Research on the Influence Factors and Coordinated Control Strategies between Unit and Grid for Isolated Power System

doi:10.4236/epe.2013.54B086

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Energy and Power Engineering, 2013, 5, 448-453

doi:10.4236/epe.2013.54B086 Published Online July 2013 (http://www.scirp.org/journal/epe)

Research on the Influence Factors and Coordinated

Control Strategies between Unit and Grid for Isolated

Power System

Ge Jin1, Xiaomei Chen2, Yongxin Feng1, Shaoxiang Deng1, Hanting Yan2, Ze x iang Cai2

1Electric Power Research Institute, Guangdong Power Grid, Guangzhou, China

2School of Electric Power, South China University of Technology, Guangzhou, China

Email: [email protected]

Received March, 2013

ABSTRACT

As the existing coordinated control strategies between grid and unit have limitations in isolated power system, this pa-

per introduces new coordinated control strategies which can improve the stability of isolated system operation. This

paper analyzes the power grid side and unit side influence factors on the isolated power system. The dynamic models

which are suitable for islanding operation are applied to simulate and analyze the stability and dynamic characteristics

of the isolated power system under the conditions of different load disturbances and governor parameters. With consid-

ering the differences of frequency characteristics between the interconnected and isolated power system, the adjusting

and optimization methods of under frequency load shedding are proposed to meet the frequency stability requirements

simultaneously in the two cases. Not only proper control strategies of the power plant but the settings of their parame-

ters are suggested to improve the operation stability of the isolated power system. To confirm the correctness and effec-

tiveness of the method mentioned above, the isolated system operation test was conducted under the real power system

condition, and the results show that the proposed coordinated control strategies can greatly improve stability of the iso-

lated power system.

Keywords: Isolated Power System; Coordinated Control Strategies; Under Frequency Load Shedding; Dynamic

Frequency Characteristics; Speed Governor Parameter Settings

1. Introduction

With the cross-regional interconnection of power system

and the continuous expansion of power grid, the connec-

tions between systems at all levels are becoming closer

and closer. And the single fault of power system, extreme

weather or manual operation errors may possibly cause

large area blackout accidents, and even cause the crash of

power system. After that blackout, the power system is

forced to take black start which is a self-help means after

the power system accident and produces huge economic

loss. However, when the large-scale power system fault

occurs, we can take reservations and reasonable meas-

ures for the local power system, and keep isolated system

operation stable. This can not only ensure the power sup-

ply for important load of the city, but also be helpful to

restoration of the whole system which can reduce the

economic loss.

In recent years, many countries around the world have

occurred many largely influenced blackout accidents,

such as the America and Canada blackout accident in

2003. In these accidents, a large number of regional grids

which have the isolated system operation condition fail

to make a full use of the coordinated control ability due

to lack of effective coordinated control measures for the

isolated system operation. And this leads to the unit trip-

ping and the power lose of those areas [1-3].

There are some problems in research of possible risks

grasp and control strategies for practical isolated power

system operation. Firstly the characteristics and relevant

coordinated control strategies of most power plants have

not been considered for isolated system operation [4-7].

Then the stable operation of the power plant depends

more on the major power system than the isolated power

system as the effective power support.

With these problems, this paper analyzes the influence

factors on the isolated system. Dynamic models which

are suitable for the isolated power system are used to

analyze the stability of frequency response under the

different conditions such as load disturbances, speed

governor parameters and logical control conditions. Op-

timization method of the under frequency load shedding

is proposed to meet the frequency stability requirements

G. JIN ET AL. 449

in interconnected and isolated system operation with

considering the differences of frequency characteristics

in the two cases[8-10]. The parameter settings and con-

trol strategies are proposed to improve the isolated op-

erational ability[11-16].

2. Frequency Characteristics of Isolated

Power System and Its Influence Factors

2.1. Frequency Characteristics

For most heavy load in regional power system, the sys-

tem response to the isolated network operation is basi-

cally a consistent frequency transient process which is

often accompanied with voltage changing highly or low-

ly.

When the generated power is shortage in isolated net-

work, the initial transient process is determined by the

response of power plants spinning reserves and low-fre-

quency low-voltage load shedding, often within seconds

reaching the lowest frequency. After that point the sys-

tem frequency response depends on the characteristics of

prime mover and speed governor. With the surplus power

in isolated system, the frequency will rise. Then the

speed governor will work and give a response to reduce

the mechanical power produced by the unit. The ability

to maintain stability instead of losing load of the isolated

network actually depends on the partly load shedding

ability of the unit.

2.2. Influences of the Power Grid

The influence of power system primarily performances

on the operation mode that is set. It is necessary for iso-

lated power grid. So when the operation mode of the

power system is made, all of the possible running situa-

tion that may occur in the isolated power system should

be considered. The automation equipments of power

system including the low-frequency low-voltage load

shedding, and low frequency disconnection device as the

last defense line of the power system are important in

affecting the operation mode of the power system. With

them, the collapse of frequency and voltage can be pre-

vented in the accident and the partly power supply can be

guaranteed. But if automation equipment such as the un-

der frequency load shedding fails to take consideration

on the frequency characteristics both of interconnected

system and isolated system, it will cause frequency drop-

ping dramatically during isolated network runtime, and

the power system will collapse in the end.

2.3. Influences of the Unit

The steam turbine of each unit under different speed de-

viation has different cultivate times in total. It shows the

lifetime of the unit. The low frequency operation of the

steam turbine is more dangerous than the high one. Be-

cause after the frequency rises due to load shedding, it

can be reduced by the governor or OPC, the overspeed

protection control system, to regulate output power.

Figure 1 shows the lifetime of steam turbine under

different frequency deviation. For power plant A, the two

horizontal lines indicate that there is no loss and the tur-

bine can run continuously between the frequency of 49.4

Hz and 50.6 Hz. When the frequency drops to 46.5 Hz,

the turbine can only run for one second, as the lifetime

curve shows at the time of frequency offset. The power

plant B has a lower frequency than 50Hz, because of

quickly action of its frequency governor. When the fre-

quency is lower than 50Hz, the life time of the turbine

will stepped reduce.

When isolated network is forming, some fixed values

of the protection devices may affect the stability of iso-

lated system operation, while current protection of the

unit and its fixed values are setting mainly based on in-

terconnected network operation. So it is necessary to set

reasonable values for the generator protection, which will

not only ensure safe operation of the unit, but also pre-

vent the unstable operation or trip of the units. The fol-

lowing generator protection equipments should be con-

sidered during the isolated system operation: electrical

overspeed trip, mechanical overspeed trip, turbine over-

speed protection control (OPC), low and over voltage

protection of generator, and auxiliary protection settings.

3. Dynamic Models of Isolated Power

System

The influence of the unbalanced power on dynamic fre-

quency characteristic of the power system at the moment

of isolated system formation can be analyzed by the

models in isolated network. Considering the unit in city

grid mainly uses the turbine generator, especially the

reheat type as shown in Figure 2. The domain time con-

stant of this turbine model is the thermal time constant

TR. The constant Km is the total gain to realize the ad-

justment of output power [8].

Figure 1. Turbine lifetime with different frequency offsets

G. JIN ET AL.

450

Typical speed governor model is shown in Figur e 3.

The most important parameter of speed governor is the

differential coefficient. It is marked with R in the feed-

back element of the above figure. And a simplified mod-

el of power system is established based on including its

important dynamic characteristics. By integrating reheat

steam turbine model with speed governor model we get

the reduced order system frequency model, as shown in

the Figure 4.

There are six constants to describe the characteristics

of this model: gain coefficient Km, damping coefficient D,

inertia constant H, thermal time constant TR, power pro-

portion of high pressure cylinder FH, and differential

coefficient R.

The incremental power setting of governor and output

electromagnetic power of generator are the two input

variables of the model. Governor incremental power

value is adjusted according to the unit instruction. The

output setting value needs to meet requirement of the

load. As at the moment of isolated network forming what

is corresponding to the disturbance are system character-

istics of just few seconds, so the governor incremental

power value can be considered to be constant. So elec-

tromagnetic power variation can be seen as a disturbance

power .

ST

Figure 2. Block Diagram of a single reh eat s team turbine.

1Ts



 





Figure 3. Block diagram of a typical speed governor.

1-F

1+T s







Figure 4. The reduced order SFR (system frequency re-

sponse) model.

For the situation that load is higher than the active

output power in isolated network, we can make d





corresponding to the stepped load change. It is op-

posite when load is less than the active output power.

We take the typical parameters of power system as:

0.95



, ,, 1.0D4.0H

8.0



,0.3



,. 0.05R

We only consider the situation when load is higher

than the active output power, and the load unbalances are

10%, 20%, 30%, 40%, 50% power deficiency. The fre-

quency response curve is shown in the Figure 5.

Take the typical parameters of power system that the

unbalanced load is given 20%, and the differential coef-

ficients are 0.05, 0.06, 0.07, 0.08, 0.09, 0.1. The fre-

quency response curve is shown in the Figure 6. From

the figure, we can see that differential coefficient has no

influence on initial slope of the frequency curve. The

main effects are shown on the recovery time, maximum

frequency deviation and the steady state frequency de-

viation.

4. Under Frequency Load Shedding Scheme

for Isolated Power System

During the isolated network operation time, the adjust

amplitude of generator unit is limited because of the lack

of units. So the influence of load disturbance on the iso-

lated network is very obvious [9]. The frequency charac-

teristics of isolated network are very dependent on the

Figure 5. The Frequency Response of Isolated Power Sys-

tem with Different Power Unbalances.

Main : Gra

0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0

-1.80

-1.60

-1.40

-1.20

-1.00

-0.80

-0.60

-0.40

-0.20

0.00

频率偏差

(Hz)

ω1(R=0.05) ω2(R=0.06) ω3(R=0.07) ω4(R=0.08) ω5(R=0.09) ω5(R=0.1)

Figure 6. The Frequency Response with Different Governor

Differential Coefficient R.

G. JIN ET AL. 451

size of the spinning reserve and frequency regulating

characteristics of the generator.

The practical parameters of isolated system operation

in a certain power system are taken as an example. The

Figure 7 has obviously shown that the generator fre-

quency regulation coefficient reached the maximum val-

ue when the frequency is about 49Hz. Then it decreased

with the gradual depletion of spinning reserve. Therefore,

when under frequency load shedding scheme is adjusted,

it is very necessary to consider the effect of spinning

reserve.

The frequency regulation effect coefficient KS of

power system is determined by both of the load and gen-

erator frequency regulation characteristics together.

Taking the practical power system as an example, we

make assessment of possible isolated system operation

on each regional network. Here we use the regional

power grid A and B to simulate. We assume that it is the

loss of all the 220 kV power supply that causes the iso-

lated system operation. The powers of network A and B

are of 50% and 80% more than power output. Table 1 is

the comparison of the under frequency load shedding

scheme between the existing and the optimizing one in

the two pieces of power system.

Figure 8(a) shows the existing under frequency load

shedding configuration scheme can not effectively re-

strain frequency from sharply falling. Figure 8( b) shows

after adopting optimization load shedding scheme in grid

A the lowest value of frequency is 48Hz after 5 seconds,

and then it returns to 49.6 Hz. Figure 9 shows the fre-

quency curves comparison between the existing and op-

timizing under frequency load shedding in grid B. The

above method is helpful to solve the frequency sharply

dropped problem caused by the possible large distur-

bance occurred when isolated power formed.

The under frequency load shedding is a provincial plan

as a whole and is distributed according to cities. For the

city field of network in which isolated network operation

may appear, the optimization under frequency load shed-

ding scheme is suitable. The deficiency power of the load

can be supplied by other pieces of network.

Figure 7. Generator frequency regulation coefficient varia-

tion curve with the system frequency variation.

Table 1. Comparison between the existing and optimizing

under frequency load shedding scheme in certain pieces of

power system.

Sheaves 1 2 3 4 5

Action Frequency(Hz) 49.048.8 48.6 48.4 48.2

Delay(s) 0.20.2 0.2 0.2 0.2

Existing load shedding

scheme of power grid A 0% 1.5% 0% 0% 10%

Optimization scheme of A4.5% 5.3% 6.1% 6.6%7%

Existing scheme of B 6.8% 0% 23.6% 9.1% 0%

Optimization scheme of B3.8% 4.4% 4.9% 5.4%5.7%

Sheaves 6 7

First

special

round

Second

special

round

Action Frequency(Hz) 48.047.8 49.0 49.0

Delay(s) 0.20.2 15 20

Existing load shedding

scheme of power grid A 0% 1.5% 3% 10%

Optimization scheme of A7.2% 7.3% 3% 3%

Existing scheme of B 12.8% 0% 12.1% 0%

Optimization scheme of B5.9% 6.1% 3% 3%

(a) the present scheme

(b) the optimized scheme

Figure 8. Frequency curve comparison of the present and

optimized under frequency load shedding schemes in Iso-

lated power system A.

G. JIN ET AL.

452

the worst conditions. So this paper mainly does the re-

search of the frequency and power response in the case

of the relatively large imbalance power disturbance when

isolated network forms, and analyzes the corresponding

control strategy.

During isolated system operation, primary frequency

regulation needs to exit from power control in time, and

turns to the valve position control. In order to ensure the

safety of the unit, and the stability of isolated system

operation, OPC can not quit. For the primary frequency

regulation of the unit, the differential coefficient is ex-

pected to set lowly. This can make the power response in

time after the frequency changes, and prevent the sig-

nificant change of frequency. Setting the dead band in a

low level can optimize the dynamic regulating process of

frequency. It is helpful for the stability of frequency to

enlarge the top and bottom frequency regulation limita-

tions, and also for reducing the oscillation. Relatively

fast response of the servo system can increase quickness

of primary frequency regulation of the unit.

(a) present scheme

There is a frequency deviation after primary frequency

regulation. But turbine is not allowed to run with this

deviation of frequency for a long time. We can put in the

frequency regulation control loop for the speed control

during isolated system operation. This is the secondary

frequency regulation. Increasing dead band and limita-

tion of primary frequency control appropriately can de-

crease the number of movements for OPC.

The OPC setting value of the 100 MW unit in one

power plant of the power system is respectively taken for

103% and 106%.The Figure 10 shows the simulation.

Increasing the OPC value appropriately by 2% - 3% can

reduce frequent opening and closing valve caused by

frequency fluctuation.

(b) optimized scheme

Figure 9. Frequency curve comparison of the present and

optimized schemes load shedding in Isolated Network B.

5. Control Strategies of Isolated System

Operation To sum up, we can make suitable isolated system op-

eration strategy by setting the parameters and control

measures of power plant units. The Table 2 takes two

certain local power plants in power system as an example,

and gives advice about parameter settings of the speed

governor.

The unbalanced power becomes the main risk for iso-

lated system operation. Generally the unbalanced power

of the load is most serious at the early period of isolated

power system. If we want the unit to be able to regulate

any kind of isolated power system, it needs to adapt to

(a) Frequency Curve of 103% OPC Setting Value (b) Frequency Curve of 106% OPC Setting Value

Figure 10. The Frequency Curve of 103% and 106% OPC Setting.

G. JIN ET AL. 453

Table 2. The recommendations of parameter value settings of two power plants in power system.

Power Plant Types Parameter Setting Adjustable Differential Coefficient Frequency Modulation Limiter

scope 4%-5%

Upper limiter:35%，no lower limiter

A Plant Mechanic hydraulicProposed value 4% 35%

Scope 4%-5%

Upper limiter:35%，no lower limiter

B Plant DEH Proposed value 4% ±35%

Power Plant Types Parameter Setting Frequency Modulation Dead Band OPC Value

scope / /

A Plant Mechanic hydraulicProposed value / /

Scope ≤ ±0.034Hz 103%-108%

B Plant DEH Proposed value ±0.017Hz 106%

6. Conclusions

This paper analyses the power grid and unit influence

factors on the isolated system operation. And the system

frequency response model is applied to analyze the spe-

cific influences. By simulating and analyzing the stabil-

ity of regional power system under different load distur-

bances, regulating system parameters and control logics,

parameter settings and control strategies are proposed to

improve the ability of isolated system operation. In the

view of the frequency characteristic difference between

the isolated system operation and networking operation,

we have proposed the optimizing scheme of under fre-

quency load shedding which has considered the fre-

quency characteristic differences. This scheme can meet

the frequency stability requirement both of networking

operation and isolated system operation. Taking the

practical power system as an example, we have con-

ducted the real isolated system operation test. This test

shows the correctness and effectiveness of the proposed

method. The coordinated control strategies improve the

ability of power grid and unit to deal with the accident.

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