27-02-2010, 06:55 AM
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TRANSFORMER PRO TECTION USING NUMERICAL RELAY
PROJECT REPORT
Submitted by
AMBILY G ASHA V N SHYAMA K
INTRODUCTION
The aim of this project is to analyze and upgrade existing transformer protection system in BPCL Kochi Refinery. Transformers are static devices totally enclosed and generally oil immersed. Therefore, chances of faults occurring on them are very rare." However the consequences of even a rare fault may be very serious unless the transformer is quickly disconnected from the system. This necessitates providing adequate automatic protection for transformer against possible faults.
Small distribution transformers are usually connected to the supply system through series fuses instead of circuit breakers. Consequently, no automatic protective relay equipment is required. However, the probability of faults on power transformers is undoubtedly more and hence protection is absolutely necessary.
The transformer is major and very important equipment in power system. It requires highly reliable protective devices. The protective scheme depends on the size of the transformer. The rating of transformers used in transmission and distribution system range from a few kVA to several hundred MVA . For small transformers of medium size ¦over current relays are used. For large transformers differential protection is recommended.
We analyzed and studied the existing transformer protection relays used in BPCL. [There different types of protection employ different types of relays.
With the technological advances being made in generation protection section, the [generator protection using new methods have gathered momentum. Here we have studied about functioning of transformer protection using numerical relays.
BPCL KOCHI REFINERY
AMBALAMUGAL, ERNAKULAM
BPCL Kochi Refinery, formerly known as Cochin Refineries Limited is a public sector enterprise which came into being as a result of a three party agreement among the Government of India, Philips Petroleum Company of the United States of America and the Duncan Brothers of Calcutta.
The formation agreement provided for the construction of a petroleum refinery in South India Petroleum and for continuing technical collaboration with Philips during a number of years. Government was to see to the marketing of all domestic products and exercise control over the company.
MILESTONES OF GROWTH
YEAR DEVELOPMENT
* 1966 Unit commissioned with Crude Oil Refining capacity
of 2.5 MMPTA
* 1973 Capacity expanded from 2.5 to 3.3 MMPTA
* 1984 Capacity expanded from 3.3 to 4.5 MMPTA
* 1989 Aromatic production commenced
* 1990 Captive power plant (26.3 MW) commissioned
* 1994 Capacity expanded to 7.5 MMPTA
1998 Steam turbine generator (17.8) commissioned
i
1999 DHDS commissioned
2000 Company renamed as Kochi Refineries Limited
Bharat Petroleum Corporation Ltd acquired the Govt's share in KRL in March 2001. With this KRL became a subsidiary of BPCL. In 2005 KRL modified its DHDS and FCCU unit to meet the EURO specifications. In 2006 KRL merged with BPCL and became a SBU viz. BPCL KR.
PRODUCTS OF BPCL
Natural Rubber Modified Bitumen (Rubberized Bitumen) NRMB
Liquefied Petroleum Gas & Kerosene for households and industrial uses
Petrol & Diesel for automobiles
Naphtha, the major raw material for fertilizer and petrochemical industries
Benzene for manufacture of caprolactum, phenol, insecticides and other chemicals
Furnace oil and low sulphur heavy stock for fuel in industries
Aviation turbine fuel (ATF) for aircrafts
Special boiling point spirit used as a solvent in tyre industry
Toluene for the manufacture of solvents and insecticides, pharmaceuticals & paints
MTO (Textile grade) and MTO (Paint grade) for use in textile and paint industiy
Poly isobutene for the manufacture of lubricants
Sulphur for use in fertilizer, sugar, chemical and tyre industry
CAPTIVE POWER PLANT (CPP)
A captive power plant of 26.3 MW was commissioned in 1991. An additional captive power plant of 17.8 MW was commissioned in 1998.
Captive Power Plant (CPP) is the heart of BPCL. It has a gas turbine generator (GTG) and a steam turbine generator (STG) which caters the electrical load of entire refinery. The captive power plant generates about 7 lakhs units of electrical energy on an average day in which the contribution of GTG is about 65% and STG about 30%. The remaining 5% is contributed by TG, which generates at 3.3KV level. BPCL also have 66KV feeders for KSEB substations, which are normally kept as emergency stand of source central maximum demand of 20 MVA. The 66KV feeder's line 1 and line 2 are tapped from (Kalamassery-Vyttila) No.l feeder and (Kalamassery-Vyttila) No.2 feeder respectively. The 66KV/11KV transformer TR-1 & TR-2 primary windings (66KV side) are kept energized always, synchronizing with state grid. This is done to draw power when required.
Power is distributed to plants through XLPE cables both buried underground and on GRP trays. Primary process substation, FCCU substation, CDUI substation, ACTP substation, PIBU substation, CPP offsite substation, Crude booster substation and P1BL substation receive 11KV supply directly from CPP. The electrical system & BPCL also consists of about 80 transformers.
66KV, 25/35MVA POWER TRANSFORMER
SPECIFICATIONS
MVA 25/35
Type of cooling ONAN/ONAF
Voltaf ie Ratio (no load)
HV LV Phase
70KV 11KV
3
Normal Current
HV 206.2/288.7 Amps
LV 1312.1/1837.0 Amps
Tap changer OLTC (BHEL)
Frequency 50Hz
Temperature rise 50/55c
Connections
HV star
LV delta
Insulation level
HV 325KVp/140KVrms
HVN 170KVp/75KVrms
LV 75KVp/28Kvrms
Make GEC (tr 1), Volt amp (tr 2)
66KV CURRENT TRANSFORMER (CT)
SPECIFICATIONS
Made to BIS 2705/1981
Max system voltage 76.5KV
Rated STC 25KA for 1 sec
Rated primary 600/450/300 Amp
Secondary 1 A/1A
Rated burden Nil/30VA
Accuracy class Core 1 PS
Min knee point volt 800/1200V
Max RCT at 75C 2.5/5ohms
Insulation level 140/325KV
Frequency 50Hz
Make Automatic Electric Ltd
66KV POTENTIAL TRANSFORMER
SPECIFICATIONS
Made to ISS 3156/1978
Phase One
Service Volt 72.5KV
Frequency 50Hz
Ratio 66/root 3Kv
Core 1 110/root 3v
Core 2 110/root3v
Core 3 110/root3v
Burden Core 1 Core 2 Core 3 100VA 100VA 50VA Class 0.5 3P 3P 3P
Type Earthed
Make Automatic Electric Ltd
NEED FOR PROTECTION
Protection is installed to detect the fault occurrences and to isolate the faulty equipments. So that the damage to the faulty equipment is limited and disruption of supplies to the adjacent unaffected equipment is minimized.
In a power system consisting of generators, motors, transformers etc. it is inevitable that sooner or later when a fault occurs it must be quickly detected and the faulty equipment must be disconnected from the system. If the faults are not detected quickly it causes unnecessary interruption of service to the customers.
Generally fuse performs the function of detection and interruption. But it is limited only to low voltage circuits. For high voltage circuits relays and circuit breakers are used.
So in brief: - Protection must detect faults and abnormal working conditions and ¦isolates faulty equipments so as to limit damage caused by fault energy and to limit effect on rest of the system.
COMMON TRANSFORMER FAULTS
Transformer may suffer only from:
i. Open circuit
ii. Over heating
iii. Winding short circuits (eg: earth- faults, phase to phase faults & inter-turn
faults)
Open circuit fault
An open circuit in one phase of a three-phase transformer may cause undesirable heating. In practice, relay protection is not provided against open circuits because this condition is relatively harmless. On the occurrence of such a fault, the transformer can be disconnected manually from the system
Over heating fault
Over heating of the transformer is usually caused by sustained overloads or short circuits & very occasionally by the failure of the cooling system. The relay protection is :also not provided against this contingency and thermal accessories are generally used to sound an alarm or control the banks of fans.
Winding short- circuit fault
Winding short-circuits (also called internal faults) on the transformer arise from deterioration of winding insulation due to over heating or mechanical injury. When an [internal fault occurs, the transformer must be disconnected quickly from the system [because a prolonged arc in the transformer may cause oil fire. Therefore, relay protection is absolutely necessary for internal faults.
PROTECTIVE RELAYS
"Protective relay is a device that detects the faults and initiates the operation of the circuit breaker to isolate the defective element from the rest of the system."
High-performance protective relaying comes into its own when it's a question of minimizing power system operating costs. Uncomplicated operation, convenient commissioning tools and flexible communication are all important elements when service and maintenance costs have to be reduced.
These relays not only handle fault detection and location tasks but also control, metering and monitoring functions. And it is these additional functions - impossible before the advent of numerical technology - which offer major cost-cutting potential.
The relays detect the abnormal conditions in the electric circuits by constantly measuring the electrical quantities which are different under normal and fault conditions. The electrical quantities which may change under fault conditions are voltage, current, frequency and phase angle. Through the changes in one or more of these quantities, the faults signal their presence, type and location to the protective relays. Having detected the fault, the relay operates to close the trip circuit of the breaker. This results in the opening iof breaker and disconnection of faulty circuit.
FUNDAMENTAL REQUIREMENS OF PROTECTIVE RELAYING
¢ Selectivity i
¢ Speed
¢ Sensitivity I
¢ Reliability
¢ Simplicity
¢ Economy
Protection relays can be classified in various ways depending on their construction, functions and are discussed below.
CLASSIFICAION OF PROTECTIVE RELAYS BASED ON TECHNOLOGY
Protective relays can be broadly classified into the following categories depending on the technology used for their construction and operation.
1. Electromagnetic relays
2. Static relays
3. Microprocessor-based relays
Electromagnetic relays:¬' It includes attracted armature, moving coil and induction disc and induction cup type relays. Electromagnetic relays contain an electromagnet (or permanent magnet) and a moving part. When the actuating quantity exceeds a certain predetermined value, an operating torque is developed which is applied on the moving part. This causes the moving part to travel and to finally close a contact to energize the trip coil of the breaker.
Static relays:-
Static relays contains electronic circuits which may contain transistors, ICs, diodes and other electronic components. There is a comparator circuit in the relay, which compares two or more voltages and gives an output, which is applied to either a slave ¦ready or a thyristor circuit. The slave ready is an electromagnetic relay which finally ;closes the contact. A static relay containing a slave ready is semi-static relay. A relay kising thyristor circuit is a wholly static relay. Static relay possess the advantage of having ilow burden on CT and PT, fast operation, absence of mechanical inertia and contact trouble, long life and less maintenance. Static relays have proved to be superior to electromagnetic relays and they are being used for the protection of important lines, power stations and substations. Yet they have not completely replaced electromagnetic relays. Static relays are treated as an addition to family of relays. Electromagnetic relays continue to be in use because of their simplicity and low cost. Their maintenance can be done by less qualified personal, where as the maintenance and repair of static relays requires personnel's trained in solid-state device.
Microprocessor-based protective relays;-
Microprocessor-based protective relays are the latest development in this area. With the development in VLSI Technology, Sophisticated and fast microprocessor are coming up. Their applications to the problems of protective relaying schemes are of current interests to power engineers. The inherit advantages of microprocessor-based relays over static relays with or a very limited range of applications, are attractive, flexibility due to their programmable approach. Microprocessor-based protective relays can provide protection at low cost and compete with conventional relays. The present downward trend in the cost of large-scale integrated circuit will encourage wide applications of microprocessor-based applications of microprocessor-based relays for the protection modem complex power network.
TECHNOLOGY COMPARISON FOR PROTECTIVE RELAYS
SI.
No SUBJECT ELECTRO MECHANICAL STATIC/ ELECTRONIC NUMERICAL
1 Measuring
elements/
Hardware Induction disc, Electromagnets, Induction cup, Balance Beam Discrete R, L, C Transistors, Analogue ICs comparators Microprocessor s, Digital ICs, Digital signal
processors
2 Measuring method Electrical Qtys converted into mechanical force, torque Level detectors, comparison with reference value in analogue comparator A/D conversion,
Numerical
algorithm
techniques
evaluate trip
criteria
Timing function Mechanical clock works, dashpot Static timers Counters
4 Sequence of events Not possible Not possible Provided
5 Visual indication Flags, targets LEDs LEDs, LCD Display
6 Trip command Additional trip duty relay required Additional trip duty relay required Trip ¦ duty contact inbuilt
7 Construction size Bulky Modular, compact Most compact
8 Self monitoring No Yes
¢ Hardware Partly
¢ Power supply Yes
¢ Hardwar e
¢ Power supply
¢ O/P
relays
¢ Firm ware
CT, PT ckts
9 Temp. Stability Yes No Yes
10 Vibration proof No Yes Yes
11 Contacts Fixed Fixed Freely
assignments marshable
12 Parameter setting *Plug setting, dial setting Thumb wheel, potentiometers, dual in line switches Keypad for numeric values, through computer/ laptop
13 Binary inputs for adaptive relaying Not available Not available Freely marshable from 24V to 250V
14 CT loading/' Burden 8 to 10 VA 1 VA < 0.5 VA
15 CT offset adjustment No No Yes
16 Harmonic immunity No Possible through analogue fdtering Yes, digital
fdtering
incorporated
17 Calibration Frequently required as settings drift due to aging Frequently required as settings drift due to aging Not required as setting are stopped in memory in digital forma T
18 Auxiliary supply Required Required Required
19 Electromagnetic/ electrostatic/ high frequency disturbances Immune Susceptible Immune
20 Multiple characteristic Not possible Not possible Possible
21 Integrated protective functions Not possible Not possible Possible
¦22 Range of settings Limited Wide Wide
23 Operational value indication Not possible Possible Possible
24 'Fault
disturbance recording Not possible Not possible Possible
25 Digital
communication port Not possible Not available Available
26 Commissioning support from relay No No Yes
VARIOUS PROTECTION SCHEMES
PERCENTAGE DIFFERENTIAL PROTECTION
Percentage differential protection is used for the protection of large power transformers having ratings of 5MVA and above. This scheme is employed for the protection of transformers against internal short circuits. It is not capable of detecting incipient faults. Figure shows schematic diagram of percentage differential protection for a Y-A transformer. The direction of current and the polarity of the CT voltage shown in fig are for s particular instant. The convention for making the polarity for upper and lower CT is the same. The current entering end has been marked as positive. The end at which current is leaving has been marked negative.
CT
The connections are made in such a way that under normal conditions or in case of external faults the current flowing in the operating coil of the relay due to CTs of the primary side is in opposition to the current flowing due to the CTs of secondary side. Consequently, the relay does not operate under such conditions. If a fault occurs on the winding, the polarity of the induced voltage of the CT of the secondary side is reversed. Now the currents in the operating coil from CTs of both primary and secondary side are in the same direction and cause the operation of the relay. To supply the matching current in the operating winding of the relay, the CT which are on the star side of the transformer are connected in delta. The CTs which are on the delta side of the transformer are connected in star. In case of Y- A connected transformer there is a phase shift of 30 degree in line currents. Also the above mentioned CTs connections also correct this phase shift. Moreover, zero sequence current flowing on the star side of the transformers does not produce current outside the delta on the other side. Therefore, the zero sequence ;current should be eliminated from the star side. This condition is also fulfilled by CTs in delta on the star side of the transformer.
In case of star/star connected transformer CTs on both sides should be connected [in delta. In case of star/star connected transformer, if star point is not earthed, CTs may [be connected in star on both sides. If the star point is earthed and CTs are connected in [star, the relay will also operate for external faults. Therefore, it is better to follow the rule
that CTs associated with star connected transformer windings should be connected in I delta and those associated with delta windings in star.
The relay settings for transformer protection are kept higher than those for [alternators. The typical value of alternator is 10% for operating coil and 5% for bias. The corresponding values for transformer may be 40% and 10% respectively. The reasons for a higher setting in the case of transformer protection are, 1. A transformer is provided with on-load tap changing gear. The CT ratio cannot be changed with varying transformation ratio of the power transformer. The CT ratio
is fixed and it is kept to suit the nominal ratio of the power transformer. Therefore, for taps other than nominal, an out of balance current flows through the operating coil of the relay during load and external fault conditions. 2. When a transformer is on no-load, there is no-load current in the relay. Therefore, its setting should be greater than no-load current.
OVERHEATING PROTECTION
The rating of a transformer depends on the temperature rise above an assumed maximum ambient temperature. Sustained overload is not allowed if the ambient temperature is equal to the assumed ambient temperature. At lower ambient temperature some over loading is permissible. The over loading will depend on the ambient temperature prevailing at the time of operation. The maximum safe over loading is that it does not over heat the winding. The maximum allowed temperature is about 95 degree Celsius. Thus the protection against over load depends on the winding temperature which is usually measured by thermal image technique.
In the thermal image technique, the temperature sensing device is placed in the transformer oil near the top of the transformer tank. A CT is employed on LV side to supply current to a small heater. Both the temperature sensing device and the heater are
placed in a small pocket. The heater produces a local temperature rise similar to that of the main winding. The temperature of the sensing element is similar to that of the [winding under all conditions. In a typical modern system the heat sensitive element is a [silicon resistor or silistor. It is incorporated with the heating element and kept in a [thermal moulded material. The whole unit forms a thermal replica of the transformer Ivinding. It is in the shape of a small cylinder and is placed in the pocket in the transformer tank about 25cm below the tank top, which is supposed to be the hottest layer [in the oil. The silistor is used as an arm of a resistance bridge supplied from a stabilized fc)C source. An indicating instrument is energized from the out of balance voltage of the bridge. Also the voltage across the silistor is applied to a static control circuit which controls cooling pumps and fans, gives warning of over heating, and ultimately trips the transformer circuit breaker.
OIL PRESSURE RELIEF DEVICES
An oil pressure relief device is fitted at the top of the transformer tank. In its simplest form, it is a frangible disc located at the end of a relief pipe protruding from the top of the transformer tank. In case of a serious fault, a surge in the oil is developed, which bursts the disc, thereby allowing the oil to discharge rapidly. This avoids the explosive rapture of the tank and the risk of lire.
The drawback of the frangible disc is that the oil which remains in the tank after rupture is left exposed to the atmosphere. This drawback can be overcome by employing a more effective device: a spring controlled pressure relief valve. It operates when the pressure exceeds 10 psi but closes automatically when the pressure falls below the critical level. The discharged oil can be ducted to a catchments pit where random discharge of oil is to be avoided. The device is commonly employed for large power transformers of the rating 2MVA and above but it can also be used for distribution transformers of 200kVA and above.
OVERFLUXING PROTECTION
The magnetic flux increases when voltage increases. This results, in increased iron [loss and magnetizing current. The core and core bolts get heated and the lamination [insulation is affected. Protection against over fluxing is required where over fluxing due [to sustained over voltage can occur. The reduction in frequency also increases the flux [density and consequently it has similar affects as those due to over voltage. The expression of flux in a transformer is given by
<j> =kE/f
Where <j)=flux, f= frequency, E=applied voltage and k=constant
Therefore, to control flux, the ratio E/f is controlled. When E/f exceeds unity, it has to be detected. Electronic circuits with suitable relays are available to measure the E/f ratio. Usually 10% of over fluxing can allowed without damage. If E/f exceeds 1.1 overfluxing protections operates. Overfluxing does not require high speed tripping and hence instantaneous operation is undesirable when momentary disturbance occur. But the transformer should be isolated in one or two minutes at the most if overfluxing persists.
BUCHHOLZ RELAY
Buchholz relay is gas-actuated relay installed in oil-immersed transformer for protection against all kind of faults. It is used to give alarm in case of slow-developing faults in transformer and to disconnect from the supply in event of severe internal faults. It is usually installed in the pipe connecting the conservator to the main tank as shown in fig. It is a universal practice to use Buchholz relays on all such oil-immersed transformers having rating in excess of 750kVA.
OPERATION
The operation is as follows.
1. In case of incipient faults within the transformers, the heat due to fault causes the decomposition of some transformer oil in the main tank. The product of decomposition contains more than 75% of Hydrogen gas. The Hydrogen gas being light tries to go into the conservative and in the process gets entrapped in the upper part of relay chamber. When a predetermined amount of gas gets accumulated, it exerts sufficient pressure on the float to cause it to tilt and close contacts of Mercury switch attached to it. This completes the alarm circuits to sound an alarm.
2. If serious faults occur in the transformer, an enormous amount of gas is generated in the main tank. The oil in the main tank rushes towards the conservator via the Buchholz relay and in doing so tilts to close the contacts of Mercury switch. This completes the trip circuit to open the circuit breaker controlling the transformer.
ADVANTAGES
1. It is the simplest form of transformer protection.
2. It detects the incipient faults at a stage much earlier than is possible with other forms of protection.
DISADVANTAGES
1. It can only be the used with oil immersed transformer equipped with conservator tanks.
2. The device can detect only faults below oil level in the transformer. Therefore, separate protection is needed for connecting cables.
EARTH-FAULT OR LEAKAGE PROTECTION
An earth-fault usually involves a partial a partial breakdown of winding insulator jto earth. The resulting leakage current is considerably less than the short circuit current. [The earth-fault may continue for a long time and cause considerable damage before if ultimately develops into a short circuit and removed from the system under these circumstances, it is profitable to employ earth fault relay is essentially an over current relay of low setting and operates as soon as an earth-fault or leak develops.
Restricted earth fault protection
Restricted earth fault protection as shown in the fig provides better protection. This scheme is used for the winding of the transformer connected in star where the |neutral point is either solidly earthed or earthed through impedance. The relay used is of high impedance type to meet the scheme stable for external fault
' supply
7^
:ffj1n_
Ml
load
Residual oveiciuient Relay
Res tiic fe A E ai th fault Relay
The residual over current relay operates only for ground fault in the transformer. The differential protection is supplemented by restricted earth fault protection in case of transformer with its neutral grounded through resistance. For such a case only about 40% of the winding is protected with a differential relay pick up setting as low as 20% of the CT winding.
OVERCURRENT RELAYS
Over current relays are used for the protection of transformers of rating lOOkVA *and below 5MVA. An earth fault tripping element is also provided in addition to the over (current feature. Such relays are used as primary protection for transformers which are not (provided with differential protection. Over current relays are also used as back-up protection where differential protection is used as primary protection.
For small transformers, over current relays are used for both overload and fault [protection. An extremely inverse relay is desirable for overload and light faults, with [instantaneous over current relay for heavy faults. A very inverse residual current relay with instantaneous relay is suitable for ground faults.
TIME-CURRENT CHARACTERISTICS
A wide variety of time-current characteristics is available for over current relays. [The name assigned to an over current relay indicates its time-current characteristics as [described below
DEFINITE TIME OVER CURRENT RELAY:-
It operates after a predetermined time when the current exceeds its pick up value. [The operating time is constant, irrespective of the magnitude of the current above the pick up value. The desired definite operating time can be set with the help of an [intentional time delay mechanism provided in the relaying unit.
The residual over current relay operates only for ground fault in the transformer. The differential protection is supplemented by restricted earth fault protection in case of transformer with its neutral grounded through resistance. For such a case only about 40% of the winding is protected with a differential relay pick up setting as low as 20% of the CT winding.
OVERCURRENT RELAYS
Over current relays are used for the protection of transformers of rating 100kVA and below 5MVA. An earth fault tripping element is also provided in addition to the over current feature. Such relays are used as primary protection for transformers which are not provided with differential protection. Over current relays are also used as back-up protection where differential protection is used as primary protection.
For small transformers, over current relays are used for both overload and fault protection. An extremely inverse relay is desirable for overload and light faults, with instantaneous over current relay for heavy faults. A very inverse residual current relay with instantaneous relay is suitable for ground faults.
TIME-CURRENT CHARACTERISTICS
A wide variety of time-current characteristics is available for over current relays. The name assigned to an over current relay indicates its time-current characteristics as [described below
DEFINITE TIME OVER CURRENT RELAY:-
It operates after a predetermined time when the current exceeds its pick up value.
The operating time is constant, irrespective of the magnitude of the current above the
pick up value. The desired definite operating time can be set with the help of an
Intentional time delay mechanism provided in the relaying unit. ,
INSTANTANEOUS OVER CURRENT RELAY:-
It operates in definite time when the current exceeds its pick up value. The operating time is constant, irrespective of the magnitude of the current. There is no intentional time delay. It operates in zero seconds or less.
Sometime a term like "high set" or "high speed" is used for very fast relays having operating times less than 0.1 sec.
INVERSE TIME OVER CURRENT RELAY:-
It operates when the current exceeds its pick up value. The operating point depends upon the magnitude of the operating current. The operating time decreases as the current increases.
INVERSE DEFINITE MINIMUM TIME (IDMT) LAG RELAY:-
In these relays the time of operation is approximately inversely proportional to the smaller values of current or other quantity causing operation and tends to be definite minimum time as the value increases without limit.
DEFINITION & BLOCK DIAGRAM OF NUMERICAL RELAY
Numerical relays are those in which measured ac quantities are sequentially converted into numeric data form. A microprocessor performs mathematical and/or operation on the data to make decision. In numerical relay there is an additional entity, the software, which runs in the background and which actually run in the relay. With the advent of numerical relay the emphasis has shifted from hardware to software. What distinguishes numerical relay from the other is software.
st fiit -TS'T^N
toveision ;
command jMiaopioce | hip
sso:
Analogue to
jligit.il
conveiter
End of reversion
Figure shows the block diagram of a numerical relay. The signals from the CTs and PTs cannot be sampled directly and converted to the digital form. This is to make sure that the signal does not contain frequency components having a frequency greater than one half of the sampling frequency. This limit is enforced by the sampling theorem. Therefore, the signals are first passed through a low-pass filter, which has to be an analogue type of filter because digital pressing can only take place after the frequency spectrum of the signal is properly shaped.
Next the analogue signal is sampled and held constant during the time the value is converted to digital form. The sample and hold circuit is an absolute must.
The sampled and hold value is passed on to the ADC through a multiplier so as to accommodate a large number of input signals. The sample and hold circuit and the ADC work under the control of the microprocessor and communicate with it with the help of control signal such as the end-of -conversion signals issued by the ADC. The ADC-passes on the digital representation of the instantaneous value of the signal to the microprocessor via as input port. The output ports of the ADC may be 4, 8, 12, 16, Or 32 bits wide or even wider. The wider the output of the ADC, the greater its resolution.
The incoming digital values from the ADC are stored in the RAM of the microprocessor and processed by the relay software in accordance with an underlying relaying algorithm. The microprocessor issues the trip signal on one of the bits of its output port which is then suitably processed so as to make it compatible with the trip coil of the CB. The microprocessor can also be used to communicate with other relays or another supervisory computer, if so desired. The relaying program or the relay software, which resides in the EPROM, can only be upgraded or modified by authorized personnel. Thus new features and functionalities can be added to an existing relay by upgrading its software.
A numerical relay can be made to run a program which periodically performs a self diagnostic test and issues an alarm signal if any discrepancy is noticed. Other features like a watch-doze timer can also be implemented, which issues an alarm if the [microprocessor does not reset it, periodically within a stipulated time and a few milliseconds. This gives an increased user confidence and improves the reliability and the relay.
Relay Setting Types
The relay requires three types of settings. The settings, described below, determine how the relay recognizes and responds to adverse operating conditions to protect your power apparatus equipment. Each relay is delivered with a set of parameters that are preprogrammed at the factory.
System settings inform the relay what it is protecting (transformer or generator/motor) and provide system information such as CT ratios and MVA ratings. The relay's differential protection function and overload function use this information to compute the system's protection requirements.
Protection settings, such as the differential current pickup and thermal overload condition, are also required. The protection functions must be set either to operate and automatically trip (ON), to operate but not trip (BLOCKED), or to not operate (OFF).
Relay configuration settings tell the relay how to process the input information land to logically associate it with the output devices. If desired, you can reassign the [binary inputs, annunciations, and the function of the relay's output signals, trip relays, [and LEDs. Configuration also is referred to in this manual as programming or marshalling the relay.
Each relay input and output setting is assigned to an address number that you [must access to display or to change the setting. Address numbers are grouped typically in blocks according to their function.
7UT61 SERIES NUMERICAL RELAY
The numerical differential protection 7UT61 is a fast and selective short-circuit protection for transformers of all voltage levels, for rotating machines, for series and ^shunt reactors, or for short lines and mini-bus bars.
Two models are available. They are
1. 7UT612
2. 7UT613
7UT612 NUMERICAL RELAY
The SIPROTEC 7UT612 differential protection relay is used for fast and selective fault clearing of short-circuits in two winding transformers of all voltage levels and also in rotating electric machines like motors and generators, for short two terminal lines and busbars up to 7 feeders.
The specific application can be chosen by parameterization. In this way an optimal adaption of the relay to the protected object can be achieved
In addition to the differential function a backup over current protection for 1 winding / star point is integrated in the relay. Optionally, a low or high-impedance restricted earth-fault protection, a negative sequence protection and a breaker failure protection can be used. With external temperature monitoring boxes (RTD-boxes) a maximum of 12 temperatures can be measured and supervised in the relay. Therefore, complete thermal supervision of a transformer is possible.
The protection relay can be parameterized for use with three-phase and single-phase transformers. In addition to this, a thermal replica is integrated for the supervision of the ohmic losses in the plant.
FUNCTION OVERVIEW
Differential protection for 2 winding transformers (3-/1-phase)
Differential protection for motors and generators
Differential protection for short two terminal lines
Differential protection for busbars up to 7 feeders (phase-segregated or with summation CT)
PROTECTION FUNCTIONS
Differential protection with phase-segregated measurement
Sensitive measuring stage for low-fault currents
Restraint against inrush of transformer
Phase-/earth overcurrent protection
[ Overload protection with or without temperature measurement
t Negative-sequence protection
Breaker failure protection
Low-or high-impedance restricted earth fault (REF)
Over excitation protection
4 Thermal monitoring of transformer via temperature measurement with external thermo-box up to 12 measuring points
CONTROL FUNCTIONS
Commands for. Ctrl, of CB and isolators
MONITORING FUNCTIONS
Self-supervision of the relay
Trip circuit supervision
Oscillographic fault recording
Permanent differential and restraint current measurement
COMMUNICATION INTERFACES
PC front port for setting with DIGSI 4
System interface protocols -IEC 60870-5-103
- PROFIBUS-FMS/-DP
- MODBUS
- DNP 3.0
Service interface for DIGSI 4 (modem)/temperature monitoring (thermo-box)
Time synchronization via IRIG-B/DCF 77
7UT613 NUMERICAL RELAY
The SIPROTEC 7UT613 differential protection relays are used for fast and selective fauld clearing of short-circuits in transformers of allj voltage levels and also in rotating electric^ machines like motors and generators, for short lines and bus bars.
The specific application can be chosen by parameterization. In this way an optimal adaptation of the relay to the protected object can be achieved.
In addition to the differential function, a backup overcurrent protection for 1 winding/ star point is integrated in the relay. Optionally, a low or high-impedance restricted earth-fault protection, a negative-sequence protection and a breaker failure protection can be used. With external temperature monitoring boxes a maximum of 12 temperatures can be measured and monitored in the relay. Therefore, complete thermal monitoring of a transformer is possible, e.g. hot-spot calculation of the oil temperature.
The protection relay can be parameterized for use with three-phase and single-phase transformers.
The integrated programmable logic (CFC) allows the users to implement their iown functions, e.g. for the automation of switchgear (interlocking). User-defined messages can be generated as well. The flexible communication interfaces are open for Imodem communication architectures with control system. Alternatively to the conventional overload protection, the relay can also provide a hot spot calculation acc. to J1EC60435
FUNCTION OVERVIEW
Differential protection for 2 or 3 winding transformers (3-/1-phase)
Differential protection for motors and generators
Differential protection for short 2 up to 3 terminal lines
Differential protection for busbars up to 9 feeders (phase-segregated or with summation CT)
PROTECTION FUNCTIONS
Differential protection with phase-segregated measurement
4 Sensitive measuring stage for low-fault currents
Fast tripping for high-fault currents
Restraint against inrush of transformer
Phase-/earth overcurrent protection
Overload protection with or without temperature measurement
Negative-sequence protection
Breaker failure protection
Low-or high-impedance restricted earth fault (REF)
OVER EXCITATION PROTECTION
Thermal monitoring of transformer via temperature measurement with external thermo-box up to 12 measuring points
CONTROL FUNCTIONS
Commands for. Ctrl, of CB and isolators
Control via keyboard, binary inputs, DIGSI 4 or SCADA system
User-defined logic with CFC
MONITORING FUNCTIONS
Self-supervision of the relay
Extensive number of operational values (I, V, P, f, coscj) etc.)
Trip circuit supervision
Oscillographic fault recording
Permanent differential and restraint current measurement
COMMUNICATION INTERFACES
PC front port for setting with DIGSI 4
System interface protocols
- IEC 60870-5-103
- PROFIBUS-FMS/-DP -MODBUS
- DNP 3.0
Service interface for DIGSI 4 (modem)/temperature monitoring (thermo-box)
Time synchronization via IRIG-B/DCF 77
Overall Operation
The numerical differential protection device SIPROTEC® 7UT612 is equipped with a powerful microcomputer system. This provides fully numerical processing of all functions in the device, from the acquisition of the measured values up to the output of commands to the circuit breakers. Figure 1-1 shows the basic structure of the device.
Analog Inputs
The measuring inputs "MI" transform the currents derived from the instrument transformers and match them to the internal signal levels for processing in the device. The device includes 8 current inputs. Three current inputs are provided for the input of the phase currents at each end of the protected zone, a further measuring input (I7) may be used for any desired current, e.g. the earth current measured between the starpoint of a transformer winding and ground. The input Ig is designed for highly sensitive current detection thus allowing, for example, the detection of small tank leakage currents of power transformers or reactors, or †with an external series resistor †processing of a voltage (e.g. for high impedance unit protection). The analog signals are then routed to the input amplifier group "IA". The input amplifier group "IA" ensures a high impedance termination for the measured signals. It contains filters which are optimized in terms of band width and speed with regard to the signal processing. The analog/digital converter group "AD" has a multiplexer, analog/digital converters and memory modules for the data transfer to the microcomputer system "pC".
Microcomputer System
Apart from processing the measured values, the microcomputer system "pC" also executes the actual protection and control functions. In particular, the following are included:
> Filtering and conditioning of measured signals.
> Continuous supervision of measured signals.
> Monitoring of the pickup conditions of each protection function.
> Conditioning of the measured signals, i.e. conversion of currents according to the connection group of the protected transformer (when used for transformer differential protection) and matching of the current amplitudes.
> Formation of the differential and restraint quantities.
> Frequency analysis of the phase currents and restraint quantities.
> Calculation of the RMS-values of the currents for thermal replica and scanning of the temperature rise of the protected object.
> Interrogation of threshold values and time sequences.
> Processing of signals for the logic functions.
> Reaching trip command decisions.
> Storage of fault messages, fault annunciations as well as oscillographic fault data for system fault analysis.
> Operating system and related function management such as e.g. data recording, real time clock, communication, interfaces etc.
> The information is provided via output amplifier "OA".
Binary Inputs and Outputs
The microcomputer system obtains external information through binary inputs such as remote resetting or blocking commands for protective elements. The "pC" issues information to external equipment via the output contacts. These outputs include, in ¦particular, trip commands to circuit breakers and signals for remote annunciation of important events and conditions.
Front Elements
Light-emitting diodes (LEDs) and a display screen (LCD) on the front panel provide information such as targets, measured values, messages related to events or faults, status, and functional status of the 7UT612. Integrated control and numeric keys in conjunction with the LCD facilitate local interaction with the 7UT612. All information of jthe device can be accessed using the integrated control and numeric keys. The. information includes protective and control settings, operating and fault messages, and measured values (see also SIPROTEC® System Manual, order-no. E50417-H1176-C151). If the device incorporates switchgear control functions, the control of circuit ibreakers and other equipment is possible from the 7UT612 front panel.
Serial Interfaces
A serial operating interface on the front panel is provided for local communications with the 7UT612 through a personal computer. Convenient operation of all functions of the device is possible using the SIPROTEC® 4 operating program DIGSI® 4. A separate serial service interface is provided for remote communications via a modem, or local communications via a substation master computer that is permanently connected to the 7UT612. DIGSI® 4 is required. All 7UT612 data can be transferred to a central master or main control system through the serial system (SCADA) interface. Various protocols and physical arrangements are available for this interface to suit the particular application. Another interface is provided for the time synchronization of the internal clock via external synchronization sources. Via additional interface modules further communication protocols may be created. The service interface may be used, alternatively, for connection of a thermobox in order to process external temperatures, e.g. in overload protection.
Power Supply
The 7UT612 can be supplied with any of the common power supply voltages. Transient dips of the supply voltage which may occur during short-circuit in the power supply system, are bridged by a capacitor.
Applications
The numerical differential protection 7UT612 is a fast and selective short-circuit protection for transformers of all voltage levels, for rotating machines, for series and shunt reactors, or for short lines and mini-busbars with two feeders.
It can also be used as a single-phase protection for busbars with up to seven feeders. The individual application can be configured, which ensures optimum matching to the protected object.
The device is also suited for two-phase connection for use in systems with 162/3 Hz rated frequency.
A major advantage of the differential protection principle is the instantaneous tripping in the event of a short-circuit at any point within the entire protected zone. The current transformers limit the protected zone at the ends towards the network. This rigid limit is the reason why the differential protection scheme shows such an ideal selectivity.
For use as transformer protection, the device is normally connected to the current transformer sets at the higher voltage side and the lower voltage side of the power transformer. The phase displacement and the interlinkage of the currents due to the winding connection of the transformer is matched in the device by calculation algorithms. The earthing conditions of the star point(s) can be adapted to the user's requirements and are automatically considered in the matching algorithms.
For use as generator or motor protection, the current in the starpoint leads of the machine and at its terminals are compared. Similar applies for series reactors. Short lines tor mini-busbars with two feeders can be protected either. "Short" means that the [connections from the CTs to the device do not cause an impermissible burden for the current transformers.
Project Report '07
1
For transformers, generators, motors, or shunt reactors with earthed starpoint, the current between the starpoint and earth can be measured and used for highly sensitive earth fault protection. The seven measured current inputs of the device allow for a single-phase protection for busbars with up to seven feeders. One 7UT612 is used per phase in this case. Alternatively, (external) summation transformers can be installed in order to allow a busbar protection for up to seven feeders with one single 7UT612 relay. An additional current input 18 is designed for very high sensitivity. This may be used e.g. for detection of small leakage currents between the tank of transformers or reactors and earth thus recognizing even high-resistance faults.
For transformers (including auto-transformers), generators, and shunt reactors, a high-impedance unit protection system can be formed using 7UT612. In this case, the currents of all current transformers (of equal design) at the ends of the protected zone feed a common (external) high-ohmic resistor the current of which is measured using the high-sensitive current input 18 of 7UT612.
The device provides backup time overcurrent protection functions for all types of protected objects. The functions can be enabled for any side. A thermal overload protection is available for any type of machine. This can be complemented by the evaluation of the hot-spot temperature and ageing rate, using an external thermobox to allow for the inclusion of the oil temperature. An unbalanced load protection enables the detection of unsymmetrical currents. Phase failures and unbalanced loads which are especially dangerous for rotating machines can thus be detected.
A version for 162/3 Hz two-phase application is available for traction supply (transformers or generators) which provides all functions suited for this application (differential protection, restricted earth fault protection, overcurrent protection, overload protection).A circuit breaker failure protection checks the reaction of one circuit breaker after a trip command. It can be assigned to any of the sides of the protected object.
Features
*t* Powerful 32-bit microprocessor system
J*. Complete numerical processing of measured values and control, from sampling and digitizing of the analog input values up to tripping commands to the circuit breakers.
>. Complete galvanic and reliable separation between internal processing circuits of the 7UT612 and external measurement, control, and power supply circuits because of the design of the analog input transducers, binary inputs and outputs, and the DC/DC or AC/DC converters.
. Suited for power transformers, generators, motors, branch-points, or smaller busbar arrangements.
J*. Simple device operation using the integrated operator panel or a connected Personal computer running DIGSI® 4.
Differential Protection for Transformers
. Current restraint tripping characteristic.
. Stabilized against in-rush currents using the second harmonic.
. Stabilized against transient and steady-state fault currents caused e.g. by over excitation of transformers, using a further harmonic: optionally the third or fifth harmonic.
. Insensitive against DC offset currents and current transformer saturation.
High stability also for different current transformer saturation.
>. High-speed instantaneous trip on high-current transformer faults.
Independent of the conditioning of the starpoint(s) of the power transformer.
High earth-fault sensitivity by detection of the starpoint current of an earthed transformer winding.
Integrated matching of the transformer connection group.
Integrated matching of the transformation ratio including different rated currents of the transformer windings.
Differential Protection for Generators and Motors
> Current restraint tripping characteristic.
High sensitivity.
Short tripping time.
> Insensitive against DC offset currents and current transformer saturation.
High stability also for different current transformer saturation.
Independent of the conditioning of the starpoint.
Differential Protection for Mini-Busbars and Short Lines
Current restraint tripping characteristic.
Short tripping time.
Insensitive against DC offset currents and current transformer saturation.
> High stability also for different current transformer saturation.
> Monitoring of the current connections with operation currents
Bus-Bar Protection
*> Single-phase differential protection for up to seven feeders of a busbar.
Either one relay per phase or one relay connected via interposed summation current transformers.
Current restraint tripping characteristic.
Short tripping time.
Insensitive against DC offset currents and current transformer saturation. High stability also for different current transformer saturation.
Monitoring of the current connections with operation currents
Restricted Earth Fault Protection
Earth fault protection for earthed transformer windings, generators, motors, shunts reactors, or starpoint formers.
Short tripping time.
> . High sensitivity for earth faults within the protected zone.
High stability against external earth faults using the magnitude and phase relationship of through-flowing earth current
High-Impedance Unit Protection
>. Highly sensitive fault current detection using a common (external) burden
resistor. t». Short tripping time.
. Insensitive against DC offset currents and current transformer saturation. *l*. high stability with optimum matching.
*l*. Suitable for earth fault detection on earthed generators, motors, shunt reactors, and transformers, including auto-transformers.
. Suitable for any voltage measurement (via the resistor current) for application of
high-impedance unit protection.
Tank Leakage Protection
For transformers or reactors the tank of which is installed isolated or high resistive against ground.
Monitoring of the leakage current flowing between the tank and ground.
. Can be connected via a "normal" current input of the device or the special highly
sensitive current input (3 mA smallest setting).
Time Overcurrent Protection for Phase Currents and Residual Current
>. Two definite time delayed overcurrent stages for each of the phase currents and the residual (threefold zero sequence) current, can be assigned to any of the sides of the protected object.
Additionally, one inverse time delayed overcurrent stage for each of the phase currents and the residual current.
Selection of various inverse time characteristics of different standards is possible, alternatively a user defined characteristic can be specified.
. All stages can be combined as desired; different characteristics can be selected for
phase currents on the one hand and the residual current on the other.
. External blocking facility for any desired stage (e.g. for reverse interlocking). . Instantaneous trip when switching on a dead fault with any desired stage.
. Inrush restraint using the second harmonic of the measured currents.
>. Dynamic switchover of the time overcurrent parameters, e.g. during cold-load startup of the power plant.
Time Overcurrent Protection for Earth Current
Two definite time delayed overcurrent stages for the earth current connected at current input 17 (e.g. current between starpoint and earth).
Additionally, one inverse time delayed overcurrent stage for the earth current.
. Selection of various inverse time characteristics of different standards is possible,
alternatively a user defined characteristic can be specified. The stages can be combined as desired.
J*. External blocking facility for any desired stage (e.g. for reverse interlocking). . Instantaneous trip when switching on a dead fault with any desired stage.
. Inrush restraint using the second harmonic of the measured current.
. Dynamic switchover of the time overcurrent parameters, e.g. during cold-load startup of the power plant.
Single-Phase Time Overcurrent Protection
. Two definite time delayed overcurrent stages can be combined as desired. *l* . For any desired single-phase overcurrent detection.
. Can be assigned to the current input 17 or the highly sensitive current input 18. . Suitable for detection of very small current (e.g. for high-impedance unit
protection or tank leakage protection, see above). > . Suitable for detection of any desired AC voltage using an external series resistor
(e.g. for high-impedance unit protection, see above). . External blocking facility for any desired stage.
Unbalanced Load Protection
. Processing of the negative sequence current of any desired side of the protected
object.
. Two definite time delayed negative sequence current stages and one additional inverse time delayed negative sequence current stage.
. Selection of various inverse time characteristics of different standards is possible,
alternatively a user defined characteristic can be specified. *l*. The stages can be combined as desired.
Thermal Overload Protection
J*. Thermal replica of current-initiated heat losses. . True RMS current calculation.
. Can be assigned to any desired side of the protected object. ¢. Adjustable thermal warning stage.
. Adjustable current warning stage.
. Alternatively evaluation of the hot-spot temperature according to IEC 60354 with
calculation of the reserve power and ageing rate (by means of external temperature sensors via thermo box).
Circuit Breaker Failure Protection
With monitoring of current flow through each breaker pole of the assigned side of the protected object.
. Supervision of the breaker position possible (if breaker auxiliary contacts available).
<. Initiation by each of the internal protection functions.
. Initiation by external trip functions possible via binary input.
External Direct Trip
Tripping of either circuit breaker by an external device via binary inputs.
Inclusion of external commands into the internal processing of information and trip commands.
With or without trip time delay.
Processing of External Information
. Combining of external signals (user defined information) into the internal
information processing. Pre-defined transformer annunciations for Buchholz protection and oil gassing. . Connection to output relays, LEDs, and via the serial system interface to a central
computer station.
User Defined Logic Functions (CFC)
. Freely programmable linkage between internal and external signals for the
implementation of user defined logic functions. . All usual logic functions.
. Time delays and measured value set point interrogation.
Commissioning Operation
. Comprehensive support facilities for operation and commissioning.
. Indication of all measured values, amplitudes and phase relation.
. Indication of the calculated differential and restraint currents.
J*. Integrated help tools can be visualized by means of a standard browser: Phasor diagrams of all currents at all ends of the protected object are displayed as a graph.
> . Connection and direction checks as well as interface check.
Monitoring Functions
. Monitoring of the internal measuring circuits, the auxiliary voltage supply, as well
as the hard- and software, resulting in increased reliability. *l* Supervision of the current transformer secondary circuits by means of symmetry checks.
. Check of the consistency of protection settings as to the protected object and the
assignment of the current inputs: blocking of the differential protection system in case of inconsistent settings which could lead to a malfunction. Trip circuit supervision is possible.
Further Functions
. Battery buffered real time clock, which may be synchronized via a
synchronization signal (e.g. DCF77, IRIG B via satellite receiver), binary
input or system interface. . Continuous calculation and display of measured quantities on the front of
the device. Indication of measured quantities of all sides of the protected
object.
. Fault event memory (trip log) for the last 8 network faults (faults in the power system), with real time stamps (ms-resolution).
Fault recording memory and data transfer for analog and user configurable binary signal traces with a maximum time range of 5 s.
Switching statistics: counter with the trip commands issued by the device, as well as record of the fault current and accumulation of the interrupted fault currents;
Communication with central control and data storage equipment via serial interfaces through the choice of data cable, modem, or optical fibers, as an option.
PROTECION SCHEME USING NUMERICAL RELAY
DIFFERENTIAL PROTECTION FOR TRANSFORMERS
Matching of the Measured Values
In power transformers, generally, the secondary currents of the current transformers are not equal when a current flows through the power transformer, but depend on the transformation ratio and the connection group of the protected power transformer, and the rated currents of the current transformers at both sides of the power transformer. The currents must, therefore, be matched in order to become comparable. Matching to the various power transformer and current transformer ratios and of the phase displacement according to the vector group of the protected transformer is performed purely mathematically. As a rule, external matching transformers are not required. The input currents are converted in relation to the power transformer rated current. This is achieved by entering the rated transformer data, such as rated power, rated voltage and rated primary current of the current transformers, into the protection device. Once the vector group has been entered, the protection is capable of performing the current comparison according to fixed formulae. Conversion of the currents is performed by programmed coefficient matrices which simulate the difference currents in the transformer windings. All conceivable vector groups (including phase exchange) are possible. In this aspect, the conditioning of the starpoint(s) of the power transformer is essential, too.
Isolated Starpoint
Figure 2-15 illustrates an example for a power transformer Yd5 (wye-delta with 150 ° phase displacement) without any earthed starpoint. The figure shows the windings and the phasor diagrams of symmetrical currents and, at the bottom, the matrix equations. The general form of these equations is
Im = k.(K).In,
Where
(Im) - Matrix of the matched currents IA, IB. IC, k - Constant factor,
(K) - Coefficient matrix, dependent on the vector group, (In) - Matrix of the phase currents IL1, IL2, IL3.
On the left (delta) winding, the matched currents IA, IB, IC are derived from the difference of the phase currents I.L1, IL2, IL3. On the right (wye) side, the matched currents are equal to the phase currents (magnitude matching not considered).
Earthed Starpoint
Figure 2-16 illustrates an example for a transformer YNd5 with an earthed starpoint on the Y-side. In this case, the zero sequence currents are eliminated. On the left side, the zero sequence currents cancel each other because of the calculation of the current differences. This complies with the fact that zero sequence current is not possible outside of the delta winding. On the right side, the zero sequence current is eliminated by the calculation rule of the matrix, e.g.
l/3-(2 IL1 - 1 IL2- 1 IL3)= 1/3 ¢ (3 IL1 - IL1 - 1L2 - IL3) = 1/3 ¢ (3 IL1 -3 I0) = (1L1 -10).
Zero sequence current elimination achieves that fault currents which flow via the transformer during earth faults in the network in case of an earth point in the protected zone (transformer starpoint or starpoint former by neutral earth reactor) are rendered harmless without any special external measures. Refer e.g. to Figure 2-17: Because of the earthed starpoint, a zero sequence current occurs on the right side during a network fault but not on the left side. Comparison of the phase currents, without zero sequence current elimination, would cause a wrong result (current difference in spite of an external fault).
Figure 2-18 shows an example of an earth fault on the delta side outside the protected zone if an earthed starpoint former (zigzag winding) is installed within the protected zone. In this arrangement, a zero sequence current occurs on the right side but not on the left, as above. If the starpoint former were outside the protected zone (i.e. CTs between power transformer and starpoint former) the zero sequence current would not pass through the measuring point (CTs) and would not have any harmful effect.
The disadvantage of elimination of the zero sequence current is that the protection becomes less sensitive (factor 2/3 because the zero sequence current amounts to 1/3) in case of an earth fault in the protected area. Therefore, elimination is suppressed in case the starpoint is not earthed (see above, Figure 2-15).
Li
Li
hm.
t52
rYY\
Figure 2-17 Example of an eann fault outside the protected transformer and current distribution
L-,
\ H
\
J
Figure 2-18 Example of an earth fault outside :ne protected transformer with a neutral earthing reactor within tie protected zone
Increasing the Ground Fault Sensitivity
Higher earth fault sensitivity in case of an earthed winding can be achieved if the starpoint current is available, i.e. if a current transformer is installed in the starpoint connection to earth and this current is fed to the device (current input 17). Figure 2-19 shows an example of a power transformer the starpoint of which is earthed on the Y-side. In this case, the zero sequence current is not eliminated. Instead of this, 1/3 of the starpoint current ISP is added for each phase.
1-1 ¢
-e-
Li
u
=C7 J "-'3
7*" H i
Figure 2-19 Example of a earth fault outside the transformer«
The matrix equation is in this case:
1=
= 1
1 0 0 0 1 0 0 0 1
/ \
1 ISP
¦SP
IL2 3'
k-2 ,
ISP corresponds to -310 but is measured in the starpoint connection of the winding and not in the phase lines. The effect is that the zero sequence current is considered in case of an internal fault (from 10 = -1/3ISP), whilst the zero sequenc
