Interphase Power Controller | Improvement and Applications

Introduction

The installation of the world’s first Interphase Power Controller (IPC) at the New York Power Authority (NYPA) Plattsburgh substation on the 115 kV tie that interconnects the NYPA and VELCO systems, is described in. This IPC belongs to the category of IPCs called the Assisted Phase Shifting Transformers (APST) where the major objective is to provide a passive solution for controlling steady state and post-contingency power flows. The basic principle of the APST is to add a high impedance in parallel with an existing PST. The impedance carries a portion of the power flow such that the installation’s capabilities are increased.

Initial Concept of IPC

The FACTS controllers based on power electronic devices offer both flexibility and high speed (of operation). There are applications where speed is not so important, but flexible operation is required. For example, in networks that are sensitive to daily or seasonal load variations, problems can arise in the regulation of power flows in a steady state.

The IPC concept involves the series connection of impedances between different phases of the two (synchronous) subnetworks that are to be interconnected, hence the name: the Interphase Power Controller.

The IPC proposed acts as a current source with the following characteristics:

1. The power flow is nearly constant (within 10%) for a wide range of (±25◦) of angle between the two subnetworks.
2. There is no significant contribution to the short circuit level by the interconnection between the two subnetworks.
3. Major contingencies on one side of the IPC have negligible impact on the voltages of the other side.
4. As no power electronic controllers are required, harmonics are not generated.

The IPC consists of three-phase reactors and capacitors each installed in series between the two subnetworks that are to be interconnected.

Improvements in IPC

To reduce the ratings of IPC and thus their costs and losses, improvements in the original concept were introduced. A major innovation is to use phase shifting transformers (PST) to apply phase-shifted voltages to the two susacceptances. This is termed injection IPC as phase shifts are introduced injection of quadrature voltages in series with the two susceptances. (see Fig).

In the injection of IPC, the phase shift angles ψ1 and ψ2 can be varied while they are fixed at ±60◦ in the IPC120. In the injection IPC, the voltages VB₁ and VB₂ are substantially smaller because ψ1 and ψ2 can be as small as ±20◦ for the same useful range of δSR (±25◦). Thus, the ratings are reduced which in turn results in the reduction of costs, and space requirement losses. These are important issues in higher-voltage transmission lines.

Application of IPC as Fault Current Limiting Transformer (FCLT)

Fault Current Limiting Transformers (FCLT) have been proposed as a special case of IPC technology. A FCLT involves a phase-shifted voltage to be applied in series with a capacitive impedance. Consider a transformer substation in which several transformers are connected in parallel to the supply load. If the additional load is to be met by adding a new transformer, there is a danger of increased fault level. This can be controlled by connecting an IPC as a FCLT (see Fig.).

A feasibility study for converting a 345/138 kV auto-transformer into an FCLT is reported. By connecting one winding of a two-winding transformer across the B phase of autotransformers 13.8 kV delta connected tertiary winding and other winding in series with A phase of autotransformers 138 kV terminal (see Fig.) a phase shifted voltage V_{se A} is obtained. Fig shows the FCLT in parallel with three autotransformers. A series capacitor of −j33.2 ohms is connected in series with the FCLT.

The leakage impedance of the autotransformer (X_ps) is 8.4 ohms while that of the series-connected two winding transformer is 1 ohm. Under normal operating conditions, the power flow in an FCLT (IPC) is a function of the injected voltage (V_se) which can be adjusted by controlling the tap position of the series-connected transformer. Thus, the FCLT has similar characteristics as the voltage injection type of IPC described earlier.

During faults, the current flow through FCLT is limited by the high series (capacitive) impedance. The FCLT has about 3 times the impedance of the autotransformer and hence will contribute only about 1/3 of the fault current provided by an autotransformer.

Applications of IPC

The power flows in transmission lines connected to a generating station or a load center are determined by the outputs of the generators or load demands which are known in advance. In case of contingencies involving tripping of line(s), the power flow in the remaining lines is regulated by tripping of generators or loads. In contrast, if a tie line is to connect two networks (or subnetworks) the power flow can vary widely depending on the phase angle (δ) difference across the two buses to which the tie line is connected.

To prevent frequent tripping of the tie line, it becomes essential to modify the power characteristics of the line. An Interphase Controller (IPC) uses only passive reactive impedances and a phase-shifting transformer to adjust the power flow characteristics of the line such that over the range of δ (determined by the network contingencies), the active power remains practically constant.

Actual implementation of a technology depends on the cost-benefit aspects and comparison with competitive technologies. For example, if speed is not essential, a phase shifting transformer (PST) which is manually controlled, may be adequate. The introduction of passive impedances (capacitive or inductive) can improve the capability of existing PST. Thus, the first installation of IPC in Plattsburgh station belongs to this category.

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