Negative Sequence Currents In Generator-Its impact, Protection practices and tripping modes-An extract from my routine Study

Operation of Generator with unbalanced three-phase currents is among one of those abnormal operating conditions to which a generator may be subjected that may not necessarily involve a fault in the generator.

This condition produce negative-phase-sequence components of current which induce a double-frequency current in the surface of the rotor, the retaining rings, the slot wedges, and to a smaller degree, in the field winding. These rotor currents may cause high and possibly dangerous temperatures in a very short time.

This article discusses situations which may lead to operation of generator with unbalanced three-phase currents and consequent generation of negative sequence current, effect of negative sequence current on generator, the typical means for detecting this abnormal operating condition and the tripping/protection practices against this condition. Following is the content of article.

1. Brief discussion about generation of negative sequence current and Post Incident Scenario

1.1  Generation of Negative Sequence Current

1.2  Post Incident Scenario and Recommendations for Operator

2.Conditions to Generate Negative Sequence Current (I2) in Generator and I2 Limits

3.Effect of Negative Sequence Current on Generator Components

4.Protection Practices against Unbalance and Negative Sequence Current (Device number 46)

5.Tripping modes

1.     Brief discussion about generation of negative sequence current and Post Incident Scenario

Unbalanced phase current flow in generator stators cause double-frequency reverse rotation currents to circulate in the rotor body that can damage the rotor forging, wedges, amortisseur windings, and retaining rings.

1.1.   Generation of Negative Sequence Current

A three-phase balanced supply voltage applied to a symmetrical three-phase winding generates a constant- magnitude flux in the airgap of the machine, which rotates at synchronous speed around the circumference of the machine (see Fig. 1)

In addition the slots and other asymmetries within the magnetic path of the flux create low-magnitude space harmonics (i.e., fluxes that rotate in both directions) of multiple frequencies of the fundamental supply frequency. In a synchronous machine under normal operation, the rotor rotates in the same direction and speed as the main (fundamental) flux.

When the supply voltage or currents are unbalanced, an additional flux of fundamental frequency appears in the airgap of the machine. However, this flux rotates in the opposite direction from the rotor. This flux induces in the rotor windings and body voltages and currents with twice the fundamental frequency. These are called negative-sequence currents (I2).

The negative sequence terminology derives from the vector analysis method of symmetrical components.This method allows an unbalanced three-phase system to be represented by positive, negative, and zero sequences.

The larger the unbalance, the higher is the negative-sequence component.

1.2.   Post Incident Scenario and Recommendations for Operator

Cylindrical rotor generators designed according to ANSI standards are capable of continuously carrying 10% negative phase sequence current. This roughly corresponds to an operating condition where two phases are carrying rated current and the third phase has 70% of rated current.

Depending on the design of the rotor (indirectly or directly cooled) generators with two phases at rated current, and no current in the third phase, can carry this unbalance for 90 to 270 seconds before damage occurs to the rotor components. Accordingly negative-phase sequence relays are necessary to protect generator rotors from damage during all possible operating conditions, including phase-to-phase and phase-to-ground faults on the transmission system.Some negative-sequence overcurrent relays provide an alarm function with a pickup value set somewhere below the trip point. This alerts the unit operator to a negative-sequence condition prior to a trip.

If the negative-sequence alarm is initiated, the operator should take the following action:

·      Notify the transmission dispatcher of the negative sequence condition and find out if there are any electrical problems on the transmission system.

·      When a negative phase sequence alarm is activated, operators should also check the phase currents for balance.

·      In addition to off-site causes for unbalance, open conductors, disconnect, or breaker poles at the site can cause the unbalance condition.

·       If no abnormalities exist, notify dispatcher that load will be reduced on the generator until the alarm clears.

·      The generation should be taken off automatic control, and load should be reduced until the alarm clears.

·      If the alarm is coincident with any electrical switching in the switchyard or within the plant, the device should be opened, and if the alarm clears, the apparatus in question should be investigated for proper operation

Electrical Engineers or technicians should:

·      verify calibration of the negative-phase sequence relay, and

·      review any data acquisition monitoring devices (protective relay digital storage or DCS trends) to verify that the unit operated with a significant current unbalance.

Following a negative phase sequence trip, the unit should not be returned to service until an investigation is completed. It may have been caused by an electrical system fault that did not clear promptly because of faulty circuit-breakers or protective relays.

2.     Conditions to Generate Negative Sequence Current (I2) in Generator and I2 Limits

There are several abnormal operating conditions that give rise to large currents flowing in the forging of the rotor, rotor wedges, teeth, end-rings, and field-windings of synchronous machines. These conditions include:

·      Unbalanced armature current (producing negative-sequence currents),

·      inadvertent energization of a machine at rest, and

·      asynchronous motoring or generation (operation with loss of field)

·      system asymmetries (untransposed lines),

·      unbalanced loads,

·      unbalanced system faults and open phases.

 These system conditions produce negative-phase-sequence components of current which induce a double-frequency current in the surface of the rotor, the retaining rings, the slot wedges, and to a smaller degree, in the field winding. These rotor currents may cause high and possibly dangerous temperatures in a very short time.

A generator shall be capable of withstanding, without injury, the effects of a continuous current unbalance corresponding to a negative-sequence current I2 of the following values, providing the rated kVA is not exceeded and the maximum current does not exceed 105% of rated current in any phase. (Negative-sequence current is expressed as a percentage of rated stator current.)

These values also express the negative-sequence current capability at reduced generator kVA capabilities.

The ability of a generator to accommodate unbalanced currents is specified by ANSI C50.12-1982 and ANSI C50.13-1989 in terms of negative-sequence current (I2). This standard specifies the continuous I2 capability of a generator and the short time capability of a generator, specified in terms (I2 )2t, as shown in figure 2.

Unbalanced fault negative-sequence current capability is expressed in per unit of rated current and time in seconds

3.     Effect of Negative Sequence Current on Generator Components

A three-phase balanced load produces a reaction field, which is approximately constant, rotating synchronously with the rotor field system.

Any unbalanced condition can be broken down into positive, negative and zero sequence components.

·      The positive component behaves similar to the balanced load.

·      The zero components produce no main armature reaction.

However, the negative component creates a reaction field, which rotates counter to the DC field, and hence produces a flux, which cuts the rotor at twice the rotational velocity.This induces double frequency currents in the field system and rotor body (Negative sequence in a generator crosses the air gap and appears in the rotoror field as a double-frequency current).

The resulting eddy currents are very large, so severe that excessive heating occurs, quickly heating the brass rotor slot wedges to the softening point where they are susceptible to being extruded under centrifugal force until they stand above the rotor surface, in danger of striking the stator iron.

When these double frequency currents tends to flow in the surface of the rotor structure, the nonmagnetic wedges, and other lower-impedance areas, generating (I2)2 R losses with rapid overheating of critical rotor components. Severe overheating and, ultimately, the melting of the wedges into the air gap can occur, causing severe damage.

If not properly controlled, serious damage to the rotor will ensue. It is therefore very important that negative

phase sequence protection be installed, to protect against unbalanced loading and its consequences.

Of particular concern is damage to the end-rings and wedges of round rotors (see Figs. 3 and 4).

4.     Protection Practices against Unbalance and Negative Sequence Current (Device number 46)

Power system s are not completely symmetrica l and loads can be unbalanced so that a small amount of negative sequence is present during normal operation. ANSI standards permit continuous I2 currents of 5%–10 % in generators and also short -time limits expressed as (I2)2= K, where I2 is the integrated negative-sequence current flowing for time t in seconds; K is a constant established by the machine design . Typical values for synchronous condensers and older turbine generators were 30–40, but for the very large generators K may be as low as 5 –10. Units subject to the:

·      Specified limit and up to 200% of the limit may be damaged, and early inspection is recommended.

·      For units more than 200%, damage can be expected.

Inverse- time–over current units, opera ting from negative-sequence current and with a time characteristic adjustable to (I2)2= K, are recommended for all generators. They are set to operate just before the specified machine (I2)2= K limit is reached. A low-level I2 auxiliary is available, operating typically at about 0.03–0.2 pu I2, for warning continued unbalance.

A positive-sequence restraint is applied for better performance. The positive-sequence restraint allows for more sensitive settings by counter balancing spurious negative and zero sequence currents resulting from:

·      System unbalances under heavy load conditions.

·      Transformation errors of current transformers (CTs).

·      Fault inception and switch-off transients.

Some modern relays like GE G60 uses unbalance element which protects the machine from rotor damage due to excessive negative-sequence current. The element has an inverse time stage which is typically used for tripping and a definite time stage typically used for alarm purposes. The inverse time stage operating characteristic is defined by the following equation:

where Inom is the generator rated current and K is the negative-sequence capability constant normally provided by the generator manufacturer.

All large synchronous machines have (should have) installed protective relays that remove the machine from operation under excessive negative sequence currents. To properly “set” the protective relays, the operator should obtain maximum allowable negative sequence I2 values from the machine’s manufacturer. The values shown in table are contained in ANSI/IEEE C50.13  as values for continuous I2 current to be withstood by a generator without injury, while exceeding neither rated MVA nor 105% of rated voltage.

This protection is a backup primarily for unbalanced system faults that are not adequately cleared; it also backs up the protection for the generator unit and associated equipment.

5.     Tripping modes

The negative-sequence relay is connected to trip the main generator breaker(s). This is the preferred tripping if the machine auxiliaries permit operation under this condition because this approach allows quick resynchronization of the unit after the unbalanced conditions have been eliminated. If the machine auxiliaries do not permit operation of the machine with the above tripping, then the negative-sequence relay must also trip the machine prime mover, the field, and transfer the auxiliaries.

This approach may not be applicable with once-through boilers, with cross-compound units, or those units that cannot transfer sufficient auxiliary loads to maintain the boiler and fuel systems. In these cases, the turbine stop valves would also be tripped. Cross-compound units with directly interconnected stator circuits can be resynchronized with the system only if the units are in synchronism with each other. If the units are out of synchronism, normal starting procedures must be used to return the units to the line. However, recent developments in the industry have established that it may be possible to resynchronize some cross-compound generators after an accidental trip without returning the two generators to turning gear speed. This procedure should be established only after very careful consideration with the manufacturer. See IEEE Std 502-1985 for further details on tripping.