System Power

*th Mediterranean Conference on Power Generation, Transmission, Distribution and Energy Conversion

MEDPOWER 2012

Improving Smart Grid Operation with New

Hierarchically Coordinated Protection Approach

Biljana Matic-Cuka and Mladen Kezunovic

interconnection point, DGs deployment is specified by the

Abstract Nowadays, power systems are characterized with standards and different guidelines, such as the IEEE

heavy loading conditions, abrupt changes in generation standard 1547 [3] and FERC order 661-A [4].

patterns, extensive switching of power system configuration Also, the transmission infrastructure upgrade has not

and high penetration of distributed generation (DG). Under

followed the increase in electric power generation. Thus, the

those conditions performance of the conventional relays that

system now needs to operate with tight margins and less

rely on predefined settings may deteriorate leading to system

redundancy under dynamic grid operating phenomena such

wide disturbances and blackouts. To ensure reliable system

as power and voltage oscillations, as well as voltage,

operation, monitoring, control and protection have to be

improved. In this paper, new Hierarchically Coordinated frequency and angular instability. In addition, introducing

Protection (HCP) approach to mitigate the effects of increased distributed generation in the distribution system changed its

grid complexity on its operation is proposed. The proposed

behavior from passive network that transfers power from

approach utilizes local and wide area measurements and relies

substation to the customers in a radial fashion to the active

on the three HCP framework levels: fault anticipation and

network with generation sources causing bidirectional flows

prediction, adaptive fault detection, and relay operation

and short circuit (SC) current levels that may vary under

correction in case of unwanted tripping. It brings intelligence to

the relays at all voltage levels and uses information and different circumstances. These new phenomenon may have

statistics from the systems such as weather, lightning, animal impact on relay operation since in some situations;

and bird migration patterns, component outage history, etc to

conventional relays may not be able to discriminate between

enhance protection system tripping dependability and security.

fault and normal conditions.

Index Terms adaptive relaying, distributed generation,

According to the historical data, relay mis-operation is

neural nets, protective relaying, synchronized sampling.

one of the major contributing factors to 70% of the brown

I. INTRODUCTION out and black out disturbances in the United States [5], [6].

Several cases of inadequacy of relay operation were quoted

D UE to the rise in energy demand and favorable

as possible causes of the disturbances. To prevent tripping

governmental policies a number of DG units have been

due the swing condition, some transmission line relays are

installed in the power systems. In the last few years, energy

armed with the blocking function. However, the relays still

from renewable sources has experienced larger percentage

may mis-operate for the faults occurring during the power

growth compared with the energy growth from conventional

swing period since they are blocked from operation [7]. The

sources. European Union (EU) heads of states assumed a

distance relay mis-operation in Zone III has caused

target of 20 % of energy generated from renewable sources

cascading events leading to major blackout [17]. Protection

by 2020 [1]. A similar plan for 25 % renewable energy

under-reach, sympathetic trips, unsuccessful clearing of

sources requirement until 2025 has been adopted in the US

faults and unintentional islanding are all major problems

[2]. In the last decade large scale wind generation is being

associated with the utilization of DGs in the distribution

rapidly installed while small scale wind generation has not

systems [8]. Study in reference [9] shows that the only way

found broad applications. On the other hand, photovoltaic

to keep existing distribution system protection philosophy in

(PV) systems have slower growth and are evolving from

presence of high DG penetration is to disconnect all DGs

very small residential units to higher generation sizes.

instantaneously in the case of the faults, even if the faults are

To smooth the DG impact on the grid operation many

temporary. It would enable the system to capture its radial

countries and utility companies have established guidelines

nature and steady short circuit current levels. However, if

while IEC, IEEE and other standard bodies are formulating

such practice continues the system reliability will be

standards for DG interconnection to the grid. The biggest

deteriorated and DG full potential will not be utilized.

issue is to make sure that DG operates in a safe environment

The main focus of this paper is the role of the protection

and that their disconnection will not worsen grid reliability.

system in mitigating reliable operation of future smart grids.

Depending on their type and technology, size and

The paper first discusses background of the interconnection

standards. The next section reviews the current relaying

Funding for this research was provided by the U.S. DOE s Office of

practices and potential problems. Then the grid protection

Electricity Delivery and Energy Reliability under the project The Future

issues in the modern grids are analyzed followed with the

Grid to Enable Sustainable Energy Systems.

Biljana Matic-Cuka and Mladen Kezunovic are with the Department of novel protection approach, a case study and conclusions.

Electrical and Computer Engineering, Texas A&M University, College

Station, TX 77843-3128 USA (e-mail: ******@****.***,

*******@***.****.***).

1

II. THE GRID INTERCONNECTION STANDARDS the case of transmission lines, relays should differentiate the

internal faults from external faults so that only the faulted

line is removed, provide the exact fault type selection so

IEEE standard 1547 2003 for Interconnecting

that advanced tripping and reclosing can be applied, and

Distribution Resources with Electric Power Systems

locate the precise fault position on the line so it can be

provides requirements for performance, operation, testing,

repaired and restored quickly.

safety and maintenance of the interconnection [3,10]. The

requirements are applicable for DGs in 60 Hz system with

generation capacity less than 10MW and they should be met

at the point of common coupling. The main idea of IEEE -

1547 is that DG should not affect operation, protection and

power quality of the distribution system and that it should be

quickly disconnected under abnormal conditions. Standard

does not allow utilization of inverter-based DG control

capabilities and it prohibits voltage regulation and reactive

power generation. There are eight complementary standards

designed to expand upon or clarify the initial standard, four

of which are published, and the other four are still in the

development phase [11], see Fig 1.

In attempt to maintain integrity of the transmission grid

with high wind energy penetration, Federal Energy

Regulatory Commission (FERC) proposed low voltage ride-

through (LVR) requirements during faults. Order 661-A

implies that wind energy sources should stay connected to

Fig 1. IEEE 1547 2003 Standard Series [11 ]

the grid during most disturbances. The wind plant should

stay connected for a grid disturbance resulting in voltage

drop of 85% for 625ms time period. Further, the wind plant

should stay connected if voltage returns to 90% of the rated

power within 3s from the beginning of the voltage drop (see

Fig. 2). Moreover the wind plant shall maintain power factor

in the range of 0.95 leading to 0.95 lagging at the point of

interconnection. To accomplish these requirements, the wind

energy source must be equipped with power electronics Fig 2. Required wind plant response to emergency low voltage by FERC

661-A [13]

converters designed to supply such a level of reactive power

or fixed and switched capacitors.

The traditional operating principles of the conventional

Although those two standards have the same objective to

relays as classified in [14] may have some selectivity and

smooth DG impact to the grid operation, their requirements

reliability of operation issues:

in case of the local fault are contradictory; IEEE 1547

Magnitude Relays: The operating logic of such

requires DG disconnection while FERC 661A expects LVR

relays is based on the comparison of the

during the fault. In the case of high DG penetration in

magnitude of one or more operating quantities to

distribution systems, simultaneous disconnection of all DGs

the threshold. For example, the overcurrent relay

under some circumstances may reduce system reliability. It

responds to the changes in the magnitude of the

may cause incorrect relay operation followed by system wide

input current, over/under voltage and frequency

disturbance, and system instability problem at transmission

relays responds to the changes in voltage and

level due to sudden increase in the load. Thus, the regulators

frequency. Such relays are sensitive to the major

for DGs at distribution level are slowly moving toward wind

changes in relay quantities that are caused by

transmission type performance requirements. FERC 661-A

normal system operation rather than the faults

and the emerging new standards for large transmission-

since in that case the safety margin for

connected PV generation still appear to be at odds with the

differentiating between the faults and normal

IEEE-1547 standard for interconnection in the distribution

operating conditions may be significantly

system [12]. On the contrary, German BDEW guidelines

reduced, as discussed in section IV.b.

have imposed a LVRT requirement, automatic real power

Directional Relays: The operating logic of such

regulation and automatic mandatory reactive power

relays is based on the comparison of the phase

contribution corresponding to 0.95 power factor.

angle between two AC inputs. The comparison

can be based on current and voltage phasor, or

III. ISSUES WITH CURRENT PROTECTIVE RELAYING

only on current phasors. Such relays if based

APPROACHES

only on current flows may create false fault

detection due to bidirectional power flows, as

The main goal of protection relay is to quickly and discussed in section IV.D

reliably detect the fault and disconnect the faulted area. In

Ratio Relays: The operating logic of such relays is

2

based on the comparison of the ratio of two B. Short Circuit Level

phasors to the thresholds. An example of a ratio Connection of DG to power grid has various impacts on

relay is the distance relay. This relay may get the performance of the existing protection schemes. The

confused with current in-feed and voltage impact depends on DG type, size and location, impedance

variation phenomena such as power swing or and configuration of the line. DGs contribute current to the

sudden increase in the load. An example of the circuit and their contribution may rise, lower or change

direction of the short circuit (SC) current. Machine based

relay mis-operation is shown in section VI.

Differential Relays: The operating logic of such DGs inject SC current levels of more than 5 times their

rated current and may contribute to the short circuit current

relays is based on the algebraic sum of two or

for long time due to high inertia. On the other head, the

more inputs. In a general form, those inputs may

inverter based DGs have lower contribution up to 2 times

be the currents entering (or leaving) a specific

rated current and trip off very quickly due to low inertia. If,

protection zone. They are sensitive to different

for example, a remote part of a distribution network is

no-fault distortions in the currents such as inrush

equipped with large inverter based DG installations, it could

currents due to component switching or distorted

happen that in case of a failure there is almost no significant

currents due to instrument transformer saturation

rise of the phase current and the fault is therefore not

Pilot Relays: The operating logic of such relays is detected by the overcurrent protection system. With DG in

based on the communicated information obtained the network, the fault impedance can also decrease due to

from the two ends of the line. The decisions parallel circuits, therefore the SC current level increases and

made by a local relay and by a remote-end relay there could be unexpected high SC currents in case of a

are combined to form the final decisions. The failure.

inside principle of each relay could be any of the

C. Power Quality

four types described above. Such relaying

The power quality problems due to DG penetration are

principle is sensitive to the errors in the

applicable to the distribution and medium voltage grid s.

communication system.

Transient voltage variations can be expected if there are

All conventional relays have in common that they operate

relatively large current changes during connection and

as a tradeoff between security (not to trip when there is no

disconnection of DGs. Two aspects of power quality usually

fault) and dependability (to trip when there is a fault), with

considered to be important during evaluation of DG impact

the bias toward dependability. This tradeoff in internal logic

on system performance are: voltage flicker conditions and

makes protective relays mis-operate since the conventional

harmonic distortion of the voltage. Depending on the

relays have predefined settings that cannot be changed

particular circumstance, a DG can either decrease or

online following the system change that requires a change in increase the quality of the voltage received by other users of

the tradeoff bias. The settings are calculated assuming the the distribution network. The effect of increasing the grid SC

worst case system conditions and cannot be easily altered current by adding generation often leads to improved power

when such conditions are changed to different worst case quality; however, it may have a negative impact on other

scenarios. Due to the complexity in system operation, such aspects of system performance. A single large DG, or

as change in topology or loading, the relay decision drawn aggregate of small DG connected to a weak grid may lead to

using only one feature, such as current magnitude may be is power quality problems during starting and stopping

insufficient to make accurate decision in the systems with conditions or output fluctuations. For certain types of DGs,

such as wind turbines or PVs, power fluctuations are a

high DG penetration.

routine part of operation due to varying wind or sunlight

conditions.

IV. OPERATION ISSUES IN A MODERN GRID

The following sections summarize issues in the modern D. Reverse Power Flow and Voltage Profile

power grid with high penetration of renewable DGs: One of the most important responsibilities for the utility is

to keep acceptable voltage range in distribution systems. The

A. Anti-islanding

inverter-based DGs may regulate voltage at point of

The islanding occurs when a part of the utility network is

common coupling (PCC). However, according to the IEEE

still energized by the DG while being disconnected from the

standard 1547 inverter-based DGs are not allowed to

main grid. In this situation, neither the voltage nor the

regulate voltage at the point of common coupling (PCC).

frequencies are controlled by the utility. Normally, islanding

The tap-changing transformers or switched capacitors are

is the consequence of a fault in the network. If DG continues

used instead. The inverter-based DGs are designed to

its operation after the utility supply was disconnected, faults

operate only at unity power factor because this condition

may not clear since the arc is still charged. The main

will produce the most real power and energy. This limitation

problems associated with unintentional islanding are:

is a matter of standardization and agreements, and it is not

unacceptable limits for voltage, frequency and other power

technical one. Generally, inverters have the capability of

quality parameters which may lead to damage of network

providing reactive power to the grid in addition to the active

and customer equipment, out -of-phase reclosing that may

power.

cause high transient inrush currents which may damage the

Introducing DGs at the load side reduces the load demand

generator and electric shock to utility workers by touching

and in turn leads to reduced losses and improved voltage

energized conductors.

profiles on the feeder. This is a true statement as long as the

DG generation coincides with the substantial load demand

3

so that the net power flow remains going from the substation A. Transmission System

to the load. As the penetration levels of DG rise, there may An example of the novel transmission system protection

be time periods during the day when the power flow is from philosophy that relies on local and wide area protection

the load towards the substation, a situation not normally methods is presented in this section. This approach allows

anticipated in the distribution system design. This will automated system-wide monitoring of system component

affects performance of standard protection schemes with condition, which assures reliable protective relay operation

directional overcurrent relays. and performs corrective actions in the case protection

Voltage regulation and voltage rise are the key factors dependability or security is compromised. The scope of the

that limit the penetration level of DG that can be connected proposed approach includes:

to the system. During heavy load conditions, with connected Predictive Protection: The system monitoring and control

DG, voltage levels may drop below acceptable limits. The tool that performs routine vulnerability analysis of operating

main concern about high DG penetration levels in condition of the whole system and individual elements [15]

distribution systems is the effect of the expected large is deployed at the control center level and alert signals are

sent to the substation level to closely monitor relays placed

randomly fluctuating real power output from these sources.

at the most vulnerable components. The prediction of where

Fluctuating power from DG generators can cause the feeder

the protection mis-operation may occur gives an early

voltage profile to change, possibly increasing the switching

warning of how the contingencies may unfold.

operations for line regulators and capacitors.

Inherently adaptive protection: At the substation level a

neural network based fault detection and classification

V. PROPOSED HIERARCHICALLY COORDINATED

algorithm is employed [16,17] . Its tripping logic is based on

PROTECTION (HCP) APPROACH

feature patterns of waveform measurements. This approach

A new HCP paradigm for smart grids is envisioned to be does not have settings and hence avoids mis-operation due to

able to deal with the grid behaviors that have not been seen inadequate settings allowing for an inherent adaptive action

before but is anticipated in the future: heavy loading to optimize the balance between dependability and security

conditions, abrupt changes in generation patterns, extensive Corrective protection: At the substation level, fast and

switching of power system configuration and high accurate synchronized sampling based fault location [18]

penetration of distributed generation. This will lead to an and event tree analysis [17] to detect incorrect line tripping

equal importance of balancing the dependability and security sequence and incorrect relay logic operation respectively are

of protective relay operations, which is hard to achieve deployed. Upon transmission line tripping, fault location

simultaneously since in the past designing protection algorithm will validate correctness of relay s operation and

systems for selected tradeoff between dependability and in case of unconfirmed fault condition; the system

security, was common. The new protection approach is component (transmission line) will be quickly restored. The

proposed to avoid relay mis-operations that may lead to relay logic will be checked as it executes and if an incorrect

blackouts. The framework of the proposed approach consists sequence is detected, the relay action will be corrected.

of the:

Predictive protection. The statistical data and As a summary, the three concepts proposed earlier are tied

information from weather related tracking systems, together in an overall solution design shown in Fig.3.

history of the component outages, and power

system operating conditions that may lead to the

major disturbances, etc are used to anticipate

occurrence of a fault condition. The local

protection is armed to respond with specific

tripping logic for each disturbance that is

anticipated.

Inherently adaptive protection. It adjusts its tripping

logic based on feature patterns of waveform

measurements that are recognized online and

matched to the patterns obtained during learning

process that includes thousands of potential fault

conditions.

Fig. 3: The Hierarchical System Protection Architecture

Corrective protection. If local protection mis-

B. Distribution System

operates, accurate and fast fault location/analysis

restores the system component(s) quickly if the The proposed framework may also be used in defining

original tripping action is determined not to be new protection scheme for distribution system. It is well

correct. known that existing protection practice with overcurrent

relays in cases of high DG penetration is ineffective due to

This approach may be utilized at all power system levels

problems in finding right settings and time-coordination. It

and any robust and reliable protection scheme may be

appears that more sophisticated and adaptive methods have

defined and developed following these three concepts. The

to be developed.

following sections describe proposed approach for

The weather tracking and prediction models have been

transmission and distribution system using such concepts.

used in the power systems to enhance the performance of

4

planning, scheduling, energy management, and feedback

control systems [19]. However, utilization of such solutions

in designing the protection system has not been explored yet.

The lightning and severe weather conditions are major cause

of the faults in distribution system. The information from the

weather satellites may be used to develop alerts to potential

protection scheme and certain strategies to better isolate the

faults in the network.

Since fuses cannot be coordinated and controlled by

external signal, they will lose their function in the system

with high DG penetration. The protection system must rely

on breakers and reclosers that will communicate with the

main relay located in the substation. The relay would sense

the fault, identify the faulted section on the feeder, and

Fig. 4: IEEE 39-bus system

isolate the faulted zone by tripping appropriate breakers.

This way, the remaining zones can still function as usual.

The relay could be designed using advanced machine pattern

recognition technique to be able to distinguish between fault

and normal condition under any circumstances.

As a part of the corrective strategy, a fast and accurate

fault location may be used [22]. In addition, highly accurate

distribution system fault location is possible by combining

Fig. 5: Event Sequence

lightning location data form the U.S. National Lightning

Detection Network with fault monitor disturbance data and

distribution feeder location (GIS) data [2 0]. The data latency

is several seconds and may be used in the corrective

protection to verify the fault location determination in the

system. Moreover, animals and birds cause large number of

outages in overhead distribution systems. The frequency of

animal and bird related outages depend on the area, season

and time of the day. The historically obtained outage

patterns and animal/bird migration patterns may be used to

verify the fault location determination in the distribution

systems [21].

Fig. 6: Trajectory of Impedance

VI. CASE STUDY

Although the two faults are not related to the healthy line

In order to illustrate the use and operational efficiency of 26-29, the power swing caused by the two faults will have an

the proposed Hierarchically Coordinated Protection scheme impact on the distance relay. It observes Zone 3 fault at

for the transmission applications, the IEEE 39-bus New 1.627s after the second fault clearing until the trajectory

England test system shown in Figure 4 is utilized [23]. The leaves Zone 3 circle at 1.998s. The distance relay may trip

two most vulnerable lines according to their vulnerable Line 26-29 when its Zone 3 timer expires. As a result, buses

indices are: Line 21-22, 28-29 [15]. The outage of those 29, 38 will be isolated from the system, including the G9 and

lines will have a large impact for the system stability since loads at bus 29. This will results in the oscillation in the rest

the original loads in those two lines will be redistributed to of the system and further cascading outage may happen.

the neighboring lines causing more overloading issues. The The mentioned situation can be prevented by the proposed

system monitoring tool will inform the local relay system and local monitoring and protection tool. When the

monitoring tool on those lines to monitor the relay first fault occurs, the faulted line 21-22 is removed and no

operations closely. other operation happens. The relay monitoring tool for the

Assume a series of disturbances occur in the system, with relay at Line 21-22 will inform the system monitoring tool

the event sequence shown in Figure 5. The related system about the relay operation for the three-phase fault. The

components are marked in Figure 4. These two faults are system security analysis is activated after the first fault. An

permanent faults and thus isolated by the relay actions. After alert signal will be sent to the local relay monitoring tool at

the line 21-22 is removed due to the first fault, the top 2 vulnerable lines at this stage. Since the first fault will not

most vulnerable lines are changed to: Line 28-29, 2-3. After degrade the system stability very much, the local relay

the line 28-29 is removed due to the second fault, the top 2 monitoring tool will not be authorized to intervene with

most vulnerable lines are changed to: Line 23-24, 26-29. relay operations at this stage. When the second fault happens

This contingency may cause relay at Bus 26 of Line 26-29 and Line 28-29 is removed, the local relay monitoring tools

to mis-operate. The trajectory of impedance seen by that for the most vulnerable lines 23-24 and 26-29 will be

relay is shown in Figure 6 with the event sequence labeled. authorized to correct the potential relay mis-operation or

unintended operation in real time since the mis-operation of

those relays will directly separate the system. After the

second fault, the local relay monitoring tool at Line 26-29

5

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X. BIOGRAPHIES

IX. REFERENCES Biljana Matic-Cuka ( S 07) received her Dipl. Ing. degree in Electrical

and Computer Engineering from University of Novi Sad, Serbia, in 2006.

[1] Renewable Energy Technology Roadmap 20% by 2020 [Online].

Currently she is pursuing the Ph.D degree in the Department of Electrical

Available:http://www.erec.org/fileadmin/erec_docs/Documents/Publi

and Computer Engineering, Texas A&M University, College Station, TX,

cations /Renewable_Energy_Technology_Roadmap.pdf

USA. Her research interests include power system protection and

[2] 25% Renewable Energy forthe United States By 2025:Agricultural

monitoring, smart grids, and distributed generation. Her industry

and Economic Impacts [Online] Available: http://www.agpolicy.org

experience includes an internship at GE Global Research, Niskayuna, NY

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(Sept 2011- Jan 2012).

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Electric Power Systems," IEEE Std 1547-2003, vol., no., pp.0_1-16,

Mladen Kezunovic (S 77-M 80 SM 85 F 99) received the Dipl. Ing.,

2003

M.S. and Ph.D. degrees in electrical engineering in 1974, 19 77 and 1980,

[4] FERC Order 661-A [Online] Aveilable: http://www.ferc.gov/

respectively. Currently, he is the Eugene E. Webb Professor, Site Director

industries/electric/indus-act/gi/wind.asp

of NSF I/UCRC Power Engineering Research Center, PSerc, and Deputy

J. Zhang and M. Kezunovic, Improving real-time fault analysis and

[5]

Director of another NSF I/UCRC Electrical Vehicles: Transportation and

validating relay operations to prevent of mitigate cascading

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LA,Oct. 2005, pp. 847 852.

of intelligent methods to power system monitoring, control, and protection.

U.S.-Canada Power System Outage Task Force, Final report on the

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He has published over 400 papers, given over 100 seminars, invited

August 14, 2003blackout in the United States and Canada: Ca uses

lectures and short courses, and consulted for over 50 companies worldwide.

and recommendations, Apr. 2004.

He is the Principal of XpertPowerTM Associates, a consulting firm

X. Lin, Y. Gao, and P. Liu, A novel scheme to identify symmetrical

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specializing in power systems data analytics. Dr. Kezunovic is a Fellow of

faults occurring during power swings, IEEE Trans. Power. Del., vol.

the IEEE, a member of CIGRE and Registered Professional Engineer in

23, no.1, pp. 73-78, Jan. 2008.

Texas

[8] Diss.Chang, Tim, Impact of Distributed Generation on Distribution

Feeder Protection, University of Toronto (Canada), 2010. 2010.

MR72537.

6

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