted on the same shaft for feeding to critical loads. The critical loads are thus completely isolated from the external system with the power quality solely dependent on the generator itself.

A bypass switch is provided to enable power from the normal source should there be any problem with the isolated generator.

FIG. 13 Electromechanical UPS system - a variant for critical loads

Another version of the combined type of system represented in the case that we just looked at is the dynamic diesel continuous power system as shown on these diagrams from Holec Corporation ( FIG. 14). We’re dealing with a system, which has critical power, but without storage battery reliance, and utilizes a combined diesel engine generator driving through an inner rotor/outer rotor clutch system to drive an output generator to serve the load.

When we look at the results of the speed/time characteristic of this continuous power system we find that while the diesel engine remains off, the inner rotor of the induction coupling is running at full 5400 rpm. Then there comes a changeover to emergency operation where the energy stored in the rotation at 5400 rpm allows the outer rotor to drive the generator set continuously at 1800 rpm while the diesel comes up to fill in the gap. The emergency operation takes place with the diesel running and then a return to normal operation occurs after that (refer FIG. 15, which illustrates this principle).

FIG. 14 Continuous power from a system deploying electrical rotary clutch

FIG. 15 Speed time characteristic of engine-based UPS system

These engine-based UPS systems are usually preferred for very high capacity requirements amounting to several hundreds of kilowatts. The main advantage of the system is that the maintenance and replacement requirements normally associated with large battery banks are totally absent. Also, the harmonic generation by static UPS systems is not an issue when using rotary UPS systems. The maintenance requirements of engine and rotating parts are specific to the rotary type and the power losses and noise are problems to contend with. It’s usual to find these systems in industrial plants producing man-made fibers to feed the fiber line drives. Some manufacturers also cite applications in critical military or aviation systems such as large radar installations.

For other applications in small and medium power range including supply to large ADP installations, static UPS systems with battery power source have become a widely deployed option. A detailed discussion on these systems follows. Also of special interest to us in this guide is the grounding requirement for static UPS systems, which depends on the type of configuration employed. Since electrical noise is a problem to contend with in ADP installations, the UPS grounding issue has received critical attention.

Solid-state UPS systems

In FIG. 16, we have the conventional block diagram for a solid-state UPS system consisting of the AC input to a rectifying device, battery bank, which forms the emergency power source, an inverter, which converts the DC to an AC sine wave output.

The rectifying device supplies the DC input to the inverter and also charges the battery bank under normal conditions.

FIG. 16 Basic solid-state UPS system

The battery storage itself is attached between the charger and the inverter. Then the inverter system converts DC back into AC, sends the AC through a static switch and a manual maintenance switch out to the AC load. The inverter, as notice, is synchronized with the input power frequency and also has a bypass line to the static switch in the event of some type of problem with the rectifier and inverter circuit. UPS systems can either be of on-line or off-line variety. In conventional on-line technology one would have the static switch, as shown, where the AC input power, the DC output from the battery charger and the AC output of the inverter all stay on line continuously. The static switch is not needed to operate, unless there is a severe inrush on the load, a fuse blowing, circuit breaker operation on the load or some problem with the actual output of the inverter part of the system. This is, by far, one of the more popular versions in which no switching takes place when the AC power from the utility is no longer available. In the case of lost input power, the battery charger simply no longer functions, the inverter looks to the DC bus maintained by the batteries and takes its energy from the batteries without any further switching operation. As an alternative, sometimes in smaller sizes and for less expense than the on-line system, there are other versions where the static switch is actually on the bypass. This is the off-line variety of UPS. This saves a good deal of the energy conversion that goes on, but gives rise to some questions as to what type of operating characteristics one can expect when there is nothing between the outside power source and the load that you are protecting. Many of the smaller systems can be built this way, in as much as the load that they are protecting does not worry about any type of power conditioning or separation from the outside, but merely is looking for the uninterruptibility of the battery-supported system. One area to watch for in off-line systems is the fact that they have a tendency to switch rather frequently back and forth. This sometimes creates disturbances because the AC input voltage rises and falls and there is no further regulation of that voltage as the system operates.

Large UPS installations seldom use the off-line model. As a matter of fact, redundant inverter modules sharing the load with a separate bypass supply operating through a static switch is the configuration that most critical installations use.

9 Multiple units for redundancy

In FIG. 17, we see a large system of several UPS units running in parallel in order to provide redundant capacity for the load. The main features are:

• Paralleled inverter outputs share the load.

• Sufficient redundant capacity is provided so that the total load will be supplied by the remaining units when any one unit is out of service for maintenance or because of failure.

• Total capacity of all units is normally available at all times, but at reduced reliability when one unit cannot be used.

• Batteries may be paralleled.

FIG. 17 Solid-state UPS system in multiple redundant configuration

Note that in this arrangement, we don’t need the bypass line as a regular function, but will always have one additional unit more than necessary to run the load. In this example, there might be three 100-kVA units where the load requires 200 kVA and all three units are running at approximately 65-70% capacity. Should one unit fail, a small load step is affected within the range of the specification of the units and the load continues to run without any switching or without transfer to bypass. It continues to maintain the protected element of the circuit. Though in the initial days of UPS systems, the inverter modules were designed using thyristor elements, the advent of the insulated gate bipolar transistor (IGBT) has all but replaced inverter grade thyristors as well as gate turn off (GTO) devices. Being functionally similar to transistors, they offer design simplicity, faster switching, lower losses and produce less audible noise. The IGBTs in high-frequency pulse width modulated type configuration is the preferred choice of today's inverter designers.

10 Considerations in selection of UPS systems for ADP facilities

Designers of facilities for large ADP installations should keep in mind a few basic facts regarding the loads, which they feed and the demands they make on the power supply equipment including the UPS. The power supply infrastructure for an installation should be designed to last at least for a decade. Typical ADP configurations change rapidly and require upgrades and replacement at least once in 3 years. New types of equipment find their way into the installation regularly. The infrastructure will thus have to cater to at least three generations of ADP equipment without major replacements or retrofits.

Different types of equipment behave differently and impose their own requirements on the infrastructure. Also, the mix of equipment is decided by the business involved. For example, a data center or an application service provider (ASP) business will consist of large number of high-capacity servers and few PCs. Typical end-user applications in any normal commercial business will most likely have predominantly desktop PCs and a few servers. Internet access providers will have a number of network switches and routers in addition to servers. The internal power supply devices may be of different designs and the facility designer must give a close look at the specifications of these devices.

The following aspects need consideration.

Power factor of loads

Most power supplies in ADP equipment are of switched mode type (SMPS) and unless corrected by suitably sized capacitors may draw currents at very low power factor, typically 0.6. For the same watt output, a device with lower power factor will draw a higher current. Accordingly, all internal and external wiring, transformers, circuit breakers and other power semiconducting devices in the connected circuits will have to be designed for higher ratings. This makes the design bulkier and more expensive.

Most high performance servers are nowadays provided with SMPS with power factor correction and draw load current at power factor close to unity. Since SMPS systems are specified in terms of watts, a lower PF will call for a higher kVA UPS (kVA = kW/PF). It should also be remembered that the selection of battery from which the inverter gets its supply should take into consideration the active power handled. If the PF of the load is taken as 0.6 PF whereas it’s actually 0.95, then a UPS system of a given kVA rating can be called upon to feed a larger kW load. It’s easy to make such a mistake since the UPS systems are usually specified in terms of kVA rating. The battery selected may become undersized and may not work satisfactorily for the specified duration of backup time (usually 15 or 30 min).

Permissible voltage

Most SMPS loads can operate at voltages from 100 to 270 V. Thus, maintaining constant voltage is not always a necessity. This aspect needs attention since bypass source need not always have to be fed through a voltage stabilizer. The UPS voltage has to be matched with bypass source voltage at all times to ensure a smooth no-break change over.

Permissible frequency

Most ADP loads are insensitive to frequency. They can accept frequencies ranging from 47 to 63 Hz. This means that the UPS with higher-frequency window will be better. Such a design will permit the UPS to track the bypass supply through a larger range of frequency and maintain synchronization between UPS and bypass source. Such synchronization ensures that whenever a change over from UPS to bypass takes place, the same can be done without a break and therefore no danger of resetting of computer systems.

The UPS should have a high slew rate (ability for quick change in output frequency) so that the fall of frequency in the electrical system can be tracked without delay thus maintaining synchronization with bypass.

Backup time for batteries

The backup time for batteries should be selected so as to ensure that there is no need to shutdown systems under any condition including failure of the standby generator to start.

Since the generators are usually started using auto mains failure (AMF) sensors, there may be a tendency to reduce the backup time by the designer. However, the failure of generator to start and need for operator intervention should be considered as a real possibility and backup time selected accordingly.

Harmonics

Since most computer loads draw currents with distorted waveforms, they impose a high degree of non-linearity in the system feeding them. Also, the harmonics from these devices can flow into the other loads of the system through capacitances (which offer a low impedance at higher frequency) and can cause unexplained heating of parts of the circuitry as well as erratic tripping of circuit breakers. Harmonics also cause the normal sinusoidal voltage waveform to get distorted thus making their effect felt throughout the rest of the system. A limit of 5% of harmonic current content must be aimed at it to avoid problems.

Standby generators feeding UPS loads

Generators feeding to ADP installations, which have large UPS equipment, will have to be designed taking into account the lower power factor and harmonic loading. Many generator manufacturers put limits on harmonic loading, which can be handled by the generators. This is due to the fact that harmonic voltages imposed by the non-linear loads can cause higher magnetic losses (due to the higher frequency) in all magnetic cores and active paths. This causes heating of the active parts and limits the generator from being run at their rated capacity. Also presence of capacitors in the load may cause resonance and the generator may fail to pick up voltage.

Short circuit behavior

It’s necessary to design a UPS system in such a way that fault in one circuit fed by it does not cause total outage of the entire loads. Generally, the ability of the UPS to maintain an output voltage under short circuit conditions is limited since excessive currents will damage the semiconductor devices and the design of UPS has to incorporate current-limiting circuitry.

This results in failure of circuit-protective devices to clear the fault since the energy flow is restricted to match the capacity of the semiconductors to handle. The resulting failure of UPS output voltage causes a change over to static bypass. The static bypass clears the short circuit by enabling adequate current flow and takes over the rest of the loads.

If the duration of voltage failure due to a short circuit (till the bypass restores it) is too long, it may cause the loads in the healthy circuit to reset also. To ensure this, the bypass must be in synchronism with the UPS at the instant of fault so that the change over takes place without a break. Hence the need to maintain large voltage and frequency window in the UPS design.

Crest factor

The UPS design should be done so that load inrush by switching of ADP equipment does not cause operation of the inverter protection. Mostly, this is not an issue since UPS systems have a crest factor of 3-5 whereas typical ADP requirements are limited to a crest factor of 2. 9.11 Grounding issues in static UPS configurations

As discussed earlier in this section, the issue of grounding UPS-derived systems is an important one as incorrectly grounded supply systems are unsafe; they result in equipment damage during faults/surges and also result in poor noise performance. We will discuss this issue in some detail.

In general, UPS configurations can be treated as separately derived source. A generator, transformer or convertor winding is a separately derived source if it has no direct electrical connection to supply conductors in another grounded system including a grounded circuit conductor. The UPS output is normally taken as a wye (star) connected winding galvanically isolated from the supply source feeding the UPS. However, many UPS systems are provided with a bypass circuit fed from the same source as the UPS without any isolation. The UPS cannot be considered as a separately derived source unless the bypass system has some form of galvanic isolation (such as a two-winding transformer). The following are applicable in the case of separately derived sources:

• The neutral (grounded circuit conductor) should be bonded to the equipment safety grounding conductor and to the local ground such as building ground network or other made electrodes.

• The grounded conductor from neutral should be connected to the grounding conductor only at the source and not at any other point since it will make the ground fault protection ineffective.

We will now detail out the common UPS configurations and recommended grounding arrangements.

UPS configurations and recommended grounding practices

Configuration 1

In this case, the bypass source and the UPS source are the same and the bypass circuit has no isolation and is connected directly to UPS output. Thus, the definition of separately derived source is not satisfied. The UPS neutral is not therefore connected to the grounding conductor of equipment or to any local grounding electrode. Since there is no isolation between the source and the loads common-mode noise, attenuation is not ensured ( FIG. 18).

FIG. 18 UPS system configuration 1

Configuration 2

In this case, the bypass supply is through a delta-wye transformer and thus there is galvanic isolation between the input supply to the UPS and the output under all conditions. The UPS can therefore be considered as a separately derived source. The neutral point of the UPS is bonded to the downstream equipment grounding wire as well as to the local grounding electrode. The bypass supply neutral is also bonded to UPS output neutral to provide a return path for neutral currents when bypass circuit is in operation (note that the static bypass switch is in-line wires only and the neutral connection is direct). Common-mode noise performance is better in this circuit if the neutral connection between bypass and UPS is kept short ( FIG. 19).

FIG. 19 UPS system configuration 2

Configuration 3

This configuration shown in FIG. 20 has a non-isolated bypass but the UPS output is taken to the loads through a distribution center, which incorporates an isolation transformer.

Thus the UPS module is not a separately derived source by itself but the secondary of the isolation transformer is a separately derived source.

FIG. 20 UPS system configuration 3

Thus the neutral of the UPS module is not bonded to the local bonding conductor.

However, the isolation transformer neutral is bonded to the grounding wire from the computer loads fed by it as well as to the local grounding electrode system.

In this configuration, the power distribution center can be placed as close to the loads as possible so that it gives a better common-mode noise protection compared to the earlier configurations. The isolation transformer can also be used as a step down transformer permitting lower voltage supplies (208/220 V) to be served by UPS modules of higher voltage (380/415/480), which improves the cost-effectiveness of the design for UPS and wiring.

FIG. 21 UPS system configuration 4

Configuration 4

This configuration ( FIG. 21) is similar to configuration 3 except that the service neutral is not brought to the UPS or the bypass module. Thus both UPS module and the distribution center can be treated as separately derived sources and neutral to ground connection is established in both these installations. Noise performance is similar to that of configuration 3.

Configuration 5

This case ( FIG. 22) is similar to configuration 2 except that the supply is from a delta connected three-wire source. Therefore, the UPS is a separately derived source and the neutral/ground connections reflect this. Noise performance is similar to that of configuration 2.

FIG. 22 UPS system configuration 5

Configuration 6

This is an example of multiple UPS modules with an isolated bypass and a standalone static transfer switch configuration.

The combination of UPS modules and bypass can be treated as a single separately derived source. In order to create a common bonding point, the neutral of the bypass source and each UPS module are brought into the static switch module and connected to a neutral bar ( FIG. 23). A separate ground bus is also provided in the same cubicle. The neutral bar is bonded to the ground bus. All equipment grounding wires from the loads and the UPS are bonded to this bus. The ground bus is also bonded to the local grounding electrode and to the service ground.

This arrangement permits any UPS module to be taken out of service without affecting the integrity of grounding connections. Common-mode noise attenuation is also achieved by separation of the service neutral from the sensitive load supply neutral.

There are other possible UPS configurations, which we won’t discuss here. But the general guiding principles in all these cases remain the same.

FIG. 23 UPS system configuration 6

Summary

In this section, we covered the need for uninterrupted power sources and their role in improving power quality and reliability. We reviewed the basics of power quality and the definitions associated with this subject. We covered the methods of improving the quality of voltage. We saw about the ways in which uninterrupted power can be made available.

We discussed about electromechanical and static equipment used for this purpose. We covered in detail the static UPS systems, these being the ones in common use, and their selection criteria when applied to ADP facilities. The issue of correct grounding practices necessary for various configurations of static UPS systems was also explained in detail.

Unless correct grounding practices are adopted for UPS fed power distribution equipment appropriate to the configuration of the UPS, noise attenuation cannot be ensured and might result in malfunctioning of systems fed by them.

While this wraps up the basic course material, a few appendices have been added for providing further reading to the participants. The participants are advised to go through these in detail. Section F contains a set of exercises, which the participants will be required to solve, which can be followed by a discussion of the solutions by the course director. Answers are also provided for verification in Section G. In addition, a set of practical problems has been given under Section H for discussion among the group, as a group activity. These problems and the method adopted to tackle them are given in the form of case studies in the next section ( Section 10). It’s recommended, therefore to take up the last module ( Section 10) for discussion after this group activity and the method adopted compared with the solutions arrived at by the group.

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