IMPLEMENTATION OF REMEDIAL ACTION … OF...Implementation of Remedial Action Scheme for Frequency Stability in TNB CEPSI 2016 1 IMPLEMENTATION OF REMEDIAL ACTION SCHEME FOR FREQUENCY

Implementation of Remedial Action Scheme for Frequency Stability in TNB

CEPSI 2016 1

IMPLEMENTATION OF REMEDIAL ACTION SCHEME FOR

FREQUENCY STABILITY IN TNB

Ahmad Zuhdi Muhamad Zamani

Tenaga Nasional Berhad

Engineering Department, Transmission Division

Level 19, Dua Sentral

Jalan Tun Sambathan, Kuala Lumpur

[email protected]

Nik Sofizan Nik Yusuf

Tenaga Nasional Berhad

Engineering Department, Transmission Division

Level 19, Dua Sentral

Jalan Tun Sambathan, Kuala Lumpur

[email protected]

ABSTRACT

One of major challenges to utilities in meeting rapid demand growth is in putting up infrastructures such as

building new power plants, substations and transmission lines. This is mainly due to economic factors and public

protest. As a result, transmission grid suffers stress and grid operations would be pushed to the limit.

This paper discusses the Remedial Action Scheme (RAS) implemented in Tenaga Nasional Berhad (TNB) to

mitigate the risk of frequency instability due to large generation loss by automatically initiating fast active power

compensation from HVDC and Hydro plants as well as Event Based Load Shedding (EBLS) scheme. EBLS scheme

calculates in real time the optimum quantum and location of load to be shed to maintain or regain the stability of

the power grid. The algorithm for the optimum quantum calculation takes into consideration the active power

reserve margin of HVDC and hydro plants, power flow in the network and preset priority of the load.

The Remedial Action Scheme integrates overload protection based on Dynamic Line Rating, adaptive load

shedding, HVDC and hydro plants control system in a single wide area protection, control and automation system.

The system consists of distributed automation processors named Real Time Application Platform (RTAP), control

intelligent electronic devices, Remote Terminal Units (RTU) as well as fast communication protocols between all

the equipment. The RAS has operated several times and prevented the power grid from frequency instability.

KEYWORDS: Remedial Action Scheme (RAS), Special Protection Scheme (SPS), System Integrity

Protection Scheme (SIPS), Transmission, Wide Area Protection, Real Time Application Platform (RTAP)

1. Introduction

Frequency stability refers to the ability of a power system to maintain the steady frequency following a severe

disturbance, which results in generation and load imbalance. The short term frequency instability refers to those

events which occur in the time frame of a few seconds. For example, insufficient generation and uncoordinated

low frequency demand control measures can cause rapid frequency decline from nominal values. This can further

lead to new cascading outages and potential catastrophic system blackouts. Another type of frequency instability

is the long term frequency instability which happens over a time frame of tens of seconds to several minutes. It

can be caused by processes with slower time responses, such as steam turbine over-speed controls or boiler

protection and control.

Figure 1 – Example of typical frequency response


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In Tenaga Nasional Berhad (TNB) Grid system, the load center is located in the West region which comprises the

national capital, Kuala Lumpur and Selangor, the country’s most developed state. As the region consumes more

than a third of the System’s total demand of 16GW, it is typically a net importer of power. On the contrary, largest

single generator of 1000 MW has recently been commissioned in Northern region and another 1000 MW generator

is to be commissioned soon at the same location making the region especially Perak area a net power exporter.

These two regions are interconnected by 500 kV double circuit transmission lines which run in parallel with the

275 kV double circuit transmission lines. Delay in building new transmission infrastructures has created network

bottleneck and made the power grid susceptible to severe disturbances such as generator out of step. Generation

constrained dispatch is not favorable since it is not economically attractive solution. Therefore, in order to

overcome the network constraint issues, network decoupling is applied to the existing network. The network

decoupling has introduced another risk to the power grid which is frequency instability due to disconnection of

large generation unit but it is relatively lesser risk to handle compared to transient or angular instability.

44%21%

7%23%EAST

SOUTH

NORTH

PERAK

CENTRAL

Load Consumption

Power Generation

Figure 2 – Overview of Power Flow

While awaiting the new transmission lines project completion, a temporary measure was set up to address the

problem. This paper describes features of the Remedial Action Scheme (RAS) that was implemented to assist the

grid operator to secure the network quickly and in the most optimal manner.

2. Network Analysis

Figure 3 shows the network diagram of the decoupled parallel transmission lines interconnecting the two region.

The decoupling is conducted at JMJG and the busbar is separated into BJMJG1 and BJMJG2. BL01 is the single

direct 500 kV transmission line transporting power into the load center. Normally, the largest 1000 MW generator

(U4) will be connected to this transmission line through BJMJG1 bus. Therefore, any N-1 contingency on BL01

transmission line will immediately cause the power grid to lose significant amount of generation. On the other

hand, losing the BL04 500kV transmission line will force all the generation at BJMJG2 bus (which normally

comprises of 3x700 MW U1, U2 and U3 generators) to flow through 275 kV transmission lines. Since there is

also large generation at DSGRI, this can cause the 275 kV transmission lines DL09 and DL10 to overload.

Subsequently, the generation at BJMJG2 and DSGRI need to be deloaded or disconnected depending of the

severity of the overload. Furthermore, N-2 contingency of BL01 and BL04 will cause the largest generation loss

and active power compensation alone might not be enough to arrest the frequency instability. With the coming of

another 1000 MW generator at BJMJG, the situation will be further aggravated if the system loses the tie line

interconnection from Singapore. Hence, load shedding at the load center is inevitable to restore the system

frequency.


CEPSI 2016 3

DBTRK1

DSGRI1

U1U4 U3 U2

1 x 1000MW

BJMJG1

3 x 700MW

BJMJG2

BATWR2

BBTRK1

BKPAR1DKULN1

DKPAR1

BL01

BL04

BL05 BL06

DL11 DL12

GB1 GB2 GB3

3 x 650MW

DATWR1

DL09 DL10

U5

1 x 1000MW (Coming)

BL02BL14BL13

Figure 3 – Network Diagram of 500kV and 275kV System

Dynamic system simulation was conducted to assess the situation accurately. Worst case scenarios where

maximum power at and from the Northern region to the load center were simulated under various contingencies.

Figure 4 – Generator Machine Angles


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Figure 4 depicts all the generator angles in the TNB power grid following the N-2 BL01 and BL04 contingency.

It can be seen that without decoupling of the busbar, generator out of step condition is detected less than 2 seconds

after the 500 kV transmission lines are loss. Generators in the central and south regions deviated from the eastern

and northers regions. Therefore, network decoupling is required to ensure generator out of step condition is

prevented.

Figure 5 – Loss of Generation

Figure 5 above shows the frequency response of the system upon losing 1400 MW and 1700 MW generation

respectively. The frequency will drop rapidly until Under Frequency Load Shedding (UFLS) Stage 1 is triggered

at 49.3 Hz. Load shedding is not desirable when active power compensation from HVDC and hydro plants can be

utilized. Figure 6 below shows that larger quantum of UFLS is required to prevent from losing the tie line at

frequency reaching 49.1. This happens in the event of losing who BJMJG. The implementation of Event Based

Load Shedding (EBLS) will prevent larger quantum of load shedding by the UFLS.

Figure 6 – Loss of Generation and Tie Line


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Figure 7 – Transmission Lines and Transformers Overloading

Transmission lines and transformers overloading following N-1 BL04 contingency can be seen in Figure 7. The

worst overload is 153% on the 275kV ATWR-BGJH lines. Therefore, generation shedding at JMJG and SGRI is

required to mitigate the overload on 275kV lines and transformers. This generation shedding will cause the

frequency to drop and active power compensation is needed to restore the frequency. Figure 8 below illustrate the

power system frequency response upon 1 unit of JMJG at 2s, 30s and 40s respectively.

Figure 8 – Frequency Response


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3. Remedial Action Scheme (RAS) System Features

The Remedial Action Scheme is a solution for the frequency instability and network overloading problem. It

consists of the following features:

Generation Deficiency Compensation (GDC)

Overload Protection based on Dynamic Line Rating (DLR)

Event Based Load Shedding (EBLS)

i) Generation Deficiency Compensation (GDC)

The GDC subsystem consists of Intelligent Electronic Devices (IED), Real Time Application Platform (RTAP -

distributed automation processors) and Remote I/O (RIO). It also utilizes IEC 61850 communication namely

GOOSE (Generic Object Oriented Substation Events) and IMAGE Protocol for RTAP to RTAP communication.

GOOSE is part of GSE (Generic Substation Events) which is a control model that provides a fast and reliable

mechanism of transferring event data over entire substation network. In this GDC subsystem, GOOSE protocol is

used for communication between IED to RTAP and RTAP to RIO. IMAGE protocol is communication protocol

specifically for RTAP to RTAP by sharing the real time database (RTDB) between the two RTAPs. It provides

almost instantaneous communication between the devices. The following Table 1 describes the function of the

devices in more details:

Table 1

Stage Equipment Description

1 Intelligent

Electronic Device

Intelligent Electronic Device (IED) for this subsystem is located at

BL01 and BL04 (both ends).

1. Calculate in real time the loading of BL01 and BL04

2. Trigger signal 1 when there is sudden load loss from above 700

MW

3. Trigger signal 2 when there is sudden load loss from above 1000

MW

4. Transmit signal 1 and signal 2 to RTAP (BDET) via GOOSE

protocol

2 RTAP (BDET) RTAP (BDET) is dedicated for the detection of events on 500 kV

system. It is located near the monitored transmission lines.

1. Receive signal 1 and signal 2 from IED via GOOSE protocol

2. Transmit signal 1 and signal 2 to RTAP (GDCS) via IMAGE

protocol

3 RTAP (GDCS) RTAP (GDCS) is dedicated for the operation of HVDC and hydro

power plants. It is located near the HVDC station.

1. Receive signal 1 and signal 2 from RTAP (GDCS) via IMAGE

protocol

2. Transmit signal 1 and signal 2 to RIO via GOOSE protocol

4 RIO RIO is located at HVDC and hydro power plant.

1. Receive signal 1 and signal 2 from RTAP (GDCS) via GOOSE

protocol.

2. Transmit signal 1 to DCS at HVDC and hydro power plant which

trigger HVDC Run Up/Run Back and hydro ramp up.

ii) Overload Protection and Generation Shedding based on Dynamic Line Rating (DLR)

Following the loss of 500kV transmission lines BL04 and BL01 or B05 and BL06, 275kV transmission lines

DL09, DL10, DL11 and DL12 as well as super grid transformers XGT1, XGT2 and XGT3 will become

significantly overloaded as previously described in network analysis. This may cause cascading trippings at the

respective lines and transformers which consequently resulting in frequency instability due to loss of large

generation from JMJG and SGRI. Therefore, the overload protection based on dynamic line rating (DLR) is

developed to prevent the loss of 275kV transmission lines and transformers by determining the actual dynamic

line rating based on true ambient condition which extends the capacity of the transmission lines more than the

static line rating capacity. The temperature of the conductor is calculated in real time using the dynamic line rating

algorithm developed by TNB.


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Therefore, the overload time can be prolonged until the conductor temperature reaches the preset value. Should

the overloading condition persists, controlled generation shedding will be executed either by deloading or tripping

the generators. The selection of generators to be shed or tripped is determined optimally by taking into

consideration the percentage of overload at the transmission lines and each generator’s loading. Therefore,

frequency instability can be prevented. The following Table 2 explained the normal step by step procedure of how

this advance protection works:

Table 2


1 Intelligent

Electronic

Device (IED)

Intelligent Electronic Device (IED) for this subsystem is located at DL09, DL10,

DL11 and DL12 (both ends) and XGT1, XGT2 and XGT3 (HV and LV).

It measures the total loading of the four lines and send it to RTAP (DDET) for

Conductor Temperature Computation (CTC) via MMS protocol.

The IEDs also transmit the severity of the overloading condition based on the

following characteristic via GOOSE protocol:

Element Loading Level Characteristic

49L Low overload level

I >110%

Time delayed Definite Time

Overcurrent

49H High overload level

I > 150%,


Overcurrent

For transformer, the IEDs monitor the winding temperature alarm and transmit the

alarm to RTAP (DDET) once activated via GOOSE protocol.

2 RTAP

(DDET)

RTAP (DDET) is dedicated for the detection of events on 275 kV system. It is

located near the monitored transmission lines.

It calculates initial conductor temperature at DL09, DL10, DL11 and DL12 lines

based on the followings:

load current

ambient temperature

solar radiation

estimated wind speed and angle at critical tower span

The Conductor Temperature Computation (CTC) which is 49M element predicts

the time it takes for the transmission lines conductor to reach certain values before

tripping of generator unit is inevitable. Its function will be elaborated more.


49M Medium overload level

130% < I < 150%

Conductor Temperature

Computation

RTAP (DDET) will determine whether the overload condition is due to N-1 BL04

loss by receiving the tripping signal of BL04 or BL01 from RTAP (BDET). RTAP

(DDET) subsequently compares the information from RTAP (BDET), IEDs and

its CTC and sends control signal to RTAP (JGSS) and RTAP (SGSS) at the JMJG

and SGRI power plants whether to trip or deload generator units based on the

severity of overload determined previously.


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3 RTAP (JGSS)

RTAP (SGSS)

RTAP (JGSS) and RTAP (SGSS) are dedicated for the control and monitoring of

the JMJG and SGRI power plants respectively. It is located near the power plants.

Both RTAPs create generation shedding table which automatically selects the most

optimum generators based on the severity of the transmission lines overload given

by RTAP (DDET) or that are armed manually by the control center.

Based on the information received from RTAP (DDET) and the generator selected,

it will send control signal to the selected generators whether to deload or trip via

GOOSE protocol.

5 RIO RIO is also located at JMJG and SGRI power plant.

1. Receive deload or trip signal from RTAP (JGSS) or RTAP (SGSS) via

GOOSE protocol

2. Transmit deload or trip signals to DCS at JMJG or SGRI power plant.

The following Figure 9 illustrates the response action of the overload protection system according to loading

level:

49L

49M

49H

5S

30m

Initiate Load Shedding

Initiate Alarm and Load Reduction



60s

Figure 9 – Response Action

Legends:


49L Low overload level

I >100%


Overcurrent

49M Medium overload level

100% < I < 150%

Conductor Temperature

Computation

49H High overload level

I > 150%,


Overcurrent

The 49M element is a Conductor Temperature Computation (CTC) type which is Dynamic Line Rating (DLR)

using measurement of some or all of the followings:

Ambient temperature

Solar radiation

Wind speed and

Wind angle

The 49M is based on Inverse Time characteristic. As the degree of overload increases, the allowable overload

duration decreases. The general relationship between overload rate and allowable duration is shown in the

following graph:


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Figure 10 – Inverse Time Characteristic

iii) Event Based Load Shedding Scheme (EBLS)

With the coming of another 1000 MW generator (U5) at JMJG and another 1200 MW generation power plants in

the further northern region, GDC scheme might not be sufficient to arrest the frequency instability upon N-2 BL01

and BL04 contingency. The situation will be further aggravated if the tie line connecting Malaysian and Singapore

power grid trips on overload. Therefore, load shedding in the load center must be done in order to prevent UFLS

from operated. This is because the load shedding quantum for UFLS is a lot more than EBLS. Based on the

simulation conducted, the load shedding quantum will be a lot less if the load shedding is operated faster (upon

detection of N-2 contingency) and before the tie line trips on overload. EBLS involves in total 28 132kV

substations in central and southern regions with total combined load of approximately 1500 MW. It is divided

into 3 preset groups depending on the contingencies and power flow. The following Table 3 describes the function

of the devices in more details:

Table 3


1 Intelligent

Electronic Device

Intelligent Electronic Device (IED) for this subsystem is located at 28

substations in central and southern region.

1. Calculate in real time the loading of the substation

2. Send the loading data to RTAP (EBLS) via MMS protocol

3. Receive tripping signal from RTAP (EBLS) if its loading is

selected and energize the trip coil.

2 RTAP (EBLS) RTAP (EBLS) is dedicated for the control and monitoring of all 28

132 kV substations involved.

1. Receive 500 kV contingency signals from RTAP (BDET) via

GOOSE protocol as previously described in GDC subsystem

2. Receive loading data from IEDs at 28 substations via MMS

protocol

3. Calculate the optimum number of load based on the given

quantum and loading data.

4. Preset the loads into 3 groups of operation using selection

algorithm.

5. Send tripping signal to the selected load following contingency

via GOOSE protocol.


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4. System Implementation

The overall system architecture of this frequency stability remedial action scheme named E-ATTEND

(Enhanced Ayer Tawar Tripping and Deloading) which consists of the Detection part, Calculation part and

Control part as shown in figure below:

ATWR_BDET_RTAP1

LAN 5

BTRK

BRTK_BL05_IED9

BRTK_BL06_IED10

LAN 5

KPAR_HVXGT4_IED31

KPAR_HVXGT5_IED33

LAN 5

KPAR

PGAUPGAU_RBCK_

HIED1

PI Historian

COIT_SMGT_PORTAL

CONTROL ROOM

ATWR

COIT_SMGT_RTAP13

PLTG

ATWR_BDET_RTAP2 ATWR_DDET_RTAP3

LAN 23

ATWR_DDET_RTAP4

LAN 24

BRTK_HVXGT6_IED43

BRTK_HVXGT7_IED45BRTK_HVXGT8_IED51

JMJG

JMJG_BL01_IED1

JMJG_BL02_IED2

LAN 5

JMJG_U4_IED37

JMJG_BL13_IED35

JMJG_BL14_IED36

BTRK

BTRK_BL01_IED7

BTRK_BL04_IED8

LAN 6

BTRK_LVXGT6_IED44

BTRK_LVXGT7_IED46BTRK_LVXGT8_IED52

ATWR_BL13_IED3

ATWR_BL02_IED4

ATWR_BL14_IED5

ATWR_LVXGT1_IED26ATWR_LVXGT2_IED28ATWR_LVXGT3_IED30

KPAR

KPAR_BL05_IED11

KPAR_BL06_IED12

LAN 6

KPAR_LVXGT4_IED32

KPAR_LVXGT5_IED34

LAN 6

ATWR_BL04_IED6

ATWR_HVXGT1_IED25

ATWR_HVXGT2_IED27

ATWR_HVXGT3_IED29

BTRK

BTRK_DL11_IED17

BTRK_DL12_IED18

LAN 23

BGJH

BGJH_DL15_IED39

BGJH_DL16_IED40

LAN 23

JJNG

JJNG_DL07_IED21

JJNG_DL08_IED22

LAN 23

SGRI

SGRI_DL09_IED13

SGRI_DL10_IED14

LAN 23

PAPN

PAPN_EL17_IED47

PAPN_EL18_IED48

LAN 23

ATWR_DL07_IED23

ATWR_DL08_IED24

KULN

KULN_DL11_IED19

KULN_DL12_IED20

LAN 24

BTRK

BTRK_DL15_IED41

BTRK_DL16_IED42

LAN 24

BJGH

BGJH_DL09_IED15

BGJH_DL10_IED16

LAN 24

PKID

PKID_EL17_IED49

PKID_EL18_IED50

LAN 24

NLDC

NLDC_SMGT_RTAP12

CORPORATE

NETWORK

COIT_SMGT_RTAP17

NEXGATE

DATA

CENTRE

Figure 11 – Overall System Architecture

The lower part of the architecture depicts the detection part while the upper part represents the control part. The

philosophical architecture for this scheme required every functional devices such as IED, RTAP and

communication LAN must have at least one redundancy. Communication is very crucial in the operation of this

scheme whereby 3 LAN connection is used to connect all the RTAPs. Meanwhile, each substation and power

plants also have their own internal LAN for communication to the RTAP. LAN 3 specifically is used as

monitoring LAN to gather all the information in this scheme and feeding it to the control center. All the

information is also recorded in the historian server and can be reviewed at any time. Access to the system is

controlled using special portal for protection against cyber security threats. Not included in the picture is the

architecture of Event Based Load Shedding Scheme which comes later after the second 1000 MW generator (U5)

at JMJG is commissioned.

The key success factor for this project is the Real Time Application Platform (RTAP). RTAP is TNB own

distributed processing unit developed in house by TNB Research group. The RTAP supports interface to other

Intelligent Electronic Devices (IED) using IEC61850, IEEEC37.118, IEC870-5-101 and Modbus protocols.

RTAP to RTAP communication enables the formation of distributed system architecture. RTAP also supports

formation of user defined algorithm and programmable logics which make it flexible to provide solution to power

system problem. All the algorithms that are applied to the RAS system such as Conductor Temperature Calculation

(CTC), Generation Deficiency Compensation (GDC) selection, and Event Based Load Shedding (EBLS) selection

run in RTAP mathematical engine. Figure 12 below illustrates the block diagram of RTAP internal structure.


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Figure 12 – RTAP internal structure

5. Results

The E-ATTEND remedial action scheme has operated several times after it was commissioned end of 2014. This

is mainly due to the instability of the operation of the newly commissioned 1000 MW (U4) generator at JMJG.

Figure 13 below shows an example of the scheme operation on November 2015 captured by the historian. Upon

the tripping of 1000 MW U4, the frequency has dropped rapidly with the lowest point of 49.623 Hz. IED

immediately sent triggering signal to HVDC and hydro generators and received within 15 milliseconds. HVDC

station immediately starts to import power (Run Up) from neighboring EGAT from 30 MW to 285 MW. 4 hydro

generators also ramped up from synchronous condenser mode until it reached close to 140 MW each. System

frequency is then recovered to its nominal value after approximately 23 seconds.

Figure 13 – Scheme Operation and Response


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6. Conclusions

The E-ATTEND project is considered a highly successful remedial action scheme (RAS) solution to manage

constraints in TNB’s grid system. The scheme has operated several times since its commissioning in 2014 and

saved the grid system from frequency instability problem or unnecessary load shedding. Therefore, the reliability

of the scheme is proven. In terms of cost savings, the returns of investment (ROI) of this project took only several

months given the fact that it has released the cheap generation that is previously supposed to be curtailed due to

N-2 contingency. In addition, by preventing unnecessary UFLS operation, the scheme has not just save TNB from

revenue loss, but also its image as the biggest utility company in Malaysia. Even though, in this case the system

serves as a temporary measure before the line upgrading project is completed, this same concept can be applied

elsewhere where contingencies beyond (N-1) is involved. This is so as no utilities would expand their transmission

networks to withstand contingencies beyond (N-1) as it would be very costly. New transmission projects can also

be deferred thus saving CAPEX cost in the long run.

In conclusion, the remedial action scheme (RAS) discussed in this paper is an invaluable tool to TNB in terms of

short term solution to operational issues; on the other hand the potential applications are limitless and can be

expended to be one of the options for planning the grid expansion. The Grid System Operator (GSO) now is able

to operate the power grid more cost effectively and therefore boosts confidence among the TNB top management

to adopt more RAS in the future.

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[8] J. Tang, J. Liu, F. Ponci, and A. Monti, “Adaptive load shedding based on combined frequency and

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IMPLEMENTATION OF REMEDIAL ACTION … OF...Implementation of Remedial Action Scheme for Frequency Stability in TNB CEPSI 2016 1 IMPLEMENTATION OF REMEDIAL ACTION SCHEME FOR FREQUENCY