L23 Interaction Between AC-DC Systems

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

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.

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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.

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• 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

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“Weak” AC System

• AC system impedance is high.

• AC system mechanical inertia low.

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AC system strength has been Classified as:

• ESCR > 3 HIGH

• 2 < ESCR < 3 LOW

• ESCR < 2 VERY LOW

[ESCR = Effective Short Circuit Ratio]

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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.

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Short circuit MVA

2ac

th

E

ZSC-MVA =

Eac = Commutation bus voltage at rated DC power

Zth = Thevenin equivalent impedance

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Problems With Low ESCR

1. High dynamic over voltages.

2. Voltage instability.

3. Harmonic resonance.

4. Objectionable voltage flicker.

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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.

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AC system Representation at Converter Busbar

Simplied diagram withshunt capacitor & synchronous compensators

Thevenin Equivalent Circuit

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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

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Voltage Interaction

Est

IqIp

I

Φ

ΦΦst

Vc/√3

Voltage Regulation at Rectifier End

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Voltage Regulation At Inverter End

I

Vc/√3

IZst

Est

Φ

Φst

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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

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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.

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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.

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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

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.

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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)

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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.

System Response to AC-intertie series Capacitor bypass

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at Square Butte, U.S.A.Apr 17, 2023 23

Torsional interaction with HVDC converter controller

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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.

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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.

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.

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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.

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Electrical damping for IPC/EPC with and without synchronising circuit

Generic AC-DC system Controller

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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)

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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

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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.

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.

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

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….

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Non characteristic harmonics are caused by: • firing angle errors,

• Negative sequence components in the converter bus AC voltage,

• Unequal converter transformer leakage impedances

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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.

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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.

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.

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Instability

• Individual firing control.

• Composite resonance.

• Cross modulation and transformer core saturation.

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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.

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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.

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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.

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.

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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.

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

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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.

Disturbances

AC-DC interaction following disturbances:

• AC side fault recovery.

• DC side fault recovery.

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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.

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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.

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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.

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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.

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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

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

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