Implementation of Variable Frequency Drives (VFD) on Boiler Feed Water Pumps for Drum Level Control

Table of Contents

Executive Summary:

Table of Figures

Introduction:
Background:
Responsibilities & Challenges:

Planning
Hardware Planning:
Software Planning:

Execution:
Sequential Logic Development
Mode Selection:
VVVF Drive ON/OFF Commands:
BFP Minimum Flow Protection:
Sequence System:
Control Logic Development
Boiler Drum Volume:
Boiler Drum Hold up Time:
Unit Run Back Logic:
Boiler Drum Water Consumption Rate:
Drum Level Controller Calculations:
Boiler Drum Level Adaptive Control:
Standby Drum Level Control:
DCS Self-sustainability:
Simulation & Commissioning:

Results & Achievements:

Executive Summary:

This is a technical report on the implementation of variable frequency drives (VFDs) for the boiler feed water pumps. It includes the planning, execution, calculations, simulation, testing and commissioning of the VVVF (variable voltage & variable frequency) drives.

The Lalpir Power Limited is an HFO fired thermal power plant having gross capacity of 365MW. It is situated Near Mahmud Kot, Tehsil Kot Addu, District Muzaffargarh, in the Province of Punjab, Pakistan.

The power plant is electrically connected with Water and Power Development Authority (WAPDA) system at 220 KV grid. It has one Generator Step-up Transformer (24 KV to 220 KV), one start-up transformer (220 KV to 11 KV), one auxiliary transformer (24 KV to 11 KV).

The boiler has a design capacity of 1200 T/H super-heated steam. To reduce plant electrical house load plant performance team suggested to install VVVF drives on motors for boiler feed water pumps. My responsibility in this task of execution team was to design, develop, test and commission the following systems for new VVVF drives including Graphic User Interface (GUI), Protection system, pump sequence control system, VVVF drive bypass system, speed control system, drum level control via VVVF drives, PID controller tuning without any support from Original Equipment Manufacturer (OEM).

The logic development was planned according to the control philosophy. The control and protections parameters were calculated as there was no data available from pumps OEM for VVVF drives operations. The drum volume, hold time and other controlling parameters were also calculated. Based on the process behavior, first time adaptive control was used for drum level control via VVVF drives.

The results of the control logic was very successful. The total deviation in the frequency control signal was 0.8 Hz at stable load which is a smooth signal for VVVF drive and pump operation also. At stable load, feed water flow standard deviation remained within 2%. The control of drum level on full load was excellent and drum level was with in 10mm band and control frequency was within 0.6 Hz range.

This is the first high level project of our plant in which we planned, design, develop and commissioned the system without any support from OEM. This is the first and only project in Pakistan in which adaptive control is used for Drum level control system and feed water pumps VVVF drives cut in /cut out automatically while controlling the drum level on frequency. There was not a signal failure in the logic execution and sequence system. The main goal of the project was to reduce the unit auxiliary load and the maximum reduction in unit auxiliary load was about 5 MW at 50% load and 3 MW at 100% load.

Table of Figures

Figure 1. Existing feed water system

Figure 2. Logic blocks in DCS subsystems

Figure 3. Mode selection logic

Figure 4. VVVF mode selection loop plate

Figure 5. Logic of ON/OFF command of 11 KV breaker

Figure 6. Graphic user interface of 11 KV breaker

Figure 7. VVVF drive manual ON/OFF command logic

Figure 8. Graphic user interface of VVVF drive ON/OFF command

Figure 9. Minimum Flow Protection Calculation & Graph

Figure 10. Minimum Flow Protection on VVVF mode

Figure 11. Sequence user Interface

Figure 12. VVVF ON END STATE

Figure 13. Feed water system main user interface

Figure 14. Drum Volume Calculations

Figure 15. Boiler Drum Hold up Time

Figure 16. Boiler Drum water Comsumption

Figure 17. Ultimate gain oscilation

Figure 18. Boiler Drum Level Loop Gain

Figure 19. Drum Level Controller

Figure 20. Feed water Controller

Figure 21. Main Level Control Valve Control

Figure 22. APC VVVF Drive ON Signal

Figure 23. VVVF drive drum level control at full load

Figure 24. Auto Cut In of BFP

Figure 25. VVVF drive duty change over sequence

Figure 26. Old Control Logic VS New Control Logic

Figure 27. BFP VVVF Drive Auto Cut off

Introduction:

Background:

Lalpir Power Limited is an HFO fired thermal power plant having capacity of 365MW. The boiler has a design capacity of 1200 T/H super-heated steam. There are three boiler feed water pumps each has a design flow capacity of 750 m3/H @ 220 Kg/cm2. The unit load can vary from 84 MW to 365 MW as per the National Power Control Center (NPCC) demand. Hence feed water demand also varies from 300 T/H to 1200 T/H according to the steam flow demand. At lower load (300 T/H) only one feed water pump is sufficient for boiler drum level. However, for full load (1200 T/H) two feed water pumps are required to maintain the feed water requirements. The third pump remains available as standby for the system. The all three pumps have individual minimum flow valves and discharge valve, but a common discharge header. All three feed water pumps operate directly through 11KV motors each has a capacity of 6200 KW. The boiler drum level is controlled via level control valve (LCV) installed on common discharge header line. Fig. 1 shows an overview of the system.

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Figure 1. Existing feed water system

To reduce the plant auxiliary load, plant performance team suggested to install Variable Voltage Variable Frequency (VVVF) drives on motors for boiler feed water pumps. The VVVF drives of Schneider Electric model # ATV1200-A2800-6666B5S were selected. Engineering team was given the task to hook up these drives with plant Distributed Control System (DCS) model Diasys Netmation of Mitsubishi Hitachi Power Systems (MHPS) Japan.

Responsibilities & Challenges:

I was assigned to lead this challenge with team of two I&C Engineers. My main responsibilities were to design, develop, test and commission following systems for new VVVF drives.

Graphic user interface

Protection system

Pump sequence control system

VVVF drive bypass system

Speed control system

Drum level control via VVVF drives

PID controller tuning

This was first major project in a sense that our engineering team was going to attempt execution in-house without OEM support. We faced following main challenges during the execution of the project:

1. DCS OEM (MHPS JAPAN) refused to share any information & guidance for VVVF hook up with the DCS system.
2. Feed water pumps OEM (WEIR Group) denied to provide any data to operate the pumps on VVVF system.
3. There was no prior reference available in Pakistan to run the feed water system automatically at VVVF system.
4. There was no data available for minimum flow of feed water pumps against pump speed.
5. We had to develop new logic in existing DCS with an option to bypass the new control logic to run the pumps on old control logic as bypass of VVVF drives with a single click at GUI.
6. The feed water pumps VVVF drives should cut in / cut out automatically depending on feed water demand.
7. In existing logic boiler drum level was being controlled by regulating the feed water flow through opening of control valves. In new logic, boiler feed water had to be controlled by changing the boiler feed pumps motor frequency. However level control valve must act as backup if drum level increase sharply due to malfunction of VVVF or load shedding due to electrical grid power supply interruption.
8. If one feed pump trips and standby pump fails to start when load is more than 50%, unit ‘Runback’ should occur and unit load drop to 180 MW.
9. There should be a system which optimize the boiler feed water pump performance at various speeds when operating in parallel.
10. The standard deviation in feed water flow must be within 2% at stable load to comply with ASME PTC 6 code.

Planning

Planning is an important phase in project management. All the required information was collected from control room and other relative departments to form the control philosophy. Several meeting were conducted with control room engineers (CRE) to finalize the graphic user interface schemes. There were following two main planning categories.

Hardware Planning:

A list of inputs / outputs was prepared for VVVF drives and hardware being reserved in the DCS. Procurement process was also initialized for the purchase of required hardware. The I/O list of one boiler feed water pump VVVF drive is as under.

Table 1. List of IOs for VVVF Drive Interface with DCS

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