L23 Interaction Between AC-DC Systems
Transcript of L23 Interaction Between AC-DC Systems
![Page 1: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/1.jpg)
High Voltage Direct Current (HVDC) Transmission (EEL 794 / EEL 452)
Interaction between AC-DC Systems
Prof. Bhim Singh, FIEEEElectrical Engineering Department
IIT Delhi, New Delhi
Apr 17, 2023 1
![Page 2: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/2.jpg)
Interaction of AC-DC systems
• AC-DC system interactions are concerned with voltage stability, over voltages, resonances and recovery from disturbances.
• Voltage stability criteria are used to determine the type of voltage control and type of reactive power supply.
• The level of temporary over voltages (TOV) influences the station design, including thyristor valve and surge arrester ratings; the lower the value of SCR the higher the potential value of TOV.
Apr 17, 2023 2
![Page 3: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/3.jpg)
Interaction of AC-DC systems
• The larger the ratio of shunt capacitor MVAR to AC system short circuit MVA, the lower will be the resonant frequency.
• Recoveries from AC-DC system faults are slower with weak systems.
• Thus the degree of interaction depends on the relative strength of the AC-DC system.
Apr 17, 2023 3
![Page 4: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/4.jpg)
• A CIGRE document released in 1992 provides valuable assistance to the understanding of these interactions.
• Their influence on station design and performance is assessed with reference to the AC-DC system strength, a relative term which is generally expressed by the Short-Circuit Ratio.
• Short Circuit Ratio is the ratio of the AC system short circuit capacity to DC-Link power.
• A strong ratio of the AC-system is defined as having an SCR above 3, and the SCRs of weak and very weak systems range between 3 & 2 and below 2 respectively.
Definition of System Strength
Apr 17, 2023 4
![Page 5: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/5.jpg)
“Weak” AC System
• AC system impedance is high.
• AC system mechanical inertia low.
Apr 17, 2023 5
![Page 6: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/6.jpg)
AC system strength has been Classified as:
• ESCR > 3 HIGH
• 2 < ESCR < 3 LOW
• ESCR < 2 VERY LOW
[ESCR = Effective Short Circuit Ratio]
Apr 17, 2023 6
![Page 7: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/7.jpg)
Issues on SCR
• Neither the short circuit capacity nor the DC-link power remains constant for a given scheme.
• The system impedance calculated from the short circuit capacity does not include the filters and other shunt components connected at the converter terminals.
• The calculated system impedance at fundamental frequency does not increase linearly with frequency.
• The presence of other converters or nonlinearities in the AC system is not taken into account in the derivation of the SCR.
Apr 17, 2023 7
![Page 8: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/8.jpg)
Short circuit MVA
2ac
th
E
ZSC-MVA =
Eac = Commutation bus voltage at rated DC power
Zth = Thevenin equivalent impedance
Apr 17, 2023 8
![Page 9: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/9.jpg)
Problems With Low ESCR
1. High dynamic over voltages.
2. Voltage instability.
3. Harmonic resonance.
4. Objectionable voltage flicker.
Apr 17, 2023 9
![Page 10: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/10.jpg)
ESCR & CSCR
• Effective Short Circuit Ratio - allows for the presence of AC filters and shunt capacitors at the converter terminals.
ESCR = S-Qs / Pd
• Critical Short Circuit Ratio – corresponds to the operation at the maximum available power (MAP), and a typical value for the inverter is 2.
• For SCR values below the CSCR the operation is in the unstable region of the AC voltage Vs DC power characteristics.
Apr 17, 2023 10
![Page 11: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/11.jpg)
AC system Representation at Converter Busbar
Simplied diagram withshunt capacitor & synchronous compensators
Thevenin Equivalent Circuit
Apr 17, 2023 11
![Page 12: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/12.jpg)
AC system Thevenin Equivalent
Zst is the equivalent of Zs (source), Xsc, Xc, and Zl in parallel.
The Eq. Thevenin source voltage Est, results from the vectorial addition of Vc/3 and I*Zst
A convenient factor in the per-unit notation is the converter rating r, defined as the ratio of converter MVA to DC Power (Pd), i.e.
2 c
d d
V Ir
V I
Apr 17, 2023 12
![Page 13: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/13.jpg)
Voltage Interaction
Est
IqIp
I
Φ
ΦΦst
Vc/√3
Voltage Regulation at Rectifier End
Apr 17, 2023 13
![Page 14: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/14.jpg)
Voltage Regulation At Inverter End
I
Vc/√3
IZst
Est
Φ
Φst
Apr 17, 2023 14
![Page 15: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/15.jpg)
AC system Thevenin Equivalent
• Zbase = Vc/(3*I) = V2c/MVAc
• Zst = V2c/MVAF
and in pu
Zst = Zst/Zbase = (V2c/MVAF)*(MVAc/V2
c) = MVAc/MVAF
Per unit system impedance Zst=r/ESCR
Apr 17, 2023 15
![Page 16: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/16.jpg)
Voltage Instability
• With DC systems connected to weak AC systems, Particularly on the inverter side, the alternating as well as direct voltages are very sensitive to Change in loading.
• An increase in direct current is accompanied by a fall of alternating voltage.
Apr 17, 2023 16
![Page 17: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/17.jpg)
Dynamic Voltage Regulation
• The maximum voltage regulation will occur if the disturbance takes place when the phase angle of the converter () is equal to the phase angle of the equivalent AC system impedance (st) and is calculated as follows
Est = V+I*Zst
Est/Vc = V/Vc + Zst*I/Vc
Or
Est/3 = (v/ 3) + (I/Vc) (r/ESCR)(V2c / MVAc)
With ESCR of 2.5 and under rated conditions
Est = 1+ 0.4 rTo calcualte ‘r’, refer DC voltage equation.
Apr 17, 2023 17
![Page 18: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/18.jpg)
Dynamic Voltage Regulation
Vd = Vc0 cos - (2Xc/) Id
Vd / Vc0 = cos - (3*Xc*Id)/(Vco*)
Substitute Vco = (32/ )Vc and the above equation becomes
1/r = cos - XcId/ 2 Vco
The commutation reactance in pu is xc = Xc/[Vc/(3I)]
and since I = (6/ )Id, then the above equation is changed to
XcId / (2 Vco) = (/6) xc
1/r = cos - (/6) xcApr 17, 2023 18
![Page 19: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/19.jpg)
Dynamic stabilization of AC system
• Power system is stable if after a disturbance it returns to a condition of equilibrium.
• If the angle between machines increases steadily, the system is transiently unstable.
Apr 17, 2023 19
![Page 20: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/20.jpg)
Apr 17, 2023 20
Harmonic and Torsional Interactions
There are certain adverse interactions at higher frequencies, which can affect the system operation:
1.Harmonic interaction at low order harmonics (2nd to 5th harmonic)
2.Torsional interactions involving turbine generator rotors and at subsynchronous frequencies (10 to 50 Hz)
![Page 21: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/21.jpg)
Apr 17, 2023 21
Torsional Interactions
The series capacitor introduce a series resonance frequency in the electrical circuit, comprising a generator stator,
transformer and the transmission line.
The self excitation phenomenon can exist even when the generator inertia is considered to be infinite, ruling out rotor oscillation.
The negative damping due to the Torsional Interactions is more severe compared to the induction generator effect.
![Page 22: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/22.jpg)
System Response to AC-intertie series Capacitor bypass
Apr 17, 2023 22
![Page 23: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/23.jpg)
at Square Butte, U.S.A.Apr 17, 2023 23
Torsional interaction with HVDC converter controller
![Page 24: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/24.jpg)
Apr 17, 2023 24
Overall conclusions of torsional tests
1. The torsional interaction is significant only at turbine-generators in the close vicinity of HVDC converters relative to other units.
2. Converter control has a major effect. The problem is more acute with EPC than IPC, although this can be compensated by adding synchronizing circuit with sufficient bandwidth.
3. The negative damping is of main concern for the torsional modes that lie in the frequency range of 10 to 20 HZ.
4. The negative damping increases with the power level.
![Page 25: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/25.jpg)
Apr 17, 2023 25
5. For a given power level, the damping introduced by the DC system is sensitive to the level of the DC voltage.
6. The adverse torsional interactions are most significant for the radial operation of the DC link.
7. Damping is reduced for monopolar operation compared to the bipolar operation.
![Page 26: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/26.jpg)
Controlled Damping of DC Interconnected systems
• With an AC tie line, if one of the interconnected systems is in difficulty following a disturbance, the line is normally tripped to prevent the disturbance affecting the other system.
• Unlike the transient stability, where the DC link must have the necessary overload capability to get through the first swing, dynamic stability can be achieved without overloading the DC link.
• If DC link is already operating at full capacity then damping can be achieved by DC power reductions at the appropriate instant.
Apr 17, 2023 26
![Page 27: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/27.jpg)
Damping of Sub Synchronous Resonance • When the electrical and mechanical resonant frequencies are
close, the torsional modes of vibration can become unstable, this phenomenon is referred as sub synchronous resonance.
• A sub synchronous damping control (SSDC) consists of a wide bandwidth controller sensitive to generator speed can provide positive damping contribution over the entire range of sub synchronous torsional frequency.
• Other method is to modulate the firing instant with a suitable phase and amplitude with regard to generator oscillations.
• Use band pass filtering technique.
Apr 17, 2023 27
![Page 28: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/28.jpg)
Apr 17, 2023 28
Electrical damping for IPC/EPC with and without synchronising circuit
![Page 29: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/29.jpg)
Generic AC-DC system Controller
Apr 17, 2023 29
![Page 30: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/30.jpg)
Apr 17, 2023 30
Control of Torsional Interaction
Adverse torsional interaction is much less compared to that caused by fixed series compensation.
The potential of adverse interaction exists only if radial operation of HVDC link connected to a turbine-generator is envisaged.
Adverse TI can be overcome by:
Modification of the converter control,
Providing a subsynchronous damping controller (SSDC)
![Page 31: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/31.jpg)
Apr 17, 2023 31
Modification of the converter control:
It is feasible only if the modification does not affect the usual function of the controller during normal and abnormal conditions.
Providing a subsynchronous damping controller (SSDC):
It is more flexible. The input signal can be taken from the rotor speed or bus frequency.
The SSDC can be designed based on: Narrow bandwidth approach, Wide bandwidth approach
![Page 32: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/32.jpg)
Apr 17, 2023 32
Torsional Interactions with MTDC systems
The major conclusions of a case study based on linearized system model:
1.A rectifier on current control contributes to maximum negative damping of torsional modes, when the generator is connected to the rectifier terminal.
2.The voltage controller at the inverter contributes to the negative damping to some extent.
3.The torsional modes are better damped when the rectifier is chosen as voltage setting terminal (VST) as opposed to the case when the inverter is chosen as VST.
![Page 33: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/33.jpg)
Apr 17, 2023 33
Torsional Interactions with VSC-HVDC:
1.DC voltage control of the VSC near the generator results in better damping of the torsional modes.
2.The constant power control of the rectifier station situated close to the generator contributes small negative damping.
3.The constant power control of the inverter station situated close to the generator can destabilize the torsional modes in a narrow range of lower frequencies.
4.Unlike the LCC based HVDC system, the problem of TI with VSC-HVDC is minor except when the converter is operated as an inverter with constant power control.
![Page 34: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/34.jpg)
Transient stabilization of AC systems
• When fault is cleared, the generation and the remaining transmission experience a transient swing which may lead to instability. Long fault clearance times can cause a loss of synchronism.
• It is better to design systems with temporary capable for overloads.
• Thyristor valves used to withstand considerable over loads without effects to avoid unnecessary protective action.
Apr 17, 2023 34
![Page 35: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/35.jpg)
Harmonic Interaction
AC SIDE DC SIDE
Positive sequence harmonic
K+1 K, 13±k 12±k, 24±k, .. 25±k12n+1±k, n=3,4…...
Negative sequence harmonics K-1 11±k 23±k 12n-1±k,n=3,4….
Apr 17, 2023 35
![Page 36: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/36.jpg)
Apr 17, 2023 36
Non characteristic harmonics are caused by: • firing angle errors,
• Negative sequence components in the converter bus AC voltage,
• Unequal converter transformer leakage impedances
![Page 37: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/37.jpg)
Apr 17, 2023 37
Some of the major factors that affect the low order harmonic resonances are:
1.Control system – generation of firing pulses
2.Saturation in converter transformers
3.The characteristics of system impedance (variation with frequency)
4.DC system characteristics – the impedance seen by the converter terminals)
5.Induction effects.
![Page 38: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/38.jpg)
Apr 17, 2023 38
The possible solutions to the problem are:
1.Modification of the control system
2.Use of additional filters, say, at 3rd harmonic
3.Use of synchronous condensers or static VAR systems at converter station.
![Page 39: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/39.jpg)
Harmonic Instabilities
Harmonic Instability: The generation and/or magnification of non characteristic frequencies by a DC system containing, initially, no balance or asymmetry.
•AC-DC systems with low short circuit ratio (SCR) often experience problems of wave form instability.
Apr 17, 2023 39
![Page 40: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/40.jpg)
Instability
• Individual firing control.
• Composite resonance.
• Cross modulation and transformer core saturation.
Apr 17, 2023 40
![Page 41: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/41.jpg)
Apr 17, 2023 41
Harmonic instability with IPC scheme of firing pulse generation
This was first studied by Ainsworth using a physical model simulator. The assumptions are as follows:
1.The DC current is constant – finite nature of the impedance of the DC system is neglected.
2.The overlap due to the leakage reactance is neglected.
3.The constant firing angle type of individual phase control scheme is considered.
![Page 42: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/42.jpg)
Apr 17, 2023 42
The major conclusion of the study are:
1.The harmonic instability can be expected with systems having low SCR at the converter bus.
The problem may be present even at moderate values of SCR if there is a resonance.
2.The firing control scheme has a major effect. The problem is worse with inverse cosine control scheme compared to the constant α control scheme.
![Page 43: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/43.jpg)
Apr 17, 2023 43
3. The performance with IPC schemes can be improved substantially using filters in the control system.
But the filters can cause errors and also slow down the response.
Furthermore, the filtering may be ineffective due to the variation in the system frequency.
4. The problem of harmonic instability is substantially solved using the EPC scheme of firing pulse generation as this eliminates the firing angle errors that are caused by the shifting of the zero crossings of the commutation voltages.
![Page 44: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/44.jpg)
Composite resonance
Composite Resonance occurs when:
A high impedance parallel resonance on the AC side coupled with a low impedance series resonance at an associated frequency on the DC side.
Apr 17, 2023 44
![Page 45: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/45.jpg)
Apr 17, 2023 45
Core Saturation Instability
Major causes:
1.The DC system having series resonance at or near the fundamental frequency.
2.Low SCR at the converter bus.
The main feature of this instability is the presence of DC components in the magnetizing current of the converter transformers causing saturation.
![Page 46: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/46.jpg)
Mechanism of core saturation instability
Ac side 2nd harmonic
impedance
Transformer core saturation
Converter switching
action
Converter switching
action
Dc side fundamental
frequency harmonic
impedance
Ac side DC side
Positive sequence 2nd
harmonic voltage
distortion
Ideal transfor-
merPositive sequence
2nd harmonic current
distortion
negative sequence 2nd
harmonic current distortion
fundamental frequency current
distortionDistortion at many
frequrncies
Apr 17, 2023 46
![Page 47: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/47.jpg)
Apr 17, 2023 47
The possible solutions to this instability are as follows:
1.Selecting smoothing reactor values to avoid the resonance in the DC system at or near the fundamental frequency.
2.Modification of the controller by adding an additional DC flux control loop.
The control signal is derived from the measured DC magnetizing current or the 2nd harmonic component.
![Page 48: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/48.jpg)
Disturbances
AC-DC interaction following disturbances:
• AC side fault recovery.
• DC side fault recovery.
Apr 17, 2023 48
![Page 49: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/49.jpg)
AC side fault recovery
• Modern DC control systems are capable of re-synchronizing and recommencing correct operation of the DC system within two cycles clearing a severe AC fault, such as a three phase to ground fault.
Control strategies to obtain good DC system recovery are:
• Reduced current level.
• Reduced power level at recovery.
• A switch of DC system control mode from constant power control to constant current control.
Apr 17, 2023 49
![Page 50: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/50.jpg)
DC side fault recovery
• DC line faults are mainly a matter of total energy loss at the receiving AC systems.
• Most common cause of DC line faults usually result in a single pole fault, with the healthy pole remaining unaffected in terms of power.
Apr 17, 2023 50
![Page 51: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/51.jpg)
Reactive Power Can Be Supplied By:
• AC filter banks.
• Shunt capacitors banks.
• Synchronous Generators.
• SVC’s or synchronous condensers.
Note: stronger the AC system, larger the switchable bank size.
Apr 17, 2023 51
![Page 52: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/52.jpg)
High Dynamic Overvoltages
Due to low ESCR systems, increase in AC voltage can be Excessive due to shunt capacitors and harmonic Filters.
It causes…
• High insulation level.
• Damage of local customer equipment.
Apr 17, 2023 52
![Page 53: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/53.jpg)
Harmonic Resonance
Due To AC Capacitors, Filters, AC System at Lower Harmonics.
So..
Avoidance of low Order Harmonic Resonance is Extremely important to Reduce Transient Over Voltages.
Apr 17, 2023 53
![Page 54: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/54.jpg)
Effective Inertia Constant (EIC)
• The ability of the AC system to maintain the required voltage and frequency depends on the rotational inertia of the AC system.
• for satisfactory performance, AC system should have minimum inertia relative to DC link.
• a measure of relative rotational inertia is the effective DC inertia constant.
• EIC generally ranges in between 2 and 3 for satisfactory operation.
• synchronous condensers have to be used to increase inertia.Apr 17, 2023 54
![Page 55: L23 Interaction Between AC-DC Systems](https://reader034.fdocuments.us/reader034/viewer/2022042422/55cf998b550346d0339dec40/html5/thumbnails/55.jpg)
Apr 17, 2023 55