Emergency generator startup study of a hydro turbine unit for a nuclear generation facility.pdf

IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 40, NO. 5, SEPTEMBER/OCTOBER 2004

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Emergency Generator Startup Study of a Hydro

Turbine Unit for a Nuclear Generation Facility

J. J. Dai , Member, IEEE , Di Xiao, Farrokh Shokooh , Fellow, IEEE , Christopher Schaeffer, and

Aldean Benge , Member, IEEE

Abstract— This paper reports the implementation of syn-

chronous generator, induction machine, hydro turbine, and

governor system, and excitation and automatic voltage regulator

system models for transient stability study. These models are

frequency dependent and are suitable for system transient studies

involving drastic frequency changes, including generator startup

and emergency load startup. A computer simulation program

has been developed using these models for a transient stability

study. The developed program is further validated and veriﬁed

using real system testing data that includes the cases of generator

startup and full-load shed in a nuclear power generation plant.

Validation results show overall an excellent correlation between

the computer simulation and the ﬁeld-testing data. As a result, the

program has been accepted by the plant for system modeling and

emergency generator startup simulation studies.

Index Terms— Frequency-dependent synchronous machine, gen-

erator startup, hydro power generation, induction machine and

network models, start emergency load, transient stability study.

ysis program has been developed to handle the conditions in

which the system undergoes considerable frequency variations

during the transient. This program includes: 1) implementation

of IEEE-type synchronous generators with damping windings

on both direct and quadratic axes, consideration of the satura-

tion effect to generator parameters and the ability to count for

frequency variation ranges from zero to a signiﬁcant overshoot;

2) implementation of frequency dependent induction machines

with different makes and conﬁgurations; and 3) implementation

of a power system’s frequency dependent network model. These

models are suitable for power system transient stability studies

in cases with and without signiﬁcant frequency changes.

The newly developed program is validated and veriﬁed by

ﬁeld-testing measurements from a real system. System conﬁgu-

ration and operating conditions are recreated including manual

and relay controlled switching actions and other time events.

Synchronous generator, induction motor, bus real power and

reactive power loadings, and some other key system dynamic

responses are plotted and compared to the actual ﬁeld measure-

ments to assess the validity of the models. An excellent correla-

tion is found between the simulation results and those from the

ﬁeld measurements. As a conclusion, the developed transient

stability analysis program is accepted by a nuclear generation

plant for emergency generator startup studies. The program can

be used to simulate any power system transients with or without

drastic frequency variation while producing reliable and satis-

factory results.

may drastically vary from its nominal value. One example

of these situations is the generator startup. In such a case, a syn-

chronous generator is started from a cold standby condition with

both the initial frequency and the voltage set at zero. Another

example in this category is the prolonged or sustained subsyn-

chronous oscillation in a large power system consisting of mul-

tiple synchronous generators. Under this condition, frequency

across the system is distributed corresponding to the locations

of different coherent synchronous generator groups in an uneven

fashion. Due to the lack of frequency dependent machines and

system models, most commercially available computer simula-

tion programs, for power system transient stability studies, fail

to produce reliable or even meaningful analytical results when

applied to the aforementioned situations.

To correctly model frequency and voltage characteristics of

power system components, a special transient stability anal-

II. T ESTING S YSTEM D ESCRIPTION

The test system is a nuclear generation plant and its emer-

gency power backup. A simpliﬁed system one-line diagram is

shown in Fig. 1.

Loads connected to 4-kV buses and the subsystems down

below are normally powered up by the grid through two 4-kV

buses, 4 kV1 and 4 kV2. These loads represent the emergency

load in the nuclear generation plant.

Generator KGEN2 located in the nearby hydro generation

station is in a cold standby condition and two circuit breakers,

1 T-7 and 2 T-7, are normally open. When there is power loss

from the grid, an emergency startup signal is received at KGEN

2 starting it immediately and two 4 kV standby buses, 1 T and

4 kV 2 T, are then energized. By setting the voltage relay con-

nected to the generator 13.2 kV terminal bus, 1 T-7 and 2 T-7

will be closed at the appropriate time to energize the emergency

loads in the plant.

Generator KGEN 2 is a hydro unit. A schematic drawing of

the studied system is shown in Fig. 2.

Paper ICPSD-ICPS-02-08, presented at the 2002 IEEE/IAS Industrial

and Commercial Power Systems Technical Conference, Savannah, GA, May

5–8, and approved for publication in the IEEE T RANSACTIONS ON I NDUSTRY

A PPLICATIONS by the Power Systems Engineering Committee of the IEEE

Industry Applications Society. Manuscript submitted for review May 15, 2002

and released for publication June 17, 2004.

J. J. Dai, D. Xiao, and F. Shokooh are with Operation Technology, Inc., Irvine,

CA 92618 USA (e-mail: JJ@etap.com; di@etap.com; Farrokh@etap.com).

C. Schaeffer is with Duke Power-Fossil Hydro Support Services, Charlotte,

NC 28214-1836 USA (e-mail: ceschaef@duke-energy.com).

A. Benge is with PLC Engineering, Greensboro, NC 27404-1108 USA

(e-mail: abenge@carolina.rr.com).

Digital Object Identiﬁer 10.1109/TIA.2004.834035

I. I NTRODUCTION

U NDER certain situations, frequency in a power system

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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 40, NO. 5, SEPTEMBER/OCTOBER 2004

Fig. 1. Simpliﬁed system one-line diagram.

Fig. 2. Schematic drawing of the hydro generation station and its load.

This system includes a synchronous generator, a hydro turbine

with a water tunnel, penstock and speed control governor, an ex-

citation system with automatic voltage regulator (AVR), trans-

mission line(s), and emergency loads. Emergency loads consist

of both induction motors and some static load. Dynamic models

have been developed to simulate each of these components for use

in the transient stability study. Due to the fact that the system fre-

quency during the generator startup process varies within a range

from 0 to as high as 120%–130% of the nominal value, all the

models must have the capability to account for this frequency vari-

ation. A brief description, of the frequency-dependent models of

these aforementioned components, is given in the next section.

DAI et al. : EMERGENCY GENERATOR STARTUP STUDY OF A HYDRO TURBINE UNIT FOR A NUCLEAR GENERATION FACILITY

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Fig. 3. Synchronous generator equivalent circuit model. (a) Generator direct-axis equivalent circuit. (b) Generator quadratic-axis equivalent circuit.

III. S YNCHRONOUS G ENERATOR ,T URBINE ,E XCITER ,

A SSOCIATED C ONTROL S YSTEMS , AND I NDUCTION

M ACHINE M ODELING

This section describes the synchronous machine, the hydro

turbine and governor system, the excitation and AVR system,

and the induction machine models used in the generator startup

study. All the models are frequency dependent.

2) dc ﬂashing circuit that is used to supply excitation voltage

to the ﬁeld winding when the generator is initially started;

a V/Hz (voltage per Hz) control circuit will switch this

from manual mode to automatic mode (connecting the

ﬁeld winding directly to the excitation and AVR system)

based on a presetting (usually 0.7–0.8 per unit); the ﬁeld

ﬂashing circuit is modeled by an RL circuit with a dc

source given in [7];

3) V/Hz limit and protection unit which reads both the gen-

erator terminal voltage and the frequency to calculate the

generator V/Hz value and, if it is too high, sends a signal

to a low value selector that tends to reduce the excitation

voltage to the ﬁeld winding;

4) load compensator that senses both the generator terminal

voltage and current to regulate the generator output

voltage to compensate for load changes;

5) stabilizing feedback loop, which based on the generator

ﬁeld winding current, to produce the necessary stabilizing

signal added to the exciter.

A. Synchronous Generator Modeling

Based on the manufacturer data sheet, the IEEE 2.1 syn-

chronous generator model is used. This model is given and

described in [4]. An equivalent circuit diagram of the model is

shown in Fig. 3.

This model does not include the frequency in its model pa-

rameters and thus allows independent consideration of changes

in frequency. Generator saturation characteristics are also ac-

commodated by adjusting model parameters.

B. Turbine and Governor System Modeling

The turbine/governor system includes water dynamics in the

water tunnel, penstock, hydro turbine, and speed governor con-

trol. As shown in Fig. 2, when the generator is started, water

from the reservoir is released ﬂowing through a single head gate

into a common power tunnel, splitting into multiple ﬂows. Each

passes a wicket gate, then a penstock, and ﬁnally enters the tur-

bine runner of the generator unit, providing mechanical power to

the generator. The wicket gate is controlled by a mechanical–hy-

draulic governor, which opens and closes the gate according to

the preprogrammed control functions as described in [7]. The

schematic diagram of the power tunnel, penstock, wicket gate,

and governor control system is illustrated in Fig. 4.

D. Induction Machine Modeling

The emergency loads are started at approximately 70%–90%

of the system rated frequency output. At this time, all induction

motors among the emergency loads are to be accelerated, or in

another words, those motors are started in an under-frequency

condition. In addition, during the motor acceleration, an over-

frequency condition can occur due to the generator’s response.

Thus, appropriate induction motor model must be employed

to study the motor starting transient for both under and over

frequency conditions. An equivalent circuit model for a double-

cage induction machine is illustrated in Fig. 6. The model

parameters used in this study are based on the estimated results

given in [7]. Essentially, in this model, the parameters are

independent of the system frequency. This concept can be

used to develop other types of induction machine frequency

dependent models. These machine models should be used in

the studies where the system frequency has large variation in

the magnitude such as the generator startup, large load shed,

or addition.

Due to the existence of the over frequency condition, machine

load models need to extend to cover the over-speed characteris-

tics, which are also correctly modeled in the program.

C. Excitation and AVR System Modeling

The excitation system of the generator is a static type and is

equipped with an AVR. The transfer function block diagram of

this excitation and AVR system is shown in Fig. 5.

This excitation and AVR system includes ﬁve main functional

blocks:

1) main loop consisting of a Rectiﬁer and an Ampliﬁer,

which, according to the voltage error signal, generates

a corrective excitation voltage and applies it to the ﬁeld

winding;

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IEEE TRANSACTIONS ON INDUSTRY APPLICATIONS, VOL. 40, NO. 5, SEPTEMBER/OCTOBER 2004

Fig. 4. Hydro turbine and governor system model diagram. (a) Speed governor and gate control system. (b) Hydro turbine and water tunnel system.

DAI et al. : EMERGENCY GENERATOR STARTUP STUDY OF A HYDRO TURBINE UNIT FOR A NUCLEAR GENERATION FACILITY

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Fig. 5. Transfer function block diagram of the excitation and AVR system.

Fig. 6. Frequency-dependent equivalent circuit model for double-cage

induction machine.

time reference 0 second by opening the water gate and applying

dc ﬂashing voltage to the generator ﬁeld winding. Water ﬂows

through the power tunnel, the penstock, and the gate, driving

the turbine to rotate the generator. At the same time, the battery

powered dc supply ﬂashes the generator ﬁeld winding, inducing

the initial rotor ﬂux inside the generator air gap and the stator

windings, resulting in a voltage at the generator terminal. As

both generator speed and terminal voltage gradually build up

and reach to certain levels, the dc ﬂashing voltage will be dis-

connected, transferring the excitation source to the exciter and

AVR system named “automatic mode,” Once the generator ter-

minal voltage reaches the pre-speciﬁed value based on the gen-

erator rated output voltage, the voltage relay mounted on the

generator terminal bus 13.2 kV closes the circuit breakers 1 T-7

and 2 T-7, thus connecting the emergency load to the generator.

Two test conditions are simulated.

• Test A (T2) The emergency loads are started at 76% of the

generator rated output voltage.

• Test B (T1) The emergency loads are started at 94% of the

generator rated output voltage.

Figs. 7–9 show generator frequency, voltage, and electrical

power responses from Test A. An excellent correlation between

the simulation and test results is clearly displayed in the ﬁgures,

indicating that the synchronous generator model, the hydro tur-

bine and governor model, and the excitation and AVR model in

IV. M ODEL V ALIDATION AND V ERIFICATIONS

Models described in the above section are all validated and

veriﬁed by comparing the simulation results of the program

with the actual ﬁeld measurements. Two ﬁeld tests are selected.

System conﬁgurations and testing conditions of these two tests

are described in this section, with the comparison of the pro-

gram simulation results and the ﬁeld measurements.

A. Generator Startup Test

This test is an actual generator startup for the system de-

scribed in Fig. 1. In this test, hydro unit KGEN2 is started at

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