Engineering Consultancy, Electrical, Electronics, Project Management, Manufacturing, Products Marketing and General Services
When an earth electrode system has been designed and installed, it is usually necessary to measure and confirm the earth resistance between the electrode and “true Earth”. The most commonly… Read more
EEP – Electrical Engineering Portal
Initially we explored the idea of three-phase power systems by connecting three voltage sources together in what is commonly known as the “Y” (or “star”) configuration. This configuration of voltage sources is characterized by a common connection point joining one side of each source. (Figure below)
Three-phase “Y” connection has three voltage sources connected to a common point.
If we draw a circuit showing each voltage source to be a coil of wire (alternator or transformer winding) and do some slight rearranging, the “Y” configuration becomes more obvious in Figure below.
Three-phase, four-wire “Y” connection uses a “common” fourth wire.
The three conductors leading away from the voltage sources (windings) toward a load are typically called lines, while the windings themselves are typically called phases. In a Y-connected system, there may or may not (Figure below) be a neutral wire attached at the junction point in the middle, although it certainly helps alleviate potential problems should one element of a three-phase load fail open, as discussed earlier.
Three-phase, three-wire “Y” connection does not use the neutral wire.
When we measure voltage and current in three-phase systems, we need to be specific as to where we’re measuring. Line voltage refers to the amount of voltage measured between any two line conductors in a balanced three-phase system. With the above circuit, the line voltage is roughly 208 volts. Phase voltage refers to the voltage measured across any one component (source winding or load impedance) in a balanced three-phase source or load. For the circuit shown above, the phase voltage is 120 volts. The terms line current and phase current follow the same logic: the former referring to current through any one line conductor, and the latter to current through any one component.
Y-connected sources and loads always have line voltages greater than phase voltages, and line currents equal to phase currents. If the Y-connected source or load is balanced, the line voltage will be equal to the phase voltage times the square root of 3:
However, the “Y” configuration is not the only valid one for connecting three-phase voltage source or load elements together. Another configuration is known as the “Delta,” for its geometric resemblance to the Greek letter of the same name (Δ). Take close notice of the polarity for each winding in Figure below.
Three-phase, three-wire Δ connection has no common.
At first glance it seems as though three voltage sources like this would create a short-circuit, electrons flowing around the triangle with nothing but the internal impedance of the windings to hold them back. Due to the phase angles of these three voltage sources, however, this is not the case.
One quick check of this is to use Kirchhoff’s Voltage Law to see if the three voltages around the loop add up to zero. If they do, then there will be no voltage available to push current around and around that loop, and consequently there will be no circulating current. Starting with the top winding and progressing counter-clockwise, our KVL expression looks something like this:
Indeed, if we add these three vector quantities together, they do add up to zero. Another way to verify the fact that these three voltage sources can be connected together in a loop without resulting in circulating currents is to open up the loop at one junction point and calculate voltage across the break: (Figure below)
Voltage across open Δ should be zero.
Starting with the right winding (120 V ∠ 120o) and progressing counter-clockwise, our KVL equation looks like this:
Sure enough, there will be zero voltage across the break, telling us that no current will circulate within the triangular loop of windings when that connection is made complete.
Having established that a Δ-connected three-phase voltage source will not burn itself to a crisp due to circulating currents, we turn to its practical use as a source of power in three-phase circuits. Because each pair of line conductors is connected directly across a single winding in a Δ circuit, the line voltage will be equal to the phase voltage. Conversely, because each line conductor attaches at a node between two windings, the line current will be the vector sum of the two joining phase currents. Not surprisingly, the resulting equations for a Δ configuration are as follows:
Let’s see how this works in an example circuit: (Figure below)
The load on the Δ source is wired in a Δ.
With each load resistance receiving 120 volts from its respective phase winding at the source, the current in each phase of this circuit will be 83.33 amps:
So each line current in this three-phase power system is equal to 144.34 amps, which is substantially more than the line currents in the Y-connected system we looked at earlier. One might wonder if we’ve lost all the advantages of three-phase power here, given the fact that we have such greater conductor currents, necessitating thicker, more costly wire. The answer is no. Although this circuit would require three number 1 gage copper conductors (at 1000 feet of distance between source and load this equates to a little over 750 pounds of copper for the whole system), it is still less than the 1000+ pounds of copper required for a single-phase system delivering the same power (30 kW) at the same voltage (120 volts conductor-to-conductor).
One distinct advantage of a Δ-connected system is its lack of a neutral wire. With a Y-connected system, a neutral wire was needed in case one of the phase loads were to fail open (or be turned off), in order to keep the phase voltages at the load from changing. This is not necessary (or even possible!) in a Δ-connected circuit. With each load phase element directly connected across a respective source phase winding, the phase voltage will be constant regardless of open failures in the load elements.
Perhaps the greatest advantage of the Δ-connected source is its fault tolerance. It is possible for one of the windings in a Δ-connected three-phase source to fail open (Figure below) without affecting load voltage or current!
Even with a source winding failure, the line voltage is still 120 V, and load phase voltage is still 120 V. The only difference is extra current in the remaining functional source windings.
The only consequence of a source winding failing open for a Δ-connected source is increased phase current in the remaining windings. Compare this fault tolerance with a Y-connected system suffering an open source winding in Figure below.
Open “Y” source winding halves the voltage on two loads of a Δ connected load.
With a Δ-connected load, two of the resistances suffer reduced voltage while one remains at the original line voltage, 208. A Y-connected load suffers an even worse fate (Figure below) with the same winding failure in a Y-connected source
Open source winding of a “Y-Y” system halves the voltage on two loads, and looses one load entirely.
In this case, two load resistances suffer reduced voltage while the third loses supply voltage completely! For this reason, Δ-connected sources are preferred for reliability. However, if dual voltages are needed (e.g. 120/208) or preferred for lower line currents, Y-connected systems are the configuration of choice.
The excitation of a synchronous generator is usually done by an AVR (Automatic Voltage Regulator) that uses generator voltage and/or current as inputs in order to control its output to a pre-set value.
AVRs include different control modes to optimise performance depending on whether the generator is connected to the grid, or in island mode. Therefore, they can be set to maintain the voltage, the PF or the reactive power.
In this report, we will analyse the principle of operation of the voltage control mode of the AVR, known as droop compensation, when one or more generators operate in island mode or are connected to the grid. Based on droop control limitations, we will study techniques to improve its performance, and compare it with the cross-current compensation method.
In the voltage control or droop mode, the AVR is regulated by a droop characteristic, which is shown in the following drawing.
Figure 1. AVR Set-point V vs reactive power Q
The droop characteristic represents a graph of the AVR voltage set-point V as a function of the generator reactive power produced. This set-point regulates the generator terminal voltage when in island mode.
The interpretation of the above graph is that as the reactive power demand from the generator increases, the generator terminal voltage decreases. The set-point in the AVR is chosen so that when the generator reactive power Q supplied is zero, the generator VN is equal to the nominal voltage. If the initial AVR set-point is not changed, VL will be the voltage due to droop that the generator terminal voltage will reach operating in island mode against reactive load QL.
The reactive power generated is calculated from the generator voltage and current signals, fed back to the AVR. Droop compensation is set as percentage drop of the nominal voltage VN for maximum reactive power QL generated. Depending on the AVR, maximum reactive power is usually defined either as the reactive power exported at rated power factor, or as the MVA rating of the generator.
Droop setting can be given values from 0%, which effectively disables the droop, to a maximum of usually 20%, which could cause VL to drop to 0.8 p.u. Typically, a setting of 4-6% is chosen.
Droop compensation is a control technique designed when the generator is connected to the grid, so it is not required when one generator is in island mode.
On the other hand, when connected to the grid, droop compensation is required and the droop characteristic is used below to explain the control of the AVR.
Figure 2. Representation of AVR control when connected to the grid.
When a generator is directly connected to the grid, the grid voltage VG is fixed and cannot be controlled by the AVR. Any requirement for reactive power from the generator will result in the AVR internal voltage set-point V to change to meet the new demand. So, in the diagram of figure 4, the increased reactive power demand QL causes the AVR set-point to increase from VG to VL because of the droop compensation control.
According to the network topology, the following operating scenarios can be identified:
These three scenarios are analysed separately below.
This is the simplest case in terms of AVR control, as there is only one active component in the circuit that can affect the busbar voltage and react to any reactive load changes.
A single synchronous machine operating in island mode is only responsible for two actions:
The diagram below presents the simple case described.
Figure 3. One generator in island mode with droop enable contact.
The AVR in this case does not require droop compensation to control its output. In order to eliminate the droop effect, which would otherwise drop the circuit voltage with any increase in the reactive load, there are two possibilities:
In the case where there is connection to the grid, the AVR needs droop compensation in order to control its output. The circuit configuration and the droop characteristic for this case are presented in figures 3 and 4 respectively.
Figure 4. One generator synchronised to the grid.
The configuration above shows that by the operation of a simple contact the droop can be enabled or disabled, allowing the flexibility to disable it when in island operation and to enable it before connecting it to the grid. This eliminates the undesired effect of lower than nominal voltages when in island operation.
For the case of island mode operation with at least two generators connected in parallel to supply the load, the control of the voltage and the reactive power requirements have to be shared between the generators in parallel.
There are two control methods for the generator AVRs to achieve this:
In this case the following assumptions have to be satisfied:
In the simplest case, the AVRs can operate in droop compensation mode to obtain equal sharing of the reactive load. The relevant diagram is shown below.
Figure 5. Island mode with two generators in parallel in droop mode.
The two generators in figure 5 share equally the reactive load connected according to the droop characteristic of the AVRs and the setting applied.
Although this control mode is ideal when there is grid connection, in island mode it results in the voltage output being dependent on the reactive power demand. So, as the requirement for reactive power increases, the output voltage from the generators decreases due to the droop compensation.
Cross-current compensation or reactive differential is a method that allows two or more paralleled generators to share equally a reactive load, given that the following assumptions are satisfied:
The secondary wiring of the compounding CTs of all the generators to be paralleled have to be interconnected. Below, the wiring configuration for two generators set up for cross-current compensation is included.
Figure 6. Island mode with two generators in parallel with cross-current compensation
According to this method, the same current develops through the compounding CT’s of the generators in parallel, since they are identical, and when the CCC contact closes, it stops flowing through the AVRs, but only flows through the CTs.
The configuration above shows that by the operation of a simple contact the CCC can be enabled or disabled, allowing the flexibility to enable it when in island operation and to disable it before connecting it to the grid. This eliminates the droop effect and allows the paralleled generators to operate in island mode at nominal voltage when the reactive load increases.
The figure below shows the complete configuration with all the techniques explained previously incorporated for maximum functionality. This includes both the droop-compensation and the CCC enable-disable contacts.
In this case, when the generators are connected to the grid, all contacts must be open.
For paralleled generators in island mode, the droop contacts must be open and the CCC contact closed.
Figure 7. Island mode with two generators in parallel with cross-current compensation and droop disable contacts.