토마스쥬마칼렌가
(Thomas Tsuma Kalenga)
1iD
장중구
(Choong-koo Chang)
†iD
-
(Dept. of Nuclear Power Plant Engineering, KEPCO International Nuclear Grduate School
(KINGS), Korea.)
Copyright © The Korean Institute of Electrical Engineers(KIEE)
Key words
Medium voltage motor, Ground fault relay, Surge arrester, VCB switching surges, and Surge simulation
1. Introduction
The reliability in operation of the auxiliary system of the nuclear power plant (NPP)
is important in the safe production of power. The reactor coolant system (RCS) provides
control of the reactor as well as a means to transfer heat energy to the steam generator.
When the reactor coolant pump (RCP) is inoperable for more than 400ms, the plant has
to go on shutdown mode. This not only affects the plant safety and reliability but
also costly due to loss of production leading to loss of revenue.
It is a general practice to protect large motors against external and internal overvoltage
transients. Surge arresters (SAs) protect operational equipment both from external
overvoltages caused by lightning strikes in overhead lines and from internal overvoltages
produced by switching operations or earth faults. SAs can degrade during its lifetime
through passage of surge currents, moisture ingress, contamination of external surface,
and overvoltages (1).
Vacuum circuit breakers (VCB) are the main isolation devices in medium voltage (MV)
switchgear. The VCBs generate high transient voltages during switching operations.
In the absence of SAs, such transients could damage the protected equipment if its
insulation withstand capacity is weak.
The nature and magnitude of the VCB switch surge in large AC motors have been greatly
studied. Naveat et al. (2) reported the influence of the connecting cable, circuit inductance, and capacitance
on the magnitude of VCB switching surge. The steep fronted surges appearing at the
motor terminals are due to closing prestrike voltages. Dick et al. (3) showed the probability of the motor experiencing prestrike voltage of different magnitude
based on the effect of breaker pole closing on the next pole to close. He also reported
the distribution of steep fronted surges on the stator winding and the inter-turn
insulation stresses in a motor due to vacuum switching. The magnitude of the surge
depends on motor and ground capacitances (4).
With several reactor shutdowns caused by SA failures and false operation of ground
fault relay (GFR) in the NPP, there is a need to perform an analysis of the switching
surges in this network and determine the cause of the false operation of the GFR.
2. Trip of RCP Motors by Ground Fault Relay
2.1 Medium Voltage Motor Protection Scheme
Reactor coolant pumps (RCP), circulating water pump (CWP), and main feedwater booster
pumps are typical loads of the 13.8 kV bus in the NPPs.
Figure 1 shows the relay and meter diagram of reactor coolant pump motor (RCPM) in a NPP.
According to the operational performance information system (OPIS) report of Korea
Institute of Nuclear Safety, RCPMs and other MV motors have failed to start or stopped
inadvertently due to malfunction of GFR (50GS) in recent years.
Fig. 1. RCP Relay and Meter Diagram
2.2 GFR Trip by Failure of Surge arrester
On September 3, 2015, failure of the SA during normal power operation caused GFR trip
and the RCP was stopped in Kori Unit 4 NPP. On January 3, 2019, the GFR was tripped
and the RCP was shut down due to the insulation failure of the SA, during normal operation
in Wolsong Unit 3 NPP (5,6).
2.3 GFR Trip by VCB Open/Close Surge
On Jan. 2018, RCP failed to start due to GFR (50GS) trip during the fourth overhaul
of Shin-Kori Unit 1. The cause of the failure was presumed to be a false trip of GFR
due to the VCB’s opening and closing surge. From 2004 to 2018, four more cases of
similar failures were reported at domestic NPPs. The GFR (50GS) is of the static type,
a relay that does not have a fault record function. Hence, there was a limitation
in analyzing the cause of the failure. The relay used for the ground fault protection
of the MV motor of Shin-kori Units 1 and 2 is ABB’s GR-5 model.
Research is being conducted to prevent false trip of GFR in the future, and in particular,
KHNP has requested relay makers to develop the intelligent ground fault relay that
has a high security and dependability.
3. Characteristic of Ground Fault Relay and Surge Arrester
It has been a general practice to install surge protection devices to motor circuits.
A motor requires surge protection, if it is exposed to lightning or capacitor switching,
is frequently started or its very critical to the process.
The RCPM is not exposed to lightning surges nor capacitor switching. It has a low
switching frequency, but is very critical to the operation of the NPP.
The crest value of the switching surges and rate of rise times in a motor circuit
depends on network parameters like the motor, the connecting cables, the switching
device, and other loads connected to the same busbar.
Thus, several factors might have caused the false operation of the GFR. The following
are analyzed:
∙ Wrong settings of the GFR,
∙ Unbalanced temporal overvoltage due to VCB switching surges,
∙ Failure of the SA,
∙ Unbalanced motor starting inrush currents.
3.1 Characteristics of Existing GFR
Ground fault protection consists of a core balance current transformer and a solid-state
ground relay. The pickup setting range of GR-5 is 5-50A and it is set at 30A tap.
Time delay settings are usually determined as part of a system coordination study.
When protecting individual loads, such as a motor, minimum settings are usually desirable,
but may not always be achievable. The time dial of the GFR for the RCP motor is set
to 0.2 seconds.
3.2 Characteristics of SA
A SA (suppressor) is a protective device for limiting surge voltages on equipment
by diverting surge current and returning the device to its original status. It is
capable of repeating these functions as specified (IEEE C62.11). During temporal over-
voltages (TOV) instances on the system, the arrester can see elevated voltages and
therefore higher 60Hz current through the unit. The magnitude and duration of the
system-generated TOV that the arrester can withstand is best expressed graphically
in Figure 2 (8).
Fig. 2. IEEE Temporal Overvoltages Capability with no Bracket
4. Coping RCP Trip by Ground Fault Relay
4.1 Surge Arrester Monitoring
If the insulation deteriorates due to aging of the SA, a breakdown may occur. The
insulation should be measured regularly. Since it is not possible to measure insulation
manually, online measurement technology is essential.
Different online monitoring methods have been proposed for the metal oxide SA in literature
(7-10). Online monitoring is based on the measurement of the total leakage current during
steady state operation and decomposing this to its capacitive and resistive components.
The resistive component of the internal continuous leakage current is a good indicator
of MOVAs status (9). Excessive resistive leakage current could lead to thermal runaway.
Decomposition of the total leakage current to its components requires measurement
of the voltage at the arrester terminal. This is a challenge in MV systems and require
the use of voltage transformers. A method is proposed using the resistive component
of the total leakage current. It beats the voltage measurement challenge by using
an analog or software generated reference signal instead of the real voltage signal.
It is the most ideal method for existing SA systems.
Exposing any nonlinear resistor to a sinusoidal voltage produces current containing
first and higher order harmonics. The ampli- tude of the third harmonic depends on
the severity of the degree of nonlinearity. The third harmonic content measured at
the continuous operating voltage Uc increases by impulse degra- dation.
Lundquist et al. proposed an online monitoring method based on 3rd harmonic analysis
with measurement of the three-phase electric field for compensation to precisely determine
the resistive component of the harmonic (10). The main challenge to this method is its high cost of implementation and only work
for specific kinds of failure. It fails to detect an increase in the resistive current
due to linear decrease of resistivity caused by chemical reaction or moisture ingress.
Lira et al. proposed two online monitoring methods based on feature extraction of
the harmonic content of the total leakage current. These do not require separation
of the resistive com- ponents thus eliminating the need for voltage measurements.
From the extracted features, method one builds a feature database that relates the
harmonic components to the arrester operating status. Then the database is used in
training of a condition classification system based on artificial neural networks
(11). Method two organizes the features into patterns to be applied to the classification
system based on self-organizing maps (SOM) (12). Both methods claim an accuracy of 98\% for correct prediction.
The temperature of the metal oxide varistor (MOV) column is affected by TOV, moisture
ingress, power loss due to aging, and energy absorption when exposed to impulses.
Therefore, the temperature of the MOV column is the best indicator of the status of
the SA. Over temperature during continuous operation leads to power loss and is independent
of the voltage shape.
Hinrichsen et al. proposed a method that allows remote measurement of the temperature
by passive sensors inserted in the MOV column (13) that are based on wireless passive surface acoustic wave (SAW) temperature sensors.
These sensors do not require power supply or hardware connection to environment.
Therefore, the resistive current monitoring method is applicable for the case of existing
MV system of the NPP while the feature extraction method and temperature monitoring
using SAW sensors is applicable for new plants after proper safety analysis.
4.2 VCB Opec/Close Surge Unbalance
The MV switchgear in the APR 1400 NPP is operated by VCB. VCB are known to generate
high switching surges. The voltage transients caused by the close operation have different
magnitudes for the different phases depending on the closing instant of the circuit
breaker poles. This unbalance could lead to operation of the GFR.
If the surge absorber fails or is removed from the circuit, it is necessary to verify
that the magnitude of VCB open/close surge generated will not destroy the insulation
of the motor winding and that the GFR will not malfunction. Figure 3 is the example of simulating surge generation during VCB closure using EMTP-RV(6).
Fig. 3. Vacuum Circuit Breaker Closing Surge Simulation Result
5. VCB Switching Surge Simulation and Basic Insulation Impulse Level of RCP Motor
Analysis
To determine whether the RCPM insulation rating can withstand the VCB open/close surges,
simulations of the network switching surges was necessary. This was achieved using
EMTP-RV software. The following subsections describe the process.
5.1 Network Description
Figure 1 showed a single line representation of the motor supply circuit. The RCPM is connected
to the 13.8 kV busbar via two- 500MCM, 100m single-core EPR coaxial cables. The SAs
are connected at the cable termination point in the switch- gear.
Figure 4 shows the equivalent circuit for the network indicating the parameters considered
during modeling.
Fig. 4. Simple Equivalent Circuit Representation of the Network
Where Rsc and Xsc are the real and imaginary parts of the short circuit impedance,
Lb, and Cb are busbar inductance and capacitance while Rc, and Cc are resistance and
capacitance of the cable respectively. Rlrm and Xlrm are the real and maginary components
of the motor locked rotor impedance.
5.2 Simulation Model
Frequency-dependent models of all the equipment in the configuration were modeled
in EMTP-RV for simulation of the system switching surges. Input parameters of the
different equipment are described in the following subsections.
Source parameters for the EMTP-RV model are taken from the short circuit analysis
of APR 1400 model simulated in ETAP at the 13.8KV busbar.
∙ Three-phase short circuit current = 39.74 kA
∙ Rated Voltage = 13.8 kV
The busbar stray capacitance and lumped inductances were approximated using [IEEE
C62.21-2003] and are as follows (14).
Busbar inductance Lb = (1+0.05n ) µH = 1.25µH for a center breaker
Busbar stray capacitance = 3200pF (15)
Where n is the number of parallel circuits connected to the same 13.8 kV busbar
The nameplate rating of the RCPM is ;
∙ Motor rating 15000HP
∙ Rated voltage 13.2 kV
∙ Starting power factor 0.17
∙ Starting current 5.76IN
∙ Running power factor 0.923
∙ Motor efficiency 97.8
∙ Motor slip at full load current 0.91
The RCPM basic insulation impulse level rating is based on the IEC 60034-15, 2009
(16). Having a voltage rating of 13.2 kV, the ratings are as follows
∙ Rated lightning impulse withstand voltage (1.2/50µs) wave Up =4V+5 kV=58 kV
where V is the rated voltage.
∙ Rated steep-front impulse withstand voltage (0.2s wave) U’p = 0.65Up=38 kV
The frequency-dependent model of the SA was adopted. It consists of non-linear resistances
𝐴0 and 𝐴1, separated by an R-L filter as shown in Figure 5
The values of the different elements were calculated using the arrester dimension
(17,18).
For this study 𝐴0 and 𝐴1 are taken from the reference paper (19) and the computed values of other parameters are shown in Table 1.
Table 1. Surge Arrester Parameters
|
Parameters
|
$R_{0}$-Ω
|
$L_{0}$-µH
|
$R_{1}$-Ω
|
$L_{1}$-mH
|
C-nF
|
|
Values
|
14
|
28
|
9.1
|
1.47
|
71.42
|
Fig. 5. Frequency-dependent Surge Arrester Mode
The behavior of the VCB is determined by; the chopping current magnitude, the high-frequency
current quenching, and the cold withstand voltage (20).
The chopping current depends on the contact material and the load surge impedance
(21). With the advancement in technology, most VCB contact material is copper chromium
that has a chopping current value of between 3-5A.
The VCB model proposed by Borghetti et al. (22) was used in creating the network model.
The dielectric strength (U) linked with VCB contacts was assumed linear and implemented
through the equation (1).
Where t is the contacts separation time, A is the rate of dielectric strength rise
and B is the transient recovery voltage withstand voltage just before contact separation
(22). The high-frequency current quenching capability of the VCB is defined by equation (2) (23).
Where C is the rate of rise of the VCB high-frequency quenching capability and D is
the VCB quenching capability just before contact separation.
The parameters A,B,C, and D are dependent on the specific VCB (22). The following typical values were assumed (21):
chopping current = 3.5 A
A = 1.3 × 10 V/s
B = 0.69 × 10
Ub_limit = 31.2 kV
C = 32 × 10A/s
D = 155 × 10 A/s
Arc quenching limit = 135 × 10 A/s
The model also includes an RLC branch in parallel to the VCB to take into account
the open contact gap stray capacitance, resistance, and relevant inductance. The value
of the RLC branch parameters are typically;
𝐿𝑠=50µH, C=100 p𝐹, and 𝑅𝑠=100 Ω
6. Simulation and Analysis Results and Discussion
From the simulations, the magnitude of the expected surges during VCB closing is below
the motor insulation voltage withstand rating. Table 2 and Table 3 gives a summary of the closing and opening surge results for different modes of operation.
The motor is exposed to higher surges when cable is grounded on both ends as highlighted
in Table 3. The surge voltage at phase ‘C’ rises close to motor withstanding capability (5.39p.u).
Figure 6 depicts the closing and opening surges experienced by the motor. The least surge
occurs when the cable is grounded at the motor end, agreeing with (24).
Table 2. Closing Surges
|
Surge Voltage Vn(kV) on CB
|
|
Condition
|
Closing
|
p.u
|
RRRV
(V/s)
|
Phases
|
|
Without SA, cable grounded motor end
|
10.99
|
1.02
|
4.68E+08
|
A
|
|
15.95
|
1.48
|
1.14E+09
|
B
|
|
15.95
|
1.48
|
6.68E+08
|
C
|
|
Without SA, cable grounded at motor & switchgear both ends
|
12.29
|
1.14
|
5.88E+08
|
A
|
|
17.13
|
1.59
|
1.17E+09
|
B
|
|
15.09
|
1.40
|
6.91E+08
|
C
|
|
With SA at switchgear, cable grounded motor end
|
11.32
|
1.05
|
5.71E+08
|
A
|
|
15.95
|
1.48
|
1.29E+09
|
B
|
|
13.47
|
1.25
|
6.19E+08
|
C
|
[Note] 1 per unit = $\sqrt{2/3}$× motor rated voltage
The reference rated rate of rise of voltage (RRRV) are 4.833 × 10 and 1.90 × 10 for
lightning impulse and steep-front withstand voltage respectively.
Table 3. Opening Surges
|
Surge Voltage Vn(kV) on CB
|
|
Condition
|
Opening
|
p.u
|
RRRV
(V/s)
|
Phases
|
|
Without SA, cable grounded motor end
|
9.48
|
0.88
|
4.32E+08
|
A
|
|
10.45
|
0.97
|
5.19E+08
|
B
|
|
9.59
|
0.89
|
3.79E+08
|
C
|
|
Without SA, cable grounded at motor & switchgear both ends
|
21.23
|
1.97
|
1.22E+09
|
A
|
|
20.80
|
1.93
|
1.18E+09
|
B
|
|
45.58
|
4.23
|
5.08E+09
|
C
|
|
With SA and motor terminal, cable grounded motor end
|
9.92
|
0.92
|
4.06E+08
|
A
|
|
10.56
|
0.98
|
4.93E+08
|
B
|
|
9.59
|
0.89
|
4.01E+08
|
C
|
Fig. 6. Closing and Opening Surges
Fig. 7. Comparison of Closing Surge with Rated Motor Impulse Insulation Withstand
Fig. 8. Ground-fault Current due to the Closing Surge Imbalance
The rate of rise of these surges were found to be lower than the RRRV for the motor
insulation withstand. A comparison is shown in Figure 7.
The ground-fault current due to the closing surge imbalance was captured in Figure 8; it has a high magnitude but is quickly damped by the system impedance. Thus does
not activate the GFR.
7. 결 론
Industrial practice of motor installation recommends using SA to protect the motor
from VCB open/close surges. After experiencing frequent RCPM trip due to failure of
SA, this study was conducted to verify the effect of VCB open/close surges on RCPM.
Expected surge analysis confirmed that the RCPM can be operated continuously without
SA. Frequent exposure to steep surges, however, may shorten motor life. In particular,
if the cable shield is grounded on both sides, the surge can severely damage the motor
insulation.
In summary, based on the study, the following conclusions were made:
The magnitude of the switching surges are below the motor insulation withstand rating.
The zero sequence current I induced on the stator due to the imbalance of the switching
surges is high (160-180A) but highly damped (50ms) hence cannot trip the GFR.
Due to the low switching frequency of the RCPM, the probability of restrikes is
also low, thus the SAs can be removed from the circuit to increase reliability until
a proper SA monitoring scheme is installed. The cable should be grounded only at the
motor end.
The RCPM has a long starting time (about 10 seconds), unbalance inrush currents
may activate the GFR. Therefore, adjustments of the GFR delay time is recommended.
Further research will be continued to develop an effective SA monitoring scheme.
Acknowledgements
This research was supported by 2020 Research Fund of the KEPCO International Nuclear
School (KINGS), Ulsan, Republic of Korea.
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저자소개
He received a M.S. in Nuclear Power Plant Engineering from KEPCO International Nuclear
Graduate School (KINGS) in 2021.
His research topic was ‘Mitigation of VCB Switching Surge and Ground Fault Relay Malfunction
on Medium Voltage Switchgear’.
He has been working for Kenya Power and Lighting Company (KPLC) for about 7 years
before joining KINGS. He holds BSc in Electrical and Electronics Engi- neering from
University of Nairobi, Kenya.
He received a M.S. in Electrical Engineering from Inha University in 1990, and a Ph.
D degree in Electrical Engineering from Myongji University in 2001.
He participated in the nuclear power plant design projects from 1985 to 1993 at KOPEC.
From 1993 to 1998 he worked as a senior engineer for Samsung Electronics’ plant control
and automation busi- ness team.
He was vice president and CTO of Sangjin Engineering from 2001 to 2012.
Since 2013, he has been a professor at the NPP Engineering Department at KEPCO International
Graduate School. (KINGS).