Interconnected System of Power Stations | Electrical Engineering

In this article we will discuss about:- 1. Introduction to Interconnectors 2. Load Sharing of Interconnectors 3. Power Limit of Interconnectors 4. Interconnectors in Parallel.

Introduction to Interconnectors:

When large loads are to be supplied from two power stations, the power stations are required to be interconnected so that there is no overloading and the loads are shared almost equally.

Consider two power stations S₁ and S₂ supplying load currents I₁ and I₂ to the load through transmission lines 1 and 2 in addition to their local loads. The power stations are also interconnected by an interconnector, which is carrying current I_i. The transfer of power is taking place as shown by direction of arrows (Fig. 13.16).

Since local loads are also connected to power stations S₁ and S₂, voltages at their bus-bars have to be maintained constant at the same value as for consumers at load end.

In order that both transmission lines 1 and 2 deliver equal power and the system operates at the same terminal voltage, it is necessary that active components of the line currents I₁ and I₂ are equal. In order that the total current in lines 1 and 2 be a minimum it is necessary that they should also have reactive components equal.

In order to achieve this in practice regulating equipment is required to be installed at the sending end of each transmission line and in the interconnector. The regulating equipment installed at the sending end of each transmission line and in the interconnector will look after the impedance drops in the transmission lines and interconnector.

As in case of alternators operating in parallel, for stable operation of the interconnected generating stations, it is necessary that the two stations must be interconnected through reactor. The incorporation of interconnector introduces angular displacement between the two stations, enabling the power to flow from one station to the other as required by operating conditions. The incorporation of impedance will cause an unnecessary loss of power.

Load Sharing of Interconnectors:

Consider two generating stations A and B operating in parallel and connected through an interconnector. Let the terminal voltage of operating stations A and B be V_A and V_B respectively, equal in magnitude but displaced from each other by an angle θ. The resultant of voltages V_A and V_B, V_AB acts along the interconnector and causes a current of I to flow through it. The current, I in the interconnector lags almost 90° behind the resultant voltage V_AB as most of the interconnectors are designed to have reactance and negligibly small resistance.

This current will be almost in phase with the voltage of the generating station, which is leading i.e., V_B in this particular case and the power transfer will result from power station b to station A in the above case. When two voltages V_A and V_B are in phase, there is no power flow between the generating stations. It is thus obvious that for power transfer between interconnected stations angular displacement between the terminal voltages of the two stations must exist. This is sometimes artificially introduced by the regulating equipment at the bus-bars of one of the stations.

To study the effect of inphase voltage boost of station B having surplus available capacity on the power transfer between the generating stations consider phasor diagram shown in Fig. 13.18. The voltage of station B is boosted from V_B to V’_B, the voltage drop in interconnector is V’_AB and the current I through the interconnector lags almost 90° behind the V’_AB in case the interconnector is reactive. The current in this situation is in phase with V_A and is in quadrature with V’_B and therefore no real power can be supplied by station B to station A in this condition.

Hence it is obvious that if the interconnector had only reactance, inphase voltage boost between the generating stations would not enable the interconnector to transfer real power between them.

To study the effect of quadrature voltage boost of station b having surplus available capacity on the power transfer be­tween the generating stations consider phasor diagram shown in Fig. 13.19. The voltage of generating stations B, V_B is in­creased to V’_B by the quadrature boost V_BV’_B, the voltage drop in the interconnector is V’_BV_B and the current through the interconnector is in phase with V_B and the power is transferred from generating station b to generating station A.

Thus, we see that quadrature voltage boost enables the real power to be transferred between power stations without changing the rotor angular displacement.

From the above discussion we conclude that:

(i) For stable operation of the interconnected power stations, it is necessary that the two stations must be interconnected through a reactor but undue increase in the value of reactance of the interconnector must not be allowed otherwise a large phase displacement between the stations will be required in order that a substantial amount of current may flow through the interconnector and large phase displacement would mean unstable operation i.e., the stations would tend to fall out of steps.

(ii) In phase voltage boost will affect the transfer of wattless current through the interconnector and will be of little help in transfer of real power between the stations.

(iii) Quadrature voltage boost will affect the transfer of real power between power stations. This method results in increase in inherent stability of the system and sudden loads can be dealt with easily.

Power Limit of Interconnectors:

The power transmitted through the interconnector depends upon the phase displacement between the voltages of two generating stations and will be maximum when the phase angle between the two voltages is 90°. If the phase displacement exceeds 90°; the increase in current is more than counterbalanced by the fall in power factor. Let the station b transmit power to station A and phase displacement between the two generating station voltages be 90°.

Under these conditions current I lags behind the voltage V_B by 45°, and leads the voltage V_A by 45°, the former station being at the sending end and the latter being at the receiving end and-

The maximum power transferred is also referred to as the synchronous capacity of the interconnector and it is defined as the kW transmitted per radian of the displacement between the two voltages of the power stations.

Interconnectors in Parallel:

It becomes sometimes necessary to connect two power stations S₁ and S₂ through two interconnectors in parallel running along the same or different routes. This situation arises when the power under transfer exceeds the capacity of a single interconnector. The load shared by two interconnectors will be in the inverse ratio of their impedances Z₁ and Z₂ as the voltages at the sending end and receiving end of the both the interconnectors are the same. The load carried by the two interconnectors can be different from this recourse if suitable voltage regulating equipment is provided in each line.

The problem of correct subdivision of load becomes complicated when the two interconnectors connecting two power stations operate at different voltages, say 132 kV and 220 kV. The conditions are diagrammatically represented in Fig. 13.21 wherein RE is the voltage regulating equipment (say tap-changing transformer or induction regulator), S₁ and S₂ are the interconnected power stations and T₁, T₂, T₃, and T₄ are power transformers of suitable rating and turn-ratios.

As regard load division, the effect of transformers can be taken into account simply by adding their resistances and reactances to the impedance (resistance and reactance) of the concerned interconnector. The equivalent system to the receiving-end voltage can be represented as in Fig. 13.22, where T is a common tap changing transformer and IR₁ and IR₂ are the induction regulators. Receiving-end current I_R is fixed and its components are I₁ and I₂ so that I_R = I₁ + I₂. The arithmetic sum of I₁ and I₂ will be minimum when they are in phase. I₁ and I₂ are assumed to be the currents which the two lines are designed to carry on the basis of correct division of load between them.

The voltage at P₁ and P₂ are given by phasors OV₁ (phasor sum of V_R and I₁Z₁) and OV₂ (phasor sum of V_R and I₂Z₂) respectively (Fig. 13.23), which can be possibly obtained by means of one common tap changing transformer T and a separate induction regulator for each line.

The voltage at P then corresponds to OD or OD’ (Fig. 13.24) and voltage at P₁ is the phasor sum of OD (or OD’) and DV₁ (or D’V₁) the latter having been obtained by means of the induction regulator IR₁. Similarly it can be shown that voltage required at P₂ is OV₂.

The same results can be obtained by using two separate tap changing transformers T₁ and T₂ -one in each line and one induction regulator IR in one of the lines, as shown in Fig. 13.25.

Let the desired voltages at Q₁ and Q₂ for proper division of load be OV₁ and OV₂ respectively and the power station be OD (Fig. 13.26). For the first line, voltage OV₁ can be obtained by increasing voltage OD’ to OD by tap changing transformer T₁ and increasing voltage OD further to OV₁ by induction regulator IR. For second line the voltage OV₂ can be had by increasing voltage OD’ to OV₂ by tap changing transformer T₂.

If however there is a provision to affect a change from OD’ to OD in the generator itself by changing excitation, tap- changing provision will not be required on transformer T₁. This arrangement is possible only if S₁ is not having any local load connected across it because otherwise its voltage will require to be maintained constant at OD’.