2.1. Parte práctica
2.1.3. Informe II y III
2.1.3.2. Descripción de los ensayos:
This concrete case is based on an electrical circuit that has a resonance frequency near 50 Hz.
This example was recorded during an on-site test on the French grid. This interesting phenomenon appeared during the preliminary phase of a black-start restoration test. This phase consists in preparing the network in order to connect the source power plan to the auxiliary transformer of the target power plant.
Network Topology
The network topology at the time of testing is shown in Figure A-9. A section of the over-head transmission grid between the substations “S0” and “S4” was de-energized for the test. This sub-network was comprised of:
163 km of 400 kV circuit between the substations “S1” and “S4”, 30 km of 225 kV circuit between the substations “S0” and “S1”, a 405/240/21 kV autotransformer (Yyd winding) at S1,
two shunt reactors located in the substation “S0” (line reactor of 80 Mvar) and in the substation “S1”
(transformer reactor of 64 Mvar connected to the tertiary winding of the 600 MVA Auto-transformer).
It should be noted that the 400 kV circuit between “S2” and “S4” substations is of double-circuit construction (i.e.
two circuits on the same tower). In other words, the double circuit line goes from the substation “S2” to the substation “S4”. The parallel circuit (on the double circuit line) remained energized during this black-start restoration test.
Figure A-9 400 kV and 275 kV sub-network de-energized for black-start test
A few measurements have been realized in three different configurations for the presented network. These configurations are obtained by opening first the line circuit breaker then the disconnector. The following voltage measurements were obtained on the theoretically “de-energized” sub-network in the “S3” substation, as shown in Figure A-9:
S2 S3
S1 S4
64 Mvar
163 km in 400 kV S0
39 km in 225 kV
80 Mvar double circuit line
225 kV 400 kV
Auto transformer YYd 405/240/21 kV
Case #1 Case #2 Case #3
Urms=346 kV phase-to-phase (i.e. 0.865 pu on a 400 kV base)
measured at “S3” substation.
With the whole line (from the
“S0” to the “S4” substation to the “S4” substation including
the bus bars of all the substations).
Urms=253 kV phase-to-phase (i.e. 0.632 pu on a 400 kV base)
measured at “S3” substation – with the whole line but without
the busbar of the “S4”
substation because the circuit breaker of the line is opened.
Urms=282 kV phase-to-phase (i.e. 0.705 pu on a 400 kV base)
measured at “S3” substation with the disconnectors opened
in the “S4” substation.
EMT Simulations done to understand the phenomenon
In order to explain the near-resonance conditions observed during the field tests, the network topology was modelled using a EMT analysis tool. See Figure A-10.
The model includes a 160 km double circuit HV line section as follows:
The first circuit (circuit #1) is connected to the main grid at the nominal voltage (i.e. 400 kV).
The second circuit (circuit #2) is supposed to be de energized. It is only connected to an auto transformer with a tertiary winding connected to a reactor of 64 Mvar at the “S1” substation and to a line reactor of 80 Mvar in “S0”.
In this model, an uncertainty of 5% was taken into account to represent the phase to earth capacitances of the overhead lines.
S4 busbar circuit breaker disconnector S3
S4 busbar circuit breaker disconnector S3
S4 busbar circuit breaker disconnector S3
Figure A-10 EMT representation of the 400 kV and 275 kV sub-networks
The double line circuit is represented by a 3-phase PI section component that is computed from the geometrical configurations of the 8 conductors, their sections and their electrical characteristics.
Simulation results are shown in Figure A-11. We can note that:
- The voltages computed on the circuit “#2 are not equal to zero
- The 3 voltages are unbalanced and they don’t have the same module.
Figure A-11 Simulation Results
1 2
Parameter Influence
power load on circuit #1 No
Voltage level on circuit #1 Yes
Length of circuit #1 connected to the grid Yes Type of tower used for the double circuit Yes Phase to earth capacitances of the over head
lines
Yes
Neutral connection of the 64 Mvar reactor No (probably because of the tertiary winding of the auto transformer where the reactor is connected.
Use of the reactor on the tertiary winding of the auto transformer
Yes
A frequency scan analysis has been done in order to identify the natural resonant frequency of the sub network (Figure A-12).
Figure A-12 Frequency scan of the sub-network
The positive sequence frequency of the subcircuit is located just above 50 Hz. That confirms the hypothesis of a resonant circuit excited at 50 Hz by the main grid. The two peaks of the impedance are probably due to the fact that the lines are not transposed in the sub network.
50 100 150 200 250 300
0 0.5 1 1.5 2
x 105
frequency (Hz)
y
PLOT
ZIN2@zinmag@1
On site tests performed after the studies in order to check the hypothesis and the conclusions of the studies
On-site measurements were carried out the two 400 kV circuits:
Circuit #1 : connected to the grid
Circuit #2 : disconnected from the grid and off line.
Figure A-13 Detailed Network Topology during Field Measurements
The measurements were taken at “S3” 400 kV substation. Line currents and phase-to-ground voltages were measured on circuit #1 (connected to the grid) and on circuit #2 (disconnected from the main grid), which is the analysed resonant circuit.
The initial conditions were the following ones:
CB1: Open
CB2 and CB3: closed
Subnetwork from S0 to S4 isolated
The following switching sequence was carried-out starting from the above initial conditions:
1) CB1 is closed in order to connect the reactor to the tertiary winding of the autotransformer in station S1.
2) Open CB3 in order to interrupt the current in circuit #1. One can note that the voltage is still different than zero.
3) Open CB2 in order to fully de-energize circuits #1 and #2.
The recorded measurements on the 3 phases are presented in Figure A-14.
S2 S3
S1 S4
64 Mvar
163 km in 400 kV S0
39 km in 225 kV
80 Mvar double circuit line
225 kV 400 kV
Auto transformer YYd 405/240/21 kV
CB1
CB2 circuit #1 CB3
circuit #2
Figure A-14 Field Measurements
After opening CB2, circuit #1 is fully de-energized but there is still voltage in circuit #2. This is probably due to the grading capacitors of the circuit breakers located at the borders of the “off line” network (circuit #2) that still couple it with the main 50 Hz grid network.
One can note that the measured voltages are unbalanced probably because the double-circuit is not transposed along the length of tested circuit.
This is probably due to the non symmetrical impedance of each phase: this impedance depends on the height of the conductors and probably the distance between the conductors located on the two circuits.
Figure A-15 and Figure A-16 below show the voltage waveforms recorded during the connection of the transformer tertiary reactor (i.e. close CB1). It can be seen that the connection of the reactor creates a resonant circuit.
Furthermore, the three phase voltages are unbalanced and they don’t have the same magnitude.
Conclusions
The resonances observed on the isolated sub-network (i.e. circuit #2) are due to:
- The source of excitation for the isolated 400 kV and 225 kV subnetwork is mainly due to intercircuit capacitive coupling within the double circuit where separation between conductors from one circuit to the second is relatively small. However, there is also some capacitive coupling provided by the grading capacitors in the circuit breakers.
mesures_TEO: VGAU2_1 mesures_TEO: VGAU2_3 mesures_TEO: VGAU2_2
mesures_TEO: VCIR2_1 mesures_TEO: VCIR2_2 mesures_TEO: VCIR2_3
mesures_TEO: JCIR2_1 mesures_TEO: JCIR2_2 mesures_TEO: JCIR2_3
0 Raccordement self d'Arrighi au réseau d'essai
0
Ouverture départ Gatinais2 à Cirroliers
0.0
17:00 17:10 17:20 17:30 17:40
6/8/06
h:m
- The voltages on the circuit #2 would have been higher if the resonance frequency had been closer to 50 Hz.
The transient before the occurrence of the steady state resonance has not been studied, however, it was during this period that the highest overvoltages were measured and were considered to be hazardous from the standpoint of equipment insulation.
Figure A-15 Voltage Waveform at Circuit #2 during connection of transformer tertiary reactor
Figure A-16 Zoom-in Voltage Waveform at Circuit #2 during connection of transformer tertiary reactor
A.4 High Temporary Overvoltages when a Distribution-Connected Generator Energizes an Isolated Ungrounded & Faulted High Voltage System
Introduction
High temporary overvoltages can occur when distribution-connected generation (such as an IPP) temporarily isolates with part of the grid, as can happen during the clearing of transmission faults. The severity of the TOV is influenced by the following factors: (1) the size of the IPP generator, (2) the shunt capacitance of the isolated system, (3) whether or not the interconnecting utility transformer provides an HV neutral ground, (4) proximity of the first series resonance point, as seen by the generator, to power frequency and (5) the available damping provided by the connected load. Figure A-17 shows an electrical single-line diagram of an actual case which was the subject of an overvoltage study with an EMT program. A 30 MW IPP proposed to connect to the utility’s grid via a 12 km 25 kV express feeder to distribution Substation “P”. The substation, which supplies residential and small commercial loads, is tapped into a 137 km 230 kV overhead transmission line between Station “B” and Station “C”. The tap point on the line is located approximately at the middle of the line. The windings of the interconnecting transformer at Substation P are configured as LV grounded star and HV delta. The minimum distribution load supplied from Station P is 4.5 MVA and no other load is supplied by the transmission line. The relevant data for the express feeder and the generator step-up transformer and the substation transformer are shown on Figure A-17.
As part of a system impact study that was carried out by the utility for the prospective IPP, simulation studies investigated the expected temporary overvoltages caused by the IPP’s generator during the clearing of faults in the transmission system. The IPP would temporarily isolate with the line because generator over-speed protection and power quality protection would be relatively slow to trip the unit. This appendix summarizes the result of one such simulation; the clearing of a sustained single line-to-ground (SLG) fault on the 230 kV transmission line.
System Modelling
The network of Figure A-17, together with the surrounding external system connected to Stations B and C, which is not shown, were modelled in an EMT program. The IPP generator is rated 32.22 MVA with a terminal voltage of 13.8 kV. The saturated value of the d-axis subtransient reactance, X”d, is 0.16 pu. The generator was modelled as a 60 Hz sinusoidal voltage source behind X”d. The 25 kV feeder and transmission line were modelled as 60 Hz distributed parameter lines. Table A-5 provides the positive and zero sequence series impedances and shunt admittances of the 230 kV circuit. The minimum distribution load of 4.5 MVA was modelled as a lumped linear impedance R + jX branch to ground. The transformers were represented by standard models comprised of three-phase inductively coupled branches. For the purposes of illustrating the basic phenomenon of high TOV on an HV ungrounded system due to series resonance near fundamental frequency in the presence of neutral shift, the effects of transformer saturation and surge arrester conduction have not been modelled.
Table A-5 137km 230kV Transmission Line 60Hz Parameters
Positive Sequence Negative Sequence
Figure A-17 Distribution IPP Connected to 25kV Feeder Supplied by a 230/25kV Substation which is tapped into a long 230kV Overhead Line
Clearing of a Transmission Line Fault
Figure A-18 and Figure A-19 show a 300 ms simulation where a sustained SLG fault occurs at time t = 0.116 s on the 230 kV line near Station B. The faulted line was assumed to be disconnected from the grid 3.5 cycles after fault inception. The IPP generator was assumed to remain connected throughout the simulation since, in practice, line protection will trip the faulted line but not the feeder breaker. Moreover, while the IPP generator breaker would eventually be opened by over-speed protection or power quality protection, this action would occur beyond the period considered by the simulation. The instantaneous phase-to-ground voltages at the 230 kV bus of Station P are shown in Figure A-18 while the corresponding voltages at the station 25 kV bus appear in Figure A-19. As is clearly shown in Figure A-18, there is a rapidly escalating TOV, reaching up to 8 pu on the unfaulted phases of the 230 kV line, commencing as soon as the generator isolates with the faulted line. All three phase-to-ground voltages become in phase. The 25 kV bus voltages exhibit temporary overvoltages on all three phases but the overvoltage is not as high in terms of per unit than for the 230 kV voltages. It is worth noting that replacing the simple generator model with a more detailed model based on a Park’s machine produces almost identical results. Also, if saturation effects and surge arresters (rated for a temporarily ungrounded application) are included in the system model, then the overvoltage on the transmission line would be expected to be limited to approximately 2.2 pu. The TOV on the Station P distribution bus would be less than 2 pu but still expected to be unacceptably high.
Figure A-18 Phase-to-Ground Voltages at Station P 230 kV Bus Before, During and After the Generator Isolates with the Faulted Transmission Line
Figure A-19 Phase-to-Ground Voltages at Station P 25 kV Bus Before, During and After the Generator Isolates with the Faulted Transmission Line
Analysis of the TOV using Sequence Networks
Insight can be gained by analysing the TOV, while the IPP energizes the isolated and faulted transmission line,
Simulation of TOV When Distribution IPP Isolates With 230 kV Ungrounded System No Saturation Effects or Surge Arrester Conduction Modelled
-2.00E+03
30 MW Distribution IPP Isolates With 137 km 230 kV Line, Neutral "Floats"
Phase A-Ground Fault on 230 kV Line
Near Power-Freq. Resonance High TOV on Unfaulted Phases
Simulation of TOV When Distribution IPP Isolates With 230 kV Ungrounded System No Saturation Effects or Surge Arrester Conduction Modelled
-1.00E+02
230 kV Line Opened at Both Ends
SLG Fault Occurs
High TOV Due to Series Resonance Near 60 Hz
isolated system can be modelled using the positive sequence, the negative sequence, and the zero sequence equivalent circuits connected in series at the fault location. Figure A-20 shows the resulting cascaded network, where resistive losses and the relatively small effects of the station load are ignored. For this analysis the generator positive sequence model is a sinusoidal 60 Hz voltage source behind X”d. For the negative sequence equivalent it is assumed that the generator negative sequence reactance is approximately X”d. It is assumed that the effects of the 230 kV line can be approximated by connecting a single lumped positive sequence charging capacitance to the substation HV terminals in the positive sequence and the negative sequence circuits. In the zero sequence circuit the zero sequence capacitance of the line is connected to the HV side of the transformer. It should be noted that the HV delta windings of the Station transformer effectively disconnects the transformer, feeder and the IPP from the zero sequence equivalent circuit. The interconnection of sequence circuits can be reduced to the simple circuit shown in Figure A-21 where the zero sequence capacitance connects the positive and the negative sequence equivalent circuits. If the driving point impedance, as seen from the generator air-gap, is scanned over a frequency range around power frequency, the resulting impedance (R and X versus frequency) plot of Figure A-22 is obtained. The frequency scan clearly demonstrates that the interconnection of equivalent sequence circuits has a series resonance at 56 Hz (where X=0), as seen by the internal voltage source of the generator. This frequency is very close to power frequency.
Figure A-20 Interconnection of Sequence Networks Representing the 230/25kV System Isolated with a Solid Single-Line-to-Ground Fault
Figure A-21 Reduced Equivalent Circuit. All Parameters are Referred to 230kV
Figure A-22 Impedance (Real & Imaginary Components) Versus Frequency of the Sequence Equivalent Circuit for the Isolated 230/25kV System in Figure A-21
Conclusions
1 When the IPP becomes isolated with the faulted 230 kV line, a series resonant circuit is formed by the combination of equivalent inductance of the source (IPP generator X”d, the leakage impedance of two transformers in series, and the feeder impedance) and the charging capacitance of the 230 kV line. The resonant frequency becomes very close to power frequency for sustained SLG line faults.
2 The sinusoidal voltage source driving the resonant circuit is the generator air-gap voltage.
3 High TOVs on both the 230 kV as well as the 25 kV distribution systems occur during the clearing of SLG line faults because of neutral shift in an ungrounded and capacitive 230 kV system having an SLG fault together with voltage amplification by the series resonance.
Impedance Scan of Isolated 230/25kV System With IPP
& Sustained SLG Fault on 230 kV Line - Sequence Equiv.
-2000 -1500 -1000 -500 0 500 1000 1500 2000
30 34 38 42 46 50 54 58 62 66 70
Frequency (Hz)
ImpedanceR&X(Ohms)
Z_RE Z_IMAG Series Resonance
56 Hz
X
R
5 Analysis using equivalent sequence networks provides useful insight into the explanation of the overvoltage.
Recommendations
1 A direct transfer trip (DTT) scheme, to be initiated by the 230 kV line protection at Stations B and C, should be implemented to trip the IPP feeder breaker at Station P. The communication medium could be power line carrier from both line terminals to Station P.
2 Opening of the faulted line should be delayed sufficiently to ensure that the IPP feeder breaker has already opened.