Phase Shifting Transformers Control and Protection Philosophy
The Indian economy is growing at a rapid pace and so in Indian power system. The installed capacity of 2.03 GW is planned by the year 2011-12 and the estimated peak demand is expected to grow to 150300 MW in the year 2011-12. With the result, the transmission & distribution is increasing many fold. However, the use of existing infrastructure to their optimal capacity rather than providing additional corridor is essential in order to have economical operation of grid.
- Dinesh Pawar
Transmission grid is used as a transport medium between generation and the load centers. The grid can be segregated in to physical group of alternate paths i.e. transmission sub-systems between the generation and the load center. The loading of parallel transmission sub-systems is governed by their impedances. The impedances are essentially determined by the type of conductor, its configuration and tower geometry. The line with the smaller impedance carries more power and vice versa. Due to uneven loading of the sub-systems in a meshed network, the total power transmission from generation to load is lower than expected.
Phase shifting transformer (PST), which is a member of FACTS family, can be used to control the active power flow in a complex power distribution network in a very efficient way. It provides a well defined phase shift between the primary & the secondary terminals. The purpose of this phase shift is usually the control of power flow over transmission lines. Both the magnitude and the direction of the power flow can be controlled by varying the phase shift.
This phase shift can be varied during operation in definite steps by use of an on load tap changer or a static tap changer and the sign of phase shift can be inverted from advance to retard and vice versa as well. The principal use of PSTs is at major inter-tie buses where the control of active power exchange is especially important.
Phase Shifting Transformers
Thus there are two main applications of PSTs: – one is to re-distribute power in parallel lines (both ends are at same voltage level) & another is to direct power from one voltage level to another (auto-transformer + PST).
The current distribution between two parallel lines will be in inverse proportion to their impedance. For example in Fig. 1a, the current distribution:
I1 = Itotal (X2/(X1+X2)) and
I2 = Itotal (X1/(X1+X2))
Because the main part of the line impedance is inductive reactance, inserting a voltage in phase with or opposite to the line voltage (changing the magnitude of the voltage) will have an impact mainly on the reactive part of currents (i.e. reactive power flows). The boost voltage with a phase angle perpendicular i.e. quadrature to the line voltage (creating a phase shift) influences mainly the real part of currents (i.e. real power flows). Thus the PST helps to vary the load as desired. Fig. 1b shows the load distribution by use of PST.
For the real power flow influence the most often used is the quadrature symmetric or the quadrature non-symmetric PST’s. A quadrature type PST is a unit where the boost voltage, which creates the phase shift between source and load terminals, is perpendicular to the line voltage at one terminal. This regulates the active power flow.
Design of Phase Shifting Transformers
The PST consists of two separate transformers: a shunt unit and a series unit. The shunt unit has its winding terminals connected so as to shift its output voltage by 90° with respect to the supply. The output of shunt unit is then applied as input to the series unit, which, because its secondary winding is in series with the main circuit, adds the phase-shifted component. The overall output voltage is hence the vector sum of the supply voltage and the 90° quadrature component. Tap connections on the shunt unit allow the magnitude of the quadrature component to be controlled, and thus the magnitude of the phase shift across the PST.
In an ideal un-loaded power transformer, the source side voltage and load side voltage are in phase. On loading the transformer, the angle between voltages (load angle) changes based on the transformer impedance and load. The difference between PST from a normal power transformer is of the non-standard phase shift angle between the source side and load side no-load voltages (phase-shift angle). There is an additional shift between source and load side voltages during load (transformer load angle). In PST there is a phase angle difference between currents of source and load side, unlike a power transformer.
Typical Arrangement of a PST in Grid Substation
Phase shifting transformer in a typical arrangement consists of a shunt unit and a series unit. The shunt unit is similar to interconnecting transformer with a control winding in addition to normal three windings (Primary, Secondary and Tertiary). The series unit is connected in conjunction with the shunt unit via its control winding so that boost voltage can be injected in quadrature. The above arrangement is supplemented with the disconnectors to offer flexibility in operation. The disconnectors are provided to ensure that the PST can also work as normal inter connecting transformer.
Figure 2 below shows the typical arrangement.
PSTSH = Shunt Unit
PSTSE = Series Unit
89TD = Disconnector for control winding
89TC & 89T = Disconnector for Series Unit
89TB = Disconnector for Bypassing PSTSE
Control of PST
Under normal conditions when the power flow is as intended, the series unit is kept in a bypass mode. In order to bypass PSTSE, disconnector 89TB is put ON and 89TC, 89T and 89TD are put OFF. This way only the PSTSH is in the circuit which acts as a normal power transformer interconnecting the two switchyards working under different voltage levels.
Under abnormal conditions when we want to have real power control from/ to switchyards connected at the either end of the PST, the bypass arrangement is no more used. And by putting OFF 89TB and putting 89TC, 89T & 89TD ON, the PSTSE is put under service. Due interlocks are taken care while operating these disconnectors as they are to be operated under no-load condition.
The on-load tap changer (OLTC) installed in the control winding is then used to control the boost voltage. This boost voltage after being injected to the line voltage in quadrature creates the phase shift between source and load terminals. For every step of tap changer, there is a definite voltage boost and corresponding phase shift. Thus every step contributes to a definite real power control. The phase shift with every step change is provided in the Rating and Diagram (R&D) plate of the PST.
Control winding is provided with a VT so that the injected voltage into the series transformer primary can be measured with voltmeter. This VT is also used for providing input to overfluxing protection.
Protection Philosophy for PST
In our typical arrangement of PST, there are two transformers, one Shunt and another Series. The protection philosophy of such an arrangement is to cover all the zones. Protection covered for each transformer can be similar to a normal transformer viz differential protection, over current protection, restricted earth fault protection, back-up earth fault protection and over fluxing protection.
Difference lies in differential protection adopted for PST. Standard transformer differential protection can not be used due to variable phase angle shift across the PST. Thus if a numerical power transformer differential relay is directly applied for the differential protection of PST, and set for Yy0 vector group compensation the differential relay will not be able to compensate for variable phase angle shift θ cause by position of on-load tap changer (OLTC) installed in the control winding. As a result a false differential current would exist which will vary in accordance with phase angle shift across the PST. However in order to apply the standard numerical transformer differential protection on a PST the following compensation shall be provided -
- Primary current magnitude difference on different sides of the protected PST.
- Phase angle shift difference across PST.
- Zero sequence current elimination.
Current magnitude difference on the two sides of PST will occur during OLTC operation and is thus variable because of different OLTC positions. Latest Differential protection relays designed has the unique feature of online OLTC position monitoring (via BCD coded binary signal) which enables the differential protection relay to automatically compensate for transformer turns ratio changes caused by OLTC movement.
Hence the standard differential relays are able to compensate current magnitude differences caused by different OLTC positions but the relay is not able to compensate for non-standard phase angle shift caused by OLTC movement in PST operating mode.
In the algorithm for differential current calculation, matrix for differential current calculation can be modified to obtain coefficients for the matrix which is variable with the phase angle shift. Every OLTC position is associated with a phase angle shift between no-load voltage on either side. This phase angle shift vis-a-vis OLTC position is depicted in rating diagram plate of PST. Relay programming is done to ensure coefficients changes with change in phase angle shift and hence differential current calculation matrix changes with each OLTC position. Thus it seems to be fully feasible to provide online compensation for actual PST phase angle shift due to OLTC movement. Zero sequence current suppression is done by providing unloaded delta tertiary winding. Due to this delta tertiary winding, the zero sequence currents from the 400KV & 220 KV side must be always excluded from the differential current calculation.
Major Protections deployed in PST
Protection is more an art than science, every individual may think differently when it comes to deploy protection scheme to a system but objective is same to protect the equipment under fault condition. Fig. 3 depicts the various protections deployed on each transformer.
- Overall Differential Protection.
- Backup Impedance Protection.
- PSTSH Differential Protection with built-in over fluxing.
- PSTSH HV winding Restricted Earth fault protection.
- PSTSH HV winding Directional Overcurrent and Earth Fault Protection.
- PSTSH LV winding Restricted Earth fault protection.
- PSTSH LV winding Overcurrent and Earth Fault Protection.
- PSTSH Control winding Restricted Earth Fault protection.
- PSTSH Control winding Directional Overcurrent and Earth Fault Protection.
- PSTSE High Impedance Differential Protection for series transformer.
- PSTSE Directional Overcurrent and Earth Fault Protection.
Phase Shifting transformer is the most economic approach to power flow management. In the coming future application of PST is going to increase manifold in India. Control and protection pf phase shifting transformer is vital from the perspective that PST is going to cost more that double the cost of normal power transformer. This article gives an overview of control and protection for a PST which will definitely be useful in this area.
Major thrust is given on differential protection which involves on-line reading OLTC position and provides online compensation for PST non-standard & variable phase shift. In practice one OLTC step means PST phase angle shift change by one or two degrees which can be easily corrected cyclically inside the modern numerical relays available now-a days.
Author is from BHEL, Piplani, Bhopal (MP).