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1 INTRODUCTION
This section considers those aspects of power quality related to disturbance of the voltage level. This includes short-term interruptions and long-term deviations from the declared voltage level. With the increasing dependence of industry and commerce on electronic equipment and processes with sophisticated controls, this is an area of increasing concern to power supply utilities and to their contractors and consultants. Some standards relating to this subject are given in Table 1. The approach of the International Electrotechnical Commission (IEC) is as explained in Section 24 -- i.e. to set limits for equipment connected to utility systems -- but the thresholds of irritability provided are equally valid for users already connected to an existing sup ply system, and indicative 'planning limits' provided are quality targets for a supplying utility. Standards of supply system quality to be expected in Europe (for systems up to 35 kV) are set out in EN 50160.
Although there are major efforts being made by various standards authorities to arrive at common criteria, the environment, priorities and requirements of different nations and their power systems mean that variations in approach are inevitable. It is absolutely essential that any consultant engineer or contractor with design or procurement responsibility checks the particular requirements of the country in which a project is to be executed, and indeed of the power network involved.
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TABLE 1 Standards Relating to Voltage Disturbances (see also Section 20)
Document Number Document Title IEC 61000-1-1 Electromagnetic compatibility (EMC) -- Part 1: General -- Section 1:
Application and interpretation of fundamental definitions and terms IEC 61000-2-8 Electromagnetic compatibility (EMC) -- Part 2-8: Environment -- Voltage dips and short interruptions on public electric power supply systems with statistical measurement results IEC 61000-3-3a Electromagnetic compatibility (EMC) _ Part 3-3: Limitation of voltage fluctuations and flicker in low-voltage supply systems for equipment with rated current #16 A IEC 61000-3-5 Electromagnetic compatibility (EMC) -- Part 3: Limits -- Section 5:
Limitation of voltage fluctuations and flicker in low-voltage power supply systems for equipment with rated current greater than 16 A IEC 61000-3-6
Electromagnetic compatibility (EMC) -- Part 3: Limits -- Section 6:
Assessment of emission limits for distorting loads in MV and HV power systems -- Basic EMC publication IEC 61000-3-7 Electromagnetic compatibility (EMC) -- Part 3: Limits -- Section 7:
Assessment of emission limits for fluctuating loads in MV and HV power systems -- Basic EMC publication IEC 61000-3-11a Electromagnetic compatibility (EMC) -- Part 3-11: Limits -- Limitation of voltage changes, voltage fluctuations and flicker in public low-voltage supply systems -- Equipment with rated current #75 A and subject to conditional connection IEC 61000-3-14 Electromagnetic compatibility (EMC) -- Limits -- Part 3-14:
Assessment of emission limits for installations connected to LV power systems IEC 61000-4-1a Electromagnetic compatibility (EMC) -- Part 4-1: Testing and measurement techniques _ Overview of IEC 61000-4 series (note there are at the time of publication 35 sections to Part 4, covering different aspects of testing and measurement; many are relevant to the present topic) IEEE 1159-1995 IEEE recommended practice on monitoring electrical power quality EN 50160 Voltage characteristics of electricity supplied by public distribution systems ER P28 Engineering recommendation on planning limits for voltage fluctuations caused by industrial, commercial and domestic equipment in the UK -- Engineering Management Conference, Utilization Consultancy group ER P29 Engineering recommendation on planning limits for voltage unbalance in the UK
a For these standards there is also an identical standard (with the same number) in the BSEN series.
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2 THE NATURE AND CAUSE OF VOLTAGE DISTURBANCES IN POWER SYSTEMS
2.1 Short-Term Interruptions and Voltage Dips and Peaks
Short interruptions (termed 'discontinuities' in some standards) to a consumer's supply, lasting not more than 1 minute (which for the purposes of power quality include short-time voltage reductions to less than 0.1 pu) are typically caused by the operation of automatic reclosing systems. Voltage dips can be caused by any of a number of events including the starting current of large motors, inrush currents occurring on connection of certain reactive loads and system faults. Faults on the transmission system result in dips of relatively high retained voltage over a large area, while distribution system faults tend to cause dips of lower retained voltage and longer duration but effective over a smaller area.
The term 'sag' is also used, particularly in American usage; it is often a synonym for 'dip,' but can sometimes be intended as a discriminatory term designating a longer-term reduction in voltage. A dip is defined in terms of duration and 'retained voltage,' the latter usually expressed as the percentage of nominal rms voltage remaining at the lowest point of the dip. Attempts have been made to standardize all these terms -- see Table 2 and 3 -- but they are not universally applicable. For the IEC, a voltage dip has an amplitude greater than 10% of the nominal voltage, and a short supply interruption is simply a dip with 100% amplitude (see FIG. 1).
Motor drives, including older variable speed drives (VSDs), are particularly susceptible to dips and interruptions because the load still requires energy and takes it from the inertia of the drive. Different machines will slow at different rates, potentially resulting in chaos in a system requiring overall process control. (Most modern electronic VSDs can tolerate a significant variation in supply voltage without changing motor speed from the set value.
The VSD maintains constant input power by increasing current as the voltage falls, the motor speed being set by the frequency of the VSD inverter.) Another area of sensitivity is Information Technology (IT) equipment, which can suffer from shutdown or a 'crash'. In this context the term 'crash' is used for loss of a computer involving data loss. 'Shutdown' is usually used for a computer system where a voltage threshold has been set at sufficiently high level that there is sufficient residual energy in the power supply for the computer system to transfer data to non-volatile memory in an orderly manner before the processor actually stops. The high voltage thresh old needed to do this often means that a relatively small voltage dip will initiate a computer shutdown denying the computer any chance of riding through the dip.
Discharge lamps will extinguish and may not restrike, contactors trip and thyristor bridges in inverter mode suffer commutation failure.
Short-term peaks, surges or swells (the term 'swell' for a voltage excursion above nominal is more common in American usage) can be caused by the sudden disconnection of load; by some types of system fault or by the mal-operation of regulating equipment; peaks of a transient nature, generally measured in milliseconds for switching surges or microseconds for impulse spikes, can also occur.
TABLE 2 Some Definitions of Power Quality Disturbances Based on IEEE Standard 1159-1995
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TABLE 3 Some Voltage Disturbance
Terms Defined (Based on IEC Standards; Compare Table 2)
Term | Definition
Voltage variation--An increase or decrease of voltage normally due to variation of the total load of a distribution system or part of it.
Rapid voltage change--A single rapid variation of the rms value of a voltage between two consecutive levels that are sustained for definite but unspecified durations.
Voltage fluctuation--A series of voltage changes or a cyclic variation of the voltage envelope.
Flicker -- Impression of unsteadiness of visual sensation induced by a light stimulus whose luminance or spectral distribution fluctuates with time.
Supply voltage dip--A sudden reduction of the supply voltage to a value between 90% and 1% of the declared voltage, followed by a voltage recovery after a short period of time.
Conventionally, the duration of a voltage dip is between 10 ms and 1 min.
Supply interruption--A condition in which the voltage at the supply terminals is lower than 1% of the declared voltage.
Temporary power frequency overvoltage
An overvoltage, at a given location, of relatively long duration.
Transient overvoltage A short duration oscillatory or non-oscillatory overvoltage, usually highly damped and with a duration of a few milliseconds or less.
Voltage unbalance In a three-phase system, a condition in which the rms values of the phase voltages of the phase angles between consecutive phases are unequal.
a Voltage fluctuation can cause changes in the luminance of lamps which can create the visual phenomenon called flicker. Above a certain threshold flicker becomes annoying. The annoyance grows very rapidly with the amplitude of the fluctuation, but at certain repetition rates even very small amplitudes can be very annoying (see FIG. 2).
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2.2 Voltage Fluctuations
Voltage fluctuations are described by the IEC as cyclical variations of the volt age envelope or a series of random voltage changes of up to 610% of the nominal. In low-voltage networks domestic appliances are significant sources, but each appliance will affect only a limited number of consumers. Fluctuations resulting from industrial loads can affect many consumers, the main effect usually being flicker (see Section 2.3). Also step voltage changes due to the connection or disconnection of large loads or capacitor banks can fall into the category of voltage fluctuations (see also Section 2.6).
Equipment with significant time constants, such as heating elements, is largely unaffected, but some electronic and IT equipment can be seriously affected.
2.3 Voltage Flicker
Flicker is fundamentally a term used to describe the behavior of lighting, and its perceived effect on the human observer. Historically various experiments have been undertaken to determine the relationship between perception or irritation on the one hand and both frequency and magnitude of lighting level disturbance on the other. The concern of the power system engineer is that the most usual cause of flicker is a distortion of the voltage waveform of the electric supply to a lamp, and flicker is the most likely cause of complaint arising from voltage fluctuations. Curves exist, derived from lighting perception results, relating percentage voltage dip, frequency of disturbance and acceptability. An example is given in FIG. 2, but such curves must be treated with caution because they make assumptions as to lamp characteristics.
The IEC approach is more sophisticated, and takes account of both short term sensitivity (a value calculated every 10 minutes) and long-term sensitivity (a combination of 12 short-term values). Intensity of flicker annoyance can be measured with a flicker meter (IEC 60868). IEC 61000-3-7 provides indicative levels for acceptable annoyance on a network, but recognizes that the absolute limits will vary between utilities depending on specifics of the loads served and the supply network. In the UK, for example, utilities apply Engineering Recommendation P28 to determine acceptability. In Europe EN 50160 gives power system flicker levels at consumers' terminals.
FIG. 1 Illustration of a voltage dip (?U1) or a voltage interruption
(?U2 5100%) (from IEC 61000-2-1).
FIG. 2 Curve illustrating the principle of the flicker problem (derived
from IEEE 141 1993).
Typical causes of annoying levels of flicker are arc furnaces, welding loads, rolling mills and processes involving frequent but random motor starts.
It is interesting to consider that since flicker irritation to humans is a function of the perceptible change in brightness of the lighting (even though the exact relationship is difficult to formalize, as stated above), and incandescent lighting is more affected by supply voltage fluctuation than fluorescent lighting due to the latter's phosphor coating, it could be that the problem of flicker may reduce as low-energy compact fluorescent lamps become more widely used.
2.4 Slow Voltage Fluctuations
So-called 'slow'-voltage variations, which fall within the limit of 610% for voltage fluctuations, but are caused over a long period by the effect of load changes in the network, are not generally considered as voltage disturbances. They will be managed by conventional network development planning, and the use of automatic tap-changing on transformers, perhaps with the addition of line-drop compensation.
2.5 Voltage Unbalance
Voltage unbalance is a condition in which the three-phase voltages differ in amplitude or are displaced from their normal 120_ phase relationship, or both. The degree of unbalance is usually defined by the ratio of the negative sequence voltage component (see Section 28) to the positive sequence component. Negative-phase sequence (NPS) unbalance appears in networks supplying such single-phase loads as modern 25 kV traction systems. NPS components influence other equipment such as rotating machines and rectifiers connected to the network. NPS voltages produce NPS currents that increase the stator and rotor losses and reduce equipment life because of the associated temperature rises involved. (A continuous operation at 10 deg. K above the normal recommended operating temperature can reduce rotating machine life by a factor of two). IEC 60034-1 imposes a 1% NPS voltage limit on the supply feeding machines. However EN 50160 points out that in some areas unbalances up to 3% can be expected, and indicates that an acceptable supply system standard is that 'under normal operating conditions, during each period of one week, 95% of the 10-minute mean rms values of the NPS component of the supply voltage shall be within the range 0 to 2% of the positive phase sequence component'.
Polyphase converters are also affected by an unbalanced supply, which causes an undesirable ripple component on the DC side and non-characteristic harmonics on the AC side.
A significant cause of voltage unbalance at medium and high voltage is single-phase railway or tramway traction load. At transmission voltages (which can then be reflected into lower-voltage levels) it can also result from unequal mutual inductance between untransposed conductors on long trans mission lines. At low-voltage (distribution) level it is usually the result of unbalanced connection of single-phase loads on a three-phase system.
2.6 Step-Change Events
Step-changes in voltage are caused by events such as capacitor switching or transformer tap-changing. Provided such events are planned, infrequent and of a controlled magnitude they will not normally constitute a nuisance and they are rarely considered when assessing a system's power quality.
However, the existence of such events in a given system does have to be considered when designing compensation equipment for other voltage disturbances. For example, 'hunting' between tap-changers and static VAr compensators has to be avoided.
3 SOLUTIONS
3.1 Energy Storage
The ideal solution to the problem of variations in supply voltage is an instantly accessible store of electrical energy which can supply energy to 'top up' volt age below the nominal and can absorb electrical power to reduce voltage above the nominal. Historically non-electrical energy storage has seemed more practical, bearing in mind the magnitude of the required store, particularly if short interruptions are to be covered. Very fast-acting systems based on flywheels have been successfully developed, but have been difficult to apply economically except in special circumstances. Hydraulic systems have not been developed on a scale sufficient for power system compensation. Chemical energy storage includes batteries, and these were widely used in the early days of direct current electricity distribution. For today, they may be applicable to small individual installations (such as UPS (Uninterruptible Power Supply) systems for sensitive IT units, where they are certainly valuable), but their main drawback is low power and energy density, resulting in space and/or cost penalties. Large-scale chemical storage techniques have been developed (e.g. the Regensys system) but at the time of publication have not been widely applied _ physical space requirement is one negative factor.
Capacitors are an option for power quality applications where the energy requirement is not large, and the capability of commercially available equipment is increasing as time passes. The Dynamic Voltage Restorer (DVRTM) is an example, with module ratings of 2 MVA or 5 MVA. It is designed to deal with voltage dips of up to 50%.
3.2 Balancing
The optimum solution of out-of-balance problems depends significantly on the nature and magnitude of the problem. At low voltage it can be remedied by re-connection of unbalanced loads or by the installation of a static balancer -- a three-phase transformer core carrying two windings per phase in zig-zag connection (FIG. 3). This is a robust and simple application particularly common in rural areas. The Steinmetz balancing system may be used in urban areas or at higher voltages.
In the case of single-phase AC railway systems, again the simplest solution is balancing the single-phase load across the three phases. Multiple infeeds to the traction overhead catenary system are made along the length of the railway line. The single-phase demand from any one source at any one time is then usually sufficiently small to avoid significant out-of-balance problems. Where such multiple infeeds are not possible inductive and capacitive reactive components have to be added across the unused phases to 'balance' out the single-phase traction load. A practical example of such a balancer arrangement, which is of the largest of its kind to be installed in the world to date, has been used for the Channel Tunnel (again, FIG. 4 explains the compensation arrangements).
FIG. 3 Application of an AC three-phase static balancer.
3.3 Static VAr Compensators (SVCs)
These devices have their origins in efforts to control the 'flicker' produced by arc furnaces, but their ability to rapidly respond to changes in reactive power loading has resulted in their widespread use as elements in power transmission systems. They have been used to deal with voltage dips, fluctuations, flicker and unbalance. Such compensators are formed from a parallel connection of capacitors and thyristor-controlled reactors. The thyristor control varies the lagging reactive current so that the compensator can either generate capacitive VArs to support the voltage or generate lagging VArs in order to reduce the voltage.
Thyristor control equipment inevitably generates its own harmonics which are very sensitive to the thyristor firing angle delay as shown in Fig. 24.2 (Section 24). The equipment capacitor arms are often split into sub units to act as the necessary harmonic filters as shown in Fig. 24.6 (Section 24) and sometimes the capacitors are also thyristor switched.
Further explanation of the principle of reactive compensation is given in Section 28 (Section 28.8.5).
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FIG. 4 Balancing of single-phase load. XC 5XL 53R.
Load; Positive phase sequence currents; Negative phase sequence currents
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3.4 The STATCOM
A development of the well-proven SVC, the STATCOM is a voltage sourced converter which uses power electronic switches (Gate Turn Off Thyristors (GTOs)) to derive an approximately sinusoidal output voltage from a DC source (typically a capacitor). It is coupled to the system being compensated via an inductive impedance of low per unit value, and acts very much as a synchronous compensator, but with a vastly faster response. It has a natural tendency to compensate for changes in system voltage, and can do so very quickly. Unlike a constant impedance device such as a capacitor or inductor whose output current will decrease with voltage, it will continue to generate its maximum output current even at low system voltages.
It is an option for compensating voltage dips, surges, unbalance and flicker and generally takes less space than an SVC, but as a more sophisticated device it is more expensive, power for power and does not yet have the proven reliability of an SVC. Nevertheless many have already been installed in transmission schemes to provide reactive compensation. An example is given in Ref. [4].
In principle, by replacing the storage capacitor with a chemical storage system or even a super-conducting storage device, the STATCOM offers the possibility of becoming a fast response system energy store.
FIG. 5 Case study -- traction supply system: initial design.
4 CASE STUDY
Tenders have been received for a large 25 kV, 80 MVAr static balancer to help balance single-phase traction loads across a three-phase incoming sup ply. The system is fed from a single relatively weak 132 kV, 800 MVA low fault level supply connected in turn from a 400 kV, 6 GVA minimum fault level primary substation source. The maximum level of allowable unbalance has been set by the requirements of IEC 60034-1 for consumers and by the Electrical Supply Utility at the 400 kV point of common coupling as a 0.25% NPS restriction. A simple relationship exists between the maximum allowable unbalance load, SLOAD (MVA), the system fault level, FSOURCE (MVA) and the percentage NPS:
Hence the maximum level of unbalance at the 400 kV source is 0.25% on 6 GVA 5 15 MVA. This is equivalent to the power demand of a single high power Channel Tunnel 'Le Shuttle' train.
The original intention has been to supply the single-phase traction loads which are spread across the three phases by three single-phase transformers connected in delta on the primary side and in star on the secondary side with a common neutral connection for the traction return current. The single line diagram is shown in FIG. 5. Specialist firms have mentioned in their tenders that whilst their balancer will reduce NPS components in accordance with the Steinmetz balancing principle shown in FIG. 4, the effective 132/25 kV single-phase transformer connections will have a high impedance to zero-phase sequence (ZPS) components (see Section 14). Such components are, of course, inherent as the return current in a traction system design and will introduce 25 kV traction supply voltage regulation difficulties.
The Project Manager calls into his office the Project Technical Manager and the Associate Director-Systems Studies to have explained to him the situation and asks what can be done to resolve the transformer problem as tenders for the equipment are to be released shortly.
1. Could the situation be improved by increasing the fault level at the primary source substation? Is this a practical proposition in the short term?
2. Could the balancer be introduced at the 132 kV level to improve the situation? What would be the relative cost implications of static var compensation at 132 kV compared to 25 kV? If a thyristor-controlled balancer generates harmonics will the necessary filters also cost more at 132 kV? Will such filters 'suck in' harmonics from the supply source as well as from the balancer itself?
3. Could an alternative transformer arrangement help to reduce the voltage regulation problem caused by ZPS losses? What alternative transformer connection would you recommend? In practice the transformer arrangement was changed from single-phase transformers to three-phase star -- zig-zag star connections. The single line diagram of the final arrangement is shown in FIG. 6.
FIG. 6 Case study -- traction supply system: final transformer connection
design.
[One phase of balancer and harmonic filters]