Converter transients imposed on HVDC converter transformers

July 10th, 2015, Published in Articles: Energize

 

High voltage direct current (HVDC) line commutated converter (LCC) systems have been in operation for over 50 years. At present there are more than 100 systems in operation transmitting more than 100 000 MW of power. Converter transients, imposed on HVDC converter transformers, can produce voltages and currents which result in equipment failure.

The reliability performance data on HVDC systems in commercial service is compiled and presented at the Cigré Paris biennial technical sessions by advisory group AG B4-04. The information gathered by this group is useful in planning, design, construction and operation of HVDC systems. The advisory group also collects data every two years on the converter transformer failures. To date the group has exclusively dealt with line commutated converters (LCC) but the scope has been changed to include voltage source converters (VSC) systems, which will be included in future reports. This report deals only with transformers used in LCC systems.

Fig. 1: Typical voltage wave-shape at the valve terminal of a converter transformer.

Fig. 1: Typical voltage wave-shape at the valve terminal of a converter transformer.

The performance data for the LCC systems has consistently shown that transformer failures contribute to over 60% of the forced energy unavailability. The performance of HVDC converter transformers was first reviewed by joint task force (JTF) 12/14.10 which surveyed all the HVDC systems operating from date of commissioning to 1990.

For the purpose of the survey, the failures were defined as actual or preventive. A failure was considered to be actual, if the failure required the removal of a unit from service because of damage to active parts. A failure was considered as preventive if the unit did not actually fail but was taken out of service to repair a potential failure of active parts following diagnostic testing such as dissolved gas-in-oil analysis (DGA), high insulation power factor, or failure of similar unit(s).

Fig. 2: Simplified PSCAD/EMTDC system model of Gui-Guang 600 kV 3150 MW HVDC link.

Fig. 2: Simplified PSCAD/EMTDC system model of Gui-Guang 600 kV 3150 MW HVDC link.

Apart from the general pertinent data, a description of the failure was requested together with the apparent cause(s) as identified by the utility and/or by the manufacturer and the relationship, if any, with the factory design/routine tests performed on that unit. The survey did not cover external bushing flashovers related to pollution.

The converter transformers are connected between an AC system and a thyristor valve based converter. The voltage wave-shapes on the AC system are well defined and contain no significant harmonics. However, the valve winding voltages to ground not only contain multiple harmonics of fundamental frequency but also contain a DC component depending on the location of the thyristor bridge from the neutral point.

The converter operation also generates high frequency transients during the commutation process. These harmonics are present during normal operation and their magnitude varies with the converter firing angle and load. The valve side windings are also subject to over voltages during various faults on the converter side.

Fig. 3: Simulated converter transformer stray capacitance parameters (1 phase).

Fig. 3: Simulated converter transformer stray capacitance parameters (1 phase).

In addition to the voltage harmonics, the converter currents also contain harmonics of fundamental frequency. The effect of these harmonics must be considered very carefully during the transformer design.

The working group’s task

The task of working group B4.51 was to indentify the voltage and current transients imposed on the converter transformer from the converter side. The working group considered the following system conditions:

  • Normal operation
  • DC voltage polarity reversal
  • Commutation failures
  • Current harmonics

A combination of an electro-magnetic transients simulation called EMTDC and actual system fault recordings were used to develop this information.

Normal operation

Under normal operation the valve side phase to ground voltages consist of a combination of DC voltage, fundamental AC voltage, harmonic voltages of fundamental frequency and high frequency transients due to commutation. The DC and fundamental harmonics voltages are as following:

Each bridge voltage has a DC component equal to Ud x (n-0,5).

Where:

Ud = Average DC voltage of the bridge
n = Bridge number starting from the ground

The fundamental component equals to the phase to neutral voltage of the valve winding side voltage.

The voltages contain a large component of harmonics of the order of 3n.

Where n = 1, 3, 5, 7, etc.

The voltages contain harmonics of the order of 6n.

Where n = 1, 2, 3, etc.

The voltages contain harmonics of the order of 6n ±1

Where n = 1, 2, 3, etc.

The magnitude of harmonic content of all the harmonics except the multiples of 12 does not change from one bridge to another.

The harmonic with multiples of 12 increase as following:

Vn = V1 x (2n-1)

Where:
V1 = Magnitude of harmonic in bridge 1
Vn = Magnitude of harmonic in bridge n
n = Bridge number counted from the ground

Fig. 4: AC filter model and parameters  at Foz de Iguacu.

Fig. 4: AC filter model and parameters
at Foz de Iguacu.

The high frequency transients are caused by the interaction between the stray capacitances and the transformer reactance when the thyristor valves turn-off. The magnitude of the commutation overshoot is dependent on the stray capacitances, thyristor RC snubber circuits and the operating parameters (firing angle α and overlap angle μ). The RC snubber circuits across the thyristors help damp these oscillations. As this information is unique to each system no general statement can be made. The information about these transients should be provided to the transformer designer after the valve design has been completed.

DC voltage polarity reversal

DC voltage polarity reversal can be caused by either DC line faults or normal power reversal in the DC systems. Both the simulations and the field measurements show that DC line faults cause only transient polarity reversal for less than 100 ms and not a sustained polarity reversal. JWG A2/B4-28 reported that polarity reversals due to DC line faults do not contribute to an additional stresses as the duration of the reversal is very small compared to the insulation time constants [1]. Therefore polarity reversal due to DC line faults is not a major concern especially for system that transmits power only in one direction.

Fig. 5: AC filter model and parameters  at Sao Roque.

Fig. 5: AC filter model and parameters
at Sao Roque.

For DC systems where power reversals occur regularly the effect of polarity reversal becomes very important. The transformer design must take into account, not only the frequency of polarity reversals but also the rate of polarity reversal.

Commutation failures

When a commutation failure occurs in a bridge a short circuit is created across the bridge. In a system with a single twelve pulse converter per pole, a commutation failure in the lower bridge results in short to ground across the bridge. This however does not result in any stresses on the valve winding of the lower bridge transformer. When a commutation failure occurs in the upper bridge, it results in pole voltage being applied to the lower bridge transformer valve winding. At the same time the lower thyristor bridge is also subjected to the pole voltage. The thyristor bridge however is protected by surge arresters.

Therefore the valve winding of the lower bridge converter transformer must also be designed to the same protective level as the thyristor valves. It is therefore essential that transformer design must be coordinated with the converter valve design. For systems using two twelve pulse converters in series per pole, the insulation level of the bottom three converter transformers must be coordinated with the corresponding thyristor bridges.

Current harmonics

HVDC converters generate characteristic current harmonics of the order of (6n ±1). In a two winding transformer, the ampere-turns are balanced if the magnetising current is neglected. The harmonic currents in the valve winding are balanced by harmonics in the line-winding; therefore eddy current enhancement factor is the same for both valve and line windings.

Fig. 6: DC filter model and parameters.

Fig. 6: DC filter model and parameters.

In a three winding transformer which includes both start and delta valve windings on the same core limb, special attention must be paid to the effect of the harmonics. In HVDC converters, the harmonics of the order of 11, 13, 23, 25, 35, 37, 47 and 49 are in phase and the harmonics of the order of 5, 7, 17, 19, 29, 31, 41 and 43 are out of phase (in opposition).

The effect of harmonic currents in phase opposition depends on the coupling between the windings and the transformer design. It is recommended that study of the effects of harmonics in opposition by means of a simulation tools should be conducted at the design stage. In addition the characteristic harmonics mentioned above, there are non-characteristic harmonics due to the following non-ideal conditions:

  • Unbalance of transformer reactance between phases and between star and delta bridges
  • Firing angle asymmetry
  • AC voltage unbalance and distortion

There is always a small amount of DC current flowing in the converter transformers. Detailed studies should be performed to investigate any possibility of core saturation instability.

Simulation of Gui-Guang system

The electromagnetic transient program PSCAD/EMTDC has been used to simulate the Gui-Guang HVDC system and to investigate the converter transformer stresses under normal operating conditions as well as the fault conditions.

Simplified Gui-Guang HVDC system

Fig. 2 shows the simplified PSCAD/EMTDC system model of Gui-Guang ±500 kV 3000 MW HVDC Link. The AC system on each side of the HVDC link is modelled as an equivalent voltage source. The main components of the simulation model are briefly described in the following sections.

Converter transformer

Converter transformers at the Anshun and Zhaoqing converter station are modelled as single phase three winding transformers connected either in star-star or star-delta configuration. The stray capacitance of the Anshun and Zhaoqing converter transformers was based on manufacturer data. Fig. 3 shows the stray capacitance of the single phase of each three winding transformers.

AC filter model

There are three types of filter banks installed on the Anhsun 500 kV AV bus as shown in Fig. 4. The final stage includes three banks of Type A filters, four banks of Type B filters and four banks of Type C capacitor banks all rated for 130 Mvar.

There are three types of filter banks installed on the Zhaoqing 500 kV AC bus as shown in Fig. 5. The final stage includes four banks of Type A filters, four banks of Type B filters and five banks of Type C capacitor banks all rated for 140 Mvar.

DC filter model

Fig. 6 shows the simulated DC filters and their parameters at the Anshun and Zhaoqing converter station as installed in Gui-Guang HVDC system.

Thyristor valve converter

Each 12-pulse valve converter is modelled with two PSCAD/EMTDC compact DC converter components connected in series. The compact DC converter component includes a built-in Graetz bridge valve group, internal phase locked oscillator (PLO), firing and valve blocking controls, and RC snubber circuits for each thyristor. The resistance and capacitance of RC snubber circuits are 2800 Ω and 0,0179 μF.

Conclusion

The converter transformers are connected to AC system on primary side and to a thyristor converter valve on the secondary side. For the theoretically calculated and simulated cases, primary side voltages are considered to be ideal. The measured values contain the effect of all the harmonics that may be present on the primary side. However the valve winding voltages to ground not only contain multiple harmonics of fundamental frequency but also contain a DC component depending on the location of the thyristor bridge from the neutral point. The converter operation also generates high frequency transients during commutation process. These harmonics are present during normal operation and their magnitude varies with the converter firing angle. The valve side windings are also subjected to overvoltages during various faults on the converter side as well as fast polarity reversal.

This report describes the voltage and current transients imposed on the converter transformers during converter operation, based on theoretical calculation, digital simulations and field measurements of typical HVDC systems. The result of this report can be used as basis for specification, design and testing of converter transformers.

Since the converter transformer voltage stresses are determined by converter operation such as expected frequency of normal and emergency power reversals, all relevant information shall be provided by the owner of HVDC system to the converter transformer designers. Moreover, converter transformer designer must be provided with the detailed information about the converter voltage transients including commutation transients, converter insulation and coordination and the current harmonics generated by the converter. The design of converter transformer needs to be closely coordinated with the design of the converter in general and the valve in specific.

References

[1]    Cigré: “Technical Brochure 406 – HVDC Converter Transformers – Design Review, Test Procedures, Ageing Evaluation and Reliability in Service”, JWG A2/B4.28, February 2010.

Acknowledgement

A summary of this article was published in Electra, April 2015, and the full article is republished here with permission.

Contact Rob Stephen, Cigré, Tel 031 563-0063, rob.stephen@eskom.co.za

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