Z. H. Li et al.

West of Henan and South of Henan, so the suitab le location of Henan power grid integrating Hami- Zhen gzho u

UHVDC is the Center of Henan power grid. However, short circuit curr ents of 500 kV buses in Center of Henan

power grid are almost close to the operation upper limitatio n. In order to decrease the short circuit cur r e nt s ef-

fectively, it’s necessary to improve and strengthen the network structure of the Center of Henan power grid and

calculate short circuit cur r ents in the new network str uct ur e [1].

2. Effect of Short Circuit Current

Short circuit current calculation and control is one of the most important functi ons in power system analysis.

Effect of short circuit current can be summarized as below [2]:

1) Short circuit is a kind of seri ous fault, it may break the stable operation of power system, break the power

supply and even bring blackout.

2) It will decrease the voltage of power system greatly, damage industry equipment and product, and debase

the quality of power energy.

3) The flaring electric arc may burn out the equipment, and if the short circuit current exceeds the interrupting

ability of breaker, it may cause more serious fault.

4) It brings thermo effect and electromotive force effect, they may damage the equipment.

5) Short circuit current may disturb communication system and ele ctronic system nearby.

However, short circuit current is far from having no benefit. Bigger short circuit current means stronger abili-

ty to tole rate serious short circuit fault for a certain power system. In order to ensure the security and stab ility of

power system, it’s necessary to keep a certain short circuit current level and control the short circuit current

within an appropriate range.

3. Analysis of Short Circuit CURRENT

3.1. Calculation Conditions of Short Circuit Current

The calculation conditions of short circuit current in the paper are as below [3] [4]:

1) Calculating result is the effective value of industrial frequency current co mpo nent injected into the short

circuit point at the moment at which the shor t circuit a rise s.

2) The thr ee -phase AC system is a symmetrical system, the structures of three phases of stator windings are

symmetrical, and the spatial position differs by 120˚ electrical angle.

3) The short circuit is a b olted fault, and the short circuit impedance in the fault point has a value of zero.

4) To suppose the quadrature axis subtransient reactance is equal to the direct axis subtransient reactance. The

model of sync hr o nous generator with the direct axis subtransient reactance a nd constant subtransient electromo-

tive force is used. The magnetic saturation effect of generator and the skin effect of co nduc t o r ar en’t considered.

5) Magnetic saturation effect of static components is n’t considered, parameters of electric equipment won’t

change with the value of current.

6) The tap of transformer is used with the actual setting.

7) The operation mode of Henan power grid is with all generators and all lines operated, the load model of

constant impedance load and induction motor is used.

3.2. Short Circuit Current of 500 kV System in Henan in 2013

The calculation results of three-phase short circuit current of some important 500 kV buses in Henan power grid

in 2013 are shown in Table 1.

From the above table, if the induction motor model was considered, then three-phase short circuit current of

some important 500 kV buses in Center of Henan power grid will exceed 63 kA. In fact the calculation cond i-

tions of short circuit current are somewhat conservative. In order to keep pace with the actual status in field, in-

duction motor may be not taken into account in the succeeding study. Even if the induction motor isn’t taken in-

to account, the maxi mu m value of thr ee -phase short circuit current in Henan power grid is close to the limit

breaking capacity of breaker. It’s necessary to decrease short circuit current by some measures.

3.3. Short Circuit Current of 500 kV System in Henan in 2014

The calculation results of three-phase short circuit current of some important 500 kV buses in Henan power grid