1. STUDY COMMITTEE A1- ROTATING ELECTRICAL MACHINES
WG A Guide for Generator On-Line Over and Under
Excitation Operation Issues
Rodica Zlatanovici (Romania), Convener
xxxxx. Paris xxxx

2. AGENDA INTRODUCTION GENERATOR CAPABILITY CHART (PQ DIAGRAM)2
AGENDA
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
AGENDA
INTRODUCTION
GENERATOR CAPABILITY CHART (PQ DIAGRAM)
ELECTROMAGNETIC PHENOMENA AT FRONTAL ZONE OF GENERATORS
GENERATOR OPERATION IN OVER AND UNDER EXCITED REGIMES
CONCLUSIONS 2

3. WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
1 - INTRODUCTION
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
MEMBERS 14
(10 countries)
1- INTRODUCTION
- EXTENSION OF WG A – GUIDE FOR THE PREVENTION OF OVERFLUXING OF GENERATORS ( 2011 ) – during the run-up
- TERMS OF REFERENCE:
GOAL:
To present the mode of utilization of the electric generators for voltage control in the electrical grid where they are connected .
CONTENT OF THE REPORT:
Includes information gained from responses to a Questionnaire on this topic sent out to the National Committees of CIGRE in 2013, and to all members and experts SCA1.
REPORT UTILITY:
Practical and theoretical aspects of operation in inductive and capacitive regimes.
Pinto Cajetan IN
Remi Tremblay CA
Wang Bo CN
Pu Jun
Juergen Weidner DE
Martin Stronge EI
Jun Matsumoto JP
Kenji Tanaka
Dan Zlatanovici RO
Cristinel Cicirone
Rodica Zlatanovici
Aalvik Geir NO
Oscar Martinez SP
Jim Edmonds US 3

4. 2 - GENERATOR CAPABILITY CHART (PQ DIAGRAM)4
2 - GENERATOR CAPABILITY CHART (PQ DIAGRAM)
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
2 - GENERATOR CAPABILITY CHART (PQ DIAGRAM)
The PQ active – reactive power diagram of the synchronous generators is
a synthetic diagram that delimits the admissible operational field of the generators, based on considerations related to the heating of the active parts and on the static stability considerations.
2.1. Theoretical PQ Diagram is valid for the rated values of the:
active power,
voltage at the generator terminals,
stator current,
field current,
frequency and
parameters of the cooling agent (temperature, pressure).
The modification of one of these parameters leads to the modification of the PQ diagram, by increasing or diminishing the admissible operational field. 4

5. In the Annex there are detailed: the significance of all curves 5
2 - GENERATOR CAPABILITY CHART (PQ DIAGRAM)
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
Theoretical PQ diagram for turbo-generators
Theoretical PQ diagram for hydro-generators
In the Annex there are detailed:
the significance of all curves
the equations of all significant curves
the mode of construction of the theoretical PQ Diagram 5

6. 2.2. The Actual PQ Diagram increased vibrations6
2 - GENERATOR CAPABILITY CHART (PQ DIAGRAM)
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
2.2. The Actual PQ Diagram
It is, in fact, a thermal diagram where the main theoretical curves represent
isotherms of maximum admissible temperatures of different active parts of
the Generator in over and under excited regimes.
But not only: other limitations, not necessarily related to the operation in over and under excited regimes, could be represented on the PQ Diagram,
due to (for example):
increased vibrations
overheating of the brushes, scintillation etc,
Actual PQ diagram with limitation
of the field current; in the shaded zones
of the diagram the generator can not operate 6

7. Other influences on the diagram limits can be added.7
2 - GENERATOR CAPABILITY CHART (PQ DIAGRAM)
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
2.3. The On-line PQ Diagram
It is the PQ diagram where all the defining parameters are directly acquired from the process and the current position of the operation point in each moment of time is shown.
This PQ diagram modifies its configuration in real time, according to the variation of the parameters from the process.
Other influences on the diagram limits can be added. 7

8. 2 - GENERATOR CAPABILITY CHART (PQ DIAGRAM) 8
2 - GENERATOR CAPABILITY CHART (PQ DIAGRAM)
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
On-line PQ diagram with rated condition (1) and three different limitations of current operation condition (2) - (4)
rated operation condition: voltage Un, frequency fn , hydrogen cooling gas temperature tn H2
reduced terminal voltage 0.95 Un
increased cooling gas temperature, 1.1 tn H2
4 increased stator cooling water temperature 1.1 tn water due to clogged hollow copper strands 8

9. low resultant magnetic field leads to a cold end zone.9
3 - ELECTROMAGNETIC PHENOMENA AT FRONTAL ZONE OF GENERATORS
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
3 - ELECTROMAGNETIC PHENOMENA AT FRONTAL ZONE OF GENERATORS
Axial magnetic field components at stator core of a generator in over excited regime:
low resultant magnetic field leads to a cold end zone. 9

10. high resultant magnetic field leads to a hot end zone.10
3 - ELECTROMAGNETIC PHENOMENA AT FRONTAL ZONE OF GENERATORS
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
Axial magnetic field components at stator core of a generator in under excited regime:
high resultant magnetic field leads to a hot end zone. 10

11. Axial magnetic field distribution at frontal stator core teeth:11
3 - ELECTROMAGNETIC PHENOMENA AT FRONTAL ZONE OF GENERATORS
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
Axial magnetic field distribution at frontal stator core teeth:
the highest magnetic edge field at opposite flank of tooth next to phase split. 11

12. due to eddy current losses at core laminations.12
3 - ELECTROMAGNETIC PHENOMENA AT FRONTAL ZONE OF GENERATORS
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
Resultant flux density on the upper tooth flat and distribution of the over temperature
due to eddy current losses at core laminations. 12

13. 4 - GENERATOR OPERATION IN OVER AND UNDER EXCITED REGIMES13
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
4 - GENERATOR OPERATION IN OVER AND UNDER EXCITED REGIMES
4.1. Characteristics of the generators used for voltage control
Turbo-generators
Hydro-generators
Nuclear
Wind generators
- Output powers > 5 MW
- Voltage: 1 – 24 kV ± 10% or 5%
4.2. Generators capability to operate continuously in over and under excited regimes
Type of generator
cos φ leading - cos φ lagging
Hydro generators
0.86 lead lag
Turbo generators in thermo power plants
0.85 lead – 0.85 lag
Turbo generators in nuclear power plants
0.95 lead lag
Wind generators
0.95 lead – 0.95 lag
4 - GENERATOR OPERATION IN OVER AND UNDER EXCITED REGIMES 13

14. 4.3. Use of generators for voltage support on the transmission system14
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
4.3. Use of generators for voltage support on the transmission system
The generators operate mainly with:
voltage set-point in the majority of the countries;
reactive power set-point in several countries;
power factor set-point.
Operation in over and under excited regimes has as consequence a greater heating of some
elements of the generator.
In comparison with operation at Q  0:
when operating at P = Pn in over excited regime with Q = Qmax lag, and
when operating at P = Pn in under excited regime with Q = Qmax lead
the stator winding, rotor winding and stator core heating increases by less than 10 deg.C
in the majority of the cases, but there are cases where the heating increases by more
than 15 deg.C.
4 - GENERATOR OPERATION IN OVER AND UNDER EXCITED REGIMES 14

15. 4.4. Long-term, on-line operation in over and under excited regimes 15
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
4.4. Long-term, on-line operation in over and under excited regimes
can cause damage to generators
During the long-term operation of a generator, both in over and in under excited regimes,
some undesirable phenomena can occur, such as:
Premature aging of the stator insulation
Premature aging of the rotor insulation
Occurrence of short-circuited turns in the rotor
Increased vibrations of the stator core
Increased vibrations of the rotor
Deterioration of the slip rings or brush system
Exciter diode fault
Heating on overhang of end bent winding.
In addition, in under excited regime other possible damages could appear:
Deterioration of the insulation between sheets in the frontal teeth
Deterioration of the insulation of the stator bars at the exit from slots
Excitation loss and asynchronous operation.
4 - GENERATOR OPERATION IN OVER AND UNDER EXCITED REGIMES 15

16. Example of Actual PQ Diagram with restrictive limits16
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
Example of Actual PQ Diagram with restrictive limits
Limitation of the field current in extreme over excited regime due to rotor vibrations and rotor winding overheating. Limitation of the reactive power in extreme under excited regime
due to the overheating of the frontal stator core teeth
1 - rated power of the turbine; 2 - stator current limit; 3 - field current limit; 4 - minimum field
current limit; 5 - thermal limit of the frontal stator core teeth; 6 - natural static stability limit with
10% reserve; 7 - minimum output power of the turbine; 8 - reduced field current; 9 - limit due
to rotor vibrations; 10 - reduced field current limit due to rotor overheating.
4 - GENERATOR OPERATION IN OVER AND UNDER EXCITED REGIMES 16

17. 4.5. Economic implications of over and under excited operation17
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
4.5. Economic implications of over and under excited operation
Additional costs for operating in over and under excited regimes consist of:
additional costs in operation;
additional costs for maintenance / repairs because of premature ageing of the generator
implications on the generator design and size and, consequently, on the cost.
4.6. Implications of over and under excited operation in generators design
In the frontal zone of the stator core the following measures are implemented:
frontal teeth with steps;
frontal teeth with slots;
slits in the sheets of the frontal teeth;
additional measures for the improvement of the frontal zone cooling.
4 - GENERATOR OPERATION IN OVER AND UNDER EXCITED REGIMES 17

18. 18
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
5 - CONCLUSIONS
All the power plants are required to participate in voltage control in the power grids where they are connected and, consequently, the generators operate in over excited regime (inductive, with generated reactive power), or under excited regime (capacitive, with absorption of reactive power).
In order to achieve the voltage control, the excitation system of the generator operates with a reactive power set-point, or with a voltage set-point, or with a power factor set-point.
The maximum amounts of produced / absorbed reactive power at different levels of the output power at the generator terminals is revealed by the Capability Chart of the generator, or so called PQ Diagram, comprising mainly the limits for: output power, stator current, field current, reactive power and static stability curve in PQ coordinates.
The PQ Diagram can be determined and used in three modes: the theoretical PQ Diagram determined by computation in the generator design phase, the actual PQ Diagram determined experimentally and reflecting the actual state of the generator and the On-line PQ Diagram calculated continuously with the actual values acquired directly from process data.
5 - CONCLUSIONS 18

19. 19
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
Operation in an inductive regime is conditioned by the field current limit and the stator current limit determined from the condition of notsurpas-sing the admissible temperatures.
Operation in a capacitive regime is conditioned by the supplementary electrical losses in the frontal zone of the stator, the stator teeth specifically. The capacitive limit is also determined by the natural static stability curve (the dP/d criterion) of the generator, and by the limit of the minimum field current.
Operation in over and under excited regimes has as consequence a greater heating of some elements of the generator in comparison with the operation with Q  0: stator and rotor windings overheating, end zone of the stator core overheating, etc.
During the long-term operation of a generator, both in over and in under excited regimes, some undesirable phenomena can occur, such as: premature aging of the stator and rotor insulation, occurrence of short-circuited turns in the rotor, increased vibrations of the rotor and stator core.
5 - CONCLUSIONS 19

20. excitation loss and asynchronous operation. 20
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
In addition, in under excited regime other possible damages could appear such as:
deterioration of the insulation between sheets because of excessive heating of the frontal teeth,
deterioration of the insulation of the stator bars at the exit from slots because of excessive heating of the frontal teeth,
excitation loss and asynchronous operation.
The monitoring of the generators operating in over and under excited regimes is achieved by means of the already classical devices or equipment, used with less or more extent.
The economic implications of the operation in under and over excited regimes are expressed only qualitatively.
5 - CONCLUSIONS 20

21. THANK YOU Rodica Zlatanovici21
WG A1.38 GUIDE FOR GENERATOR ON-LINE OVER AND UNDER
EXCITATION OPERATION ISSUES
SCA1 – ROTATING ELECTRICAL MACHINES
TUTORIAL
THANK YOU
Rodica Zlatanovici
WG A1.38 Convener 21