Generator And Generator Transformers Protection 9b4

GENERATOR AND GENERATOR TRANSFORMERS PROTECTION General 1.1

1.2

Generators are designed to run at high load factor for a large number of years and permit certain incidences of abnormal working conditions. The machine and its auxiliaries are supervised by monitoring devices to keep the incidences of abnormal working conditions down to a minimum. Despite the monitoring, electrical and mechanical faults may occur, and the generators must be provided with protective relays, which, in case of a fault, quickly initiate a disconnection of the machine from the system and, if necessary, initiate a complete shutdown of the machine. Recommendations contained herein for electrical protective system of generator and generator transformer are intended to be used for generators of 10 MVA and above. It describes requirements of various protections, special comments to help in determining applications of these for units of various types and sizes, setting criteria and tripping principles.

The protection requirements of machine used in pumped storage schemes need further attention and the recommendations given below do not cover these details. 1.2.1

Generator can be connected to the primary transmission system in the following two ways: I. Unit Scheme In this scheme no switchgear is provided between the generator and generator transformer which are treated as a unit; a unit transformer is trapped off the interconnection for the supply of power to auxiliary plant. II. Generator Circuit Breakers Scheme In this scheme, a generator circuit breaker is provided between the generator and generater transformer.

1.3

In view of generator unit being a complex system, various electrical hazards require consideration. These are given below: a. Stator insulation failure b. Overvoltage c. Rotor faults d. Loss of synchronism e. Over/under frequency f. Overload g. Unbalanced loading h. Loss of excitation i. Reverse power protection j. Inadvertent energisation of generator

1.4

Following are the various protections recommended for the generator and generator transformer protection.

Type of Fault GENERATOR STATOR Short circuits

Asymmetry Stator overload

ANSI Device No. 87 G 87 GT 21 G 51/27 G 46 G 51 G

Device No.

Protection Functions Generator differential Overall differential Minimum impedance(or alternatively over current/under voltage) Negative sequence Overload

Earth fault stator

64 G1 64 G2 40 G 98 G 32 G/37 G

Loss of excitation Out of step Monitoring Blade fatigue Inter turn fault Mag.circuits Higher voltage Accidental energisation Monitoring GENERATOR ROTOR Rotor ground GENERATOR TRANSFORMER Short circuits

Ground fault Overhang UNIT AUXILIARY TRANSFORMER Short circuit Ground fault

81 G 59 G/87 GT 99 G 59 G 27/50 G 60 G

95% stator earth fault 100% stator earth fault Loss of excitation Pole slip Low forward power/reverse power (double protection for large generators) Minimum frequency Over voltage or under current Overfluxing volt/Hz Overvoltage Dead machine PT fuse failure

64 F

Rotor earth fault

87 GT 51 GT 87 T 51 NGT 87 NT 87 HV

Overall differential Overcurrent Transformer differential Earth fault overcurrent Restricted earth fault HV winding cum overhang differential

87 UAT 51 UAT 51 UAT 64 UAT

Transformer differential Overcurrent Residual overcurrent Restricted earth fault

The transformers guards like Buchholtz protection, winding temperature high, oil temperature high, etc. are also important and shall be provided as per transformer manufacturer’s recommendations. 1.5

Recommendations regarding selection of the generator protections for units of different types and sizes are summarized in the table given below: Functions

differential 95% stator E/F 100% stator E/F Interturn faults Backup impedance Voltage controlled O/C Negative sequence Field Failure Reverse power Pole slipping overload overvoltage Under frequency Dead machine Rotar earth fault overfluxing

Steam Small(<1 0 MVA) Y Y N Y N

Gas Medium(<10100 MVA) Y Y Y/N Y Y

Turbines Large(>100 MVA) Y Y Y Y Y

Hydro Small(<10 MVA) Y Y N Y N

Turbines Medium(<10100 MVA) Y Y Y/N Y Y

Large(>100 MVA) Y Y Y Y Y

Y

N

N

Y

N

N

Y

Y

Y

Y

Y

Y

Y Y N N Y Y N Y N

Y Y N N Y Y N Y Y

Y Y Y N Y Y Y Y Y

Y Y N Y Y Y N Y N

Y Y N Y Y Y N Y Y

Y Y Y Y Y Y Y Y Y

2. Detailed requirements 2.1 Generator stator 2.1.1 Generator differential protection(87 G) This is a unit type protection, instantaneous in operation, covering the stator winding for phase to phase faults. The generator differential relay is not sensitive to phase earth faults due to high neutral earthing resistance arrangement. As this protection operates for generator internal faults, opening the generator breaker in class-A eliminates the system in-feed to the fault(if the unit is synchronized). For all machines of ratings 10 MVA and above, this protection shall be provided. Requirements : i) ii) iii) iv) v) vi)

2.1.2

Be triple pole type with individual phase indication Have operating time less than 30 milliseconds at 2 times setting Be high or low impedance principle based Operating current 5 to 10% of nominal current High stability against maximum through fault condition, CT saturation, harmonics and DC transients Be provided with suitable voltage dependent resistors across the relay to limit the voltage to safe level, in case of high impedance type relay

Generator transformer differential protection (87 GT) This is a unit type protection with coverage from the generator terminals up to the HV breaker and the generator transformer HV terminals. It will detect phase faults on both sides of the generator transformer and single phase to earth faults of the HV side only (the earth fault current on the LV side is very small, due to the high neutral earthling resistance). Generator transformer differential relays have harmonic restraint circuits to prevent incorrect operation when the generator and the unit transformers are energized from the system. For all machines of ratings MVA and above, this shall be provided. The protection need not include Unit Auxiliary Transformer (UAT) in its zone and UAT should be covered by a separate protection. In case of breaker-and-half switching scheme, the CT’s associated with main and tie breakers shall be connected to separate bias winding and these should not be paralleled in order to avoid false operation due to dissimilar CT transient response. Requirements : i) ii) iii)

iv) v) vi)

Be triple pole type with individual phase indication Have through fault restraint features for external faults with an adjustable or multi-bias setting Have magnetizing inrush restraint features and also be stable for inrush under normal over fluxing conditions, magnetizing inrush proof feature shall not be achieved through any intentional time delay e.g. use of timers to block relay operation Shall have unrestrained instantaneous highest overcurrent unit operation which is unaffected by inrush Have one bias winding per phase and per CT input (However UAT CT need not have separate input and maybe parallel with the generator CT). Have an adjustable operating current

vii) viii)

2.1.3

Have an operating time not greater than 30 milliseconds at 5 times setting Have facility for ratio and phase angle correction eiter through auxiliary transformer or through inbuilt provisions.

Backup protection for short circuits For machines of 10 MVA above this shall be provided. For machines of ratings less than 10 MVA voltage restrained over current relay may be used in place of minimum impedance relay.

2.1.3.1 Backup impedance protection (21 G) This operates for phase faults in the unit, in the HV yard or in the adjacent transmission lines with a suitable delay, for cases when the corresponding main protection fails to operate. For faults on the high voltage side of the delta-star connected generator transformer, the impedance measured by the relay is influenced by the relay connection, the fault type and generator and system source impedance. For this reason a circular characteristic the center of which corresponds to connection position of VT should be used. The impedance relay shall have fuse-fail interlock. Requirements : i) ii) iii) iv) v)

Be triple pole type Be of single step under impedance type Shall have two adjustable definite time delay of 0.5-5 seconds Be suitable for measuring two or three phase faults within a set distance from the point of installation Shall be able to operate for fault currents down to 0.3 in.

Setting recommendations The impedance relay shall have reach setting to cover the longest HV outgoing line or 70% of rated generator load impedance -

Time relay of step 1 = 0.5 sec Time relay of step 2 shall be set to coordinate with third zone time of distance relay or back up o/c relay of the outgoing line

2.1.3.2 Overcurrent/undervoltage protection (51/27 )G i) ii) iii) iv) 2.1.4

Be triple pole type Should be able to operate when the fault current from the generator terminals becomes low due to excitation system characteristic Shall have undervoltage criteria Shall reset to de-energized position if under-voltage criteria disappears

Negative phase sequence protection (46 G) The negative phase sequence protection safeguards the generator rotor against overheating caused by the induced double frequency (100 Hz) currents when negative phase sequence currents are present in the stator. The negative phase sequence current can appear due to un-balanced single phase loads or transmission line unsymmetrical faults. For all machines of ratings 10 MVA and above this shall be provided. Requirements:

i) ii) iii) iv) v)

Be triple pole type Have an alarm unit and a trap unit Have a continuously adjustable negative sequence current setting Have a definite time setting range of 1-10 seconds associated with an alarm unit Have alarm unit range covering negative phase sequence current of 5-10 % of generator rated current continuously adjustable

vi)

Have a trip unit with a variable time current characteristic matching with the generator

2

I2 t

characteristic Setting recommendations The relay should be set to the NPS capacity of the generator. The NPS capability of the machines varies considerably from one machine to another and no required settings are thus suggested. Alarm unit shall be set at 50% of continuous withstand capability value of the machine and time delay for alarm can be = 3 seconds. 2.1.5

Generator overload protection (51 G) As an additional check of the stator winding temperatures, overload relay is used. This may be provided for hydro units where there are high head variations and connected for alarm. It may also be connected for run back. In case of thermal set there is no chance of thermal overloading as the machine is provided with number of limiters and therefore is not recommended. Requirements:

2.1.6

i) Be single pole type ii) Be of definite time over-current type iii) Have a continuously adjustable setting range of 50-200% iv) Have a drop-off/pick-up ratio greater than 95% v) Have an adjustable time setting range of 2.5 to 25 seconds Generator stator earth fault protection The high neutral earthling resistance arrangement limits the generator earth fault current to less than 10 amperes, therefore minimizing damage to the core lamination. Although a single phase earth fault is not critical, it requires clearance within a short time, due to the following: -

It may develop into a phase to phase fault (due to presence of ionized air) If a second earth fault occurs the current is no longer limited by the earthing resistor Fire may result from the earth arc

Two different types of stator earth fault relays are recommended both installed in the secondary circuit of the generator. 2.1.6.1 0-95% stator earth fault protection (64 G1) This protection zone is limited to approximately 95% of the stator winding due to the danger of false tripping. This shall be voltage relay monitoring the voltage developed across the earthing resistor by the neutral return current. This is normally in the back up protection. It also covers the generator bus, low voltage winding of the unit transformer and the high voltage winding of the UAT. When connected to open delta winding of generation PT, the protection shall be blocked for PT fuse failure. For faults within 10% of the generator neutral, the resulting current is not enough to operate the relay.

For all machines of ratings 10 MVA and above this shall be provided. Requirements: i) ii) iii) iv) v)

Single pole type Shall have independently adjustable voltage and time setting Suitable to protect 95% of stator winding Be suitable for operating from broken-delta voltage transformers or neutral grounding transformer secondary Shall be insensitive to 3rd harmonic voltage

Setting recommendations Pick-up voltage of the relay = 5% of maximum neutral voltage Time delay – 0.3-0.5 seconds 2.1.6.2 100% stator earth fault protection (64 G2) This protects the whole stator winding and the generator neutral. The relay generally operates on the principle of low frequency signal injection into the secondary of the earthing transformer, detecting the corresponding current if an earth fault occurs. The relay is set in of insulation resistance. This is normally in the main protection. Alternatively, a protection based on change in magnitude/distribution of 3rd harmonic voltage caused by an earth fault is used. When provided, this shall have voltage check or current check unit as applicable, to prevent faulty operation of the relay at generator stand still or during the machine running down period. For machines above 100 MVA the protection shall always be provided. For machines of smaller sizes the option is left to the utility depending upon the importance of the machine. For machines of 200 MVA and above low frequency current injection based relays are recommended. Where 95-100% relay is separately available, this may be connected for alarm and operator can take the machine out. Requirements: i) ii) iii)

iv) v)

vi)

Be suitable to protect 100% of stator winding Be insensitive to external faults, transients and inherent harmonic currents. Shall be based on low frequency current injection principle/alternatively shall operate on the principle of detecting change in the magnitude/distribution of third harmonic voltage caused by an earth fault. Have continuously adjustable time delay range 1-20 seconds If based on injection principle o Shall continuously monitor ground circuit o Shall continuously monitor injection signal and injection equipment o Shall be in service at standstill, startup and stop If based on 3rd harmonic voltage principle o Shall have under voltage or over current check

Setting recommendations 100% stator earth fault relay (injection principle) Pick up level of the relay = 500 ohms

Time delay – 2 seconds (greater than 3rd zone of distance relay) 100% stator earth fault relay (3rd harmonic principle)

2.1.7

Setting 0.45 V Time delay 2.0 sec Voltage check unit 80% of rated voltage Current check unit 20% of rated current Loss of excitation protection (40 G) A complete loss of excitation may occur as a result of unintentional opening of the field breaker, an open circuit or a short circuit of the main field or a fault in AVR. When a generator with sufficient active load loses the field current it goes out of synchronism and starts to run on asynchronously at a speed higher than the system absorbing reactive power for the excitation from the system. Under these conditions the stator end regions and part od the rotor get over heated. This is recommended for machines of all sizes above 10 MVA. Requirements: i)

ii) iii)

Have mho characteristic lying in the 3rd and 4th quadrant of impedance diagram with adjustable reach and offset. Alternatively, this protection shall be based on directional current unit with setting range to match generator capability curve. Shall have an under voltage relay and/or over current relay as an additional check. Shall have timer with adjustable range of 1-10 seconds to distinguish loss of excitation from power swings.

Setting recommendations i)

2.1.8

For offset mho type relay o Diameter of mho circle = Xd o Offset of the mho circle from origin = Xd/2 o Time delay = 1.0 second o Under voltage relay = 70% ii) For directional current type relay o directional current relay is set to match with the generator capability curve in 4th quardrand o time delay 0.5 – 1 second o under voltage 70% o over current 110 – 115% Pole slipping protection (98G) The loss of field protection shall be supplemented by an additional out of step function which detects all pole slips. Pole slipping of generators with respect to the system, can be caused by a number of conditions leading to an increase in rotor angular position beyond the generator transient stability limits. Some of the causes of pole slipping are: a) Large network disturbances b) Faults on electrical network close to the generator c) Weak tie between the network and the generator (tripping of transmission lines) d) Loss of generator field (field winding or excitation supply failure) e) Operating the generator in an excessive under excited mode.

This is recommended for machines of 100 MVA and above. For hydro machines utilities can decide depending on machine parameter.

Requirements i) ii)

Shall be capable of detecting a power swing which can lead to instability in addition to being able to detect an actual pole slip By varying size of the characteristic it shall be possible to ensure that a trip command is given to the circuit breakers in such a way that seperation of poles occurs at a controlled angle at any time.

Setting requirements i)

If the source of oscillation lies within a generator/transformer unit, the machine has to be isolated from the network after the first slip

Forward reach of relay characteristics shall cover generator/generator transformer. Tripping in this zone shall be in the first pole slip. The reach of this zone is = 0.7 XT ii) 2.1.9

If the source of oscillation lies outside the unit in the network, the generator should not be switched off or atleast not until several pole slips have occurred. Low forward power/reverse power interlock relays (32 G/37 G) The low forward power interlock is recommended for thermal machines and reverse power protection may be used for hydro machines to protect against motoring. When the steam flow through turbine is interrupted by closing the ESVs or the governor valves, the remaining energy stored in the set is delivered to the system and the machine enters into a motoring condition drawing power from the system to supply its losses while keeping the turbo alternator shaft at synchronous speed. The low forward power relay detects that the unitis motoring and must therefore be shutdown Tripping for mechanical faults and abnormal conditions

For faults in the turbine or boiler, the turbine protection closes the ESVs. For abnormal conditions, the generator protection closes the ESVs. When the generator develops low forward power, the relay after a short time delay trips the generator breaker. A protection field suppression signal may also be required if the AVR does not have a built-in facility to reduce the generator field current as the speed decreases, to avoid overfluxing conditions. Requirements i) ii) iii) iv) v) vi)

be single phase power measuring type have a power setting of approximately 0.5 – 1 % of rated active power of generator unit have independent time delay relay with setting range of 1 - 10 seconds and 0 – 20 seconds respectively on pick up have one more common timer with a pick up setting range 5 – 50 seconds for annunciation that the turbo generator set has started motoring have suitable arrangement for preventing the operation of this protection during startup and synchronizing of the unit shall be provided with possibility of angle correction to facilitate measurement of power accurately.

Setting recommendations:

2.1.10

low forward power relay pickup < 0.05 x Pn timer t1 = 2 seconds, t2 = 2 seconds Under frequency protection The frequency under protection Prevents the steam turbine and generator from exceeding the permissible operating time at reduced frequencies

-

Ensures that the generating unit is separated from the network at a present value of frequency that is less than the final stage of system load shedding. Prevents the AVR from exciting the machine at reduced speeds when some protective relays may not perform at all Prevent over fluxing of the generator. The over fluxing relay is used to protect against small over fluxing for long periods while the over voltage and under frequency relays als protect against large over fluxing for short times

The stator under frequency relay measures the frequency of the stator terminal voltage. Though under frequency tripping is recommended by turbine manufactures, care should be taken by grid operating personnel in ensuring that machines are not run at lower frequencies and instead resort to means like load shedding in the event of overload. Requirements: i) ii) iii)

Have one alarm stage and two tripping stages Shall have setting of range of 45 Hz – 55 Hz with a least count of 0.1 Hz for each stage Timer for alarm stage have a range of 0.5 – 5 seconds with a least count of 0.5 second. Timers for each tripping stage shall have a range of 1 to 10 seconds with a least count of 0.1 second Shall have under voltage blocking

iv)

Setting recommendations: -

Stator under frequency relay pickup level = 48.5 Hz Time delay Alarm -2 seconds

For tripping stages recommendations of turbine manufacturers may be followed. 2.1.11

Inter turn fault protection (59 G) It is generally considered difficult to obtain reliable protection against short circuit of one turn if the stator winding has large number of turns per phase. Inter turn fault protection is recommended only for machines where there is a split winding and all the six terminals are brought out on the neutral side. For the generator with split neutral, conventional inter-turn fault protective scheme comprises a time delayed low set over-current relay which senses the current flowing in the connection between the neutrals of the stator winding. Alternatively, a split phase differential protection may be used. Requirements: i) ii) iii)

Over current relay with time delay Over current relay shall have built in filters to reject higher harmonics Range of over current shall be selected depending on maximum spill current for external fault

Setting recommendations Over current relay shall be set to maximum unbalanced current in case of external fault. Time delay 0.2 – 0.4 seconds 2.1.12

Generator transformer over fluxing protection (99 GT) Over fluxing protection is provided to safeguard the generator, generator transformer and unit transformer against operation at flux densities which may cause accumulative damage to the core.

From the fundamental equation V = 4.44 x f x n x ϕ, the level of flux is proportional to the ratio of terminal voltage to frequency (v/f). This ratio is monitored by the protective relay. An over fluxing condition is more likely to occur while the generator is separated from the system and the speed is allowed to drop, but it can also happen with the machine on load if the tap changer of the generator transformer (HV side) is on a low tap position and the excitation of the generator is manually increased. In this case the increased generator terminal voltage may cause over fluxing tripping at nominal frequency. The over fluxing protection operates with a time delay after which the tripping functions are executed. This protection must be provided for generator-transformer of size 10 MVA and above. Requirements: i) ii) iii) iv) v)

Over fluxing protection shall be phase to phase connected Operate on the principle of measurement of voltage to frequency ratio Have inverse time characteristics compatible with generator transformer over fluxing withstand capability for tripping Provide an independent alarm with a definite time delay of value of V/f between 100% to 130% of rated value Have a high resetting ratio of 98% or better

Setting recommendations: The over fluxing capability of the transformer must be checked and the characteristic matched accordingly for both alarm and trip. 2.1.13

Generator over voltage protection (59 G)