Power Management Systems

PMS Objective

The Power Management System (PMS) objective is to enhance electrical power system safety and reliability. The SYSTEM uses computers and networking technology for enhancing plant electrical system safety, reliability, operability and maintainability.

Design Aim

The aim of the design will be to achieve the following:

  • Personnel safety
  • Code compliance
  • Reliability
  • Operability
  • Low maintenance

Design Codes

The design will conform to the relevant IEC standards, national and local regulations.

Electrical System safety and reliability enhancement shall be achieved by increased monitoring, interlocking and distributed control and protection.

Plant operability shall be enhanced by providing operator assistance for safe and secure (error free) operation of the plant electrical system. Automatic functions are provided where necessary which aid in faster recovery. PMS includes functionality to automatically and manually generate reports to reduce operator workload.

Plant electrical system maintainability shall be enhanced by collating maintenance data (from condition based monitoring systems and historical plant data) and providing appropriate tools for data analysis and maintenance record management.

PMS includes features and functionality for performing off-line engineering analysis to predict electrical system performance based on plant data (on-line, historical, fault and disturbance data). Engineering analysis tools are being used to further enhance electrical system reliability by fine-tuning electrical system controls and protection set points and also aid in future electrical system expansions and upgrades. PMS also provides engineering access to the intelligent electronic devices (IED) for settings, configuration and other non-operational data such as fault records.

Load shedding and load blocking systems are provided as part of the overall generation and distribution system and will inhibit motor starting and /or remove potentially excess load to avoid a loss of stability and collapse of the power system. Shutdown of the load shedding and load blocking systems shall not cause disturbance such as tripping to any running loads. The load shedding system inputs will include circuit breaker status and power flow. It will be configured to respond to:

  • Fast changes, for example, electrical trip or generator trip.
  • If required, under-frequency beyond preset duration
  • If required, gradual loading toward / beyond maximum generation capability. Gradual load shedding is applicable to the Combustion Turbine Generators (CTGs) and is provided to ensure that the plant load does not cause the available generating capacity to be overloaded. Where required for system stability, load shedding will be extended to appropriate levels.

Following a load shed event, appropriate automatic changeovers and motor restarts are blocked to prevent system overloading.

After restoration of units, the electrical power system shall be manually restored to its normal state. Loads tripped via the automatic load shedding and loads blocked by the load blocking system shall require a manual reset operation at the PMS HMI. The PMS will provide PCS with information on blocking signals issued / removed and available spare generation capacity to assist in this process.

Load shedding controllers and system architecture are dual redundant. Failure or maintenance outage of any redundant component shall not cause any loss of functionality or degradation of performance.

Load shedding are implemented in an independent PLC with hardwired trip signals to circuit breakers. A detail load shedding and load blocking systems are developed during Detail Engineering phase.

Fast Acting Load Shedding

Following loss of one or more CTGs or occurrence of the other contingencies, if such act will cause running load to exceed power generation, sufficient load shall be immediately shed according to load shed priority tables to ensure continued operation of the remaining units. Load shed decision shall also consider step loading capabilities of the remaining CTGs.

The fast acting load shedding shall be executed no more than 200 milliseconds from occurrence of contingency to the opening of circuit breaker, including circuit breaker opening time. If stability analysis requires faster operation of the load shedding (<200 ms) to prevent collapse of the electrical system, then the load shedding system shall be designed to meet this requirement.

Load shedding / load reduction

The PMS shall trip load feeders to avert cascade failure of generation system. Load shedding shall be activated in the following situation:

  • Sudden loss of a generator leading to an overload of remaining generators on line.

The loads are automatically pre-selected based on a set priority. Changes to the priority load list can only be carried out by authorized operators.

The kW loading of all sheddable loads are monitored via hardwired KW signals from the respective HV switchboard cubicles.

Failure of power to the PMS shall not shut down the power system or inhibit control from the PCS.

Failure of any single system component, including IED, Ethernet switches, protocol converters, cables etc shall not result in loss of communication to the multiple IED.

Security procedures, access control, firewalls are aligned with the project information technology standards and procedures.

Performance, expandability and testing requirements are developed and incorporated into PMS specifications, drawings and documents.

Load Shedding

Load shedding will be implemented in case of trip of an on-line generator leading to an overload on the remaining generators.

The software will perform the following functions:

  • Determine the maximum power capability of an on-line power generating plant using generator available power.
  • Continuously calculate the load shed requirement based on the measured load (KW). The total load to be shed must reduce the loading of the remaining generators to a settable value – e.g. 95%. In certain cases, load reduction is applied instead of load shedding.
  • The above calculated results shall be displayed in real time on the PMS HMI showing ina different colour the consumers that would be subject to shedding or load reduction if a generator was going to trip.
  • The load shedding will be active also with bus tie-open and one bus de-energized. The PMS is not to be designed for split bus operation.
  • In the event of a generator trip the load shedding requirement is instantly activated; the load reduction is initiated and may take a short time to be achieved. If the load reduction is not achieved within a short time delay due to malfunction of the communication links, the PMS shall proceed with shedding the next consumers in the list. Once the power settable value is reached, the sequence must stop and restart from that point in case of second generator fault.

Control Functions

The PMS provides provides the following control functions

  • Generator kW load sharing on an equal percentage or base load;
  • Generator kVAr load sharing on either on an equal percentage or base load;
  • Load shedding
  • Load reduction
  • Prevention of large HV motor start in case of insufficient spinning reserve
  • Load increase limitation
  • Advice of high and low load conditions such that plant can be regulated by the operator (e.g. start / stop generators or reduce load);
  • Essential generator sequential starting, synchronizing and paralleling with transformers

The frequency will be adjusted by the PMS by means of raise / lower commands to the turbine governor.

The voltage will be controlled by the AVRs. Voltage setting will be adjusted by the PMS by means of raise / lower commands to the AVR.

 Generator active power load sharing

The real power load sharing is accomplished by the PMS, by measuring the active power (kW) delivered by the unit and adjusting speed as needed to achieve either an equal load sharing or base loading.

Generator reaktive power load sharing

The reactive power sharing is accomplished by the PMS, by measuring the reactive power (kVAR) delivered by the unit and adjusting the voltage as needed to achieve either an equal reacting load sharing or to keep constant the reactive power of base loaded (kW) sets.If there is a short fall in generator capacity due to the trip of an 11KV generator, the PMS will re-calculate the power system’s capacity and trip sufficient loads based on the running KW and shed priority.

Gradual overload of a generator is alarmed to the operator station. If a gradual overload occurs and is maintained for a time which is inversely proportional to the percentage overload, an overload alarm is issued.

The PMS calculates the available power from the number of generators in parallel and their individual capability as received from the turbine UCPs. The absorbed load of each consumer is measured using signals from the HV switchboard and summed to provide the load consumption.

If the spinning reserve i.e the difference between the available power and the consumed power is below a preset value, starting of large motors is inhibited.

For large motor starting, in addition to the spinning reserve criteria, a minimum number of generators are to be operating before a load can be started.

Status and Alarm functions

The PMS acquires the status of various network parameters such as:

  • Status (open, close, tripped) of all relevant circuit breakers (volt free contact from switchboards)
  • Gas turbine available power (4-20mA signal)  from the relevant UCP
  • Generator kW, kVAR and current
  • Consumers kW
  • Switchboards voltage

Alarms are included to indicate possible problems with the power system the PMS is controlling. The alarms are active for the duration of the problem and transmitted to the ICSS.

Operating modes

The system will have two operating modes, automatic and manual, selected from HMI.

  • In Automatic mode, the auto-start sequence is provided automatically;
  • In manual mode, the operator takes control and execute all the operation manually.

Functions and basic facilities

The following functions shall be provided:

  • Generator automatic / sequential start and paralleling in case of main power blackout.
  • Generator manual start and paralleling for testing purposes.

The following facilities are provided for each essential generator:

  • Generator start, stop & emergency stop pushbuttons
  • Generator status indication
  • Generator multi-function display meter (kW, kVAr, ect)
  • Generator auto synchronizer
  • Manual/auto operation selection
  • Supply voltage and frequency indication
  • Generator manual speed and voltage adjustment (including motorized potentiometer)
  • Generator circuit breaker open and close pushbuttons and associated status lamps

The following facilities are provided for all the CB’s.

  • Mimic representation (on HMI)
  • Open & close pushbuttons and associated status lamps
  • Manual/ Auto synch selector switch
  • Synchronizing command (synchronize and close the related CB)
  • Synchroscopes complete with double V& F displays and indicating lamps
  • Synchronize selector switch

Essential generator auto-start sequence details

The essential generators auto-start are performed in case of condition of dead-bus. This condition shall be determined by taking into the account the status of all circuit breakers associated with power sources that can feed the essential switchboard. If all generator circuit breakers, all transformer circuit breakers and the link to the emergency switchboard are all open, the bus is dead.

A “deadbus” condition detected by the PMS panel at the essential switchboard shall cause an output signal to actuate the required start air valve, subject to the generator being “ready to start”.

The PMS then will then send in this order:

  • A simultaneous start command to the three generators, waiting for the ready condition on each machine to be reached;
  • A closing command to essential switchboard tie-breaker;
  • A closing command to the circuit breaker of the first diesel in the a.m. sequence.

The remaining diesel generator sets shall then auto-synchronize and close onto the switchboard in accordance with the selected sequence. In the event that all diesel sets are unavailable or fail to start, an alarm to ICSS is provided after time period has expired.

Event log and trending

The PMS uses a windows application for sequentially logging and time stamping of events. If the application is online, the event will be displayed as and when they occur.

Each change of state of a digital input are logged and time stamped.

All analogue inputs, analogue outputs and digital input are logged at 1 second interval for trending purposes.

The event logger will have sufficient memory to record the last 7 days and playback predefined subsets of parameters.

It is possible to merge time stamped events together with trended values and export them for further analysis.