Generator Negative Sequence Currents: Effects, Safety Procedures, and Trip Modes

Generator Negative Sequence Currents: Effects, Safety Procedures, and Trip Modes

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 ,
•      Inadvertent energization of a machine at rest, and
•     Asynchronous motoring or generation 
•      System asymmetries ,
•       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. 

No alt text provided for this image

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

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 .

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 tend to flow in the surface of the rotor structure, the nonmagnetic wedges, and other lower-impedance areas, generating2 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 

4. Protection Practices against Unbalance and Negative Sequence Current

Power systems are not completely symmetrical 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–overcurrent units, operating 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 positive-sequence restraint is applied for better performance. The positive-sequence restraint allows for more sensitive settings by counterbalancing 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 use an unbalanced 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 the 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.