Practical Evaluation of Transient Voltage Distribution in HVDC/UHVDC Thyristor Valve

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1 Practical Evaluation of Transient Voltage Distribution in HVDC/UHVDC Thyristor Valve Huan Guo China Electric Power Research Institute August 31,2011

Introduction Voltage distribution characteristic between different thyristor levels is significant, especially for a valve with a large quantity of thyristor.

2 Introduction Voltage distribution characteristic between different thyristor levels is significant, especially for a valve with a large quantity of thyristor. Even distribution can enhance reliability, minimize thyristor number and valve cost. Stray capacitances are the main factor that affects the transient voltage distribution; Its accurate calculation is necessary for a better understanding and optimized design.

3 BEM Usually Boundary Element Method (BEM) is adopted d for such external electromagnetic problems Detailed element segmentation ti is required on the object boundaries only. stray capacitances of avalve structure t can be derived d by integrating surface charges directly.

4 Efficiency improvement method Matrix dimension of typical BEM is usually too large to resolve easily. Following methods applied can minimize the matrix dimension and computation tti load. consequently enhance the computing accuracy with defined computing capability. Shielding effect Window truncation Reuse technology

5 Shielding Effect Supposing ψ (i) = 1 and ψ ( j ) = ψ (k) = 0 in Fig.1, then the surface charge for conductor j is pkk pij pjk pik (5) Qj = Qk p jj p ik p jk p ik And the potential coefficients being respectively p jj = p kk, p ij = p ji 1/ (1 + d )andp jk = p kj 1 / d. Substituting these potential coefficients into equation (5) then (6) Qj = Qk( pkk )/( pjj ) Qk 1+ d d d 1+ d 1+ d i k j 1 Fig. 1 An electrostatic system with three conductors d

6 Shielding Effect The contribution due to conductor j on the potential of conductor i is P ij Q j Q k / (1 + d ) 2, changing inversely proportionally to the distance squared between them. Influence due to the non-neighbouring conductors are much less comparing to these due to the neighbouring conductors.

7 Window truncation if stray capacitances relative to conductor i are calculated, and conductor i is defined dfi das a reference then all the conductors outside an assumed truncating window with radius r e are ignored. Similarly, conductor i is assumed to have a potential of one, whereas the rest of them have a potential of zero, then jy i jx r e PQ 0 M = ei = 1 ith row M 0 P [ p xx ] [ p xy ] = [ pxy ] [ pyy ] (7) Fig. 2 An electrostatic system with truncating window applied

Window truncation Once the surface charges Q for all conductors in the system are obtained, mutual coupling and self capacitances can be obtained in accordance with equations (3) and (4) {{ } { }}

8 Window truncation Once the surface charges Q for all conductors in the system are obtained, mutual coupling and self capacitances can be obtained in accordance with equations (3) and (4) {{ } { }} Cij = Qj, j jx, jy, j i C i0 = Qj + Q (8) k j jx k jy {jx} Q p xx Q{jx} = [[ pxx ] [ pxy ]] (9) % Q{jy} * Δ Q = Q Q, {{jx},{jy}} j j j j p Δ Q + [ p ( Qk)] = 0 (10) xx {jx} xy kth column k {jy} p ( ), { } xx {jx} xy th column ( k ) Δ Q + p k ( Qk) = 0 k { jy} (11)

9 Window truncation Δ Q < 0, j {jx} ( k ) j j ( k ) Qk Qj j {jx} ( k ) j 0, {jx} Qk k jy k jy j jx Δ Q = Δ Q < j > Δ (12) Combining equations (2) and (3) yields C * * ij = Qj, j {jx}, j i * * Ci0 = Qj j jx Substituting equation (8) into the above equation, then: < Δ Q (13) j (14) * Cij Cij = Δ Qj > 0, j {jx}, j i * (15) Ci0 Ci0 = ΔQj Qk > 0 j jx k jy

10 Window truncation Once the window truncationti is applied, mutual capacitances relative to the conductors outside the truncating window will be omitted; self and mutual capacitances of the remaining conductors will increase. The valve transient voltage distribution is instead mainly influenced by the stray capacitances existing between the valve structure and earth. Derived results are generally larger than their actual values, which h will result in a more pessimistic i study for transient t voltage distributions, and lead to a more conservative and safe design of a valve structure.

11 Reuse Technology Reuse technology means that similar questions of a system need to be studied once only. The conclusions obtained for one question could be extended to other similar questions of the system with or without t aminimum ii supplementary correction. Once mutual stray capacitances relative to a conductor in one layer are obtained, the same results could be reused for the stray capacitances for the identical conductor in other layer. The stray capacitances obtained for one module in a layer could be reused for the other module in the same layer.

12 Simulation circuit Once the stray capacitances are obtained, equivalent circuit of converter valves can be built using the field-circuit couple method. Fig.3 shows the simplified equivalent circuit for A5000 converter valve structure using in China Jing-Su ±800kV UHVDC project. 11

13 Simulation circuit C hs C hs C g C g R g R d C d C SS C T T TE C SE CTT CTT T C SS TT SS CSS C C R g R d C d T T CTE 1 2 T9 C SE Fig. 4 Equivalent circuit for A5000 converter valve structure 12

14 Transient Voltage Distribution voltage uneven coefficient (VUC) is introduced. ut max VUC = u Tav (16) For a valve structure with a defined internal design, the VUC will be different if the amount of series-connected valve modules or the valve-to-earth th stray capacitances change. The amount of series-connected valve modules in a valve structure is dependent on the overvoltage protection level and the converter topology of a HVDC scheme. 13

15 Transient Voltage Distribution (22) With steep front impulse With lightning impulse With switching impulse VUC Module quantity in a valve Fig.5 Impact of valve modules on VUC in A5000 ±800kV UHVDC valve structure t 14

16 Transient Voltage Distribution If the VUC of a large valve structure increases, (fast) grading capacitances are usually implemented to control the VUC to acceptable levels, C g (nf) N N m Fig.6 Required Cg for different amount of valve modules N m for A5000 ±800kV UHVDC valve structure. 15

17 conclusion A practical approach for the assessment of transient voltage distributions was proposed, which includes three aspects: stray capacitance calculation using efficiency improved methods, analysis of transient voltage distributions, and grading capacitance design. Three different efficiency improvement methods, including shading hdi effect, window id truncation, ti and reuse technology, were introduced for the stray capacitance calculation. 16

18 conclusion Voltageg uneven coefficients for the A5000 ±800kV UHVDC valve structures with different amount of valve modules were calculated by building the analogue circuit using the field-circuit coupling method. The required grading capacitance is proposed for A5000 ±800kV thyristor valve, In order to constrain the voltage stresses applied to the worst-case thyristor t levell of avalve structure. t For Chinese Jing-Su ±800kV UHVDC project, grading capacitance is unnecessary. 17

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