Types of Transformer Faults.Transformer Protection.

Any number of conditions have been the reason for an electrical transformer failure.

- winding failures.

- voltage regulating load tap changers, when supplied.

- transformer bushings.

- transformer core problems.

Other miscellaneous failures have been caused by current transformers, oil leakage due to inadequate tank welds, oil contamination from metal particles, overloads, and overvoltage.

Types of Transformer Protection

Electrical methods.

Fuse: Power fuses have been used for many years to provide transformer fault protection. Generally it is recommended that transformers sized larger than 10 MVA be protected with more sensitive devices such as the differential relay. Fuses provide a low maintenance, economical solution for protection. Protection and control devices, circuit breakers, and station batteries are not required.

However, fuses provide limited protection for some internal transformer faults. A fuse is also a single phase device. Certain system faults may only operate one fuse. This will result in single phase service to connected three phase customers.

Fuse selection criteria include: adequate interrupting capability, calculating load currents during peak and emergency conditions, performing coordination studies that include source and low side protection equipment, and expected transformer size and winding configuration.

Overcurrent Protection: Overcurrent relays generally provide the same level of protection as power fuses. Higher sensitivity and fault clearing times can be achieved in some instances by using an overcurrent relay connected to measure residual current. This application allows pick up settings to be lower than expected maximum load current. It is also possible to apply an instantaneous overcurrent relay set to respond only to faults within the first 75% of the transformer. This solution, for which careful fault current calculations are needed, does not require coordination with low side protective devices.

Overcurrent relays do not have the same maintenance and cost advantages found with power fuses. Protection and control devices, circuit breakers, and station batteries are required. The overcurrent relays are a small part of the total cost and when this alternative is chosen, differential relays are generally added to enhance transformer protection. In this instance, the overcurrent relays will provide backup protection for the differentials.

DifferentialProtection: The most widely accepted device for transformer protection is called a restrained dif­ferential relay. This relay compares current values flowing into and out of the transformer windings. To assure protection under varying conditions, the main protection element has a multislope restrained characteristic. The initial slope ensures sensitivity for internal faults while allowing for up to 15% mismatch when the power transformer is at the limit of its tap range (if supplied with a load tap changer).

However, misoperation of the differential element is possible during transformer energization. High inrush currents may occur, depending on the point on wave of switching as well as the magnetic state of the transformer core. The use of traditional second harmonic restraint to block the relay during inrush conditions may result in a significant slowing of the relay during heavy internal faults due to the possible presence of second harmonics as a result of saturation of the line current transformers. To overcome this, some relays use a waveform recognition technique to detect the inrush condition.

It is highly recommended that separate relay input connections be used for each set used to protect the transformer. Failure to follow this practice may result in incorrect differential relay response. Appro­priate testing of a protective relay for such configuration is another challenging task for the relay engineer.

Overexcitation Protection: Overexcitation can be caused by an increase in system voltage or a reduction in frequency. It follows, therefore, that transformers can withstand an increase in voltage with a correspond­ing increase in frequency but not an increase in voltage with a decrease in frequency. Operation cannot be sustained when the ratio of voltage to frequency exceeds more than a small amount.

Protection against overflux conditions does not require high-speed tripping. In fact, instantaneous tripping is undesirable, as it would cause tripping for transient system disturbances, which are not damaging to the transformer.

An alarm is triggered at a lower level than the trip setting and is used to initiate corrective action. The alarm has a definite time delay, while the trip characteristic generally has a choice of definite time delay or inverse time characteristic.

Mechanical methods.

Accumulated gases and pressure relays are generally accepted methods used to detect transformer faults using mechanical methods. These detection methods provide sensitive fault detection and compliment protection provided by differential or overcurrent relays.

Thermal methods.

There are several thermal methods to detect transformer faults. They are: Hot Spot-Temperature, Heating due to Overexcitation, Heating Due to Current Harmonic Content, Heating Due to Solar Induced Currents, Load Tap-changer Overheating.

Backup Protection

Backup protection, typically overcurrent or impedance relays applied to one or both sides of the trans­former, perform two functions. One function is to backup the primary protection, most likely a differ­ential relay, and operate in event of its failure to trip.

The second function is protection for thermal or mechanical damage to the transformer. Protection that can detect these external faults and operate in time to prevent transformer damage should be considered. The protection must be set to operate before the through-fault withstand capability of the transformer is reached. If, because of its large size or importance, only differential protection is applied to a transformer, clearing of external faults before transformer damage can occur by other protective devices must be ensured.

 

1. Read and translate the text.

2. Find the English equivalents in the text:

Повреждение изоляции, износ изоляции, трансформаторный ввод, реле максимального тока, реле сопротивления,защита электрического генератора от перевозбуждения, повреждение обмотки,дифференциальная защита, чувствительный, устройство переключения отпаек/отводов (трансформатора).

3. Correspond the fault with its reasons.

Winding failures	General aging, contamination, cracking, internal moisture, and loss of oil. Two other possible reasons are vandalism and animals.
Tap changer failures	Insulation deterioration, often the result of moisture, overheating, vibration, voltage surges, and mechanical stress created during transformer through faults
Transformer  bushings	have been attributed to core insulation failure,  an open ground strap, or shorted laminations
Transformer core problems	a malfunction of the mechanical switching mechanism, high resistance load contacts, insulation tracking, overheating, or contamination of the insulating oil.

4. Put the following words in the correct order to form a sentence.

1) device, the, this, what, function,is, of?

2) provide, detection,this, fault, method, does, sensitive?

3) for,most, protection, what, the, widely, is, device, used, transformer?

4) protection, several, and, perform, control, functions, devices, usually.

5) also, fuse, is,a, a, phase,single, device.

 

5. Write down advantages and drawbacks of different types of protection.

 

TEXT 22. Transmission Line ProtectionStanley H. Horowitz

 

The study of transmission line protection presents many fundamental relaying considerations that apply, in one degree or another, to the protection of other types of power system protection. Each electrical element, of course, will have problems unique to itself, but the concepts of reliability, selectivity, local and remote backup, zones of protection, coordination and speed which may be present in the protection of one or more other electrical apparatus are all present in the considerations surrounding transmission line protection.

Since transmission lines are also the links to adjacent lines or connected equipment, transmission line protection must be compatible with the protection of all of these other elements. This requires coordi­nation of settings, operating times and characteristics.

The purpose of power system protection is to detect faults or abnormal operating conditions and to initiate corrective action. Relays must be able to evaluate a wide variety of parameters to establish that corrective action is required. The most common parameters which reflect the presence of a fault are the voltages and currents at the terminals of the protected apparatus or at the appropriate zone boundaries. The fundamental problem in power system protection is to define the quantities that can differentiate between normal and abnormal conditions. This problem is compounded by the fact that “normal” in the present sense means outside the zone of protection. This aspect, which is of the greatest significance in designing a secure relaying system, dominates the design of all protection systems.

The Nature of Relaying Reliability

Reliability, in system protection parlance, has special definitions which differ from the usual planning or operating usage. A relay can misoperate in two ways: it can fail to operate when it is required to do so, or it can operate when it is not required or desirable for it to do so. To cover both situations, there are two components in defining reliability:

Dependability - which refers to the certainty that a relay will respond correctly for all faults for which it is designed and applied to operate; and

Security - which is the measure that a relay will not operate incorrectly for any fault.

Most relays and relay schemes are designed to be dependable since the system itself is robust enough to withstand an incorrect tripout, whereas a failure to trip (loss of dependability) may be catastrophic in terms of system performance.

The relay must separate the meaningful and significant information contained in the waveforms upon which a secure relaying decision must be based. These considerations demand that the relay take a certain amount of time to arrive at a decision with the necessary degree of certainty. The relationship between the relay response time and its degree of certainty is an inverse one and is one of the most basic properties of all protection systems.

Although the operating time of relays often varies between wide limits, relays are generally classified by their speed of operation as follows:

1.  Instantaneous — These relays operate as soon as a secure decision is made. No intentional time delay is introduced to slow down the relay response.

2.  Time-delay — An intentional time delay is inserted between the relay decision time and the initiation of the trip action.

3.  High-speed — A relay that operates in less than a specified time. The specified time in present practice is 50 milliseconds (3 cycles on a 60 Hz system).

4.  Ultra high-speed — This term is not included in the Relay Standards but is commonly considered to be operation in 4 milliseconds or less.

Primary and Backup Protection

The main protection system for a given zone of protection is called the primary protection system. It operates in the fastest time possible and removes the least amount of equipment from service. On Extra High Voltage (EHV) systems, i.e., 345kV and above, it is common to use duplicate primary protection systems in case a component in one primary protection chain fails to operate. This duplication is therefore intended to cover the failure of the relays themselves. One may use relays from a different manufacturer, or relays based on a different principle of operation to avoid common-mode failures. The operating time and the tripping logic of both the primary and its duplicate system are the same.

It is not always practical to duplicate every element of the protection chain. On High Voltage (HV) and EHV systems, the costs of transducers and circuit breakers are very expensive and the cost of duplicate equipment may not be justified. On lower voltage systems, even the relays themselves may not be duplicated. In such situations, a backup set of relays will be used. Backup relays are slower than the primary relays and may remove more of the system elementsthan is necessary to clear the fault.

Remote Backup relays are located in a separate location and are completely independent of the relays, transducers, batteries, and circuit breakers that they are backing up. There are no common failures that can affect both sets of relays. However, complex system configurations may significantly affect the ability of a remote relay to “see” all faults for which backup is desired. In addition, remote backup may remove more sources of the system than can be allowed.

Local Backup relays do not suffer from the same difficulties as remote backup, but they are installed in the same substation and use some of the same elements as the primary protection. They may then fail to operate for the same reasons as the primary protection.

1. Read and translate the text.

2. Put questions to the words underlined.

1) The purpose of power system protection is to detect faults.

2) Relays are generally classified by their speed of operation.

3) Most relays and relay schemes are designed to be dependable.

4)  This requires coordi­nation of settings, operating times and characteristics.

5) They may then fail to operate for the same reasons as the primary protection.

3. Match the words from the text with their corresponding definitions.

Instantaneous	loss of security
Tripout	sturdy and strong in form, constitution, or construction;
adjacent	occurring with almost no delay; immediate
withstand	Possession of the means or skill to do something
transduсer	being near or close, especially having a common boundary
Relay	a device that converts a signal from one form of energy to another.
Ability	to resist or oppose with determined effort.
Robust	an electrical device, typically incorporatingan electromagnet, which is activated by a current or signal in one circuit to open or close another circuit.

4. Findtheantonyms.

normal	cheap
common	close
conpatible	undependable
remote	unsuitable, improper
expensive	similar
presens	abnormal
dependable	rare
different	absence

 

5. Put the following words in the correct order to form a sentence.

1) Significant, separate,the relay, information, must, the, meaningful, and.

2) Slower, relays, than,backup, are, relays,primary?

3) Two, in,there, defining, are, components, reliability.

 

6. Prepare a talk on one of the types of transmission line protection.

TEXT 23.Power electronics.

Power semiconductor devices are the most important functional elements in all power conversion applications. The power devices are mainly used as switches to convert power from one form to another. They are used in motor control systems, uninterrupted power supplies, high-voltage DC transmission, power supplies, induction heating, and in many other power conversion applications.

The thyristor, also called a silicon-controlled rectifier (SCR), is basically a four-layer three-junction pnpndevice. It has three terminals: anode, cathode, and gate. The device is turned on by applying a short pulse across the gate and cathode. Once the device turns on, the gate loses its control to turn off the device. The turn-off is achieved by applying a reverse voltage across the anode and cathode. There are basically two classifications of thyristors: converter grade and inverter grade. The difference between a converter-grade and an inverter- grade thyristor is the low turn-off time (on the order of a few microseconds) for the latter. The converter- grade thyristors are slow type and are used in natural commutation (or phase-controlled) applications. Inverter-grade thyristors are used in forced commutation applications such as DC-DCchoppers and DC-AC inverters. The inverter-grade thyristors are turned off by forcing the current to zero using an external commutation circuit. This requires additional commutating components, thus resulting in additional losses in the inverter.

The Gate Turn-off (GTO) thyristor is a power switching device that can be turned on by a short pulse of gate current and turned off by a reverse gate pulse. This reverse gate current amplitude is dependent on the anode current to be turned off. Hence there is no need for an external commutation circuit to turn it off. Because turn-off is provided by bypassing carriers directly to the gate circuit, its turn-off time is short, thus giving it more capability for high-frequency operation than thyristors.

Reverse-Conducting Thyristor (RCT) and Asymmetrical Silicon-Controlled Rectifier (ASCR).

Normally in inverter applications, a diode in antiparallel is connected to the thyristor for commuta­tion/freewheeling purposes. In RCTs, the diode is integrated with a fast switching thyristor in a singlesilicon chip. Thus, the number of power devices could be reduced. This integration brings forth a substantial improvement of the static and dynamic characteristics as well as its overall circuit performance.

The RCTs are designed mainly for specific applications such as traction drives. The antiparallel diode limits the reverse voltage across the thyristor to 1 to 2 V. Also, because of the reverse recovery behavior of the diodes, the thyristor may see very high reapplied dv/dt when the diode recovers from its reverse voltage.

Power transistors are used in applications ranging from a few to several hundred kilowatts and switching frequencies up to about 10 kHz. Power transistors used in power conversion applications are generally npn type. The power transistor is turned on by supplying sufficient base current, and this base drive has to be maintained throughout its conduction period. It is turned off by removing the base drive and making the base voltage slightly negative. The saturation voltage of the device is normally 0.5 to 2.5 V and increases as the current increases. Hence, the on-state losses increase more than proportionately with current. The transistor off-state losses are much lower than the on-state losses because the leakage current of the device is of the order of a few milliamperes. Because of relatively larger switching times, the switching loss significantly increases with switching frequency. Power transistors can block only forward voltages. The reverse peak voltage rating of these devices is as low as 5 to 10 V. Power transistors do not have Pt withstand capability. In other words, they can absorb only very little energy before breakdown. Therefore, they cannot be protected by semiconductor fuses, and thus an electronic protection method has to be used.

Power MOSFETs are marketed by different manufacturers with differences in internal geometry and with different names such as MegaMOS, HEXFET, SIPMOS, and TMOS. They have unique features that make them potentially attractive for switching applications. They are essentially voltage-driven rather than current-driven devices, unlike bipolar transistors.

The gate of a MOSFET is isolated electrically from the source by a layer of silicon oxide. The gate draws only a minute leakage current on the order of nanoamperes. Hence, the gate drive circuit is simple and power loss in the gate control circuit is practically negligible. Although in steady state the gate draws virtually no current, this is not so under transient conditions. The gate-to-source and gate-to-drain capacitances have to be charged and discharged appropriately to obtain the desired switching speed, and the drive circuit must have a sufficiently low output impedance to supply the required charging and discharging currents.

The Insulated-Gate Bipolar Transistor (IGBT) has the high input impedance and high-speed characteristics of a MOSFET with the conduc­tivity characteristic (low saturation voltage) of a bipolar transistor. The IGBT is turned on by applying a positive voltage between the gate and emitter and, as in the MOSFET, it is turned off by making the gate signal zero or slightly negative. The IGBT has a much lower voltage drop than a MOSFET of similar ratings. The structure of an IGBT is more like a thyristor and MOSFET. For a given IGBT, there is a critical value of collector current that will cause a large enough voltage drop to activate the thyristor. Hence, the device manufacturer specifies the peak allowable collector current that can flow without latch- up occurring. There is also a corresponding gate source voltage that permits this current to flow that should not be exceeded.

Like the power MOSFET, the IGBT does not exhibit the secondary breakdown phenomenon common to bipolar transistors. However, care should be taken not to exceed the maximum power dissipation and specified maximum junction temperature of the device under all conditions for guaranteed reliable operation. The on-state voltage of the IGBT is heavily dependent on the gate voltage. To obtain a low on-state voltage, a sufficiently high gate voltage must be applied.

MOS-Controlled Thyristor (MCT) is a new type of power semiconductor device that combines the capabilities of thyristor voltage and current with MOS gated turn-on and turn-off. It is a high power, high frequency, low conduction drop and a rugged device, which is more likely to be used in the future for medium and high power applications.

 

1. Read and translate the text.

2. Answer the following questions.

1) What is the difference between converter-grade and an inverter- grade thyristor?

2) Why canthe number of power devices in RCT be reduced?

3) How is the power transistor turned on?

4) What method is used to protect power transistors?

5) What is the structure of an IGBT similar to?

6) What is the on-state voltage of the IGBT dependent on?

3. Findthe syn on y ms.

Negligible	fall, decrease
exceed	different
withstand	concerning, with regards to
drop	Acceptable, suitable
unlike	get
with respect to	tiny, unimportant
obtain	survive, resist, endure
attractive	surpass, overfulfill
rugged	durable, reliable, sturdy

 

4. Complete the table below, using a dictionary.

Verb	Noun (idea)	Adjective	Adverb
-	-	allowable	-
require	-	-	-
combine	-	-	-
-	conduc­tivity	-	-
-	capability	-	-
-	-	relatively	-

5. Write down the characteristic features of SCR, GTO,RCT, MOSFET, IGBT and MCT in a given table:

SCR	GTO	RCT	MOSFET	IGBT	MCT

TEXT 24. Power Quality. Harmonics in Power Systems

Electric power quality has emerged as a major area of electric power engineering. The predominant reason for this emergence is the increase in sensitivity of end-use equipment. While grounding, voltage sags, harmonics, voltage flicker, and long-term monitoring do not cover all aspects of power quality, they provide a broad - based overview that should serve to increase overall understanding of problems related to power quality.

Proper grounding of equipment is essential for safe and proper operation of sensitive electronic equipment.According to the National Electric Code it is essential to insure proper and trouble-free equip­ment operation, and also to insure the safety of associated personnel.

Other than poor grounding practices, voltage sags due primarily to system faults are probably the most significant of all power quality problems. Voltage sags due to short circuits are often seen at distances very remote from the fault point, thereby affecting a potentially large number of utility customers. Coupled with the wide-area impact of a fault event is the fact that there is no effective preventive for all power system faults. End-use equipment will, therefore, be exposed to short periods of reduced voltage which may or may not lead to malfunctions.

Like voltage sags, the concerns associated with flicker are also related to voltage variations. Voltage flicker, however, is tied to the likelihood of a human observer to become annoyed by the variations in the output of a lamp when the supply voltage amplitude is varying. In most cases, voltage flicker considers (at least approximately) periodic voltage fluctuations with frequencies less than about 30-35 Hz that aresmall in size. Human perception, rather than equipment malfunction, is the relevant factor when con­sidering voltage flicker.

For many periodic waveform (either voltage or current) variations, the power of classical Fourier series theory can be applied. The terms in the Fourier series are called harmonics; relevant harmonic terms may have frequencies above or below the fundamental power system frequency. In most cases, nonfun­damental frequency equipment currents produce voltages in the power delivery system at those same frequencies. This voltage distortion is present in the supply to other end-use equipment and can lead to improper operation of the equipment.

Power system harmonics are not a new topic, but the proliferation of high-power electronics used in motor drives and power controllers has necessitated increased research and development in many areas relating to harmonics. For many years, high-voltage direct current (HVDC) stations have been a major focus area for the study of power system harmonics due to their rectifier and inverter stations. Roughly two decades ago, electronic devices that could handle several kW up to several MW became commercially viable and reliable products. This technological advance in electronics led to the widespread use of numerous converter topologies, all of which represent nonlinear elements in the power system.

Even though the power semiconductor converter is largely responsible for the large-scale interest in power system harmonics, other types of equipment also present a nonlinear characteristic to the power system. In broad terms, loads that produce harmonics can be grouped into three main categories: coveringarcing loads, semiconductor converter loads, and loads with magnetic saturation of iron cores.

Arcing loads, like electric arc furnaces and florescent lamps, tend to produce harmonics across a wide range of frequencies with a generally decreasing relationship with frequency. Semiconductor loads, such as adjustable-speed motor drives, tend to produce certain harmonic patterns with relatively predictable amplitudes at known harmonics. Saturated magnetic elements, like overexcited transformers, also tend to produce certain “characteristic” harmonics. Like arcing loads, both semiconductor converters and saturated magnetics produce harmonics that generally decrease with frequency.

 

1. Read and translate the text.

2. Read and decide if the following statements are true (T) or false (F).

1) Proper grounding of equipment is essential for safe operation of equipment.

2) Voltage sags due to short circuits are hardly seen at distances very remote from the fault point.

3) Voltage flicker is irritating for people.

4) The power of classical Fourier series theory can be used for several periodic waveform variations.

5) Voltage distortion can lead to incorrect operation of the equipment.

6) Power system harmonics have been studied for a long time.

7) Nonlinear elements in the power system became widespread not long ago.

3.  Put the following words in the correct order to form a sentence.

1) Insure, it, the, essential, to, proper, is, operation, of, equipment.

2) Voltage, customers, of, sags, affect, large, can, number, utility.

3) Power, largely, semiconductor, the, converter, responsible, is?

4) by, cannot, transistors,protected, semiconductor, fuses, power, be.

5) Semiconductor, elements, most, are, devices, power, the,important, functional.

 

4. Match the words from the text with their corresponding definitions.

Voltage sag	the power output of a generator or power plant.
adjustable	To state, tell about, or make known in advance, especially on the basis of special knowledge
harmonic	Any of a series of periodic waves whose frequencies are integral multiples of a fundamental frequency.
load	Unable to hold or contain more; full
predictable	to move or change (something) so as to be in a more effective arrangement or desired condition
Saturated	is a short duration reduction in rms voltage which can be caused by a short circuit, overload or starting of electric motors
large-scale	wide-ranging or extensive.

 

5. Write the summary of the text.