Research of 750kV controllable high voltage shunt reactor

Qin rui Gansu Electric Power Research Institute

Gansu lanzhou, China qinrui1025@sina.com

Guo wenke, Chen guoping Lanzhou University of Technology

Gansu lanzhou, China guowenke321@126.com

Abstract—This paper describes a stepped switching controllable high voltage shunt reactor's basic operating principle, the device can regulate the system reactive power, voltage and frequency over inhibition of arc current, with a continuous smooth power adjustable harmonic current small and the advantages of fast response. It analyzes the stepped switching controllable high voltage shunt reactor protection features and functionality, protection and circuit breaker protection, Article uses Fourier series decomposition method. This method analyzed about current harmonic content of 300Mvar 750kV controllable high voltage shunt reactor’s work winding which will put into operation in Hexi 750kV transmission project Dunhuang substation. Keywords-Controlled Shunt Reactor; stepped switching controllable; protection system; harmonic current

I. INTRODUCTION To solve the coal, water, wind and other load centers and

distribution of primary energy imbalance problem, the backbone of our AC power grid system should adopt the super/ultra-high voltage compact lines to achieve long-distance, high-capacity transmission. According to the national power grid development plan, construction of 750 kV put into operation in Gansu Hexi and Xinjiang compact transmission line of AC power interconnection[1]. Super/UHV transmission line capacitive charging huge power, the trend of dramatic change and limited insulation margin to the power system reactive power regulation, over-voltage suppression of the higher requirements.

In the super/ultra-high voltage grid, controllable shunt reactors (CSR) could simplify the system reactive power and voltage control, suppress frequency over-voltage and operating over-voltage, eliminate the generator self-excitation, the dynamically compensate circuit charging power, inhibit the arc current, damp resonance system, etc. In addition to the traditional advantages of shunt reactor outside, the CSR could smoothly adjust reactive power of system, and the system disturbance caused by the rapid response to dynamic stability. It improved system reliability, increased transmission capacity and improved grid operation efficiency. So it had a very broad application prospects [2-5].

According to the different working principle, controllable high voltage shunt reactor can be divided into stepped controllable shunt reactor (SCSR) and magnetically controlled shunt reactor (MCSR).With the SCSR’s control is simple, fast response, high accuracy, high reliability, etc. It had selected One of the important equipment for China's UHV

transmission. And it would put into Dunhuang substation of Gansu Hexi 750kV transmission project.

Ⅱ. SCSR's structure and working principle The SCSR is devised transformer and reactor into one. the

design of the transformer leakage reactance is 100% which full used the antihypertensive effect of the transformer, Make the thyristor valve work at low voltages, In the transformer low voltage side, access the thyristor valve to adjust and achieve the inductive reactive power control [6]. The SCSR principle wiring diagram shows in Figure 1.

Figure1. Stepped controllable switching high voltage shunt reactor

principle of single-phase wiring In this figure: The work winding terminals A, X of SCSR

directly connected to bus in Dunhuang substation. High- voltage side of the three-phase winding used star wiring, the neutral point directly to ground (according to the needs of the installation of a small neutral reactance), and low-voltage side used star wiring, directly grounded neutral point. XK1, XK2, XK3 as limiting reactor, and the control winding terminals a, x of SCSR connected in parallel. TR1, TR2, TR3 are the thyristor valve, respectively, 50% step, 75% step and 100% step, Its role is to achieve rapid adjustment by quick off shunt reactor impedance effect.QF1,QF2 and QF3 are breakers. Their role is to bypass TR1, TR2, TR3, which will switch to the circuit breaker current; K1, K2, K3 for the isolation switch, whose role is to TR1, TR2, TR3 access system, for switching operation between circuits, to change the system operating mode, and also can prevent the control of operational errors caused by electrical faults.

When the controllable high resistance to run-time adjustment output capacity, need to exit the corresponding reactance, the Triac fast conduction, the corresponding

978-1-4577-0547-2/12/$31.00 ©2012 IEEE

reactance bypass, after conducting Triac and connected at both ends of the bypass circuit breaker gate, the long-term take on to short circuit current, Triac out of operation. As the load changes from the no-load to rated power, regular control of TR1, TR2, TR3 is turned on or off, thereby changing the work winding current to achieve the purpose of sub-regulating the work winding current. For example, in this circuit, if the disconnect TR1, TR2 turns on, regardless of TR3 turns on or not, at this time limiting reactor only XK3 access control winding, then you can adjust the conduction angle of TR2 can be reached gradually adjustment the reactive power purpose. The current limiting reactor XK1, XK2, XK3 can only access one by one, so the high resistance of the controllable reactive power control is segmented [7].

Ⅲ. SCSR protection system features and functions

A．SCSR protection system features

At present, China has embarked on the development and production of 750kV stepped switching controllable high voltage shunt reactor. Its protection configuration is also under development, the protection of research has the following characteristics:

First, as opposed to traditional fixed high resistance, 750kV stepped switching controllable high voltage shunt reactor in addition to a coil of but also have secondary windings, so the protection configuration of 750kV stepped switching controllable high voltage shunt reactor becomes complicated.

Second, according to the work principle of 750kV stepped switching controllable high voltage shunt reactor known which essentially 100% working capacity is under short-circuit the secondary winding of the work in the state of high impedance transformer. In order to achieve capacity control, the secondary winding series reactance values are based on different levels regulate the reactor working volume, thus, the biggest feature of the secondary winding is that the secondary winding in a variety of capacity under short-circuit current is small work under its current capacity at all levels. Because the traditional short-circuit protection are based on the case of lower voltage and current increase to achieve, so the configuration of this feature to protect the increased difficulty.

Third, 750kV stepped switching controllable high voltage shunt reactor biggest feature is that in 100% working capacity under 50% of secondary winding metallic inter-turn short-circuit, short-circuit loop current is zero, This time secondary current for rated current protection, it is difficult to detect such failure.

B. SCSR protection system functions In order to guarantee 750kV stepped switching controllable

high voltage shunt reactor operation of security and stability, the protection system mainly provide the following function protection[8-13]: valve refused to trigger protection; valve continued to trigger protection; valve margin insufficient protection; valve overvoltage protection; circuit breaker malfunction protection; circuit breaker failure protection; transient protection.

1) Valve protection

Controllable high resistance thyristor valve in switch state, used to work for the cut with parallel reactor. The valve protection mainly by the valve refused to trigger protection, the valve continued to trigger protection, the valve margin insufficient protection and valve overvoltage protection, etc. Among them, the valve refused to trigger protection and the valve continued to trigger mainly used to protect the fault of valve trigger system; valve margin insufficient protection is mainly used to protect when the valve margin is insufficient to avoid the damage that caused to withstand over-voltage; valve overvoltage protection is mainly used to protect the load reactor series break caused by the valve over-voltage.

2) Circuit breaker protection Circuit breaker protection mainly includes the circuit

breaker malfunction protection and circuit breaker failure protection. The circuit breaker malfunction protection is divided into circuit breaker misclassification protection and circuit breaker false switching protection. Circuit breaker failure protection is divided into close brake failure protection and break-brake failure protection.

IV. HARMONIC ANALYSIS In one cycle, when the thyristor is always in the on-state,

the winding current can be considered as non-harmonic components. However, if the thyristor only part of a cycle period is turned on, this control winding current will contain harmonics [7]. According to stepped controllable high voltage shunt reactor principle, to draw the equivalent circuit 2, to facilitate its harmonic analysis. Define the current symbol: Inj, inj, where “I” is RMS, “i” is instantaneous value, “n” is the winding number (shown in Figure 2, n=1,2,3,4,5), “k” indicates there are a total of k control winding short-circuit state (k = 2,3,4), k does not take number 1, because the 25% level of control winding is direct transition from no-load to its rated value, there is not adjusted by the thyristor, so that also avoid at light loads will have a great high-order harmonic current injection current. For example, i1, 2 indicates that when the 25% level and 50% level in two control windings are shorted, the work of winding in the current instantaneous value.

100%级

75%级

50%级

25%级X

X1

X2

X3

TR1

TR2

TR3

i1

u1

i5

i4

i3

i2K

Figure 2. Stepped controllable switching high voltage shunt reactor

equivalence principle Suppose the k-th state regulation in the control winding,

and TRk+1 trigger angle, ignoring the excitation current. From the figure 2, we get the current for work windings:

(0 ,,

2 2 )

tt

t

ω φπ φ ω π φ

π φ ω π

< <− < < +

− < <

tφ ω π φ< < −

2tπ φ ω π φ+ < < −

1, 1, 1

1, 1, 1

1, 1, 1

1, 1, 1

1, 1, 1

2 sin

2 sin

( 2 2 ) sin

2 sin

( 2 2 ) sin

k k

k k

k k

k k

k k

i I t

i I t

I I

i I t

I I

ω

ω

φ

ω

φ

−

−

−

−

−

⎧⎪⎪⎪⎪

=

= −

−

= +

⎪⎨⎪

⎩ −

⎪⎪⎪⎪

Definition *1, 1, 1, 1

1, 1, 1

/ 2/

k k k

k k k

i i II Iβ

−

−

==

Then (1) becomes

(0 ,,

2 2 )

tt

t

ω φπ φ ω π φ

π φ ω π

< <− < < +

− < <

tφ ω π φ< < −

2tπ φ ω π φ+ < < −

*1,

*1,

*1,

1 sin

1sin (1 )sin

1sin (1 )sin

kk

kk

kk

i t

i t

i t

ωβ

ω φβ

ω φβ

⎧=⎪

⎪⎪

= − −⎨⎪⎪

= + −⎪⎩

By (3), we get

1, 1, 1 1, 1, 1( 3 ) / ( 3 ) /k k k k kUI UI Q Qβ − −= =

kβ is the capacity increase coefficient, when we known the rated capacity of controllable high voltage shunt reactor, inter-stage capacity increase coefficient will determine the capacity at all levels, that determines the number of control winding, the final decision of the controllable high voltage shunt reactor, to current injected into the grid harmonic coefficient, in this article 300Mvar 750kV stepped controllable high voltage shunt reactor 2 3 42, 1.5, 1.33β β β= = = .

To (4) the Fourier series transform, obtain the relative value of work winding current fundamental wave amplitude value:

21 1,* *

1 1, 1,01,

1 sin2

1 1 1 2 1(2 sin 2 ) ( 2 sin 2 ) (1 )sin 2

m km k k

k

k k

II i td t

I

π

ω ωπ

φ φ π φ φ φπ β π π β

= =

= − + − + − −

∫

Work winding h times harmonic current amplitude:

21,* *

1, 1,01,

1 sinh2

2 1 1 1[ sin(1 ) sin(1 ) ]1 1

2 1 1[ sin(1 ) sin(1 ) ]1 1

4 1(1 )sin cosh

hm jhm j j

j

k

k

II i td t

I

h hh h

h hh h

h

π

ω ωπ

φ φπ β

φ φπ

φ φπ β

= =

= − − + +− −

− + + + −− +

−

∫

Make

1, 1, 1

1, 1, 1, 1

/( ) /

k k k k

k k k k k

K I II I Iα

+ −

+ −

== −

From (3), (8), (9) can be obtained

k k kKβ α= +

Equation (6), (7) becomes

*1 1,

1 1 [ ]

2( 1) ( 1)sin 2 ]

m j k kk

k k k k

I K

K K

α ππ β

α φ α φ

= + −

+ − − + −

*1,

2 1 ( 1)

1 1[ sin(1 ) sin(1 ) ]1 1

hm j k kk

I Kh

h hh h

απ β

φ φ

= − + − ⋅

− + +− +

(12)

From (10), (11), (12) can get work winding current h times

harmonic coefficient: *

1, 1,*

1 1, 1 1,

2 1 sin( 1) sin( 1)( 1)[ ]1 1

11 ( 1)( 2 sin 2 )

hm j hm jh

m j m j

j jj

k k

I IF

I Ih hK

h h h

K

φ φαπ β

α π φ φπ

= =

− +− + − +− +

=+ + − − −

So the harmonic coefficient and k-th control winding

current and the NO.1,2, ..., k-1 control winding current is already in short-circuit state has a relationship.

According to Fourier analysis, the use of thyristors as switching devices is inevitable in the control winding voltage and current contain harmonics, and the number of harmonics contained in the odd. As the thyristor is a semi-controlled switch, once it cannot turn off on their own, turn-off thyristor is set to be forced off the circuit, so current will contain a large number of low harmonics, the lower order harmonics, the greater the amplitude of harmonic; when larger thyristor control angle, the greater the amplitude of the harmonic; but with the increase in the number of harmonics, the harmonic content will reduced; these are due to the characteristics of thyristor phase-shift control.

Based on the above formula available, when φ = 30 °, F3 = 13.8%, whenφ = 18 °, F5 = 5%, combined with Fourier analysis shows that, when the thyristor firing angle between the change in 0°~180°, the amplitude of even harmonics of the size of the order of magnitude is 10-3, which is approximately 0, can be ignored; when thyristor control angle between the change in 50°~160°, the third harmonic amplitude is still very large, in some thyristor control angles, the proportion of harmonic relative fundamental wave amplitude reached 50%~70%, the maximum proportion of 5 times harmonic amplitude relative to the fundamental wave amplitude reached 60%. With the increasing number of harmonics, the harmonic amplitude smaller, according to the theoretical analysis of these trends of data can be found that exist low-order harmonic pollution, high harmonic pollution relatively smaller in the control winding of stepped controllable high voltage shunt reactor.

However, for the total capacity of stepped controllable high voltage shunt reactor, the capacity of each control winding only a part, and each stable state has only one winding in the adjustable state, the other winding either always short circuit or always open, so, despite the current of control winding in the adjustable state exists harmonic pollution, and the possession of the harmonic components of the control winding fundamental wave current proportion can be great, but from the work winding side view, the current harmonics compared with the total current is not too great, to meet the power grid operation for power quality harmonic requirements[7]. Also, the smooth adjustment of stepped controllable high voltage shunt reactor is designed in order to reduce when the part of cycle a thyristor in conduction arising from the harmonic content.

V. CONCLUSION

1）The SCSR is devised transformer and reactor into one. The design of the transformer leakage reactance is 100%, according to its working principle and characteristics we can see, in theory, can be realized without the transition process control, with the harmonic current small, fast response, small power loss, etc.

2）Because characteristics of SCSR, it is configured with a certain difficulty to protect and research value. To our super/ UHV fully prepared to work, study of SCSR protection is particularly important.

3）The output current harmonic content of SCSR winding work is connected with in the conduction state of the whole number and capacity, in adjust winding capacity accounted for the total reactor volume ratio, in adjust the firing angle of thyristor winding state and other factors. From the work winding side view, the current harmonics compared with the total current is not too great, to meet the power grid operation for power quality harmonic requirements.

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