Secondary Control in AC Microgrids

Challenges and Solutions

Omid Palizban and Kimmo Kauhaniemi

Department of Electrical Engineering and Energy Technology, University of Vaasa, Vaasa, Finland

Keywords: Hierarchical Control, Decentralized Control, Micro Grid, Secondary Control.

Abstract: The hierarchical control structure of a microgrid can be described as consisting of four levels: processing,

sensing and adjusting, monitoring and supervising, and maintenance and optimizing. This paper focuses on

the secondary control level, which can be classified as centralized or decentralized control. A

comprehensive investigation of both centralized and decentralized control is presented in this paper.

Decentralized control is proposed in order to deal with some of the disadvantages of central control, such as

the high risk of unplanned interruption arising from a Microgrid Central Controller (MGCC) malfunction.

However, decentralized control is not yet complete, and some challenges to its implementation remain. This

paper also looks at these challenges and proposes some solutions that may help to improve the performance

of decentralized control and overcome its disadvantages. Finally, a general methodology of microgrid

control is modeled.

1 INTRODUCTION

In previous decades, fossil fuels were the basic

resource for generating energy in the world.

However, taking into account the increasing costs of

energy and the associated environmental concerns,

industrial experts and researchers are presently

seeking to replace fossil sources by Renewable

Energy Sources (RESs). In this regard, the European

Union (EU) has established the so-called 20-20-20

energy target, which requires a 20% decrease in

greenhouse gas release, a 20% increase in the

consumption of energy from RESs, and a 20%

decrease in primary energy consumption through

improvements in power efficiency by 2020.

Moreover, based on the energy roadmap for 2050,

which aims for a 41% decrease in energy demand

through increases in energy efficiency, the role of

RESs will be more significant in future (Lise, van

der Laan et al. 2013).

To solve the problem and to increase both efficiency

and power quality, RESs can now be integrated into

the main network in the form of Distributed

Generators (DG) or Microgrids (MG). A MG

consists of a methodical organization of such DG

systems—an organization that leads to increases in

system capacity and achieves the aimed-for high

power quality (Li and Kao 2009). Moreover, the

Energy Storage System (ESS) is an important

component of the microgrid, which satisfies

requirements in many instances, such as ancillary

services like load following, operational reserve,

frequency regulation, peak shaving, black start

during island mode, renewable integration, and

relieving congestion and constraints.

Controlling the DGs and MGs is a critical topic, and

it is necessary to implement a hierarchical control

system in order to achieve this. A comprehensive

analysis of hierarchical microgrid control, applying

the IEC/ISO 62264 standard, is presented in

(Palizban, Kauhaniemi et al. 2014; Palizban,

Kauhaniemi et al. 2014). Based on the microgrid

control scheme proposed in those papers,

hierarchical control consists of four different levels.

There are many different technical possibilities for

each of the control levels. A complete review of

primary techniques is presented in (Vandoorn, De

Kooning et al. 2013). Based on the secondary

control level, which is investigated in (Guerrero,

Vasquez et al. 2011; Shafiee, Guerrero et al. 2014),

control levels can be classified into two types:

centralized and decentralized. The definition of

(de)centralization is based on the position of the

MGCC. A significant disadvantage of the

centralized method is that the system will go

completely out of service if any problem occurs in

the MGCC. To overcome this drawback, a

294

Palizban O. and Kauhaniemi K..

Secondary Control in AC Microgrids - Challenges and Solutions.

DOI: 10.5220/0005488102940299

In Proceedings of the 4th International Conference on Smart Cities and Green ICT Systems (SMARTGREENS-2015), pages 294-299

ISBN: 978-989-758-105-2

 2015 SCITEPRESS (Science and Technology Publications, Lda.)

decentralized control method has been proposed, but

this cannot support the system in all its function. In

this regard, the present paper investigates the

challenges to microgrids using decentralized

secondary control, and introduces some solutions to

deal with the disadvantages. This paper is organized

as follows: a preliminary discussion of hierarchical

control is presented in Section 2. Section 3 discusses

secondary control in detail. Hierarchical control of

ESS is presented in Section 4. The challenges and

solution to secondary control are described in

Section 5; in Section 6, a general hierarchical control

in microgrid is modeled. Finally, the paper is

concluded in Section 7.

2 HIERARCHICAL CONTROL

LEVELS

The principle of the classification of the control

levels in MGs is shown in Fig. 1.The hierarchical

control structure of MGs can be classified into four

levels, which are explained as below:

The source operating point control is the first step in

MG control, and is designed using power electronic

devices. Generally, the control level is achieved

through the inner current and voltage control loop,

and the main goal of this control level is to manage

the output power of the micro sources (MSs). In the

next control level the primary control accounts for

any variation in the system on the order of

milliseconds, so the fastest response is provided by

this primary control. The aim of this control level is

to adjust the reference values from the amplitude of

the voltage and the frequency in order to feed the

inner control loop. Supervising and monitoring the

frequency and voltage is the main duty of the

secondary control level. In fact, during any variation

in load or generation in the MG, the secondary

control regulates the deviation in the appropriate

limitation range—for example, by ± 0.1 Hz in

Nordel (North of Europe) or by ± 0.2 Hz in UCTE

(Continental Europe) (Guerrero, Vasquez et al.

2011). Compared to the primary control level, the

secondary control has a slower dynamic response to

variation. This control level can be classified into

centralized and decentralized controls, which are

discussed in depth in Section 3. In the last, the third

control level is the final and slowest. It regulates the

frequency and voltage when the MG is connected to

the main grid. This adjustment is based on

measuring the active and reactive power from the

main grid.

Figure 1: The hierarchical control structure of MGs.

3 SECONDARY CONTROL

As mentioned previously, centralized, and

decentralized control are methods of secondary

control for adjusting the output of each different DG.

The principle of centralized control methods in MGs

is very similar to in the inner control loop. The

MGCC plays the main role in managing the power

between the different DGs and interfaces with main

grid (Fig.2).

Figure 2: The principle of centralized control methods in

MGs.

The MGCC detects all the values from the main grid

and DGs, and provides a reference value to send to

the primary and inner control loops. Fig.3 illustrates

the processing of the secondary control level in the

central method.

The significant disadvantage of this method is the

dependence of the control on MGCC, which means

that the system faces a problem when there is any

malfunction in the central control. Moreover, a

communications link between all DGs and MGCC is

required for centralized mode, making the method

more unreliable (Bidram, Davoudi et al. 2013). To

overcome these problems, distributed secondary

control has recently been proposed in(Shafiee,

Vasquez et al. 2012; Bidram, Davoudi et al. 2013;

Bidram, Davoudi et al. 2013; Shafiee, Guerrero et al.

2014).

SecondaryControlinACMicrogrids-ChallengesandSolutions

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ref

VΔ

fΔ

Figure 3: The secondary control level in the central

method.

Figure 4: The principle of decentralized control methods

in MGs.

Figure 5: combining the decentralized control for the DGs

and ESS.

Figure 6: A chart of the challenges and solutions.

The duty of the control in the second level, whether it

is centralized or decentralized, is almost the same,

though there are some differences between the

operating types.

In the decentralized set-up, the MGCC for

controlling and supporting the primary control has

been removed and the secondary control is

transferred to beside each primary control (Fig.4). In

centralized control, the output voltage of each DG is

measured using remote sensing and is sent to the

MGCC, while in decentralized control, the sensing

and measurement of the voltage is performed at the

terminal of each DG unit.

4 HIERARCHICAL CONTROL OF

ENERGY STORAGE SYSTEM

As illustrated later in Section 6, the DGs units have

either some delayed response or slow controllability

to variation inside the MG. Hence, it is necessary to

cover the gap during island mode using ESS. The

system is managed by charging and discharging the

ESS. Hierarchical control of ESS is included in two

levels: primary and secondary. The third control level

in ESS is the removal of the storage control, because

the energy storage is a local resource and does not

cooperate with the main grid. The voltage and the

frequency of the MG are adjusted though the ESS

primary control. As presented in (Loh and Blaabjerg

2011), the frequency of the system (f) is used to

measure the microgrid’s capacity. Based on this

sensing, if the frequency of the network is near to the

maximum value (f

max

), then the power generation is

higher than the power demand, so the energy storage

system must absorb the additional energy and remain

in charging power mode throughout its current state

of charge (SoC). A frequency that lies between the

maximum and minimum value is a normal situation

that does not need energy to be absorbed or injected

by the energy storage system. However, if the

frequency drops close to the minimum value (f

min

this means that the power demand has increased and

that the energy storage system should inject stored

power and move from charging mode to discharging

mode. However, since the energy capacity of the

energy storage is limited, the output power of the

ESS must return back to zero or some other fixed

value needed to maintain the SoC with the increasing

output power of the DGs. Decreasing the power is

the duty of the secondary control level of the energy

storage system (Kim, Jeon et al. 2010). As described

in Section 2, increasing the output power from the

DGs is the responsibility of the primary control level.

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5 CHALLENGES AND

SOLUTIONS

As described above, the responsibility of secondary

control is to support the MG in achieving reliable

output from the different DGs. The control level

modeled in the next section is based on a general

approach to a central control for MGs. Based on the

results of this model, a centralized control method is

proposed to manage the output power from DGs;

however, their response time is not fast enough, and

it is necessary to add some ancillary service to cover

the delay time. The ESS is the best solution for a fast

response to cover the delay. Power management and

the balancing power between DGs and ESS is also

the duty of the MGCC. One challenge to centralized

control that might prevent the system operating at

the optimum level with high efficiency and good

performance is the high risk to system stability.

Fig.6 presents a chart of the challenges and

solutions. The decentralized control is proposed for

improving performance and enhancing the efficiency

of the system. Although this control type can help in

removing the MGCC from the microgrid, there are

some challenges to implementing the control

strategy—such as operating the microgrid using a

distributed control strategy during grid connection

mode and the synchronization process between DGs

and the main grid. Indeed, most studies of this

control method have focused on the island mode.

Moreover, The inability of controlling the system

during the transient and persistent faults that cause

blackout situations in microgrids (as well as black-

start coordination) without an MGCC and some of

the other management functions of the MG are

challenges to implementing a fully decentralized

control system (Shafiee, Guerrero et al. 2014). One

way of dealing with these challenges and achieving

a greater level of control in practice is to use a

distributed energy storage system, which would be

installed beside each DG. Based on the structure of

the system (see Fig.5), the arrangement consists of a

set of DGs with storage, along with separate primary

and secondary controls. Although a distributed

energy storage system has recently been proposed by

(Morstyn, Hredzak et al. 2014; Xu, Zhang et al.

2015), however in these studies, only the

possibilities of this system is investigated whereas

MGs are controlled centrally. The benefit of

combining the decentralized control for the DGs and

ESS is that it may help increase the efficiency of the

decentralized secondary control, by solving the

previous problems of this control level, as well as

increasing the power reliability by decreasing the

error rate and the number of unplanned interruptions

of the system, controlling the system during

transient faults and coordinating the system in black-

start situations. Finally, the faster response and

easier energy management in island mode will also

assist in critical situations.

Figure 7: The structure of the simulation case study.

6 MODELING OF

HIERARCHICAL CONTROL

The regulation of voltage and frequency in

hierarchical control, as described in Section 2, is

modeled in this section on the basis of the general

approach presented recently in many papers. In the

simulation, the centralized secondary control was

utilized, and some parts from models previously

created in joint projects between the University of

Vaasa, VTT, and ABB were employed. The network

examined in this paper is shown in Fig.7.

It consists of a transformer (MV/LV) with an 800

kVA capability and a circuit breaker (CB) controlled

by the MGCC. During the simulation, the islanded

microgrid is disconnected from the main grid by this

breaker. The inverter-based DGs operate on the

basis of active and reactive power control in grid

connection mode, and on the basis of voltage and

frequency control mode during island mode. Fig.8

shows the control-block diagram modeled in

PSCAD. The results obtained from the dynamic

system study are as follows: To investigate the

system in both modes, the system was connected to

SecondaryControlinACMicrogrids-ChallengesandSolutions

297

gri

ref

grid

DGd

−

DGq

−

Figure 8: The Control block diagram of the MGs in the grid connection and island mode.

the main grid at the beginning of the simulation and

disconnected after 4 seconds. The system did, in

fact, operate in island mode for the rest of the

running time.

The voltage on the LV side of the transformer is

shown in Figure 9. After the isolation of the system

from the main grid, power balancing became the

responsibility of the energy storage system

However, since this paper investigates only

centralized and decentralized secondary control, the

storage system result is not described here. The

output voltage, current and active power of the DGs

is presented in figures 10, 11, and 12, respectively.

The slow response of DGs for needed power

increase, which constitutes a challenge to supporting

the system, can be seen clearly in Fig.12. DGs are

not able to increase the power output immediately

when the system is disconnected from grid and is

operating in island mode. Since the simplified model

applied here does not consider the power increase

rate limits of the primary energy source the problem

will be even bigger in practical system. In

frequency control part, Fig. 13 shows the frequency

of DGs during grid connection and island mode.

7 CONCLUSIONS

Based on the hierarchical control strategy—an

important feature of microgrid operation—this paper

focuses on the secondary control, which can operate

on the basis of centralized and decentralized control.

The challenges to the complete implementation of

secondary control are discussed in this paper. Of

these, the challenge of coordinating the control level

during black-start situations is the most important.

Based on this study, a combination of distributed

energy storage and the MG, which is controlled by a

decentralized secondary control, may help overcome

these implementation challenges. It is expected that

these solutions will improve efficiency, power

reliability, and response time, while decreasing lost

power and the error rate of the system. The

investigations so far are technical in nature; research

into the economic aspects of this arrangement is thus

a key question for future work.

Volta

e of

rid

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

100

150

200

250

300

350

400

450

(V)

V_rms_Tr

Figure 9: The voltage on the LV side of the transformer.

DG1 DG2

Time

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

100

200

300

400

500

(V)

Vrms_DG1 V_rms_DG2

Figure 10: The output voltage of each DG.

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Current

Tim

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

100

120

(A)

IvaRMS IvbRMS Ivc RMS

Figure 11: Output current of DG.

Po w e r

Time

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

-20

100

120

140

160

(KW)

P_grid P_ DG 1 P_ DG 2

Figure 12: Active power of the DGs and network.

DGs

Tim

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0

49.9700

49.9750

49.9800

49.9850

49.9900

49.9950

50.0000

Fr e

Figure 13: The frequency of DGs.

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