A Concept 'Smart Switch' for Single-Phase Transformer and Reactor Control

Introduced to electrical power networks at the turn of the century, the use of power electronic devices to control and regulate single-phase networks has trailed behind. A likely reason for this outcome is the relative cost of 'smart devices' in a system of low earned revenue. Now a feature of grid modernisation projects, interest in 'smart devices' as a means to extend the useful life of distribution assets, delay capital expenditure, lower operating costs and to improve the supply reliability, is growing. Described in this paper for the control of single-phase transformers and reactors is a concept 'smart switch' that uses a low voltage low power thyristor. Built with a novel magnetic core and winding arrangement, the disconnection and reconnection of a transformer or reactor is controlled by the semiconductor switch. The concept design is demonstrated in this paper for a thermal overload and switched shunt reactor transformer applications.


I. INTRODUCTION
o release additional line capacity as the demand for power increases, fixed shunt reactors, which are used to mitigate the Ferranti effect in Single Wire Earth Return Systems (SWER), must be disconnected. In a low cost design of low earned revenue however, the expense of a high voltage automated switch for this purpose is unreasonable. An automation barrier, shunt load reactors of lower voltage and rating are instead connected at points of low voltage supply where they can be switched economically [1].
Illegal electricity connections and meter tampering usually in low voltage networks is a worldwide problem that deprives utilities of revenue. A safety hazard, the consequence of the illegal practice is poor supply quality and damage to equipment [2] as an overload will usually result in a supply trip or the failure of equipment if not detected. Remote from a service depot, an interruption in supply because of an overload condition can be prolonged.
Presented in this paper a concept 'smart switch' for singlephase transformers offers an economic solution to the problem of fixed shunt reactors, area wide supply trips because of an overload condition and thermal damage to equipment. Equipped with a voltage or overload sensor, a smart transformer or reactor is with power electronic technology [3], [4] able to regulate and control the voltage and power in a single-phase distribution network autonomously.

I. AUTONOMOUS SMART DEVICE
A smart device in autonomous electricity grid architecture is with the latest sensor technologies largely able to control itself [5]. Equipped with a communication link, the smart switch device proposed in this paper equates to:  A load management tool for single-phase distribution networks.  A voltage management tool for single-phase distribution networks.  A power quality management tool for single-phase distribution networks.

II. CONCEPT SWITCH
In Figure 1 (a), (b) and (c), the interaction between windings W 2 , W 3 and W 4 , for a voltage applied to winding W 1 , is a function of the magnetic core fluxes φ 1 , φ 2 and φ 3 , which for current to flow in the windings must be in balance [6]- [8]. A product of the winding current I W2 , I W3 , I W4 and connected load impedance Z L2 , Z L3 and Z L4 , Figure 2, the voltage drop -(I W2 Z L2 + I W3 Z L3 + I W4 Z L4 ) is equal to the terminal voltage V Wn . The terminal voltage V Wn is V Wn = E Wn -(I Wn R Wn + jI Wn X Wn ) T where E Wn is the induced winding voltage, R Wn the winding resistance and X Wn the winding leakage reactance. Fig. 4(a). The equivalent circuit of a load ZLn across each winding Wn (b) when one winding is short circuit, and (c) when one winding is open circuit. The winding resistance RWn and leakage reactance XWn is not included.
Across each winding, the voltage drop -I Wn is 0.3V Wn for a load impedance Z L2 = Z L3 = Z L4 and winding turns N W2 = N W3 = N W4 .
The load impedance Z L2 with winding W 2 open circuit, Figure  4(b), is Z L2 = ∞ Ω (). The winding current I W2 , I W3 and I W4 is for this condition I W2 = I W3 = I W4 = 0, which satisfies the condition of magnetic balance for an open circuit winding, e.g. the core flux φ 1 = φ 2 = φ 3 = 0.
Winding W 2 is in Figure 4(c) short circuit, and for this condition the terminal voltage V W1 and winding current I W1 are zero because the voltage drop across winding W 3 and W 4 is 0.5V Wn for a load impedance Z L3 = Z L4 .
The volt per turn is for turns N W1 = N W2 = N W3 = N W4 : The ampere-turn relationship for magnetic balance is: The winding current I W1 is for a voltage V W1 : The load impedance Z Ln is in Figure 5 referred to winding W 1 . The referred load impedance Z Ln ' is: Shown in Figure 6(a), (b) and (c), for an alternating voltage V applied across W 1 , current circulates in winding W 3 and W 4 if the solid state switch S 1 conducts (the switch is closed). The flow of current ceases when switch S1 is open (the solid state switch does not conduct). Fig. 6. The core flux for (a) a conducting solid state switch S1 across winding W2, and (b) a load impedance ZL3 across winding W3, and (c) a load impedance ZL4 across winding W4 with a voltage V applied across windings W1.

A. Transformer simulation test
In Figure 7 is a test model of the transformer described. Wound around the inner limbs of three single-phase magnetic circuits are two windings W 11 and W 22 Figure 8. The windings have an equal number of turns. Wound over the outer limbs are windings W 2 , W 3 and W 4 , which also have the same number of turns. The turn ratio between the inner and outer limb windings is 0.5:1. The three magnetic circuits, which are shown, mutually displaced by 120 o , have no effect on the transformer action. A different design arrangement is possible.  The test results for a resistive load connected across winding W 3 and W 4 , with winding W 2 short circuit and winding W 11 open circuit are shown in Figure 9(b). The circuit has a 220 V single-phase voltage source connected across winding W 12 . Fig. 9(b). The test results of a resistive load connected across windings W3 and W4 with winding W2 short circuit and a 220 V single-phase voltage applied across W12 The test results for a resistive load connected across winding W 2 , W 3 and W 4 , with winding W 11 open circuit, are shown in Figure 9(c). The results obtained in Fig 9(a) and Fig. 9(b) confirm the hypothesis of a concept a switch previously described. Connected across winding W 2 a solid state switch of low voltage and low power can be used to control the current in windings W 3 and W 4 and by association the current in winding W 11 . The results also demonstrate the load dependent distribution of voltage across each transformer winding.

II. SHUNT REACTOR TRANSFORMER CASE
The core and winding arrangement of a novel shunt reactor transformer design for SWER line voltage regulation is shown in Figure 10.  The primary winding, which is grounded, is connected to a high voltage line. An intelligent electronic device (IED), which is connected to the secondary winding, controls the flow of an inductive current in the primary winding by way of a voltage sensor, inductors L 11 , L 12 and L 13 and a solid state switch SW 1 . Switch SW 2 is an option for a stepped voltage adjustment. The tertiary winding current T n is zero when switch SW 1 conducts, e.g. I T1 = I T2 = I T3 = 0 Figure 12(a). Rated for a highly inductive load, the power rating of switch SW 1 is low as the voltage drop across the load inductors L 12 and L 13 is equal to the induced voltage. Switch SW 1 has a blocking voltage equal to or greater than the highest anticipated open circuit winding voltage, which in Figure  12(b) is 800 V. Built for an inductive rating of 25 KVAr, the reactor transformer has with switch SW 2 and inductor L 11 a stepped rating of 25 KVAr and 16 KVAr, Figure 12(c). Switch SW 2 is rated for the load current of the (shunt) load inductor L 11 and open circuit voltage of tertiary winding T 1 . The shunt reactor transformer design has with off-the-shelf 400 V 41 mH (shunt) load reactors the same rating as a fixed shunt reactor, which is 25 kVAr [9].The tertiary windings are sized for the connected load. The state of the solid state bidirectional triode thyristor (TRIAC) switches is controlled by a voltage sensor with hysteresis and time delay settings.
Because the revenue earned from energy retailing has with the introduction of battery storage and solar PV technologies declined steadily power electronic devices are today viewed as a viable upgrade path to release network capacity, [10]- [17]. Discussed in this paper, is a battery energy storage system (BESS) that may be integrated in the shunt reactor transformer design Fig 11 as an energy storage solution to release additional line capacity.

A. Line voltage regulation case studies
Invented in New Zealand in 1925, the Single Wire Earth Return (SWER) distribution system is widely used to supply rural loads of low load density. Connected between a single conductor and ground, the load current of a single-phase transformer in a SWER system flows back to the ground terminal of an isolating transformer, Figure 13, or the ground terminal of a three-phase supply [18]. Although a cost effective supply solution, SWER designs often have a poor voltage regulation and suffer from high losses and capacity constraints because of a high line charging current and high line impedance. Fig. 13. A single line SWER distribution system that is constructed with an isolating transformer The phase voltage of a 33 kV distribution system is 19.1 kV or 12.7 kV for a 22 kV system or 6.7 kV for an 11 kV system. A load flow model of a SWER distribution line [19], [20] that is supplied from two phases of a three-phase 33 kV distribution line is shown in Figure 14. Included in the model is a 33/19.1 kV 150 kVA isolating transformer and two line sections, L1 and L2. The branch currents in this model are: By substitution Is 1 is: and rewritten Is 1 is: Is 1 = (y/2)Vs 1 + (y/2)Vr 1 + (y/2)Vs 2 + (y/2)Vr 2 + 0 Considering a homogeneous line L where L = L1 + L2, then the current I S is: The node voltage Vs is by substitution: At no load, the node voltage Vs is Vs = (1 + (Z L .Y/2)) Vr and the voltage drop across line L is: Neglecting the line resistance, the voltage drop by substitution is: For a negative sign, the receiving voltage Vr is greater than the sending voltage Vs.
The study case parameters of a typical medium length SWER distribution line Fig. 14 are listed in Table 1, Table 2 and  Table 3.   Seen in Table 2, the use of high tensile light weight stranded Steel Core Galvanised Zinc (SC/GZ) or stranded Steel Core Aluminium Clad (SC/AC) conductors to increase the span distance results in a high line a.c. resistance. Using a MATHCAD software program, the node voltages and branch currents are determined from a Backward/Forward load flow sweep method [21], [22]; where the node voltages, in an initial backward walk from node A2 L2 to A1 T , are kept the same before they are then adjusted in a forward walk keeping the branch currents the same for recursive iterations.

1) Case study 1 -Line charging current
At no-load the receiving end voltage V r2 is higher than the sending end voltage Vs 1 Table 4. Across line L1 and L2, the voltage drop for a source voltage V T = 1.0, is Vs 1 -V r2 = -(0.03 + j0.14). Given a contract voltage of +5% of nominal voltage V n , the customer supply transformers at nodes Vr 1 , Vs 2 and Vr 2 must be set on a minus voltage tap to stay within limit. Table 4 No-load node voltages VT = 1.0 (Fig. 14 The line charging current is in Table 5 higher at the sending end than at the receiving end. Table 5 No-load branch currents VT = 1.0 (Fig. 14 For a fully loaded three-phase system where the source voltage V T = 0.9V n , the supply transformers must now be set on a plus voltage tap to stay within limit. Table 6 No-load node voltages VT = 0.9 (Fig. 14 The line charging current Table 7 is now lower than in Table 5 for an un-loaded three-phase system. Table 7 No-load branch currents VT = 0.9 (Fig. 14

2) Case study 2 -Shunt reactors at no load
With 25 kVAr shunt reactors installed at nodes Vr 1 , Vs 2 and Vr 2 , the SWER line has a flat voltage profile when the three-phase system is at no-load. Set on a plus voltage tap, a voltage drop when the line is loaded is allowed for at points of customer supply. Table 8 No-load node voltages VT = 1.0 (Fig. 14.)  The branch currents I 2 and I 5 , are with shunt lower Table 9. Table 9 No-load branch currents VT = 1.0 (Fig. 14.)

3) Case study 3 -Load demand and line capacity
With the customer supply transformers on a maximum +5% voltage tap at nodes Vr 1 , Vs 2 and Vr2 the supply voltage is below the acceptable limit when both the three-phase and single-phase networks are fully loaded Table 10. Table 10 Node voltages VT = 0.9 (Fig. 14 Table 11 Branch currents VT = 0.9 (Fig. 14

4) Case study 4 -Battery Energy Storage System
Released at high load demand, the energy stored in a battery energy storage system (BESS), Figure 15, provides additional line capacity to support the line voltage Table 12. The current I 2 at peak-load, Table 13, is with a 24 kW BESS at node Vr 1 and Vr 2 , Fig. 14, lower than the line current I 2 Table  11. The voltage drop across line L1 and L2 is with both BESSs in service lower.
The battery bank may be charged from the power grid at a lagging power factor in times of light load or from a solar charging system when the irradiance for this purpose is sufficient. With the batteries fully charged, excess capacity is then available to be exported to the power grid.
A leading power factor setting is appropriate for the export of power to the power grid because with a unity power factor setting the imaginary voltage drop remains the same. Fig. 15. A line shunt reactor transformer with a battery energy storage system connected to the secondary winding. Switch SW1 across tertiary winding T1 with Fig. 11 is not conducting.  Table 13 Branch currents (Fig. 14

III. THERMAL OVERLOAD CASE
Over temperature, accelerated aging and the risk of transformer failure, can have an adverse effect on the operation of an electricity power grid. Rated for a maximum temperature rise above ambient temperature, the insulation-life of a transformer is shortened by an operating temperature above the design limit. In Figure 16, either the top oil or hotspot temperature measurement functions to protect the transformer against an overload by opening switch SW 1 . Switch SW 1 operates autonomously to isolate an overload before the load is restored after an appropriate cooling period. An ′Adaptive Transformer Thermal Overload' protection scheme, which measures the ambient temperature, has only one setting -a per unit loss-of-life factor.
Equipped with a communication link in a Supervisory Control and Data Acquisition (SCADA) system, an operator is informed of an overload when it occurs. The switch may also in this system be used to redistribute single-phase loads and to establish a balance between the supply and demand for energy. It is also a tool to dissuade illegal connections.
While numeric thermal overload relays typically use a combination of current, ambient temperature and transformer top oil temperature to detect the presence of an over-load, the operating temperature of a transformer can also be estimated by a thermal-replica model. In this model a maximum temperature rise is calculated from a measured current. A simple representation of the operating temperature of a transformer, the model does not account for variations in the ambient temperature and is not a true top-oil or winding or hot-spot temperature measurement [23]. Fig. 15.
A single-phase distribution transformer equipped with an autonomous thermal overload protection scheme

IV. CONCLUSION
A low voltage, low power bipolar solid state switch for single-phase transformer and reactor control was presented in this paper. Central in the switch concept design is a novel magnetic core and winding arrangement, which was demonstrated for an autonomous SWER line shunt reactor application and a 'smart' single-phase transformer overload protection and control system.
The concept switch deals with the unacceptable high cost of a high voltage switch, which is needed in SWER distribution networks for line shunt reactors to release additional line capacity as the demand for power grows.
As an intelligent control switch it also addresses the need for an autonomous overload protection scheme to protect single-phase transformers against overloads, dissuade illegal electricity connections, and to re-distribute or balance load with supply.