Operation of an Auxiliary Electrical System

of Power Plant in Emergency Conditions

of Power System

J. Buchta, M. Pawlik and R. Szubert

Abstract--The paper presents analysis of operational conditions of

auxiliary electrical system of power plant during load shedding

by generation unit. Load shedding of power unit refers to the

situation in which generation unit is disconnected from a grid

when frequency falls below preset level and power unit safe-

guards auxiliary service supply in order to be able to restore

power system after its blackout. Detailed analysis of auxiliary

power system operation is advisable for operational reasons. The

paper presents results of simulation performed for 360 MW

power unit during load shedding. Matlab-Simulink environment

has been applied to prepare simulation.

Index Terms— load shedding, modeling, power generation auxil-

iary systems, power plants, rotating machine transient analysis.

I. INTRODUCTION

Electric load shedding is implemented in power system grid

when load demand is higher than electrical supply. When the

frequency falls below preset level, a predetermined amount of

load is removed to restore frequency. This problem is widely

represented in bibliography both in theoretical approach and

dynamical simulations [1, 2]. Load shedding of power unit

refers to the situation in which generation unit is disconnected

from a grid when frequency falls below preset level and power

unit safeguards auxiliary service supply in order to be able to

restore power system [3]. That problem isn’t represented as

widely in bibliography. Most of papers are focused on ex-

perimental test, which were done in power plants [4, 5]. To the

author’s knowledge, there is no papers referring to the dy-

namical simulation of such a states. This paper is an attempt to

build a mathematical model and perform simulations focused

on operation of auxiliary electrical system of power unit dur-

ing load shedding.

Problems associated with the control of load shedding of

power units to the load required by their auxiliary systems are

not new in Polish energy sector. This issue has been discussed

This work was supported by the Polish Ministry of Science and Higher

Education under Grant PBZ-MNiEN-4/2/2006

J. Buchta, M. Pawlik, and R. Szubert are with the Faculty of Electrical,

Electronic, Computer and Control Engineering, Technical University of Lodz, Poland, 90-924 Lodz, 18/22 Stefanowskiego Street, (e-mails:

janusz.buchta@p.lodz.pl; maciej.pawlik@p.lodz.pl; rafszubert@wp.pl).

for more than 40 years. During the period, when power units

with a capacity 200 MW were installed in Polish power sys-

tem, the issue became paramount, but with entirely different

reasons than today. In the 60s, no one thought about the inter-

connection of Polish power system with transmission systems

of western Europe and the guarantee the strict UCTE stan-

dards. Power units installed in that period were equipped with

various, often unnecessary protections, and that were based on

unreliable systems. This was the main cause of frequent power

units shutdowns. Then the idea arose, not to shut off the whole

unit but only that part which was in danger of damage. In

particular, the steam boiler shouldn’t be shut off when it was

able to operate, as well as turbine, because this meant the

long-term start-up of power unit and significant costs incur-

rence. Serious failures, resulting in turbine damages, as well as

steam boiler or synchronous generator damages occurred after

power unit disconnection from transmission system and at-

tempt of its reconnection. For this reason, a selective protec-

tion system originates, which makes possible power unit op-

eration after load shedding in auxiliary service supply proce-

dure. Currently, the ability of the generation unit transition

after a successful load shedding to auxiliary system supply

without connection with a power system is one of the services

required by transmission system operator expressed in Trans-

mission System Operation and Maintenance Instruction [6].

According to UCTE Operation Handbook, power stations in

emergency conditions in power system automatically discon-

nect at 47.5 Hz, and shall safeguard auxiliary service supply.

Therefore, power plant auxiliary service supply operation

mode is the part of power system restoration strategy. Several

of national power plants are capable to provide this service

[4, 5].

II. OPERATION OF POWER PLANT DURING LOAD SHEDDING

AND AUXILIARY SERVICE SUPPLY

Power system failures can be understood very widely, rang-

ing from relatively not serious disturbance such as shutdown

of a single power unit to the total disintegration of power sys-

tem. Power plants should be prepared for the complete outage

of a grid called blackout. In case of an outage, power plant

restarts become the most important task. Among the factors

that influence on fast restoration of power generation one can

2009 IEEE Electrical Power & Energy Conference 978-1-4244-4509-7/09/$25.00 ©2009 IEEE

indicate the ability to keep as much turbine sets as possible in

auxiliary service supply or in island operation.

The rapid load shedding of a power unit from its nominal

load to the load of its auxiliaries is a difficult operation and

not always successfully finished. In order to ensure a success-

ful load shedding, a number of conditions needs to be met,

ranging from an increase of turbine speed to be stopped by

speed governor and correct work of damping station (steam

by-pass of a turbine), following by mazout burners ignition in

order to stabilize the flame after reducing an output of steam

boiler, and ending with the performance of many systems that

ensure stable operation of the unit supplying only auxiliary

system for several hours.

Power plant auxiliary service supply is particularly interest-

ing operational mode because it concerns work of a power unit

in a small and relatively weak, isolated power system, which

consist of auxiliary drives. This is extremely important from

the point of view of the fast restoration of the system in emer-

gency conditions. In order to continue auxiliary service supply

of power unit for a long period, the steam boiler must have a

load much higher, about 40% of its nominal load. However,

6% of power unit rating is enough to supply its auxiliaries,

especially that energy consumption by auxiliary drives is re-

duced, and some of mills are shut off. The steam boiler must

operate with higher load in order to maintain its thermal per-

formance in normal range and ensure its automatic control.

The excess of steam produced in boiler must be piped away by

the damping station into condenser. Power units which are

able to perform auxiliary service supply use high-speed damp-

ing stations that need 3-4 seconds to open and which are

adapted to boiler capacity. This creates a good conditions to

deal with load shedding, because the whole process may take

place at constant steam pressure, which helps to keep water

level in the separator and steam temperature. In power units,

which are not equipped with rapid damping stations, load

shedding often causes opening of safety-valves on the steam

boiler.

A separated grid forms small and weak power system,

which sets specific requirements to power unit. It is very diffi-

cult to balance such a system, because even a small imbalance

in energy generation and consumption leads to large fluctua-

tions in voltage frequency. Large frequency deviation of sev-

eral Hz, may cause total closing of turbine valves.

III. MODEL OF POWER UNIT FOR LOAD SHEDDING ANALYSIS

The block diagram of large power unit is shown in Fig .1.

The diagram includes thermal installation: boiler (B) and

steam turbine (T). An electrical system consist of synchronous

generator (G), main transformer (MTr), excitation transformer

(ETr), excitation system (E), unit transformer (UTr), auxiliary

electrical system (AES) supplying induction motors (M). The

diagram also includes speed governor (SG) and voltage regu-

lator (VR). Fig. 1 shows a conventional layout of auxiliary

electrical system. The generator supplies main and unit trans-

formers. The auxiliary transformer takes power directly from

the generator terminals to supply unit auxiliaries. The auxil-

iary board is sectioned, so that it’s an electrical extension of

double-track system of steam boiler and turbine. The auxiliary

power system supplies the equipment on which the generation

of electricity is directly dependent, i.e. mills, fans, pumps, etc.

Auxiliaries are generally driven by MV induction motors for

which the capacity of single motor is often greater than 1 MW.

Typical auxiliary power system, applied in Polish power

plants rated at 120 MWe to 500 MWe, covers the 6 kV and 0.4

kV ac level. The voltage level is related with the capacity of

the largest induction motor. For Polish power plants, the maxi-

mum capacity of a single motor has been amounted to

6.3 MW. Therefore the voltage level of 6 kV has been suitable

for former Polish power plants.

Model of a power unit for an analysis of load shedding was

created in the MATLAB-Simulink. Model contains the fol-

lowing elements:

• model of an auxiliary electrical system supplying induction

motors, which takes into account non-linear characteristics

of load torque of auxiliary drives,

• model of electrical energy output system

• model of steam turbine with synchronous generator,

• models of turbo set speed governor and voltage regulator.

E

P gs

U gs I g

U g

P g

ω s

α

ω

ETr

MTr

AES

UTr

VR SG

T

B

G M M M M

Board I Board II

Fig.1. Block diagram of large steam power unit

Simulink built-in models of synchronous generator, induc-

tion motor and transformer have been implemented. In case of

steam turbine, voltage regulator and speed governor standard

models have been used [7-9]. Type ST1A excitation system

model have been used and transmittance model of steam tur-

bine. Detailed schemes and equations are presented in bibliog-

raphy [7-9]. Block diagram of the simulation model prepared

in Simulink to analyze an operation of auxiliary system during

power unit load shedding is presented in Fig. 2. The scheme

includes blocks representing steam turbine, synchronous gen-

erator, governors, main transformer, power system, breaker

and auxiliary system of power station. Additionally there are

the standard blocks used for measurement in Simulink envi-

ronment. Fig. 3 shows detailed scheme of the block labeled

Auxiliary System. One can recognize two auxiliary boards

from which respective induction motors are supplied. Model-

ing of load torque of auxiliary drives is an additional problem,

particularly important in simulation of transients. The figure

explains the way in which load torque is modeled as a quad-

ratic function of motor rotational speed.

IV. SIMULATION RESEARCH OF 360 MW POWER UNIT LOAD

SHEDDING TO AUXILIARY SERVICE SUPPLY

The model has been applied to simulate transients during

load shedding for 360 MW power unit and following this

transition to auxiliary service supply.

Fig.2. Block diagram of the Simulink model

Fig.3. Detailed scheme of the block “Auxiliary System” presented in Fig.2 (abbreviations in compliance with Table 1).

TABLE 1

BASIC DATA OF 6 KV AUXILIARY DRIVES OF POWER PLANT

Number of motors (n) and total capacities of drives

Installed motors Motors operating

in normal power plant state

Motor power

rating

Board I Board II Board I Board II

Drive

kW n kW n kW n kW n kW

Mill 1500 4 6000 4 6000 3 4500 3 4500

FD fan (FDF) 1800 1 1800 1 1800 1 1800 1 1800

ID fan (IDF) 3150 1 3150 1 3150 1 3150 1 3150

Feedwater pump (FWP) 6300 1 6300 - - - - - -

Condensate pump (CNP) 1000 1 1000 1 1000 1 1000 - -

Cooling water pump (CWP) 2000 1 2000 1 2000 1 2000 1 2000

Circulating pump (CP) 780 1 780 1 780 - - 1 780

Basic data of main auxiliary drives of a power unit under

investigation are presented in Table 1.The largest induction

motor driving feedwater pump is connected to the board I.

Simulations of power unit load shedding have been per-

formed. The results of simulations are presented in figures

containing plots of several characteristic quantities. The open-

ing of the breaker that initiates load shedding occurred at

t = 10s. The start-up of feedwater pump has been initiated at

t = 12s when induction motor rated at 6,300 kW has been

supplied from auxiliary board I. Results of the simulation are

shown in Fig. 4-13. Following the breaker opening in power

unit output system a sudden imbalance occurs between turbine

torque and generator load. This causes a rapid increase in

speed of turbine. There is also an increase of voltage at gen-

erator terminals and auxiliary electrical system busbars due to

a decrease in the generator load current. As a result of turbine

speed increase, the frequency of voltage also rises, as well as

rotational speed of motors supplied by auxiliary system. This

results in increase of motor load torque, motor currents and

energy consumption.

Turbine speed governor shuts valves and reduces the steam

flow to the turbine. The increase of turbine speed is stopped in

consequence. The rise of motor speeds causes the increase of

motor currents, which are set in at slightly higher level than

before disturbance.

Transition of the power unit to auxiliary service supply af-

ter load shedding causes the start-up of motor driving feedwa-

ter pump. Plots presented in Fig.8-11 show, that at the moment

of load shedding, motors supplied from auxiliary electrical

system pass through transient state associated with the change

of motors load and conditions of power supply. There is an

increase of voltage level at busbars and current consumed by

motors, that cause an increase of electromagnetic torque.

Start-up motor driving feedwater pump, which occurred 2s

after load shedding caused decrease of voltage at auxiliary

system busbars, that is shown in Fig. 10. The rotational speed

of the turbine set after load shedding amounts to 104.4% of

the nominal speed in steady state. Maximum speed of the

turbine does not exceed 10% of the speed rating. Turbine set

load during auxiliary service supply amounts to 25 MW,

which represents approximately 7% of generator capacity.

Comparing Fig. 9 with Fig. 11 one can state that distur-

bances in auxiliary drives operation are more intense for

board I from which the starting motor is supplied.

Auxiliary system stabilizes after about 6 s, when the feed-

water pump start-up ends and the motor current is fixed. The

biggest disturbance in auxiliary system operation is taking

place within several seconds after load shedding, when sudden

frequency change is observed as well as voltage rises at gen-

erator terminals, and following the start-up of the motor driv-

ing feedwater pump. The simulation refers only to the initial

state of power plant auxiliary service supply, and some sys-

tems have been ignored in the model. Control systems of

steam boiler have been neglected. Steam boiler is an object

with large thermal inertia and change of its output at the initial

period of auxiliary service supply can be omitted. An assump-

tion has been made that after load shedding, the damping sta-

tion opens rapidly and the excess of steam generated by boiler

flows through the station.

Fig.4. Mechanical power of a steam turbine

Fig.5. Electrical power of a generator

Fig.6. Rotational speed of a steam turbine

Fig.7. Voltage at generator terminals

Fig.8. Voltage at board II busbars

Fig.9. Total current of motors connected to board II busbars

Fig.10. Voltage at board I busbars

Fig.11. Total current of motors connected to board I busbars

Fig. 12. Rotational speed of the motor driving feedwater pump

Fig. 13. Rotational speed of the motor driving cooling water pump (board I)

V. CONCLUSIONS

The capability of load shedding by power unit enables

power system restoration in case of blackout. Power plant

operates using its output to power its own auxiliaries till the

moment of resynchronization. This capability of power plant

is regarded as a service and may be an additional source of

income for power plant.

Computer simulation can’t exclude experimental tests

checking generation unit availability for load shedding and

auxiliary service supply. However, results of computer simula-

tion reliably reproduce the dynamics of the unit, therefore can

be complementary for the experimental tests performed at

power plant. Simulation research may reduce the number of

such a tests for the expenses necessary to carry out experi-

ments.

REFERENCES

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[2] N. Perumal, Aliza Che Amran, “Automatic Load Shedding in Power

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gi, Malaysia, pp. 211-216.

[3] Joint Working Group 39/11. Exchange of services between large electric-

ity generating plants and high voltage electric power systems. CIGRE,

1999. 138.

[4] W. Bekasiak and E. Mazurkiewicz, „Wykonane w Elektrowni Bełchatów

SA prace do obrony i restytucji elektrowni w stanach katastrofalnych Kra-

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[8] IEEE Recommended Practice for Excitation System Models for Power

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BIOGRAPHIES

Janusz Buchta (non IEEE member) graduated from

the Technical University of Lodz, Poland. There he

received the Ph.D. degree in electrical power engi-

neering. Presently he is a senior lecturer at the Insti-

tute of Electrical Power Engineering, TU of Lodz.

His area of interest includes modeling and simulation

of transients in power plant auxiliaries.

Maciej Pawlik (non IEEE member) received Ph.D.

and D.Sc. degrees from the Technical University of

Lodz. At present he is Professor in TU of Lodz and

Head of the Institute of Electrical Power Engineer-

ing. He is a member of Committee of Power Engi-

neering Problems of the Polish Academy of Science

and member of the Council for Science at Ministry

of Science and Higher Education. His field of inter-

est includes electricity generation technologies.

Rafał Szubert (non IEEE member) graduated from

the Technical University of Lodz with M.Sc. degree.

Presently he is a Ph.D. student at the Institute of

Electrical Power Engineering of TU of Lodz. His

research interests lie in the area of power system

modeling and simulation.