5.7 Various operating states (Normal, alert, emergency, in-extremis and restorative) – State transition diagram showing various state transitions and control strategies
A properly designed and operated power system must meet the following requirements:
Table 1.1: Various elements of power system operation and control
S.No.
Operation and control action
Time period
1
Relaying execution control, system voltage control
Multi seconds
2
System frequency control tie-line power control
Few seconds to few minutes
3
Economic dispatch
Few minutes to few hours
4
System security analysis
Few minutes to few hours
5
Unit commitment
Few hours to few weeks
6
Maintenance scheduling
One month to one year
7
System planning
One year to 10 years
(a) The system must have adequate capability to meet the continuously varying active and reactive power demand of system load. This requires maintaining and approximately controlling adequate spinning reserve of active and reactive power at all time instants.
(b) The system should be designed and operated so as to supply electrical energy at minimum cost and with minimum adverse ecological impact.
(c) The electrical power supplied to the consumers must meet certain minimum quality standards with respect to the following:
i) The network frequency should be maintained within a range of ±3 percent of its ‘nominal’ value.
ii) The voltage magnitudes should be maintained within a range of ±10 percent of the corresponding ‘nominal’ value at each network bus bar.
iii) The supply should meet a desired level of reliability to ensure supply continuity as far as possible.
(d) It should maintain scheduled tie-line flow and contractual power exchange.
To meet the requirements at points i), ii) and iii) above, several levels of controls incorporating a large number of devices are needed.
Generating unit controls
The controls provided in generating units consist of prime mover control and excitation controls as shown in Figure. The controls are also called as local frequency control (LFC) and automatic voltage control (AVC).
These controllers are set for a particular operating condition and maintain the frequency and voltage magnitude within the specified limits following small changes in load demand. If the input to the prime mover is constant, then an increase in the active power of load at the generator terminals results in a drop in the prime mover speed. This then, causes a reduction in the frequency.
Controls in a Power System
On the other hand, an increase in reactive power demand at the generator results in the reduction of terminal voltage, if the excitation (generation field current) is kept constant. As the time constant of excitation system is much smaller than that of prime mover system, the coupling between LFC and AVC loop is negligible and hence they are considered independently.
Load frequency control
In LFC, two feedback loops namely, primary and secondary loops are provided. Both the loops help in maintaining the real power balance by adjusting the turbine input power. The primary LFC loop senses the generator speed and accordingly controls the turbine input. This is a faster loop and operates in the order of seconds.
Generator controls
But this loop provides only a coarse frequency control. The secondary LFC loop which senses the system frequency and tie-time power, fine tunes the frequency back to the nominal value. This is a slower loop and may take minutes to eliminate frequency error.
Automatic voltage control
In AVC, the bus voltage is measured and compared to a reference. The resulting error voltage is then amplified and applied to the excitation control system. The output of the exciter controls the generator field current. An increase in the reactive power load of the generator causes the terminal voltage to decrease and this results in generation of voltage error signal. The amplified error signal then increases the exciter field current which in turn increases the exciter terminal voltage. This increases the generator field current, which results in an increase in the generated emf. The reactive power generation of the generator is thus increased and the terminal voltage is brought back to its nominal value. The generation control maintains the active power balance in the system. It also controls the division of load active power between the generators in the system to ensure economic operation.
Economic dispatch
Economic operation and planning of electric energy generating system has been accorded due importance by the power system operators. Power systems need to be operated economically to make electrical energy cost-effective to the consumer and profitable for the operator. The operational economics that deals with power generation and delivery can be divided into two sub-problems.
One dealing with minimum cost of power generation and other dealing with delivery of power with minimum power loss. The problem of minimum production cost is solved using economic dispatch.
The main aim of economic dispatch problem is to minimize the total cost of generating real power at different plants in the system while maintaining the real power balance in the system. For system having hydro-plants, a coordinated dispatch of hydro-thermal units is carried out. The economic dispatch and minimum loss problems can be solved by means of optimal power flow (OPF) method. The OPF calculations involve a sequence of load flow solutions in which certain controllable parameters are automatically adjusted to satisfy the network constraints while minimizing a specified objective function.
The power system control objectives are dependent on the operating state of the system. Under normal operating conditions, the controller tries to operate the system as economically as possible with voltages and frequency maintained close to nominal values. But abnormal conditions like outage of a larger generator, of a major transmission line or sudden increase or reduction of system load can cause havoc in the system, if not properly controlled.
Different operating objectives have to be met in order to restore the system to normal operation after the occurrence of such contingencies.
Security analysis and contingency evaluation
For the analysis of power system security and development of approximate control systems, the system operating conditions are classified into five states: normal, alert, emergency, in extremis and restorative. The state and the transitions between them are shown in Figure.
Various operating stages:
Power system state transition diagram
1. Normal state
2. Alert state
3. Emergency state
4. Extremis state
5. Restorative state
Normal state
A system is said to be in normal if both load and operating constraints are satisfied. It is one in which the total demand on the system is met by satisfying all the operating constraints.
Alert state
A normal state of the system said to be in alert state if one or more of the postulated contingency states, consists of the constraint limits violated. When the system security level falls below a certain level or the probability of disturbance increases, the system may be in alert state.
All equalities and inequalities are satisfied but on the event of a disturbance, the system may not have all the inequality constraints satisfied. If severe disturbance occurs, the system will push into emergency state. To bring back the system to secure state, preventive control action is carried out.
Emergency state
The system is said to be in emergency state if one or more operating constraints are violated, but the load constraint is satisfied. In this state, the equality constraints are unchanged. The system will return to the normal or alert state by means of corrective actions, disconnection of faulted section or load sharing.
Extremis state
When the system is in emergency, if no proper corrective action is taken in time then it goes to either emergency state or extremis state. In this regard neither the load or nor the operating constraint is satisfied, this result is islanding.
Also the generating units are strained beyond their capacity. So emergency control action is done to bring back the system state either to the emergency state or normal state.
Restorative state:
From this state, the system may be brought back either to alert state or secure state. The latter is a slow process. Hence, in certain cases, first the system is brought back to alert state and then to the secure state. This is done using restorative control action.
Unit Commitment
The total load in the power system varies throughout a day and its value also changes with the day of the week and season. Hence, it is not economical to run all the units available all the time. Thus, the problem of unit commitment is to determine in advance, the start and the shut down sequence of the available generators such that the load demand is met and the cost of generation is minimum.
Maintenance scheduling
Preventive maintenance has to be carried out on power system components to ensure that they continue to operate efficiently and reliably. Generators are usually put on maintenance once every year. Their maintenance has to be so scheduled such that the available generation is sufficient to meet the system load demand. The problem of maintenance scheduling deals with the sequencing of generator maintenance such that sufficient generation is always available to meet the load demand and the cost of maintenances and cost of lost generation is minimum.