R

FIGURE 5.13 Isolated operation.

Isolated operation involves no interaction with the utility's distribution system because the generator does not operate in parallel with the utility. In some isolated systems, the generator is sized for a specific load that is always powered from the generator and never from the utility. There are two ways of transferring load to isolated operation:

• A break-before-make transfer switch (also known as open transition switching), disconnects the load from the utility prior to making the new connection with the onsite electric generating facility.

• A momentary-parallel (or closed transition) switch, a control system starts the customer's generator and parallels it with the utility's distribution system, quickly ramps the generator output power to meet the customer's load demand and then disconnects the load from the utility.

5.2.3.2 Roll-Over Operation

When a roll-over connection exists (Figure 5.14), the load can be connected only to one of the two sources (grid or DG system) at any given moment. Both the sources are connected to a load control center with a load transfer switch. When the source that is feeding the load fails, this device makes the commutation, ensuring that the other source feeds the load. This commutation can be automatic or manual, to allow the interchange between the two sources due to technical or economical reasons, even when a failure does not exist. In this configuration, the DG unit provides power to load 2 for peaking, base-load, or backup power, and the utility provides power to load 1 and occasionally to load 2.

The automatic commutation devices achieve the fast transition between sources, but there is always a time period in which the load is not fed. This kind of connection does not allow the decrease of

Utility

Load 2

Load 2

Load1

FIGURE 5.14 Roll-over operation.

the frequency of interruption, but reduces considerably their duration. To eliminate the feeding interruption, the installation of energy storage devices between the switch and the load would be necessary. In this configuration, the transfer time of the load switch is typically 0.1-0.15 s.

The roll-over operation is cheaper and simpler because it does not need a high number of control, protection, and coordination equipment. This type of operation can easily ensure the impossibility of the DG system injecting energy into the grid when the grid is out of service. This phenomenon, named backfeed or islanding, can be harmful to the grid operation and can put people and goods at risk. For example, if a line is disconnected due to technical reasons, a worker can be electrocuted by assuming that the line does not have voltage.

5.2.3.3 Parallel Operation

When a parallel operation exists, the sources are interconnected and both are connected with the load. If one of the sources fails, the load passes to be instantaneously fed exclusively by the other, without any interruption in the load supply.

The fact of the two sources operating in parallel implies that the DG unit will be in operation and in synchronism with the grid, aggregating the necessary conditions to feed the load at the moment of the grid failure. This kind of operation is more expensive because besides the necessary additional protection and control equipment, there are additional fuel and equipment wear out costs that occur in generating equipment, even without electricity generation.

The parallel operation requires a large quantity of monitoring, control, synchronization, and protection devices. Both the sources must be protected against the failures of the other, including the backfeed phenomenon. This kind of connection is necessary in the cases in which the DG unit owner wants to sell energy to the grid.

Several types of parallel connections are available (Figure 5.15), depending on the DG unit localization and the possibility of selling energy to the grid. In the first configuration, the DG unit operates in parallel with the grid, supplying energy to all the loads or to some loads, particularly providing the utility supplemental or backup power. In this configuration, it is impossible to supply energy to the grid.

In the second configuration, the DG unit operates in parallel with the grid, supplying energy to all the loads. With this configuration, it is possible to supply energy to the grid. The DG unit provides peaking or base-load power to load and exports power to the grid, providing the utility supplemental and backup power. In the third configuration, the DG unit operates in parallel with the grid, supplying energy to the grid and to the consumer. In this configuration, the DG unit does not usually belong to the consumer.

5.2.4 Ancillary Services

Ancillary services is the designation given to a number of functions that are necessary to support the reliable and efficient operations of the power system network. Besides energy (kWh) and capacity (kW), DG can provide other additional benefits, including spinning reserve capacity, peaking, load following, reactive power, and voltage support and other ancillary services.

Generally, a customer can use DG in conjunction with the traditional utility service or as a separate service. There are two ways of DG utilization: to supply power and energy during peak periods or during the entire demand period. Distributed generation equipment can also be used as backup or standby power. Some ancillary services provided by the conventional generators can also be provided by DG, thus minimizing the cost of supplying ancillary services.

In addition to generating energy, DG operation can provide the following benefits:

• Eliminate the need to upgrade the size of feeders

• Improve voltage levels at the feeder ends

• Eliminate the need for capacitor banks

• Provide reactive power compensation

• Eliminate the need for voltage regulators

FIGURE 5.15 Parallel operation.

• Reduce feeder loading and delay replacement

• Reduce line losses and transmission system load

The main types of ancillary services, which can be provided by generators include: Regulation Service and Frequency Response. They provide generation capacity that is available and running, and that can be used to maintain real-time balance in the transmission system. As system loads fluctuate minute-by-minute, generators must be available to match instantly the fluctuations due to the increase or decrease of the loads. An AGC reacts to perceived system fluctuations by adjusting its output to oppose or dampen the fluctuation, whether it is caused by load changes or changes in the output of other bulk system generators as they ramp up or down. Generation units equipped with automated generation control can follow load variations on the time scale of seconds. The load balance is a critical service to the stability of day-to-day grid operation.

The load-following capability service is associated with the "Regulation service and spinning reserves." Load following is the use of available generation capacity to meet the variations in system load. A more detailed description is given below.

Spinning Reserves. This refers to supplemental generation capacity that is ready to quickly ramp up at short notice. Spinning reserves is the incremental generating supply that an active unit can ramp up to within 10 min and then sustain, typically for 30-120 min.

An amount of generating capacity must be kept fully warmed up and ready to take over within seconds in the event of a generator or transmission line failure. The term spinning refers to the fact that the generator is on, spinning at rated speed (in the case of turbine generators), and synchronized to the grid. It only needs to adjust its power output to the prescribed level.

Large DG and aggregated small DG alike can provide spinning reserve service. Implicit in the definition, however, is the availability of the capacity to be called upon at any time. Therefore, for example, a DG unit cannot use its full capacity for peak shaving a local load, and at the same time qualify that capacity for spinning reserve.

This limitation is true for nonspinning and replacement reserve services as well. A generator designed to run at 80% of its normal capacity for local purposes can qualify the remaining 20% capacity for spinning reserve, as long as it is synchronized to the grid for the defined reserve period.

Some quick-release hydro units allow a change from zero to full power in 1 min. Alternatively, quick-response loads (using demand response controls) can also contribute to achieve a fast balance between supply and demand.

Supplementary (Nonspinning) Reserves. This refers to generation that is available but not running. Generation kept on standby so that it can be started rapidly in the event that generators or lines suddenly fail, but not as rapidly as spinning reserves above. The incremental generation that can be achieved by units with slower responses, and those requiring start-up, is considered nonspinning reserve. Jurisdictions differ as to the ramping time allowed, varying from 10 to 30 min.

Generators that can start, synchronize, and ramp to full power in short time periods can therefore participate in the quick-response reserve market without running at all times. Fast-start combustion turbines can serve this function. In addition, customers in the form of medium fast-response load may provide nonspinning reserve services.

Nonspinning reserve, in most cases, will be a more appropriate choice over spinning reserve for unused DG capacity. Most distribution level DG technologies do not require 10 min to start-up, and therefore would not gain from remaining synchronized to the grid when not needed. Nonspinning reserves further provide ample opportunity for generators installed as emergency backup systems to participate in the reserve market, where they would not under spinning reserve. These generators are designed to remain off under normal circumstances and serve the customer's load only if the utility experiences an outage; therefore, their capacity during normal utility operation is always available.

Replacement or Operating Reserves. Replacement reserve is the incremental generation that can be obtained in the next hour to replace spinning and nonspinning reserves used in the current hour. Replacement reserve is very similar to nonspinning reserve with the exception that the generator has 60 min to start- and ramp-up instead of only 10 min. Each resource providing replacement reserve must be capable of supplying any level of output up to and including its full reserved capacity within 60 min after issue of dispatch instructions by the independent system operator (ISO). Each resource providing replacement reserve must be capable of sustaining the required output for at least 2 h.

Replacement reserve may be supplied from resources already providing another ancillary service, such as spinning reserve. However, the sum of the ancillary service capacity plus the replacement reserve cannot exceed the capacity of said resource.

Replacement reserve can be provided by large and aggregated small DG that requires more than 10 min (and less than 1 h) to start and ramp to full power. This would be appropriate in cases where the generator technology itself has ramping limitations, or where the generator starting functions are not automated in response to a signal from the ISO, and therefore require delayed manual intervention.

Voltage Support. These services are required to maintain transmission voltage level margins within the criteria in force. Dispatchers at the control center alter the settings on transformers, transmission lines, and other downstream grid-connected equipment, as well as provide sufficient reactive power in areas where needed.

Reactive Power Support. Reactive power support is the injection or absorption of reactive power from generators to maintain transmission-system voltage within required ranges. Generators and loads maybe dispatched and operated within a prescribed power factor range to boost the voltage during heavy load periods, or reduce the voltage during light load periods. The service can be provided by generators, loads, and utility distribution companies alike, as long as they have the proper power factor adjustment capabilities.

Black-Start Generation Capability. Black-start generation capability is the ability of a generating unit to go from a shutdown condition to an operating condition without assistance from the electrical grid and to then energize the grid to help other units start after a blackout occurs.

Generators are started in a sequence so that each subsequent generator has an energized bus with which to synchronize. Strategically located black-start generators are a key factor for ensuring timely restoration after a major outage. Each black-start generating unit must be able to start-up with a dead primary and station service bus within 10 min of issue of a dispatch instruction by the ISO requiring a black start.

Each slack-start generating unit must provide sufficient reactive capability to keep the energized transmission bus voltages within emergency voltage limits over the range of no-load to full load.

Each black-start generating unit must be capable of sustaining its output for a minimum period of 12 h from the time when it first starts delivering energy.

The other characteristics that may influence the adoption of DG technologies for ancillary service applications will vary according to the service performed and the ultimate shape of the ancillary service market. Start-up time for all electrical generators is an extremely important parameter to determine if the particular unit can be used as the reserve or can operate in the load following mode.

The part-load capabilities of DG technologies and the start-up time periods of each are presented in Table 5.6.

In addition to their high fuel efficiency, fuel cells appear to offer technical capabilities. Their flexible size enables them to be located close to the load, which can reduce energy losses and transmission and distribution costs. One of the most significant characteristics of the fuel cell is its ability to operate efficiently at part-load, i.e., to respond to sudden increases or decreases in power demands. In addition to meeting changes in power demand, the fuel cell's spinning reverse and load following capabilities enable it to complement effectively the variable output from other renewable power sources, such as solar energy and wind farms.

5.2.5 Advantages of the Grid Interconnection

5.2.5.1 Economical Advantages

The cost of the electricity provided by the grid can be smaller than the cost of the local production in some time periods. Thus, during the periods in which the marginal production cost in the local unit is superior to the grid electricity cost, it makes sense to use the grid energy. During a peak period, the cost of the grid electricity is higher when local production becomes advantageous.

Sometimes is not advantageous to size the DG unit to meet all the required power by the loads. In this situation, when the requested power is higher, the additional required energy is provided by the grid. When the unit is working below the full capacity and the marginal production cost is lesser than the grid price, it is possible to increase the local production thus increasing the profits.

5.2.5.2 Voltage Regulation

The electric power grid is projected to approach an ideal voltage source, with lower internal impedance. In this kind of source, the voltage is the same to all the connected loads, ensuring that the start of a large

TABLE 5.6 Summary Table of Some Performance Characteristics by Distributed Generation Technology Type

Technology

Steam Turbine

Diesel Engine

Natural Gas Engine

Gas Turbine

Micro turbine

PAFC

MCFC

SOFC Tubular SOFC Planar

PEMFC

Part-load

Satisfactory

Good

Satisfactory

Poor

Satisfactory

Satisfactory

Poor

Satisfactory

Start-up time

lh-ld

10 s

10 s

10 min-1 h

60s

1-4 h

More than 10 h

5-10 h Not available

<0.1 h

Source: From US Environmental Protection Agency, Introduction to CHP Technologies, California Energy Commission, DER Equipment.

Source: From US Environmental Protection Agency, Introduction to CHP Technologies, California Energy Commission, DER Equipment.

load does not disturb the feeding voltage to the other loads. In a nonideal voltage source, the start of a large load (e.g., a large induction motor) causes a momentary voltage sag, affecting the other loads negatively.

In general, the grid achieves a good voltage regulation, because as the power of the load varies, the grid adjusts the power flows automatically, with very small variations in the voltage. Only at some points of the grid, especially toward the end of long lines, this variation is relatively high.

In general, DG units do not have as good a voltage regulation as the grid. As the load varies, the unit controller monitors the output voltage, that tends to decrease with the load increase and increase with the load reduction. When the voltage varies, the unit controller automatically responds, but it is almost impossible to equal the nearly instantaneous response of the grid.

The interconnection, with the DG unit working in parallel with the grid, solves the voltage regulation problem, even in the cases in which all the consumed energy is provided by the DG unit. When the load varies, the grid ensures the transitory instantaneous response, allowing the unit to make a relatively slow change.

5.2.5.3 Reliability

When properly managed, two energy sources work better together than isolated. Either with DG reserve units or with normal operating units, the DG system owner can view the grid as a reserve energy source. In fact, in almost all of the sites, the grid reliability is higher than the reliability of any isolated DG unit. In the DG projects, it is common to use values of 92%-93% for the unit's availability. Considering an availability of 99%, which is extremely difficult to obtain, the corresponding unavailability will be higher than 80 h per year, which is unacceptable to most of the appliances.

Even in areas with poor performance of the electric grid, the grid availability is normally higher than the DG unit, being many times higher than the availability of a DG system, even with several units. The grid utilization like reserve source makes sense in most of the cases, if the charged costs by the grid operator are reasonable.

5.2.6 Disadvantages of the Grid Interconnection

5.2.6.1 Costs with the Grid Operator

In any market, the grid operator will charge a considerable value for interconnection with the grid. The DG unit owner, making the interconnection, will normally pay not only for the energy supplied by the grid, but also a charge dependent upon the maximum power delivered by the grid.

5.2.6.2 Additional Equipment and Maintenance

Distributed generation system operation with interconnection with the grid is more complex than an isolated operation. Besides all the necessary equipment for the system operation, it is necessary to install additional control, metering, and protection devices to isolate the DG system from the grid. Usually the additional equipment, besides increasing the cost, increases the system complexity, thereby increasing the maintenance needs.

5.2.6.3 Increasing Maintenance Needs to Ensure High Reliability Levels

The potential reliability improvement achieved by the interconnection does not appear automatically. It is necessary to have a constrained management of the DG/grid combination to obtain the expected reliability levels. There are three essential points:

• The DG system operating in parallel with the grid needs an exhaustive monitoring of the grid operation conditions of the DG system, the load and the interconnection. The existence of two sources and a load cause a complex control problem that is easily resolved with the modern electronic devices, but is a potential weakness. Problems can occur either because the system is not correctly programmed or due to a failure in a key element of the system that can deactivate the entire system. In critical DG units interconnected with the grid, redundant control equipment is usually installed with auto-monitoring.

• Problems at grid points distant from the DG installation can cause perturbations in the DG unit operation. The most common problems are the atmospheric discharges. Reaching a line, a lightning discharge causes a current impulse that, without the appropriated protection, can reach the DG system. To mitigate this problem, additional protection equipment is needed, increasing the total cost of the system.

• Undesirable events in the grid can disturb the DG unit operation, especially if the control and protection equipment is very sensitive. Momentary failure of a line can cause an abrupt fall in the voltage that can be interpreted by the control system as a risk situation to the DG unit, resulting in its removal from service, or it may be interpreted as an abrupt increase in the load. In the last situation, the control device responds by increasing the power of the DG unit. After the automatic reclosure, the voltage level increases too fast and the DG unit cannot react in a timely manner, while it continues to try follow what it seems a load increase. An overvoltage occurs that is detected by the control system which removes the unit from service, and the installation is then supplied by the grid.

5.2.7 IEEE Standard for Interconnecting Distributed Resources with Electric Power Systems

The IEEE Standards Board approved the IEEE 1547 Standard for Interconnecting Distributed Resources (DR) with Electric Power Systems (EPS) in June, 2003. It was then approved as an American National Standard in October 2003. Many of the technical concerns that the companies of distributed electricity usually raise for the DR interconnection with the grid are related to reliability, security, and quality of service. The IEEE P1547/D07 defines interconnection technical specifications and requirements that are universally needed for interconnection of DR. This standard constitutes an important step to overcome the barriers and increase the development of DG installations.

This American standard establishes criteria and requirements for interconnection of DR with EPS. It provides requirements relevant to the performance, operation, testing, safety considerations, and maintenance of the interconnection. The standard applies to all DR technologies, with aggregate capacity of 10 MVA or smaller at the point of common coupling, and to all EPS at typical primary and/or secondary distribution voltages.

The standard defines interconnection technical specifications and requirements that all interconnection systems and DRs shall meet. General requirements are related to voltage regulation, integration with area electric power system grounding, synchronization, DR on secondary grid and spot networks, inadvertent energization, and reconnection to area EPS, monitoring, and isolation device.

Abnormal conditions can arise on the area EPS that require a response from the connected DR. This response contributes to the safety of utility maintenance personnel and the general public, as well as the avoidance of damage to the connected equipment, including the DR. The abnormal conditions of concern are voltage and frequency excursions above or below the values stated in the IEEE P1547 (Table 5.7), and the isolation of a portion of the area EPS with some DR, presenting the potential for an unintended island.

TABLE 5.7 Interconnection System Response to Abnormal Voltages

Voltage Range (% of Base Voltage)

Clearing Time (s)a

V< 50

0.16

50 < F < 88

2

110 < y < 120

1

V> 120

0.16

a Clearing time: Time between the start of the abnormal condition and the DR ceasing to energize the area EPS.

a Clearing time: Time between the start of the abnormal condition and the DR ceasing to energize the area EPS.

TABLE 5.8 Maximum Harmonic Current Distortion in Percent of Current (I)

Individual Harmonic Order (Odd Harmonics)

< 11

11< h < 17

17< h < 23

23< h < 35

35< h

Total Demand Distortion (TDD)

Percent (%)

4.0

2.0

1.5

0.6

0.3

5.0

I is the greater of the local EPS maximum load current integrated demand (15-30 min) without the DR unit, or the DR unit rated current capacity (transformed to the PCC when a transformer exists between the DR unit and the PCC); even harmonics are limited to 25% of the odd harmonic limits above.

I is the greater of the local EPS maximum load current integrated demand (15-30 min) without the DR unit, or the DR unit rated current capacity (transformed to the PCC when a transformer exists between the DR unit and the PCC); even harmonics are limited to 25% of the odd harmonic limits above.

Power quality issues are also addressed in the IEEE P1547, namely, limitation of DC injection, voltage flicker induced by the DR, harmonic current injection (Table 5.8), immunity protection and surge capabilities, as well as the islanding considerations.

The standard also provides test requirements for an interconnection system to demonstrate that it meets all the requirements. The following tests are required for all interconnection systems:

• Interconnection test

• Production tests

• Interconnection installation evaluation

• Commissioning tests

• Periodic interconnection tests

IEEE 1547 is the first in a family of IEEE interconnection standards for DR (Figure 5.16). Other standards in the family currently underway are:

• IEEE P1547.1 Draft Standard for Conformance Tests Procedures for Equipment Interconnecting DR with EPS. This standard specifies the type, production, and commissioning tests that shall be performed to demonstrate that the interconnection functions and equipment of a distributed resource conform to IEEE Standard P1547.

• IEEE P1547.2 Draft Application Guide for IEEE 1547 Standard for Interconnecting DR with EPS. This guide provides technical background and application details to support the understanding of IEEE 1547 Standard for Interconnecting DR with EPS.

• IEEE P1547.3 Draft Guide For Monitoring, Information Exchange, and Control of DR Interconnected with EPS. This document provides guidelines for monitoring, information exchange, and control for DR interconnected with EPS.

• IEEE P1547.4 Draft Guide for Design, Operation, and Integration of Distributed Resource Island Systems with EPS. This document provides alternative approaches and good practices for the design, operation, and integration of distributed resource island systems with EPS.

• IEEE P1547.5 Draft Technical Guidelines for Interconnection of Electric Power Sources Greater than 10 MVA to the Power Transmission Grid. This document provides guidelines regarding the technical requirements, including design, construction, commissioning acceptance testing, and maintenance/performance requirements, for interconnecting dispatchable electric power sources with a capacity of more than 10 MVA to a bulk power transmission grid.

• IEEE P1561 Draft Guide for Sizing Hybrid Stand-Alone Energy Systems. This guide provides the rationale and guidance for operating lead-acid batteries in remote hybrid systems considering the system's load, and the capacities of its renewable-energy generator(s), dispatchable generator(s), and battery(s).

5.2.8 Power Quality Applications

Power quality-related issues are currently of great concern. The widespread use of electronic equipment, such as information technology equipment, power electronics such as adjustable speed drives (ASDs), programmable logic controllers (PLCs), and energy-efficient lighting led to a complete change of electric

IEEE Std 1547™ (2003) Standard for interconnecting distributed resources with electric power systems

P1547.6 Draft recommended practice for interconnecting distributed resources with electric power systems distribution secondary networks

P1547.3 Draft guide for monitoring, information exchange, and control of DR interconncted with EPS

P1547.2 Draft application guide for IEEE 1547 standard for interconnecting distributed resources with electric power systems

P1547.6 Draft recommended practice for interconnecting distributed resources with electric power systems distribution secondary networks

P1547.3 Draft guide for monitoring, information exchange, and control of DR interconncted with EPS

Guide for impacts

P1547.4 Draft guide for design, operation, and integration of distributed resource island systems with electric power systems electric power systems_

P1547.5 Draft technical guidelines for interconnection of electric power sources greater than 10 MVA to the power transmission grid

Guide for impacts

P1547.4 Draft guide for design, operation, and integration of distributed resource island systems with electric power systems

P1547.5 Draft technical guidelines for interconnection of electric power sources greater than 10 MVA to the power transmission grid

IEEE Std 1547.1™ (2005) standard for conformance test procedures for equipment interconnecting distributed resources with electric power systems_

(Publication year in parentheses: P1547.X are under development: other topics are under consideration by SCC21 work group members)

DP specifications & performance (includes modeling)

FIGURE 5.16 IEEE SCC21 1547 Series of Interconnection Standards. (From Institute of Electric and Electronics Engineers (IEEE), IEEE P1547/D07. Standard for Interconnecting Distributed Resources with Electric Power Systems. IEEE Standards Coordinating Committee 21 (IEEE SCC21) on Fuel Cells, Photovoltaics, Dispersed Generation, and Energy Storage of the IEEE Standards Association, New York, 2001. With permission.)

loads nature. These loads are simultaneously the major causers and the major victims of PQ problems. Due to their nonlinearity, all these loads cause disturbances in the voltage waveform.

Along with technology advance, the organization of the worldwide economy has evolved towards globalization and the profit margins of many activities tend to decrease. The increased sensitivity of the vast majority of processes (industrial, services, and even residential) to PQ problems turns the availability of electric power with quality a crucial factor for competitiveness in every activity sector. The most critical areas are the continuous process industry and the information technology services. When a disturbance occurs, huge financial losses may occur, with the consequent loss of productivity and competitiveness.

Although many efforts have been taken by utilities, some consumers require a level of PQ higher than the level provided by modern electric networks. This implies that some measures must be taken in order to achieve higher levels of PQ.

The most common types of PQ problems are presented in Table 5.9.

TABLE 5.9 Most Common Power Quality Problems

2. Very short interruptions y y

3. Long interruptions

Description: A decrease of the normal voltage level between 10 and 90% of the nominal rms voltage at the power frequency, for durations of 0,5 cycle to 1 min Causes: Faults in the transmission or distribution network (most of the times on parallel feeders). Faults in consumer's installation. Connection of heavy loads and start-up of large motors

Consequences: Malfunction of information technology equipment, namely microprocessor-based control systems (PCs, programmable logic controllers (PLCs), adjustable speed drives (ASDs), etc.) that may lead to a process stoppage. Tripping of contactors and electromechanical relays. Disconnection and loss of efficiency in electric rotating machines Description: Total interruption of electrical supply for duration from few milliseconds to one or two seconds

Causes: Mainly due to the opening and automatic reclosure of protection devices to decommission a faulty section of the network. The main fault causes are insulation failure, lightning, and insulator flashover Consequences: Tripping of protection devices, loss of information, and malfunction of data processing equipment. Stoppage of sensitive equipment, such as ASDs, PCs, PLCs, if they are not prepared to deal with this situation Description: Total interruption of electrical supply for duration greater than 1-2 s Causes: Equipment failure in the power system network, storms, and objects (trees, cars, etc.) striking lines or poles, fire, human error, bad coordination or failure of protection devices Consequences: Stoppage of all equipment

4. Voltage spike

5. Voltage swell

6. Harmonic distortion

7. Voltage fluctuation

Description: Very fast variation of the voltage value for durations from several microseconds to few milliseconds. These variations may reach thousands of volts, even in low voltage Causes: Lightning, switching of lines or power factor correction capacitors, disconnection of heavy loads Consequences: Destruction of components (particularly electronic components) and of insulation materials, data processing errors or data loss, electromagnetic interference Description: Momentary increase of the voltage, at the power frequency, outside the normal tolerances, with duration of more than one cycle and typically less than a few seconds

Causes: Start/stop of heavy loads, badly dimensioned power sources, badly regulated transformers (mainly during off-peak hours) Consequences: Data loss, flickering of lighting and screens, stoppage or damage of sensitive equipment, if the voltage values are too high Description: Voltage or current waveforms assume nonsinusoidal shape. The waveform corresponds to the sum of different sine-waves with different magnitudes and phases, having frequencies that are multiples of power-system frequency Causes: Classic sources: electric machines working above the knee of the magnetization curve (magnetic saturation), arc furnaces, welding machines, rectifiers, and DC brush motors. Modern sources: all nonlinear loads, such as power electronics equipment including ASDs, switched mode power supplies, data processing equipment, high efficiency lighting Consequences: Increased probability of occurrence of resonance, neutral overload in three-phase systems, overheating of all cables and equipment, loss of efficiency in electric machines, electromagnetic interference with communication systems, errors in measures when using average reading meters, nuisance tripping of thermal protections

Description: Oscillation of voltage value, amplitude modulated by a signal with frequency of 0-30 Hz Causes: Arc furnaces, frequent start/stop of electric motors (for instance elevators), oscillating loads

Consequences: Most consequences are common to undervoltages. The most perceptible consequence is the flickering of lighting and screens, giving the impression of unsteadiness of visual perception

TABLE 5.9 (Continued)

8. Noise

8. Noise

9. Voltage unbalance

9. Voltage unbalance

Description: Superimposition of high frequency signals on the waveform of the powersystem frequency

Causes: Electromagnetic interferences provoked by Hertzian waves, such as microwaves, television diffusion, and radiation due to welding machines, arc furnaces, and electronic equipment. Improper grounding may also be a cause Consequences: Disturbances on sensitive electronic equipment, usually not destructive.

May cause data loss and data processing errors Description: A voltage variation in a three-phase system in which the three voltage magnitudes or the phase-angle differences between them are not equal Causes: Large single-phase loads (induction furnaces, traction loads), incorrect distribution of all single-phase loads by the three phases of the system (this may be also due to a fault)

Consequences: Unbalanced systems imply the existence of a negative sequence that is harmful to all three-phase loads. The most affected loads are three-phase induction machines

Even the most advanced transmission and distribution systems are not able to provide electrical energy with the desired level of reliability for the proper functioning of the loads in the modern society. Modern T&D (transmission and distribution) systems are projected for 99.9%-99.99% availability. This value is highly dependant of redundancy level of the network, which is different according to the geographical location and the voltage level (availability is higher at the HV network). In some remote sites, availability of T&D systems may be as low as 99%. Even with a 99.99% level, there is an equivalent interruption time of 52 min per year. The most demanding processes in the modern digital economy need electrical energy with 99.9999999% availability (9-nines reliability) to function properly.

The mitigation of PQ problems may take place at different levels: transmission, distribution, and the end use equipment. As seen in Figure 5.17, several measures can be taken at these levels. Many PQ problems have origin in the transmission or distribution grid. Thus, a proper transmission and distribution grid, with adequate planning and maintenance, is essential to minimize the occurrence of PQ problems.

Distributed

Power

End- ^^

Transmission

Distribution

resources

quality

use

interface

devices ^^

Develop codes and standards

FIGURE 5.17 Solutions for digital power.

Develop enhanced interface devices

Develop codes and standards

Develop enhanced interface devices

CZ5 Issue O Solutions

FIGURE 5.17 Solutions for digital power.

Make end-use devices less sensitive

FIGURE 5.18 Restoring technologies principle.

Interest in the use of distributed energy resources has increased substantially over the last few years because of their potential to provide increased reliability. These resources include DG and energy storage systems. Energy storage systems, also known as restoring technologies, are used to provide the electric loads with ride-through capability in poor PQ environment. Recent technological advances in power electronics and storage technologies are turning the restoring technologies as one of the premium solutions to mitigate PQ problems (Figure 5.18).

Distributed generation units can be used to provide clean power to critical loads, isolating them from disturbances with origin in the grid. Distributed generation units can also be used as backup generators to assure energy supply to critical loads during sustained outages. Additionally, DG units can be used for load management purposed to decrease the peak demand.

At present, the reciprocating engine is the prevalent technology in DG market, but with technology advancements, other technologies are becoming more attractive, such as photovoltaics, microturbines, or fuel cells.

If DG units are to be used as backup generation, a storage unit must be used to provide energy to the loads during the period between the origin of the disturbance and the start-up of the emergency generator.

The most common solution is the combination of electrochemical batteries UPS and a diesel genset. At present, the integration of a flywheel and a diesel genset in a single unit (Figure 5.19 and Figure 5.20) is also becoming a popular solution, offered by many manufacturers.

http://www. geindustrial.com. With permission.)"/>
FIGURE 5.19 Scheme of a continuous power system, using a flywheel and a diesel genset. (From http://www. geindustrial.com. With permission.)

Diesel Clutch Electric

Diesel Clutch Electric

http://www.hitecups.com/?RubriekID=1991. With permission.)"/>
FIGURE 5.20 Dynamic UPS, by Hitec Power Protection. (From http://www.hitecups.com/?RubriekID=1991. With permission.)
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