Analysis of the Primary Restorative Transmission System

Copyright © IFAC Power Plants and Power Systems Control, Seoul, Korea. 2003

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ANALYSIS OF THE PRIMARY RESTORATIVE TRANSMISSION SYSTEM H.J. Lee, S.M. Park, K.S. Lee,

Department ofElectrical Engineering Kwangwoon University, NPT Center

J.C. Bai, B.W. Whang

I.J. Song, N.H. Lee,

Power System Operation Criteria Team Power System Stabilization Group Korea Power Exchange Korea Electric Power Research Institute

Abstract: Service restoration following a complete or partial collapse starts with reenergizing a transmission line from black-start generators. Ferranti effect and line charging problems can arise as unloaded transmission line is reenergized. This paper presents analytical results on the primary restorative transmission system focused on the problems during the early restoration process. Methodologies are presented that handle load pick-up to compensate Ferranti effect, and terminal voltage and reactive capability limitation of black-start generators about self-excitation. An algorithm for static analysis about these methodologies is developed, and EMTDC simulation for verifying the efficiency of the algorithm is performed. Copyright © 2003 IFA C Keywords: blackout, subsystem, reenergization, black-start

the planning of PRT systems. The efficiency of the algorithm is verified using EMTDC simulation.

1. Introduction As the restoration method is dependent on the fault, voltage level, the structure of transmission network, and generators of the country, the strategy is different in each country. However, it is clear that during the fust stage of restoration, Black-Start Generators (BSGs) which usually are hydro type supply some generators of big capacity through so called Primary Restorative Transmission (PRT) lines in case of wide area blackout.

2. Charging Capacity of the PRT line Overvoltage can be induced at the PRT line by Ferranti effect as unloaded transmission line is reenergized, and this problem should be compensated by lagging reactive power such as picking-up loads at substations. Therefore, charging capacity of the PRT line can be found when compensation by load pickup and reactance of transformers and lines of the PRT system is considered.

The static analysis of the PRT systems can be focussed on three problems. One is the overvoltage of the PRT systems, another is the PRT line charging, and the other is the reactive capability of a BSG. In the researches about the problems, (Adibi et aI., 1987a; Adibi et aI., 1987b; Adibi et aI., 1991; Adibi et aI., 1994a) proposed methods, which are energizing fewer high voltage lines, operating generators at minimum voltage levels, deactivating switched static capacitors, connecting shunt reactors, and adjusting transformer taps to appropriate positions to prevent generator self-excitation by balancing reactive power, (Morin, 1987; Huang et aI., 1991) showed voltage profile of PRT lines considering picking-up loads, and (Adibi et aI., 1994b) showed quantitative Reactive Capability Limitation (RCL), but did not consider generator pole type.

2. I Load Pick-up at Substations

Single line diagram consisting of two BSGs, step-up transformers, and TILs separated to several sections is shown in Fig. 1. Each section of the PRT line is

[;;]<[~~+~~YNll+:;YN ][~:~] ZN

=[ IVNratedl~J·

(I)

(2)

SZNL
IZN =

vN

ZN

In this paper, we proposed the advanced static analysis about PRT line charging capacity considering the voltage profile and lagging reactive power compensation by reactance of lines and transformers, and RCL considering generator pole type. An algorithm is developed that can be used in

where Vg =

Terminal voltage of the BSGs

SLL
House load of the BSGs Load picked-up at each substation Rated voltage at the marked VN bus

SZNL4>ZN =

vNrated = 519

(3)

3. RCL Of Generators

~y~J.~."'"'~ y~~ y

The RCL of BSGs should be larger than charging capacity of the PRT line to prevent self-excitation, and this RCL is variable depending on rotor type of the BSG. When a limitation( Qlim ) by stator end core heating is given, let the Qlim is the RCL of the BSG if it is smaller than the RCL found by next equations.

.

s.';;~

Fig.!. Single Line Diagram ofa PRT system expressed as a pi network, load pick-up at each bus is arbitrary, and house load is supplied from step-up transformer's secondary side, the marked ~ bus. Vend , the voltage of the PRT line end which generators of big capacity located in, can be set by manual operation, and I RN is zero on condition that a PRT line is reenergized initially. vN and ISN from Eq. (I), and IZN from Eqs. (2) and (3) can be found. As Eqs. (1)-(3) are applied for each section of the PRT line, VI, ISI, IZI and Istation are also fow!d.

3.1. Non-Salient Pole A rotor of steam unit has usually non-salient type, and a RCL of a generator having a non-salient pole is found as follows. Apparent power which a generator supplies is expressed by Eq. (8). The simultaneous equation is set from P, Q ofEq. (8), and the RCL can be found from Eq. (9). In case the generator is connected to a infinite bus, 8 is normally keep within 20° to prevent loss of synchronism under normal operating condition. But, loss of synchronism is not be considered in case a T/L is reenergized initially. Therefore, 8 max can be 90° , and the RCL of the BSG is expressed by Eq. (10).

2.2 Terminal Voltage ofthe BSGs Korean Power Systems adopt the all-open switching strategy except the CBs which are inserted in a PRT line to make power system structure simple after wide area blackout. When a T/L is reenergized under all-open switching condition, overvoltage can be induced at the receiving end. This overvoltage can cause the dielectric problem of the buses and transformers of each substation, and generators of big capacity which are connected to the receiving end of the T/L. So, terminal voltage of the BSGs should be determined to keep voltage of the all components of the PRT system under operating limit.

- -" 1

ErV ]·

S=P+jQ=vI =v - . jXd

2

VEl [VEj V ] =--sin8+j --cos8-Xd

Xd

(8)

Xd

where, Xd =

Ef

Synchronous reactance

= EfL8 (Generator internal voltage)

V= VLO° (Generator terminal voltage) Terminal voltage of the BSGs is found from Eqs. (4)-(7) showing relation between some factors, which are sending voltage( VI), step-up transformer impedance, and current through jXT' Also, voltage ratio of the transformer should be considered. Let ISI + IZI + Istation is Is· IgI +lg 2 =Is

(4)

V -Vt V -Vt I - g I - g gl - jXTJ ' g2 - jXT2

(5)

V -Vt V -VI I gl +lg2 =-g-.-+-~--=Is JXTJ jXT2 V g

=V,

I

+1 (jX TJ ·jXT2 ) S jXTJ + jXT2

v2

1

Q =--P-max tan8max Xd

v2

Qmax =- X

d

3.2. Salient Pole

(6) (7)

2.3 Charging Capacity ofthe PRT line Charging capacity can be found when the lagging reactive components such as picked-up load and inductance of T/Ls and transformers are considered. Therefore, T/L Charging Capacity (TCC) to consider compensating effect is the same as reactive power which BSGs supply. Current from each BSG can be found when Vg of Eq. (7) is substituted for Eq. (5).

Fig. 2. Phasor diagram for a salient pole

520

(9) (10)

A rotor of hydro unit has commonly salient type, and a generator's internal voltage having this type is expressed by Eq. (11). Eq. (12) is substituted for Eq. (11). Eq. (14) shows that AXrXq'f,i is on q axis. Eq is the same because angle of El is 0 from Eq.

(13). Eq. (17) is classified into active and reactive power. Eq. (20) is substituted for Eq. (19). When omax is 90° , the RCL of the generator which has a salient pole is expressed by Eq. (21). (11) where

1= IL -I/'

(Generator terminal current)

=IdLo _90° (d axis component of I) Iq = IqLo (q axis component of I) Id

(12)

Iq =I-Id

El = AXrxq'f,i + jXql+V = AXrXq'f,i+Eq (13)

Fig. 3. Static analysis algorithm flow chart

AXd - xq}i;; =(Xd - XqYdL(900+0 -90°) =

Id = I sin(o +1/') El

network is shown in Fig. 4. Voltage profile and charging capacity of the PRT line, and the RCL of BSGs are analyzed. EMTDC simulation is performed to verify this analysis and consider switching dynamics.

(14)

(XrXqYd LO

(15) (16)

= Eq +(XrXqYd

S=Vl =V[IqLO-j!dLO)" = VL-O[Iq +j!dl vsino Ef-VCOSO) = (Vcoso-jVsino - - + j - - ' - - - { Xq Xd 2

(1 1)

VEf v - - - sin20 P=--sino+Xd 2 X q Xd 2

v

2

+2

(1

5.1 Static Analysis Reactive power supplied by BSGs, its margin, and power angle and terminal voltage of BSGs are shown in Table 1, and receiving end voltage considering switching transient voltage based on EMTDC simulation is set. Voltage profiles of No.l primary TIL are shown in Fig. 5.

(18)

2

Q= VEf cosO_V 2 (COS 0 _sin 0) Xd Xd Xq VEl P Xd = sine

(17)

1)sin20

Xd - x q

sino

v2

Qmax =-Xq

(19) (20)

5.2. EMTDC Simulation

(21)

There is voltage transient state by CB switching when the PRT line is reenergized, and dielectric breakdown can be caused by excessive transient

4. Program Flow Chart The flow chart of an algorithm analyzing a PRT system is shown in Fig. 3. Whether a PRT system is available or not can be decided by conditions, which are comparing TCC with RCL of generators, and generator terminal voltage with UVR setting value. The PRT system is available when it only satisfies all of those two conditions.

Table 1 Study results of the sample network PRT

NO.2

NO.1

BSGNO

5. Case Study When a total blackout occurs, a decentralized restoration manner is commonly adopted to recover from it rapidly. KEPCO has the same strategy, so Korean power grid consists of seven subsystems for restoration. In this case study, one of them is selected as a sample network. The structure of the sample network is 521

2

Receiving end Vtg. [kV] [%]

330.766 95.87

Supplied Q [MVAR]

78.624

9.025

9.025

Qmargin [MVAR]

125.476

21.075

21.075

Power angle[ 0]

3.12

3.26

3.26

Generator terminal Vtg.[kV] [%]

16.239 90.22

152.990 99.34

11.393 86.31

BoRyunQSfT TeAnSfTo

#1-4

,..J3oRyungSfT

U#3-6

0

DangJinSfT

#1.2

MuJu

PIP

0 o

7172

7572 7672

BoRyung GfT#7.8

SinNamWonS/S

DeaChung

HIP

0

152-2

627 637

Fig. 4. The primary T/Ls at the sample network


Recei~ng End

42[MVAR]). Charging capacity of the PRT system and the RCL of the BSGs, voltage profile, and suitable terminal voltage of the BSGs are examined. Developed algorithm can be used in the planning of PRT systems.

95.87%>

ACKNOWLEDGMENT Authors would like to thank Korea Ministry of Science and Technology and Korea Science and Engineering Foundation for their support through ERC program.

Fig. 5. Voltage profile of the No.l primary T/L voltage. Therefore, the exciter of BSGs should be set to keep it within a operating limitation of voltage. Parameters used in EMTDC simulation are from PSS/E data.

REFERENCES M.M. Adibi et a1. (1987a). Power System Restoration - A Task Force Report, Vol. 2, No. 2, pp. 271277 IEEE Transactions on Power Systems. M.M. Adibi et a1. (1987b). Power System Restoration - The Second Task Force Report, Vol. 2, No. 4, pp. 927-933, IEEE Transactions on Power Systems. Gaston Morin (1987). Service Restoration Following a Major Failure on the Hydro-Quebec Power System, Vol. 2, No. 2, pp. 454-462, IEEE Transactions on Power Delivery. M.M. Adibi et a1. (1991). Power System Restoration Issues, Vol. 4, No. 2, pp. 19-24, IEEE Computer Applications in Power. I.A. Huang et a1. (1991). Power System Restoration Incorporating Interactive Graphics and Optimization, pp. 216-222, IEEE Power Industry Computer Applications Conference. M.M. Adibi et a1. (1994a). Special Considerations in Power System Restoration - The Second Working Group Report, Vol. 9, No. 1, pp. 15-21, IEEE Transactions on Power Systems. M.M. Adibi et a1. (1994b). Reactive Capability Limitation of Synchronous Machines, Vol. 9, No. 1, pp. 29-40, IEEE Transactions on Power Systems.

5.3. Voltage Profile

No.1 and No.2 PRT line's Voltage RMS errors between the proposed algorithm and EMTDC are about 0.5[%]. These results mean that the static analysis by the proposed algorithm and EMTDC simulation are almost the same.

6. Conclusion PRT systems are the basic skeleton for restoration when the power system grid is totally or regionwidely collapsed. In this paper, T/L charging capacity considering compensation by lagging reactive power caused by picked-up loads and inductances of PRT systems, and RCL of the BSG depending on the pole type are presented. In the case study, one of seven subsystems consisting of Korean power system is selected. The subsystem has no voltage problem based on the proposed algorithm, and there are enough reactive power margins(No.1 is about 125[MVAR], No.2 is about 522