In parallel with the development of the society, it is expected that more electricity demand. The expending scale of the distributed network will be needed to provide the electricity for the different level demand, but the power system will faced more challenge to remain the stable operation after subjected by the disturbance. The islanded power system may easy to be formed if the system suffers large disturbance and a part of the distribution network losses the connection with the main power system. Without the back system, generation and load becomes unbalance in the islanded system before it reconnects. In most conditions, the system can adjust the balance automatically, but if the amount of demand is too much, the frequency and voltage are decreased beyond the limitation value, which will cause the blackout or system break down. In recent years, consider the environment pollution issue, the renewable energy generation technologies are used to supply the electricity. However, it also brings more unstable factors to the power system. In order to remain the temporary islanded power system stable, some loads have to be shed. The load shedding scheme is used as a means of protection to protect the islanded system operation. The existed load shedding schemes can be separated into three types: undervoltage load shedding (UVLS) scheme, underfrequency load shedding (UFLS) scheme and combinational load shedding (CLS) scheme, but each scheme has its own limitation.
The aim of this project is based on the literature review, investigate, design and model the development of load shedding scheme to assist islanded operation of power systems. This report is mainly focus on the literature review, which is organized as following section. In section 2, some basic principles of the power system will be introduced, as well as the different renewable energy generation technologies. In section 3, 4, 5, will talk about the three load shedding scheme. Section 6 is mainly discussing the different load shedding scheme, the preliminary system modeling is in section 7.
Review of power system
Basically, a complete power system is consisted with three parts: the generation; the transmission network, which includes the sub-transmission system and the distribution system, and the utilization .
The turbine acts as the primary motor provides the mechanical power and the governor determine the turbine speed by controlling the amount of steam, gas or water input based on the reference speed. The generator aims to convert the mechanical power to electrical power to transmit to the load. The excitation system supply the excitation current to the rotor of the generator. And the function of the automatic voltage regulator (AVR) is to maintain the synchronous generator voltage at the predetermined value, or change the voltage according to the predefined plan. When the voltage of generator and the reactive power change, AVR will control the output excite current depending on the feedback signal to adjust the voltage and reactive power. When the amount of the system demand changed, the system frequency and voltage will be adjusted to balance the real and reactive power in the power system.
The transmission network connects the generation part and the load to transmit the electrical power which can be separated into the transmission system, which connect the all major generation part; the sub-transmission system is to connect the large customers with high voltage, such as the large industries; and the distribution system, which with the medium voltage to the individual customers, the primary side can provide the electricity to small power system, and the secondary side can supply to the commercial customers. The transmission line reactance is larger than the resistance, which indicates the transmission line mainly consume the reactive power from the power system.
The load in the power system can be divided in to the resistive load and motor load . The resistive load only has the resistance, such as the lighting, which indicates that this load only consumes the real power. However, the motor load, like the air conditioner or some equipment of the industry. This kind of load not only needs the real power but also requires the reactive power from the power system.
Renewable energy generating technology
With the development of the society, the renewable resources with its environmental friendly and low cost characters become more and more popular, for example, the wind, the biomass, the hydro, etc. Those nature energies can be used to create the electricity.
Wind is a kind of unlimited resource which through the wind turbine to creates the kinetic energy. Since the wind turbine through the gearbox drives the generator, the kinetic energy will be transformed into the electrical power. The wind turbine can be divided into horizontal axis wind turbine (HAWT) and vertical axis wind turbine (VAWT), the HAWT type is used more widely than another. Most of the wind generators are the induction machine. Wind turbine uses the High Voltage Direct Current Transmission (HVDC) decouples the frequency to the grid side, which results in no frequency response at generator side. Therefore, the system frequency drop has an obvious effect on the turbine.
Biomass is organic waste which can be used to create the electricity. Burning the Biomass to heat the water in the boiler, the steam through the steam turbine create the kinetic energy, and the steam turbine drive the rotor of the synchronous generator, then the mechanical power will be converted to the electrical power. Using Biomass to generate the electricity is stable and it reduces the amount of using fossil fuels. However, the collecting and growing the resource in enough quantities is difficult .
The process of using hydro to produce the electricity can be described as the energy transfer: potential energy - kinetic energy- electrical energy. The dam will be set to raise the water level. Water in the high level will be collected in the conduit to the low level, the water pressure will be created when it through the penstock and drives the hydro turbine. The hydro turbine connects to the generator, and the electrical power will be produced. After through the hydro turbine, water will flow into the draft tube to the tail-water reservoir. The advantage of using hydro energy is no resource wastes and pollutant emits. Using hydro energy to provide the electricity is not only stable, but also can supplement the power shortage. When the system power is shortage, the governor system opens the guide vane to increase the power, however, for the hydro turbine; it cannot provide the power as soon as possible, but decrease for a certain stage then begin increase . The cost of dam build is expensive and the quality of the water may be influenced.
System stability refers to the ability of power system can remain the stable operation under the initial operation condition, and also can regain a normal state of equilibrium after disturbing . In , the system stability can be divided into three types: the steady-state stability, the transient stability and the dynamic stability.
The steady-state stability means the ability for power system that whether or not the power system could continue operating in the stable condition after subjected by the small disturbances, such as load changes, but not over load.
The transient stability respects the ability of power system that whether or not every synchronous machine can keeping the synchronous operation reach in the new steady-state or recover to the original operation condition, when the power system is suffered from the large disturbances. Those disturbances include the loss of generator or large load; short-circuit on transmission line and loss of the tie line between two subsystems, etc.
Comparing with the transient stability, the dynamic stability stand for the ability of the power system can remain the short-term (sec-min) stable operation by the automatic adjustment after get the small disturbances or change the parameters of the controller.
Power system islanding and load shedding scheme
The Power system islanding refers to when a part of the distribution network is subjected by large disturbance then disconnect with the main system network. This disconnected part of the system is called islanded power system. Once the islanded system is formed, before it re-connected with the main power system, it still need to provide electricity, but the generation part may not be able to afford the adequate real and reactive power for the system demand, as well as the real power and reactive power compensation; the frequency and voltage will be dropped and system becomes unstable.
Load shedding scheme will be used to protect the system balance and against the disturbances, especially for the islanded power system. Load shedding scheme should make sure the system recovered in short time, loads need to be shed with the fast speed. Load shedding process also needs to be separated into several steps, and make sure the amount of disconnected load should be as less as possible. Set the rank for the load is important: non-vital, semi-vital and vital ; and the non-vital loads can be shed first to recover the power system balance, like the residential electricity. The vital loads include some important equipment and large industries.
Undervoltage load shedding (UVLS)
Voltage decrease in power system
Since with the increasing demand of the electricity, the number of transmission line will be increased, and the long transmission line also be needed to transmit the electrical power for customers. Therefore the power system is more depended on the transmission network. However, if the transmission line is tripped, based on the character of transmission line which mentioned in section 2, the reactive power losses will be increased, and result in the significant voltage drop at the receiving end. In addition, due to the motor type load installed in the power system, after the transmission line short circuits, the motor cannot trip immediately, thus, the more reactive power will be loosed .
The change of the reactive power will influence the system voltage. The relationship between reactive and voltage is described below. Assume X is the total reactance of the power system. Based on the following vector diagram, the real power and reactive power are :
Figure the voltage vector diagram
If the real power is constant, then the reactive power equals to
Assume the value of E is constant; therefore, if the reactive power decrease, the voltage also be dropped. Thus, it is important to keep the balance of the reactive power, some compensation equipments in the power system to compensate the reactive power are the shunt capacitors and the synchronous condensers, the AVR and excitation system in the generation part will also help to balance the voltage.
UVLS scheme is a cost-effective protection measure to avoid the long-term voltage collapse resulted in the large disturbances . When the power system cannot provide enough reactive power for the load, the voltage is decreased to the per-determined level in the per-identified period.
UVLS scheme types
UVLS scheme can be separated into two types: centralized and decentralized .
For the decentralized load shedding scheme, the undervoltage relays are installed in the load side, and measure the different load voltage. When the voltage decreases, the pre-determined load will be shed.
However, comparing with the decentralized scheme, centralized load shedding scheme has only one undervoltage relay, which with the logic control is installed at the key position in the power system and collects the voltage information from other buses. If the voltage drops too low, the load trip signal will be sent from the relay and transmit to the load side. The disconnected loads are not need to per-determined, and it will depend on the voltage drop information which collects in the relay.
The centralized load shedding scheme with high speed than the decentralized scheme, but it need the communication equipment and more complicated than the decentralized scheme.
Distributed UVLS scheme
For this load shedding scheme, the three important factors should be taken into consideration: the time, the location and the amount of load shedding. In , based on the typical rule which mentioned above, this distributed UVLS scheme, which with the distributed controllers to observe the voltage and control the load in the power system, can adjust to the disturbance location and severity.
For the single controller, set the T0 as the time when the measured voltage is smaller than the per-determined threshold voltage. Td is the delay time which depends on the time evaluation of the voltage. The logic equations of the controller are shown below. The amount of voltage change from T0 to T0+Td is constant C, which can be adjusted. The equation is:
From the equation (3), it can be seen that the more voltage drop, the smaller value of the constant C will be get, which indicates the faster shedding action will be taken. The condition is to avoid the other neighboring fault influence the controller. The two factors when set the voltage threshold value need to be considered: make sure the load shedding action can be taken and prevent the excessive shedding. Thus, the value of should be set a little bit smaller than the low voltage when the power system is suffered from the predicted disturbances.
The average voltage drop in this period will be calculated:
Thus, the amount of load shedding is:
Where the constant K also can be determined.
From the equation (5) and (6), the larger value of voltage drop result in the higher average voltage decrease and the more load shed. In order to minimize the amount of the load shedding, the value of K needs to be set large.
The amount of load shedding also should be bounded, to avoid the undesired transients.
Pk respects the single load power at Kth controller, â-³Psingle indicates the limited load shedding amount in single step, and â-³Pmax is the maximum of the total load shedding amount.
The working process of the controller can be separated into three states: the idle state, the preparation state and the shed state. The flow chart is shown below.
Figure the working process of single UVLS relay 
Obviously, when the measured voltage is equal or larger than the limited value, the controller stays at the idle state, unless the voltage is smaller than the threshold value, and the controller starts to working and takes the action to shed the load. In order to avoid shedding the load for other smaller disturbances, which may also cause the voltage drop, thus, another condition to limit the relay take action is. If thetime delay is elapsed since T0, the relay will remain at the preparation state until the voltage decrease keep for some time. The current time is act as new values of T0 to make the controller shed the load again if the system is remain unstable.
For the two closest controllers, the structure is shown below.
Figure the structure of the UVLS scheme 
Assume the controller1and controller2 are set at the bus A and bus B, respectively. Since the voltage is declined at the bus A, the voltage also will be dropped at the bus B, thus, if the controller1 shed some load, the voltage at bus A will be increased, and also influence the bus B voltage increase, which indicates the value of at bus B is larger, and the load shedding action will be delayed, thus, the amount of load shedding at bus B is smaller than before it determined. Therefore, the total amount load shedding also decreased.
This method has better performance to adjust the serious situation; what is more, the distributed structure also can measure the disturbance location faster. Moreover, it is simpler and does not depending on the communication equipments, the cost is not expensive. The drawback of this scheme is not considering the frequency drop when the system is disturbed.
Underfrequency load shedding (UFLS)
Factors influence the frequency in power system
The shortage of the real power cause by the loss of generator, or the distribution network disconnect with the main system. The spinning reserve cannot afford the enough real power for the small isolated distribution system; thus, the real power is unbalanced at the generation part and the load part. The real power deficit will cause the frequency drop in the power system. The relationship between the frequency and real power can be explained by the swing equation. "The swing equation is the fundamental equation that describes the rotor dynamics in transient stability ".Based on the Newton's second law, which determines the rotor motion, can be written as:
For the generation operation, the value of Te and Ta are both positive. In steady-state operation, those conditions need to be satisfied:
Obviously, in this situation, the speed of the rotor remains a constant, which called synchronous speed. If the difference value of Tm and Te is large than zero, the value of rotor angle acceleration is increased, as well as the corresponded rotor velocity is increased. On the other hand, the decreased value of illustrate the mechanical torque is larger than the electrical torque.
The swing equation is given as:
The mechanical power is the difference value between prime mover and mechanical losses, and the electrical power is the sum of the generator electrical power output and the electrical power losses.
The swing equation can be written as:
It can be seen that the value of frequency- , is influenced by themechanical power, electrical power and inertia constant. The unbalanced demand and generation provided is reflected to the unequaled mechanical torque and electrical torque. The more real power is needed from the generation part, which will result in the speed of the rotor increased. In order to keep the system operate at the steady-state, the mechanical torque also need to be accelerated, however, the turbine cannot afford such higher speed if it exceed the limited value, and then the governor cannot control the speed, the system becomes unstable and even breakdown.
With the purpose of avoiding the system frequency beyond from the acceptable range, UFLS scheme as a means of protection. The underfrequency relay with the logic block control or the communication equipment is installed in the power system to measure the frequency and decided whether or not the system should shed the load.
UFLS scheme type
Same as the UVLS scheme, UFLS scheme also can be divided in two types: centralized and decentralized . In addition, the UFLS scheme can also be separated to the traditional scheme, the semi-adaptive scheme and adaptive scheme .
Traditional UFLS scheme, which depends on the experience and simulation, is shed the certain amount of the load in per determined steps at the per-determined the frequency threshold when the measured frequency is decreased. The underfrequency relay does not need the complex logic and structure inside. Therefore, this scheme is useful and with low cost, but its limitation is also obvious. It cannot deal with the sudden and devastating damage due to the inflexibility. Shed the per-identified loads in limited steps may not enough if the frequency remain decrease. On the other hand, when the frequency dropped under the threshold value, shed a part of load is enough to recover the system frequency, however, because the amount of load shedding is determined, more loads will be shed finally.
A forward shed scheme is semi-adaptive UFLS scheme, which depends on the frequency response mode to measure the value of the first frequency threshold derivative. This value indicates the speed of the frequency declined and whether or not the power system will be subjected by a serious disturbance. Based on this value, the amount of load will be determined, however, other steps frequency limitation value and shedding amount need to decided before, like the traditional scheme.
Amore advanced scheme is the rate of change of frequency load shedding (ROCOFLS) scheme, which also depends on the frequency response mode, but it relies on measuring the initial slop of frequency decline, and to determine the size of disturbance. This method used widely and can easier to recognize the balance of the incipient real power. The principle of will be introduced in following section.
Rate of change of frequency load shedding scheme
For ROCOFLS scheme, those items should be taken into consideration: the minimum acceptable system frequency value for the stable operation, the number of steps, and the amount of load shedding. In , , , the ROCOFLS scheme is introduced and applied at the 60Hz, which is the standard frequency of USA.
The basic system frequency model is shown below.
Figure the system frequency logic model 
The function of boiler, turbine, governor, generator and network are mentioned before. The red area of the model is the swing equation with the damping factor D. this equation can be written as :
The damping factor influences the speed deviation between the generator and turbine, and respectsthe frequency changes.
The frequency response mode act as the important role in ROCOFLS scheme, the reduced order of its logic block is shown below.
Figure the frequency response mode 
Assume the disturbance power act as the input power, but it should be the negative value. Fhp respects the high pressure power fraction. Therefore, the frequency change equals to
Because the load power which should be shed equals the disturbance power, suppose Pshed respect the amount of load shedding power. Thus,
Transfer to the time domain
Therefore, the initial slop of frequency drop can be calculated:
The total inertia of the generation part is
Due to the standard frequency is 60Hz- the normal frequency, assume A is the initial slop of frequency drop, and the inertia is constant, the equation (126.96.36.199) can be written as:
Thus, the amount of load shedding power- the size of disturbance will be getting:
From the discussion above, obviously, the inertia plays a significant role in the frequency response model and also influences the amount of load shedding. Inertia is stands for the stored kinetic energy at the synchronous speed divided by generator volt-ampere rating . The lager value of inertia respects the less frequency decline and more stable system is.
In the practical application, the spinning reserve also needs to be considered. Onlyuse the value of initial slop of frequency decline to decide the Pshed is not enough, because the spinning reserve will compensate the real power. The secure margin value of slop of frequency decline is A0, and also the corresponding amount of load shedding is Pshed 0, therefore,
Those conditions aim to make sure the load will not be shed in one stroke. As mentioned above scheme will be separate in several steps, usually, the more load will be shed at the first two steps to ensure the system recover in short time, the amount of load shedding in other steps will be calculated.
The drawback of this scheme is that the voltage is not considered, and the amount of load shedding is more depended on the inertia value. If the inertia of the generation unit is changed, the shed amount may not accurate.
Combinational load shedding scheme
As mentioned above, after disturbed the power system, the voltage and frequency are both influenced, however, because the UFLS and UVLS are mainly designed for underfrequency and undervoltage condition, respectively. Therefore, the voltage drop and the frequency decrease will not be considered in UFLS scheme and UVLS scheme. With the development of the load shedding scheme, in , , the combinational load shedding scheme measures both frequency and voltage to control the load, which need to be shed.
The structure of the combinational load shedding scheme is shown below.
Figure the combinational load shedding relay structure 
The whole structure can be divided into three parts: the frequency calculation part- the yellow line area, the voltage calculation part- the blue line area and the comparison part- the green line area.
In the measurement unit, the voltage and frequency will be measured as well as the rate of change is also calculated. The purpose of the block: is to avoid the wrong value of the frequency derivative totrigger the MAX block and trip the load. Oncethe measured frequency larger than the limitation value, trigger the MAX and the corresponding frequency derivative also be sent to the MAX block to get the. The frequency offset value will be calculate based on the, which will be sent to the sum block.
In the blue line area, the voltage signal is sent to the sum block, and on the other hand, it will through the filter1 and the difference value will be got. Appreciate time constant T=0.3sec. The function of filter 2 is to get the integral of the voltage drop. The reset block for the filter is to in case of the slightly voltage decline. From the combinational function:
The relationship between frequency and is:
Hence, the initial frequency setting is
For the green line area, add the frequency offset and the initial frequency setting, the result should through the limit condition:
Then the final FSETTING will be got, which will be compared with the measured frequency. If the measured frequency smaller than the setting frequency, the load will be shed.
Obviously, this combinational scheme is based on the UFLS, but considers the voltage collapse. This scheme avoids the drawbacks of the UVLS scheme and UFLS scheme.
Comparison and discussion
In this section will focus on the difference between the UFLS scheme, the UVLS scheme and the combination load shedding scheme. Compare each scheme in following aspects and discuss the advantages and disadvantages
Voltage and frequency recover
As the three different methods introduced before, obviously, the UVLS scheme is mainly focus on the voltage drop, shed the load to recover the reactive power balance. For the UFLS scheme, which is to protect the real power balance in the power system, and have a remarkable effect to recover the frequency drop. However, the UFLS scheme may not have the influence on the voltage collapse, as well as the UVLS scheme for the frequency recover. The combinational load shedding scheme can remedy this defect, which is based on the UFLS scheme, consider the voltage drop in the power system. This scheme not only recovers the frequency but also improves the voltage stability margin of the power system and the power system security during the system is suffered by the shortage of generation and transmission lines tripped at the same time.
The central relay setting can prioritize the load and decides which is shed first. It relay on the communication equipment to achieve the control automatically and save the maintain cost. Comparing with distributed relay installed, the distributed structure is more simple and flexible to monitoring the voltage and frequency, but it will spend more time to gather the information and more maintenance costs.
In reality, when the relay is set in the power system, usually, the undervoltage relay need to be set at the primary side of the transformer, which is in case of the transformer wastage and the slightly load change result in the voltage collapse. Comparing with the underfrequency relay, because the frequency on the both sides of the transformer is equal, the underfrequency relay can be set at both sides.
The underfrequency relay, for example the electromechanical ones, is designed for the nominal voltage, which has no effect on the voltage drop , as well as the undervoltage relay. Whereas, the combinational load shedding relay can measures both frequency and voltage and synthetic judges the amount of load shedding.
Different capacity of power system
Since for the large distribution network, in order to make sure the system stable, the frequency should be controlled at the stranded frequency level (US: 60Hz; UK: 50Hz) with the acceptable range in everywhere. However, the voltage is different due to the different demand needs, the voltage drop may only have serious effect on the area network, and the voltage at other part of the system also will be influenced, but not obviously. However, if the frequency decreases shortly, the whole distribution will be sunk into a dangerous situation. Therefore, the UVLS scheme is an appropriate method to protect the local power system, but the UFLS scheme has an apparent effect on protecting the large distribution network.
Economy and development
The development of UFLS scheme is faster than the UVLS scheme, lots of adaptive UFLS scheme with logic was be launched. The complex logic function and structure result in the high cost of the adaptive load shedding scheme. Since the load shedding control will be tent to automatically, the communication equipment will be used in UFLS and UVLS widely.
In order to make sure the voltage and frequency both can be recovered, the combinational load shedding scheme is an appropriate load shedding scheme to against the combinational disturbance, even some serious damages. The current combinational does not rely on the communication equipment, but it also has the complex logic.
The real and reactive power balance will be illustrated by motioning the voltage and frequency in islanded power system. The frequency drop is mainly influence the balance of mechanical torque and electrical torque. Therefore, the unbalance between generation and load will result in the system breakdown or the blackout. Load shedding scheme plays a significant role to assist the islanded power system operation. Three different load shedding schemes are introduced in this report, the UVLS scheme - the distributed load shedding scheme and UFLS scheme - the ROCOFLS scheme are the basic scheme to shed the load, more adaptive schemes are mainly based on those scheme, like the adaptive SCADA load shedding scheme . The combinational load shedding scheme with its special characters is more popular in recent year, which considers both voltage and frequency drop to improve the system security.
Future work plan
The process of the project in this semester is almost finish, and in next semester is mainly focus on the design and modeling the islanded power system, as well as the load shedding scheme. Keep on reading, and the power system design is begin at the winter holiday. The first stage is to design the islanded power system, the second stage is design the load shedding scheme. The load shedding design will be based on the UFLS scheme, and consider the voltage collapse in the power system. The third stage is to optimize and modify the load shedding scheme. The Gantt chart will be given in the appendix.
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